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SEISMOIDGY AND TBCI’ONICS
OF THE NORTH AMERICAN PLATE IN THE ARCTIC:
NORTHEAST SIBERIA AND ALASKA

By

David Bruce Cook

A DISSERTATION

Submitted to
Michi an State Universi
in partial ent of the reqmrements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Geological Sciences
1988

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ABSTRACT
SEISMOIDGY AND TECTONICS

OF THE NORTH AMERICAN PLATE IN THE ARCTIC:
NORTHEAST SIBERIA AND ALASKA

By
David Bruce Cook

Through the use of geological and geophysical methods, new conclusions
have been reached regarding the tectonic setting and history of three regions
of the North American plate within the Arctic. The settings of these regions,
the western Bering Sea margin, interior Alaska and the North American plate
boundary in northeast Siberia, are independently examined using seismological
methods or tectonostratigraphic terrane analysis. The results of each
investigation are summarized as follows:

The western Bering Sea margin is composed of five terranes of oceanic
affinity which were accreted during a period of Cretaceous to Miocene
convergence. Presently, extension is taking place in the western Bering Sea,
consistent with results from the remainder of the Bering Sea region.

In northern and western Alaska, however, intraplate earthquake focal
mechanism solutions indicate three distinct regions of stress orientation. A
northwest-southeast horizontal stress in much of interior alaska is created by
Pacific plate subduction at the Aleutian trench. Stresses in northern and
eastern Alaska are northeast-southwest, similar to the mid-continent region of
North American. The Yukon-Koyukuk province and the Bering Sea are
currently undergoing extension.

Interplate earthquake focal mechanism solutions in northeast Siberia and
its adjacent seas are used to delineate the position and character of the North

American plate boundary in Siberia. The onshore continuation of the Arctic

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mid-ocean ridge does not represent a continental rift, and is instead expressed
as a compressional zone within the Cherskii Mountains. This transition is
caused by the uniqueness of this boundary where the pole of rotation lies very
close to the plate boundary, causing an abrupt transition in faulting style. A

complete data set extends the Arctic mid-ocean ridge to the Aleutian-Kuril

arc-arc junction best fits a three plate system, with an independent Sea of
Okhotsk plate.

To Grandpa Cook,
who

iii

efined persistence and pride . . . .

ACKNOWLEDGEMENTS

When the completion of a challenge so engrossing becomes a reality,
everyone with whom I shared even a brief time must be thanked, for you all
somehow helped me through the past eight years.

Naming names is especially difficult, but what better place to start than at
the beginning: John McGuire and Uncle Ken, you are the ones who introduced
me to and gave me access to Geology - thanks for putting me on the right
track!

A strong influence was Kaz, who grabbed me as a Junior and forced me to
make decisions which were rarely easy but invaluable as lessons. As an
advisor, no one else will make you work so hard; yet Kaz plays more than an
advisor’s role, and for that he gets iii/y sincere ap reciation. I also thank the
rest of my committee, John Wilband, ike Velbel, ill Cambray and Larry Ruff,
who seemed to make the bad times worse, yet definitely made the result all the

better.

There are many other people within the department who deserve
tremendous thanks for their influence and friendship: Stoney, my "anchor man,"
Tom Vogel, who taught me to disprove before ever proving, and four people
whose assistance is irreplaceable: Loretta, the "woman in black" (don’t we still
have a date on the pool table?); Cath (who forced me to face reality and
volleyballs when I often ought not have ; Mona, who can always bring a smile
to her face and add a little cheer to the day; and fellow cave dweller and THE
Dungeon Master, Diane.

There were so many other friends who shared ood times and various
emotions during all this. Thanks to all of you, especrally Jim (m com uter
mentor), Bruce (who’ll g; down in belch histo ), Tim, Soo Meen, Cindy, on,
Jim, Tim, Frank, Amy, 11, Bob, Bill, Jim, Mi e, Beth, John, Dana, Ken, Deb,
Paul, Paige and Scott for a few thoughts on science, and alot of thoughts of
other things. Thanks to Chris S. for friendship and Philosophy on a starry
Texas night with California wine. Mitch was a great ab asmstant, but is an
even greater friend. Special thanks to Erin, who often was my verbal
Punchinlgh bag, yet still ave me a couch to sleep on when the nights got too
013g. anks also to t e staff at Eureka Elementary School, for encouraging
an helping both Shellon and myself.

Many people had a part in the scientific cooperation necessary to
complete thlS project. I thank Boris Baranov and L. Parfenov (whose verbal
and written input occasional] changed arm-waving into fact), and Chris
”Curly" Wolf (friend and consu tant in hardware, software, volleyball and a few
million other things). Computer software was rovided by Seth Stein (Surface
wave analysis), Glenn Kroeger (Bod -wave synt etics) and John Wilband ("XY"
and others). Thanks also goes to buck DeMets, and the NUVEL group, for
data prior to Bublication. For a wide variety of reasons, thanks also goes to
Norm Sleep, ave Stone, Art Grantz, Dave Scholl, Chuck Estabrook, Doug
Christensen, Bill Harbert, G. Leonard Johnson, Fumiko Tajima, Hiroo Kanamon
and Jack Sweeny.

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I am extremely grateful to Amoco Production Compan , which provided me
with funding for purchase of satellite imagery as well as t e facilities in which
to process the images (thereby gracing me with the first "sights" of my field
area). Assistantships and support were provided by the Department of
Geological Sciences, Michi an tate Universrty, and by a Continumg Doctoral
Fellowship awarded by t e College of Natural Science, Michigan State
University. Further su port was Brovided by Dr. Ted "Tornado" Fujita (who
also provided wit). 1s researc was funded, in part, by Office of Naval
Research contract no. N00014-83-K-0693 and by National Science Foundation
grant no. 87-21119.

Of course, my family was more than supportive throughout my whole
student career. Mom and Dad tau t me responsibility early, and let me make
my own mistakes and successes - love you for that. The rest of the family
sat through many a one-sided conversation and provided encouragement and
humor as necessary. First in my life, although she may not have always
believed it, was Shellon, who probably spent less time with me the two years
we’ve been married than she had before we were together. SSC, you’ve put up
with alot, and you are to be congratulated long before myself.

Finally, Dr. Zapp was a lifesaver more than once - thanks. Oh, and I
shant forget Lobachevsky, who showed me the light .....

 

:7/‘2?

TABLE OF CONTENTS

INTRODUCTION TO RESEARCH 1

R NE
TERRANES OF THE WESTERN BERING SEA MARGIN:
CONSTRAINTS ON THE EVOLUTION OF THE BERING SEA

Introduction 4
Bering Sea Setting 7
Western Bering Sea Terranes 18
Evolution 53
Conclusions 61
List of References 62

W
INTRAPLATE SEISMICI'TY OF ALASKA:
IMPLICATIONS FOR REGIONAL STRESSES

Introduction 69
New Focal Mechanisms 72
Stress Indicators 121
Stress Regimes 124
Conclusions 131

List of References 133
W '
SEISMICITY AND FOCAL MECHANISMS OF THE
NORTH AMERICAN-EURASIAN PLATE BOUNDARY IN SIBERIA
Introduction 138

Tectonic Setting 143

Seismicity

Focal Mechanism Solutions

Tectonic Implications

Role of Pre-existing Structure

Conclusions

Appendix 3A: Discrepancies Between Moment Tensors
and P-wave Data: Potential Causes and

Implications

Appendix 3B: Determination of Possible Range of Fault
Slip Using the Method of Angelier (1979)

List of References

Wmm INTERACTIONS IN NORTHEAST ASIA:
NORTH AMERICAN, EURASIAN AND OKHOTSK PLATES
Introduction
Seismicity and Focal Mechanisms
Slip Vectors and Poles of Rotation
Discussion

Conclusions

List of References

vii

145
146
184
195
205

207

210
216

220
224
233
239
245
247

 

Table ll. 1

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LIST OF TABLES

Table 2-1. Earthquake parameters for Alaskan earthquakes
Table 2-2. Well elongation stress data, northwest Canada
Table 3-1. Focal mechanisms and slip vectors

Table 3-2. Fault parameters in the Cherskii Mountain system
Table 3-3. North American - Eurasian pole studies

Table 4—1. Focal mechanisms and slip vectors

Table 4-2. Comparison of poles of rotation

viii

83
122
149
187
204
23 1
240

Figure 1-1.
Figure 1-2.
Figure 1-3.
Figure 1-4.

Figure 1-5

LIST OF FIGURES

Regional Geography of the Bering Sea
Geography of the Western Bering Sea
Satellite mosaic of the Olyutorsk region

Map of terranes identified in this study

. Strati aphic columns of terranes

a. Lithologic sym 015

b. Ukelayat-Central Kamchatka terrane

c. Cretaceous-Paleogene Sediments terrane
d. Goven-Eastern Ranges terrane

e. Tertiary Accretion

Complex terrane

f. East Olyutorsk-Shirs ov terrane
g. Miocene volcanics

Figure 1-6.
Figure 1-7.
Figure 2-1.
Figure 2-2.
Figure 2-3.
Figure 2-4.
Figure 2-5.
Figure 2-6.
Figure 2-7.
Figure 2-8.
Figure 2-9.

LANDSAT image of the KPS terrane

Tectonic evolution of the western Bering Sea
Regional geography of Alaska

Synthetic seismograms for 1968 Rampart event
Rayleigh wave radiation pattern for Rampart event
Master event relocation for Rampart event

Focal mechanisms for Rampart aftershocks
Synthetic seismograms at station AKU

Focal mechanisms for the March 9, 1985 earthquake
Cross sections from Estabrook (1985)

Focal mechanism for the December 20, 1966 earthquake

Figure 2-10. Focal mechanism for the 1981 Kobuk Trough earthquake

Figure 2-11. Master event relocation for the Kobuk Trough earthquake

 

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Figure 2-13. P-wave mechanism and synthetics, December 13, 1964 event
Figure 2-14. Map of the Kigluaik-Bendeleben fault system

Figure 2-15. Ritsema (1962) solution for April 7, 1958 earthquake

Figure 2-16. Focal mechanism of the April 7, 1958 Huslia earthquake
Figure 2-17. P—wave first motions for the April 11, 1973 event

Figure 2-18. First motions and Rayleigh wave pattern for the
January 22, 1968 Barter Island earthquake

Figure 2-19. Synthetic seismogram at State College, Pennsylvania
Figure 2-20. Map and seismic section in Barter Island vicinity
Figure 2-21. Plot of principle stress orientations

Figure 2-22. Proposed distribution of stresses in Alaska

Figure 3-1. Regional geographic map of northeast Siberia

Figure 3-2. Shallow and intermediate teleseismic seismicity
of northeast Siberia and adjacent Arctic mid-ocean ridge

Figure 3-3. Distribution of seismicity for the year 1976

Figure 34. Solutions and synthetics for the August 25, 1964 event
Figure 3-5. Solutions for the December 3, 1960, event

Figure 3-6. Solutions for the April 7, 1969, event

Figure 3-7. Synthetics for the April 7, 1969, event

Figure 3-8. Synthetics for the April 7, 1969, event

Figure 3-9. Solutions for the June 10, 1983, event -

Figure 3-10. P-wave solution and Rayleigh wave radiation pattern
for the July 21, 1964, Bour Khaya Bay earthquake

Figure 3-11. Synthetics for the July 21, 1964, event
Figure 3-12. Waveform dependence on focal mechanism solution

Figure 3-13. P-wave solution and Rayleigh wave radiation pattern
for the February 1, 1980, Lena River delta earthquake

100
103
105
107
110
112

114
117
119
123
125
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147
152
155
157
159
163
165

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172

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Figure 3-14. P-wave solutions for the May 5, 1963, and August 12,
1975, earthquakes

Figure 3-15. P-wave solution for the April 19, 1962, event
Figure 3-16. Solutions for the November 22, 1984, event
Figure 3-17. Solutions for the January 21, 1976, event
Figure 3-18. Solutions for the September 9, 1968, event
Figure 3-19. Synthetics for the September 9, 1968, event

Figure 3-20. Horizontal slip and fault azimuths in the northern
and southern Cherskii Mountains

Figure 3-21. Predicted range of slip azimuth for the
northern Cherskii Mountains

Figure 3-22. Predicted range of slip azimuth for the
southern Cherskii Mountains

Figure 3-23. Graphical representation of Angelier’s (1979) method

Figure 3-24. P-wave solution and Raylei wave radiation pattern
of the December 15, 1973, New Siberian Islands earthquake

Figure 4-1. Index map of northeast Siberia

Figure 4-2. Shallow and intermediate teleseismic seismicity
of northeast Siberia and the adjacent Arctic

Figure 4-3. Focal mechanisms of earthquakes in northeast Siberia
and the adjacent Arctic

Figure 4-4. Horizontal projections of slip vectors and
poles computed in this study

Figure 4-5. Schematic plate configuration in northeast Siberia

176
180
182
185
188
190

197

200

202
212

214
221

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228

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INTRODUCTION TO RESEARCH

The North American plate occupies a major portion of the globe north of
the Arctic circle. However, the exact location and nature of its boundaries,
particularly within the Soviet Union, is poorly known. This is due, in part, to
the general remoteness of the Arctic region as well as to the complete
inaccessibility to regions of northeastern Siberia, USSR to western scientists.
These two factors combined have prevented any major accumulation of reliable
geological or geophysical data over an area of millions of square kilometers.
The goal of this dissertation is to examine, to whatever extent is possible, the
geology and tectonics of the North American plate in two neighboring regions
of the Arctic: Alaska and northeast Siberia. ,

A logical place to begin is within the Bering Sea. This body of water
spans the gap between the two continental regions in question. Chapter One,
Terranes of the Western Bering Sea Margin: Constraints on the Evolution of
the Bering Sea, incorporates a previously underutilized data source, Soviet
publications, with relatively recent LANDSAT imagery data. Commonly, Soviet
investigators will not recognize plate tectonic theory, and would rather explain
all geological observations in terms of "vertical tectonics" and ”oceanization."
Therefore, Soviet publications were important more for their data and
descriptions than for their interpretations. Completion of this phase of the
research laid a good groundwork from which to expand both east (into Alaska)

and west (into Siberia). More importantly, I became exposed to the intricacies,

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however unpleasant and frustrating, of working with Soviet literature dealing
with an area of complete physical inaccessibility.

Alaska, relatively less remote and less difficult to access, provides a
great deal more background than its neighbor to the west. In Chapter Two,
Intraplate Seismicity of Alaska: Implications for Regional Stresses, the
fundamentals of seismological investigations were developed for the Arctic
regions. Since a great deal of data is available from Alaska, the reliability of
the data could be more easily verified. This, in turn, outlined potential
limitations of further seismological investigations in areas of lower data density,
such as northeast Siberia. The intraplate seismicity of Alaska is not of great
magnitude. In fact, nearly all events are less than Mb 6.0, and many are in the
Mb 4.5 - 5.0 range. There are inherent difficulties in working with such small
events. Signal-to-noise ratios are small, data are often constrained to
short-period waveforms, and the limits of advanced numerical techniques (e.g.,
surface-wave analysis and body-wave modelling) are often strained if not
exceeded.

In light of such constraints, it is important to note that the conclusions
based on seismological investigations of Alaska and northeast Siberia are by no
means definitive or unique. In fact, the interpretation of any one focal
mechanism solution may give an ambiguous conclusion at best. However, the
consistency observed among many mechanisms, despite their individual
limitations, is where the strength of this type of investigation is expressed.
Even more encouraging is the realization that many of the numerical methods
are found to work down to magnitudes of 5.5 and often as low as 5.0.

The final application, having laid the foundation and tested it, is in

Chapters Three and Four. The seismicity of the North American plate boundary

 

 

3

in northeast Siberia had been poorly defined. The determination of epicentral
distribution and source parameters was necessary in order to identify the
present-day kinematics of the plate boundary. Chapter Three, Seismicity and
Focal Mechanisms of the North American-Eurasian Plate Boundary in Siberia,
examines the seismicity in detail, arriving at conclusions regarding the seismic
character along sub-segments of the plate boundary in northeast Siberia and the
Laptev Sea. Chapter Four, Present-Day Plate Interactions in Northeast
Asia: North American, Eurasian and Okhotsk Plates, utilizes the results of
Chapter Three and of previous studies in order to examine the present nature
of the North American plate boundary in northeast Siberia. Interestingly
enough, the plate boundary ends where Chapter One began - in the western
Bering Sea.

CHAPTER ONE

TERRANES OF THE WESTERN BERING SEA MARGIN:
CONSTRAINTS ON THE EVOLUTION OF THE BERING SEA

INTRODUCTION

The western margin of the Bering Sea consists of an amalgamation of
tectonostratigraphic terranes (Jones et al., 1977) which form present-day
Kamchatka Peninsula and Koryakia (Koryak Highlands). Along this margin,
subduction, rifting, and volcanism (island arc, Andean-type and intraplate) have
each been active for some time period since the Cretaceous. The complexities
of this varied tectonic history are preserved within the terranes bounding the
western Bering Sea. This paper delineates the terranes of the
Kayak-Kamchatka system which have accreted since Cretaceous time, and uses
characteristics of these terranes as an aid in constraining the tectonic
evolution of the Bering Sea region.

Koryakia is composed of several terranes with oceanic affinities which lie
between the Okhotsk-Chukotsk volcanic belt and the Bering Sea (Figure 1-1).
The oceanic terranes’ basement ages and times of accretion young in a seaward
direction. Northwest of the Okhotsk-Chukotsk volcanic belt, terranes with
continental affinities (e.g., Omolon and Prikolymsk; Fujita and Newberry, 1983)
predominate. To the south is Kamchatka Peninsula, which is composed of ten
terranes having oceanic affinity that have accreted since Mesozoic time
(Watson, 1985; Watson and Fujita, 1985). Numerous volcanic arcs, presumed to
have developed through interactions of the Farallon, Izanagi, Kula and Pacific
plates, are preserved within the Kamchatka terranes.

The terranes of the western Bering Sea margin provide new insights on
the evolution of the Bering Sea region. Many investigators have examined the
general terrane makeup within northeast Siberia, but not at sufficiently fine
scale to constrain the regions Cenozoic evolution. Within Koryakia, Churkin

and Trexler (1981), Chuan (1983), Fujita and Newberry (1983) and Natal’in

 

Figure 1-1: Regional geography of the Bering Sea Re ion. The Ohkotsk -
Chukotsk volcanic belt is shown in the s aded area. Arrows
represent present-day Pacific convergence direction of Engebretson,
et al., 1985. Basins and canyons abbreviated as follows: NT-
Norton Basin; NV - Navarin Basin; PC - Pribilof Canyon; SG - St.
George Basin; ZC - Zemchug Canyon.

 

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and Parfenov (1983) have all identified the accretion of "Olyutorsk" (Figure
1-1) in the Cenozoic. Kazimirov (1985) considered Olyutorsk to be a volcanic
and siliceous-sediment filled depression, fault bounded to the north. Only very
recently did Koltypin and Kononov (1986) identify several discrete terranes in
Olyutorsk which accreted independently during Cenozoic time. I shall use
”Olyutorsk” to represent a subregion of Koryakia rather than a distinct terrane,
and define it as the area between the Ukelayat and Vyvenka river basins to the
north and the Bering Sea to the south (Figure 1-1).

Since the area of interest is not accessible to western scientists, I have
used two primary methods of investigation: published literature (both in
Russian and in translation), and satellite image interpretation. Satellite
imagery at scales of 1:1,000,000 (mosaics) and 1:250,000 (LANDSAT
Multi-Spectral Scanner (MSS)) enabled me to identify structural trends and
surface morphologies useful in terrane identification and characterization.
These two primary data sources were supplemented by Soviet geologic maps. I
recognize that there are limitations in this method of terrane analysis, such as
non-uniform data distribution and inability to verify some of the information.
However, for a region which is so remote and inaccessible, such an approach
can provide useful and otherwise unobtainable information on the geologic

make-up and evolution.

BERING SEA SETTING
The Bering Sea is separated from the Pacific basin by the Aleutian island
arc, and is bounded to the northeast by the Alaskan shelf (Beringian margin).
Within the Bering Sea there are three major basins (Aleutian, Bowers and

Komandorskii), each of which apparently have distinct tectonic histories. There

8
are also two prominant undersea rises, the Shirshov and Bowers ridges (Figure
1-1).

Determining the genesis and present geologic character of these features
is crucial for developing a model of the Bering Sea’s evolution. Below, I
summarize present knowledge regarding these features, which I combine with
terrane data from the western Bering Sea to develop a model for the evolution
of the Bering Sea region.

B . . I I 'n

A series of northwest trending basins line the outer margin of the
Alaskan shelf. These basins, originally thought to have formed during the
Paleogene, now appear to have been more active in the Miocene (Harbert et
al., 1987). The oldest sediments identified from the Cost wells within the
Navarin, Norton and St. George basins are Eocene (Turner et al., 1984).
However, these Eocene sediments form a thin, discontinuous layer in most of
the basins, and the dominant sediment thickness is of Miocene age. Thus, the
basins appear to have developed around Eocene time, with renewed or increased
subsidence during Miocene time.

Miocene to present extension is also suggested further inland by basaltic
volcanism in the Yukon-Koyukuk province of Alaska (Nakamura et al., 1980) and
by the occurance of extensional earthquakes (Sykes and Sbar, 1974; Liu and
Kanamori, .1979) . Additionally, extensional features, discussed below, are also
present in the Komandorskii basin to the west. Therefore, extension is taking
place on a regional scale in the Bering Sea and along many of its margins.

51 . E .
The Aleutian basin is the largest of the three major Bering Sea basins,

and has been intensely investigated by a number of workers (e.g., Scholl et al.,

9

1975; Marlow et al., 1982; Cooper et al., 1976a). These investigations have
shown the basin to be underlain by ”normal” oceanic crust (Scholl et al., 1975)
with a depth to Moho of about 16 km (Cooper et al., 1977; Bogdonov and
Neprochnov, 1984), about 4 km deeper than under the Pacific plate. Kienle
(1971) found that the free-air gravity anomaly within the Aleutian Basin was
approximately zero, and concluded that the basin was in isostatic equilibrium.
Magnetic anomalies trending nearly north-south have been identified within the
basin by Cooper et al. (1976a), who identified them as M1 to M13 (117-132 Ma).
More recent work in conjunction with GLORIA has supported the existence of
these anomalies (Cochrane et al., 1987), although age modelling has not yet
been completed.

The sediment layer within the Aleutian basin is quite thick, ranging from 4
to 9 (Scholl et al., 1975) or 11 km (Bogdanov and Neprochnov, 1984). The
oldest of these sediments are probably Cretaceous to Tertiary in age (Scholl et
al., 1975). Heat flow within the basin is not unusual, ranging from 41 to 75
milliwatts per meter squared (mW/mz) (Ben-Avraham and Cooper, 1981) and
averaging 62 mW/mz.

The history of the Aleutian basin has been debated by many authors (e.g.,
Scholl et al., 1975; Cooper et al., 1976b; Marlow et al., 1982; Ben-Avraham and
Cooper, 1981). A critical point is the age of the underlying crust. It has been
suggested that the crust is a trapped remanent of either the Kula plate
(e.g., Scholl et al., 1975; Ben-Avraham and Cooper, 1981) or Izanagi plate (Rea
and Dixon, 1983). If the Cretaceous age reported by Cooper et al. (1976a) is
correct, the trapping hypothesis is plausible. If, however, the crust is younger,

a different evolution must be explored. In any case, there is no good evidence

10
for a spreading center within the basin, seemingly eliminating the possibility of
in situ formation.
H w B . ”3.!

Located in the south-central Bering Sea, adjacent to the Aleutian arc, are
the Bowers basin and ridge. Bowers basin is poorly studied relative to the
other major basins of the Bering Sea.

Bowers ridge stands more than 2.0 km above the surrounding sea floor
(Cooper et al., 1981), and may be a remnant volcanic arc (Kienle, 1971;
Ben-Avraham and Cooper, 1981). Seismic reflection and refraction data
(Ludwig et al., 1971a; Scholl et al., 1975; Cooper et al., 1981), gravity surveys
(Kienle, 1971) and magnetic profiles (Kienle, 1971; Cooper et al., 1976a, 1981)
suggest that westward directed subduction occurred under the eastern flank of
the ridge. Sediments of at least early Tertiary age are found within summit
basins of the ridge (Scholl et al., 1975). *

Bowers ridge extends from the Aleutian ridge in an arcuate shape, convex
northeastwardly, then narrows and plunges westward into a channel separating
it from the southern tip of the Shirshov ridge. Within this channel, the
continuation of the structure is expressed as a small ridge with a slight
southward convexity. ‘ Data from GLORIA surveys (Cochrane et al., 1987)
indicates there is a basement continuity between the two structures, although
Rabinowitz (1974) argues from seismic profiling that they are separate
structures.

Bowers basin may be intermediate between the Aleutian and Komandorskii
basins. There is a small variation (-30 to +30 mGals) of the free-air anomaly
within the basin, and magnetic signatures, if present, are weak. Cooper et al.

(1976a) suggest that any magnetic anomalies may, have been effectively

11
demagnetized by the thermal influence of Pacific subduction under the Aleutian
arc. Sediment thickness within Bowers basin is slightly less than in the
Aleutian basin (Scholl et al., 1975), although depth to basement appears to be
the same.
AlentianALc

The southern boundary of the Bering Sea is formed by the Aleutian island
are, which extends westward from Alaska Peninsula to Kamchatka Peninsula.
The total distance along the strike of the arc is more than 2000 km, and
Pacific plate convergence ranges from nearly orthogonal in the east to parallel
in the west, along the Komandorskii Islands. Structural trends and
geochemical similarities indicate that Cape Kamchatka (Figure 1—2) may actually
be an accreted fragment of the westernmost Aleutian arc (Watson and Fujita,
1985; Zinkevich et al., 1987).

The are is thought to have formed at least 50 my. ago (Eocene), based
upon ages obtained from the Finger Bay volcanics on Adak Island. Since these
are the oldest exposed rocks, an even older initiation of arc formation is
plausible. Cretaceous sediments found within the Aleutian basin to the north
(Scholl et al., 1975) may also suggest an earlier, Cretaceous, formation of the
Aleutian arc (Scholl et al., 1983). A cessation of volcanism and associated
uplift and erosion along the ridge beginning at 15 Ma is indicated by the high
erosional rates and broadly deformed rocks of that time (Marlow et al., 1973).
This volcanic quiescence, which continued until approximately 3 Ma, was
suggested by Anderson (1970) to be the result of plutonic emplacement along
the length of the are. An equally speculative hypothesis is that such broad
Uplift and deformation may have accompanied the subduction of the extinct

Pacific-Kula ridge (DeI.ong et al., 1978), a small segment of which is still

12
preserved south of the central Aleutians (Moore et al., 1987). Since 3 Ma, new
stratovolcanos have formed north of the previous volcanic centers, lying
unconformably above the erosional remnants of the earlier volcanic arc (Scholl
et al., 1975).

