Christeson, G. L., S. P. S. Gulick, H. J. A. Van Avendonk, L. L. Worthington, R. S. Reece, and T. L. Pavlis, The Yakutat terrane: Dramatic change in crustal thickness across the transition fault, Alaska, Geology, 38, 895-898, 2010, 3 citations, doi:10.1130/G31170.1, #2267 
We present new constraints on the crustal structure of the Yakutat terrane and evidence of the role of the Transition fault in southern Alaska. The Yakutat terrane south of Yakutat Bay includes crystalline crust that is 24ââ¬â27 km thick overlain by sedimentary units that are 4.5ââ¬â7.5 km thick. The Yakutat terrane crustal thickness and velocity structure are consistent with an oceanic plateau origin. The southern edge of the Yakutat terrane is bounded by the Transition fault, which is imaged as a near-vertical fault zone ~1 km wide. The Transition fault is coincident with a dramatic change in Moho depth from 32 km for Yakutat oceanic plateau crust to 11.5 km for Pacific Ocean crust occurring over a horizontal distance of 0ââ¬â5 km. There is no evidence for underthrusting of the Pacific Ocean crust beneath the Yakutat terrane at the Transition fault. We argue that the Yakutat terrane formed on the Kula or Farallon plate and was later juxtaposed next to younger Pacific Ocean crust by the Transition fault.
Cowan, E. A., K. C. Seramur, R. Powell, B. A. Willems, S. P. S. Gulick, and J. Jaeger, Fjords as temporary sediment traps: History of glacial erosion and deposition in Muir Inlet, Glacier Bay National Park, southeastern Alaska, Geol. Soc. Amer. Bull., 122, 1067-1080, 2010, 1 citation, doi:10.1130/B26595.1, #2147
Glacimarine sedimentary deposits within the basins of Muir Inlet, a 48-km-long silled fjord, are interpreted from complimentary sets of high-resolution, seismic-refl ection profiles using known glacial-advance and retreat history. Two prominent glacial erosion surfaces are identifi ed: the lowest attributed to the Last Glacial Maximum (LGM) advance and the upper coincident with the Little Ice Age (LIA) advance. The LGM ice sheet, which advanced onto the continental shelf, was 1700 m thick in Muir Inlet and eroded bedrock, whereas the thinner LIA ice did not. LGM deposits >300 m thick occur beneath the LIA erosion surface in the deepest basins. Evidence for earlier Neo glacial advances is present in subaerial deposits; however, Neoglacial sediments preserved within the marine record are restricted to one depositional package on the entrance sill. Volumes of LIA retreat sediments were calculated within basins. An average annual sediment fl ux was calculated by modeling the duration of sediment contributed from Muir Glacier and from tributary glaciers and side-entry sources. The annual sediment flux ranged from 1.3 ÃÆââ¬â 106 m3/yr to 4.6 ÃÆââ¬â 107 m3/yr and increases logarithmically with increasing drainage basin area, similar to fluvial systems. This sediment flux does not only represent bedrock erosion. Additional sediment is contributed from persistent tributary glaciers and from LGM sediment stored within deeper basins. Basin-wide reflections characterize the most common seismic facies and indicate that strata are horizontal and continuous across each basin, confi rming the importance of sediment gravity fl ows originating from sills and sloping fjord walls.
Dean, S. M., L. C. McNeill, T. J. Henstock, J. M. Bull, S. P. S. Gulick, J. A. Austin, N. L. Bangs, Y. S. Djajadihardja, and H. Permana, Contrasting decollement and prism properties over the Sumatra 2004-2005 earthquake rupture boundary
, Science, 329, 207-210, 2010, 7 citations, doi:10.1126/science.1189373, #2280
Styles of subduction zone deformation and earthquake rupture dynamics are strongly linked, jointly influencing hazard potential. Seismic reflection profiles across the trench west of Sumatra, Indonesia, show differences across the boundary between the major 2004 and 2005 plate interface earthquakes, which exhibited contrasting earthquake rupture and tsunami generation. In the southern part of the 2004 rupture, we interpret a negative-polarity sedimentary reflector ~500 meters above the subducting oceanic basement as the seaward extension of the plate interface. This pred̮̩̉̉collement reflector corresponds to unusual prism structure, morphology, and seismogenic behavior that are absent along the 2005 rupture zone. Although margins like the 2004 rupture zone are globally rare, our results suggest that sediment properties influence earthquake rupture, tsunami hazard, and prism development at subducting plate boundaries.
