~;_. :42. .. . ‘.' q .A .A- < ' '_,_ -- . ‘3‘ -I~l: ,.n .3: I I... [m . LI“! II. uh}; ‘rnv ‘ IQ? 3" I I. 2" - 91:“ Ufn' ”Mn N SSW .‘(I’)’ ' "o..‘ H ','n”n‘¢:I '0' I I! “Hm.“ I ”'I‘ . 'I Q“. ““43. . II ' ',I ‘3. on fl'a... - I _Iu' 1" 'b 'lé;:y\ " I! :IIII I ‘ I I}, Juli” ' -\ ”2:5" 3-" 'I‘I’I nun-{mu -. v-d "Q? a ”i % ,Z;:-.I~ 1 15.9“ I'n. ‘vl $241.“! .iLu‘bhizri V R 1'} :I'IIIT. MWII’I' 'I3I"I". p. . V'v y... .u‘ .3; .3: '7‘" ' .m. “vi? 9 .‘l. , \' NWJMV’ ICHIGAN STAYE UNIVERSITY LIBRARIE IIIIIIIIIIII III I IIIIIIIIIIIII . ,1 III III 3 1293 00550 2798 uaaAR ; Michigan Stu in... University This is to certify that the thesis entitled Finite Strain Estimations for Archean Mona Schist Pillows and Early Proterozoic Enchantment Lake Formation Metawackes in the Eastern Upper Peninsula of Michigan Date presented by Paul J. Carter, Jr. has been accepted towards fulfillment of the requirements for Master's Geology degree in M W of essor 2/22/89 0-7 639 MS U is an Affirmative Action/Equal Opportunity Institution -<.-.v~' . 1V‘ESI.J BEIURNING MATERIALS: Place in book drop to LJBRARJES remove this checkout from All-KSlI-L. your record. FINES will be charged if book is returned after the date stamped below. FINITE STRAIN ESTIMATIONS FOR ARCHEAN MONA SCHIST PILLOWS AND EARLY PROTEROZOIC ENCHANTMENT LAKE FORMATION METAWACKES IN THE EASTERN UPPER PENINSULA OF MICHIGAN BY Paul J. Carter, Jr. A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Geological Sciences 1989 ABSTRACT FINITE STRAIN ESTIMATIONS FOR ARCHEAN MONA SCHIST PILLOWS AND EARLY PROTEROZOIC ENCHANTMENT LAKE FORMATION METAWACKES IN THE EASTERN UPPER PENINSULA OF MICHIGAN By Paul J. Carter, Jr. Finite strain ellipsoid patterns for pillows of the lower member of the Archean Hena Schist and metawackes of the early Proterozoic Enchantment Lake Formation were used to test for transpression and heterogeneity of strain during deformation associated with the Penokean Orogeny. Eleven Mona Schist and seven Enchantment Lake Formation samples were collected close to the northern contact of the Marquette Trough and Archean basement rocks. Finite strain analyses indicate 25-40% north-south horizontal shortening was accompanied by 40-70% vertical lengthening for pillows of the Hena Schist, with little or no change of length horizontally east-west. Analyses of metawackes from the Enchantment Lake Formation yielded similar magnitudes. Paul J. Carter, Jr. Magnitudes and orientations of finite strain ellipsoids are similar to previous estimates for slates and quartzites of the Mesnard Quartzite, Rona Formation, and Ajibik Quartzite (Westjohn, 1978, 1986, 1987). It is proposed that near plane strain conditions probably prevailed in pillows of the Mona Schist and plane to flattening strain dominated the Enchantment Lake Formation. Rf/phi plots do not indicate more than one deformation for pillows of the Mona Schist north of the trough. Finite strain ellipsoid symmetries and orientations from the Mona Schist and Enchantment Lake Formation north of the Marquette Trough are consistent with a deformation model involving transpression. Small folds in the Mona Schist south of the trough and in the Enchantment Lake Formation near Negaunee (Larue and Cambray, 1979) are also consistent with transpression. -In this model transpression is described by horizontal simple shear imposed on initial plane to flattening strains. ACKNOWLEDGEMENTS I have many people to thank for the support and guidance I received on the way to finishing this degree. Dr. John Palmquist sparked my interest in structure and strain analysis in the upper peninsula of Michigan, and in geology in general. Thanks to Dr. Palmquist, and my advisor Dr. F.W. Cambray, I accomplished my goal of obtaining a M.S. in geology. I thank Dr. Cambray for his advise, opinions, encouragement, patience and friendship throughout the project. I also thank my committee members, Dr. Duncan Sibley and Dr. Micheal Velbel, for their interest, comments, and questions. Dr. Tom Vegel and Dr. John Wilband are to be thanked for advice and review of my proposal. Dr. Wilband is also thanked for the development of computer software, used by so many graduate students at M.S.U., and his advice and friendship during field camp. I appreciate the helpful advise of the departmental office staff - Loretta, Cathy, and Mona. Thanks also goes to Diane and the rest of the library staff. My office mates (Dave Westjohn, Bob Cunniff, Bill Sack, Jim Guentert, Guo Anlin, and Marco Antonellini) and other ii graduate students (Steve Young, Tim Flood, Jim Mills, and many others) are all thanked for their interest, comments, and friendship. Dave Westjohn deserves a special thanks for critical review of a variety of projects related to academics, work, and life in general. Grants provided by M.S.U. and Chevron helped expenses related to field work and manuscript preparation. Dr. Robert Bauer and Dan Schultz-Ela are thanked for computer software used to construct ellipsoids. Last, but not least, thanks goes to my girlfriend Shelly, and my parents and family for love and support. Paul J. Carter, Jr. February, 1989 iii TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES INTRODUCTION . . . . . . . . . . . Purpose . . . . . . . . . . . Hypothesis . . . . . . . . . . Previous strain measurements . GEOLOGIC FRAMEWORK . . . . . . . . Archean rocks . . . . . . . . Early Proterozoic rocks . . . Tectonic setting . . . . . . SAMPLE DESCRIPTION . . . . . . . . Mona Schist . . . . . . . . . Enchantment Lake Formation . . Kitchi Schist . . . . . . . . SAMPLE LOCATIONS . . . . . . . . . Mona Schist . . . . . . . . . Enchantment Lake Formation . . Kitchi Schist . . . . . . . . SAMPLE PREPARATION . . . . . . . . Mona Schist . . . . . . . . . Enchantment Lake Formation and STRAIN ANALYSIS METHODS. . . . . . Definitions . . . . . . . . . Mona Schist . . . . . . . . . Enchantment Lake Formation and Kitchi Schist Kitchi Schist 11 11 15 15 18 18 20 21 23 23 25 25 26 26 27 28 28 29 35 TABLE OF CONTENTS (continued) DATA AND RESULTS OF ANALYSES . . . . . . . . Mena Schist . . . . . . . . . . . . . . Enchantment Lake Formation . . . . . . . DISCUSSION . . . . . . . . . . . . . . . . . Mona Schist . . . . . . . . . . . Superimposition of strain Simple shear imposed on flattening or plane strain . . . . . . . . . Structure at Harvey quarry . . . . Enchantment Lake Formation . . . . . . . Kitchi Schist . . . . . . . . . . . . . CONCLUSIONS . . . . . . . . . . . . . . . . Mena Schist . . . . . . . . . . . . . . Enchantment Lake Formation . . . . . . . Kitchi Schist . . . . . . . . . . . . . SUMMARY . . . . . . . . . . . . . . . . . . APPENDIX . . . . . . . . . . . . . . . . . . LIST OF ”PMCES O O O O O O O O O C O O O 38 38 61 66 66 70 7O 72 77 84 85 85 86 87 88 91 92 Table 1. Table 2. Table 3. Table 4. LIST OF TABLES "Development of stratigraphic nomenclature in the Negaunee quadrangle." From Puffett (p. 7, 197‘) O O O O O I O O O O O O O O I O O I O O 0 3 Section plane and calculated data from eleven samples of variolitic Mona Schist pillows. Isym - index of symmetry (from Lisle, 1985). Strain - strained axial lengths. G.o.fit a goodness-of-fit. k-value - (a - 1)/(b - 1), where a - X/Y and b - Y/z (after Flinn, 1964) . . . . . . . . . . . . . . . . . . . . 39 From Lisle (1985). Cut-off values of Isym used to test for symmetry of Rf/phi plots. Only 5% (and 10%, bracketed) of randomly tested samples gave Isym values lower than those in table 3 . . . . . . . . . . . . . . . 41 Section plane and calculated data from seven samples of Enchantment Lake Formation metawackes. Abbreviations as in table 2. Isym from Lisle (1985) and k-value from Flinn (1964) . . . . . . . . . . . . . . . . . . . . 62 vi Figure Figure Figure Figure Figure Figure Figure Figure 1. 2. Generalized geologic map of the study area Transpression geometry, taken from Sanderson LIST OF FIGURES and Marchini (1984). initially equal to the confining blocks in size, is deformed by compression parallel to The stippled block, Y-axis and shearing parallel to x-axis. Volume is conserved by lengthening parallel The amount of shear strain is to z-axis. equal to the tangent of psi . From Cambray (1984). Range Supergroup . . Pillow in near vertical, north striking outcrop surface at the Midway Industrial Diagram illustrating geometry of Cambray's transpression model used to explain asymmetry and orientation of f2 folds in rocks of the Marquette Park (section 24 T48N R26W). elongated vertically. discussed) Secondary mineral growth on quartz and feldspar grains in metawackes of the Enchantment Lake Formation . Calcite crystals growing perpendicular to parts of a broken and stretched feldspar crystal. jigsaw fit of restored crystal . Elongation is vertical: note Sample location map From Lisle (1985). labelled R1 and theta curves". phi vii "Deformation grid with Center of plot coincides with the intersection of Rs-value for that particular grid and zero The pillow is approximately 15 inches in width and is Note selvages which separate pillows and concentration of deformed varioles in pillow centers (to be 2 5 6 19 22 22 24 30 Figure Figure Figure Figure Figure Figure Figure 10. 11. 12. 13. 14. 15. LIST OF FIGURES (continued) From Lisle (1985). Graph which demonstrates theoretical overestimation of Rs by harmonic mean of Rf. Ri values are shown: abbreviations defined in text . . . . . . . Plot of the log mean of undeformed ellipse (BESTELL of Owen, 1984) versus internal inconsistency (PASE7 of Bauer and Sheehan, after PASES of Roberts and Siddans, 1970) and best-fit line. One odd data point was not included but the slope of the best-fit line did not change significantly . . . . . Quadrants A, B, C, and D, used to assess symmetry of Rf/phi plots, are defined by the harmonic mean of Rf and the vector phi mean Rf/phi plots from measurment of varioles in pillows in the lower member of the Mona Schist. Labels on the top of plots indicates sample number and number of varioles (ellipses) measured. Hmean - harmonic mean, Fmax - maximum fluctuation of phi-values . . . . . . . . . . . . . . . . . Equal area lower hemisphere plots of x (triangles), 2 (squares), and z (crosses) axes of calculated strain ellipsoids. Strain ellipsoids determined from the measurment of varioles in pillows in the lower member of the Mona Schist. Figure 13b is a contoured plot of figure 13a . . . . . From Westjohn (1978). Equal area lower hemisphere plot of x, Y, and z axes of calculated strain ellipsoids. Data from reduction spots and deformed vein sets in slates . . . . . . . . . . . . . . . . . . . From Westjohn (1978). Wood (1974) plots of data shown in figure 14. LLPrs 8 Little Lake Plessier reduction spots, Nrs = Negaunee reduction spots, Hrs - Harvey reduction spots, and Hv - Harvey veins. Percentages of axial extensions are valid assuming homogenous deformation of passive markers and negligible volume loss . . . . . viii 30 36 4O 43 56 58 59 Figure Figure Figure Figure Figure Figure Figure 17. 18. 19. 20. 21. 