The genesis of the Aleutian arc has been debated by many investigators
who seem to disagree on the latitude of its initiation. Some favor a southern
formation of the arc, with subsequent northward migration as part of a
megaterrane which was emplaced along southern Alaska and northeast Siberia
(Moore et al., 1983). Others argue that the arc has formed nearly in situ
(Harbert, 1987), possibly along an old transform fault on the Izanagi plate
(Woods and Davies, 1977). Recent paleomagnetic data (Harbert, 1987), tend to
support the in situ formation hypothesis.

Shmhonflidge

Shirshov Ridge is a highly asymmetric feature which separates the
Aleutian basin on the east from the Komandorskii basin on the west. It rises
some 1 to 2 km above the surrounding basins, and is a bathymetric (Scholl et
al., 1975), gravimetric (Ben-Avraham and Cooper, 1981) and magnetic (Cooper
et al., 1976c) offshore extension of the Olyutorsk Peninsula (Figure 1-2). The
eastern margin of the ridge slopes gradually into the Aleutian basin; some of
the uppermost overlying sedimentary cover continues undisrupted from the ridge
to the Aleutian basin (Savostin et al., 1986). Some sedimentary layers to the
east of the ridge are folded (Ludwig et al., 1971b). The western flank of
Shirshov ridge drops steeply into the Komandorskii basin. This is due to a
large number of faults which dissect and downthrow the ridge’s western margin
into the Komandorskii basin. Savostin et al. (1986) report that these are listric

normal faults. Some of the sedimentary cover on the Shirshov ridge is thicker

5m 1-

13

Figure 1-2: Detailed eogaphy of the western Bering Sea re 'on. "DSDP 191"
is the 19 2 eep Sea Drilling Project site 1511 Note Pacific
convergence (arrow; Engebretson et al., 1985) is nearly parallel to
western Aleutian trench strike.

114

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15
(up to a maximum of 2 km; Neprochnov et al., 1985) than in the Komandorskii
basin (Bogdanov and Neprochnov, 1984). There is, however, variation of the
sediment thicknesses within the summit basins of Shirshov ridge, with generally
thinner deposits on the western flank (Ludwig et al., 1971b).

Extensive dredging has been conducted on Shirshov ridge, recovering
reportedly basement samples and overlying sediments. However, due to
extensive glaciation, the potential for ice rafted debris necessitates extreme
caution in using these rocks for dating or characterization of their source.
Neprochnov et al. (1985) and Peyve and Yurkova (1987) describe "freshly
chipped” basement recoveries which range from gabbro to low-titanium basalts,
similar to those found onshore in the Olyutorsk Peninsula. Neprochnov et
al. (1985) also discarded many samples which they suspected to be
allochthonous.

Peyve and Yurkova (1987) presented a detailed analysis of the dredge
samples, and assigned a Cretaceous age to the basalts of Shirshov ridge.
Using a variety of major and minor element (e.g., Ti, Al, Na, K, Cr, Ni, Zr, Y)
analyses, Peyve and Yurkova (1987) attempted to assign a tectonic setting to
basalts of both Shirshov ridge and Olyutorsk range. Many of their
classifications were based upon highly mobile elements (e.g., Sr and K),
therefore, I ignored results based upon these elements. However, when using
less mobile elements (e.g., Cr and Ti), basalts of the Olyutorsk range
(Machevna complex) clearly clustered along Peyve and Yurkova’s (1987) basalt
differentiation trend of Pacific island arcs. Shirshov ridge dredge samples did
not cluster as tightly, but ranged from the island arc differentiation trend to a
subalkalic trend similar to ocean island basalts. A second group of basalts from

Olyutorsk Peninsula (Olyutorka complex) were classified as "transitional" (Peyve

16
and Yurkova, 1987), due to their variation from the island arc differentiation
trend to an ocean island basalt trend.

A multitude of origins have been proposed for Shirshov ridge, including a
hot spot trace (Ben-Avraham and Cooper, 1981), a spreading ridge (Kienle,
1971; Neprochnov et al., 1985), a remnant arc (Karig, 1972), an ophiolitic "pile"
(Peyve and Yurkova, 1987) and a remnant continent (Nur and Ben-Avraham,
1972). Whatever its origin, basalt geochemical similarities (Peyve and Yurkova,
1987) and geophysical signatures (Ben-Avraham and Cooper, 1981; Cooper et al.,
1976c) between onshore East Olyutorsk terrane and Shirshov ridge strongly
support the interpretation of Shirshov ridge as an offshore continuation of the
East Olyutorsk terrane.

There is an apparent basement structural correlation, first noted by
Ludwig et al. (1971b), between Bowers and Shirshov ridges. GLORIA data
(Cochrane et al., 1987) show that the northern end of Bowers ridge plunges
downward and curves westward toward the southern tip of Shirshov ridge. This
apparent topographic connection is further supported by a continuous magnetic
anomaly, identified by Cooper et al. (1976a), which extends along the crest of
both Bowers and Shirshov ridges as well as along the correlative basement
axis. This suggests that the two ridges may have had a common genesis, and
were separated at some later period of their evolution. Rabinowitz (1974) used
seismic reflection data to identify two basement peaks separated by less than 10
km and, therefore, suggested differing structural origins for the Bowers and
Shirshov ridges. However, the southern extent of Shirshov ridge, as noted by
Rabinowitz (1974), has dual peaks, and may continue into the basement in the

same manner,

17
I; l l .. B .

The Komandorskii basin extends westward from Shirshov ridge to the
continental margin of Kamchatka Peninsula. Its southern and northern
boundaries are the Aleutian arc and Koryakia, respectively. The basin is
apparently floored by oceanic crust (Ludwig et al., 1971b; Creager and Scholl,
1973), exhibits high heat flow (85 to 210 mW/mz; Rabinowitz and Cooper, 1977;
Bogdanov and Neprochnov, 1984), and has a thin (I to 2 km) sedimentary cover
of Miocene and younger age (Scholl et. al., 1975). Weak magnetic anomalies
(generally less than 200 gammas; CoOper et al., 1976a) are found within the
basin, paralleling north-south trending basement ridges of 0.5 to 1.0 km
amplitudes (Cooper et al., 1976a). The depth to basement in the Komandorskii
basin is apparently shallow (4-5 km), contrasting with greater depths (greater
than 7 km) in both the Aleutian and Bowers basins (Cooper et al., 1976a).

Data obtained from the Komandorskii basin since 1982 has drastically
changed the view of its evolution. New structural data obtained by a Soviet
expedition (29th cruise of the R/V Dmitrii Mendeleyev; Savostin et. al., 1986)
have shown ”systematic extensional features” which converge in the southeast
corner of the basin. Seliverstov (1987) presents seismic profiles showing
volcanic edifices north of Bering Island within the Komandorskii basin;
down-dropped graben structures and fumaroles are also present (Bogdanov,
1986). Furthermore, redating of the original DSDP basement basalts (DSDP Site
191) has shown them to be Miocene (approximately 9.2 Ma), rather than the
originally reported Oligocene (29 Ma) age (D. Scholl, pers. comm). These data
all point to the existence of Miocene to present rifts within the Komandorskii

basin. These rifts, north of Bering Island and in southeastern Komandorskii

18
Basin, trend nearly due north and are semi-orthogonal to the Pacific

convergence direction.

WESTERN BERING SEA TERRANES

Koryakia’s Cretaceous development is described by Natal’in and Parfenov
(1983) as a Andean-type margin. Fujita and Newberry (1983) identified the
Koryak terrane, a forearc basin sequence composed of coarse clastics, tuffs
and sediments derived from the Okhotsk-Chukotsk volcanic belt (Fujita and
Newberry, 1983; Natal’in and Parfenov, 1983). These rocks are the
continentward abutment against which the terranes of Olyutorsk have been
accreted.

Olyutorsk has commonly been considered a single oceanic terrane which
accreted in the Cenozoic (e.g., Churkin and Trexler, 1981; Fujita and Newberry,
1983) along a thrust boundary preserved in the Ukelayat and Vyvenka River
basins. Extrapolation of this terrane south into Kamchatka has often been
proposed (Natal’in and Parfenov, 1983; Koltypin and Kononov, 1986), but not
explicitly proven. Here I identify a number of smaller terranes that compose
Olyutorsk (Figure 1-3), and present new evidence for their correlation to
terranes in Kamchatka (Figure 1—4) as identified by Watson (1985) and Watson
and Fujita (1985).

Wu:

The Ukelayat-Central Kamchatka (UCK) terrane extends from southern
Kamchatka northward through Kamchatka and western Karaginskii Island, then
northeast and east through Olyutorsk. The eastern boundary of this terrane is
defined by large thrust faults along its entire length. These thrusts dip 30° to
40° east-southeast in Kamchatka (Tsikunov and Petrov, 1973) and 15° to 40°

19

Figure 1-3: 1:1,000,000 satellite mosaic of the Olyutorsk region.
Boundaries of terranes composing Olyutorsk are shown by dotted
lines. OP - Ol torsk Peninsula; GP - Goven Peninsula; A - Apuka
Mountains; P - akhachinsk Plateau.

20

’l,‘

‘u Eveline}! \e‘
V :.- ‘5 ..

 

Figure 1-3

Flt

21

Figure 14: Map of terranes identified by this study. Dashed lines are
inferred boundaries.

22

 

p0

 

500 Km

 

 

 

 

Figure 1-4

23
(Aleksandrov et al., 1980) or possibly 75° (Koltypin and Kononov, 1986)
southeast in Olyutorsk.

In Kamchatka, a small basement exposure is found in the Khavyven uplift.

The uplift consists of metamorphic greenschists and microquartzites containing
mafic and ultramafic intrusives (Gnibidenko and Marakhanov, 1973; Shul’diner et
al., 1979). These basement rocks are likely to be of Cretaceous or Jurassic age
(Watson and Fujita, 1985). On western Karaginskii Island, Dolmatov et al.
(1969) identify a Cretaceous basal layer of mafic green tuffs and "cherty
cement" which may correlate to the Khavyven exposure in Kamchatka. No
basement is identified in Olyutorsk, and in general, basement rocks are not well
exposed, and are interpreted to lie under the extent of the terrane in
Kamchatka based on geophysical data (Utnasin et al., 1975; Rivosh, 1964).

Andesites, basalts, tuffs, conglomerates and sandstones of unknown age lie
unconformably upon the basement rocks in Kamchatka (Figure l-Sb). Eocene
(?) to Oligocene rocks of the Kelmenskaya suite contain tuffs and sandy clastic
rocks (Watson, 1985; Watson and Fujita, 1985). Finally, the Miocene and Late
Miocene to Pliocene Yelovsk and Kavran suites consist of tuffaceous
sandstones, siltstones and tuffs with occasional interbedded conglomerates
(Watson, 1985).

In Olyutorsk, Koltypin and Kononov (1986) identify a Cretaceous
(Santonian to Danian) flyschoid formation as the oldest rocks in that region of
the UCK terrane. No details are given as to the lithologies which compose
this formation. Paleocene to Eocene rocks consist of a molasse complex
(Koltypin and Kononov, 1986), including silts, shales and fine sands. Above

these lie Eocene to Oligocene clastics, volcanics and tuffs.

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24

Serova et al. (1975) detail the lithology and micropaleontology of the
Paleogene to Miocene on Karaginskii Island. The Eocene is reported to
contain volcanic ash tuffs with interbedded sandstones and siltstones. This
section is overlain conformably by an Oligocene sandstone unit with
intermittent interbedded sandstone and siltstone. The Miocene is thin (350 m),
but contains predominant tuffs of basic composition with interbedded
sandstones and siltstones. The benthic foraminifera found in these sections on
Karaginskii Island are also noted extensively in much of Koryakia.

The compositions of post-Cretaceous rocks in Kamchatka, Olyurtorsk and
on Karaginskii Island are similar (Figure 1-5b). This similarity indicates that
this is one terrane extending from southern Kamchatka to Dezhneva Bay in
eastern Olyutorsk. A continuous magnetic anomaly of greater than 500 gammas
(Cooper et. al., 1976c) along the terrane’s length also support the identification
of UCK as a single terrane. Basement rocks, where exposed, suggest that the
terrane is oceanic; overlying volcanics, marine and non-marine sedimentary
rocks are consistent with this interpretation, but suggest a nearby terrigenous
source as well. The basement of the UCK terrane most likely began formation
in the Jurassic or Early Cretaceous (Watson and Fujita, 1985). In Kamchatka,
Gnibidenko and Marakhanov (1973) obtained K-Ar dates of 92-122 Ma within the
Khavyven basement sequence. This age is probably a metamorphic date
(Gnibidenko and Marakhanov, 1973), and may represent the time of this
terrane’s accretion. Watson (1985) suggested the terrane may have represented
either normal oceanic crust or a back-arc basin. The data presented here, when
considered in light of constraints imposed by adjacent seaward terranes, support
interpretation of the terrane as an abducted fragment of normal oceanic crust.

A sedimentary sequence (Cretaceous to Paleogene Sediments terrane) and an

“gm

25

Figure 16;}; Lithologic symbols used in the stratigraphic columns in Figures
.- 8°

26

 

 

°. 2 .....‘.o. I, ‘s “
. ,' - j - Sandstone Ii “ “ Tuff
J n .

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

L * - H ‘ ‘
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_':.':::'_::::.‘_:. Silistonc Basalt
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+ + +

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Figure 1-5a

Flt

27

Figure 1-5b: Stratigra hic columns for the Ukelayat-Central Kamchatka
terrane. Co umns are summaries for each region. Suite names
beside each column are given in full below. .
Kamchatka: Kv - Kavron; Y1 - Yelovsk; Kl - Kelmenskaya

UKELAYAT - CENTRAL KAMCHATKA

28

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

< t ’: 9:. - . , Suite . . . ' ..
x Pratt U09“ Koryakia kziiiiclizitkzi Nami Karaginskii
{.13 ACE
Quaternary
LJIC . , . . ,_-_
Early .{x . .‘x. ‘8 “ Kv
en . it , *5 ,
0 Middle _-_- .. -, -, a, .fi
22’ n ” it “ t
0 $3 $5 :5 o 0 ° 0 Y]
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U ’: LJIC ‘3 ”P ‘ q 5‘ 3‘ IT. .: I)
3 ru/“ 3: 2'”: 2-1 : Bu . .
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o '0. ... . 0 .°. . .0... ...° . ' .O.‘.o
2 “TC .. ..o o. ..... o. ' . .° .0.- : ..
a , ”T"._.—_'..—_:_— 1‘ ——————— . -
0 Meddlc “.‘_'_—.’_'::'.:_:-.° 63 - GEE .-
3 o . . ° 0 ° ‘ F: :-— .7... 1:...-
63 ° ; __ _ _ _'_ _
C- Ehdy - ' .»\ .;:;::::1:::::
1.... 3313.23}: «7?? o -9. _o_ _o_ .9.
——————— f) O .o -°.. :° -©_—-——‘T
Early ’ 3,; ‘ . . - - '. it
. . . o .. f‘ ‘3 ‘ { ( ’ ’ I‘ ‘S
Maastrichtian Flyschmd u “ ” H
g Campanian Formation' _ ii ' 31 l ' i {t
3 8 Santonian m ”5‘“ f? ” ‘S
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u G Turoninn I 63 ‘ “
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Jurassic

 

Figure 1-5b

Figure

29

Figure l-Sc: Stratigra hic columns for the Cretaceous - Paleogene Sediments
t.errane olumns are summaries for each region. Suite names
beside each column are given in full below.

Koryakia: Al - Aluga; Vl - Val’en; Ml - Mil’gemay; Tv - Tavensk;
A -Ayaon
Kamchat a: Dr - Drozdovsk; Kp - Khapitsk

 

 

 

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30

CRETACEOUS - PALEOGEN E SEDIMENTS

 

 

 

 

 

  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

) 0 ‘II 0 Suite Suite . ..
1 ER! EIAGEE ; l Koryakia Nam Kamchatka Mm Karaginskii
()uatcrnary l '
Late
Early
o LJIC
: O
o
50 .
0 Middle
0
Z
.S.’
8 Early
8
Q g Late
l‘ Early
8 Late
&
0 Middle
2
8 Early
Late
Early
Maastrichtian
ES’ Campanian
O
3 8 Santonian
.3 .‘3 Coniacian
. 0 .
u 6 Turonian
:c'} Ccnomanian
a m Albian
A (an
2 ,3 P' ,
7; 8 Barrcmian
‘3 a . .
m :3 Hautcrman
L. o 0
U Valanginian
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I;nc
Juranfic

 

 

 

 

 

 

 

Figure 1-5e

 

31

Figure l-5d: Stratigraphic columns for the Goven - Eastern Ran es terrane.
Columns are summaries for each region. Suite names eside each
column are 'ven in full below.

Koryakia: - Aluga; Gv - Goven; Lv - Lavrov; Py - Pylga
Kamchatka: Os - Osipov; Ch - Chazhmin; St - Stanislavsk; Bs -
Bushuyka

32

GOVEN - EASTERN RANGES

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

H o SUIIC SUIIC
zEVER AGE Koryakia Nam; Kamchatka Name
Quaternary
Late
Early , :
0 ”TC 1... OS
3 .|—-—|‘..... °.. ..‘ . . ..'. ill——
0 Middle s; 'o f: ’ 6;,- .n {rs '.‘
i) .i-gz'riflr Al -,'- -n-: 1.}; ‘1' Ch
:3 em, :—:,—:—:—:=:: -_'.-_-_l-':.:'_'—_'_—'-
3b ' ::::‘_-"
o ..-.' .'.'o 0. o th- _______
U; Late 0 no 11 no —_'._———_._.
o __ N o n u , _-_-.-_—.__—.
‘— Early N I‘ :'. ..". ..o '
0 Late it N n N -_Z'-._'—_—'.:":L- 5‘
5 —— o O D". 7:? 7.: TI?»—
3° Middle ”nib“ 4331:! Lv A” n O
2 “£7 "P rflv O n 85
5‘} Early no 23 "o 2:; N n
?
Late Wu H p rip (l ”I
5“” n-fi-‘L— - .__PY
Maastrichtian
g Campanian
3 3 Santonian
.3 g Coniacian
(‘3 Turonian
°" Ccnornanian
Albian
U m '
A tan
2 ,3 P' ,
7: 8 Barrcmian
a a - -
m .6 Hautcrrvran
6 Valanginian ..
Berriasian
Lam:
Jurassic {

 

 

 

Figure 1-5d

33

Figure l-Se: Stratigraphic columns for the Tertiary Accretionary Complex
terrane. Columns are summaries for each region. Suite names
beside each column are given in full below.

Koryakia: Al- -Aluga
Kamchatka: Ts- -Tyushevka series; Ch- Chazhmin; Bg- Bogachevka

34

TERTIARY ACCRETIONARY COMPLEX

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

, ‘H K ki Suite- 7 h kn Suite
{13‘ ER AGE orya 8 Name kamc at Name
Quaternary
Late I
Early 1
Lam: '33 '33-1: :3‘ . .
o O . . . . _ , . , .
5' ll O H o no“ v-°:_-;-_;-_:':._"':..-.'_‘l
§° Middle iii-155 A1+.‘—'.=-:':.-?.-=.=
Z . . . . . . u ' o ' . o o o . .
8 Early
S
o S . . . .-
5 Late ' .
o . . I O
(- Early . " ° '
§ Late :' .
m , u . 0 9
.3 MlddlC Z? I“, III '
PS Early
me
Early
Maastn'chtian
‘3 Campanian
O
3 8 Santonian
,3 .3 Coniacian
u 0’ Turonian
§ Cenomanian
é Albian
g '3 Aptian
4 >‘O .
Z 8 Barrerman
L3 g Hauterivian
(‘3 Valanginian
Berriasian
Lau:
Jurassic L

 

 

 

 

Figure 1-5e

35

Figure l-Sf: Stratigraphic column for the East Olyutorsk - Shirshov terrane.
Suite names beside each column are given 1n full below.
Koryakia: Al - Aluga

36

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

EAST OLYUTORSK - SHIRSHOV
‘H K o SUllC
o alua
a PE“ AGE 'V Nam
Quaternary
Late
Early
o Lille °...’.: : ..:.o'.o...
5 92.3.8.0. .33.-
8" Middle e:—:— = r:— A]
o .... _ _—_—_-_'—_4
z 777-1-127-
'- 12 t‘
N Early t’O ’ t‘ o
o ’ ° 21 n
r: 2‘ o 21 O i...__J
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U 5 Late 0 '
l- Early
: Late
&
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£2
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Maastrichtian ‘9 "‘9 “f,"
m , it ’1’ ’1’,
:3 Campaman
o . l ‘9
3 3 Santoman “ ’I’F’ {tab
.353 Coniacian '°
.3 . 1 1
U U Turoman l 1
"- Ccnomanian t t
Albian ‘ ‘ ‘
a m A 'an
2 ,‘3 P" ,
7: 8 Barremtan
fig .3 Hautcrivian
O Valang'nian
Berriasian
Late
Jurassic h

 

 

Figure 1-5f

37

Figure 1-5g: Stratigraphic column for the Miocene volcanics.

38

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

MlOCENE VOLCANICS
'- pER H Ko aki
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Figure 1-53

39
island are complex (Goven-Eastern Ranges terrane) are found southeast of the
UCK terrane.
W

A narrow Cretaceous to Paleogene sedimentary (KPS) terrane which
contains numerous allochthonous ultramafic and gabbroic rocks is thrust over
the UCK terrane. The KPS terrane ranges from 20 to 60 km in width, and
extends from central Kamchatka (Tumrok Range) northward through central
Karaginskii Island and into Olyutorsk where it follows the basin of the
Vyvenka River to the northeast (Figure 1-6). The KPS terrane is overlain for
a small distance by younger volcanics, and is then identified further east
between the Il’pi and Vatyna river valleys in easternmost Olyutorsk (Figure
1-4). Yermakov and Mishin (1974) traced the units laterally from eastern
Olyutorsk to the Kamchatka isthmus based on mineralogic consistencies within
the rocks. In Kamchatka, Watson (1985) included these sediments and
ultramafics within the Eastern Ranges terrane. However, faults bound these
sediments, and considering the lithologic correlations to Olyutorsk and
Karaginskii Island, it is suggested that the Kamchatka KPS deposits should be
considered separate from the Eastern Ranges.

In Olyutorsk, the oldest rocks of the KPS terrane are Albian to
Cenomanian basalts and andesites, with occasional lenses of carbonates and
cherts (Bogdanov et al., 1982). Above these volcanics are 1 to 1.5 km of
tuffs, cherts and fine elastic rocks which microfossil data suggest are
Cenomanian to Maastrichtian in age (Bogdanov et al., 1982). The contact
between these rocks and the overlying Ayaon suite (Yermakov and Mishin, 1974)

is not described.

LLO

Figure 1-6: 1:250,000 LANDSAT image close-up showing the linear deformation
of the KPS terrane. MV denotes a small Miocene volcanic pile.

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41

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Figure 1-6

42

The thickest (up to 8 km) Paleogene section of the KPS terrane is found
in Olyutorsk, where Yermakov and Mishin (1974) have divided the complex into
four suites (Figure l-Sc), consisting primarily of sandstones, siltstones, and
argillites, with lesser amounts of conglomerates, tuffs, tuffaceous sandstones and
cherty-argillaceous rocks. These suites, the Ayaon, Tavensk, Mil’gernay and
Val’en, are summarized below.

The Ayaon suite is composed of fine-grained clastics. They are dark grey
in color, and contain a matrix of organic compounds and silt-sized grains.
Common grain types include quartz, plagioclase and volcanic fragments with
basic composition. A lesser amount of poorly sorted sandstone is also found
within the suite. The sandstone is composed of quartz, plagioclase, extrusive
volcanic rock fragments (acidic, intermediate and basic), chert and other
fine-grained material.

The overlying Tavensk suite contains coarser grained sediments including
sandstones, gravellites and conglomerates. The sandstones are poorly sorted,
and are made up of extrusive volcanic clasts, chert, quartz, plagioclase and
silt. Rare thin beds of fine-grained clastics are similar to the rocks of the
Ayaon suite.

Parallel or cross-bedded, occasionally massive, fine- to medium-grained
sandstones and siltstones dominate the Mil’gernay suite. Matrix is minimal
within the rocks, constituting less than 10% of the total make-up. Grains are
usually quartz, plagioclase, acid volcanics, lesser basic and intermediate rocks,
chert and shaley rock fragments. The siltstones of the Mal’gernay suite are
sometimes microstratified, distinguishing them from siltstones of the Ayaon or

Tavensk suites.

43

The Val’en suite is also composed of coarse-grained rocks: sandstones,
gravellites and conglomerates. The sandstones are poorly sorted with quartz,
plagioclase, acid volcanic fragments (rarely basic or intermediate), chert and
fine-grained elastic rock fragments. No data are given on the coarser
sedimentary rocks.

Several investigators (e.g., Dundo et. al., 1982; Fujita and Newberry, 1983;
Palandzhjan, 1986; Koltypin and Kononov, 1986) have identified ultramafic
ophiolite blocks (e.g., Vatyna, Vyvenka) within the sediments of the KPS
terrane in Olyutorsk. These blocks sometimes have associated pillow
structures, siliceous and/or carbonaceous rocks (Aleksandrov et. al., 1980), and
are older than the sediments of the KPS terrane (Alekseyev, 1979).

Bogdanov et al. (1982) identify an east-dipping thrust fault running
north-south through Karaginskii Island which separates the KPS terrane from
the UCK terrane. On the eastern (upthrown) side of the thrust, Bogdanov et
al. (1982) identify volcanic and siliceous rocks and abundant ultramafic
exposures. Within this same central region, Serova et al. (1975) describe
north-south trending Paleogene tufts, tuff-breccias, siltstones and interbedded
siltstones and sandstones. Dolmatov et al. (1969) and Dundo et al. ( 1982)
further describe Late Cretaceous mafic and ultramafic blocks among the
Cretaceous to Paleocene deposits on the island.