Goff, J. A., M. A. Allison, and S. P. S. Gulick, Offshore transport of sediment during cyclonic storms: Hurricane Ike (2008), Texas Gulf Coast, USA, Geology, 38, 351-354, 2010, 3 citations, doi:10.1130/G30632.1, #2162
Extreme storms can have a large impact on coastal sediment budgets and the character of strata preserved in the geologic record. Here, we investigate the impact of a cyclonic storm-surge flood and ebb on sediment transport in a microtidal beach barrierâshelf system. Hurricane Ike made landfall on the Texas (United States) coast on 13 September 2008. The accompanying storm surge flooded Galveston Bay with up to 5 m of water above sea level. The surge flood and ebb preferentially flowed over a low-elevation, bay-fronting spit known as the Bolivar Peninsula, destroying buildings and eroding sediments. Surge waters also flowed through Bolivar Roads tidal inlet, the main passageway through the barrier system that separates the gulf and the bay. Bathymetry, Chirp data, and samples were collected in Bolivar Roads tidal inlet 9 to 10 d after the storm, and we compare them here to data collected 4 mo prior. Additional data were collected offshore of Bolivar Peninsula in October 2008. Our results document the dominance of the storm-surge ebb in forcing sediment transport through the inlet, which is not considered in models of beach-barrier evolution. Shoreface sands appear to have been incised by the storm, and advected with beach-barrier sediments sufficiently offshore by the storm-surge ebb that they cannot be reincorporated, indicating a significant loss to the barrier system's sediment budget as a result of a single storm.
Gulick, S. P. S., N. L. Bangs, G. F. Moore, J. Ashi, K. M. Martin, D. S. Sawyer, H. J. Tobin, S. Kuramoto, and A. Taira, Rapid forearc basin uplift and megasplay fault development from 3D seismic images of Nankai margin off Kii Peninsula, Japan, Earth Planet. Sci. Lett., 300, 55-62, 2010, 3 citations, doi:10.1016/j.epsl.2010.09.034, #2283
Offshore Kii Peninsula, Japan, a large thrust within the overriding forearc, the megasplay fault, appears to move coseismically during great earthquakes. 3D seismic images of the Kumano forearc basin that overlies the megasplay, correlated with IODP drilling data, are a potential record of the history of large-scale motion along this structure. In the early Quaternary, uplift occurred in the southwest portion of the basin that may be a preliminary phase of motion along the megasplay. More extensive landward tilting of the outer basin sediments across the seismic volume occurred over ~ 300 kyr in the middle to late Quaternary (1.3ââ¬â1 Ma); this tilting event may represent the major period of motion along the megasplay that formed the modern fault geometry. Extensive normal faulting that cuts the forearc basin sediments clearly formed subsequent to the late Quaternary tilting and in many cases offset the modern seafloor; these faults may form either due to gravitational response to the uplift or as a by-product of sediment underthusting. These results suggest that the megasplay is a recently formed and transient structure and support the idea that out-of-sequence thrusts serving as the dominant structure for convergence-driven shortening in a subduction zone may be short-lived geologically but dominate a margin during these intervals.
