!!!   THE ORIGIN AND IMPLICATIONS OF A HIGH AMPLITUDE MAGNETIC ANOMALY ON THE EASTERN NORTH AMERICAN MARGIN  By  Matthew R. Karl                 A THESIS  Submitted to  Michigan State University in partial fulfillment of the requirements  for the degree of   Geological Sciences Ð Master of Science  2016ABSTRACT !!"#$!%&'(')!*)+!',-.'/*"'%)0!%1!*!#'(#!*,-.'"2+$!,*()$"'/!*)%,*.3!%)!"#$!$*0"$&)!)%&"#!*,$&'/*)!,*&(')!! By   Matthew R. Karl  Understanding the origin of the Hudson Fan Magnetic Anomaly Highs (HFMAH) on the Eastern North American Margin (ENAM) has implications for the rifting processes that formed the Atlantic seafloor. The origin and implications of HFMAH were explored in this research using magnetic forward modeling based on newly acquired high-resolution sea surface magnetic anomaly data with multi-channel seismic (MCS) for a source geometry constraint. The modeling results show that the two peaks of the HFMAH are reproducible by two highly magnetized bodies in the crust, the locations of which coincide with two zones of rough basement topography observed in MCS. It is proposed that the HFMAH is due to the crust emplaced by a propagating rift that accommodated rapid changes in directions and spreading rates during the very early stage of the Atlantic opening that formed the ENAM.             ACKNOWLEDGEMENTS !444! I thank the Department of Geological Sciences at Michigan State University for the support during this research. I also thank the captain and crews on R/V Langseth MGL1407 cruise.                               TABLE OF CONTENTS !45!LIST OF TABLESÉÉÉÉÉÉÉÉÉÉÉÉ..ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ...v  LIST OF FIGURESÉÉÉÉÉÉÉÉÉÉ.ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ.vi  KEY TO ABBREVIATIONSÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ..ix  INTRODUCTIONÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ.....1  BACKGROUNDÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ3  METHODSÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ.5  RESULTSÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ...7  DISCUSSION ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ....8  CONCLUSIONÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ15  APPENDIXÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ..16  BIBLIOGRAPHYÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ...28               LIST OF TABLES !5!Table 1: Multi-Channel Seismic Acquisition ParametersÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ17          !!!!!!!!!!!!!!!!!!!!!LIST OF FIGURES !54!Figure 1: (A) Magnetic anomaly map of the ENAM with prominent features indicated (Maus et al., 2009).  ECMA: East Coast Magnetic Anomaly. BMA: Brunswick Magnetic Anomaly. BSMA: Blake Spur Magnetic Anomaly. JQZ: Jurassic magnetic Quiet Zone. NESC: New England Seamount chain. Thick lines parallel to margin are M-series isochrons. M25: 156.6 Ma; M21: 149.9 Ma; M16: 141.9 Ma; M10: 131.7 Ma; M4: 126.5; Anomaly 100: 118.7 Ma (Muller et al., 1997). Bathymetry indicated by thin black contours: annotated and contoured at 500 m intervals (Amante and Eakins, 2009). Black lines running perpendicular to the margin are fracture zone locations from (Klitgord and Schouten, 1986) thick are major, thin are minor. White box indicates the region containing the Hudson Fan Magnetic Anomaly High (HFMAH). (B) Magnetic anomaly map with reduced scale for ENAM, which more accurately represents the amplitude and scale of the HFMAH and itÕs situation within the Jurassic Quiet Zone (JQZ). (C) Gravity anomaly map of the ENAM (Sandwell et al., 2014). White box indicates the region containing the HFMAH; note that there is no significant anomaly observed beneath the HFMAH ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ............18  Figure 2: (A) Location of MGL1407 seismic reflection survey lines in the area of the HFMAH. Bathymetry with contours drawn at 100m and annotated at 200m intervals (Amante and Eakins, 2009). Black lines running perpendicular to the margin are fracture zone locations, thick are major, thin are minor (Klitgord and Schouten 1986). M-series isochron M25 (156.6 Ma) is visible in the lower right. White dots represent DSDP borehole locations in the area. (B) 3D grid view of HFMAH (Maus et al., 2009). Processed sea-surface magnetics data from along MGL1407 cruise is displayed as black wiggles; these are included to compare with the shape of the HFMAH; because of the grid view, they are offset slightly from their track line positions. (C) Bathymetry data in region of HFMAH from NGDC NOAA website.ÉÉÉÉÉÉ..................19! Figure 3: (A: Upper Panel) 2D magnetic anomaly profiles sampled from EMAG2 grid (Maus et al., 2009) represented by dashed blue line and processed sea-surface magnetics data over MGL1407 MCS Lines 2 and 3 represented by solid blue line (see Figure 2A for location map).  Black line represents 2D forward solution for constant layer 2 thickness with varying magnetization. (A: Lower Panel) 2D block model used to represent the sedimentary package, layer 2 with a constant thickness of 2.5 km, and a layer 3 extending to the moho. Magnetizations of each 2D block are listed inside each block with units of A/m. (B: Upper Panel) Displays the same information using the same line and color scheme as A: Upper Panel; with the black line representing the 2D forward solution for a constant magnetization and varying layer 2 thickness. !