The Khapitsk suite in Kamchatka (Watson, 1985) is equivalent to the
Cretaceous units of the KPS terrane identified in Olyutorsk. Watson (1985)
included these rocks within the Eastern Ranges terrane of Kamchatka
Peninsula. However, the Khapitsk suite has a differing lithology and is
apparently fault bounded. The suite includes layers of tuffs, tuffaceous

clastics and cherts among andesitic and basaltic volcanics (Shapiro, 1981), and

le<

rej

44

occasional exposures of ultramafic rocks (e.g., Tumrok Range; Watson, 1985).
Limestone blocks are often found in association with these ultramafics. This
led Rotman et al. (1973) to suggest the ultramafies and associated rocks may
represent a subduction zone melange complex. The Drozdovsk suite contains
alternating deposits of sandstones and argillites, with occasional tuffaceous or
calcareous horizons. Benthic foraminifera suggest a Danian to Early Paleocene
age for the suite (Serova et al., 1970).

The ultramafics (e.g. Vatyna, Karaginskii, Tumrok) complexes can be used
as a marker for lateral correlation of the KPS terrane. Additionally, correlation
is aided through identification of extensive surficial expressions of folding of
the sediments observed on LANDSAT and mosaic imagery (Figure 1-3 and 1-6).
Finally, aeromagnetic values (Cooper et. al. 1976c) consistently range from 100
to 300 gammas over the KPS terrane. Within the faulted boundaries, the
presence of ultramfic blocks, decrease in magnetic intensity and distinct
sedimentologic lithologies characterize the KPS terrane.

Faulting and folding are found internally throughout the terrane, although
lateral slip seems to dominate in Kamchatka (Watson, 1985). Deformation of the
rocks is extensive in Olyutorsk (Aleksandrov et al., 1980; Mitrofanov et al.,
1980), and fold wavelengths of 1 to 8 km are observed in the Vyvenka River
basin (Figure 1-6). Filatova and Yegorov (1983) note that field evidence
suggests maximum shortening within this complex occurred in eastern Olyutorsk
(between the Il’pi and Vatyna rivers), indicating that plate motion was most
likely perpendicular to the northern margin during emplacement.

The KPS terrane is thrust over the UCK terrane to the west. The age of
the sediments and thrust faults ties very well with the timing (Cretaceous -

Paleogene boundary) of compression and major thrusting in southern Koryakia

45

(Harbert et al., 1987; Kazimirov, 1985), as well as in Kamchatka (Watson and
Fujita, 1985). Subduction, back-are and oceanic settings must be considered as
possible sources for the KPS terrane sediments. The sediments must have
originally lain stratigraphically above the UCK terrane, and were then thrust
over the UCK terrane during obduction. Therefore, I suggest these were
accretionary wedge sediments which were lying upon the UCK terrane. This
presents a simple solution to their present structural position and is consistent
with the observed lithologies.

Although not examined in this investigation, Yermakov (1975) and
Yermakov and Suprunenko (1975) propose that this terrane extends along the
Alaskan Shelf, through Zemchug and Pribilof canyons, southeastward to
Shumagin and Kodiak islands. If this is the ease, the accretion of the
Goven-Eastern Ranges (GER) terrane may be synchronous with the docking of
the Peninsular terrane along southern Alaska and the Beringian margin.
We

Watson (1985) identified the Eastern Ranges terrane in Kamchatka, and
suggested that it represented an island are complex. Here I show that
lithologic and geophysical correlations are consistent with a northward
extension of the terrane from Kamchatka into Olyutorsk, and that the data
support Watson’s (1985) interpretation.

Within Olyutorsk, the terrane extends eastward from the Goven Peninsula
to the Pakhachinskii Plateau and Range, where it is buried beneath younger
volcanics (Figure 1-3). I therefore have named the entire terrane
Goven-Eastern Ranges (GER). The terrane is thrust upon the KPS terrane to
the west, and an accretionary complex underthrusts the terrane to the east

(Figure 1-4). If lateral succession is consistent in the western Bering Sea,

sil

Cu

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46
then it is likely the terrane also is found on eastern Karaginskii Island. The
only data which could be obtained describe eastern Karaginskii Island as a
”flysch complex” (Bogdanov et al., 1982), and no specific lithologic or age data
are known.

In Olyutorsk, the Paleocene to Miocene rocks of the GER terrane consist
of the Pylga, Lavrov, Goven and Kylan suites (Figure l-Sd). The Pylga suite is
found near the coastline of Olyutorsk Bay and extends from the southern tip of
Goven Peninsula to the Apuka River in the east. The suite consists mostly of
marine sediments, including argillites, siltstones, sandy shales, tuffs and
andesites (Pichugina et al., 1974b). The Pylga suite has been dated as Early
Paleocene based on planktonie foraminifera (Pichugina et al., 1974b), and is
approximately 2000 meters thick.

Above the Early Paleocene Pylga are later Paleogene suites, the Lavrov
and Goven. The Lavrov suite consists of volcanic and/or cherty rocks, while
the overlying Goven suite is dominated by elastic rocks: sandstones, siltstones
and argillites (Pichugina et al., 1974a). The distinct lithologic change and a
lack of fossils in the Lavrov and Goven suites distinguish them from the older
Pylga suite.

The Early to Middle Miocene Aluga series, primarily sandstones and
siltstones, has an extensive distribution and consistent facies over Il’pinski,
Goven and Olyutorsk Peninsulas, thereby suggesting it is an overlap succession.
The thickness of the Aluga series varies greatly, from 1200 m in the Il’pi
Peninsula to 10000 m in the Apuka Range region (Pichugina et al., 1974a). This
distribution pattern and the age of the sediments correlates quite well with the

onset of Miocene volcanism in central Olyutorsk. The uplift associated with the

47
volcanism may, therefore, represent the source of the sediments composing the
Aluga series.

Descriptions of correlative lithologies in Kamchatka are given by Watson
(1985). Watson (1985) and Watson and Fujita (1985) identify underlying
Cretaceous volcanic rocks, the Vetlovsk suite, in Kamchatka which have not
been described in Olyutorsk. It is possible that similar rocks in Olyutorsk are
overlain by the extensive Miocene basalts which border the northeastern
margin of the terrane or are concealed under the continental shelf west of the
Komandorskii Basin.

Overlying the Vetlovsk suite with ”minor unconformity" (Watson, 1985)
are the Bushuyka and Stanislavsk suites. The Bushuyka suite is composed of
andesitic tuffs and cherts. Again, as seen in the Pylga and Lavrov suites in
Olyutorsk, a distinct lithologic change marks the horizon between the Vetlovsk
and Bushuyka suites in Kamchatka. Conformably overlying the Bushuyka suite is
the Stanislavsk suite, consisting of sandstones and siltstones (Shapiro and
Seliverstov, 1975), and lacking any paleontological remains.

Finally in Kamchatka, a sequence of fine elasties composing the Chazhmin
suite is found overlying the Stanislavsk suite. This unit has been assigned an
Early Miocene age by Watson (1985). A thick (greater than 2000 m) overlap
succession of elastic sediments (the Osipov suite) lies unconformably on the
Chazhmin suite in Kamchatka; the Chazmin and Osipov are suggested to be
analogs of the Aluga suite in Olyutorsk.

Thus, there exists a one-to-one correlation between the rocks comprising
the Goven terrane in Olyutorsk and those of the Eastern Ranges of Kamchatka
and, therefore, a single GovenoEastern Ranges terrane is pr0posed. The

volcanic and elastic-siliceous sedimentary rocks of the GER terrane are very

similar
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48

similar to the are facies which today comprise the Aleutian Island are.
Therefore, by analogy, it seems reasonable to suggest that the GER terrane
represents an accreted island are complex. Basement ultramafics, basalts and
tuffs (Watson and Fujita, 1985) found in Kamchatka, and possibly buried by
young volcanics in Olyutorsk, support the conclusion. The thickness and
diversity of sediments may indicate a lengthy evolution with at least one
period of volcanic quiescence and erosion. 8
W

A large northwest dipping thrust fault separates the GER terrane from a
Tertiary (Eocene (?) to Miocene) accretionary complex (TAC) to located to the
east (Figure 1-4). The TAC is identified as a narrow (less than 50 km) band
southeast of the GER terrane and west of Kronotsky Peninsula and Cape
Kamchatka. The gravimetrieally identified East Kamchatka synclinorium along
the shelf east of Karaginskii Island (Shapiro, 1976) may be the offshore
continuation of the TAC, connecting the Kamchatka rocks with similar
lithologies along the easternmost margin of the Govena Peninsula. Within
Olyutorsk, the TAC extends a short distance northeast and and is terminated
abruptly by overlying Miocene volcanics.

The TAC consists almost entirely of siltstones, sandstones, tuffs, cherts
and argillites (Shapiro and Seliverstov, 1975; Koltypin and Kononov, 1986), both
in Kamchatka and Koryakia (Figure l-Se). The Miocene Aluga suite is an
overlap succession composed of sandstones and siltstones, and is the youngest
unit of the TAC in Olyutorsk. Watson (1985) identified this terrane in
Kamchatka as the East Kamchatka basin.

It is important to note that the Aleutian island are came into existence

just before or during the formation and emplacement of the TAC terrane.

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49

Therefore, the extension of this terrane from central Kamchatka into Koryakia
implies that westward subduction along the entire Kamchatka-Koryak margin
must have continued until at least early Miocene. In order to accomplish this,
one of two geometries must have existed during this time. In the first
geometry, a unitary fragment of oceanic crust would be present along the
entire western Bering Sea margin. This fragment would have had uniform
convergence and rate relative to this boundary until such time as it was
completely subducted. In the second geometry, two separate plates would be
subdueting along the Koryak-Kamchatka boundary: one north of the Aleutian
subduction zone and one to the south. The northern convergence could have
been due to the relative compression between the Eurasian and North American
plates (Harbert et al., 1987). The southern zone would have been related to
Pacific subduction along the eastern margin of Kamchatka. It is not presently
possible to distinguish between these two models. The solution, however, has
important implications for the accretion mechanisms of Kronotsky Peninsula and
Cape Kamchatka of Kamchatka, as well as for the evolution of the Komandorskii
basin and east Kamchatka synclinorium of Shapiro (1976).
Eastflbmtnrle-Shiflhnfleflm

The lithologic information in the easternmost extent of Olyutorsk
(hereafter refered to as "East Olyutorsk," extending south from the Vatyna
River to Olyutorsk Peninsula; Figure 1-2 and 1-4) is less detailed than in other
regions of Olyutorsk. However, East Olyutorsk appears to have a less complex
stratigraphy, with Cretaceous to Paleocene basic to intermediate volcanics
dominating. Here I will primarily discuss the onshore extent of the East
Olyutorsk - Shirshov (EOS) terrane; the details of the Shirshov Ridge were

discussed earlier in the paper.

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50

Four volcanic complexes are identified in East Olyutorsk by Peyve and
Yurkova (1987): Maehevna, Gytgynskii, Nichakvayam and Olyutorka. All are
basic volcanics of Cretaceous age. The Machevna complex comprises the bulk
of the EOS terrane. These low-potassium basalts often occur as pillows, and
are very similar to basalts dredged from the Shirshov Ridge. These basalts fall
along the island-arc basalt differentiation trend of the Ti-Cr plot, and
therefore, Peyve and Yurkova (1987) identify them as island-arc volcanics.
'Ihe Gytgynskii and Nichakvayam complexes are small exposures of low-silica
tholeiites located at the extreme north and west boundaries of the EOS
terrane. There have been no basalts similar to the Gytgynskii or Nichakvayam
complexes yet recovered from dredgings along the Shirshov Ridge.

The very southern tip of Olyutorsk Peninsula is composed of the
Olyutorka complex, which is composed of transitional low-silica, high-titaniaum
basalts (Peyve and Yurkova, 1987). It is these rocks which are chemically
similar the sub-alkalic basalts dredged from the Shirshov Ridge. There are
extensive formations of andesite-basalt tuffs, as well as volcanic and siliceous
sediments, associated with the onshore volcanics, which range from Santonian to
Maastriehtian in age (Koltypin and Kononov, 1986).

Thrust faults are found in easternmost Olyutorsk and on Olyutorsk
Peninsula in the south, however, a major thrust bounds the EOS terrane,
separating it from the KPS terrane on the north (Palandzhjan, 1986; Koltypin
and Kononov, 1986). The thrust fault on Olyutorsk Peninsula defines a
tectonic contact between the Olyutorka complex volcanics and the Machevna
group to the north. Along the eastern margin of the EOS terrane there is a
west-dipping fault where the Machevna group has been thrust over a narrow

band of silty sehists (Koltypin and Kononov, 1986). The western boundary of

51
the terrane, which is truncated along the Apuka River valley (Figure 1-2 and
1-4), is most likely buried beneath the Miocene volcanics of the Pakhachinskii
plateau and Apuka Mountains (Figure 1-3).

I consider the Shirshov Ridge to be an offshore continuation of East
Olyutorsk for the following reasons: 1) topographic continuity (Scholl et al.,
1975); 2) continuous gravity and magnetic signature (Ben-Avraham and Cooper,
1981; Cooper et al., 1976c); 3) dredge samples from the Shirshov Ridge are
mineralogically similar to those on the southern portion of the Olyutorsk
Peninsula (Neprochnov et al., 1985); 4) major and trace element chemistries of
dredge rocks from Shirshov Ridge are similar to onshore chemistries in East
Olyutorsk (Peyve and Yurkova, 1987); and 5) ages of the Shirshov volcanics and
overlying sediments suggest common ages of genesis for both the Shirshov and
East Olyutorsk rocks (Bogdanov and Neprochnov, 1984).

The presence of andesite and basalt volcanics and their directly
associated tuffs and sediments strengthen Peyve and Yurkova’s (1987)
interpretation of the East Olyutorsk region as a remnant island are. The
geochemical similarity to other island arcs and presence of pillow basalts seem
to provide conclusive evidence for this classification. Shirshov ridge has been
interpreted by many (e.g., Scholl et al., 1975; Dickinson, 1978; Ben-Avraham and
Cooper, 1981) to be a volcanic are, which is consistent with the onshore
conclusions. .

Stratigraphic succession of the Aluga series across the EOS and GER
terranes suggest that they were joined by Middle Miocene time (Figure 1-Sf).
Whether the accretion of the GER and EOS terranes to Olyutorsk was
independent or if they had become a super-terrane, with the TAC pinched

between them, before colliding with Kamchatka-Olyutorsk is uncertain, and is

52
unlikely to be determined due to the obstruction of their spatial contact by a
sequence of Miocene volcanics.
If I! l .

Laterally extensive Miocene volcanics cover south-central Olyutorsk from
the central coastline of Olyutorsk Bay on the south to the Vyvenka and
Ukelayat river basins on the north (Figure 1-4). To the east the volcanics
terminate sharply along the Apuka and Aehayvayam river valleys, while to the
west the volcanics make an irregular boundary with the underlying GER and
TAC terranes.

These volcanics are predominantly tholeiitic and alkaline basalts (Dundo
et al., 1982; Bogdanov and Neprochnov, 1984). Although no major or trace
element geochemical data are available, the volcanics are visible on satellite
imagery (Figure 1-3), and can be seen as laterally extensive and lobate flows.
Such charateristics are consistent with the expected behavior of relatively low
viscosity basalts (Fink, 1987).

The stratigraphically lowest volcanics (Figure 1-5g) are sometimes
described as intermediate volcanics (Bogdanov and Neprochnov, 1984), while the
overlying flows are similar to plateau basalts. This may reflect source
chamber inversions during the eruption process (Smith, 1979) or early
contamination (of the intermediate volcanics) by the country rock. It is
interesting to note that the Miocene age of these volcanics is very similar to
other extensional indicators in the Bering Sea, including the age of rifting in

the Komandorskii basin to the south.

EVOLUTION

Based upon the character of the terranes identified in Kamchatka and
Koryakia (Figures 1-4 and 1-5), in addition to the geological and geophysical
data available on the Bering Sea, a Cretaceous to present evolution for the
Bering Sea region is proposed (Figure 1-7). I realize this evolution is, in part,
similar to reconstructions done by the many others (e.g., Scholl et al., 1975;
Marlow and C00per, 1980; Marlow et al., 1982) who have investigated the Bering
Sea region. The uniqueness of this model lies in the timing and tectonic
settings which I derive from the western Bering Sea margin terranes and in the
Neogene events of the western Bering Sea. In this model, I assume that a
simple four plate system (Farallon, Izanagi, Kula and Pacific) has been involved
since Early Cretaceous. Plate motion and convergence data have been taken
from Engebretson et al. (1987) and Harbert et al. (in press).

Weenies

In the Early Cretaceous, the Farallon plate interacted with the northern
Pacific basin margins, with the Farallon-Izanagi spreading ridge located to the
south. The Farallon plate converged with Koryakia in a northerly direction, at
a rate of 37 to 111 km/my (Harbert et al., in press). The Farallon-Izanagi
ridge was rotating in a clockwise direction, and formed a northward migrating
triple junction along the Eurasian coast.

Watson (1985) suggested that the triple junction formed by the
intersection of the Farallon-Izanagi spreading center with the Eurasian coast
migrated northward through Kamchatka between 115 Ma and 100 Ma. This is
based on the lack of Albian to Turonian volcanic rocks in Kamchatka, and
Albian to ”middle Cretaceous” elastic sediments, possibly related to uplift as the

ridge was subducted, found in the Omgon and Kvakhon terranes of Kamchatka

S3

Figure 1-7: Tectonic evolution of the western Berin Sea since Early
Cretaceous time. Schematic cross-sections for e time period are
shown throu h Paleogene time. A)Early Cretaceous B) Late
Cretaceous ) Paleogene D) Neogene to Present, including a
generalized present-day cross-sectron approximately east-west
through Olyutorsk derived from this study and a cross-section
throng}; Kamchatka at the latitude of Kronotsky Peninsula, modified
from atson (1985).

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56

(Watson and Fujita, 1985). In addition, from 115 to 100 Ma the convergence
rate between the Farallon plate and Koryakia was uncharacteristically slow
(approximately 10 km/my; Harbert et al., in press), and may have resulted from
subduction of a thermally buoyant ridge.

Synchronous with the passing of the triple junction was the formation of
the Okhotsk-Chukotsk volcanic belt within northeast Siberia. There was a
wide arc-trench gap associated with the Okhotsk-Chukotsk are resulting from
the shallow angle of subduction (Fujita and Newberry, 1983) of the Izanagi
plate. This is, however, consistent with what may be expected since the
convergence rate of the Izanagi plate was very rapid (192 to 222 km/my;
Harbert et al., in press) compared to the Farallon plate convergence.
Mam

As the Izanagi plate subducted beneath the Koryakia-Kamchatka margin,
the Kula plate was located to the south, and was subducting under the Goven -
Eastern Ranges island arc (Figure 1-7a). At the end of Cretaceous or
beginning of the Paleogene, a fragment of the Izanagi plate (UCK terrane) and
accretionary wedge sediments (KPS terrane) were obducted onto the Koryakia
margin as the Goven-Eastern Ranges island are (GER terrane) collided with the
zone of Izanagi subduction (Figure 1-7b). Subduetion of the Izanagi plate
ceased, and the Kula plate continued subduction under the Goven-Eastern
Ranges are, becoming the new Olyutorsk-Kamchatka margin. The timing of this
event is based upon the age of the youngest marine sediments found in the KPS
terrane. It is also possible that the cessation of volcanism in the
Okhotsk-Chukotsk arc correlates with end of the Izanagi plate’s rapid
convergence and the comparitively slower (157 km/my) northwesterly

convergence of the Kula plate along the Koryakia-Kamchatka margin.

57
Paleogene

Although collision of the Goven-Eastern Ranges island are occurred by
Early Paleogene, sediments as young as Miocene are found within the TAC
terrane, which represents the frontal accretionary wedge of the Goven-Eastern
Ranges island are. This suggests that westward subduction continued under
the Goven-Eastern Ranges are until Miocene time. However, during the
Paleocene to Miocene formation of the Tertiary accretionary complex, the
Aleutian island are also developed, trapping the fragment of oceanic crust
which now floors the Aleutian basin (Cooper et al., 1976a). In order for
subduction to continue until Miocene along the Olyutorsk-Kamchatka margin, as
indicated by the extent of the TAC terrane, either 1) A small subducting
fragment of Kula (?) plate must have remained to the west of the Bering Sea
and the Pacific plate, or, 2) synchronous westward subduction of two different
plates (Pacific and North America (?)) occurred north and south of the Aleutian
subduction zone, or, 3) back-arc spreading was inititated in the area of the
Komandorskii Basin. As of yet, there is no evidence conclusively supporting any
model. However, the westward translation and accretion of the Cape Kamchatka
terrane (Watson, 1985) in Eocene to Oligocene time (Watson and Fujita, 1985) is
consistent with continued westward convergence under the Goven-Eastern
Ranges are at least through Paleogene time.

The East Olyutorsk-Shirshov are, which had initiated its formation in the
Senonian, became inactive during the Paleogene, possibly after being trapped on
the Kula plate fragment north of the Aleutian arc. However, convergence
continued between the floor of the Bering Sea and the Olyutorsk margin from
60 to at least 40 my ago, thereby causing the East Olyutorsk-Shirshov arc to

impinge upon the eastern Olyutorsk margin. Oceanic crust to the west of the

58

East Olyutorsk-Shirshov island arc was subducted along the TAC terrane locus,
which may correlate offshore to the "East Kamchatka synclinorium” (Shapiro,
1976) extending northward from Cape Kamchatka to the east of Karaginskii
Island toward Goven Peninsula. Shortening also continued to be taken up along
the eastern Kamchatka margin. The western convergence of the
paleo-Komandorskii basin and the Komandorskii Islands also brought Cape
Kamchatka into its present position during Eocene to Oligocene (Watson and
Fujita, 1985). This was the final step in bringing the pre-Miocene features of
the Bering Sea to their present positions, and was nearly the end of the
compressional period within the Bering Sea.

An apparent basement correlation between the Shirshov and Bowers ridge
(Cooper et al., 1976a) presents the interesting possibility that Bowers ridge was
also part of the East Olyutorsk-Shirshov island are system. If this was the
case, southern Bowers ridge may have impinged upon the Aleutian island are
and separated from the Shirshov ridge along a left-lateral strike-slip fault
(Kienle, 1971) which is now preserved in the basement rocks between Shirshov
and Bowers ridges. Shortening along Bowers ridge was accomodated by
renewed subduction under its eastern and northern flanks, while the Shirshov
ridge remained tectonically inactive. Miocene rifting in the Komandorskii basin
and related normal faulting (Savostin et al., 1986) has also structurally
broadened Shirshov ridge, as first noted by Ludwig et al. (1971a). Thus
differing Paleogene to Neogene evolutions of the Shirshov and Bowers ridges
can explain the discrepencies in their observed geophysical (gravity, magnetic

and seismic) character (Ludwig et al., 1971b; Kienle, 1971; Rabinowitz, 1974).

59
Witnesses

The emplacement of the EOS terrane brought the major structural features
into their present positions within the Bering Sea region. This emplacement
was also followed closely by the end of compression, and the transition by
mid-Miocene to large scale extension in the whole Bering Sea region.
Evidence for extension is found in the Komandorskii basin, where rift
volcanism is presently taking place and 9 Ma basalts have been recovered at
DSDP site 191, in central Olyutorsk, where there is extensive Miocene basaltic
volcanism, in the Beringian margin basins, which have large thicknesses of
Miocene sediments, and in Alaska’s Yukon-Koyukuk province, where extensional
earthquake mechanisms have been determined. This geologic evidence is in
contrast to the Neogene compression between North America and Siberia
predicted by the models of Pitrnan and Talwani (1972), Vink (1984) or
Engebretson and Wallace (1984).

Within the Komandorskii basin, the present rifting, and the possibility of
related onshore activity in Olyutorsk, can now explain the high heat flow
values and shallow depth to basement (relative to the Aleutian basin) noted
earlier. Seliverstov (1987) presents seismic profiles across a volcanic edifice
within a 20 km wide graben structure which may be associated with the
modern rift north of Bering Island (Bogdanov, 1986). The rift runs near the
DSDP site 191 drill hole, and is shown by Savostin et a1. (1986) to be offset
by transforms which may also intersect the Shirshov ridge.

Miocene volcanism is also found in central Olyutorsk (Bogdanov and
Neprochnov, 1984; Koltypin and Kononov, 1986), where it covers portions of
the EOS, TAC, KPS and GER terranes. The volcanics are basic in composition,

and appear as laterally extensive flows in the region of the Apuka Mountains

60
and Pachachinsk Plateau. Whether there is a direct correlation between
volcanism in the Komandorskii basin and central Olyutorsk is unclear at
present.

The Beringian margin basins are thought to have originated in Eocene
time based upon the oldest sediments flooring the basins. However, Miocene
sediments have substantial thicknesses, and represent a renewed or increased
extensional period at the Beringian Margin. The basins do not exhibit typical
spreading structure, and are inconsistent with models of back-arc extension
which require spreading perpendicular to convergence direction (Jurdy, 1979),
and may instead be the response of a pre-existing weakness to extensional
back-are forces. This may be similar to extensional back-arc features oriented
parallel to relative convergence noted by Karig et al. (1978) in the Mariana
trough.

In the Yukon-Koyukuk province of Alaska, extensional focal mechanisms of
earthquakes have been determined by numerous investigators (e.g. Liu and
Kanamori, 1979; Biswas et al., 1986; Estabrook, 1985; and Chapter 2, this
thesis). Thus it appears that the extension observed in the western Bering Sea
and the Beringian margin extends even farther eastward to interior Alaska. The
consistent extensional earthquakes of western Alaska are enigmatic when viewed
in terms of Alaskan tectonics. A regional extensional episode, however, could
best explain such events. Lockhart (1984), in his study of the Seward Peninsula
concluded that the extension observed in Seward Peninsula is not caused by
localized rifting, but by a regional extension. Such a conclusion is consistent

with observations over the whole Bering Sea region.

CONCLUSIONS

The western Bering Sea margin is composed of five terranes of oceanic
affinities which have accreted since Cretaceous time. Of the five, two
represent island arcs (GER and EOS), two are packages of marine sediments
(KPS and T AC) and one is a fragment of oceanic crust. Since Miocene time,
volcanism has covered much of the exposure of these terranes in Olyutorsk,
where extensional basaltic eruptions are observed, and in Kamchatka where
island-are type volcanics predominate.