Henstock, T., L. M. McNeill, S. M. Dean, P. J. Barton, F. Tilmann, A. Rietbrock, D. Robinson, S. P. S. Gulick, J. A. Austin, D. Natawidjaja, H. Permana, A. Bonneville, F. Lucazeau, M. Weber, and Y. S. Djajadihardja, Exploring structural controls on Sumatran earthquakes, Eos, Trans. Amer. Geophys. Un., 91, 405-406, 2010, doi:10.1029/2010EO440002, #2416
A series of linked marine and land studies have recently targeted the Sumatra subduction zone, focusing on the 2004 and 2005 plate boundary earthquake ruptures in Indonesia. A collaborative research effort by scientists from the United Kingdom (UK Sumatra Consortium), Indonesia, United States, France, and Germany is focusing on imaging the crustal structure of the margin to examine controls on along-strike and updip earthquake rupture propagation. The fundamental science objective is to examine how margin architecture and properties control earthquake rupture location and propagation.
Hornbach, M. J., N. Braudy, R. W. Briggs, M.-H. Cormier, M. B. Davis, J. B. Diebold, N. Dieudonne, R. Douilly, C. Frohlich, S. P. S. Gulick, H. E. Johnson, P. Mann, C. M. G. McHugh, K. Ryan-Mishkin, C. S. Prentice, L. Seeber, C. C. Sorlien, M. S. Steckler, S. J. Symithe, F. W. Taylor, and J. Templeton, High tsunami frequency as a result of combined strike-slip faulting and coastal landslides, Nature Geoscience, 3, 783-788, 2010, 5 citations, doi:10.1038/NGEO975, #2281
Earthquakes on strike-slip faults can produce devastating natural hazards. However, because they consist predominantly of lateral motion, these faults are rarely associated with significant uplift or tsunami generation. And although submarine slides can generate tsunami, only a few per cent of all tsunami are believed to be triggered in this way. The 12 January Mw 7.0 Haiti earthquake exhibited primarily strike-slip motion but nevertheless generated a tsunami. Here we present data from a comprehensive field survey that covered the onshore and offshore area around the epicentre to document that modest uplift together with slope failure caused tsunamigenesis. Submarine landslides caused the most severe tsunami locally. Our analysis suggests that slide-generated tsunami occur an order-of-magnitude more frequently along the Gonave microplate than global estimates predict. Uplift was generated because of the earthquake's location, where the Caribbean and Gonave microplates collide obliquely. The earthquake also caused liquefaction at several river deltas that prograde rapidly and are prone to failure. We conclude that coastal strike-slip fault systems such as the Enriquillo-Plantain Garden fault produce relief conducive to rapid sedimentation, erosion and slope failure, so that even modest predominantly strike-slip earthquakes can cause potentially catastrophic slide-generated tsunami, a risk that is underestimated at present.
Martin, K. M., S. P. S. Gulick, N. L. Bangs, G. F. Moore, J. Ashi, J.-O. Park, S. Kuramoto, and A. Taira, Possible strain partitioning structure between the Kumano fore-arc basin and the slope of the Nankai Trough accretionary prism, Geochem., Geophys., Geosyst., 11, Q0AD02, 2010, 10 citations, doi:10.1029/2009GC002668, #2185
A 12 km wide, 56 km long, three-dimensional (3-D) seismic volume acquired over the Nankai Trough offshore the Kii Peninsula, Japan, images the accretionary prism, fore-arc basin, and subducting Philippine Sea Plate. We have analyzed an unusual, trench-parallel depression (a ââ¬Ånotchââ¬Â) along the seaward edge of the fore-arc Kumano Basin, just landward of the megasplay fault system. This bathymetric feature varies along strike, from a single, steep-walled, ∼3.5 km wide notch in the northeast to a broader, ∼5 km wide zone with several shallower linear depressions in the southwest. Below the notch we found both vertical faults and faults which dip toward the central axis of the depression. Dipping faults appear to have normal offset, consistent with the extension required to form a bathymetric low. Some of these dipping faults may join the central vertical fault(s) at depth, creating apparent flower structures. Offset on the vertical faults is difficult to determine, but the along-strike geometry of these faults makes predominantly normal or thrust motion unlikely. We conclude, therefore, that the notch feature is the bathymetric expression of a transtensional fault system. By considering only the along-strike variability of the megasplay fault, we could not explain a transform feature at the scale of the notch. Strike-slip faulting at the seaward edge of fore-arc basins is also observed in Sumatra and is there attributed to strain partitioning due to oblique convergence. The wedge and décollement strength variations which control the location of the fore-arc basins may therefore play a role in the position where an along-strike component of strain is localized. While the obliquity of convergence in the Nankai Trough is comparatively small (∼15ð), we believe it generated the Kumano Basin Edge Fault Zone, which has implications for interpreting local measured stress orientations and suggests potential locations for strain-partitioning-related deformation in other subduction zones.