(B: Lower Panel) 2D block model used to represent the sedimentary package, layer 2 with a variable thickness, and a variable thickness layer 3 extending to the moho. Constant magnetizations were applied to layer 2 and 3 and are listed inside each block. (C: Upper Panel) Displays the same information using the same line and color scheme as A: Upper Panel; with the black line representing the 2D forward solution for geologically reasonable magnetization based on Johnson and Pariso, (1993) and geologically reasonable layer 2 thicknesses based on Purdy et al., (1992). (C: Lower Panel) 2D block model used to represent the sedimentary package, layer 2 !544!with a variable thickness, and a variable thickness layer 3 extending to the moho. Variable magnetizations of each 2D block are listed inside each block with units of A/m. (D) MGL1407 processed MCS two-way travel time (TWT) data along Lines 2 and 3.  The green horizon below 5.5 seconds TWT is interpreted as the seafloor reflector, the red horizon at approximately 7.5 seconds TWT is interpreted as the basement reflector, and the blue horizon at approximately 10 seconds TWT is interpreted as the moho reflector.  Features of interest in the sedimentary package include the relatively flat seafloor morphology; onlapping strata situated between 140 and 150 km along profile.  Rough basement topography is observed between 110 and 140 km along profile. Also note that the moho reflector is relatively flat along profile. (E) Cartoon of the ENAM along MGL1407 MCS Lines 1, 2 and 3. Location of MCS Lines 2 and 3 are indicated by the black box. SDR: Seaward dipping reflector, basalt layers that are thought to source the East Coast Magnetic Anomaly (ECMA)ÉÉÉÉÉÉÉÉÉÉÉÉÉÉ...ÉÉÉÉÉÉÉÉÉ20  Figure 4: (A1) A sketch of the 3D layer model used to represent ÒnormalÓ oceanic crust based on seismic horizon picks and ETOPO1 grid bathymetry data (Amante and Eakins, 2009). (A2) A sketch of the 3D layer model used to represent ÒfaultedÓ oceanic crust represented by a 1km fault offset of the oceanic crust. (B1) 3D forward solution for ÒnormalÓ oceanic crust layer model (see A1) creates highest peak anomaly of the 3D layer models.(B2) 3D forward solution for ÒfaultedÓ oceanic crust layer model (A2) and is similar in shape to the ÒnormalÓ oceanic crust 3D forward solution, but with a slightly lower anomaly signal. (C) 2D magnetic anomaly profiles sampled from EMAG2 grid (Maus et al., 2009) represented by dashed blue line and processed sea-surface magnetics data over MGL1407 MCS Lines 2 and 3 represented by solid blue line (see Figure 2A for location map). The solid black line represents the 3D forward model solution for the ÒnormalÓ oceanic crust model sampled along ship track. The dashed red line represents the 3D forward model solution for the ÒfaultedÓ oceanic crust model sampled along ship track. Both 3D forward profiles have been scaled up by 30 nTÉÉÉÉ..ÉÉÉÉÉÉÉÉÉÉÉÉÉÉ...22  Figure 5: 6*7!Magnetic anomaly map of the northeast Pacific with prominent features indicated (Maus et al., 2009). (B) Plate boundaries, fracture zones, pseudofaults and failed rifts superimposed over magnetic anomaly pattern, modified from Hey and Wilson (1982). (C) Pattern of magnetic stripes predicted by model presented in Hey and Wilson (1982)ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ..ÉÉÉ..ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ.23  Figure 6: (A) Magnetic anomaly map of the Cocos-Nazca ridge with spreading center and transform fault indicated in black (Maus et al., 2009). (B) Interpretive contour of observed magnetic anomalies near the tip of the propagating rift. Normally magnetized crust is stippled; reversely magnetized is white. Dashed lines show track control; thick lines are the active rise axis and transform fault. Modified from Hey et al. (1980). (C) Magnetic anomaly pattern predicted by plate reconstruction model in Hey et al. (1980)ÉÉÉÉÉÉÉÉÉÉÉÉÉÉ24 Figure 7: (A)  Plate reconstruction of ENAM at M25 time (~154 Ma)  using GPlates  1.5 Software and Datasets with EMAG2 magnetic anomaly dataset. (B) Cartoon of opening scenarioat M25 (~154 Ma) time based on Labails et al., (2010) with seafloor segments from Mueller et al., (2008). (C) Cartoon of opening scenario at M25 (~154 Ma) time based on Labails et al.,(2010) showing illustration of a propagating rift. The HFMAH and its conjugate on the Eastern Atlantic (Pink outlines, line up within the ÒV-patternÓ of the propagating riftÉÉÉÉÉÉÉ.25!!! !5444!1489:;!<=!(A) The EMAG2 magnetic anomaly grid (Maus et al., 2009) color map and bathymetry contours (Amante and Eakins, 2009) in the region of the Murray and Molokai Fracture Zones. Note the short-lived transform faults (pseudo-faulting) shown between the Murray and Molokai Fracture Zones, as displayed by offset sections of isochrons (thick black lines). Also note magnetic anomaly highs generated by the oblique spreading centers due to this pseudo-faulting. (B) The same region with gravity grid (Sandwell et al., 2014). Note how magnetic anomaly highs in (A) are not always associated with gravity anomaly highsÉÉÉÉÉÉÉÉÉÉÉÉÉ...26  Figure 9: Upper panels indicate the rifting and spreading history of the North American and African continents at (A) ECMA (~190 Ma),  (B) in between BSMA (~170 Ma) and M25 (~154 Ma), and (C) at M25 (~154 Ma) time. Lower panels indicate detailed seafloor spreading and propagation history during the stage B above.  