22. LIST OF FIGURES (continued) From Westjohn (1978). data from Westjohn (1978) and pillows of Abbreviations for Westjohn's this study. data as in figure 15 . Equal area lower hemisphere plots of x (triangles), Y (squares), and z (crosses) axes of calculated strain ellipsoids. Strain ellipsoids determined for seven samples of the Enchantment Lake Formation from means of grain ratios and orientations. Figure 17b is a contoured plot of figure 17a . From westjohn (1978). data from Westjohn (1978) and seven samples Wood (1974) plot of Wood (1974) plot of of the Enchantment Lake Formation. Abbreviations for Westjohn’s data as in figure 15 Sample 626ml from an abandoned quarry in section 22 T488 R25W. spherical as in sample 51xm1. of sample 626ml are shown in figure 12k Sample 620m1 from an outcrop on Front Street in Marquette, immediately south of highway Upright isoclinal fold in fine-grained schist is in contact with pillows 41. Outcrop sketch of structures in the Mona ' Schist at Harvey quarry in section 6 T47N Features are described in the text R24W. Equal-area lower hemisphere plot of poles to planar structures in fine-grained Mona Varioles are nearly Rf/phi plots Schist at Harvey quarry. Mona Schist ix Squares are poles to older, crenulated schisotsity or layering ($1 of figure 21); crosses are poles to younger (Penokean), crenulation schistosity or foliation; and triangles are poles to axial planes of small quartzose folds in the 60 64 65 67 68 73 75 Figure 23. Figure 24. LIST OF FIGURES (continued) Equal-area lower hemisphere plot of linear structures in fine-grained Mona Schist at Harvey quarry. Lineations represent the intersection of 81 and 82 (triangles) and fold axes (cross) of small quartzose folds . . . . . . . . . . . . . . . . . . . 76 Equal area lower hemisphere plots of X (triangles), Y (squares), and z (crosses) axes of calculated strain ellipsoids. Strain ellipsoids determined for seven samples of the Enchantment Lake Formation from means of grain ratios and orientations. Multiple symbols in each plot correspond to the number of calculated solutions and large variability indicates variability of solutions . . . . . . . . . . . . . . . . . 78 INTRODUCTION Purpose The purpose of this study is to test for transpressive deformation along the northern margin of the Marquette Trough during the Penokean Orogeny, by quantifying finite strain in the early Proterozoic Enchantment Lake Formation and pillows of the lower member of the Archean Mona Schist, in the eastern upper peninsula of Michigan between Marquette and Negaunee (see Figure 1). The Penokean Orogeny occurred approximately 1.85-1.9 b.y.b.p. (Van Schmus, 1976: Sims et al., 1980) at the close of the early Proterozoic (Precambrian x, U.S.G.S.). The Enchantment Lake Formation was deposited unconformably on an Archean basement and is the basal formation of the Marquette Range Supergroup (Cannon and Gair, 1970), which is composed of the Chocolay, Menominee, and Baraga Groups near Marquette (see Table 1). Eleven Mona Schist and seven Enchantment Lake Formation samples were analyzed to define finite strain ellipsoids. The finite strain ellipsoid patterns of the Mena Schist and Enchantment Lake Formation, together with schistosity surfaces and small folds in the Mona Schist south of the Marquette Trough, are used to test for transpression and heterogeneity of strain in rocks differing in age and lithology. The results of finite x FLORENCE Figure 1. Generalized geologic map of the study area. .Aeema .b .mv avenues scum :.oaocmuoooo oocoaomz on» Ca ouaucaocoao: panmcumfiucuum no acoamodo>oos .H dance .53. 52.... . < 9. 5.8. $8.. .58. x 33.5.8: 23.3.8... 25.3.28... 88.8 5.33 83.8 538 5.33 :33 36.38 .38 39.18 22.3.. B...- 9.8 22.2.32 .8532 as... .8532 :2! 3.5.4.8 as... saga...- utup-Sa 22."... . «=9. c.1428: 39... flap-«.5 33.8.. 2.98 .528 #30 >30 nah—838 <23. #30 £838 <20: .0030 I £838 <20: .0020 I .830 I I m m Ba... as: m 2.18 «is m m use. use; m m a 5282... m m 321 £35... a .s. 2 892.8 m 33:30 :22 m m 8.8. 25.52 m m 5.855 2.22 m m .538 asap-<2. :35. m .598. m £;.OI<3 SHOE-““3 ‘980 HP; 935% .Iflwwma- fi has. 22.—432.29.. 55.32.22. 9.238.. 2.3.. 9.3. 3:382 .. 23¢ 32.293... 1.30:8 at). «2.22.6... (waver. «who 3 803.... 65m 5 how can c0530 300: cases... use 3.0 .ooo...usaa. 4 strain estimations are compared to previous estimates of finite strain for rocks of the Chocolay and Menominee Groups (Westjohn, 1978, 1986, 1987). Hypothesis Harland (1971) coined the phrase "simple transpression" to describe deformation resulting from oblique plate convergence. Transpression is defined by Sanderson and Marchini (1984) as ”transcurrent shear accompanied by horizontal shortening across, and vertical lengthening along the shear plane". Figure 2 explains transpression geometry. Cambray (1984) proposed transpression to account for variation in fold patterns (Larue and Cambray, 1979) of early Proterozoic metasedimentary rocks of the Chocolay Group in the eastern upper peninsula of Michigan. Recently, Hudleston (et al., 1987) characterized deformation in Archean greenschists of the Vermillion district of Minnesota "as one of transpression: oblique compression between two more rigid lithospheric blocks". Cambray's transpression model (1984) has been proposed as a tectonic model for rocks of the Marquette Range Supergroup and may account for the variation in fold orientations of the complex synclinorium known as the Marquette Trough (see Figure 3). Passive margin deposition of the Chocolay Group occurred on an Archean craton prior to existence of the trough (Van Schmus, 1976; Cambray, 1978; Larue and 81083, 1980a). Formation of Archean block fault uplifts and basin subsidence was accompanied by \r Figure 2. Transpression geometry, taken from Sanderson and Marchini (1984). The stippled block, initially equal to the confining blocks in size, is deformed by compression parallel to Y-axis and shearing parallel to X-axis. Volume is conserved by lengthening parallel to z-axis. The amount of shear strain is equal to the tangent of psi. Figure 3. From Cambray (1984). Diagram illustrating geometry of Cambray's transpression model used to explain asymmetry and orientation of f2 folds in rocks of the Marquette Range Supergroup. 7 deposition of the Menominee and Baraga Groups in the Marquette Trough. In this model subsequent Penokean compression closed the trough and late-stage simple shear induced left-lateral motion along trough margins. The major compression direction would be slightly oblique to trough margins. This is supported by the work of Myers (1984), which suggests the major compression direction to be north-northeast during Penokean deformation of the Marquette Trough. Evidence for left-lateral simple shear is the asymmetry and orientaton of superimposed folds along the trough margins (Larue and Cambray, 1979). Steeply plunging f2. folds in the Enchantment Lake Formation and Kona Formation near Negaunee, the Mesnard Quartzite near Marquette, and Siamo Slate near Teal Lake, exhibit axial planes subparallel to trough margins and indicate left-lateral horizontal shearing (Larue and Cambray, 1979: Cambray, 1984). Cambray (1984) states that "the variable orientation of the latter (f2 folds) is due to the variations in the initial bedding produced by F1 (Ramsay, 1967) and their location may be controlled by irregularities in the trough margin." Important to this model is the finite strain state of the rocks involved. Assuming no irregularities in the trough margin, compression directed perfectly orthogonal to margins of the trough would result in flattening or plane strains at every point along the trough margin. Simple shear imposed on previous flattening strains as a result of non-orthogonal 8 compression on trough margins, or resulting from irregularities in trough margins, could create a variation in strain symmetry and orientation at trough margins as with fold orientations reported by Larue and Cambray (1979). The deformation history may involve more than one event or a continuous episode. Structures one might expect as a result of transpression are outlined by Sanderson and Marchini (1984). Variability of strain ellipsoid orientation is expected as with folds. Previous strain measurements Due to an apparent lack of suitable strain markers, complexity of structure, and laborious calculations prior to modern computers, few attempts have been made to quantify finite strain in Archean rocks of the upper peninsula of Michigan. Rocks of the early Proterozoic Marquette Range Supergroup contain numerous finite strain markers, some of which have been used to estimate finite strain. Reduction spots in slates (westjohn, 1978, 1986, 1987; Nefe, 1980: Meyer, 1983), and deformed vein sets (Westjohn, 1978) and quartzites (Westjohn, 1986, 1987), have been used to construct finite strain ellipsoids. These studies have concentrated on the Kona Formation and Mesnard Quartzite of the Chocolay Group, and Ajibik Quartzite of the Menominee Group. Westjohn (1978) determined strain ellipsoid axial extensions from reduction spots in Kona slates at Harvey quarry as +61% x, +17% Y, and -47% z. The orientation of the x-axis was determined 9 to be close to vertical, and the Y and z axes nearly horizontal with Y trending east and z trending north. Results of measurements of deformed vein sets were similar. Reduction spots from samples near Negaunee and Little Lake Plessier showed greater vertical extension. Percentage of z-axis shortening is less pronounced for strain ellipsoids determined from reduction spots and quartz-grain subfabrics in quartzites (-10 to -15%), and Westjohn (1986) uses these values and values from reduction spots in slates as a range of ”flattening strains in supracrustal rocks of the Marquette Range Supergroup". Westjohn (1986) relates measured strains to partial closure of the trough during Penokean deformation and concludes that-near plane strains dominate in fold limbs. Meyer (1983) determined strain ellipsoid axial lengths from reduction spots in four samples of Kona slates from a quarry 5-6 miles southeast of Harvey quarry. These lengths are 1) 1.30-1.41 x, 0.99-1.00 Y, and 0.70-0.78 z in flat lying beds and 2) 1.18-1.34 x, 1.10-1.16 Y, and 0.68-0.73 Z in shallow dipping (@ 20 degrees) beds. The strained lengths, which assume 1.0 initial axial length, relate to extensional percentages of +24 to +38% x, -3 to +14% Y, and -26 to -29% Z. Recent measurements (Carter and Palmquist, 1986) have shown that finite strain in one pillow of the lower member of the Mona Schist is similar in magnitude and orientation to finite strain documented for rocks of the Chocolay and Menominee Groups. No finite strain estimates have been 10 made for the Enchantment Lake Formation or greenschist in the eastern upper peninsula near Marquette, Michigan. GEOLOGIC FRAMEWORK Archean rocks The Precambrian rocks of the southern Lake Superior region have been studied extensively since the 1800's (Williams, 1890: Van Hise and Bayley, 1895; Van Hise et al., 1897: Van Hise and Leith, 1911). Archean rocks mapped by Gair and Thaden (1968) between Marquette and Negaunee consist of the Mona Schist, Kitchi Schist, and Compeau Creek Gneiss (see Figure 1). Archean rocks immediately south of the Marquette Trough are gneissic, except for a small section of Mona Schist that is in contact with rocks of the Chocolay Group in a quarry at the north end of Harvey, immediately south of Marquette (northwest 1/4 section 6 T47N R24W). North of the trough the Mena Schist separates the Marquette Range Supergroup from the Compeau Creek Gneiss. According to Hammond and Van Schmus (1978) "U-Pb isotope studies on zircons from samples of the gneisses and volcanics do not indicate an age any greater than 2,750 m.y. for rocks of the Northern Complex". Therefore, the basement rocks north of the trough are assumed to be older than the youngest sediments and younger than approximately 2,750 million years. Gair and Thaden (1968) considered the Mona Schist to be the oldest formation they mapped and in fault contact with the Kitchi 11 12 Schist near Negaunee. Puffet (1974) believed exposures in the Negaunee quadrangle were insufficient to determine the relative age of the schists. The contact between the schists is obscure and their protoliths may have been deposited in one episode. Gair and Thaden (1968) believed that the Mona Schist and Kitchi Schist were metamorphosed to greenschist facies prior to emplacement of tonalite-granodiorite plutons, now known as the Compeau Creek Gneiss, because of greenschist xenoliths or "septa" in the gneiss. The origin of these xenoliths of greenschist in the gneiss is not clear. Recently, the xenoliths have been interpreted as mafic intrusions based on appearance of xenolith margins (Baxter and Bornhorst,.1988). Gair and Thaden (1968) also proposed that metamorphism of the Mena Schist occurred during emplacement of the Compeau Creek Gneiss. This proposal is based on the fact that a range in metamorphic grade exists between the Compeau Creek Gneiss and the lower member of the Mona Schist. The metamorphic facies decreases in intensity away from the gneiss, which records lower amphibolite facies. The upper member of the Mona Schist adjacent to the gneiss records upper greenschist facies, and the lower member of the Mona Schist reached only lower greenschist facies. Therefore, according to Gair and Thaden (1968), the Mena Schist experienced two metamorphisms prior to the Penokean Orogeny. It is assumed that penetrative deformation accompanied Archean metamorphism in the Marquette area. The Marquette 13 Trough parallels the contact between the Archean granite-greenschist complex to the north of the trough and gneiss complex to the south (Morey and Sims, 1976), and the "Great Lakes tectonic zone" (Sims et al., 1980), which is interpreted to stretch from.Minnesota to Ontario. This parallelism has been cited as evidence for east-west trending structure prior to deposition of the Marquette Range Supergroup. Archean deformation resulting in vertical east-west foliation is well documented in Minnesota and Ontario (Van Hise et al., 1897: Van Hise and Leith, 1911). Sims (1980) reports folds due to Archean deformation at Marenisco, Michigan as trending northeast, subparallel to the Great Lakes tectonic zone. These rocks are dated by 2,700 m.y. old granites which cut folded rocks (Sims et al., 1977). Evidence for foliation in gneisses near Republic (which parallels the margins of the Republic Trough) being Archean is that the foliation "has only been affected by the later deformation within a few centimeters of" early Proterozoic diabase dikes (Cambray, 1984). In contrast to gneissic rocks south of the trough, and metavolcanics and gneisses in the Great Lakes tectonic zone near Marenisco, rocks of the granite-greenschist complex north of the trough are assumed to have experienced little or no ductile deformation subsequent to 2,500 m.y. ago (Sims et al., 1980). The majority of strain experienced by Archean basement rocks during the Penokean Orogeny has been attributed by Cambray (1978) to ductile shear of mafic dikes. 14 Although magnitudes of strain are not reported, the Compeau Creek Gneiss north of Marquette and metavolcanics northwest of Negaunee (mapped as the sheared rhyolite tuff member of the Mona Schist by Puffett (1974)) appear strongly deformed. Multiple Archean deformational events have been proposed for rocks of the "Marquette Greenstone Belt" in northern Marquette County by Bornhorst (1988). These rocks are located near large-scale shear zones. Many localized "shear zones" or zones of high shearing strains are not shown on available geologic maps and shear direction indicators are not common. This paper is concerned with the lower member of the Mona Schist, a massive metavolcanic unit composed of pillowed and massive metabasalt, and the Enchantment Lake Formation. The Mena Schist was named for the Mona Hills in southern parts of sections 27 and 28 T48N R25W (van Hise and Bayley, 1895). The lower member of the Mona-Schist contains an abundance of pillows, suggesting a sub-aqueous origin in part. The upper member of the Mona Schist, the Lighthouse Point Member, is mostly layered amphibolite schist. Minor lithologies in the Mona Schist include chloritic, actinolitic, and quartz schists, and slates and mafic meta-agglomerate (Gair and Thaden, 1968). The Kitchi Schist was named for the Kitchi Hills in section 33 T48N R27W (Van Hise and Bayley, 1895). Samples of the Kitchi Schist, from an outcrop north of Beverly Hills, were collected for comparison to the Mona Schist. In this outcrop the Kitchi Schist contains white subhedral to 15 euhedral feldspar crystals in a schistose matrix and is probably the crystal feldspar tuff of Puffett (1974). Thickness of the Mona Schist is estimated to be 13-21,000 feet in the Marquette quadrangle (Gair and Thaden, 1968) and 24,000 feet in the Negaunee quadrangle (Puffett, 1974). The "maximum mapped width" of the Kitchi Schist is 4,800 feet (Puffett, 1974). Early Proterozoic rocks Lithologies of the Enchantment Lake Formation range from conglomerates, slates, arkoses, and sericitic quartzites (Gair and Thaden, 1968) and it is the basal formation of the early Proterozoic Marquette Range Supergroup. Maximum thickness at the type locality in section 29 T48N R25W is 500 feet (Gair and Thaden, 1968). In section 31 T488 R26W the thickness of the Enchantment Lake Formation is only 150 feet (Puffett, 1974). The Marquette Range Supergroup was deposited unconformably on Archean basement composed dominantly of gneissic and metavolcanic rock. It is composed of a variety of detrital and chemical lithologies including conglomerates, slates, quartzites, siltstones, graded wackes, carbonates, and iron formation. Tectonic setting Many theories have been proposed to account for the depositional and deformational environments of the IMarquette Range Supergroup. James (1954) postulated that 16 the Marquette Range Supergroup was deposited in a subsiding basin, bound on north and south by high angle reverse faults, and was subsequently folded into a complex synclinorium when the rocks were compressed between uplifted Archean blocks.' The Archean blocks were assumed to have behaved in a brittle manner; like the jaws of a vise. Cannon (1973) claimed early Proterozoic metadiabase feeder dikes are not deformed in Archean basement gneisses, as sills associated with the dikes are in the Marquette Range Supergroup (see Plates 1 and 6 of Gair, 1975). Cambray (1978, 1984) proposed that the dikes, although not folded, are deformed in the gneiss based on observed oblique foliation. According to Cambray (1984) this indicates ductility differences and inhomogeneity of basement deformation. Both authors believe that older structures were utilized during Penokean deformation. Sims (et al., 1980) proposed intracratonic deposition and deformation of the Marquette Range Supergroup, based on continuity of continental crust beneath the Penokean deformation belt and absence of oceanic crust development. Van Schmus (1976) proposed a back-arc environment for deposition and Cambray (1978) proposed collision tectonics similar to the Phanerozoic. Recently, Hoffman (1988) proposed that the Marquette Range Supergroup may be a foredeep sequence, deposited between a prograding fold-thrust belt to the south and a downwarped Archean foreland to the north. In all models the basal Chocolay Group would be considered a passive margin sequence. The 17 Menominee Group contains abundant turbidites and iron formation and sedimentary thicknesses and paleocurrent directions indicate the Marquette Trough existed during deposition (Larue and Sloss, 1980a). Deposition of the Baraga Group may represent more widespread subsidence. Cannon (1973) and Klasner (1978) support a deformational model which assumes gravity sliding and soft sediment deformation prior to the major compressional event. Basement blocks are assumed by these authors to have behaved in a brittle manner. The reader is referred to Sims and Peterman (1983) and Cambray (1984) concerning detailed arguments for and against a plate margin setting in the upper peninsula during the early Precambrian. SAMPLE DESCRIPTION Mona Schist On sub-horizontal and vertical outcrop surfaces pillows define an east-west striking vertical foliation (Figure 4). Pillow cusps and rounded tops can be seen in some areas and pillows are interpreted as having tops to the north (Gair and Thaden, 1968). Pillows generally range in size from 1-4 feet in length (Gair and Thaden, 1968; Puffett, 1974) although larger pillows are common. There is no recognizable foliation within individual pillows other than that defined by sub-ellipsoidal varioles. Varioles, originally interpreted as filled-in vesicles or amygdules (Gair and Thaden, 1968; Puffett, 1974; Carter and Palmquist, 1986), are parallel to foliation defined by pillows (Figure 4). These sub-ellipsoidal markers are used in this study to define the finite strain ellipsoid for pillows. Varioles are easily recognized in outcrop and hand specimen as small (0.2 - 1.0 cm), white or pale green ellipses, in sharp contrast to dark green pillow matrix. Both varioles and matrix are very fine-grained and mineralogy is not easily determined. The reader is referred to Borradaile and Poulsen (1981) and Borradaile (1982) concerning the tectonic deformation of pillows. Spheroidal textures or structures in volcanic rocks are 18 19 Figure 4. Pillow in near vertical, north striking outcrop surface at the Midway Industrial Park (section 24 T48N R26W). The pillow is approximately 15 inches in width and is elongated vertically. Note selvages which separate pillows and concentration of deformed varioles in pillow centers (to be discussed). 20 common (Augustithis, 1982). In pillow basalts spherulites are common and form by devitrification of glass or as clinopyroxenes nucleate around minerals such as plagioclase and olivine (Yeats et al., 1973; Gelinas and Brooks, 1974). Many spherulites are sheaf-like in form and their margins are generally rough and pitted, in contrast to the smooth outline or margins of varioles (Yeats et al., 1973). Varioles are believed to form as immiscibile droplets during crystallization (Yeats et al., 1973). The varioles in the Mena Schist are regarded as such for several reasons. No odd or sheaf-like forms are noted. Variole margins are smooth and are concentrated in pillow interiors (as opposed to crowding in margins common to vesicles or spherulites), becoming too crowded in pillow centers for determination of margins or aspect ratios (Figure 4). The varioles demonstrate a consistent ellipsoidal form and near-spherical varioles are recognized in some areas. Enchantment Lake Although some quartzites were sampled, the majority of samples analyzed contain considerable matrix material. Quartz and feldspar grains are supported by schistose matrix dominated by chlorite, sericite, and quartz. Some samples’ matrix has been replaced by calcite. Rocks sampled were the quartz-rich metawackes described by Gair and Thaden (1968). All samples, except 625e4, are from the northern limb of the trough where bedding and cleavage are 21 near vertical and strike east-west. Secondary fibrous mineral growth on the ends of grains and between grains are common (Figure 5). Kitchi Schist Subhedral to euhedral plagioclase feldspar crystals are aligned parallel to near-vertical schistosity in the outcrop sample for this study. Feldspars are replaced by sericite and calcite and twinning is still recognizable. Detailed descriptions are given by Puffett (1974). Many broken and extended feldspar crystals can be seen in thin-section and weathered outcrop surfaces. Extensional fibers between and in broken crystals are sericite, calcite, and quartz (Figure 6). Fine-grained blebs of chlorite are elongated parallel to schistosity and are associated with Opaques. Matrix minerals are dominantly fine-grained chlorite, sericite, epidote, opaques, and quartz. 