Schulte, P., L. Alegret, I. Arenillas, J. Antonio Arz, P. R. Barton, P. R. Bown, T. J. Bralower, G. L. Christeson, P. Claeys, C. S. Cockell, G. S. Collins, A. Deutsch, T. J. Goldin, K. Goto, J. M. Grajales-Nishimura, R. A. F. Grieve, S. P. S. Gulick, K. R. Johnson, W. Kiessling, C. Koeberl, D. A. Kring, K. G. MacLeod, T. Matsui, J. Melosh, A. Montanari, C. R. Neal, D. J. Nichols, R. D. Norris, E. Pierazzo, G. Ravizza, M. Rebolledo-Vieyra, W. U. Reimold, E. Robin, T. Salge, R. P. Speijer, A. R. Sweet, J. Urrutia-Fucugauchi, V. Vajda, M. T. Whalen, and P. S. Willumsen, The Chicxulub asteroid impact and mass extinction at the Cretaceous-Paleogene boundary, Science, 327, 1214-1218, 2010, 48 citations, doi:10.1126/science.1177265, #2184
The Cretaceous-Paleogene boundary ~65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction.
Worthington, L. L., S. P. S. Gulick, and T. L. Pavlis, Coupled stratigraphic and structural evolution of a glaciated orogenic wedge, offshore St. Elias orogen, Alaska, Tectonics, 29, TC6013, 2010, 1 citation, doi:10.1029/2010TC002723, #2279
The St. Elias orogen is the result of ~10 Myr of oblique convergence and flat-slab subduction in the Gulf of Alaska between North America and the Yakutat microplate. Extensive glaciation and a complex tectonic environment make this region a unique case study in which to examine the details of terrane accretion and the possible coupled influence of climate and tectonic drivers on the structural and topographic evolution of an orogenic wedge. Reflection seismic profiles across the offshore Pamplona zone fold-thrust belt, the frontal St. Elias orogenic wedge, provide constraints for quantifying Pleistocene deformation recorded in the glaciomarine Yakataga formation. The total amount of Pleistocene shortening observed varies from ~3 to 5 mm/yr, compared to the current GPS-derived Yakutat-North America convergence rate across the St. Elias orogen of ~45 mm/yr. Growth strata and kinematic fold analysis allow comparison of relative timing of fault activity, which reveals temporal and spatial shifting of active deformation during the glacial period: faulting localized adjacent to the coastline and at the current submarine deformation front. The abandoned, currently inactive region is colocated with the major glacial depocenter in the region, the Bering Trough. These observations imply that glacial processes such as sediment loading and focused erosion during advance-retreat cycles has a direct effect on the evolution of individual faults within the Pamplona zone and the overall deformation pattern in the offshore St. Elias margin. This information provides key constraints for understanding how climatic shifts may have affected the evolution of margin architecture during Pleistocene glacial-interglacial periods.