Underlying color map is EMAG2 magnetic anomaly grid (Maus et al., 2009), overlaid with sea surface magnetic anomaly profiles, including the MGL1407 cruise line (blue thick solid line). The gray ovals indicate areas of rugged basement topography. White and black solid lines are proposed isochrons among correlatable anomaly peaks and troughsÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ...27 KEY TO ABBREVIATIONS !4>!ABSMA: AFRICAN BLAKE SPUR MAGNETIC ANOMALY  BSMA: BLAKE SPUR MAGNETIC ANOMALY  CDP: COMMON DEPTH POINT  DSDP: DEEP SEA DRILLING PROGRAM  ECMA: EAST COAST MAGNETIC ANOMALY  EMAG2: EARTH MAGNETIC ANOMALY GRID 2-ARC MIN RESOLUTION  ENAM:  EASTERN NORTH AMERICAN MARGIN  HFMAH: HUDSON FAN MAGNETIC ANOMALY HIGH  MAR: MID ATLANTIC RIDGE  MCS: MULTI CHANNEL SEISMIC  TWT: TWO WAY TRAVEL TIME  WACMA: WEST AFRICAN COAST MAGENTIC ANOMALY !?!INTRODUCTION The Eastern North American Margin (ENAM), a consequence of the break-up of the last supercontinent, Pangaea, represents a mature passive margin that records the complete history of rift evolution and post-rift processes (Klitgord and Schouten, 1986; Bird et al., 2007; Schettino and Turco, 2009; Labails et al., 2010) (Fig. 1A). Geophysical evidence collected over the past forty years have documented key geological signatures that resulted from the rifting on the ENAM, including seaward dipping reflectors (SDR) (Sheridan et al., 1993; Holbrook et al., 1994b; Oh et al., 1995; Lizarralde and Holbrook, 1997) and prominent magnetic anomalies: the East Coast Magnetic Anomaly (ECMA) (Drake et al., 1959; McBride and Nelson, 1988; Austin et al., 1990; Holbrook et al.,1994a; Talwani et al., 1995) and the Blake Spur Magnetic Anomaly (BSMA)  (Klitgord et al., 1988; Labails et al., 2010) (Figs. 1A-B).  The conjugate side of the Atlantic, the Northwest African Margin (NWAM), although not as thoroughly studied, has matching counterparts to the ECMA and BSMA: the West African Coast Magnetic Anomaly (WACMA) and the African Blake Spur Magnetic Anomaly (ABSMA) (Sahabi et al., 2004; Labails et al., 2010).  The ability to match anomalies on conjugate sides of the mid ocean spreading center is key to plate tectonic reconstructions.  The ENAM seafloor younger than the ECMA (~190 Ma) is magnetically quiet, and except for the BSMA, there is no coherent margin-scale magnetic lineation determined until M25 (~154 Ma) (Fig. 1A-B) (cf., M28-M40 by Bird et al., 2007). This magnetically featureless seafloor  !@!known as the Jurassic Quiet Zone (JQZ) was the result of a low intensity magnetic field combined with rapid polarity reversals during Jurassic time (Tivey et al., 2006) (Fig. 1A-B). The JQZ has been a major arena for debate over different proposals to explain the magmatism and plate configurations in the early stage of the Atlantic opening tectonics, particularly during the earliest phase of seafloor spreading and margin scale plate tectonics (Klitgord and Schouten, 1986; Bird et al., 2007; Schettino and Turco, 2009; Labails et al., 2010).   !There is a distinctive, high magnetic anomaly that was historically identified yet unexplained in the magnetically quiet seafloor of the JQZ (Figs. 1 and 2). This magnetic anomaly high, almost coinciding with the location of the Hudson Fan (hereafter called Hudson Fan Magnetic Anomaly High (HFMAH)), encompasses an area approximately 180 km long and 70 km wide, and is located at the base of the continental rise (Figs 1A-B and 2A). The amplitude of the HFMAH is similar to that of the ECMA and major volcanic features, i.e., the New England Seamounts (Duncan, 1984; Swift et al., 1986) (Fig. 1A-B). However, no significant gravity anomaly has been identified under the HFMAH (Sandwell et al., 2014), suggesting that substantial igneous addition by late-stage magmatism, unlike the New England Seamounts, may not be its cause (Fig. 1C). !!The purpose of this research was to investigate possible origins of the HFMAH using newly acquired magnetic and seismic reflection data. The goal was to understand which possible source Ð in situ structure, lithology, magnetic polarities, and/or any combination of any of these Ð would  !A!be most plausible in elucidating the origin and implications of the HFMAH in the formation and evolutionary history of ENAM.  BACKGROUND Marine magnetic anomaly analyses have played an important role in advancing our understanding of the ENAM rifting processes (e.g., Hutchinson et al., 1982; Talwani et al., 1995; Behn and Lin, 2000). Two distinctive magnetic anomalies define the extent of the ENAM: the ECMA and the BSMA (Figs. 1A-B).  The ECMA is located approximately 200 km off the east coast of North America and is a positive magnetic anomaly with maximum amplitudes ranging from 200-300 nanotesla, with peaks as high as 350 nanotesla (Labails et al., 2010); it extends from Nova Scotia to Georgia (Drake et al., 1959), over 2500 km. It has been thought to be the continent-ocean crust boundary as a consequence of rifting process (Holbrook et al., 1994a; Talwani et al., 1995). Recent magnetic modeling of across-margin seismic profiles (e.g., EDGE experiment) strongly support rift-related volcanic material, the basalts of the SDRs, as the source of the ECMA (Austin et al., 1990; Talwani et al., 1995). The ECMA displays segmentation which is similar to that observed at the Mid-Atlantic Ridge (MAR) (Fig.1C) (Behn and Lin, 2000). While the origin of the segmentation remains uncertain, it has been attributed to rock mechanical strength variations in the rifted lithosphere (Wyer and Watts, 2006). This segmentation is also characterized by the presence of salt deposits in the Carolina Trough to the south, and in the Scotian Basin to the north of the Kelvin Seamounts (Sahabi et al., 2004; Labails et al., 2010). The salt deposits are used to date the age of the onset of seafloor spreading; with the  !B!rationale that seafloor spreading began at the point at which the water became too deep to deposit salt. The formation and evolutionary history of the ENAM, particularly those leading to different scenarios of the Atlantic opening, have been discussed by identifying prominent magnetic anomalies including the ECMA, the BSMA and the M-series anomalies (Klitgord and Schouten, 1986; Bird et al., 2007; Labails et al., 2010). Over the past four decades, two major competing scenarios have emerged to explain the Atlantic opening: 1) a reconfiguration of the