The geology of these terranes preserves information which better
constrains the Cretaceous and Cenozoic evolution of the Bering Sea. Obducted
oceanic crust and sediments, possibly remnents of the Izanagi plate, have been
identified in northern Olyutorsk. Evidence within the GER (island arc) and
TAC (accretionary wedge) terranes indicates subduction continued along the
Kamchatka-Olyutorsk margin until Miocene time. Miocene volcanism stitches
Olyutorsk together with another Cretaceous island are (EOS terrane) which
collided with the margin pre-Miocene. The Bowers Ridge may also be a
remnant of this island are, offset from the Shirshov Ridge along a left-lateral
fault. Some of the terranes preserved in the western Bering Sea may also
continue under the Beringian Margin and/or be preserved in southern Alaska

and its margins.

61

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Transactions (Doklady) of the USSR Academy of Sciences, Earth Science
Section, 285, 89-92, 1985.

CHAPTER TWO

INTRAPLATE SEISMICITY OF ALASKA:
IMPLICATIONS FOR REGIONAL STRESSES

INTRODUCTION

Alaska is characterized by a gradient of seismicity which decreases from
south to north. This undoubtedly arises partially from the decreasing effect of
Pacific subduction away from the Aleutian trench (Figure 2-1). Realizing
Alaska is tectonically and geologically complex, and due to the rapid northward
decrease in seismicity, any extrapolation of a coherent stress pattern beyond
the vicinity of the Aleutian Arc may at first seem unwarranted. However, as
has been shown in earlier studies (e.g., Gedney and Berg, 1969; Gedney, 1970;
Nakamura et al., 1980), consistent regional stress trends are observed south of
the Brooks Range. The manifestation of these stresses in structural, volcanic,
gravimetric and seismic indicators presents the opportunity to evaluate sources
of stress.

The various methods utilized in evaluating lithospheric stresses commonly
compliment each other. Early analyses of stresses often were limited to a
single method (e.g., first motion studies (Scheidegger, 1964)) which could create
a systematic bias in the interpretation. However, massive compilations and
analyses, such as those done by M. L. Zoback (Zoback and Zoback, 1980; Zoback
et al., 1984, 1986) suggest that consistent stresses are indicated by differing
methods, and that these stresses often encompass vast regions (e.g., the
mid-continent, Zoback et al., 1986).

This investigation presents new source parameters for 11 earthquakes
north of 64°N in Alaska. These data are combined with borehole elongation
orientations and focal mechanism solutions in northern Alaska and northwest
Canada in order to define the stress regimes within Alaska. The results

modify the studies of Nakamura et al. (1980) and Biswas et al. (1986). The

69

70

Figure 2—1: Regional geo ra by of Alaska and surroundin areas. Faults are
from Grantz (1 6 and Bro an et al. (1975). B - Barter Island; H -
Huslia; B-K - Bendeleben- 'gluaik Mountains; R - Rampart; SC -
Stevens Creek; F - Fairbanks '

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72
gross delineation of stresses are, however, in good agreement with even the

earliest Alaskan stress studies (e.g., Gedney and Berg, 1969).

NEW FOCAL MECHANISMS

New source parameters were obtained for eleven earthquakes of magnitude
4.2 to 6.0 (Table 2-1). For three events (nos. 1, 4, 9), focal mechanisms have
been presented by other investigators prior to this study, and are modified in
this study in light of new or re-evaluated data. Source parameters were
obtained through the use of P-wave first motions, surface wave analysis,
aftershock studies and body-wave modelling. Brief explanations of these
methods are given, followed by discussions of each of the 11 earthquakes to be
used as indicators of the stress regimes within Alaska.

Methnds

Four methods have been used to constrain source parameters: P-wave
first motions, surface wave analysis, master event relocations, and synthetic
seismogram (long and short period) computations. Due to the small size of the
events (Mb 4.2 to 6.3) studied, the signal to noise ratio is often poor.
Therefore, independent confirmation of results using different techniques is
essential.

First motions were read from World Wide Standardized Seismograph
Network (WWSSN) and Canadian network seismograms. This was supplemented
by first motions reported in the International Seismological Center (ISC)
Bulletin and the International Seismological Summary (185). Long and short
period data are used in all cases.

Rayleigh waves are analyzed for some post-1964 events, which are

recorded on standardized instruments. Those Rayleigh waves with periods

73

Figure 2-2: Synthetic seismograms for the October 29, 1968, Rampart
mainshock. STF is the source-time function, given as rise, run and
fall times, in seconds. Crustal structure parameters include P-wa e
velocity (or, km s), S-wave velocity (3, km/s), density (p, gm/cm )
and thrckness ( , km).

74

 

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75

between 20 and 100 seconds were used. The amplitude spectra were computed
and equalized to a common station magnification of 1500 and 90 degrees. To
differentiate between signal and incoherent noise at individual stations,
spectral amplitudes were plotted against frequency and examined. The final
plot of spectral amplitude vs. station azimuth is then generated for several
frequency windows. The resultant radiation pattern is then compared to the
theoretical radiation pattern generated from the fault strike, dip, and slip
angle.

Master Event relocations are used to identify the fault plane accurately.
The largest (”master") event of a sequence of earthquakes is assumed to be best
located and have the most representative travel time residuals. By assuming
that the travel paths of the master event and its aftershocks are similar (valid
over a small focal area), the master event residuals are used as corrections for
travel-times of smaller events for matching source-receiver pairs. This yields
better relative locations by effectively removing the effects of lateral
inhomogeneities in the earth.

Synthetic seismograms are computed for long and short period body waves
using the method of Kroeger (1987). These further constrain the focal
mechanism and focal depth. Detailed crustal structure is not important in
calculating long period synthetic seismograms since the period of the waveform
is long relative to the thicknesses of layers within the crust. This treatment is
not appropriate, however, for short period synthetics. Good constraints on
composition, density, seismic velocity, and thickness are important (Seno and
Kroeger, 1983). These better constrained parameters are, therefore, used in all
synthetic seismogram calculations. The crustal constraints are obtained from

the works of previous authors and are noted in the discussions.

76

MW

Event one is the October 29, 1968, Mb=6.0 earthquake referred to as the
Rampart event. Gedney (1970) arrived at a strike-slip solution, with the P-axis
trending NW-SE (Figure 2-2). Rayleigh wave radiation patterns generated for
various periods between 25 and 60 seconds produce a four-lobed pattern (Figure
2-3) which fits well with the theoretical pattern based on Gedney’s (1970)
solution. To better constrain the fault plane, a Master Event relocation of the
140 associated aftershocks was also done. Although the majority of ISC
reported aftershock epicenters trended NNE-SSW relative to the mainshock, a
disturbing E-W trend was observed with two of the larger (Mb > 4.2)
aftershocks. Following the relocation, however, these two shocks took on an
approximately 005° trend relative to the mainshock, as did the smaller
aftershocks (Figure 2-4). This trend matches well with the NOSE striking nodal
plane in the focal mechanism, and this nodal plane is taken as the fault plane.
The relocated aftershock trend is also sub-parallel to the Minook Creek valley
and a northward trending segment of the adjoining Yukon River. Master event
relocation therefore supports Gedney’s (1970) conclusion that the Minook Creek
valley is as the surface expression of the fault. Focal mechanisms were
constrained for two aftershocks (events 2 and 3, magnitude 4.6 and 4.4
respectively) using P-wave first motions. Both of these solutions are very
similar to the mainshock (Figure 2-5), with nearly north-south trending nodal
planes chosen as the fault planes.

Both long- and short-period synthetic seismograms were computed to
constrain the depth of the Rampart mainshock (no. 1). Excellent matches are
obtained using the listed fault parameters. Crustal structure, from Jones et

al. (1981), source-time-functions, and resulting seismograms are shown in

77

Figure 2-3: Rayleigh wave radiation pattern for the October 29, 1968 Rampart
earthquake. Stippled pattern is the theoretical radiation pattern for
the so ution given in Table 2-1.

78

 

 

 

 

 

 

 

29 OCTOBER 1968
PERIOD = 60 SECONDS

Figure 2-3

79

Figure 2—4: Master event relocation of the October 29, 1968 Rampart mainshock
and 140 aftershocks. Note the north-south realignment of the four
largest aftershocks (small stars) after relocation.

lSC LOCAHONS

80

RELOCATED

 

 

 

EVENT

I 1 I

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81

Figure 2-5: P-wave focal mechanisms for two Rampart aftershocks. Small
circles are ISC reported compressions (filled) and dilatations
(open). Lar e circles are compressions (filled) and dilatations
(open read rom WWSSN and Canadian network seismograms. P
indicates the position of the compressional stress axis.

82

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83

Table 2-1: Eartrhfiuake parameters for north, central and western Alaska.
LA. - Latitude in degrees north
LON .GT - Longitude in degrees west
M.AG - Ma nitude: Mb unless noted as L (Local magnitude) or S

(Sur ace wave magnitude)

PLANEx - Orientation of the nodal plane, using the convention of

Aki and Richards (1980) (Dip down to the right of
strike).

REFS - References, as follows:
A40: Adkins, 1940

B62: Balakina, 1962

D60: Davis, 1960

D285: Dziewonski et al., 1985
E85: Estabrook, 1985

G85: Gedney, 1985

H79: Hasegawa et al., 1979
J68: Jordan et al., 1968
LK80: Liu and Kanamori, 1980
LW74: Leblanc and Wetmiller, 1974
R62: Ritsema, 1962

S61: Schaffncr, 1961

S64: Stevens, 1964

$874: Sykes and Sbar, 1974

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1 001029 221010.3 05.10 150.07 0.0 11. 000/90 090/02 321/00 052/00 100/02
2 001031 002513.7 05.71 119.91 1.0 310/70 252/00 300/00 200/20 015/00
3 001103 073739.1 05.71 119.00 1.1 001/01 091/90 321/17 222/17 091/01
1 050309 110001.0 00.3 119.0 3.0 7. 010/71 100/90 331/12 211/10 100/71 105
110001.3 00.2 150.0 5.9 210/00 120/05 315/01 075/00 250/01 NEIC
110007.2 00.53 119.91 211/79 305/71 100/22 250/00 002/07 0205
3. 001220 002020.5 00.02 110.1 1.0 050/00 110/00 013/15 101/00 191/70
0 001220 005753.0 00.79 110.1 1.9
0 010712 012750 1 07.71 101.20 5.2 100/05 190/07 331/01 002/00 231/01
7 000020 101932 00.71 102.7 3.0 200/01 190/00 110/12 053/03 311/79
0 011213 003320.9 01.00 103.37 3.3 17. 203/57 011/00 233/17 139/02 017/11
9 300107 153010 00.03 150.59 7.3 220/30 030/53 332/79 113/09 231/05 102
10 730111 051210.1 01.01 100.01 1.2 011/30 230/70 199/53 319/19 039/20
11 000122 231130.1 70.3 111.1 1.7 <10 201/05 010/51 231/29 331/19 091/53
110mu1 mums “0 1UJ 31 my» nun 1nn0 unu onma 1”
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11 370722 170920 01.55 117.20 7.3 031/07 131/77 351/27 201/07 150/02 110
01 721120 133537.1 05.7 115.5 5.0s 015/70 112/70 331/22 001/03 100/00 :05
10 730309 111911.9 05.9 119.7 1.0L 000/01 102/02 320/25 232/12 110/02 :05
07 050211 030102.2 00.3 119.0 5.1 031/90 303/90 100/00 230/00 9:11 105
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191 720720 101021.7 00 5 130 0 1 0 320/00 230/01 190/21 099/11 337/01 1971
s55 051005 oo1715.1 05.1 131.0 5.2 0. 011/32 272/50 330/01 250/03 007/20 3571
550 050110 232210.0 01.09 100.23 5.0 12. 137/25 305/00 200/09 027/22 307/01 ss71

85

Figure 2-2. A best fit between observed and computed waveforms is found for
a focal depth of 14 km. Of particular interest is the fit obtained with the
short period synthetics. Good matches for a full 40 seconds of record are
observed for AKU and STU (Figure 2-6). A well constrained crustal structure
is an important factor in obtaining this match. Seno and Kroeger (1983) also
found that good short period synthetics could be computed with goodcrustal
constraint at the source.

Event 4 is a 1985 earthquake located very near the 1968 Rampart series.
A centroid moment tensor solution has been determined by Dziewonski et
al. (1985), and P-wave solutions have been presented by Estabrook (1985) and
also in the ISC bulletin (March, 1985) by the National Earthquake Information
Center (NEIC) (Table 2-1; Figure 2-7). A series of 22 aftershocks, which
occurred within two months after the mainshock and ranged in magnitude from
3.4 to 5.1 (Estabrook, 1985), fall along a trend of 040 degrees. This trend is
very similar to the trends of nodal planes determined by NEIC (030°) and
Dziewonski et al. (1985) (031°), although somewhat discordant with Estabrook
(1985) (016°). Estabrook (1985) does state that the northerly trending nodal
plane is likely to represent the fault plane due to parallelism with the active
Dall Mountain fault and other seismic trends in the vicinity. Cross sections
through the hypocentral region (Figure 2-8; Estabrook, 1985) indicate a steep
northwest dip of the fault plane. Estabrook’s (1985) solution indicates a
southeast dipping plane, and thus, the mainshock focal mechanism solutions of
the NEIC and Dziewonski (1985) fit the aftershock data better. Brogan et al.
(1975) suggest that the Ball Mountain fault has a normal faulting component,

down to the west. This further supports nodal planes chosen by the NEIC and

86

Figure 2-6: Short-period synthetic seismograms calculated at various depths for
the station AKU, using the given solution (Table 2-1) of the October
29, 1968 Rampart event.

8?

 

 

AKU. SPZ -

21 WW

 

Figure 2-6

 

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88

Figure 2-7: Focal mechanism solutions for the March 9, 1985 earthquake (#4,
Table 2-1). All solutions indicate left-lateral strike-slip along the
north-northeast to northeast trending probable fault plane.

89

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Figure 2-8: Cross sections from Estabrook (1985) for the March 9, 1985,
earthquake and aftershocks. The A-A.’ section suggests a northeast
trending fault plane with a steep northwest dip.

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92
Dziewonski (1985), although all three solutions indicate left-lateral slip along
the north to northeast trending plane.

Events 5a and b were an earthquake doublet known as the Caro series
which occurred approximately 200 km NNE of the Rampart (nos. 1, 2 and 3)
events. The pair, occurring on December 20, 1966, are separated by 31
minutes and are of magnitudes 4.8 and 4.9, respectively. Only the focal
mechanism for the second event (Figure 2-9) is well constrained due to the
lack of good P-wave first motions recorded for the first event. A remarkable
similarity between long period waveforms, however, leads Coley (1983) to
believe that both of the events are of the same mechanism. The first 60
seconds of long period waveform recorded at COL shown in Figure 2-9,
exemplifies these similarities.

Geologic mapping in this area by Beikman (1980) shows a predominance of
NSSE trending faults. This is consistent with the N60E striking nodal plane
constrained for this event, which is therefore taken as the fault plane.

An Mb=S.2 earthquake (no. 6) occurred in the western Brooks Range on
July 12, 1981. This event appears to have occurred along a nearly east-west
striking right-lateral strike-slip fault. Reading WWSSN seismograms permitted a
solution to be obtained despite ISC reported first motions being inconsistent
within the focal sphere. Additionally, an observed lack of Rayleigh wave
energy with high amplitude Love waves at TUC and ISC reported long- and
short-period dilitations at 3.11 helped to constrain the east-west trending nodal
plane (Figure 2-10).

Master Event relocation of aftershocks for this event shows dramatic
results. The ISC reported epicenters of 14 aftershocks show no linear trends.

Following relocation, however, an approximate 110 degree trend is identified

93

Figure 2-9: P-wave focal mechanism for the second (00h 57m 53s) December 20,
1966 earthquake (#Sb). Long-period waveforms at Colle e Station
(COL) are shown for both events. The remarkable simi arity lead
Coley (1983) to suggest the two events had similar focal
mechanisms.

91+

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95

Figure 2-10: P-wave mechanism for the 1981 Kobuk Trough earthquake (#6).
The large open square indicates a reported long- and short-period
dilatation at BJI. The small star is a nodal pick at TUC based on
a large Love/Rayleigh wave ratio. Other conventions are as in
Figure 2-5.

96

 

 

 

12 JULY 1981 ‘

Figure 2-10

97
(Figure 2-11). This trend is very consistent with the 106 degree striking nodal
plane, which is chosen to be the fault plane for this earthquake. The right
lateral strike-slip motion is also consistent with the observations of Oldow
(pers. comm.), who has mapped a number of Recent strike-slip faults within the
Brooks Range.

A centroid moment tensor solution for the July 12, 1981, earthquake by
Dziewonski et al. (1988) presents a best double couple solution with nodal
planes oriented 2470/730 and 1540/790. This solution is in poor agreement
with aftershock distribution (Figure 2-11) and geologic data which identifies
the Kobuk trough as a nearly east-west trending fault (Brogan et al., 1975).
Hinzen (1986) has shown that many moment tensor solutions differ from P—wave
and surface wave solutions. Moment tensors represent a mean solution for the
entire rupture process, whereas P-waves are indicative of conditions at rupture
initiation. Therefore, the discrepancy of the moment tensor solution may be
due to a change in slip direction or fault geometry (e.g., curved fault planes)
after initial rupture.

An earthquake on Baldwin Peninsula in 1966 (no. 7) is the westernmost
strike-slip solution found in northern Alaska. Focal mechanism constraint
using P-wave first motions is fairly good, and allows only slight variation of
the nodal planes (Figure 2-12).

The relative motion of this event (no. 7) is similar to other events
(nos. E9, E11 and 6) in the western Brooks range, and may represent
right-lateral motion on a western extension of the fault-controlled Kobuk
trough (Grantz, 1966; Brogan et al., 1975). There is a normal faulting
component to the focal mechanism, which may be induced by the vertical
stresses in Seward Peninsula (Lockhart, 1984) just south of Baldwin Peninsula.

98

Figure 2-11: Master event relocation for the aftershock sequence of the 1981
Kobuk Trough earthquake. ISC reported epicentral positions
relocate to an approximate N70W trend, very close to the N74W
trending nodal plane.

99

 

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Figure 2—11

100

Figure 2-12: P-wave mechanism for the August 26, 1966 Baldwin Peninsula
earthquake. Conventions are as in Figure 2-5.

 

I

 

 

101

 

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26 AUGUST 1966

Figure 2-12

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1981
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102

The December 13, 1964, Mb=5.3 Seward Peninsula event (no. 8) has a
normal dip-slip mechanism which is very well constrained by P-wave first
motions (Figure 2-13). Short period body-wave synthetic seismograms computed
for a good azimuthal distribution of stations indicated a best fit depth of 17 km
(Figure 2-13), and are consistent with the P-wave focal mechanism.

This earthquake occurred in the Bendeleben-Kigluiak Mountain Ranges, and
is probably located on the nearly east-west striking Kigluiak normal fault
system within these mountains (Hudson and Plafker, 1978; Nakamura et al.,
1980). Hudson and Plafker (1978) map the Kigluiak fault in the vicinity of the
epicenter as an ENE striking, northward dipping plane (Figure 2-14), which is in
excellent agreement with one nodal plane (strike/dip = 2630/570), which is
thereby presumed to represent the fault plane. Hudson and Plafker (1978)
describe modern offset along the Kigluiak fault, including "abruptly separated
alluvium,” and scarps which juxtapose bedrock along unconsolidated deposits and
moraines, which is consistent with its interpretation as the active fault of this
1964 earthquake.

Event 9 is the largest earthquake of an extensive series of earthquakes to
occur in 1958. This event, often referred to as the Huslia earthquake, had a
magnitude of 7.3, and was followed by many aftershocks, including a magnitude
6.5 on April 13, 1958. The event has been investigated by many (e.g.,
Schaffner, 1961; Balakina, 1962; Ritsema, 1962; Stevens, 1964), resulting in a
variety of focal mechanism solutions. Coley (1983) modified the solution
proposed by Ritsema (1962), which was a proposed single-couple based on
P-wave first motions and S-wave polarization angles (Figure 2—15), in order to
obtain a more consistent fit with the S-wave data. Coley’s (1983) solution,

however, has non-orthogonal nodal planes. The solution presented here (Table

103

Figure 2-13: P-wave first motions and short-period synthetic seismograms for
the December 13, 1964, Seward Peninsula earthquake (#8).
Conventions are as in Figures 2-2 and 2-5.

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105

Figure 2-14: Map of the Ki uaik-Bendeleben fault system from Hudson and
Plafker (1978). e star represents the eplcenter of the December
13, 1964 earthquake.

106

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107

Figure 2-15: Shear-wave olarizations and focal mechanism of Ritsema (1962)
for the April , 1958, Huslia earthguake (#9). Dashed line shows
the fit of the preferred solution ( igure 2-16) to the P-wave and
S-wave data.

108

 

07 APRIL 1958
RITSEMA SOLUTION

Figure 2-15

109
2-1; Figure 2-16) is a slight modification of Ritsema’s (1962) and Coley’s (1983),
and has been chosen to best fit all the data and to maintain orthogonality of
the nodal planes.

Event 10 was a small (Mb=4.2) event which occurred at almost exactly the
same epicenter as a 1965 magnitude 5.8 earthquake (Sykes and Sbar, 1974;
no. SS6) east of Seward Peninsula. Despite the small size of the event, a
moderately well constrained focal mechanism was determined by reading P-wave
first motions and using ISC reported first motions (Figure 2-17). The solution
is dominantly normal faulting, with a steeply dipping nodal plane trending 070°,
and a less well constrained gently dipping plane trending 014°. The east-west
trending plane is constrained mainly by ISC reported first motions. The nodal
planes are nearly orthogonal to Sykes and Sbar’s (1974) solution for the nearby
1965 event, but the north-trending nodal plane parallels the dominant fault
trend (Bickel and Patton, 1957) in the epicentral region. Furthermore, the
mechanism is consistent with other earthquake solutions, including the 1965
event, which indicate extension in western Alaska.

The January 22, 1968, Barter Island earthquake (Mb=4.7; no. 11) was
located about 2 km offshore of northeastern Alaska, at the northern end of
the Canning Displacement Zone (CDZ) of Grantz and May (1982). First motions
indimte this event has a strike-slip solution with a large normal faulting
component (Figure 2-18). Rayleigh waves were analyzed to further constrain
the focal mechanism obtained from first motions. Although data in the
southwest quadrant are sparse, a fair match is found between the observed
radiation pattern and the theoretical pattern generated from the focal
mechanism (Figure 2-18). A node is present in the radiation pattern, and is

not inconsistent with what is expected from the dip slip mechanism shown.

110

Figure 2-16: Focal mechanism of the Huslia earth uake derived from
orthogonalizing the nodal planes of Coley (1 83), whose solution
was based upon the best fit of P-wave first motions and Ritsema’s
(1962) S-wave polarizations.

 

111

 

07 APRIL 1958
THIS STUDY

Figure 2-16

 

112

Figure 2-17: P-wave first motions for the Mb=4.2, April 11, 1973 event (#10)
east of the Seward Peninsula. Conventions are as in Figure 2-5.

113

 

 

 

 

 

11 APRIL 1973

Figure 2-17

 

1111

Figure 2-18: P-wave first motion plot and Rayleigh wave radiation pattern for
the January 22, 1968, Barter Island earthquake (#11). Conventions
are as in Figure 2-5.

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116

The small magnitude of this event prevented a good distribution of body
waves which could be used for synthetic seismogram computation. Seismograms
calculated at two stations, using either of the proposed solutions, do match
with observed records when placed at very shallow (<5 km) focal depths
(Figure 2-19). Crustal structure is taken from Grantz and May (1982).

Reflection seismic lines from the R/V S.P. Lee show approximately
east-west striking, steeply dipping normal faults (Grantz and May, 1982; Figure
2-20). This corresponds with the 095° striking nodal plane of the mechanism,
which is therefore taken as the fault plane for this event. Grantz et al. (1987)
describe west-northwest trending normal faults and listric growth faults which
drop the northern wall down along the continental slope. Thrusting, if active,
occurs only on low-angle detachment faults, which do not match the dip of the
fault plane in the solution. Quaternary deposits are offset by as much as 10
meters, consistent with recent activity on the faults. The sense of strike-slip
motion on this event is opposite that which is expected if it were due to
stresses/ strains along the CD2.

Biswas et al. (1986) proposed a nearly pure strike-slip solution for the
Barter Island earthquake, with nodal planes of 050°/60° and 315°/80°. Such a
solution is inconsistent with P—wave first motions read from seismograms
(specifically at MBC, CMC, DUG and FSJ), with the nodal azimuths on the
Rayleigh wave radiation pattern, and with the trends of any of the faults
identified in the Barter Island or Camden Bay region. Due to these
inconsistencies, the solution of Biswas et al. (1986) is not considered in this

study.

117

Figure 2-19: Synthetic seismo ram at State College, Pennsylvania, calculated
for t e Barter Islan earth uake focal mechanism solutions of Biswas
et al. (1986) and from 11115 study. Fits for either mechanism can
only be obtained when a very shallow (< 5 km) depth is used.

118

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119

Figure 2—20: Map and seismic section in the vicinity of the 1968 Barter Island
earthquake. The motion alon the north-south nodal plane is
oppos1te that of the Canning isplacement Zone. The east-west
nodal plane parallels the strike of the steeply dipping normal faults
in the seismic section, and is most likey the fault plane.

120

   

 

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STRESS INDICATORS

In addition to the earthquake source parameters presented above, I have
included parameters of 16 earthquakes from other sources and 9 borehole
elongation stress determinations from northwestern Canada (Table 2-1 and 2-2;
Figure 2-21).

The earthquakes, ranging in magnitude from 3.8 to 7.3, have well
constrained nodal plane orientations. The solutions represent compressive
maximum horizontal stress if one of the two following criteria for the
principal stress axes’ dips are met:

P, T < 30° (Strike slip faulting)

T > 30° > P ((?lghrust faulting)
where P is the principal compressive stress and T is the principal tensile
stress. If the principal stress axes’ dips meet the criteria

P > 30 > T (Normal faulting)
then the maximum compressive stress is considered vertical.

The borehole elongation data (Table 2-2) are summarized from Zoback et
al. (1984). Details of the theory behind the interpretation of these
measurements can be found in Bell and Gough (1979, 1981). Briefly stated, the
axis of elongation found within well drill holes, at depths greater than a few
hundred meters, is assumed to represent the minimum horizontal stress (Sh)
(Bell and Gough, 1979; Gough and Bell, 1981). The maximum stress must then
be perpendicular to this orientation, and is thought to be horizontal (SH) rather
than vertical (Bell and Gough, 1981) in northwest Canada.

121

122

n i
132.500
134.000
134.700
135.200
138.400
140.300 W
140.200 W
135.000 W
135.500 W

W
W
W
W
W

All;
354
349
354
038
040
000
345
062
056

MEAN AZI OF ALL DATA = 17.556

 

Data from: Zoback et al., 1984.