Bangs, N. L., G. F. Moore, S. P. S. Gulick, E. M. Pangborn, H. J. Tobin, S. Kuramoto, and A. Taira, Broad, weak regions of the Nankai megathrust and implications for shallow coseismic slip, Earth Planet. Sci. Lett., 284, 44-49, 2009, 18 citations, doi:10.1016/j.epsl.2009.04.026, #2073
Deep within the Nankai Trough subduction zone the plate-boundary thrust slips along a well-imaged megasplay fault system during the megathrust earthquakes that regularly strike southwest Japan. The routing of the active plate-boundary thrust along an upward-branching splay fault causes deep underthrusting of an unusually thick section of material attached to the subducting ocean crust. Here we present three-dimensional seismic reflection data that shows this unusually thick section is fluid-rich sediment that results in broad, weakly-coupled regions of the megathrust down into the updip end of the seismogenic zone. The weakly coupled regions lie above an underthrust low seismic impedance (presumed to be low-velocity and low-density) sediment section that is between one and two kilometers thick and covers ~ 3300 km2, at least one-eighth of the total rupture area of the 1944 Mw 8.1 Tonankai earthquake. This underthrust sediment section is broader and much deeper than inferred at other margins. Sediment underthrusting into the normally well-coupled seismogenic zone likely releases fluids and elevates fluid pressure, which reduces inter-plate coupling along portions of the megasplay fault and allows coseismic rupture to propagate to unusually shallow depths and generate large tsunami as inferred for the 1944 Tonankai event. Splay faults may be a common, yet transient mechanism for developing weak subduction zone thrusts.
Christeson, G. L., G. S. Collins, J. V. Morgan, S. P. S. Gulick, P. J. Barton, and M. R. Warner, Mantle deformation beneath the Chicxulub impact crater, Earth Planet. Sci. Lett., 284, 249-257, 2009, 2 citations, doi:10.1016/j.epsl.2009.04.033, #2068
The surface expression of impact craters is well-known from visual images of the Moon, Venus, and other planets and planetary bodies, but constraints on deep structure of these craters is largely limited to interpretations of gravity data. Although the gravity models are non-unique, they do suggest that large impact craters are associated with structure at the base of the crust. We use seismic data to map Moho (crustâmantle interface) topography beneath the Chicxulub crater, the youngest and best preserved of the three largest known terrestrial impact craters. The Moho is upwarped by ~ 1.5â2 km near the center of the Chicxulub crater, and depressed by ~ 0.5â1.0 km at a distance of ~ 30â55 km from the crater center. A comparison with numerical modeling results reveal that immediately following impact a transient crater reached a maximum depth of at least 30 km prior to collapse, and that subsequent collapse of the transient crater uplifted target material from deep below the crater floor. These results demonstrate that deformation from large terrestrial impacts can extend to the base of the continental crust. A similar Moho topography is also modeled for some large lunar and Martian craters, which suggests that mantle deformation may play a prominent role in large crater formation.
Moore, G. F., J.-O. Park, N. L. Bangs, S. P. S. Gulick, H. J. Tobin, Y. Nakamura, S. Sato, T. Tsuji, T. Yoro, H. Tanaka, S. Uraki, Y. Kido, Y. Sanada, S. Kuramoto, and A. Taira, Structural and seismic stratigraphic framework of the NanTroSEIZE Stage 1 transect, In Kinoshita, M., H. Tobin, J. Ashi, G. Kimura, S. Lallement, E. J. Screaton, D. Curewitz, H. Masago, K. T. Moe, and the Expedition 314/315/316 scientists, Proc. Int. Ocean Drilling Prog., 314/315/316, 2009, doi:10.2204/iodp.proc.314315316.102.2009, #2051
The location of the Integrated Ocean Drilling Program's (IODP) Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) was based on regional two-dimensional seismic reflection surveys carried out by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). Final site locations were chosen based on a three-dimensional (3-D) seismic reflection survey acquired across the seaward margin of Kumano Basin and the accretionary prism from the basin to the deformation front. This survey covered a region 12 km wide (approximately parallel to the regional structural strike) and 56 km long (approximately perpendicular to the regional strike) and provided detailed images of the structure and seismic stratigraphy of the drill sites. Sites were drilled in the frontal thrust zone at the toe of the accretionary prism, the frontal region of the megasplay fault zone, and the forearc basin. The 3-D seismic data volume images a main frontal thrust at the prism toe with the hanging wall thrust at least 7.5 km seaward over the trench. This configuration is different from that in other parts of the Nankai prism. At the shallow end of the megasplay, the data images a complex thrust system that truncates older structures in the underlying accretionary prism and shows that the hanging wall block has overridden more than 1250 m of young slope sediments. At the forearc basin site, we interpret landward-dipping forearc basin strata onlapping older slope sediments, which in turn overlie an older part of the accretionary prism.