22 l l . O A .-. rap “"‘ Figure 5. Secondary mineral growth on quartz and feldspar grains in metawackes of the Enchantment Lake Formation. Figure 6. Calcite crystals growing perpendicular to parts of a broken and stretched feldspar crystal. Elongation is vertical; note jigsaw fit of crystal. SAMPLE LOCATIONS - refer to Figure 7 Mona Schist Sample 725 was collected from an outcrop located in the Midway Industrial Park, north of highway 41, in the northwest 1/4 of the southwest 1/4 of section 24 T48N R26W. Subsequently, samples 726ml and ccmip were collected from the same outcrop. Samples 725, 726ml, and ccmip all are from the top edge of pillows. Immediately east of the Midway Industrial Park, in the eastern half of section 24 T48N R26W and western half of section 19 T48N R25W, samples 710m2, 701m3, and 710m4 were collected. Samples 710m3 and 710m4 are from the top edge of pillows. Samples 914m3 and 914m4 were collected from an outcrop on the property of the Windbergs, located near the corner of Hamilton and Linden streets in Marquette. Sample 915M1 was collected from an east-west trending ridge, which lies approximately 1/4 mile west of the Windberg outcrop, on the property of the Houlmonts . Sample 914m3 is one-half to two-thirds of a medium-sized pillow. Sample 914m4 is from the top edge of a small pillow and sample 915ml is from the south facing side or bottom of a pillow (assuming tops to the north). Samples 626ml and 511ml were collected from an 23 24 do... coaumooH magnum .5. 025.2 >39. 3mm: .5on 33m Afi mu so 2.5.... >33: 22:. 131.3.D130 ODCQ O Ozcufl . m z s. 22: Moflmm oomwc moomo X 5:950“. ox...— EmEEacocw moods 025.662 483 « ll 4 M95...“ M NEE—em QleW—Q .0 Be: a a . q _ w x ._ F «No .5259: 8.6. .mEow aces 2.91 .mEow 22.x / q q Scams Eon :53 4 «emmw 23.6.22 .5 B a .795: /\J lirli/ [Kl// 2 can 0:. too a .620...” «:02 / a. so some / 22050.... m e. r. N p o w . _ . _ _ _ _ 3238. e353 .. < 3:... m N F o 235 scans.— 25 abandoned quarry in the northeast 1/4 of the southwest 1/4 of the southwest 1/4 of section 22 T48N R25W. Varioles appear undeformed and are nearly spherical in both samples. Samples 513m1 and 513m2 were collected from outcrops southeast of Pine Hill Quarry in the southwest 1/4 of the northwest 1/4 of T48N R26W. Sample 513m1 is from the pinched out western edge of a pillow and sample 513m2 is from the north facing side or top of a pillow. Enchantment Lake Formation Samples 701e2 and 701e7 were collected from section 34 T48N R25W and are fine to medium-grained quartz-rich metawackes. Sample 709e5 is from section 32 T48N R26W and is a medium-grained quartz-rich metawacke. Samples 626e0 and 626e3 are from section 31 T48N R26W and are similar to samples 701e2 and 701e7. Sample 626e4 is a fine-grained metawacke from section 28 T48N R25W. Sample 625e4 is a medium-grained quartzite from the southern limb of an anticline in section 3 T48N R25W. This sample may be Mesnard Quartzite. Kitchi Schist Samples 916k2, 916k4, and 916k5 were all collected from a large outcrop immediately north of Valley Road in the southeast 1/4 of section 25 T48N R27W. SAMPLE PREPARATION Orientation marks were placed on all samples prior to collection and were subsequently out such that three closely orthogonal planes resulted. Section plane orientations were derived from field marks using lower hemisphere equal area stereonets and contact goniometer. Mona Schist Samples of the Mona Schist were cut as close as possible to apparent principal planes, as defined by the varioles in hand specimen. Each sample was cut so that one horizontal and two vertical section planes resulted. The azimuth of the vertical planes was generally north and east, but varied between samples depending on hand specimen inspection of variole orientation. In all samples, the number of measurable varioles was maximised by making a succession of parallel cuts; the thickness of each cut being greater than that of the varioles. In most cases fewer than 100 varioles per section plane cut could be measured due to sample size. Difficulty obtaining samples of pillows, due to hardness, pillow morphology and glacial polishing, limited the number of measurable varioles for some samples. Mylar overlays were placed on section planes and the long and short axes of each variole marked and 26 27 digitized. The reference line used was a section plane strike line. Enchantment Lake Formation and Kitchi Schist Samples of the Enchantment Lake Formation and Kitchi Schist were cut in a fashion similar to that employed on the Mona Schist. Due to lack of strain markers in the Enchantment Lake Formation, samples had to be thin-sectioned, so that measurement of the orientation and aspect ratio of grains could be used to estimate finite strain. For coarse-grained samples thin-sections were placed in a photographic enlarger between polarizing plates and images were projected directly on to photographic paper for development. Fine-grained samples needed to be photographed and the negatives enlarged and projected onto photographic paper. Samples of the Kitchi Schist were thin-sectioned and photographed and compared to the Mona Schist. STRAIN ANALMSIS METHODS Definitions Rs - strain ratio of finite strain ellipsoid, Rf - marker axial ratio in final state, Ri - marker axial ratio in initial state, phi - angle formed by measured marker long axis and reference line in final state, theta - angle formed by marker long axis and reference line in initial state, x - long axis of ellipse or ellipsoid, Y - intermediate axis of ellipse or ellipsoid, z - short axis of ellipse or ellipsoid, en - extension of axes where n = 1 (X), 2 (Y), or 3 (Z). a 'X/Y or (1+31)/ (1+32), b - Y/z or (1 + e2) / (1 + e3), 1: -(a-1)/(b-l) Ellipsoid symmetries considered in strain analysis are prolate or constrictional ellipsoids and oblate or flattened ellipsoids. Constrictional, or cigar-shaped, ellipsoids have been extended in one direction and shortened in the plane perpendicular to extension. Perfect or pure constriction of an initially spherical marker would result in x > Y a z. Flattened, or pancaked, ellipsoids have been extended in two directions (generally considered the XY or cleavage plane) and shortened in the mutually perpendicular direction. Pure flattening would result in X - Y > 2. Assuming no volume loss, plane strain conditions exist when the length of the Y-axis of the finite strain markers does not change overall, while the x-axis extends 28 29 and z-axis shortens (or x > Y - 1 > 2). Several authors (Dunnett and Siddans, 1971: Sanderson, 1976; Holst, 1985) have discussed the effects of superimposition of strains on strain symmetry and deformation paths. Mona Schist The Rf/phi method (Ramsay, 1967: Dunnett, 1969; Dunnett and Siddans, 1971) was used to estimate section finite strain ellipses. The method presumes homogenous deformation of passive markers with their matrix and an initially random orientation of marker long axes to make the best estimate of Rs. For this study random is defined as large fluctuation of initial marker long axes orientations. Imagine a perfectly homogenous deformation of passive markers with identical initial ellipticities (Ri). Because of differences in initial orientations of marker long axes (theta), Rf/phi values will vary but form a predictable girdle known as a Rf/phi curve. The theoretical curves are produced from equations relating Rf, phi, Ri, theta, and Rs (equations are given in appendix). In reality there will be a range of initial Ri values and a scatter of data points. A theoretically derived Rf/phi curve with a Ri value greater than that of all initial markers, should enclose all data points (assuming an appropriate Rs value is chosen). Markers with the same initial orientation define theta curves. The Rf/phi curves and theta curves together define a deformation grid and are shown in Figure 8 (from Lisle, 1985). 3O .e a" J.- 4T.-. 10-. C 0 all Ian; j-“' X Z X l 3.19 4:4; - O \ \ \ \ \ 1 / / / / C, O 00' it)” Figure 8. From Lisle (1985). "Deformation grid with labelled Ri and theta curves”. Center of plot coincides with the intersection of Rs-value for that particular grid and zero phi. ‘ HARMONIC MEAN OF RF MI 1 3* 3 41 5 STRAIN RATIO, RS Figure 9. From Lisle (1985). Graph which demonstrates theoretical overestimation of Rs by harmonic mean of Rf. Ri values are shown; abbreviations defined in text. 31 The Rf/phi method allows the user to estimate initial ellipticities of subellipsoial markers and asymmetry of plots has been shown to indicate non-random orientations of marker axes prior to deformation. For each section plane the vector phi mean of variole long axis directions was used to estimate the long axis direction of the section finite strain ellipse. vector phi means are calculated using: v. mean phi - [arctan (sum sin(2phi) / sum cos(2phi)) ]/2. Fluctuation of phi-values (or a range of phi-values) greater than 90 degrees has been shown by Ramsay (1967) to indicate Ri > Rs. The section finite strain ratio, Rs, may be estimated in several ways. Ramsey and Huber (1983) explain that means of marker ellipses will always overestimate Rs because most markers possess ellipticities higher than that of the strain ellipse. One estimation can be made from the harmonic mean of marker axial ratios (Rf) where: Hmean = [n / (Rfl'l + sz'l + ... an'1)], n being the total number of markers measured. As Ramsay and Huber (1983) state, "there is no logical reason to choose this mean in preference to others, except that the harmonic mean takes on a lower value that the arithmetic and geometric means and lies closer to the true value of 32 Rs". Lisle (1977) has shown the harmonic mean to be an overestimation of the true strain ellipse at low strains (Figure 9, Rs <=I 2.5). Another estimation can be made by visually fitting theoretical curves to Rf/phi section plots (Dunnett, 1969). Curves of varying finite Rs are overlaid on Rf/phi plots until a best eye-ball fit is produced (Figure 8). Each overlay contains a group of Ri and theta curves that should contour the Rf/phi pairs symmetrically if the markers have behaved passively during a homogenous deformation and initial long axes directions were random. The center of the overlay coincides with the