Table 2: WELL ELONGATION PRINCIPLE COMPRESSIVE
STRESS DATA FROM NORTHWEST CANADA

Reffi
CN61

CN 62
CN 63
CN 64
CN 65
CN66
CN 67
CN 68
CN 69

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STRESS REGIMES

Compression due to Pacific subduction and related back-arc extension
represent two principal tectonic sources which would probably be incorporated
into any hypothesis regarding the distribution of stresses within south-central
Alaska. Conclusions of previous investigations have indicated these sources are
indeed affecting the regional stress (e.g., Nakamura et al. (1980), Gedney ( 1970),
Biswas et al. (1986)). Further north and east, however, Pacific induced stresses
should decrease, and other sources, such as ridge puSh from the Arctic
mid-ocean ridge or continental margin sediment loading may influence the
regional stress (Hasegawa et al., 1979). Local stresses or structural geometries
could be responsible for small variations in the overall stress pattern within
Alaska and northwest Canada.

The data presented here delineate distinct regions of compression and
extension associated with the subduction of the Pacific plate. Stress
orientations in northeast Alaska indicate that stresses identified east of the
Canadian Rockies and in the Mackenzie delta region (Gough et al., 1983) may
also continue north and west of the Davidson mountains to at least Camden
Bay. Thus, three separate regional stress regimes, central Alaska, western
Alaska and northern Alaska, are identified (Figure 2-22).

Wash

Source parameters for 17 earthquakes (nos. 1, 2, 3, 4, 5, 6, 7, Gl, 62, J 1,
12, A1, E1, E6, E7, E9, AND E11) are used to define the central Alaska stress
regime (Figure 2-22). All the earthquakes occurred north of 64°N and south of
68°N. The westernmost of these earthquakes (nos. 6 and 7) occur at the

western end of the Kobuk Valley, northeast of Seward Peninsula.

124

125

Figure 2-22: Proposed distribution of stresses in Alaska and surrounding areas.
Hatchured region is under northwest-southeast horizontal
compression associated with Pacific subduction. Northern Alaska
and the Beaufort Sea is undergoing north-northeast to northeast
horizontal compression, and western Alaska and the Bering Sea are
undergoing extension. Long dashed boundary is the corn ression to
extensron transition proposed by Nakamura et a1. (19 0). Short
dashed line is the transxtion as proposed by Biswas et al. (1986).
SP8 is the stress province boundary of Gough ct al. (1983).

127

Of the 17 events, 16 have near horizontal P-axis orientations with a
northwest-southeast (2840-354°) trend (Table 2-1). This indicates that the
compressive stresses generated by the subduction of the Pacific plate are
reflected in earthquakes for hundreds of kilometers not only north and
northeast of the Aleutian trench, but also into the southwestern Brooks
Range. Compressive horizontal stress directions (Table 2-1) are in good
agreement with the stress trajectories proposed for the Fairbanks and Rampart
areas (Gedney, 1970; Nakamura et al., 1980; Biswas et al., 1986), but horizontal
compression extends further north and west than Nakamura et al.’s (1980)
proposed boundary marking the transition from compression to extension.
Biswas et al. (1986) redefine the transition boundary to extend north to the
Arctic coast, but maintain extension in the western Brooks Range where
earthquakes (nos. 6, 7, E9, E11; see also Gedney and Marshall, 1981) indicate
NW-SE directed compression. Therefore, results of this study indicate the
Brooks Range must be included within the compressional regime (Figure 2-22).

An interesting consistency should be noted within the central Alaska
region. There is a slight, yet systematic variation of P-axis orientations
between earthquakes in the Fairbanks region (nos. A1, I 1, 12, G], GZ) and
those in the Rampart region (nos. 1, 2, 3, 4, E6, E7). Focal mechanisms of
earthquakes in the Fairbanks region, which lie south of the Kaltag-Stevens
Creek-Tintina fault system, have a more north-northwest P-axis trend (mean
orientation = 339°, range of 323° to 354°) than the Rampart region solutions
(mean P—axis orientation = 326°, range 300° to 348°) north of the
Kaltag-Stevens Creek-Tintina fault system. Furthermore, four of the five
events (J 1, J2, GI, 62) in the Fairbanks region are part of earthquake pairs

which occurred at the same epicentral locations. For each pair, the second

128

event (J2 and 62) has a more west-northwest P-axis trend similar to that
north of the Kaltag-Stevens Creek-Tintina fault system. Gedney (1970) has
shown that the strain north of the Kaltag—Stevens Creek-Tintina fault system
is significantly less than south of the system. Thus, it appears that in regions
of less strain (e.g., north of the Kaltag-Stevens Creek-Tintina fault (Gedney,
1970)) or in areas where strain has been released (after the first of an
earthquake pair) the P-axes orient in a more northwest to west-northwest
direction.

W3

Northwest of the Kaltag fault, normal faulting earthquakes (nos. 8, 9, 10,
SS6) indicate extension is the active tectonic process. This region, which is
nearly 1000 km from the Aleutian trench axis, includes Seward Peninsula and
the northern Yukon-Koyukuk province. Uyeda and Kanamori (1979) identified
two stress regimes associated with the Aleutian subduction zone. The first,
near the volcanic arc, is compressional. Further from the arc, however, tension
is indicated, despite the Bering sea being an "inactive” (Uyeda and Kanamori,
1979) back-arc region.

There is additional evidence of recent extension within the western
Alaska - Bering Sea region. Hudson and Plafker (1978) identified major normal
faults (Kigluaik and Bendeleben) in southern Seward Peninsula (Figure 2-14). A
December, 1964, magnitude 5.3 earthquake (no. 8) most likely occurred on the
Kiguaik fault. The cast - west trending, north dipping nodal plane matches
very well with the mapped character of the Kigluaik fault (Hudson and Plafker,
1978). Quaternary basaltic volcanism has also been noted in Seward Peninsula
and northern Yukon - Koyukuk province (Hopkins et al., 1971; Hudson, 1977;
Beikman, 1980). Finally, Lockhart (1984) used gravity anomalies to propose

129
extension in the Seward Peninsula, and suggested such stresses must exist on a
regional scale.

Nakamura et a1. (1980) and Biswas et al. (1986) both included western
Alaska, with Seward Peninsula, northern Yukon-Koyukuk province and the
western Brooks Range, in the extensional regimes of their Alaskan stress
models. As discussed earlier, data from this investigation suggest that the
western Brooks Range, at least north of the Kobuk Trench, supports horizontal
compressive stresses and is therefore not part of the western Alaska
extensional region (Figure 2-22). The distribution of Tertiary to Quaternary
basalts in the Yukon-Koyukuk province and Seward Peninsula, but not within
the western Brooks Range (Stone and Wallace, 1987), supports the placement of
a northern boundary on the extensional region. It is possible that the
compression is preferentially transmitted along the Brooks Range due to a more
homogenous basement and/or lithospheric differences from Seward Peninsula and
Yukon-Koyukuk province. However, data do indicate that Seward Peninsula and
northern Yukon-Koyukuk province are undergoing extension as concluded by
Nakamura et al. (1980) and Biwas et al. (1986). The extension is related to
regional back-arc stresses (Lockhart, 1984), which are acting over the entire
Bering Sea region (Chapter 1).

mm

The stresses south of the Brooks Range have been determined by numerous
investigations using a variety of methods (e.g., Gedney, 1970; Nakamura et al.,
1980; Biswas et al., 1986; this study). Although each of these investigations
may differ in detail, the general Pacific compression (central Alaska) and

back-arc extension (western Alaska) models seem to apply in every case. North

130
of the Brooks Range, however, the determination of the stress regime(s) has
been hampered by the sparse availability of data.

Examination of two moderate earthquakes (#11, this study; #E19,
Estabrook, 1985) located offshore northern Alaska near Camden Bay indicates
stress orientations similar to those derived from borehole elongations in
northwest Canada. Both earthquakes are predominantly strike-slip, and have a
northeast trending, moderately (29° to 30°) dipping principal compressive stress
orientation. This orientation is similar to stresses derived from drill hole
elongations located in the Yukon and Northwest Territories (Table 2-2; Figure
2-21) as well as to that of the 1985 Nahanni earthquake in the Northwest
Territories (Wetmiller et al., 1988; Dziewonski et al., 1986).

The orientation of principal horizontal stress in northeast Alaska differs
from that predicted by ridge push forces (compression approximately
north-south) or loading of the continental margin (extension approximately
north-south). It is likely that the stresses in northern Alaska are more
complex than within the Mid-Continent region, and cannot be ascribed solely
to sediment loading or ridge push forces. The solutions of the two southern
Beaufort Sea earthquakes have fault planes, with normal faulting components,
parallel to the continental margin, although the tensional axes are not
perpendicular to the margin. It may be suggested that tensional sediment
loading stresses are nearly equal to compressive ridge push stresses
perpendicular to the margin. These two opposing stresses may in effect cancel
one another, allowing yet another source to be reflected in the observed stress
orientations.

Gough et al. (1983) identified a ”stress province boundary" which

separated the Mid-Continent stress (NE-SW) regime from an Alaskan stress

131
(NW-SE) regime (Figure 2-22). This extended the Mid-Continent stress
orientation (NE-SW) northwest to the Mackenzie Delta region, with the Rocky
Mountain front approximating the position of the stress province boundary.
The two Beaufort Sea earthquakes (nos. 11 and E19) also exhibit the
Mid-Continent stress orientation. I therefore suggest that the stresses within
southern Beaufort Sea may be similar to those presently operating in the Yukon
and Northwest Territories of Canada, and that this stress regime is separated
from Alaskan stresses (Gough et al., 1983) by a line which runs along the

Davidson Mountains to the northern Brooks Range.

CONCLUSIONS

This study has improved upon earlier evaluations of Alaskan stresses by
evaluating earthquake source parameters and borehole elongations in Alaska
and northwestern Canada. Previous studies within Alaska by means of
seismicity (e.g., Gedney, 1970; Biswas et al., 1986), volcanic alignments
(Nakamura et al., 1980) and theoretical modelling (e.g., Richardson et al., 1979)
have extrapolated stresses over large areas void of data. Additional. data
presented in this investigation have filled portions of the void and allowed a
more detailed outline of the stress regimes to be obtained. Compressive
stresses derived from the subduction of the Pacific plate at the Aleutian trench
radiate north and northeast into central Alaska. The compressive stresses may
also be preferentially transmitted westward along the Brooks Range, thus
explaining continued strike-slip faulting as far west as Kotzebue Sound.
Therefore, extension cannot be generalized throughout western Alaska, as had

been done previously (Nakamura etal., 1980; Biswas et al., 1986).

132

Western Alaska, south of approximately 66°N, is undergoing extension
related to back-arc stresses. The extension is manifested not only in the
seismicity, which is dominated by normal faulting earthquakes, but also in the
geology (Hudson and Plafker, 1978) and the gravimetric signature (Lochhart,
1984). Lockhart’s (1984) conclusion that the back-arc stresses must be of
regional extent is supported by investigations elsewhere in the Bering Sea
(Chapter 1; Savostin et al., 1986).

The Mid-Continent stress regime (Zoback et al., 1986) appears to extend
into the Beaufort Sea. Thus, the stress province boundary identified by Gough
et al. (1983) extends beyond northwest Canada into northern Alaska. This
boundary separates the Mid-Continent stresses from Pacific induced stresses
along the entire length of the Rocky Mountains, and may continue along the
northeastern Brooks Range as well. If northern Alaska is influenced by multiple
sources of stress, including ridge push and continental margin extension, these

opposing stresses appear to cancel one another.

HST OF REFERENCES

LIST OF REFERENCES

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Hudson, T. and G. Plafker, 1978. Kigluaik and Bendeleben faults, Seward
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Jones, D. L, N. J. Silberling, H. C. Berg and G. Plafker, 1981. Map showing
tectonostratigraphic terrains of Alaska, columnar sections, and sum
description of terrains, U.S. Geol. Sur. Open File Report 81-792, 20p. wit
maps.

Jordan, J. N., G. J. Dunphy and S. T. Harding, 1968. ' The Fairbanks, Alaska,
earthquakes of June 21, 1967, Preliminary Engineering Seismological

Report. 60 pp-

Kroegkr, G. C., 1987. Synthesis and Analysis of Teleseismic Body Wave
ismograms, PhD Dissertation, Stanford University, Palo Alto, CA, 135

e
PP-

136

Liu, H. L., and H. Kanamori, 1980. Determination of source arameters of
mid-plate earthquakes from the waveforms of ody waves,
Bull. Seis. Soc. Amer., 70, 1989-2004.

Lockhart, A. B., 1984. A gravity survey of the central Seward Peninsula,
Alaska (Abstract), Geol. Soc. Am. Cordill. Sect., 16, 319.

Nakamura, K., K. H. Jacob and J. N. Davies, 1977. Volcanoes as possible

indicators of tectonic stress orientation - Aleutians and Alaska, Pageoph,
115, 87-112.

Nakamura, K., G. Plafker, K. H. Jacob and J. N. Davies, 1980. A tectonic
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Packer, D. R., G. E. Brogan and D. B. Stone, 1975. New data on plate
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Patton, W. W., 1973. Reconnaissance geolo of the northern Yukon-Koyukuk
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Patton, W. A., I. L. Tailleur, W. P. Bros e and M. A. Lamphere, 1977.
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R. G. Coleman and W. P. Irwin, eds., W State of
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Richardson, R. M., S. C. Solomon and N. H. Slee , 1979. Tectonic stress in
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Savostin, L. A., B. V. Baranov, T. Z. Gri oryan and L. R. Merklin, Tectonics
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137

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W D. Reidel Pubhshing, Holland, 323 pp.

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U.S. Geol. Sun Prof. Paper 84-157, 2p

CHAPTER THREE

SEISMICITY AND FOCAL MECHANISMS OF THE
NORTH AMERICAN - EURASIAN PLATE BOUNDARY IN SIBERIA

INTRODUCTION

The seismicity of the North American plate boundary in the Arctic and
Siberia has been the subject of only limited study due to the scarcity of large
(mb > 6.0) earthquakes. The majority of Arctic earthquakes occur on the Arctic
mid-ocean ridge, separating the North American and Eurasian plates (Fujita et
al., in press). This narrow (< 50 km wide) linear trend is potentially the
longest such trend of the mid-ocean ridge system (Sykes, 1965), and bisects the
Eurasian basin and enters Siberia at the continental slope of the Laptev Sea
(Figure 3-1). Within northeast Siberia, the seismicity becomes spatially diffuse,
extending as a broad trend from the Lena River delta to the southern Cherskii
Mountains where there is a bifurcation of epicenters to the south and east
(Figure 3-2).

Since the installation of the World Wide Standardized Seismograph
Network (WWSSN) only two earthquakes (April 19, 1962 and August 25, 1964) of
magnitude greater 6.0 have occurred in this region. Source parameters for the
1964 event, and a select few other earthquakes, have been determined by
various western investigators (e.g., Sykes, 1967; Chapman and Solomon, 1976;
Jemsek et al., 1986). Soviet investigations (Lazareva and Misharina, 1965;
Savostin and Karasik, 1981; Savsotin et al., 1983; Koz’min, 1984) also have been
sparse, and appear to utilize International Seismological Center (ISC) Bulletin
and Bureau Central International De Seismologie (BCIS) reported first motions
and/or data from the Soviet northeast Siberian local network. Results of these
studies are frequently inconsistent with each other, as well as with western
studies, for both large and intermediate size events.

Several investigators (e.g., Chapman and Solomon, 1976; Savostin and

Karasik, 1981) have utilized this restricted set of focal mechanism solutions in

138

139

Figure 3-1: Regional geographic map of northeast Siberia showing key features
of interest. B-T denotes the location of Balagan-Tas vo cano and
EP denotes the El’ginskii Plateau. The position of the star indicates
the Aleutian-Kurile arc-arc junction.

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Figure 3—2: Shallow and intermediate (h < 100 km) teleseismic seismicity of
northeast Siberia and adjacent Arctic mid-ocean ridge. Epicenters
are from the ISC only. Large dots with stars are events used in
this study. Open stars are events used from McMullen (1985). The
shaded region along the Kurile and Aleutian arc denotes numerous
events.

142

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an attempt to define the kinematics of the North American plate within
northeast Siberia. Others (e.g., Morgan, 1972; Minster et al., 1974; Minster and
Jordan, 1978 and DeMets et al., in press) have used data exclusive of
northeastern Siberia to define the North American - Eurasian pole of rotation.
However, the proximity of the pole of rotation to the physical plate boundary
(DeMets et al., in press) and the potential existence of a Sea of Okhotsk plate
(Savostin and Karasik, 1981; McMullen, 1985; Seno, 1985) could negatively affect
the results of such broad studies.

This investigation attempts to better define the seismicity associated with
the North American plate boundary in northeast Siberia through the
determination of source parameters for the intermediate size (4.5 < mb < 6.0)
earthquakes. The result increases the data set available for investigation of the
North American plate boundary the Arctic and Siberia, may sheds new light on

the reliability of data from regions which are relatively inaccessible.

TECTONIC SEITING

The exact position of the North American plate boundary in northeast
Siberia has long been debated by numerous workers (e.g., Chapman and Solomon,
'1976; Zonenshain et al., 1978; Savostin and Karasik, 1981). Despite its
presently undefined position, there is basic agreement upon the general trend
between the Arctic ocean and the southern Cherskii Mountains. The setting of
this area includes extremely rugged topography contrasted by large depressions
with thick accumulations of sediments.

To the north, the Arctic mid-ocean ridge enters the continental shelf of
northeast Siberia in the central Laptev Sea (Figure 3-1). The Laptev Sea is
thought to be floored by grabens as old as Eocene (Savostin and Karasik,

144
1981). It is bounded to the east by the New Siberian Islands and to the
southwest by the Lena River delta. The sea’s southernmost extent is Bour
Khaya Bay.

South and southeast from Bour Khaya Bay extends a series of depressions
(Savostin and Karasik, 1981) bounded by prominent features such as the
Cherskii Mountains and Moma Range (Figure 3—1). The general trend. of the
depressions and en echelon uplifts within the Cherskii Mountains is northwest
- southeast. The depressions are bounded by normal faults, and are often cut
by faults described as strike-slip or transform (Zonenshain et al., 1978),
although they rarely are traced beyond the depression walls (Savostin and
Karasik, 1981). If the depressions are structurally assymetric as Grachev
(1973) notes within the Moma Rift, these transverse faults may be better
described as transfer faults (Lister et al., 1986) rather than transform faults.
I... Parfenov (pers. com., 1986) reports that thrust and strike-slip faulting is
now active within these depressions.

Sediments within the depressions range from Eocene (?) in the Laptev
Sea, Oligocene in the Lena River delta region, Oligocene to Miocene in the
northernmost Cherskii Mountains, and Pliocene in the southern regions of the
Cherskii Mountains. The deposits are described only as sands and
unconsolidated cobbles (Savostin and Karasik, 1981). Large (> 5 km) alluvial
fans on the borders of the depressions are observable on LANDSAT imagery, as
are a few small volcanic cones.

Numerous authors have suggested that the series of southeast trending
depressions are the continental continuation of the Arctic mid-ocean ridge
(e.g., Grachev et al., 1970; Churkin, 1972; Chapman and Solomon, 1976; Savostin
and Karasik, 1981; Grachev, 1982). This conclusion is based on high heat flow,

145
basaltic volcanism and relative subsidence (Argunov and Gavrikov, 1960;
Naymark, 1976; Savostin and Karasik, 1981) found throughout the system.
However, as early as the 1960’s, investigators were finding evidence of
compression and uplift (Lazareva and Misharina, 1965; Rezanov and Kochetkov,
1962) which may contradict the interpretation of the Cherskii - Moma system as

an active rift, although the uplift could be associated with thermal doming.

SEISMICITY

The Arctic mid-ocean ridge is reflected in a very narrow, linear band of
seismicity as it crosses the Arctic basin. However, the linearity ends as the
ridge impinges upon the continental shelf at the Laptev Sea. Southward from
this point, the epicenters become quite dispersed, extending from the New
Siberian Islands west to the Lena River delta. Upon entering continental
Siberia, there is a local decrease in seismicity, and then increased seismicity
follows the general southeast trend of the Cherskii Mountains in a diffuse, 400
km wide band.

A first-order observation of this epicentral trend camouflages some more
important points. When the epicentral distribution is examined as a function
of magnitude it becomes quite evident that the larger (Mb>4.5) events form a
linear band along the western margin of the northern Cherskii Mountains
(Figure 3-2). This immediately questions the location accuracy of the smaller
earthquakes which are often detected only by Soviet local networks, since
larger events are recorded and located by a multitude of global stations. A
study by Fujita et al. (1987) examined the seismicity in northeast Siberia and
concluded there was a bias of epicentral location as a function of spatial and

temporal local station distribution. Their results also suggested that many

146
epicenters in northeast Siberia were located using P-wave polarizations at a
single station of the Soviet northeast seismograph network. This method
creates annular rings (Figure 3-3) around stations because the critical refracted
arrival of very close events artificially locates the epicenter at a minimum
radius of 90 km (assuming a 33 km thick crust with velocity = 6 km/s and an
upper mantle velocity = 7.5 km/s). Additionally, temporal gaps in seismicity
developed as stations opened and closed. Therefore, the diffuse distribution of
smaller earthquakes may be due, in part, to location capabilities. The trend of
the larger (Mb > 4.5) events is more representative of the actual interplate

epicentral distribution in the northern Cherskii Mountains and Laptev Sea.

FOCAL MECHANISM SOLUTIONS

From the interplate earthquakes in the Laptev Sea and northern Cherskii
Mountains, twelve events large enough for study (magnitude 5.0-6.2) have been
identified (Table 3-1). These were chosen from epicentral data available in the
International Seismological Summary (188), the International Seismological
Center (ISC) Bulletin, the Preliminary Determination of Epicenters (PDE) and
Zemletryaseniya v SSSR (USSR Academy of Sciences earthquake annual).
Seven new focal mechanism solutions are presented (Table 3-1), and solutions
for the other five events are from other sources. Alternative mechanisms are
presented for some of the moment tensor solutions since inconsistencies were
observed between these solutions and mechanisms obtained through studies based
on P-wave first motions (Appendix 3A).

The methods used to determine mechanisms for earthquakes in this study
are detailed in Chapter 2. To summarize, the general approach includes first

motion studies using data read from the WWSSN and Canadian Seismograph

1’47

Figure 3—3: Distribution of seismicity reported in the Seismologicheskii
Bulleten for the car 1976. Size of cross is linear with increasing
event size. Filed and open circles are locations of Soviet
seismograph stations. From Fujita et al. (1987).

OS

 

 

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148

 

 

 

 

Figure 3-3

149
Table 3-1: FOCAL MECHANISMS AND SLIP VECTORS

wa

01 1964 08 25 78.15 126.65 05 6.2 338/54 184/58 068 2
346/48 164/42 075 1

338/34 184/58 068 3

176/67 033/36 121 C

02 1960 12 03 76.64 131.24 28 5 070/60 175/65 086 4
167/72 347/18 * C

03 1969 04 07 76.55 130.86 11 5. 314/48 157/45 067 1
020/70 120/65 030 C

016/40 160/50 070 3

300/64 152/30 057 5

011/38 268/80 178 6

330/60 195/60 061 7

04 1983 06 10 75.53 122.75 25 5. 144/72 029/39 118 1
142/59 008/40 108 8

150/70 034/40 060 C

05 1964 07 21 72.10 130.10 12 5. 170/45 326/50 056 C
06 1980 02 01 73.06 122.59 22 5. 168/50 274/71 005 C
Alternate 011/75 274/72 005 C

07 1963 05 20 72.20 126.25 10 5. 174/60 270/80 000 C
Alternate 012/80 282/90 010 C

08 1975 08 12 70.76 127.12 16 5. 164/72 286/30 018 C
Alternate 167/70 067/70 159 C

09 1962 04 19 69.80 138.98 17 6. 170/40 350/50 * C
10 1984 11 22 68.53 140.88 26 5. 087/75 341/45 066 9
058/90 328/84 058 C

11 1976 01 21 67.70 140.00 18 5. 090/45 338/70 068 C
106/34 348/72 078 3

088/58 180/87 091 6

12 1968 09 09 66.17 142.13 06 5. 241/74 146/62 054 C
284/68 174/50 084 3

274/69 162/47 071 6

Plane 1 is the presumed fault plane
Focal depth

h:

Azimuth of the horizontal projection of the slip vector.
* Denotes azimuth indeterminate due to poor nodal plane
constraint ’ ‘

A2:

8: Reference- C This study
1 Jemsek et al., 1986
2 Sykes, 1967
3 Savostin and Karasik, 1981
4 Lazareva and Misharina, 1965
5 Chapman and Solomon, 1976
6 Koz’min, 1984
7 Conant. 1972
8 Dziewonski et al., 1983
9 Dziewonski et al., 1985

150
Network, and supplemented with ISC Bulletin (or ISS or Seismologicheskii
Bulleten, Akademiya Nauk SSSR) reports. For this study, some seismograms
from the Soviet seismograph network were also available and were used in both
first motion analysis and body-wave modelling. Whenever possible, synthetic
seismograms and Rayleigh wave radiation patterns were computed in order to
better constrain source mechanism and focal depth. ‘

One additional method used in the northern Cherskii Mountains was
interpretation of LANDSAT multi-spectral scanner imagery. Surface mapping of
the region using this data allowed the delineation of recent and past fault
trends and precipitated the choice of fault plane when fault traces were
identified in close proximity to epicenters.

Focal mechanism solutions from this chapter will be used in Chapter 4, in
conjunction with solutions from McMullen (1985) and other investigators, to
constrain the present-day plate geometry and pole(s) of rotation of the North
American plate in northeast Siberia.

Lamm

Four earthquakes (Table 3-1, nos. 1-4) span the transition zone between
the Arctic mid-ocean ridge seismicity and the more dispersive pattern found on
the continental shelf. Of these, the northernmost event (no. 1) occurs within
the linear band of earthquakes bisecting the Eurasian Basin. Of the other three
events, two (nos. 2 and 3) are on the extrapolation of the Arctic mid-ocean
ridge into the continental shelf. The third event (no. 4) occurred southwest of
the other shelf events, well off any linear extrapolation of the ridge axis.