direction of plate motion and spreading rate between North America and Africa around 170 Ma (Sahabi et al., 2004; Labails et al., 2010; Gaina et al., 2013) and 2) a basin-scale ridge jump to the east around 170 Ma, which left crust from both ridge flanks on the ENAM (Vogt et al., 1971; Klitgord and Schouten, 1986; Bird et al., 2007). !!The present-day ENAM continental shelf is shaped by active sediment transport from river inlets, to the continental shelf, to the shelf break, and to the abyssal plain via submarine canyon systems (Brothers et al., 2013). Since the Atlantic opened, terrigeneous sediments have been continuously carried to the ENAM continental margin by the nascent- and proto-Hudson river (Sirkin and Bokuniewicz, 2006). The location of the Hudson Canyon became entrenched within the continental shelf (Fig. 2C), surviving in the same locale through the Pleistocene glaciations to the present; it currently terminates in the Hudson Fan (Fig. 2C). At the seaward end of the Hudson Fan, seafloor morphology exhibits a well-developed contourite, the shallower distribution limit of which follows the 4600 m bathymetry contour, marking the boundary between the continental slope and abyssal plain (Fig. 2C).  !C!METHODS!Total field magnetic and multichannel seismic (MCS) reflection data were collected concurrently during the cruise along the Lines 2 and 3 of cruise MGL1407 of the R/V Marcus G. Langseth. A Geometrics G882 cesium vapor marine magnetometer was towed 213 m behind the navigation reference point of the shipÕs Seapath 200 system. Raw total magnetic field data were corrected for the ship-to-magnetometer offset so that the position of each observation of the magnetic field was accurately matched to the location it was collected from. Outliers in the magnetic data (e.g., electrical noise) were removed. A correction was applied using the international geomagnetic reference field (IGRF-11 model, Finlay et al., 2010), which corrected for the contribution to the total magnetic field from the EarthÕs core. A correction was also applied for the diurnal field variation based on data collected from the NASA Stennis Space Center, Mississippi to account for the contribution to the total magnetic field from the daily ionization of EarthÕs atmosphere by the Sun. Resulting total field magnetic anomalies have the range of -100 to +200 nanotesla in the crustal magnetic anomalies, which matches the expected range based on comparison with the Earth magnetic anomaly grid (EMAG2) (Maus et al., 2009) (Figs. 1 and 2).   The MCS data were collected using four air-gun arrays with a total capacity of 6600 cubic inches and an 8 km hydrophone streamer (Table 1). Eliminating bad traces and removing low-frequency noise processed the MCS data. A normal move out correction was applied to create common depth point (CDP) gathers. Inner mutes, refraction mutes and FK filtering were applied as needed. Velocity analyses were conducted for every 500 to 1000 CDP to generate relatively  !D!smooth stacking velocity models to be used in post-stack Kirchoff time migrations. Additionally, 34 sonobuoy refraction measurements were used to aid in calculating the velocities used for the major reflectors at major lithological boundaries: sediment, basement, crust and upper mantle. The stacked, migrated images exhibit unmistakable seafloor, sediment-basement interface and Mohorovicic discontinuity or ÒmohoÓ reflectors throughout Lines 2 and 3. These reflectors were used to identify the major structural boundaries of the oceanic lithosphere: layer 1, the sedimentary package, layer 2, pillow basalts and sheeted dikes and layer 3, gabbros. There is no particular magmatic and/or structural disturbance observed in the neatly layered oceanic lithosphere. The two-way travel time (TWT) was converted for each of these major reflectors using the velocity model of Lizzaralde and Holbrook (1997) to estimate the depths and geometries of these layers for subsequent magnetic models (Fig. 3A-C). Two rough basement topography zones were observed above the bottom of the slope break (Fig. 3D). These areas contain vertical displacements up to ~ 400 m with respect to the adjacent flat seafloor with a 6.5 km/s crustal velocity (Lizarralde and Holbrook, 1997).  To test the validity of the average magnetizations for layer 2 and 3 proposed in Johnson and Pariso (1993), a three-dimensional inversion based on the method of Parker and Huestis (1974) was carried out. A three-dimensional forward model was used to show that the shape and amplitude of the HFMAH could be reproduced using those calculated magnetizations, and to test the impact of a 1 km fault offset on a forward model of the HFMAH (Fig. 4B1-B2). To further investigate the architecture of the magnetic source, two-dimensional magnetic forward models  !E!were calculated from first principles (e.g., Talwani and Heirtzler, 1964). The MCS interpretation was used to constrain the geometry of the major lithological layers. Expanding the crustal model with constant crustal thickness used in the three-dimensional modeling, two end-member models were constructed: 1) a model with constant layer 2 thickness and various magnetization values, and 2) a model with constant magnetization values and various layer 2 thickness. For the model that assumed constant layer 2 thicknesses, a value of 2.5 km was used to conservatively estimate the thickness predicted for crust formed at the MAR from Purdy et al. (1992). !"