intersection of the horizontal and vertical lines corresponding to the Rs and vector phi mean respectively. Lisle (1977) developed a more objective method to estimate the section finite strain ratio. Assuming a homogenous deformation at the scale of measurements, and an initially random fabric or a random distribution of markers initial long axes (theta), the overlay that produces a symmetric theta distribution should be the best estimate of the true finite strain ellipse. Lisle uses a chi-squared test to test for a symmetric theta distribution. Siddans (1980) found the overestimation of Rs "expressed as the percentage error (Lisle, 1977)", to be a maximum of 12.4% for harmonic means and 6.6% for fitting of standard Rf/phi curves. The percents were calculated by comparing two-dimensional section plane data predicted by a mathematical model involving homogenous deformation of random ellipses (with constant initial axial ratios) to two-dimensional data 33 estimated by harmonic means and fitting of curves. Symmetrical Rf/phi plots may indicate initially random orientation of marker long axes. Lisle (1985), after Dunnett and Siddans (1971), uses an ”index of symmetry" as an initial check on symmetry of Rf/phi plots. High values of the index (close to 1.0) indicate the degree of symmetry of plots. However, symmetrical Rf/phi plots can also result from strain imposed coaxially upon non-random initial fabrics or by a unique case of non-orthogonal straining of asymmetric initial fabrics. Symmetrical Rf/phi plots do not prove initially random fabrics but help to limit the possibilities. Harmonic means of variole Rf were chosen as estimates of Rs, since means on all section planes were within the uncertainty range (+/- 0.1) of Rs estimated using best-fit overlays, and randomly tested samples passed chi-squared tests (after Lisle, 1977) using overlays where Rs equaled harmonic means. The vector phi mean and estimated finite Rs for each section plane were used to construct the finite strain ellipsoid that mathematically gave the best-fit. The best-fit finite strain ellipsoid was calculated using two separate computer programs. Both programs assume initially spherical markers and conservation of volume. One was written by Owens (1984, TRYELL-BESTELL) and the other was adapted by Bauer and Sheehan (PASE7) from a program written by Roberts and Siddans (1971, PASES). PASES uses matrix scaling and combining routines supplied by Owens (Siddans, 34 1980). Both programs contain internal checks or parameters that are used as indicators of how well the section finite strain ellipses combined to form the finite strain ellipsoid. Owens’ initial program (TRYELL), accepts Rs estimates and long axis pitch from any number of randomly oriented planes. The six independent trial solutions returned are used to iteratively calculate the best-fit ellipsoid (BESTELL). All six solutions should converge to the same calculated Rs. The better the solution the fewer iterations needed for convergence. The program calculates the section ellipses predicted by the ellipsoid solution from observed section data, and then uses them to unstrain the observed ellipses. If the calculated and observed section ellipses are identical the undeformed ellipse ratio should be 1.0. This indicates that the observed (measured) section ellipses combined easily, or very well, to form an ellipsoid. Owens (1984) used the log mean of undeformed ellipses as an internal check on goodness-of-fit of the data. An empirically derived log mean of undeformed ellipse ratio of 1.10 or less is considered by Owens (1984) to be acceptable. PASE7 requires 3 mutually orthogonal section planes and lambda values as input instead of axial ratios (Rf). Lambda values are defined as; lambdax/y - (1+e1)2/(1+e2)2]. Internal inconsistencies of two-dimensional strain data results in independent strain estimates being different 35 (Siddans, 1980). The "internal inconsistency" percent value returned by PASE7 is based on the deviation of each independent solution from the mean of all solutions (Bauer, pers. comm. 1987). Bauer (pers. comm.) considers an internal inconsistency value of 10% or less to be acceptable. Figure 10 shows the relationship between goodness-of-fit indicators for Mona Schist data. The best-fit line suggests 1.10 log mean of undeformed ellipses is equivalent to 15% internal inconsistency (best-fit line goodness-of-fit - 0.89, corr. coef. - 0.94). Failure of measurements to conform to PASE7 requirements of orthogonal section planes and known principal extensions (lambda values) may contribute to error. The Rf/phi method was used to estimate two dimensional finite strain for pillows of the Mona Schist for reasons discussed. There are many other methods one may use to estimate two dimensional finite strain (Gay, 1968; Elliot, 1970; Matthews et al., 1974; Shimamoto and Ikeda, 1976; Robin, 1977; Tobisch et al., 1977; Fry, 1979a,b; Holst, 1982). The reader is referred to several papers which discuss the relative merits and accuracies of these methods (Hanna and Fry, 1979: Siddans, 1980: Paterson, 1983; Babaie, 1986). Enchantment Lake Formation and Kitchi Schist Harmonic means of grain axial ratios and vector phi means of grain long axes directions were used to estimate section finite strain ellipses for the Enchantment Lake 36 Goodness-of—fit parameters 1J2 . . . ... O D 1.04 log mean undeformed ellipse 1.00 L L 1 0 5 10 15 internal inconsistency (Z) Figure 10. Plot of the log mean of undeformed ellipse (BESTELL of Owen, 1984) versus internal inconsistency (PASE7 of Bauer and Sheehan, after PASES of Roberts and Siddans, 1970) and best-fit line. 37 Formation. Determinations made from Fry (1979) plots are compared to estimations. The effect of initial grain shape and compaction of detrital sediments are discussed by several authors in light of strain analysis (Lisle, 1979; Sanderson, 1976). Samples of the Kitchi Schist were examined qualitatively and extensional directions and magnitudes (from feldspars only) are compared to the Mona Schist. DATA AND RESULTS OF ANALYSES Mona Schist Table 2 shows the number of varioles measured per section plane, index of symmetry (Lisle, 1985), estimated and calculated section strain ellipse ratios (Rs), calculated strain ellipsoid axial lengths ('Strain‘ of X, Y, z where 1.0 - unchanged length), an indication of goodness-of-fit of the data (G.o.fit), and calculated ellipsoid symmetry for each sample (k-value of Flinn (1962)). The Isym values in table 2 were determined using: Isym = 1 - ('na - “b. + ”no - nd') / N, where na-d are the number of points on an Rf/phi plot falling in the quadrants defined by the horizontal and vertical lines corresponding to the harmonic and vector phi mean, and N is the total number of points measured in the section plane. High values of Isym (close to 1) indicate a symmetrical pattern. Figure 11 shows the relationship of the variables and Table 3 (from Lisle, 1985) contains acceptable values of Isym. The table was generated from "two hundred trial calculations of Isym for randomly sampled markers from a uniform orientation distribution" (Lisle, 1985). All but three section planes do not meet 38 39 Table 2. Section plane and calculated data from eleven samples of variolitic Mona Schist pillows. Isym = index of symmetry (from Lisle, 1985). Strain - strained axial lengths. G.o.fit = goodness-of-fit. k-value = (a - 1)/(b - 1), where a = X/Y and b = Y/z (after Flinn, 1964). Number _Ss.9_tiszn_Es__ WWW of var curve harm. BEST BEST PASE7 BESTELL BESTELL -ioles Isym fit mean -ELL -ELL PASE7 PASE7 125 X/Y 88 .91 1.50 1.54 1.43 1.42 1.19 1.07 1.13 Y/Z 46 .83 1.50 1.50 1.39 .99 1.00 10.85% 1.10 X/Z 78 .90 1.80 1.86 2.00 .71 .84 12§M1 X/Y 49 .90 1.70 1.70 1.68 1.60 1.26 1.02 1.13 Y/Z 26 .92 1.60 1.58 1.56 .99 .99 2.55% 1.13 X/Z 45 .93 2.30 2.35 2.38 .64 .80 QQMIB X/Y 102 .96 1.50 1.54 1.52 1.45 1.19 1.02 1.32 Y/Z 69 .90 1.45 1.47 1.44 .97 .99 6.15% 1.20 X/Z 149 .90 1.80 1.79 1.82 .71 .85 ZIQMZ ' X/Y 39 .87 1.60 1.59 1.46 1.43 1.20 1.10 1.21 Y/Z 45 .84 1.50 1.52 1.39 .98 .99 14.08% 1.18 X/Z 65 .92 1.80 1.80 1.97 .71 .84 Zlflfll X/Y 89 .94 1.70 1.65 1.62 1.54 1.26 1.02 1.37 Y/Z 73 .88 1.45 1.46 1.43 ‘ .97 .97 11.07% 1.53 X/Z 89 .92 2.20 2.22 2.26 .67 .81 .Zlflfli X/Y 42 .86 2.00 2.01 1.84 1.68 1.29 1.11 2.39 Y/Z 47 .77 1.50 1.50 1.40 .90 .95 15.64% 2.11 X/Z 44 .95 2.10 2.05 2.25 .66 .81 21531 X/Y 239 .89 1.70 1.72 1.57 1.56 1.25 1.11 1.59 Y/Z 109 .90 1.50 1.57 1.42 .95 .98 12.89% 1.48 X/Z 348 .87 2.10 2.04 2.24 .67 .82 Slifli X/Y 75 .88 1.45 1.44 1.42 1.40 1.18 1.03 0.84 Y/Z 143 .94 1.50 1.52 1.48 1.02 1.01 6.20% 0.85 X/Z 146 .78 1.70 1.74 1.80 .71 .84 21§M1 X/Y 109 .84 1.70 1.68 1.69 1.41 1.20 1.01 1.03 Y/z 100 .76 1.40 1.40 1.40 1.00 1.00 0.22% 1.08 X/Z 160 .91 1.80 1.78 1.77 .71 .84 filifll X/Y 167 .99 2.40 2.41 2.35 2.20 1.47 1.03 1.79 Y/Z 120 .85 1.80 1.85 1.80 .91 .96 1.45% 1.52 X/Z 111 .86 4.20 4.26 4.36 .50 .71 illflz X/Y 183' .78 1.60 1.58 1.48 1.54 1.24 1.07 0.69 Y/Z 90 .91 1.80 1.80 1.68 1.05 1.02 10.25% 0.73 X/Z 108 .93 2.30 2.31 2.47 .62 .79 4O Rf vecuw‘ rnean I I I I e l I . . I A : :8 . I .0 beings--- -- "Ii...- ______ mean of Rf .. 0‘ If. 1’ .:?.g..e so I . C I D I I I l I I I j V I [I T T -60 -40 -20 0 20 40 an 9' Figure 11. Quadrants A, B, C, and D, used to assess symmetry of Rf/phi plots, are defined by the harmonic mean of Rf and the vector phi mean. 41 Table 3. From Lisle (1985). Cut-off values of Isym used to test for symmetry of Rf/phi plots. Only 5% (and 10%, bracketed) of randomly tested samples gave Isym values lower than those in table 3. Sample Size M 20 35 60 100 200 0.3 0.51 0.60 0.74 0.82 1.5 (0.4) (0.63) (0.67) (0.78) (0.85) 0.5 0.63 0.73 0.80 0.86 2.0 (0.5) (0.63) (0.77) (0.82) (0.88) 0.5 0.63 '0.73 0.80 0.87 Rs 3.0 (0.6) (0.63) (0.77) (0.82) (0.88) 0.5 0.63 0.73 0.82 0.87 5.0 (0.6) (0.63) (0.77) (0.82) (0.88) 0.6 0.63 0.73 0.82 0.87 10.0 (0.6) (0.63) (0.77) (0.84) (0.89) 42' Isym criteria. Figure 12 shows Rf/phi plots for section planes of all samples and Figure 13a is a three axis plot of mathematically calculated strain ellipsoids for all Mona Schist samples. In Figure 12, Rf/phi plots are, from top to bottom, the XY, Y2, and X2 apparent principal section plane data. In Figure 13a calculated XY planes are near vertical and within 30 degrees of east-west. Extension directions are near vertical to steeply plunging,,trending dominantly northwest (Figure 13b). Two of eleven best-fit solutions (samples 710m4 and 914m3) do not meet the goodness-of-fit criteria established by Owens (1984). If 15% internal inconsistency is taken as an equivalent of Owens criteria (based on Figure 10 and the fact that PASE7 requires lambda values and orthogonal section planes), then one sample does not meet the goodness-of-fit criteria established for PASE7. A large difference between estimated and calculated section strain ratios and large values of goodness-of-fit parameters is taken to indicate that a best-fit solution is not acceptable as an estimation of the finite strain ellipsoid. Although Y axial lengths returned are consistent between computer programs, and indicate strain symmetry or strain ellipsoid type (Y < 1 indicates constriction, Y > 1 flattening, and Y a 1 plane strain), there is a significant difference in calculated X and Z axial lengths. The X and z axial lengths returned by PASE7 may be meaningless since known principal extensions are the usual input. 