The August 25, 1964 earthquake is one of the largest to occur in the
oceanic part of the Arctic. Several investigators have presented solutions for

this event, including a body-wave inversion by Jemsek et al. (1986) which

151
indicates nearly pure normal faulting (Figure 3-4). The P-wave first motion
distribution, which cannot constrain any eastward dipping plane, does not fit
the west dipping nodal plane of the Jemsek et al. (1986) solution (Figure 3-4).
Other solutions by Sykes (1967) and Savostin and Karasik (1981) have a more
steeply west-dipping nodal plane which better fits the P-wave first motion data.

Body-wave modelling of the 1964 event proved to be unsuccessful using
published solutions. P-waves from six stations were modelled using a focal
mechanism constrained using first motions picked from seismograms, including
Moscow, and supplemented with ISC Bulletin data (Figure 34). Fair matches
were obtained using a simple trapezoidal source-time function and a focal
depth of 13 km. A better match may be possible using a more complex
source-time function as suggested by Jemsek et al. (1986).

Sykes (1967) and Solomon and Julian (1974) have noted that P-wave
solutions of ridge crest events may have non-orthogonal nodal planes due to
erroneous take-off angles resulting from lateral variability of the upper mantle
structure beneath the ridge. Sykes’ (1967) solution for the August, 1964,
earthquake has nodal planes separated by 73°, citing anomalous upper mantle
structure below the rift as a cause of distorted raypaths. Since the velocity
profile of this thermal regime is poorly known, it is not possible to correct for
the anomalous take-off angle. Althoug there is a better fit of the P-wave data,
the solution of Sykes (1967) cannot be preferred until new take-off angles can
be calculated.

Lazareva and Misharina (1965) present a solution for a magnitude 5
earthquake (December 3, 1960; no. 2) located further into the shelf of the
Laptev Sea. The solution, which is evidently constrained by using P-wave first

motions from the ISS and BCIS, is presented by Lazareva and Misharina (1965)

152

Figure 34: Solutions and synthetics for the August 25, 1964 event. Filled
circles are compressions, open circles are dilatations. Large circles
are first motions picked from seismograms. Small circles are ISC
Bulletin reported first motions. Lower hemisphere projection.
Synthetics are for the solution presented in this study.

-- - - Sykes, 1967
— ' —- Jemsek et al., 1986
—— This study

153

 

25 AUGUST 1964
All Solutions

GDH LPZ

%

SCP LPZ

\sz

 

154

with non-orthogonal nodal planes (Figure 3-5), as in the case of Sykes’ (1967)
solution for the August, 1964, earthquake discussed above. Lazareva and
Misharina’s (1965) solution has a large strike-slip component which is apparently
an attempt to fit a compression reported at Matsishiro (MAT) (Figure 3-5).
However, the residual at MAT is 5 minutes, suggesting an incorrect phase has
been choosen as the initial P-wave. I present an alternative normal faulting
solution, although the data are insufficient to constrain the east-dipping nodal
plane. The better constrained strike of the west-dipping plane, presumed to be
the fault plane, parallels the trend of the Arctic mid-ocean ridge.

Event 3 occurred on April 7, 1969, and has been studied by numerous
workers (e. g., Conant, 1972; Chapman and Solomon, 1976; Savostin and
Karasik, 1981; Koz’min, 1984; Jemsek et al., 1986). The solutions range from
strike-slip to normal faulting. A body-wave inversion is presented in Jemsek
et al. (1986), who note that the solution is extremely uncertain due to
waveform complexity and poor station distribution. Like the August, 1964,
event, the body-wave inversion does not fit the P-wave first motion data
(Figure 3—6).

Short- and long-period body wave synthetic seismograms were calculated
at three WWSSN stations, limited to the western quadrants, using a best fit
P-wave source mechanism (Figure 3-7). There is a good fit, particularly in the
short-period waveforms, suggesting that the P-wave solution and focal depth (12
km) are plausible models for the initial rupture. However, the poor azimuthal
distribution of the stations prevent tight control on the slip. The decreasing
fit of long-period records, and the differing body-wave inversion solution, may

indicate a complex rupture process.

155

Figure 3-5: Solutions for the December 3, 1960, event on the Arctic
mid-ocean ridge. Data are from the ISS and BCIS. Double triangle
represents conflicting polarities reported in ISS and BCIS. Small
"M" denotes position of MAT, which reported a compression with a
residual of 5 minutes.

— — - Lazareva and Misharina, 1965

This study

 

156

 

 

 

 

3 DECEMBER 1960
All Solutions

Figure 3-5

 

 

157

Figure 3-6: Solutions for the April 7, 1969, event. Data are the same on each
plot. Conventions as in Figure 3—4.
. o .. : Conant, 1972
— - Chapman and Solomon, 1976
' - ° - Jemsek et al., 1986
— -‘ 9 -- Savostin and Karasik, 1981
- — -- - Koz’min, 1984
-— This study

158

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159

Figure 3-7: Synthetics for the April 7, 1969, event.

160

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Synthetic seismograms were also calculated at four Soviet stations for
which records were available. The Soviet stations do not operate a
standardized instrument, and therefore, the instrument response of each station
varied. All the instruments are quasi-broadband, and response curves and
galvanometer periods were available (Prilozhenie k Seismologicheskomu
Bulletenu). The resulting waveform matches (Figure 3-8; note time increases
right to left) were very encouraging. In every case, the general waveform
shape was similar; only the wavelength seemed to vary. This may be due to
use of an incorrect instrument response in the calculation, since Soviet station
equipment changes may not have been reported immediately, or because of pooly
calibrated equipment. The general agreement, however, is consistent with the
WWSSN synthetics and suggest the P-wave mechanism in Table 3—1 is a well
constrained solution. .

The last of the outer shelf events included in this study occurred on June
10, 1983. Centroid moment tensor and body-wave inversion solutions
(Dziewonski et al., 1983; Jemsek et al., 1986) both suggest normal faulting with
a significant strike-slip component, very simiilar to the P-wave solution of the
April, 1969, earthquake. The focal depths of 12 km and 22 km (Dziewonski et
al., 1983 and Jemsek et al., 1986, respectively) differ dramatically. Jemsek
attributes the difference to the data used, noting that WWSSN data are not
low-pass filtered as are the Global Digital Seismograph Network (GDSN) data
used by Dziewonski et al. (1983). Jemsek et al. (1986) therefore conclude that
the WWSSN data are better at resolving "small" events. Only P-wave first
motion data reported in the ISC Bulletin and Seismological Bulletin of the USSR

are used to constrain the mechanism presented in this study (Table 3-1). These

162
P-wave data are better fit to the moment tensor solutions of Dziewonski et
al. (1983) and Jemsek et al. (1986) than had been observed for events 1 and 3.
W

Earthquakes around the Lena River Delta are characterized by a decrease
in magnitude relative to those on the shelf to the north. I have examined
three earthquakes (nos. 6, 7 and 8) on the southern extent of Lena River delta
(Figure 3-2), and one (no. 5) to the east in Bour Khaya Bay. The epicentral
trend of the three earthquakes on the delta follow the hinge line of the
Siberian platform (Andreev, 1983), suggesting there may be some structural
control on their occurrence.

On July 21, 1964, a magnitude 5.4 earthquake (no. 5) occurred in Bour
Khaya Bay at the southern extent of Laptev Sea shelf seismicity. Although
the event lies on a linear extrapolation of the Arctic mid-ocean ridge axis,
epicenters to the north are distributed in a broad pattern extending from the
New Siberian Islands to west of the Lena River delta. Kogan
(1974) determined the upper crustal structure in the Bour Khaya Bay area using
seismic refraction data. Through these studies Kogan ( 1974) delineated a low
velocity zone which continued to approximately 20 km depth and which is
flanked by "normal" continental crust and shelf sediments. The identification of
this low velocity zone beneath the projection of the ridge axis is consistent
with Sykes’ (1967) and Lazareva and Misharina’s (1965) use of non-orthogonal
nodal planes on other Laptev Sea shelf events (nos. 1 and 2).

A number of P-wave first motions were read from WWSSN and Canadian
Network seismograms in an attempt to constrain the nodal planes. All of
these were dilitational, however, as were 70 percent of the ISC and BCIS

reported first motions (Figure 3-10). The data was consistent with a normal

163

Figure 3-8: Synthetics for the April 7, 1969, event calculated for Soviet
seismograph stations.

164

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165

Figure 3-9: Solutions for the June 10, 1983, event. Data are from the
Seismologicheskii Bulleten SSSR and the ISC Bulletin. Conventions
as in Figure 3-4.
. -- ' — Dziewonski et al., 1983
- - — Jemsek et al., 1986
—— This study

166

 

 

 

 

 

 

 

1 0 JUNE 1 983
All Solutions

Figure 3—9

167
fault, although nodal plane orientations were completely unconstrained.
However, the Rayleigh wave radiation pattern was quite good and indicated a
nearly pure normal faulting mechanism with north-northwest striking nodal
planes (Figure 3-10).

Using the crustal model of Kogan (1974), short-period synthetic
seismograms were computed at six stations distributed in all but the southeast
quadrant of the focal sphere. The match between the synthetic and observed
seismograms are very good through the first 15 to 20 seconds of waveform
(Figure 3-11). The depth is constrained at 12 km. Since the P-wave and
Raleigh wave data did not constrain the amount of slip for the mechanism,
variations of the preferred solution were calculated using a variety of slip
directions. As shown in Figure 3—12, the variations drastically affect the match
of the synthetic waveform, suggesting the preferred solution is quite robust.

Earthquakes in 1980, 1963 and 1975 (nos. 6, 7 and 8, respectively)
occurred along the hinge line of the Siberian platform south of the Lena River
delta. Due to the smaller magnitudes of events 7 and 8 (Mb 5.0 and 5.1,
respectively), only P-wave first motion studies could be conducted. The
February 1, 1980, event (no. 6) had magnitudes of 5.4 (Mb) and 5.3 (Ms) and
was conducive to Rayleigh wave analysis as well. The lack of a second tightly
constrained nodal plane seems to make the solutions for earthquakes 6 through
8 non-unique. For events 6 and 7, only the east striking nodal plane is
constrained; the north striking nodal plane could dip either east or west (Figure
3-13 and 3-14). Examination of the Rayleigh wave radiation pattern for event 6
(Figure 3-13) suggests a west dipping nodal plane would be the preferred
solution. Event 8 (August, 1975) differs in that the north striking nodal plane

is relatively well constrained, and the second nodal plane may dip north, east

168

Figure 3-10: P-wave solution and Rayleigh wave radiation pattern for the July
21, 1964, earthquake in Bour Khaya Bay. S uares are data points
used in Rayleigh wave analysis. Solid line on ayleilgh wave pattern
corresponds to theoretical radiation based on the -wave solution.
First motion conventions as in Figure 3—4.

169

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oom mm "P
wm><3 Io_w4><m

 

 

 

 

 

 

 

Yong:

 

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z

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170

Figure 3-11: Synthetics for the July 21, 1964, event.

171

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172

Figure 3-12: Waveform de ndence on focal mechanism solution. Small
variations of the ocal mechanism produced pronounced change in
waveform character.

N3

 

 

SYNTHETICS FOR 64 - JULY-2|

MECHANISM

OBSERVED

SE I SB}; '~ FA“.

 

STRIKE OF FAULT PLANE: 170‘
DIP 0F FAULT PLANE: QS'
SLIP: 300°

SQIQTIQN I 2

STRIKE 0F FAULT PLANE: 170'

DIP OF FAULT PLANE: 55°
SLIP: 330'

SQ'ILITIS‘N t 2

STRIKE OF FAULT PLANE: 170'

DIP 0F FAULT PLANE: 55'
SLIP: 285'

GOL

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Figure 3-12

 

174

Figure 3-13: P—wave solution and Rayleigh wave radiation pattern for the
February 1, 1980, earthquake south of the Lena River delta.
Dashed line represents alternative solution for the P-wave
mechanism. Conventions as in Figure 3-4 and Figure 3-10.

175

onu-

13m 0' :20 kguuvzzhm

.moom no"...

mm><>> roar—Cum

31m 8&5

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176

Figure 3-14: P-wave solutions for the May 5, 1963, and All} st 12, 1975,
earthquakes south of the Lena River delta. Dashed i'nes represent
alternative solutions. Conventions as in Figure 3-4.

 

mum? ...w3034 NP

darn

chamfim

nmmw ><E m

 

 

178
or south (Figure 3-14). For each of these earthquakes, either mechanism can
be chosen without dramatically affecting the azimuth of the horizontal
projection of the slip vector (Table 3-1).

There is a reason to select preferred solutions to events 6 through 8
despite the poor constraint offered by P-wave first motions. For each event,
one solution has a north-northwest trending, west-dipping nodal plane. This
is, in fact, the preferred solution for the February, 1980, earthquake based
upon the distribution of Rayleigh wave energy. The north-northwest
orientation is also nearly parallel to the strike of the hinge line of the
Siberian platform (335°). Therefore, not only can a preferred solution be
offered, but the nodal plane paralleling the hinge line may be chosen as the
fault plane for each of these events.

rn " n i p

The seismicity of the northern Cherskii Mountains trends southeast in a
400 km wide band toward the Sea of Okhotsk. As discussed earlier, this
distribution may be caused by poor epicentral locations. The larger
earthquakes (Mb > 4.5) define a linear trend along the western margin of the
Cherskii Mountains which is more likely to represent the position of the North
American plate boundary in northeast Siberia. Solutions are presented for four
earthquakes (nos. 9-12) within this structurally complex region, which has
commonly been regarded as the onshore extension of the Arctic mid-ocean ridge
(Grachev et al., 1970; Churkin, 1972; Chapman and Solomon, 1976; Savostin and
Karasik, 1981; Grachev, 1982). All of the solutions have compressional
mechanisms which are difficult to explain in a rifting regime.

A magnitude 6.2 (Mb) earthquake on April 19, 1962, (no. 9) is the largest

of the Cherskii Mountain events chosen for study. Nearly all of the observed

179
and ISS reported first motions are compressions (Figure 3-15). The lack of
dilitations and nodal picks therefore prevents any constraint on the nodal plane
orientations. Compressions fill the northwest and northeast quandrants of the
focal sphere, and would allow only slight, if any, strike-slip component in any
solution.

An earthquake on November 22, 1984 was the most recent large (Mb > 5.0)
event in the northern Cherskii Mountains. A Centroid-Moment Tensor solution
by Dziewonski et al. (1985) indicates a thrust mechanism with a
shallow-dipping, northeast trending P-axis (Table 3-1). As was noted for
Laptev Sea earthquakes (nos. 1, 3, 4), P-wave first-motion distribution is
inconsistent with the solution obtained through moment tensor inversion. ISC
reported first-motions are more consistent with a nearly pure strike-slip
solution (Figure 3-16, Table 3-1) with north-northwest and east-northeast
trending nodal planes. Either solution is consistent with fault orientations
reported in Zonenshain et a1. (1978) and Savostin and Karasik (1981), and with
fault azimuths mapped from LANDSAT images (Path 126, Row 13 and Path 128,
Row 12). Therefore, either or both (Appendix 3A) of the solutions may be
allowed since both are consistent with observed geology. The possible reasons
for discrepancies between moment tensor inversions and P-wave first-motions,
first discussed by Hinzen (1986), are addressed in Appendix 3A.

A small (Mb 5.0) earthquake on January 21, 1976, has been studied by
Savostin and Karasik (1981) and Koz’min (1984), who utilized Soviet local
network data and ISC Bulletin reported first motions. I have supplemented
these solutions by picking P-wave first motions from WWSSN records. All
three solutions are fairly consistent with the P-wave first motion data, and do

not vary more than 25° in the azimuth of their slip vectors. The local

180

Figure 3-15: P-wave solution for the April 19, 1962, event. Conventions as in
Figure 3-4.

 

 

181

 

N
O 3% 0o.
0 fig.
, I
I
19 APRIL 1962

Figure 3-15

 

 

182

Figure 3-16: Solutions for the November 22, 1984, event. Conventions as in
Figure 3-4.
- -- — Dziewonski et al., 1985

This study

 

183

 

 

 

I

 

22 NOVEMBER 1984
All Solutions

Figure 3-16

184
network data of Koz’min (1984) also fit the solution constrained in this study
(Figure 3-17). The mechanism indicates thrusting with some strike-slip
component. The 090° strike of one of the nodal planes is consistent with the
general trend of azimuths (070° to 085°; Table 3-2) of large faults identified on
LANDSAT imagery (Path 126, Row 13 and Path 128, Row 12) or reported in the
epicentral region (Zonenshain et al., 1978; Savostin and Karasik, 1981),. and is
presumed to be the strike of the fault plane.

The final event examined in the northern Cherskii Mountains occurred on
September 9, 1968. This earthquake has also been examined by Savostin and
Karasik (1981) and Koz’min (1984). Both studies, however, determined
northwest trending thrust fault mechanisms which do not fit P-wave first
motion data which have been read from seismograms (Figure 3-18).

The solution presented has a large strike-slip component along a 241°
trending nodal plane, which is presumed to be the fault plane. This plane,
again, trends sub-parallel to the azimuth of faults mapped (Table 3-2;
Zonenshain et al., 1978; Savostin and Karasik, 1981) in the epicentral region.
Body-wave modelling of short-period seismograms (Figure 3-19) was very
sensitive to the slip angle of the solution, and best fit at a focal depth of

about 5 km.

TECI‘ONIC IMPLICATIONS
The seismicity from the Arctic mid-ocean ridge to the Cherskii Mountains
exhibits varying characteristics along its trend. The narrow band of epicenters
along the Arctic mid-ocean ridge gives way to broadly distributed seismicity
within the Laptev Sea. A small gap marks the transition from extensional

mechanisms on the Laptev Sea shelf to compression in the northern Cherskii

185

Figure 3-17: Solutions for the January 21, 1976, event. Conventions as in
Figure 3-4.
-—— - Savostin and Karasik,1981
- . . - . Koz’min,1984
-— This study

186

 

 

 

21 JANUARY 1 976

Figure 3-17

187

Table 3-2

FAULT PARAMETERS FROM LITERATURE
(DATA FROM ZONENSIIAIN, 1978 AND SAVOSTIN AND KARASIK, I981)

161121111013 mummmm
67.00 N 136.00 E 65.0
67.41 N 138.50 E 68.0
67.66 N 14025 F. 71.0
67.97 N 141.00 1: 78.0
68.34 N 137.50 1; 59.0
64.05 N 146.00 F. 117.0
64.45 N 144.00 6 112.5
63.32 N 144.00 B 113.0
66.09 N 144.00 E 98.0
65.69 N 146.59 B 128.0
64.81 N 147.00 E 115.0
64.69 N 14750 E 120.0
64.42 N 148.00 B 125.0
64.00 N 149.25 E 131.0

MAJOR FAULTS IDENTIFIED IN THIS STUDY

WWW

P115 R17 61.0 N 152.0 B 79.00
. 61.5 N 152.5 8 110.00
6 62.5 N 151.0 E 136.00
P1 16 R1 63.3 N 152.0 B 126.00
- 61.5 N 1465 E . 107.00
”22 R“ 61.5 N 147.0 E 98.00
65.5 N 145.0 B 158.00‘
P126 R13 67.5 N 142.0 E 85.00
67.7 N 140.0 E 70.00

9128 R12

188

Figure 3-18: Solutions for the September 9, 1968, event. Conventions as in
Figure 3-4.
-— —~ - Savostin and Karasik, 1981
. . . . - 9 Koz’min, 1984
——- This study

 

189

 

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......... fl)?”\ 0
...... \‘o..
o : . ..\ \9
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09 SEPTEMBER 1968

Figure 3-18

 

190

Figure 3-19: Synthetics for the September 9, 1968, event.

 

5;: ..T‘

191

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192

Mountains. Focal mechanism solutions are consistent with distinguishing three
regions of differing seismic character along the plate boundary. These
solutions, especially when combined with other geological and geophysical data,
are important in understanding the tectonics of each segment of the plate
boundary. There is possibly a unique transition of earthquake slip along the
boundary due to the proximity of the proposed North America - Eurasia pole of
rotation with the plate boundary (e.g., DeMets et al., in press). The discussion
of the tectonic implications of such a pole position, in light of the data
presented here, are discussed in Chapter 4. Here I discuss only the tectonic
implications of each independent region.
1.2mm

Focal mechanism solutions for events 1 through 4 indicate that normal
faulting associated with the Arctic mid-ocean ridge continues onto the
continental slope and shelf of the Laptev Sea. Complex ruptures and /or fault
geometries are indicated by the inconsistent solutions obtained through moment
tensor inversions and P-wave first motion studies. The discrepancies, which
may result from moment tensor solutions that average the entire rupture
process as opposed to initial rupture patterns indicated by first motion data
(Appendix 3A), may prove useful in more detailed studies of the ocean to
continent rift transition. Increased lower crustal and upper mantle
temperatures below the rift are indicated by the non-orthogonal nodal planes
of two focal mechanism solutions.
W

The combined seismicity in the Laptev Sea and Lena River delta must be
used when modelling the transition from oceanic to continental crust.

Unfortunately, the three earthquakes on the Lena River delta (nos. 6, 7 and 8)

193

cannot be used without some question as to their true position along the plate
boundary. Options to consider include 1) the events are intraplate, not related
to the North American interplate boundary seismicity; 2) the events are due to
interplate deformation, but are controlled by the structures of the Siberian
platform hinge line; 3) the events are true interplate earthquakes representative
of a definitive North American boundary; and, 4) the events are interplate
earthquakes associated with a more diffuse plate boundary.

Options 1 and 3 seem unlikely. The earthquakes’ proximity to other
interplate seismicity makes it unreasonable to assume that they are not, at
least to some degree, controlled by interplate dynamics. On the other hand,
the distribution of the interplate seismicity does not in the least suggest a
distinct, simple plate boundary geometry in the Laptev Sea-Lena River delta
region.

Option 2 cannot be disregarded regardless of conclusions reached on
option 4. Should option 4 hold true, the pre-existing structure of the Siberian
platform may still control some of the plate geometry. Therefore, option 2 can
only be examined in light of models developed for option 4.

The 4th option seems plausible given the broad distribution of larger (Mb
> 4.5) earthquakes in the Laptev Sea-Lena River delta area. The boundary is
not likely to be diffuse as in the Indian Ocean usage of Wiens et al. (1985),
who distribute the Australian - Indo-Arabian interplate deformation throughout
a zone 800 km wide and over 1000 km long. Rather, a more systematic
distribution may be modelled such as has been suggested by Engeln et al. (1988)
or Lister et al. (1986). Greatly simplified, each of these models incorporates
assymetric deformation broadly around a newly evolving boundary. More

detailed study of hypocentral distribution and source mechanisms of earthquakes

194
in Laptev Sea and Lena River Delta must be conducted in order to evaluate
these models.

South of the Lena River delta region is a small decrease in seismicity
between the Arctic mid-ocean ridge and the northern Cherskii Mountains. The
relatively few events in this 300 km wide zone are less than Mb 4.5, and are
recorded only by local Soviet stations. This apparent ”gap" in seismicity may
be due to either a large (Mb 6.3) earthquake which reportedly occurred in the
region in 1918, which released accumulated stresses, or to the proximity of the
zone to the proposed North American - Eurasian pole of rotation (Chapter 4;
DeMets et al., in press). If the pole of rotation were in this zone the relative
motion would approach zero, thereby limiting the stresses to very small values
and decreasing seismic potential. This. position for the pole of rotation is also
supported by the transition from normal faulting to the north to thrust faulting
in the south (Chapter 4).

N r 'i n i

The northern Cherskii Mountains are located along the southern boundary
of the Moma ”rift," a series of depressions extending from approximately 70°N,
138°E, to 63°N, 153°E, which are paralleled by the seismicity. The
depressions, bounded by apparent normal faults and cut by ”transform" faults
(Zonenshain et al., 1978; Savostin and Karasik, 1981), are sites of relative
subsidence, high heat flow and limited basaltic volcanism. These geological
and geophysical indicators have led numerous investigators (Demenitskaya and
Karasik, 1969; Grachev et al., 1970; Zonenshain et al., 1978; Savostin and
Karasik, 1981; Grachev, 1982) to consider this region as an extension of the

Arctic mid-ocean ridge and thus to interpret the depressions as rift grabens.

195

In contrast to the above interpretations, however, focal mechanism
solutions (nos. 9-12) for the northern Cherskii Mountains all indicate
compression approximately perpendicular to the axis of the depressions. These
mechanisms could potentially result from two differing hypotheses. The first is
that extension is still taking place along the Moma rift system, and that the
earthquakes all represent strike-slip motion along continental transform faults.
Although possible, it seems unusual that no normal faulting mechanisms would
be determined anywhere in the rift. The second hypothesis is that the rift is
no longer undergoing extension. In this case, a transition from extension to
compression in the recent past could explain both the geologic and geophysical
data. Such a scenario is consistent with the results of Rezanov and Kochetkov
(1962) who find that recent tectonic movements, based on the elevation of the
pre-Pleistocene peneplanation surface, indicate uplift in the Cherskii Mountains
and Mom "rift.” Furthermore, L. Parfenov (pers. comm., 1986) reports that the
Moma "rift" is inactive, and that active faults are strike-slip and thrust. A
more detailed discussion of the implications of such a recent transition, and
the effect on the North American - Eurasian pole of rotation, is presented in

Chapter 4.

ROLE OF PRE-EXISTING STRUCTURE
As discussed earlier, the distribution of larger earthquakes in the Cherskii
Mountains can be interpreted as representing a distinct, rather than diffuse,
North American plate boundary. However, structures associated with the Moma
”rift” may act as pre-existing weaknesses which control the location of and slip
along the present-day plate boundary. Additionally, poles of rotation calculated

using the strike of transform faults within the Moma ”rift" (e.g. Zonenshain et

196

al., 1978; Savostin and Karasik, 1981) actually describe the pole of the past
rifting event rather than the present plate configuration. To determine what
influence pre-existing structure had on 1) earthquake slip vector azimuths, and
correspondingly, 2) previous computations of North American poles of rotation
(e.g., Pitman and Talwani, 1972; Minster et al., 1974; Karasik, et al., 1975;
Chapman and Solomon, 1976; Chase, 1978; Savostin and Karasik, 1981), I utilized
fault data from literature (Zonenshain et al., 1978; Savostin and Karasik, 1981)
and from LANDSAT imagery (Table 3-2). This was combined with seven
Cherskii Mountain earthquake slip vectors determined in this study (nos. 10, 11
and 12) and in McMullen (1985; earthquakes of May 18, 1971, June 5, 1970,
January 13, 1972 and June 19, 1974). The slip associated with earthquakes is
assumed to be induced by regional stress acting along faults.