#$%&'$!The new sea surface magnetic anomaly data display two anomaly peaks within the originally recognized HFMAHÕs long wavelength single peak (e.g. EMAG2 grid, Maus et al., 2009) (Fig. 2B). The two end-member 2D magnetic forward modeling results indicate that the magnetic source bodies that create HFMAH are confined directly under the two peaks of the HFMAH, the locations of which coincide with the two rough basement topography zones on the MCS (Fig. 3D and 9). In the model with constant crustal thickness, the highest magnetization values (4 A/m) are located beneath the two peaks (Fig. 3A) and in the model with constant magnetization layer 2 is thickest (5 km) beneath the two peaks (Fig. 3B). Based on these results, a forward model that lies in between the two-end member models was used to describe a reasonable range of thicknesses for layer 2 based on Purdy et al. (1992) with magnetizations in the range calculated by the 3D inversion.    !<!DISCUSSION!Despite being a prominent feature on the ENAM the existence and origin of the HFMAH has not been discussed in previous literature. Two obvious marine magnetic sources, a geomagneticpolarity reversal and a late-stage igneous emplacement are, however, immediately ruled out. Given its isolation in the magnetically quiet JQZ seafloor (Fig. 1A-B), the HFMAH being sourced from a geomagnetic polarity reversal does not match with the observed extent or the current understanding of the cause of the JQZ (frequent reversals during a period of low field strength). If late-stage magmatic events emplaced excess igneous material (i.e., seamounts, igneous provinces), such material could produce high magnetic anomalies as seen in the New England Seamount chain, and the ECMA (Talwani et al., 1995; Lizarralde and Holbrook et al., 1997). However, the both the gravity and the MCS data collected along Lines 2 and 3 indicate no excess igneous crust emplacement under the HFMAH.   A major structural boundary, such as a normal fault associated with fracture zones formed during the early stage of rifting and seafloor-spreading regimes (e.g., Larson and Pitman, 1972; Behn and Lin, 2000) can also be a significant source of high amplitude magnetic anomalies from its edge effect (Grauch et al., 2006). Indeed, the HFMAH is located across multiple predicted extensions of the MAR fracture zones (Fig. 1A) (Klitgord and Schouten, 1986); however, the directions of the predicted extension of the fracture zones are normal (perpendicular) to the expected direction of a normal fault that could produce the observed two-peak magnetic anomaly  !F!highs (e.g., Grauch et al., 2006). However, the MCS data does not contain reflectors offset by normal faulting beneath the HFMAH.   To further assess the possibility of a normal fault that occurred out of phase from the observed 2D seismic lines, three-dimensional magnetic inversion and forward modeling were conducted using the method of Parker and Huestis, (1974).  The three-dimensional inversion modeling was used to calculate magnetization distribution and to investigate source structure (Fig. 4A1-A2). The three-dimensional forward modeling results show that a 1-km normal faulted oceanic crust model displays very similar (only a few nanotesla difference) anomaly when compared to the normal oceanic crust model(Fig 4 C.). The few nanotesla difference is likely due probably to the 5.5-km thick sediment packet and ~ 4.5 km water depth resulting in a significant Earth filter, eliminating a simple correlation between the origin of the HFMAH and crust offset by normal faulting.  !The thick (~5.5 km) sedimentary package contains a sufficient volume of iron-bearing minerals to contribute to marine magnetic anomalies on the ENAM (Elmore et al., 2012). Due to the coincidence of the HFMAH and the Hudson Fan (Fig. 2)G!I believe such sedimentary contribution to the remnant magnetization is warranted in the forward modeling. The sedimentary contribution to the remnant magnetization may be a unique consequence of iron-rich sediment transport from the continent to the continental slope through the Hudson Canyon system. The MCS imagery captures a mound that marks the shallower depth limit of the contourite  !?H!distribution (Figs. 2C and 3E) and has dammed sediments from Hudson Canyon system over time (Fig. 2). Nearby Deep Sea Drilling Program (DSDP) Holes 11-106 (Fig. 2A) (Hollister et al., 1972) collected from the upper 1015 m of sediments beneath the Hudson Fan, exhibit zones with iron bearing sediments (5-15 weight percent), the amount of which falls into the range of magnetite present in basalts.  Based on the results of DSDP Hole 11-106 and the similarity in location of the Hudson Fan and the HFMAH, for our modeling I estimate that the ~5.5 km sedimentary package makes up ~10 percent of the crustal magnetic anomaly.   Based on the information above, I suggest that the primary source of the HFMAH is due to the difference between the remnant magnetization of the underlying versus surrounding oceanic crust. The layers of the oceanic lithosphere that possess the highest remnant magnetization values are layer 2 and layer 3 due to the amount of magnetite present in the lithologies of those layers. The pillow basalts and sheeted dikes of layer 2 contain between 5-14 weight percent magnetite while the gabbros of layer 3 contain between 1-2 weight percent. The relative thickness of layer 2 to 3 could then produce magnetic anomalies, depending on the thickness of those layers and the amount of magnetite present. In the two-dimensional forward model with constant magnetizations, the layer 2 thicknesses can be modeled as thick as 5 km beneath the two rough basement topography zones under the HFMAH where a thinned layer 3 is required to fit with the observed flat Moho reflector (Fig. 3B).  