43 Figures 12a-l. Rf/phi plots from measurment of varioles in pillows in the lower member of the Mona Schist. Labels on the top of plots indicates sample number and number of varioles measured. Hmean - harmonic mean, Fmax - maximum fluctuation of phi-values. 44 725A 88 varioles w mvm.. u come: ..m .. 1 .. . \ it. a \ N..& a x to . m . \ \ “3’“. m4. t new... H. ...... _ twee... 8......” / I. Q .n / an... F a / ‘z-'.M EM L L I a 7258 46 varioles ' h a \\ \ -M§6§ .me x 4...... u . : ....wee .. I..-., // / .«fl§-'§.L / .0. $ .n / '8‘% p / 4’ a / v E M "I m w GOV.“ I CMDEI .um 7250 78 varioles 0 80 N00." I CMQEI .um \ I m .\ .. .\\ “a f ”RN N \N I w 2 "on m “J F ...... 2...... //, mm / .fl E- . Figure 12a. 45 726M1A 49 varioles 10 1 7‘5. / : \ I : __.// ‘ \\\J _, ..j a 1~9‘0;- - ‘ - :60“ ’ ~30 0 30 '-"""'"' .90 Phi. Fmax - 38.383 10 726M18 26 varioles a": / ‘x . / \ \ T _.// :‘ \\\ g .— I. 22' v 1 ., 19i: ...... -so -3o 0 so ........ so Phi. Fmax - 35.117 10 726MIC 45 varioles : IIIIIIII In a . I"! i . I ‘iilunill ' ”WM“ ; (98%») a Mime)» E I 4:936.» \ ..- Q e".~n“~ s .--." 1.. .. .. . .1. " .. Phi. Fmax - 28.268 Figure 12b. Sample 726ml. 10 Hnean - 1.536 Hf. Rf. Hmean - 1.467 10 Hmean - 1.788 Rf. CCMIPA 102 varioles // /’ \\ \ \‘ '\\ \. \\ Phi. a so Fmax - 38.185 CCMIPB 69 varioles -mI o :m Phi. Fmax - 30.449 CCMIPC 149 varioles \ Phi. Figure 12c. Fmax - 29 30 .522 Sample ccmip. 47 710M28 39 varioles ,_ E . .-.--.0—1 e ’O D--.--". W 5mm.« I come: .hm o :w Fmax - 27.013 <fl Phi. 710M20 45 varioles u .\, \. .A'V A7 L N \ ~,~ L“”U%%PW Ax. ./ Ix. LflfiflWSzv hum." I came: ..m o Fmax - 29.796 4» Phi. 4m 710M2A 65 varioles P P P P h D /’ P! ' 88888 l 60 ”a 30 b : Fmax - 31.267‘ 1‘;--- am Phi. 4W m vom.. I came: ..m 1 4” Figure 12d. Sample 710m2. 48 710M30 89 varioles m , , t 1 I?) %—-\\ \ ‘ I P/ /~~\ \,. 1 g . I 1 E + 1 ------- 1". ' 1 1 .‘."'r--—--—fi -so ~50 -so 0 so so Phi. Fmax - 26.044 710M3A 73 varioles 10 l T I I I / x In ’ /I‘ .\\~ 9 ’ //// ./ ~. ~\\\ ' I / / \ \ “ z’ I #/ / \ \‘I S » / __ o.) s .z E .. ------ ' . -u 1 l 1_ 4m -xI 0 so 1» Phi. Fmax - 42.224 710M38 89 varioles 1° F r - I I 1 m ’ l a I 1 m D I 5 L.-’ Q '1 \1 s I , i . EE 1 l 1 l L 4» 4m so Figure 12e. an o :w Phi. Fmax - 29.860 Sample 710m3. 10 Rf. Hmean = 2.007 ... 0 Hi. Hnean - 1.501 p O Rt. Hmean - 2.048 49 710M4A 42 varioles ‘eo o :w Phi. Fmax - 24.089 710M4B 47 V31" 10 185 - 0 so Phi. Fmax - 58.123 710M4C 44 varioles ‘ \...- ../ '60 Figure 12f. an o :m Phi. Fmax - 49.274 Sample 710m4. 50 914M38 239 varioles w ‘3 /’ N\ r- \ ‘ A /’ \\ I _// \. ‘ C 4 a: Q) s S-— I I E + 1 ------ 1" L j "P------i -s1 4» 4m 0 :m so Phi. Fmax - 37.393 914M34 109 varioles 10 T I I I // ‘\ P // \\ é /’ // " \\ \\ /"“"1I~. : '1'- g l———' q \.‘_ s z. . O ‘— o ..e 4 “ E ' *9 + I ***** '” ‘*~- 1 1 L l l -mI 4m - o :w so so Phi. Fnax - 78.545 914M3C 348 varioles :0 1 l V ‘ J m 1 m . 9 (\l ' 4 C (D __/ \: 2 I J E “...- -..-I 1 l l 1 1 4» so Figure 12g. 4» 0 so Phi. Fmax - 35.203 Sample 914m3. 10 I I I I / '\ m // \\ 3 // / L \ \\ g // / ‘ \ \\ I I—d/ / \ \ \‘1 o: . __ _ e I ‘1 E x‘ r """ ‘ «t --. . “- 1-90 -3o 0 so so so Phi. Fmax = 46.559 914M4A 143 varioles 10 U r I I /” \ /‘ ~\ g / "\ ' \ w: ,/ x \ \\ .; / / N \ \ I ...—d/ / ~\ \M C "' —« P I ‘ i - U —. E flu‘o‘fl n‘~.~§ ‘ 4‘ 9‘s“ 1.": '0 ~ -’ ‘ Qé-ia’x-zéé ~ ‘3 "Ei’u‘hdLaLf:Z‘lb‘i’l..' ....... J-T‘y‘. ---.. i l l l 1 4m 4» -m) o a) so so Phi. Fmax - 30.317 914M4B 146 varioles m a: ~~ : N .; N I ___// ..N \‘____ C h . M Q) E \— I 1». . E ’ . - L 31 1 ------- 1"“, l L "“r------- -so ~50 -3 so 51 914M40 75 varioles Figure 12h. o a so Phi. Fmax - 48.409 Sample 914m4. Hmean - 1.681 Rf. Hmean ' 1.401 Hf. Hmean - 1.782 Hf. 52 915M1C 109 varioles 10 / . / \ l “N // // \\ 1 / / / h\ 1 L J/ / ~\ \\—I ___/ / \. + { .' 4» ‘ 1 L ------ 1"." 1 1 ."’r---=;1 '90 '60 90 an 0 1» Phi. Fmax - 28.967 915M1A 100 varioles o 0 so Phi. Fmax - 25.581 Figure 121. Sample 915ml. 4» 4» ~31 0 so so Phi. Fmax - 70.054 0 915M18 180 varioles . , , L ”N r // .r. "\ _fl,,// . ‘\\\. !‘ 1 ‘1‘ ----- -°1j 1 -«: 4w -3 so 53 513M1A 167 varioles m m . = 1 m l c P m a: E I 1 E .- 1 l l l l -m: 4» 4m 0 in so so Phi. Fmax - 21.733 513M18 120 varioles m . 8 / ‘ a. // . — ; _-"/,/ \\\ on Q) E I ’4 + E o 1‘ L .‘1 ----- “-‘f/ 4» 4w -m: o a) so Phi. Fmax - 41.908 513M1C 111 varioles 10 . ”W I.” (D F g; ' On Q a". a ’ '5; s ' fill; O E I E 1 1 l l I -so ~50 «so 0 so so so Phi. Fmax - 14.874 Figure 12j. Sample 513m1. 54 513M2A 183 varioles 3. ‘.I..' .3. f. \ :é ...... \\\ .\ ..u-aummv mad \, \. .nxsmmamm‘_ "u \ \ \ .m»flmmn\v - lll'll?.~mw....m...fi o x 4. a a z nlnfis:mfi. M / / / “ F x / gs / «00¢ . i; 10 mmm.« I GENE: .fim 513M28 90 varioles / / P \ 1-.0~-- -- ..-- -60 w mm~.. . came: .~¢ 30 Fmax - 43.907 ~30 Phi. 513M20 108 varioles O ‘ ....... Q C ‘Q- -v" -30 Phi. -60 mom.m u come: .hm 30 Fmax - 33.583 0 Figure 12k. Sample 513m2. Hmean - 1.162 Rf. Hmean - 1.120 Rf. Hmean - 1.160 Rf. 55 626MIA 50 varioles am 0 1» Phi. Fmax = 174.022 626M18 16 varioles 30 - 0 so Phi. Fmax s 145.416 626M1C 16 varioles 4m -31 0 so Phi. Fmax - 113.292 Figure 121. Sample 626ml. . .\ ;.~ ._ {cit-T: m. c _ j‘\‘ 7 ./" ' .v/ {:1 3-41 5 5-6: ........ ...... ......... .......... ................... .......... ................ ........ 4!}: '1. ::::f::;::§:.€:.’;:':f:' ”14 11-121 I: 13-15: ............ u ...... ............ ....... Figures 13a-b. Equal area lower hemisphere plots of X (triangles), Y (squares), and z (crosses) axes of. Strain ellipsoids determined calculated strain ellipsoids. from the measurment of varioles in pillows in the lower member of the Mona Schist. Figure 13b is a contoured plot of figure 13a. 57 Considering that BESTELL accepts ratios from any number of randomly oriented planes, axial lengths returned by BESTELL are considered a better estimate. This conclusion is supported by the closeness of principal plane strain ratios determined by BESTELL (ratioed ‘Strain' axes lengths) with calculated section strain ratios and harmonic means of measured section ellipse ratios. Nine of eleven samples are modeled as constrictional. Flinn's (1962) k-values indicate plane strain when k - l, constriction when k > 1, and flattening when k < 1 (the opposite of Y axial lengths). Figures 14 and 15 show data for Kona Formation slates (Westjohn, 1978). This data is supported by work of Nefe (1980). Work by Westjohn (1986, 1987) on reduction spots and quartz sub-fabrics in the Mesnard and Ajibik Quartzites indicates smaller strain magnitudes (-10 to -15% z-axis shortening) and similar orientations in fold limbs. Figures 15 and 16 are modified from Wood (1974) and Westjohn (1978) and are a modified form of Flinn’s (1962) diagram. Wood (1574) plots log (X/Y) versus log (Z/Y) (instead of log (Y/Z)), so that "lines representing equal changes of length of the three dimensions X, Y, and z are straight". The advantage of the Wood plot is that axial extensions can be read directly off the plot. Comparison of strain magnitudes reported for quartzites (Westjohn, 1986, 1987) and reduction spots (Westjohn, 1978) with magnitudes from varioles (see Figures 15 and 16) indicate that strain magnitudes calculated for the Mona Schist are bracketed by those documented for 58 turn: um: ruxflu autumn: 3114mm mm (mm «mum urn (vuusi 7." 5111mm: can: Figure 14. From Westjohn (1978). Equal area lower hemisphere plot of x, Y, and z axes of calculated strain ellipsoids. Data from reduction spots and deformed vein sets in slates. 59 llCIIAL' 0.1! 0.63 0.30 0.00 lo)! 0.33 0.30 '\ I ' ‘3 . 1 0.: , . 0:, .o‘ ... a.“ .' ‘ .. \ ‘ . 2. 0.4 ‘r " ... .4 a.” '2 t I s‘ o ’39 b 3 ‘ ‘L a oo o.) v ‘ . 3 ‘3- '0 cl ‘ ‘. i : ~‘ "0 «t I. I 0.: J° '3 - . 1.30 'g. H ‘ " 0.1 fl .. 8.86 l ; l l J‘ . 1' ..o’ ...: ..., -.o‘ ..., .... ..., use an Figure 15. From Westjohn (1978). WOod (1974) plot of data shown in figure 14. LLPrs = Little Lake Plessier reduction spots, Nrs - Negaunee reduction spots, Hrs = Harvey reduction spots, and Hv 2 Harvey veins. Percentages of axial extensions are valid assuming homogenous deformation of passive markers and negligible volume loss. 60 P365” ..7’ Co" '0’. .... ..31 ..33 0.1. 's I ' " 4 0.3 , . . t ‘ ' ._ s.“ . ‘. \ ‘3. s s. '0‘ up ' ‘ ‘ '0,‘ 1 . F g ” 9‘ 2 ‘0: ‘ u o s ‘o o . 3.0. 3 fl 3- Co .. d c . K ‘ * ‘9 é ‘ . ‘ I 30 L . ‘0’ .. ‘ n ' . ‘0’. ‘o C ’ ..- o.x . -' H .. 1.86 J i 1 1- v v L :— l l -.o‘ ‘ -..: -.., ..o‘ ..., ..o‘ -.., m m Figure 16. From Westjohn (1978). Wood (1974) plot of data from Westjohn (1978) and pillows of this study. Abbreviations for Westjohn's data as in figure 15. 61 slates and quartzites mentioned. Comparison of Figures 13 and 14 show that orientations are similar. The important difference to note is strain ellipsoid symmetry or type. Although two samples are modeled as flattening strains, nine are slightly constrictional. Unlike flattened . ellipsoids, constrictional ellipsoids generally indicate' superimposition of strain (assuming initially near spherical markers and homogenous deformation). Constriction may have resulted from imposition of tectonic strain on an earlier fabric, or superimposition may have occured in a continuous episode. It should be pointed out that comparing strain data from different lithologies is only valid when negligible volume loss is assumed. Enchantment Lake Formation Harmonic means of grain axial ratios and vector phi means of grain long axes were used to estimate the section finite strain ellipses. Table 4 shows number of measured grains, estimated and calculated section plane strain ratios, calculated axial lengths ('Strain’), goodness-of-fit (G.o.fit), and ellipsoid type. Three of seven samples do not pass the goodness-of-fit criteria established for the Mona Schist. The differences in the best-fit solutions are related to the quality of data and data input requirements (lambda values for PASE7). Poor best-fit solutions are due to variability of section plane data. As with samples of the Mona Schist magnitudes of strain returned by each program are different and results 62 Table 4. Section plane and calculated data from seven samples of Enchantment Lake Formation metawackes. Abbreviations as in table 2. Isym from Lisle (1985) and k-value from Flinn (1964). Number 5.9mm 1.2.11.1; - of harm. BEST BEST- PASE7 BESTELL BESTELL grains mean -ELL ELL PASE7 PASE7 Z9122 X/Y 150 1.62 1.56 1.48 1.23 1.10 1.21 Y/Z 150 1.63 1.49 .98 .98 13.21% 1.44 X/Z 150 1.70 1.86 .69 .83 19121 , X/Y 81 1.74 1.47 1.49 1.25 1.20 1.16 Y/Z 57 1.72 2.02 .99 .98 26.28% 1.49 X/Z 50 1.68 1.50 .68 .82 92921 X/Y 60 1.59 1.45 1.38 1.18 1.13 1.21 Y/Z 60 1.50 1.37 .98 .99 18.08% 1.18 X/Z 70 1.54 1.73 .74 .86 .1922: X/Y 113 1.60 1.60 1.39 1.18 1.00 0.62 Y/Z 170 1,56 1.56 1.05 1.03 0.17% 0.60 X/Z 60 1.60 1.60 .69 .83 92929 X/Y 61 1.56 1.47 1.42 1.19 1.10 1.35 Y/Z 200 1.53 1.40 .97 .98 14.99% 1.37 X/Z 158 1.60 1.76 .73 .85 92921 X/Y 150 1.51 1.50 1.55 1.21 1.02 1.58 Y/Z 150 1.73 1.77 .95 1.01 1.08% 0.87 X/Z 60 1.64 1.60 .68 .82 92921 X/Y 120 1.51 1.37 1.43 1.20 1.14 1.31 Y/Z 150 1.59 1.40 .98 .99 16.78% 1.18 X/Z 151 1.69 1.89 .72 .84 63 from BESTELL are considered more accurate. Calculated strain ellipsoid axial lengths are similar to lengths calculated for the Mona Schist (X 8 1.4, Y 8 0.97 z 9 0.7). However, these lengths are a product of initial grain shape, compaction, and tectonic strain. Five of seven samples of the Enchantment Lake Formation are modeled as constrictional. Figure 17 is a three axis plot of calculated strain ellipsoids for all samples. Calculated strain ellipsoid XY planes are near vertical and east-west for all samples except 709e5, and are consistent with local and regional structure and previous work on slates (see Figure 14). Calculated z-axes for all samples except 709e5 are plunging less than 30 degrees and trend within 20 degrees of south (5 samples) or north. The direction of the long or x-axis of calculated best-fit ellipsoids for four of seven samples is approximately 60 degrees at-N80-90E and two of the remaining three samples have Y-axes that have flipped with the x-axis in the XY plane. Figure 18 is a Wood plot of all Enchantment Lake Formation samples. 