Initial inspection of the relationship between slip vector orientation and
fault azimuth suggests a very high correlation (Figure 3-20). It is clear that
there exist two clusters of fault orientations (Figure 3-20, Table 3-2). Faults
in the southern Cherskii Mountains, south of 66°N, have a mean orientation of
117° (standard deviation = 10.1°) while those the northern Cherskii Mountains,
north 67°N, have a mean orientation of 68° (standard deviation = 7.0).
Interestingly, the transition between the two groups occurs at the same position
(approximately 66°N, 140°E) which was independently found to define a change
in earthquake slip vector azimuth (Table 3-1). Thus, azimuthal and spatial
agreement between the fault and slip vector data can possibly be interpreted as
evidence for structural control on the earthquake slip.

A calculation of the theoretically predicted range of slip, using the
method of Angelier (1979), was done using a simplified model of the fault

orientations and regional stress (Appendix 3B). The regional stress, assumed to

197

Figure 3-20: Horizontal slip and fault azimuths in the northern and southern
Cherskii Mountains. Stippled bars are earthquake slip vector
azimuths, cross-hatching are fault azimuths.

198

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BEEN/WIN

199
be spatially and temporally constant over short periods of geologic time, and
the fault azimuths are combined to predict the range of slip direction along the
fault plane. The fault dip is not constrained from the fault azimuth, and is
therefore allowed to vary from vertical to 42°. This is then compared to the
slip vectors of the earthquakes within the region. A more detailed explanation
of this application of Angelier’s (1979) method is presented in Appendix 3B.

The method was applied to both southern and northern Cherskii fault
groups. The results are shown in Figures 3-21 and 3-22. The relation
between the predicted range of slip for the northern Cherskii faults and
earthquake slip vectors is ambiguous. The three slip vectors scatter inside and
outside the predicted range of slip, and no statistically significant conclusion
can be drawn. The result of the southern Cherskii data is significant, however,
as all four of the earthquake slip vectors fall outside the predicted range of
slip on pre-existing faults. From this it can be suggested that earthquakes
south of 66°N along the North American plate boundary are occurring on new
fractures, and are not controlled by the pre-existing structural fabric. This
conclusion is consistent with the results of Chapter 4, which suggest the
southern Cherskii Mountain region is a newly developing plate boundary.

An auxiliary result of this was an explanation of why there have
generally been two locations, southern and northern, proposed for the North
American - Eurasian pole of rotation. As shown in Table 3-3, data used in the
determinations of the North America - Eurasia relative motion have been
obtained from the Atlantic Ocean basin, the Eurasia basin and in continental
Eurasia. Realizing that rifting is no longer active in the Cherskii Mountain

system, and therefore that the continental transforms describe past relative

200

Figure 3-21: Predicted range of slip azimuth for the northern Cherskii
Mountains. Shaded area is predicted range of slip as calculated
using the method of Angelier (1979). Stars represent the dip and
slip azimuth of earthquakes in the northern Cherskii Mountains
(events 10, 11 and 12).

201

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202

Figure 3-22: Predicted range of slip azimuth for the southern Cherskii
Mountains. Shaded area is predicted range of slip as calculated
using the method of Angelier (1979). Stars represent the dip and

slip azimuth of earthquakes in the s0uthern Cherskii Mountains
(data from McMullen (1985)).

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NORTH AMERICAN - EURASIAN I’OLE STUDIES

STUDY

l'ilmzm 81 Talwani, 72

Minster ('1 al., 74
Cook ct al., 86

DcMcts ct al., in prep.
Minster &- Jordan, 78

Karasik ct al., 75

Chapman 8: Solomon, 76
Chase, 78
Zonenshain, 73

Savostin & Karasik 81

PLATES: NA = NORTH AMERICA EU = EURASIA OK = OKITOTSK

2011

Table 3-3

1111.1:

NORTT TERIA'
201.55
63.0, l37.0

(1')}, 128.0
71.2, I32.I

69.3, 130.6

65.9, 132.4

65.5. I39.S

SOUTHERLY
L’QLES
61.3, 130.0

53.7, 137.3
62.2, 143.6

59.5, 140.3

N ATI-IS

NAEU

NAJ-IU

NA,EU,OK

NA.EU.OK

NA,EU

NA,EU

NA.EU

NA.EU

NA,EU,OK

NA,EU,OK

MI-ZTIIOIIS

A'I'I. ARC EUR

MM‘.
1'1‘0
MAC
TFO

SV
TFO

MAC
SV
TFO

MAC

SV
TFO

MAC
TFO

MAC
TFO

SV

MAC

MAC

SV

MAC
SV

MAC

MAC
TTO

MAC
TFO

SV

1111110115: 01550111183 r111: DATA 051:0 IN mm or 11111121: REGIONS:

AT'I. = ATLANTIC OCEAN
ARC = ARCTIC OCEAN ,
11118 = l-ZlIRASlA (NORTHEAST 51111211111)

THE DATA TTII‘ZMSEIA'I-IS INCLUDE:

sv = EARTHQUAKE sur VECTORS
MAC = OCEANIC MAGNETIC ANOMAUI-ZS

TFO = OCT-ZANIC TRANSFORM STRIKES
'I’FC = CONTINENTALTRANSFORM STRIKES

SV

SV

SV

SV
TFC

SV
TFC

205
plate motions and not present motion, new conclusions may be drawn about
previous pole determinations.

Southern poles calculations such as those by Zonenshain et al. (1978) and
Savostin and Karasik (1981) utilized the strike of continental transforms in the
Moma rift system. These faults bias the pole southward toward the paleopole
which described the rifting event. Chapman and Solomon (1976) and Chase
(1978) also calculated southernly positions for the North American - Eurasian
pole of rotation. In these cases, the investigators did not include a Sea of
Okhotsk plate in their models, but included data from the southern Cherskii
Mountains which are ascribed to North American - Sea of Okhotsk relative
motion (McMullen, 1985). The inclusion Of this data will shift the North
American - Eurasian pole toward the position of the North American - Sea of
Okhotsk pole. Therefore, North American - Eurasian pole calculations which do
not utilize fault strikes from the Cherskii Mountains and which do attribute
southern Cherskii Mountain slip vector data to North American - Okhotsk
relative motion result in a northern position for the Euler pole. With this in
mind, a new North American - Eurasian pole of rotation is determined in

Chapter 4.

CONCLUSIONS
Models which consider the depressions of the Cherskii Mountains as the
onshore continuation of the Arctic mid-ocean ridge cannot account for
compression indicated by earthquake focal mechanisms. Instead, extension
along the Arctic mid-ocean ridge terminates in the southernmost Laptev Sea
after a broad distribution over the Laptev Sea shelf. A small "gap" in

seismicity southeast of Buor Khaya Bay may be indicative of a nearby Euler

206

pole describing the relative motion along the plate boundary. Increased
seismicity within the Cherskii Mountains is characterized by compressive
mechanisms, inconsistent with the Observed geology. Additionally, the slip
direction determined from focal mechanisms in the northern Cherskii Mountains
is opposite the slip in the southern Cherskii Mountains (McMullen, 1985;
Chapman and Solomon, 1976). This is possibly indicative of motion between
differing plate pairs (i.e., North America - Eurasia and North America -
Okhotsk).

The structural complexity in the Cherskii Mountains may play a role in
the location and direction of present-day slip. Simplified models suggest that
earthquakes in the northern Cherskii Mountains are not inconsistent with slip
along the pre-existing structure, although the earthquakes in the southern
Cherskii’s appear to be occurring on recent fractures.

Complex structure and/or complex rupture along much of the plate
boundary is indicated by inconsistent P-wave solutions and moment tensor
inversions in the Laptev Sea and the Cherskii Mountains. Rather than being
mutually exclusive, it is suggested that such discrepancies may instead be
helpful in understanding the source processes in large continental earthquakes.
An apparent slow velocity in the upper mantle below the Arctic mid-ocean
ridge, indicated by non—orthoganal focal mechanism nodal planes, further
complicates the understanding of the present plate geometry within the Laptev

Sea.

APPENDICES

APPENDIX 3A

DISCREPANCIES BETWEEN MOMENT TENSORS AND P-WAVE DATA:
POTENTIAL CAUSES AND IMPLICATIONS

Four earthquakes included in this study (nos. 1, 3, 4 and 10; Table 3-1)
had either Centroid-Moment Tensor (CMT; Dziewonski et al., 1981) or
body-wave inversion (Jemsek et al., 1986) solutions derived from previous
studies. For each of these events, there was some degree of misfit between the
CMT or body-wave inversion and solutions consistent with P-wave first motions.

Teleseismic P-wave first motion plots often cannot well constrain the
nodal planes when either of the planes are shallow dipping. Therefore, these
solutions often cannot be considered good solutions to the faulting process
without support from other methods (e.g., synthetic seismograms, surface-wave
analysis). Similarly, moment tensor analysis also has problems when inverting
small, shallow earthquakes. Certain components of the moment tensor
(specifically the sz and Myz terms; see for example Kennett (1983)) become
indeterminate when the hypocenter is shallow (<30 km). Better constraint can
’be Obtained by inverting shorter wavelengths, as the minimun usable depth
decreases in proportion to the wavelength. However, there exist further
trade-Offs as these shorter wavelengths are more susceptible to lateral
heterogenieties.

The above explanation may sufficiently explain the discrepancy observed
between inversion and P-wave first motion solutions. However, interesting
observations in this study warrant further speculation on the discrepancy

between P-wave first motion solutions and those based on other methods.

207

208

Chung and Kanamori (1978) noted that waves of different periods carry
separate information, and should therefore not be expected to provide the same
results. It is now known that many earthquakes, especially those of great
magnitude, have complex rupture histories of long duration and numerous
sub-events. These complexities are components of moment tensor and inversion
determinations, which account for such processes over the entire rupture
duration. P-wave first motion studies, however, are blind to such complex
histories, and instead best record the initial rupture process. This initial
rupture is undoubtedly an important aspect of understanding the complete
source mechanism, albeit potentially more influenced by local dynamics (e.g.,
over-burden stresses or pre-existing weaknesses). It is more likely, however,
that the moment tensor and inversion determinations better reflect the true
plate dynamics through the incorporation of an average solution which is not
necessarily constrained to be double couple (Hinzen, 1986).

More importantly, it may be possible to predict where discrepancies
between first motion solutions and moment tensors or inversions will occur.
Hinzen (1986) concluded that differing solutions from the two methods are
most likely attributable to changes in fault plane geometry or slip direction
after initial rupture. Fewer differences were noted as a function of
non-double couple component or earthquake strength. A possible negative
correlation between depth and misfit was also suggested. The conclusions of
Hinzen (1986) may be further interpreted to indicate a correlation between
tectonic setting and misfit between first motion and moment tensor or
inversion solutions.

Many shallow earthquakes occur in rift settings of either oceanic or

continental affinity. Complex fault geometries (e.g., curved faults) potentially

209
characterize rift settings, and strong inhomogenieties exist in continental crust.
It can therefore be expected that shallow earthquakes have more complex
rupture processes, and thus greater discrepancies between first motion and
inversion solutions may be expected beyond the limitations of the inversion
process as discussed above. This is in agreement with Hinzen’s (1986)
conclusion regarding depth and misfit. An exception to this rule may be found
in large strike-slip settings (e.g., mid-oceanic ridge transforms, western
California or southern Alaska) which may have fault geometries which are
simple in comparison to rifts. Discrepancies may still exist in areas of
continental crust (e.g., western California).

Dziewonski and co-workers (e.g., Dziewonski et al., 1983; Dziewonski et
al., 1985) have compiled an extensive database of moment tensor solutions for
nearly the past decade. An comparison between these moment tensor, or other
inversion, determinations and P-wave first motiOn solutions in well defined

tectonic settings is suggested to test the above speculation.

APPENDIX 3B

DETERMINATION OF POSSIBLE RANGE OF FAULT SLIP
USING THE METHOD OF ANGELIER (1979)

Angelier (1979) evaluates the validity of approximating the relative motion
on a fault as being due to a regional mean stress (defined as a tensor). A
number of simplifying assumptions are applied in the method, and some
limitations are implied. However, the results of Angelier’s (1979) study indicate
the approximating model gives satisfactory results.

Angelier (1979) makes the assumptions that 1) the orientation of fault
planes does not contain any information regarding the regional stress (i.e.,
pre-existing zones of weaknesses are present), 2) the magnitude and direction
of stress remains constant throughout the rupture time, and 3) the relative
sizes of the principal stresses ( ¢ ) are not explicity known. Assumption 3
limits the method in that only a range of predicted slip, dependent on the
value of shear stress, can be resolved. Similar assumptions made in
seismological studies of fault orientation and regional stress direction (e.g.,
Gephart and Forsyth, 1984; Vasseur et al., 1983) demonstrate the
interchangability of the method between focal mechanism solutions and field
data. More recent structural studies (e.g., Michael, 1984) have simplified the
computational aspect of Angelier’s (1979) method by further assuming that the
tangential traction on individual faults is similar throughout the region.
However, for my investigation a graphical method (Angelier, 1979) was utilized
to obtain the predicted range of slip, and the assumptions of Michael (1984) do
not change the results.

The graphic method defines windows of potential slip through the
intersection of great circles (lower hemisphere Schmidt projection) between the

210

211

pole to the fault plane and the P- and T-axes of the regional stress (Figure
3-23). The value of the range of slip is expressed by

COS 6 = TAN a; TAN a, Angelier (1979)
The regional stress directions (P, T and B representing maximum, minimum and
null directions) are derived from an intraplate event (December 12, 1975;
Figure 3-24) in the nearby New Siberian Islands. Although a somewhat
circular proof, these stress orientations are taken as valid due to their
consistency with the P-axis orientation Of the interplate events (Table 3-1)
included in this study. The use Of a horizontal orientation of the P-axis even
at shallow depths is supported by the results of Engelder and Sbar (1984), who
utilize in situ measurements and Obtain horizontal stresses several times the
vertical stress as shallow as 100 m.

In order to Obtain the predicted ranges shown on Figures 321 and 3-22,
great circles were drawn between the P- and T-axes and the pole to the fault
plane. The intersection of these circles with the fault plane then defined the
range of slip. Since the fault plane dip is not obtainable, in most cases, from
LANDSAT imagery, nor was it reported in the literature, slip ranges were

determined for fault plane dips ranging from 90° to 42° in 8° increments.

212

Figure 3-23: Graphical representation of Angelier’s (1979 method for
determirung ranges Of slip. P: P-axis azimuth; T: -axis azimuth;
B: null axis; 11: pole to the fault plane, f.

213

 

 

 

 

 

 

GRAPHICAL METHOD OF ANGELIER (1979)

Figure 3-23

2114

Figure 3-24: P-wave solution and Rayleigh wave radiation pattern of the
December 15, 1973 New Siberian Islands intraplate earthquake. The
epicenter’s location in the New Siberian Islands is shown on the
map.

215

 

 

 

 

 

 

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Figure 3-20

LIST OF REFERENCES

LIST OF REFERENCES

Andreev, V. S., 1983. Boguchanskii glubinn ' razlom (in Russian), Vestnik
Moskva Universitet, ser. 4, Geologiya, 2, 7 -80.

Angelier, J., 1979. Determination of the mean principal stress directions of
stresses for a given fault population, Teetonophysics, 56, T17-T‘26.

Argunov, M.S. and S. I. Gavrikov, 1960. Balagan-Tas, an Early Quaternary
volcano, Izv. Acad. Sci. USSR, (8), 72-74.

Chapman, ME. and S. C. Solomon, 1976. North American - Eurasian plate
boundary in northeast Asia. J. Geophys. Res., 81, 921-930.

Chase, CG, 1978. Plate kinematics: the Americas, East Africa, and the rest of
the world. Earth Planet. Sci. Lett., 37, 355-368.

Chung, W.-Y. and H. Kanamori, 1978. Subduction rocess of a fracture zone
and aseismic ridges - the focal mechanism an source characteristics of
the New Hebrides earthquake of 1969 January 19 and some related events,
Geophys. J. Royal Astr. Soc., 54, 221-240.

Churkin, M. Jr.,1972. Western boundary of the North American continental
plate in Asia, Geol. Soc. Amer. Bull., 83, 1027-1036.

Conant, D. A., 1972. Six new focal mechanism solutions for the Arctic and a
center of rotation for plate movement, M. A. Thesis, Columbia
University, New York, 18 pp.

DeMets, G, Gordon, R.G., Stein, 8., Ar 5, DR, Engeln, J., Lundgren, P.,
Quible, D.G., Stein, G, Wiens, D., einstein, S.A., and Woods, D.F., in
press. Current plate motions. J. Geophys. Res.

Dziewonski, A. M., T.-A. Chou and J. H. Woodhouse, 1981. Determination of
earthquake source parameters from waveform data for studies of global
and regional seismicity, J. Geophys. Res., 86, 2825-2852.

Dziewonski, A.M., J. E. Franzen and J. H. Woodhouse, 1983. Centroid-moment
tensor solutions for April - June, 1983, Phys. Earth Planet. Inter.,

33: 243-249.
Dziewonski, A.M., J. E. Franzen and J. H. Woodhouse, 1985. Centroid-moment
tensor solutions for October - December, 1984, Phys. Earth

Planet. Inter., 39, 147-156.

216

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Engelder, T. and M. L. Sbar, 1984. Near-surface in situ stress: introduction, J.
Geophys. Res., 89, 9321-9322.

Engeln, J. F., S. Stein, J. Werner and R. G. Gordon, 1988. Microplate and

shear zone models for oceanic spreading center reorganizations,
J. Geophys. Res., 93, 2839-2856.

Fujita, K., E. A. Lynch and D. B. Cook, 1987. History of the Soviet
seismograph network in northeast Siberia (abstract), EOS, 66, p. 1062.

Fujita, K., D. B. Cook, H. Hasegawa, D. Forsyth and R. Wetmiller, in ress.
Seismicity and focal mechanisms of the Arctic region and the orth
American plate boundary in Asia, in, A. Grantz et al. (eds.), The Geology
(éf Sgrthcfiamerica, vol. L, The Arctic Ocean Region; Geol. Soc. Amer.,

0 er, .

Gephart, J. W. and D. W. Forsyth, 1984. An improved method for determining
the regional stress tensor using earthquake focal mechanism data:

Agplication to the San Fernando earthquake sequence, J. Geophys. Res.,
8 , 9305-9320.

Grachev, A. F., 1973. Momskii materikovii rift (severo-vostok SSSR), _i_n,
R. M. Demenitskaya (ed.), Geofizicheskie Metody Razvedki v Arktike,
vyp. 8, NIIGA, Leningrad, 56-75.

Grachev, A. F., 1982. Geodynamics of the transitional zone from the Moma
rift to the Gakkel Ridge, Amer. Assoc. Pet. Geol Mem. 34, 103-113.

Grachev, A.F., Demenitskaya, RM. and Karasik, A.M., 1970. The Mid-Arctic
ridge and its continental continuation, Geomorphology, 1, 30-32.

Hinzen, K.-G., 1986. Comparison of fault-plane solutions and moment tensors,
J. Geophys., 59, 112-118.

Jemsek, J. P., E. A. Bergman, J. L. Nabalek and S. C. Solomon, 1986. Focal
depths and mechanisms of large earthquakes on the Arctic mid-ocean
ri ge system, J. Geophys. Res., 9 , 13993-14005.

Kennett, B. L. N., 1983. Seismic Wave Propagation in Stratified Media,
Cambridge University Press, London, pp. 342.

Kogan, AL, 1974. Postanovka seismicheskikh rabot metodom KMPV-GSZ s
morskogo l’da na shel’fe Arkticheskikh Morei (opyt rabot v More
Laptevykh)(in Russian), Geofiz. Metody Razv. Arktiki, 9, 33-38.

Karasik, A.M., Rozhdestvenskiy, SS. and Donets, E.G., 1975. The structure of
the ma etic anomaly field and the geometry of the spreading of the
Mohn idge in the Norwegian-Greenland Sea, Izv. Acad. Sc1. USSR,
EarthPhys1cs,11, 115-123.

Koz’min, B.M., 1984. Seismicheskie poyasa Yakutii i mekhanismy ochagov ikh
zemletryasenii (in Russian), Nauka, Moscow, 126 p.

218

Lazareva, A. P., and L. A Misharana, 1965. Stresses in earth uake foci in
the Arctic seismic belt, Izvestiya Academy of Sciences 0 the USSR,
Physics of the Solid Earth, 1, 84-8 .

Lister, G. S., M. A. Etheridge and P. A. 2S‘ymonds, 1986. Detachment faulting
and the evolution of passive continent m,arg1ns Geology, 14, 246-250.

McMullen, C. A., 1985. Seismicity and Tectonics of the Northeastern Sea of
Okhotsk. M. S. Thesis, 107p., Michigan State Univ., E. Lansing.

Michael, A. J.,1984. Determination of stress from slip data. faults and folds,
J. Geophys. Res., 89, 11517-11526.

Minster, J.B., Jordan, T.H., Molnar, P. and Haines, E., 1974. Numerical
gnodsellin 7of instantaneous plate tectonics, Geophys. J. Roy. Aston. Soc.,
6, 41- 6.

Minster, J.B. and Jordan, T.H., 1978. Present-day plate motions,
J. Geophys. Res., 83, 5331-5354.

Morgan, W.J., 1968. Rises, trenches, great faults, and crustal blocks,
J. GeOphys. Res., 73, 1959-1982.

Naymark, A.A.,1976. Neotectonics of the Moma region, northeastern USSR,
Doklady Acad. Sci. USSR, Earth Science Sections, 229, 39-42.

Pitman, WC. III, and Talwani, M, 1972. Sea-floor spreading in the North
Atlantic, Geol. Soc. Am. Bull., 83, 619- 646.

Prilozhenie k Seismologicheskomu Bulletenu, 1983. Parametri, AmplitudOnie and
Fazovie Kharakteristiki Pribo v O rnikh Seismicheskikh Stantsii SSSR,
1981 (in Russian), Akademiya auk SSR, Moscow.

Rezanov, LA. and Kochetkov, V.M., 1962. Recent tectonics and seismic
regionalization in the north- eastern region of the USSR,
Izv. Acad. Sci. USSR, Geophys. Ser., 1045-1052.

Savostin, LA. and Karasik, A.M.,1981. Recent plate tectonics of the Arctic
basin and of northeastern Asia, Tectonophysics,74,111- 145.

Savostin, 1.., Zonenshain, L. and Baranov, B., 1983. Geology and plate
tlectogigcszgf the Sea of Okhotsk, Am. Geophys. Union, Geodynamics Ser,
1 1 - 1

Seismologicheskii Bulleten’, Akademiya Nauk SSSR Institut of Fiziki fo the
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Seno, T., 1985. Is northern Honshu a microplate?, Tectonophysics,
115, 177-196.

219

Stein, S. 1987. Introduction to Seismology, Earthquakes and Earth Structure,
September, 1987, draft text, 522 pp.

Solomon, S. C. and B. R. Julian, 1974. Seismic constraints on ocean- -r_idge
mantle structure: anaomalous fault lane solutions from first motions,
Geophys. J. Royal Astr. Soc., 38, 265- 5.

Sykesgg... 5R§ 1965. The seismicity of the Arctic, Bull. Seismol. Soc. Amer., 55,
1- 1 .

Sykes, L. R, 1967. Mechanisms of earth uakes and nature of faulting on the
mid-oceanic ridges, J. Geophys. Res., 2, 2131-2153.

Vasseur, G., A. Etchecopar and H. Philip, 1983. Stress state inferred from
multiple focal mechanisms, Annales Geophysicae, 1, 291-298.

Wiens, D. A., C. DeMets, R. G. Gordon, S. Stein, D. Argu J. F. En nsgeln,
P. Lundren, D. Quible, C. Stein, S. Weinstein and D. F.gu Woods, 198 A
diffuse glate boundary model for Indian Ocean tectonics,
Geophys. es. Letters, 12, 429-432.

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Recent plate tectonics o northeastern Asia in connection with the

Openigt of the northern Atlantic and Arctic basins, Oceanology, 18,
846 8

CHAPTER FOUR

PRESENT-DAY PLATE INTERACTIONS IN NORTHEAST ASIA:
NORTH AMERICAN, EURASIAN AND OKHOTSK PLATES

INTRODUCTION

Previous studies have delineated the North America - Eurasia plate
boundary along the Arctic Mid-Ocean Ridge from the North Atlantic to the
Lena River delta (Figure 4-1; Sykes, 1965; Zonenshain et al., 1978). The
seismicity along this segment follows a narrow band, not exceeding 50 km in
width (Figure 4-2). As the ridge impinges on the continental shelf of Eurasia,
the depth of seismicity increases (Jemsek et al., 1984) and the zone of
seismicity increases in width. In the continental part of northeast Asia,
between the Sette-Daban and the Cherskii Mountains, the zone of seismicity
reaches widths of about 500 km and has been suggested to be a diffuse plate
boundary (Chapman and Solomon, 1976). Along the northeastern edge of this
zone of seismicity is a series of depressions (Figure 4-1; Moma "rift"), first
suggested by Grachev et al. ( 1970) to be a continental continuation of the
Arctic Mid-Ocean Ridge. South of the Sette-Daban Mountains the seismicity
appears to decrease. Active seismicity has been noted, however, along Sakhalin
Island (Chapman and Solomon, 1976) and along the eastern coast of Kamchatka
(Cormier, 1975) north of the Aleutian-Kurile arc-arc junction (here referred to
as the northeast Kamchatka seismic zone). These observations led Chapman and
Solomon (1976) to postulate that the Sea of Okhotsk was part of the North
American plate and most global plate inversions have followed this
configuration.

In the past decade, however, a series of moderate (5 < mb < 6)
earthquakes occurred along the northern coast of the Sea of Okhotsk (Figure
4-2) in a zone between the southern Cherskii Mountains and the northeast

Kamchatka seismic zone. In addition, several well constrained focal

220

221

Figure +1: Index map of northeast Siberia showin grabens (Stippledg as
shown by Savostin and Karasik (1981) an Savostin et al. (1 83).
B-T denotes location of Balagan-Tas volcano. EP denotes the
El’ginskii Plateau. Star indicates the position of the
Aleutian-Kurile arc-arc junction.

222

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224
mechanisms in the Cherskii Mountains have been determined that allow the rift
hypothesis for the Cherskii Mountains to be tested.

In this chapter, events occurring in northeastern Siberia are examined to
determine the present nature of the North American plate boundary in
northeast Siberia and whether an independent Okhotsk plate exists.
Additionally, preliminary estimates concerning the relative motion of the

Okhotsk plate with respect to the North American plate are made.