Thinned or missing layer 3 has been observed in ultraslow spreading environments (e.g., Gakkel Ridge, Jokat et al., 2003) with half spreading rate of 6-7 mm/yr (Dick et al., 2003). However, the wide range of half spreading rates of 4-18  !??!mm/yr compiled by recent researchers in Labails et al. (2010) makes it difficult to support a scenario of the initiation and/or succession of ultraslow spreading periods as the source of the HFMAH. If more M-series lineation could be interpreted within the JQZ, the half spreading rate could be calculated with higher accuracy and this possibility tested further.In the constant layer 2 thickness model, the highest magnetizations (4 A/m) are located directly under the two rough basement topography zones beneath the HFMAH (Fig. 3A). The highest magnetizations (3.5-3 A/m) are similarly situated in the final forward model with variable layer 2 thicknesses that are capped at 2.5 km for the MAR based on Purdy et al. (1992). Alteration processes in the crust may explain the differences between the adjacent magnetization bodies, which could result in the two peaks observed in the HFMAH. The rough basement topography associated with intermediate to slow spreading rates (Macdonald, 1986; Buck et al., 2005) is caused by short wavelength faulting which enhances hydrothermal circulation and penetrates to greater depth, (e.g., Karson et al., 2002) resulting in a decrease in the remnant magnetization and magnetic anomaly observed. In layer 2, high-temperature, pervasive or long-term, low-temperature (Johnson and Pariso, 1993) modes of hydrothermal circulation can decrease crustal magnetization. Arguably, a known process to form strongly magnetized bodies by hydrothermal circulation can be attributed to serpentinization of uplifted mantle rocks (Oufi et al., 2002; Maffione et al., 2014). However, that would require an uplifted mantle and corresponding Moho reflector, which is not observed in the MCS data.   !?@!What possible scenario could explain the difference between the remnant magnetization of the crust underlying the HFMAH versus the surrounding crust? In order to answer this question, other regions characterized by unusual magnetic anomaly patterns, such as the Cocos-Nazca Ridge near the Galapagos, the Juan de Fuca Ridge and the East Pacific Rise spreading centers were investigated (Hey, 1977; Hey et al., 1980; Sinton et al., 1983; Wanless et al., 2012).  These areas are characterized by magnetic anomaly high patterns that are oriented obliquely to the spreading direction, similar to the HFMAH (Fig. 5; Fig 6). This distinctive magnetic anomaly pattern has been interpreted as a consequence of a propagating ridge segment. A propagating ridge segment occurs when a growing spreading center is attached to a dying spreading center by transform fault. As the growing spreading center advances, the transform fault becomes inactive and it, along with a segment of the dying spreading center is added to the surrounding oceanic lithosphere. The zone of active transform faulting moves to the tip of the propagating ridge segment. This process is repeated and the growing spreading center gets longer and the dying spreading center shrinks. The propagating rift hypothesis has been tested in plate reconstructions of the Cocos-Nazca Ridge (Fig. 6) and the Juan de Fuca Ridge (Fig. 5) to reproduce the observed magnetic anomaly pattern in those areas. I also examined modern analogues of short-lived pseudo-faults with discontinuous propagation geometry (Hey, 1977) located in between Molokai and Murray Fracture Zones (Fig. 8). Neither recent high-resolution seafloor bathymetry (Amante and Eakins, 2009) nor satellite-based gravity anomalies (and vertical gravity gradient) (Sandwell et al., 2014) show indications of additional volcanic material, similar to the HFMAH. However, the EMAG2 magnetic anomaly grid clearly exhibits numerous short-lived transform faults or  !?A!pseudo-faults offsetting isochrons associated with propagating rifts (Fig. 8) Keeping the observation that margin-wide tectonic change had to have occurred in the JQZ in mind, it is possible that the HFMAHÕs structural features and strongly magnetized bodies represent a consequence of a short-lived propagating ridge segment.   In order to test the feasibility of this hypothesis, a propagating rift was drawn for the early Atlantic opening reconstruction of Labails et al. (2010) (Fig. 7C) and was compared to the seafloor age grid of Mueller et al. (2008) and the location of prominent magnetic anomalies observed in the EMAG2 grid (Fig. 7B). Additionally the GPlates software (Williams et al., 2012) and datasets were used to reconstruct back to 154 Ma to compare the magnetic anomaly pattern on both the ENAM and NWAM (Fig. 7A). Using this reconstruction, we observe a magnetic anomaly high on the African side that can be interpreted as the conjugate magnetic anomaly high to the HFMAH (Fig. 7). Similar to the WACMA and ABSMA, the conjugate to the HFMAH is lower amplitude than its counterpart on the ENAM.  The cartoon illustrated in (Fig. 9) shows that the propagating rift scenario fits within the framework of current plate reconstructions, and the HFMAH and its conjugate anomaly fall into the propagator wake left by the propagating ridge segment. In this cartoon, the HFMAH is hypothesized to be oriented along the trend of the pseudo-faults between the propagating mid ocean ridge segment and the retreating mid ocean ridge segment (Fig. 9). This is supported by the location of rough basement topography as observed in the MCS data, which could possibly represent the surface expression of the proposed pseudo-fault (Fig. 9).   !?B!A series of pseudo-fault segments can often be associated with unusually high amplitude magnetic anomalies, correlated to basalts enriched in iron (Fe) and titanium (Ti) (Hey et al., 1980) that create abundant single domain magnetite (Horen and Fleutelot, 1998). This is thought to be the result of renewed crystal fractionation resulting from the propagating rift breaking through older, thicker crust (Hey et al., 1980; Sinton et al., 1983; Wanless et al., 2012) (Fig. 7C). The concomitance of the HFMAH basement topography and magnetic signature remarkably fits the characters of the propagating segment. The modeled thickened layer 2 (Fig. 3) beneath the two rough basement topography zones could represent the igneous alteration commonly observed at propagating rift tips such as on the Cocos-Nazca spreading center (Hey et al., 1980; Hey and Wilson, 1995) and the East Pacific Rise spreading center (Roland et al., 2012). The oblique, ellipsoidal magnetic anomaly highs observed in the EMAG2 grid (Fig. 9) and the new shipboard magnetic dataset in our survey area are similar to magnetic anomalies associated with propagating rifts (e.g. Hey et al., 1980). The observation that the location of the propagating rift lines up with the bend observed in the ECMA at New York-New Jersey bight could possibly be representing a tectonic corridor of preexisting weakness (Fig. 7), maybe a fabric inherited from the Appalachian mountain bend, that could be investigated in future plate tectonic studies of the ENAM.  However, the ultimate origin of the HFMAH will be left until the magnetic signal is ground-truthed by rock samples recovered from drilling the oceanic crust on the ENAM. Nevertheless, associating the origin of HFMAH to the strongly magnetized crust formed by a relatively short-lived propagating rift segment during the early Atlantic opening between BSMA (~170 Ma) and M25 (~154 Ma) sheds new light on the formation and evolution of ENAM. The  !?C!magnetic anomaly pattern indicated by the HFMAH and its conjugate suggest that the propagating rift segment progressed from the south to the north, with the tip of the propagating segment penetrating oceanic crust emplaced ~5 Ma previously (Fig. 7).    CONCLUSION!Newly acquired high-resolution sea surface magnetic anomaly data were compared to forward model solutions using new multi-channel seismic (MCS) as a constraint on source geometry. From the magnetic forward models results, a structural origin of HFMAH can be ruled out while an origin due to the difference in remnant magnetization in adjacent blocks of oceanic crust is supported. The most likely source of the difference in remnant magnetization is as the result of a propagating rift that proceeded from south to north of the New York-New Jersey bight. The magnetic anomaly pattern left in the wake of the propagating spreading ridge center is interpreted to have been caused by increased fractionation of the oceanic crust, resulting in additional Fe-Ti rich basalts to be erupted in layer 2 of the oceanic lithosphere. Future investigations of the ENAM can test this hypothesis by identifying more M-series lineation in the JQZ or by obtaining rock samples of the oceanic crust.  !?D!                  APPENDIX                      APPENDIX !?E!                  Table 1. Multi-Channel Seismic Acquisition Parameters Cruise Acquistion ID # channels  Shot interval (m) Sample interval (ms) Record Length (s) MGL1407 Seq 1 480 25 1 9.5 Seq 2-7 636 50 2 20 Seq 8-17 636 50 2 18 Seq 18-20 636 50 1 18        !"#!$%&'()!"*!(A) Magnetic anomaly map of the ENAM with prominent features indicated (Maus et al., 2009).  ECMA: East Coast Magnetic Anomaly. BMA: Brunswick Magnetic Anomaly. BSMA: Blake Spur Magnetic Anomaly. JQZ: Jurassic magnetic Quiet Zone. NESC: New England Seamount chain. Thick lines parallel to margin are M-series isochron. M25: 156.6 Ma; M21: 149.9 Ma; M16: 141.9 Ma; M10: 131.7 Ma; M4: 126.5; Anomaly 100: 118.7 Ma (Muller et al., 1997). Bathymetry indicated by thin black contours: annotated and contoured at 500 m intervals (Amante and Eakins, 2009). Black lines running perpendicular to the margin are fracture zone locations from (Klitgord and Schouten, 1986) thick are major, thin are minor. White box indicates the region containing the Hudson Fan Magnetic Anomaly High (HFMAH). (B) Magnetic anomaly map with reduced scale for ENAM, which more accurately represents the amplitude and scale of the HFMAH and itÕs situation within the Jurassic Quiet Zone (JQZ). (C) Gravity anomaly map of the ENAM (Sandwell et al., 2014). White box indicates the region containing the HFMAH; note that there is no significant anomaly observed beneath the HFMAH.  !"#! Figure 2: (A) Location of MGL1407 seismic reflection survey lines in the area of the HFMAH. Bathymetry with contours drawn at 100m and annotated at 200m intervals (Amante and Eakins, 2009). Black lines running perpendicular to the margin are fracture zone locations, thick are major, thin are minor (Klitgord and Schouten 1986). M-series isochron M25 (156.6 Ma) is visible in the lower right. White dots represent DSDP borehole locations in the area. (B) 3D grid view of HFMAH (Maus et al., 2009). Processed sea-surface magnetics data from along MGL1407 cruise is displayed as black wiggles; these are included to compare with the shape of the HFMAH; because of the grid view, they are offset slightly from their track line positions. (C) Bathymetry data in region of HFMAH from NGDC NOAA website.!        !$%!Figure 3: (A: Upper Panel) 2D magnetic anomaly profiles sampled from EMAG2 grid (Maus et al., 2009) represented by dashed blue line and processed sea-surface magnetics data over MGL1407 MCS Lines 2 and 3 represented by solid blue line (see Figure 2A for location map).  Black line represents 2D forward solution for constant layer 2 thicknesses with varying magnetization. (A: Lower Panel) 2D block model used to represent the sedimentary package, layer 2 with a constant thickness of 2.5 km, and a layer 3 extending to the moho. Magnetizations of each 2D block are listed inside each block with units of A/m. (B: Upper Panel) Displays the same information using the same line and color scheme as A: Upper Panel; with the black line representing the 2D forward solution for a constant magnetization and varying layer 2 thickness. !