5-7: 5 9-101 mm 11-13: 1145: \\‘ .. u. . a. ;¢) . .- , ”/11: 17-19: * men E3 23-24: 17b . Figures 17a-b.‘ Equal area lower hemisphere plots of X (triangles), Y (squares), and z (crosses) axes of calculated strain ellipsoids. Strain ellipsoids determined for seven samples of the Enchantment Lake Formation from means of grain ratios and orientations. Figure 17b is a contoured plot of figure 17a. Col. L v . 5 .M « assessed ..7’ I. J— I V . $4” for westjohn’s samples of the Abbreviations . _. . 4% my...” )4 J 2%». a od Figure 18. From Westjohn (1978). Wood (1974) plot of a a from Westjohn (1978) and seven ntment Lake Formation. a as in figure 15. d t Encha d t DISCUSSION Mona Schist The close grouping of determined extension directions (X) (Figure 13) and XY planes of estimated finite strain ellipsoids are consistent with regional Penokean structure. Calculated strain ratios are similar in magnitude to previous strain estimations of Penokean deformation (Westjohn, 1978, 1986, 1987). Compaction of markers during deposition and burial can contribute to measured strain. Based on symmetry of Rf/phi plots and near spherical varioles in samples 626ml (Figure 19) and 511ml, it is inferred that variole long axes were initially random and that initial aspect ratios were less than 1.5 (see Figure 12k, Hmeans are < 1.2). The samples with near spherical varioles were collected from an abandoned quarry 1/2 mile west of an outcrop of fine-grained Mona Schist (located at the intersection of highway 41 and Front Street in Marquette). The fine-grained rock contains an upright isoclinal fold (Figure 20) and a penetrative schistosity. This zone of apparently high strain is in contact with pillows and runs west into the south pit of the abandoned quarry. Fine-grained chloritic slates or schists are interlayered with Mona Schist pillows (Gair and Thaden, 1968) and appear to be recording much of the strain. More 66 67 Figure 19. Sample 626ml from an abandoned quarry in section 22 T48N R25W. Varioles are nearly spherical as in sample 511ml. Rf/phi plots of sample 626ml are shown in figure 12k. 68 Figure 20. Sample 620m1 from an outcrop on Front Street in Marquette, immediately south of highway 41. Upright isoclinal fold in fine-grained schist is in contact with pillows. 69 massive metabasalt in the Mona Schist has no foliation (Gair and Thaden, 1968). The pillows in the lower member of the Mona Schist near Marquette either experienced no deformation prior to the Penokean Orogeny or measured strains are Archean, and the Mona Schist behaved in a brittle fashion during Penokean deformation. Because of the closeness of determined strain ellipsoid axes orientations and magnitudes to those of rocks of the Marquette Range Supergroup, neither possiblity can be rejected. It has been shown that with volume loss apparent flattening strains may come to lie in the field of apparent constriction on Flinn diagrams (Ramsay and Wood, 1973). In other words, with volume loss calculated constrictional strains would become that much more constrictional. However, strains calculated for pillows of the Marquette Range Supergroup are considered accurate because of the nature and morphology of pillows and low magnitudes of strain. Being discrete units enclosed by initially glassy selvages, diffusion of material out of pillows is considered minimal under such low strains. Constrictional ellipsoids, or modeled constrictional strains, may be due to: 1) non-orthogonal superimposition of strain: 2) superimposition of strain in unique magnitudes oriented 90 degrees from initial strain: 3) simple shear imposed upon flattening or plane strains in a continuous episode of deformation (transpression). It is important to note that although nine of eleven modeled 70 strains are constrictional seven of those nine are near plane strain. Superimposition of strain Superimposition of strain implies previously compacted or deformed markers. The presence of pillows with apparently undeformed varioles (Hmeans < 1.2) and symmetry of Rf/phi plots does not suggest previous compaction or deformation of markers. No refolded folds or other structures are recognized within the lower member of the Mona Schist, north of the Marquette Trough between Marquette and Negaunee, to suggest more than one deformational event. Simple shear imposed on flattening or plane strain Simple shearing of initially spherical markers, without volume loss, results in plane strain. Whether Mona Schist varioles were significantly flattened during early stages of deformation is unknown. Horizontal shearing imposed on flattening strains, without vertical lengthening, should increase horizontal Yz section strain ellipse ellipticity, while ellipticities in the XY and X2 sections remain unchanged. This apparent increase in Y-axis length, in the absence of vertical lengthening, would result in modeled strains being more flattened. Vertical lengthening, accompanied by vertical shearing parallel to the X-axis or extension direction, would result in an increase in X2 section ellipticities and more constrictional modeled 71 strains. In the absence of initial flattening, the principal extension direction (X) would be sub-parallel to the direction of shearing. Transpression-has been proposed by Cambray (1984) to account for variation in fold patterns on the margins of the Marquette trough. In transpression there is no extension horizontally and volume is conserved by vertical lengthening (Sanderson and Marchini, 1984). Strain data for pillows of the Mena Schist are consistent with late-stage horizontally directed simple shear (sub-parallel to the Y-axis of the strain ellipse and trough margins), accompanied by vertical lengthening. Structural analyses of metasedimentary rocks by Hudleston (1976) and Schultz-Ela (1986) in the vermillion district of Minnesota indicates a change in "magnitude and symmetry of cumulative strain measured from deformed clasts' across a major fault described by ductile simple shear (Hudleston et al., 1987). Fabric symmetry in this area changes from planar to linear tectonite "corresponding to a change in strain symmetry from flattening to constrictional" (Hudleston et al., 1987). Evidence for significant north-south shortening also exists leading the authors to believe that deformation of the Vermillion district was transpressive. Deformation is not assumed to be regionally homogenous leading to "juxtaposition of zones of constrictional and flattening strains". Data from strain analyses of late Archean pillow basalts and agglomerates, also from the Vermillion district, are 72 consistent with a two-stage simple shear model (Bidwell and Bauer, 1987). Fifteen strain ellipsoids were determined using varioles, clasts, and phenocrysts. Near plane strain symmetries are proposed to have resulted during an initial simple shear event. Subsequent shearing resulted in variation of X and Y-axes of the strain ellipsoids and high flattening strains. This data and interpretation is supported by measured mineral lineations. It is obvious that in regions of variable lithology and structure, many strain histories or deformation paths will result. Without independent shear indicators in the pillows of the Mona Schist shearing directions and magnitudes cannot be determined and it is assumed that the majority of lateral motion experienced by the Mona Schist was in fine-grained rock between more massive basalt. Structure at Harvey quarry .In the northwest 1/4 of section 6 T47N R24W, greenschist is in fault contact with rocks of the Chocolay Group and is mapped as Mona Schist (Gair and Thaden, 1968). The Mona Schist immediately south of the fault contains a near vertical east-west cleavage or schistosity (82) that crenulates an older layering or schistosity (81) (Figure 21). The later schistosity ($2) is closely parallel to Penokean age slaty cleavage in rocks of the Chocolay Group north of the fault, and is interpreted to have developed at the same time. Parallel to this apparently Penokean age cleavage are axial planes of small 73 ..uxou on» :w conwuomop one mousuoom .zemm zhwe m coauoom cw >uumsv >o>hmz umsumwnom osoz may cu mounuosuum no nouoxm monouso .Hm ousmfim *oow 001, of m 322 .332. . Titz - .m .m an \I .u a... an euwwv r ..< .m .m 9+ 154.5% ma: 83 «2.ch 3.; on $1."... ] to. m A vie¢ % A on ”039,6 ms. 43 as. _ .34 M a»; . fl . moi 74 (1-2 foot amplitudes) folds of quartz-rich layers and lenses, which are parallel to the older schistosity or layering. Greater than 350 feet south of the fault this older planar feature is not crenulated, appears undeformed, and has an apparent dip of 30-40 degrees south. Figure 22 is a lower hemisphere plot of poles to planar features in the Mona Schist at Harvey and Figure 23 is a plot of linear features. The crenulated layering (31) dips approximately 60 degrees to the north within 45 feet of the fault and 30-40 degrees south greater than 85 feet south of the fault. Poles to axial planes of small folds and the crenulation cleavage are coincident, plunging less than 30 degrees north or south. It appears that an older layering or schistosity (81) has been deformed by Penokean deformation, represented by the east-west vertical crenulation cleavage ($2) and small-scale folds near fault contact with rocks of the Chocolay Group. Crenulated schistosities or other structures indicating more than one deformation are not recognized in the lower member of the Mona Schist between Marquette and Negaunee. Evidence for lateral movement exists in the greenschist at Harvey quarry. Immediately south of a sheared vertical dike, on a vertical east-west outcrop face (340 feet south of fault contact with the Chocolay Group), are small folds with axial planes that strike east-west and axes that plunge less than 30 degrees south (see Figure 21). Adjacent to these folds are folds with orientations as the ones described previously. This may be evidence for 75 Figure 22. Equal-area lower hemisphere plot of poles to planar structures in fine-grained Mona Schist at Harvey quarry. Squares are poles to older, crenulated schisotsity or layering (81 of figure 21); crosses are poles to younger (Penokean), crenulation schistosity or foliation: and triangles are poles to axial planes of small folds in the Mona Schist. 76 Figure 23. Equal-area lower hemisphere plot of linear structures in fine-grained Mona Schist at Harvey quarry. Lineations represent the intersection of $1 and 82 (triangles) and fold axes (cross) of small folds. 