SEISMICITY AND FOCAL MECHANISMS
In the last 25 years about two dozen events of magnitude 5 or greater
have occurred along the continental extension of the Arctic seismic zone.
These events are concentrated on the northeastern edge of the zone of diffuse
seismicity. The largest, Ms = 6.6, occurred on May 18, 1971, in the southern
Cherskii Mountains. Details of the seismicity within the diffuse seismic zone
are unclear. Although Soviet workers have tabulated the seismicity of the
region since about 1700 (Koz’min and Andreev, 1977) and with a fair degree of
macroseismic data since about 1900 (Kochetkov, 1968; Koz’min, 1984), the
detection and location capability within northeast Siberia varies greatly in time
and space. The apparent concentration of microseismic activity northeast of
Magadan shown by Koz’min (1984) has been shown by Fujita et al. (1983) to
result from station distribution and detection capability in the region, as does

the apparent lack of microseismicity in northern Shelikhov Bay.
Several large events occurred prior to the 19605 when there were few
stations in the area. Particularly destructive earthquakes occurred in the

19205 in the northern part of the region, near the Arctic Ocean, and several

225
damaging earthquakes have occurred along the northern shores of the Sea of
Okhotsk (Koz’min, 1984).

Prior to the 19705 there were no instrumentally recorded moderate-sized
teleseismic events from the Shelikhov Bay and northwestern Kamchatka
regions. In the past 15 years, however, six moderate-sized events (Figure 4-2)
have been recorded which form a lineation from the southern Cherskii
Mountains to Karaginskii Island. These events suggest that the Sea of
Okhotsk may not be part of the North American plate.

Based on the North America-Eurasia pole of rotation computed by Chapman
and Solomon (1976) the Cherskii Mountains should represent a tensional regime
or have right-lateral strike-slip faulting, assuming a northwest striking fault
plane. Savostin and Karasik (1981) proposed that these depressions were
rift-grabens along a continental extension of the Arctic Mid-Ocean Ridge, based
on the presence of down-dropped blocks with high heat flow and basaltic
volcanism (Naymark, 1976; Grachev, 1982). Using strikes of "transform faults"
offsetting these grabens, Savostin and Karasik (1981) obtained a North
America-Eurasia pole of rotation in the southern Sette-Daban Mountains, that is
not too dissimilar from the Chapman and Solomon (1976) pole.

Examination of published focal mechanisms by Savostin and Karasik
(1981), Chapman and Solomon (1976), and Filson and Frasier (1972) indicate
that the sense of motion on the earthquakes is opposite to what was expected;
i.e., thrust faulting and left-lateral strike-slip faulting. Therefore, this
investigation seeks to verify the accuracy of these mechanisms and uses
additional solutions presented in Chapter 3 and McMullen (1985) to increase

the data set.

n—V

“_to—- in...

f’)

226

Focal mechanisms from Chapter 3 and McMullen (1985) have been
determined using P-wave first motions, Rayleigh wave amplitude radiation
patterns, and synthetic seismograms. P-wave first motions were reread from
World-Wide Standardized Seismograph Network (WWSSN) records whenever
possible. Except for the largest events, first motions could only be read from
short-period records. Due to the small size of the events, bulletin-reported
first motions from the International Seismological Center (ISC) Bulletin and the
Soviet Operational Catalog were needed to supplement the reread first
motions. Rayleigh wave radiation patterns (Kanamori, 1970) are obtained for
events of M5 5.5 and greater. Due to the size of the events and station
distribution, data for most events are obtained only in two or three
quadrants. Finally, synthetic seismograms are computed for several events to
confirm or further constrain the mechanisms and obtain focal depths.
Short-period P-waves were modelled using the layered-media algorithm of
Kroeger and Geller (1983). As noted by Seno and Kroeger (1983), this requires
some knowledge of the crustal structure, therefore results of Soviet refraction
surveys (e.g., Kogan, 1974) are used where possible. Short-period synthetics
prove to be extremely useful in confirming mechanisms. Each earthquake is
studied with as many of the above methods as possible. Due to the small
magnitudes of the events, however, uncertainties of 10-20 degrees are common
and some results should be viewed as preliminary. In all of the cases
presented, however, the type of faulting is clear. The methods used to
constrain the focal mechanisms are presented in Chapter 3, and detailed
explanations of the solutions found in Chapter 3 and McMullen (1985) are not

repeated here.

 

227

Figure 4-3 shows both new and previously published focal mechanisms
determined for the interplate region of northeast Siberia. These indicate a
transition from nearly pure normal faulting in the Laptev Sea to thrust and
strike-slip mechanisms in the Cherskii Mountains. Along the lower Lena River,
some events show a large strike-slip component as well. In Shelikhov Bay and
northwestern Kamchatka, mechanisms are poorly constrained but are not
inconsistent with thrusting and left-lateral strike-slip motion, while the largest
events in the northeast Kamchatka seismic zone show thrusting.

The events west of the Bour Khaya Bay, in the Lena River delta, lie on
the hinge line of the Siberian platform along a series of fractures (Andreev,
1983). They appear to be associated with reactivated faulting due to the
opening of the Eurasia basin of the Arctic Ocean. The plane parallel to the
hinge line is chosen as the fault plane.

In the northern Cherskii Mountains, all events show a thrust faulting
component. Although one nodal plane of these solutions (Table 4-1, nr. 3032)
is usually very close to the strike of the "grabens" of Savostin and Karasik
(1981) (Figure 4-1), choosing these planes as the fault plane results in major
inconsistencies in plate motion calculations. Thus, the east-west striking plane
is chosen, which is close to the strike of the faults offsetting the "grabens". A
large event which occurred in the northern Cherskii Mountains in 1962 has
only been studied using first motions reported in the International Seismological
Summary (ISS). The mechanism is poorly constrained but also appears to be a
thrust of indeterminate orientation; it is not used in these calculations.

In the southern Cherskii Mountains, all mechanisms yields strike-slip
solutions. Due to the size of the events, all of them are relatively well

constrained and show northeast-southwest and northwest—southeast striking

228

Figure 4—3: Focal mechanisms of earthquakes in northeast Siberia and the
adjacent Arctic. Compressional quadrants are solid;
stip led com ressional quadrants denote less certain mechanisms and
das ed noda planes denote that the orientation of the plane is
ind)eterminate. Events used in this study are numbered (see Table
4-1 .

229

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230
nodal planes. McMullen’s (1985) solutions are essentially identical to those
obtained by Chapman and Solomon (1976). The northwest - southeast striking
nodal plane is chosen as the fault plane based on aftershocks of the May 18,
1971, event located using a portable array by Koz’min et al. (1975) and on
directivity studies by Filson and Frasier (1972).

Across Shelikhov Bay and northwestern Kamchatka, no obvious trends or
lineations parallel the nodal planes. Inspection of space shuttle imagery
(STS—9), however, shows lineations in northern Kamchatka oriented N155°E and
N880E. Two thrust events (Figure 4-3) have a nodal plane close to the first
trend, while an event near Karaginskii Island (Table 4-1, nr. 36) appears to be
left-lateral strike slip on a nearly east-west plane. Thus, the east-west
lineations may be strike slip features, while the NISSOE lineations may be
steeply dipping reverse faults. The 1976 sequence on Karaginskii Island is
problematic in that quite different mechanisms have been proposed by various
workers. The mechanism proposed by Koz’min (1984) has nodal planes close to
both lineation strikes in Kamchatka. However, observable lineaments on
Karaginskii Island and the aftershock distribution mapped by Fedotov et
al. (1980) strike northeast-southwest. It is also possible that this sequence is
an intraplate series (McMullen, 1985). Due to these uncertainties, the 1976
sequence is not used in this investigation. South of Karaginskii Island, in the
northeast Kamchatka seismic zone, the chosen fault planes are parallel to the
seismic zone (and the coastline) which results in convergence between

Kamchatka and the Bering Sea.

 

231

Table 4-1: Focal mechanisms and sli vectors. Plane 1 is the presumed fault
lane. h: focal depth; Q: uality (Good, Fair, Poor, N denotes not
own, - denotes inversion); Az: Azimuth of the horizontal
grqjection of the slip vector; Va: Variance of the strike;
: eference - C This study
1 McMullen, 1985
2 Savostin and Karasik, 1981
3 Jemsek et al., 1984
4 Koz’min, 1984
5 Cormier, 1975
6 Veith, 1974
7 Dziewonski et al., 1985
8 Stauder and Mualchin, 1976
9 Minster et al., 1974
10 Chapman and Solomon, 1976

 

Nr
01
02
O3
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40

Table

Date
Transform
Transform

1967 02 13
1963 03 28
Transform
1969 05 05
Transform

1971
1970
1975
1968
1970

03
02
O4
01
10

23
22
16
O3
21

Transform

1970
1967
1967
1976
1973
1964
1968
1975
1970
1964
1969
1983
1964
1980
1963
1975
1984
1976
1968
1971
1970
1972
1974
1972
1969
1970
1969

10
10
11
O9
11
07
06
02
O4
08
04
O6
O7
02
05
08
11
01
09
05
06
01
06
08
11
01
12

26
18
23
16
09
31
08
26
23
25
07
10
21
01
20
12
22
21
09
18
05
13
19
O3
22
27
23

4-1:

Lat N
52.5

52.5

52.82
66.29
66.5

66.91
71.0

70.97
71.23
71.49
72.22
74.62
79.0

79.80
79.81
80.20
84.30
86.05
86.47
87.00
84.98
80.65
78.15
76.55
75.53
72.10
73.06
72.20
70.76
68.53
67.70
66.17
63.92
63.26
61.94
63.14
59.51
57.70
57.64
57.34

232

FOCAL MECHANISMS AND SLIP VECTORS

Long E h mb Plane 1 Plane 2 A2

-35.0 ------------------- 96
-33.5 ------------------- 95
-34.25 17 5.6 095/888 005/90v 95
-19.86 15 7.3 106/86N 017/7BE 107
-20.0 ------------------- 98
-18.17 33 5.2 112/82N 025/725 115
-08.0 ------------------- 115
-06.86 29 5.9 120/748 026/728 116
-08.21 33 5.1 111/875 020/74E 110
~10.36 15 6.0 107/808 016/818 106
001.55 33 5.2 142/58w 030/6OE 120
008.56 33 5.4 133/61W 030/64E 122
002.5 ------------------- 128
002.9 34 5.6 138/76W 017/81N 139
002.9 42 5.7 134
-00.7 16 5.7 128/74w 040/84N 130
000.9 01 5.4 038/483 224/42W 134
032.8 01 5.4 058/488 248/43N 157
040.7 07 5.2 331/533 089/593 178
051.4 32 5.2 248/54N 096/408 006
098.5 02 5.3 308/40N 126/508 036
122.0 27 5.2 226/SON 332/725 063
126.65 05 6.2 171/46w 341/455 072
130.86 11 5.4 142/48w 343/425 072
122.75 25 5.4 003/338 156/60W 065
130.10 12 5.4 170/45w 326/50E 056
122.59 22 5.4 168/50W 274/71N 005
126.25 10 5.0 174/60W 270/80N 000
127.12 16 5.1 164/72W 286/30N 018
140.88 26 5.4 087/758 341/4SE 066
140.00 18 5.0 090/458 338/708 068
142.13 06 5.0 238/73N 140/65W 050
146.10 10 5.9 300/82N 210/88W 122
146.18 04 5.4 316/708 218/72W 130
147.04 33 5.3 100/808 010/82E 109
150.92 17 4.9 146/77W 236/90V 146
163.10 11 5.2 287/72N 197/82W 104
163.56 51 6.3 032/75E 172/19W 82
163.60 51 5.0 010/45E 190/45W 100
163.14 13 5.4 033/823 293/41N 123

Va

10
10
10
10

10
10
10
10
10
10
10
10
10

10
10

10
10

10

10
10
10
10
10
10

15
15
10

10
20

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SLIP VECTORS AND POLES OF ROTATION

Slip vectors computed for the newly determined mechanisms are based on
the above discussed choice of fault plane (Table 4-1) and are combined with
data from the North Atlantic and the Arctic Oceans obtained or cited by
previous workers (e.g., Savostin and Karasik, 1981; Chapman and Solomon,
1976). Slip vector uncertainties are assigned based on the quality of the focal
mechanism. The horizontal projection of the slip vectors for the Arctic Ocean
and northeast Siberia are shown in Figure 4-4. Unnumbered events in Figure
4-3 are not used since either the constraint on the slip vector is weak or the
mechanism is poorly constrained.

Poles of rotation are computed using the strikes of the slip vectors using
the algorithm of Morgan (1968). Error ellipses are estimated based on a value
of F = 1.25 'Fmin. where Fmin is the minimum r.m.s. error obtained in the pole
calculation (LePichon et al., 1973). The final data set consists of S transform
fault strikes from the North Atlantic and 34 slip vectors (Table 4-1).

The data were first divided into five subsets based on geographic regions
and combinations of regions. Best fitting results are obtained by dividing the
region into two subsets: the North Atlantic and Arctic Oceans t0 the northern
Cherskii Mountains, and the southern Cherskii Mountains to the northeast
Kamchatka seismic zone. Inclusion of the southern Cherskii Mountains events
in the Arctic data set results in large misfits as does including the Lena River
events in the southern Cherskii data set. The northern Cherskii Mountains
events can be included in either data set, depending on the choice of fault
plane. However, their inclusion with the southern Cherskii events results in
inconsistent motions along the plate boundary defined by those events. An

uncertainty exists with the northeast Kamchatka seismic zone. A fairly good fit

233

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0.2.2....

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l.

 

234

Figure 4—4: Horizontal projections of slip vectors (solid lines) and poles
com uted in this study (circled stars). Numbers identify
eart quakes with Table 4-1.

235

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236

for the November, 1969, event is obtained by choosing the N032°E striking
nodal plane as the fault plane. This results in a high angle thrust that is
admissible based on other data (Cormier, 1975) and is supported by the
narrowness of the northeast Kamchatka seismic zone. Use of the low-angle
thrust plane yields a much poorer fit and should be accompanied by a wider
seismic zone representing the surface projection of the fault plane. The
December, 1969, event is less well constrained (Cormier, 1975) and may have a
mechanism more similar to the November event. More study of these events
and this region is needed.

For the Arctic data set, a pole in the Lena River delta (71.240N,
132.050E) is obtained and for the southern Cherskii-Kamchatka data set a pole
is obtained off the coast of western Chukotka (72.40N, 169.80E; Figure 4-5).
The fit for the southern Cherskii pole is poorer with up to 25° variations from
the model azimuth. The three Lena River delta events (Table 4-1, nrs. 27-29)
are included in the data set although two of them yield large (25°) misfits.
Exclusion of these events only moves the pole slightly (71.420N, 131.530E).
They are therefore retained, as they probably represent tension at the edge of
the rift zone.

The region between the November 22, 1984 (northernmost thrust event),
and the August 12, 1975 (southernmost normal faulting event), events
represents a 500 km-wide sector where no large earthquakes have occurred
since the installation of the WWSSN. Because of this, the exact nature of the
transition zone is not clear. Large events have been located in, or near, the
region, in 1918- and 1927, and Grachev (1982) proposes that it represents an
incipient transform fault. More likely, the region is relatively aseismic due to

its proximity to the pole of rotation.

237

Figure 4-5: Schematic late configuration in northeast Siberian and les of
rotation for orth America - Eurasia and North America - gokhotsk.
Plate boundaries shown dashed where uncertain. Arrowsheads along
plate boundaries denote approximate relative motions. Solid dots
show locations of North America - Eurasia poles computed by
grevious workers, triangles show paleo ole posnions computed b
avostin et al. (1984), and 0 en circ es show Okhotsk - Nort
America dpoles computed by ot er workers. Starred poles are those
camputg in this study with the Stippled regions denoting an error
e 1psm .

238

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239

The relative consistency of all the slip vectors between the southern
Cherskii Mountains and eastern Kamchatka with respect to a pole off western
Chukotka suggests that all of these events reflect motion between common
plates. The distinction of this pole from that between North America and
Eurasia then indicates that the Suntar-Khayata Mountains and the Sea of
Okhotsk lie on a common plate. Following previous workers (Savostin et al.,
1982, 1983) this is called the Okhotsk plate, although the boundaries are
different. The Okhotsk plate is undergoing counterclockwise rotation with
respect to North America.

An important observation is that the Suntar-Khayata Mountains and the
southwestern Cherskii Mountains appear to be part of the Okhotsk plate. This
means that the triple junction between the Okhotsk, North American, and
Eurasian plates probably lies in the central Cherskii Mountains, near the
epicenter of the September 9, 1968, event (Table 4-1, nr. 32). The exact
location and orientation of the plate boundaries near the triple junction is
indeterminate, thus the relative motion between Eurasia and Okhotsk can not
be determined. However, an event occurred in 1959 several hundred kilometers
west of the 1968 event and has first motions reported in the ISS that are
consistent with a thrust mechanism. This suggests a more north-northeast

orientation for the Okhotsk-Eurasia boundary near the triple junction.

DISCUSSION
The North America - Eurasia pole of rotation obtained here is
considerably further north of most recent determinations (Table 4-2), the
notable exception being the pole of DeMets et al. (in press). It is further

north of the Chapman and Solomon (1976) pole, which does not explain the

‘

I‘L— “T”?!

2140

Table 4-2: COMPARISON OF POLES OF ROTATION

E . 'H l E .
68.00N 137.00E
69.30N 128.00E
65.50N 139.50E
61.80N 130.00E
65.850N 132.440E
53.70N 137.30E
62.20N 143.60E
59.480N 140.830E
59.60N 141.40E
59.46ON 141.260E
69.300N 130.560E
71.240N 132.050E
h ri —

45.80N 145.30E
47.090N 144.850E
72.40N 169.80E

0270/ Ma

0.2480 / Ma
0.23 1° / Ma
0.2290 / Ma

0.1890/Ma

0.210 Ma
0.21 / Ma
0.2660/ Ma

0.410 Ma
0.478 / Ma

Pitman and Talwani,
1972
Minster et al., 1974
Karasik et al., 1975
Chapman and Solomon,
197
Minster and Jordan,
1976 (global pole)
Chase, 1978
Zonenshain et al.,
1978
Savostin and Karasik,
1981
Savostin et al., 1982
Savostin et al., 1983
DeMets et al., in

ress (best fit pole)

is study

Savostin et al., 1982
Savostin et al., 1983
This study

241

compression in the Cherskii Mountains or the slip vectors from the Lena River
delta, and was derived using slip vectors from Sakhalin Island which may
represent motion between the Okhotsk and Amurian plates (Savostin et al.,
1982, 1983). In addition, Chapman and Solomon (1976) noted that their pole
does not satisfy the strike-slip mechanisms in the southern Cherskii
Mountains. The addition of an Okhotsk plate solves these problems. The pole
determined here is also close to the recently determined pole by DeMets et
al. (in press) which uses slip-vector, transform orientation, and magnetic
lineations from the North Atlantic and Arctic Oceans. A similar pole, located
near the Laptev Sea, was obtained earlier by Minster et al. (1974) based only
on North Atlantic data.

The Soviet poles (Savostin and Karasik, 1981; Savostin et al., 1982, 1983)
also differ from that proposed here and are located in the southern
Sette-Daban Mountains. These poles were obtained using strikes of faults
offsetting the depressions in the Cherskii Mountains, i.e., considering them to
be transform faults. This approach is only valid if the faults are presently
active and represent interplate motion. The thrust focal mechanisms in the
Cherskii Mountains make untenable the interpretation that they are pure
transform earthquakes, and the slip vectors indicate that that the northern and
southern segments represent motion between different plate pairs. Thus it may
be concluded that the premise is in error. Instead, it is suggested that these
faults may represent past plate motions.

This investigation indicates the existence of a separate plate
encompassing the Sea of Okhotsk and the Suntar-Khayata Mountains. Although
the northeastern boundary of this plate is well defined by the seismic zone

discussed above, its full western boundary is not clearly defined. The southern

242

part of its western boundary is well defined by a seismic zone along Sakhalin
Island (Chapman and Solomon, 1976). At the northern tip of Sakhalin Island,
this seismic zone turns west and continues in a linear band to the Baikal rift
zone. This has led to the identification of a separate Amurian block or plate
(Savostin et al., 1982, 1983; Ishikawa and Yu, 1984; Tapponier et al., 1982)
which includes the Maritime Provinces and western Sakhalin. Between the
northern tip of Sakhalin and the Cherskii Mountains, discontinuous segments of
seismicity are known in the Sette-Daban Mountains and in the El’ginskii
Plateau, which may be occurring on the Okhotsk-Eurasia plate boundary.

It is also interesting to note that the El’ginskii Plateau and the
Suntar-Khayata Mountains display considerable diffuse microseismicity (Koz’min,
1984) and that the largest events, studied in this paper, lie along the northern
and eastern edge. The western edge of this diffuse zone connects the central
Cherskii Mountains and the Sette-Daban Mountains and was the site of large
earthquakes in 1951. If the boundaries of this diffuse zone are not a result of
the epicentral location process, then it is possible that the relatively aseismic
regions represent the North American and Eurasian plates, and that the
seismically active zone represents the northern part of the Okhotsk plate being
compressed between its larger neighbors. Further study is required before this
problem can be clarified.

The rate of relative motion between North America and the Okhotsk plate
is not clear. However, the low level of seismicity along Shelikhov Bay and the
relative consistency of slip vectors and convergence rates between the Pacific
plate and the Sea of Okhotsk, computed assuming that the Sea of Okhotsk is
part of the North American plate (Minster and Jordan, 1976; Seno, 1985),

suggest that the rate of motion is fairly small. Soviet workers (Savostin et al.,

- I"! w

 

J

 

243
1982, 1983) have estimated a rate of about 1-1.5 cm/yr along the northern Sea
of Okhotsk, however, their Okhotsk-North America pole differs significantly
from this investigation’s and their plate model for the region incorporates
different plates and configurations. In addition, the rate data were estimated
from a regional inversion using measured offsets on thrust faults on Sakhalin
Island. Further analysis will be needed, especially of the Okhotsk-Eurasian
interaction, before accurate rate data can be derived since the Okhotsk plate
has no easily measureable rate information.

Finally, it is important to address the apparent discord between the
observed geology and focal mechanism solutions for the Cherskii Mountains.
Soviet workers have mapped a series of grabens extending from Bour Khaya Bay
through the Cherskii Mountains (e.g., Moma Depression) and into northwestern
Kamchatka (Figure 4-1; Savostin and Karasik, 1981; Grachev, 1982; Savostin et
al., 1983). The grabens are filled with Pliocene sediments in the Moma
depression and are older (Oligocene) near the Lena River delta and may be
Eocene in the Laptev Sea (Zonenshain et al., 1978). These grabens are
associated with high heat flow and several volcanic edifices are known from
the Morna region. One of them, Balagan-Tas, is reported to have been active
in the Early Quaternary (Argunov and Gavrikov, 1960) and perhaps as recently
as 1770 (Vlodavetz and Piip, 1959). Thus, a rifting environment in the not too
distant past is indicated. The focal mechanisms presented above, in addition to
mechanisms determined for some of the same events by Savostin and Karasik
(1981) and Koz’min (1984), are clearly thrust faults or events with a thrusting
component. In addition, the sense of motion on the fault planes contradict the

mapped offsets of the grabens.

244

One possible explanation of this discrepancy, assuming that North
America, Eurasia, and Okhotsk are the only plates in the system, is a change
in the pole of rotation in the recent past. Peters and Vink (1985) have
proposed that the North America-Eurasia pole of rotation was located near its
present position since Oligocene (anomaly 13) time. Alternatively, Savostin et
al. (1984) have suggested that the pole of rotation between North America and
Eurasia migrated steadily towards northeastern Siberia over at least the last 36
my I propose that the pole was located in continental Eurasia, near Bour
Khaya Bay, in the Miocene (anomaly 5 time) and then began, or continued, a
migration towards the southeast, opening up the Moma and associated
depressions. In the last few million years the migration continued to the
Pacific basin, creating a tensional regime throughout northeast Asia and
separating the Sea of Okhotsk from the North American plate. Due to the
resulting change in plate configuration, to North America-Okhotsk-Eurasia, the
pole of rotation changed to its present site in the Lena River delta region;
perhaps 0 to 3 Ma.

Most pre-Tertiary structural lineations in Shelikhov Bay and northwestern
Kamchatka strike northeast - southwest (Parfenov, 1983). The grabens in this
area are only weakly manifested (Savostin et al., 1983) in topography or
observable lineaments (Filatova et al., 1980) indicating that they are either very
young (Savostin et al., 1982) or were extremely short lived. Lineaments visible
on space shuttle photography are consistent with focal mechanism solutions and
indicate that the region now consists of east-west strike-slip segments
offsetting northwest-southeast striking thrust faults. The variability of the slip

vector azimuths in the Cherskii Mountains and the northern Sea of Okhotsk

245
may also reflect the recent change, i.e., that good strike-slip faults have not
developed and some pre-existing faults are still being used.

Based on the congruence between one nodal plane for each of the
earthquakes of the Cherskii Mountains and the strike of faults offsetting the
grabens, I infer that much of the present-day faulting is occurring along
reactivated faults from the rifting episode (Cook and Fujita, 1987). In
addition, numerous strike-slip and thrust faults from prior episodes of plate
accretion (Fujita and Newberry, 1983) strike northwest-southeast (Parfenov,
1983) and some seismic activity is probably occurring along reactivated
segments of these faults. In addition, Rezanov and Kochetkov (1962) note that
the Pliocene peneplation surface has been uplifted some 1500-2000 m in the

Cherskii Mountains, a possible indicator of compression.

CONCLUSIONS

On the basis of newly determined focal mechanisms and trends in
seismicity, it is proposed that there exists a separate Okhotsk plate which
incorporates the Sea of Okhotsk, Kamchatka Peninsula, and the Suntar-Khayata
Mountains of the northeast USSR. Based on slip vectors from these
earthquakes, I have calculated a present-day North America-Eurasia pole of
rotation in the Lena River delta region and a North America-Okhotsk pole off
of western Chukotka.

Structural and geologic data from northeast Siberia suggest the possibility
that the North America-Eurasia pole of rotation migrated across northeast
Siberia until a tensional regime separated the North American and Okhotsk
plates within the past 3 million years. Due to the plate reconfiguration, the

North America-Eurasia pole of rotation shifted back to the Lena River delta

‘ W

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246
leaving residual evidence of rifting across the northeast USSR. Present day
seismicity, in many places, appears to be occurring on reactivated faults from

prior tectonic episodes.

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