Berger, A. L., S. P. S. Gulick, J. Spotila, P. Upton, J. Jaeger, J. B. Chapman, L. L. Worthington, T. L. Pavlis, K. Ridgway, B. A. Willems, and R. J. McAleer, Quaternary tectonic response to intensified glacial erosion in an orogenic wedge, Nature Geoscience, 1, 793-799, 2008, 21 citations, doi:10.1038/ngeo334, #2024
Active orogens are thought to behave as internally deforming critical-taper wedges that are in rough long-term equilibrium with tectonic influx and erosional outflux. Spatial and temporal variations in climate are therefore hypothesized to have a significant influence on denudation, topography and deformation of orogens, thereby affecting wedge taper. However, the impact of the most severe transition in Northern Hemisphere climate during the Cenozoic eraâthe onset of glaciationâhas hitherto not been empirically documented. Here we analyse the spatial patterns of denudation and deformation, and their temporal variations, in the heavily glaciated St Elias orogen in southern Alaska. Low-temperature thermochronometry, thermokinematic modelling and offshore seismic reflection and borehole data suggest that the global-scale intensification of glaciation in the middle Pleistocene epoch enhanced glacier growth and caused ice streams to advance to the edge of the continental shelf. This led to focused denudation across the subaerial reaches of the orogen and burial of the actively deforming wedge toe by the eroded sediment. We propose that this climatically driven mass redistribution forced a structural reorganization of the orogen to maintain critical taper. Our empirical results thus support decades of numerical model predictions of orogenesis and provide compelling field evidence for the significant impact of climate change on tectonics.
Chapman, J. B., T. L. Pavlis, S. P. S. Gulick, A. Burger, L. A. Lowe, J. Spotila, R. L. Bruhn, M. Vorkink, P. Koons, A. K. Barker, K. Picornel, K. Ridgeway, B. Hallet, J. Jaeger, and J. McAlpin, Neotectonics of the Yakutat collision: Changes in deformation driven by mass redistribution, in Active Tectonics and Seismic Potential of Alaska, edited by J. T. Freymueller, P. J. Haeussler, R. L. Wesson, and G. Ekstrom, 350 pp., Geophysical Monograph Series, Volume, 179, 2008, #1935
Collins, G. S., J. V. Morgan, P. J. Barton, G. L. Christeson, S. P. S. Gulick, J. Urrutia-Fucugauchi, M. R. Warner, and K. Wunnemann, Dynamic modeling suggests terrace zone asymmetry in the Chicxulub crater is caused by target heterogeneity, Earth Planet. Sci. Lett., 270, 221-230, 2008, 20 citations, doi:10.1016/j.epsl.2008.03.032, #1934
We investigate the cause of terrace zone asymmetry in the Chicxulub impact crater using dynamic models of crater formation. Marine seismic data acquired across the crater show that the geometry of the crater's terrace zone, a series of sedimentary megablocks that slumped into the crater from the crater rim, varies significantly around the offshore half of the crater. The seismic data also reveal that, at the time of impact, both the water depth and sediment thickness varied with azimuth around the impact site. To test whether the observed heterogeneity in the pre-impact target might have affected terrace zone geometry we constructed two end-member models of upper-target structure at Chicxulub, based on the seismic data at different azimuths. One model, representing the northwest sector, had no water layer and a 3-km thick sediment layer; the other model, representing the northeast sector, had a 2-km water layer above a 4-km sediment layer. Numerical models of vertical impacts into these two targets produced final craters that differ substantially in terrace zone geometry, suggesting that the initial water depth and sediment thickness variations affected the structure of the terrace zone at Chicxulub. Moreover, the differences in terrace zone geometry between the two numerical models are consistent with the observed differences in the geometry of the terrace zone at different azimuths around the Chicxulub crater. We conclude that asymmetry in the pre-impact target rocks at Chicxulub is likely to be the primary cause of asymmetry in the terrace zone.