77 translational motion in the Mona Schist near the margin of the trough. It is not clear whether the Enchantment Lake Formation is present in Harvey quarry or is faulted or sheared out. Enchantment Lake Formation Ductility contrasts between competent grains and fine-grained matrix results in poor finite strain magnitude estimates. Strain magnitudes reported previously for rocks of the Chocolay Group (Westjohn, 1978) are low. Considering ranges of error, finite strain ratios may only reflect mean initial grain aspect ratios and/or compaction. Tectonic finite strain may or may not be recorded by grain shape. However, some samples do contain a fabric. In these samples quartz and feldspar grains are aligned and fibers or crystals of mica, calcite, and quartz on opposite edges of grains are subparallel to external foliation and extension directions estimated by grain long axes (Figure 5). This parallelism suggests that secondary mineral growth on grains accompanied the formation of tectonic foliation and that matrix material absorbed much of the strain. It should be noted that post-deformational mineral growth may occur parallel to cleavage and/or extension directions to accentuate deformational fabric (Wood, 1974). Three-axis lower hemisphere plots of samples 701e2, 701e7, 626e4, 626e0, and 626e3 are shown in Figure 24. Strains are constrictional and the plots are similar except 78 Figures 24a-g. Equal area lower hemisphere plots of X (triangles), Y (squares), and z (crosses) axes of calculated strain ellipsoids. Strain ellipsoids determined for seven samples of the Enchantment Lake Formation from means of grain ratios and orientations. Multiple symbols in each plot correspond to the number of calculated solutions and large variability indicates variability of solutions. 79 Figure 24a. Sample 701e2. DJ Figure 24b. Sample 701e7. 1 M‘ Figure 24c. Figure 24d. 80 Sample 625e4. bl Sample 709e5. 81 Figure 24e. Sample 626e0. hJ Figure 24f. Sample 626e3. 82 Figure 249. Sample 626e4. 83 for a switch of x-y axes in samples 701e7 and 626e3. Solutions range from good to poor and samples 701e7 and 626e4 do not meet the goodness-of-fit criteria. Strain magnitudes and the degree of constriction is similar between samples. In apparent xz and XY planes (XY dips steeply north and strikes slightly north of west) of all samples, mica and quartz extensional fibers have grown perpendicular to grains and the fibers are aligned near vertical. Section Y2 planes do not show extensional fibers supporting constrictional modeled strains. Sample 709e5 is modeled as a flattening strain. The solution is excellent in terms of goodness-of-fit criteria and the Y2 plane has been rotated 45 degrees counter-clockwise about x. Extensional fibers on grains in this sample are similar to the above samples but no fibers are recognized in the.Yz plane. Sample 625e4 is the only quartzite analyzed and no extensional fibers are recognized. It is modeled as constrictional and the solution is poor. This sample may be Mesnard Quartzite. Consistent with extension directions determined by vector phi means of grain long axes for some samples of the Enchantment Lake Formation, are extension directions returned by a normalized Fry plot (Erslev, 1987, after Fry, 1979). Anti-clustering and grain-to-grain contacts are necessary criteria for correct application of the Fry method (Fry, 1979). Table 4 shows the pitches of calculated strain ellipsoids axes in section planes 84 determined by the normalized Fry method and by vector phi means of grain long axes. Correspondence between the two is good in some samples but the rocks analyzed are not ideal for utilizing the Fry method. This is because grains are not in contact and several grain size populations exist in some samples (Crespi, 1986). All grains in one portion of a thin-section were digitized because Crespi (1986) has shown that for greywackes all grains constituted the most anti-clustered population. Strain ratios are incorrectly estimated and plots circular or not well defined if the Fry method is improperly used. It is suggested here that principal directions (whether of the strain ellipsoid or mean grain orientation) determined using vector phi means of grain long axes are accurate for some samples based on: 1) the observance of parallel orientations of fibrous secondary minerals and host grains, 2) low internal inconsistencies of mathematical solutions, and 3) determinations from normalized Fry plots. Kitchi Schist Samples of the Kitchi Schist contain feldspar crystals that show minimum extensions of 50% vertically. The lack of stretched feldspars and extensional fibers in horizontal planes and minimum vertical extensions of 50% indicates similar behavior (or non-flattening strains) of the Kitchi Schist in response to compression. CONCLUSIONS Mona Schist 1) Strain magnitudes and orientations estimated for pillows of the lower member of the Mona Schist between Marquette and Negaunee are consistent with those documented for slates of the Kona Formation and quartzites of the Mesnard and Ajibik Quartzites (westjohn, 1978; 1986: 1987). Eight of eleven strain ellipsoid solutions for pillows show horizontal shortening of 27-36%, vertical extension of 40-60%, and near plane strain. 2) Although constrictional strains were determined for nine samples, and flattening strains for two other samples, the strains are close to plane strain. Symmetry of Rf/phi plots and near spherical varioles are evidence against significant amounts of initial compaction and/or previous deformation. Although constrictional strains are consistent with a deformational model involving transpression, without independent shear indicators in the Mona Schist between Marquette and Negaunee, it is not conclusive that strain ellipsoid symmetries resulted from transpression. However, folds in greenschist mapped as Mona Schist south of the Marquette Trough may indicate translational motion. 3) Strain is assumed homogenous in Mona Schist pillows 85 86 at the hand specimen scale for measurements, but strain concentration is discontinuous and inhomogenous throughout the lower member of the Mona Schist. Fine-grained rock is strongly schistose and shearing and lateral displacement in fine-grained rock is probably more pronounced than in massive metabasalt or pillows. Folds on east-west striking vertical faces south of the trough indicate lateral motion sub-parallel to trough margins. Enchantment Lake Formation 1) Strain magnitudes and orientations estimated for the Enchantment Lake Formation on the northern margin of the Marquette Trough are similar to those of the slates and quartzites of the Chocolay and Menominee Groups and the pillows of the Mena Schist. Without independent indicators or estimates of volume loss, calculated strains should be considered tenuous. 2) As with pillow samples of the Mona Schist, strain ellipsoid symmetry is variable and strain ellipsoid z-axes are consistently directed north or south with shallow plunges. Measured strain is inhomogeneous; x and Y axes have interchanged between steeply plunging east and shallowly plunging west directions. Flipping of x and Y axes is consistent with transpression. 3) Heterogeneities may be due to: a) transpression, b) considerable competence contrast between grains and matrix, 87 c) low strain magnitudes. Kitchi Schist 1) The Kitchi Schist contains a schistosity that parallels regional and local structure. vertical extensions of 50% are documented by feldspars and are consistent with extensions determined for the Mona Schist and Marquette Range Supergroup. SUMMARY Strain analysis of pillows in the lower member of the Mona Schist indicates strain ellipsoid orientations and magnitudes are similar to strain ellipsoids determined for the Chocolay and Menominee Groups (Westjohn, 1978, 1986, 1987), which were deformed during the Penokean Orogeny. In pillows, north-south horizontal shortening of 25-40% (z-axis of finite strain ellipsoid) was accompanied by northwest steeply plunging to vertical extensions of 40-70% (x-axis of finite strain ellipsoid). It is concluded that the pillows were deformed with little or no volume loss under near plane strain conditions in which the length of the Y-axis of the finite strain ellipsoid remained relatively unchanged. The Y-axes are close to horizontal but trend dominantly east with shallow plunges. The variability of strain ellipsoid symmetry may be the result of restricted amounts of variable simple shear imposed on low strains. Amount of shearing cannot be determined and strain is inhomogenous throughout the Mona Schist. Calculated strain ellipsoid axial extensions similar to those of the Mona Schist were determined for seven samples of the Enchantment Lake Formation. The Enchantment Lake Formation probably suffered some volume loss though no evidence is documented. Extensional fibers are parallel to 88 89 extensional directions determined from grain long axes directions for the Enchantment Lake Formation. Calculated strains may be overestimations since grains were not initially spherical, but amount of strain suffered by matrix material is unknown. The crenulated schistosity in the Mona Schist south of the trough argues for previous structure in the Mona Schsit. This schistosity appears relatively undeformed away from the trough margin and has a shallow dip to the south. North of the trough, between Marquette and Negaunee, the Mona Schist appears to have experienced only one deformation. Rf/phi plots do not indicate that more than one deformation has been recorded by varioles in pillows of the lower member of the Mona Schist. Strain magnitudes and orientations are very similar to Penokean strains and are not consistent with the orientation of undeformed schistosity south of the trough. Transpression of the Marquette Trough has been attributed by Cambray (1984) to compression oriented .north-northeast to east-west striking trough margins. Transpression is described by vertical lengthening and east-west horizontal shearing. Small folds in the Mona Schist south of the trough, and in the Enchantment Lake Formation near Negaunee (Larue and Cambray, 1979), indicate translational motion and are consistent with transpression near the trough margin. Variability of finite strain ellipsoid symmetry in the Mona Schist and Enchantment Lake Formation, and the flipping of the x and Y axes of the 9O finite strain ellipsoid in the Enchantment Lake Formation, are also consistent with transpression. The directions of shearing are probably variable depending on previous structure and local stress directions. APPENDIX APPENDIX Equations relating Rs, Ri, theta, Rf, and phi were developed by Ramsay (1967) and are listed below. tan (2*phi) = [2*Rs*(Riz-l)*sin(2*theta)] [(Ri2+1)(Rsz-1) + (Riz-l)(Rs2+1)*cos(2*theta)] Rf = [tanzphi*(1 + Riztanztheta) - Rs2*(tan2theta + Ri2)]1/2 [Rsztanzphi(tanztheta+Ri2)-(1+Ri2tan2theta)] Markers with identical Ri define Rf/phi curves given by: cos (2*phi) .. [(Rf + 1/Rf)(Rs + 1/Rs) - 2(Ri + 1/Ri)] [(Rf - l/Rf)(Rs - l/Rs)]. 91 LIST OF REFERENCES LIST OF REFERENCES Augustithis, 3.8., 1982. ”Atlas of Sphearoidal Textures and Structures and their Genetic Significance". Theophrastus Publ. S.A., Athens, 329 p. 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