(B: Lower Panel) 2D block model used to represent the sedimentary package, layer 2 with a variable thickness, and a variable thickness layer 3 extending to the moho.   !$"!Figure 3 (contÕd) Constant magnetizations were applied to layer 2 and 3 and are listed inside each block. (C: Upper Panel) Displays the same information using the same line and color scheme as A: Upper Panel; with the black line representing the 2D forward solution for geologically reasonable magnetization based on Johnson and Pariso, (1993) and geologically reasonable layer 2 thicknesses based on Purdy et al., (1992). (C: Lower Panel) 2D block model used to represent the sedimentary package, layer 2 with a variable thickness, and a variable thickness layer 3 extending to the moho. Variable magnetizations of each 2D block are listed inside each block with units of A/m. (D) MGL1407 processed MCS two-way travel time (TWT) data along Lines 2 and 3.  The green horizon below 5.5 seconds TWT is interpreted as the seafloor reflector, the red horizon at approximately 7.5 seconds TWT is interpreted as the basement reflector, and the blue horizon at approximately 10 seconds TWT is interpreted as the moho reflector.  Features of interest in the sedimentary package include the relatively flat seafloor morphology; onlapping strata situated between 140 and 150 km along profile.  Rough basement topography is observed between 110 and 140 km along profile. Also note that the moho reflector is relatively flat along profile. (E) Cartoon of the ENAM along MGL1407 MCS Lines 1, 2 and 3. Location of MCS Lines 2 and 3 are indicated by the black box. SDR: Seaward dipping reflector, basalt layers that are thought to source the East Coast Magnetic Anomaly (ECMA).           !$$!Figure 4: (A1) A sketch of the 3D layer model used to represent ÒnormalÓ oceanic crust based on seismic horizon picks and ETOPO1 grid bathymetry data (Amante and Eakins, 2009). (A2) A sketch of the 3D layer model used to represent ÒfaultedÓ oceanic crust represented by a 1km fault offset of the oceanic crust. (B1) 3D forward solution for ÒnormalÓ oceanic crust layer model (see A1) creates highest peak anomaly of the 3D layer models. (B2) 3D forward solution for ÒfaultedÓ oceanic crust layer model (A2) and is similar in shape to the ÒnormalÓ oceanic crust 3D forward solution, but with a slightly lower anomaly signal. (C) 2D magnetic anomaly profiles sampled from EMAG2 grid (Maus et al., 2009) represented by dashed blue line and processed sea-surface magnetics data over MGL1407 MCS Lines 2 and 3 represented by solid blue line (see Figure 2A for location map). The solid black line represents the 3D forward model solution for the ÒnormalÓ oceanic crust model sampled along ship track. The dashed red line represents the 3D forward model solution for the ÒfaultedÓ oceanic crust model sampled along ship track. Both 3D forward profiles have been scaled up by 30 nT. !"#!!Figure 5: (A) Magnetic anomaly map of the northeast Pacific with prominent features indicated (Maus et al., 2009). (B) Plate boundaries, fracture zones, pseudofaults and failed rifts superimposed over magnetic anomaly pattern, modified from Hey and Wilson (1982).  (C) Pattern of magnetic stripes predicted by model presented in Hey and Wilson (1982) !"$! !Figure 6: (A) Magnetic anomaly map of the Cocos-Nazca ridge with spreading center and transform fault indicated in black (Maus et al., 2009). (B) Interpretive contour of observed magnetic anomalies near the tip of the propagating rift. Normally magnetized crust is stippled; reversely magnetized is white. Dashed lines show track control; thick lines are the active rise axis and transform fault. Modified from Hey et al. (1980). (C) Magnetic anomaly pattern predicted by plate reconstruction model in Hey et al. (1980).   !"%!!!Figure 7: (A) Plate reconstruction of ENAM at M25 time (~154 Ma) using GPlates 1.5 Software and Datasets with EMAG2 magnetic anomaly dataset. (B) Cartoon of opening scenario at M25 (~154 Ma) time based on Labails et al., (2010) with seafloor segments from Mueller et al., (2008). (C) Cartoon of opening scenario at M25 (~154 Ma) time based on Labails et al., (2010) showing illustration of a propagating rift. The HFMAH and its conjugate on the Eastern Atlantic (Pink outlines, line up within the ÒV-patternÓ of the propagating rift. !"#!!Figure 8: (A) The EMAG2 magnetic anomaly grid (Maus et al., 2009) color map and bathymetry contours (Amante and Eakins, 2009) in the region of the Murray and Molokai Fracture Zones. Note the short-lived transform faults (pseudo-faulting) shown between the Murray and Molokai Fracture Zones, as displayed by offset sections of isochrons (thick black lines). Also note magnetic anomaly highs generated due to the oblique pattern of mid-ocean spreading centers due to this pseudo-faulting. (B) The same region with gravity grid (Sandwell et al., 2014). Note how magnetic anomaly highs in (A) are not always associated with gravity highs.   !"$!            !Figure 9: Upper panels indicate the rifting and spreading history of the North American and African continents at (A) ECMA (~190 Ma),  (B) in between BSMA (~170 Ma) and M25 (~154 Ma), and (C) at M25 (~154 Ma) time. Lower panels indicate detailed seafloor spreading and propagation history during the stage B above.  Underlying color map is EMAG2 magnetic anomaly grid (Maus et al., 2009), overlaid with sea surface magnetic anomaly profiles, including the MGL1407 cruise line (blue thick solid line). 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