Gulick, S. P. S., P. J. Barton, G. L. Christeson, J. V. Morgan, M. A. McDonald, K. Mendoza-Cervantes, Z. F. Pearson, A. T. Surendra, J. Urrutia-Fucugauchi, P. M. Vermeesch, and M. R. Warner, Importance of pre-impact crustal structure for the asymmetry of the Chicxulub impact crater, Nature Geoscience, 1, 131-135, 2008, 31 citations, doi:10.1038/ngeo103, #1889
Impact craters are observed on the surfaces of all rocky planets and satellites in our Solar System1; some impacts on Earth, such as the Cretaceous/Tertiary one that formed the Chicxulub impact crater2, 3, have been implicated in mass extinctions4, 5, 6, 7, 8, 9, 10, 11, 12. The direction and angle of the impactâor its trajectoryâis an important determinant of the severity of the consequent environmental damage, both in the downrange direction (direction bolide travels) and in the amount of material that enters the plume of material vaporized on impact2, 13, 14, 15. The trajectory of the Chicxulub impact has previously been inferred largely from asymmetries in the gravity anomalies over the crater2, 3. Here, we use seismic data to image the Chicxulub crater in three dimensions and demonstrate that the strong asymmetry of its subsurface correlates with significant pre-existing undulations on the end-Cretaceous continental shelf that was the site of this impact. These results suggest that for rocky planets, geological and geomorphological heterogeneities at the target site may play an important role in determining impact crater structure, in addition to impact trajectories. In those cases where heterogeneous targets are inferred, deciphering impact trajectories from final crater geometries alone may be difficult and require further data such as the distribution of ejecta.
McDonald, M. A., H. J. Melosh, and S. P. S. Gulick, Oblique impacts and peak ring position: Venus and Chicxulub, Geophys. Res. Lett., 35, L07203, 2008, 7 citations, doi:10.1029/2008GL033346, #1979
The circular peak ring of the Chicxulub impact crater is not concentric with the outer rim of the structure, as defined by both gravity and seismic data. Although this has been attributed to an oblique impact, and used to define the direction of impactor approach, different groups have suggested radically different approach azimuths based on the same data sets. We investigate the putative correlation between peak ring offset and impactor azimuth by examining a suite of 19 peak ring craters on Venus, for which the impactor's approach direction is known from their asymmetric external ejecta. We find no statistically significant correlation between peak ring offset and the azimuth of the impactor's approach.
Mosher, D. C., J. A. Austin, D. Fisher, and S. P. S. Gulick, Deformation of the northern Sumatra accretionary prism from high-resolution reflection profiles and ROV observations, Marine Geol., 252, 89-99, 2008, 11 citations, doi:10.1016/j.margeo.2008.03.014, #1983
Worthington, L. L., S. P. S. Gulick, T. L. Pavlis, and R. Bruns, Localized deformation zones in the offshore leading edge of the Yakutat microplate, in Gulf of Alaska Active Tectonics and Seismic Potential of Alaska, edited by J. T. Freymueller, P. J. Haessler, R. L. Wesson and G. Ekstrom, Geophys. Monograph Ser., 179, 2008, #2128
Fisher, D., D. C. Mosher, J. A. Austin, S. P. S. Gulick, T. MasterLark, and K. Moran, Active deformation across the Sumatran forearc over the December 2004 Mw9.2 rupture, Geology, 35, 99-102, 2007, 10 citations, doi:10.1130/G22993A.1, #1831
A 220-km-long, single-channel seismic reflection profile crosses the northern Sumatra margin and presumed rupture zone of the December 2004 Mw9.2 tsunamigenic earthquake and images active deformation across the forearc. At the largest wavelength (tens of kilometers), the forearc surface is defined by a steep, 55-km-wide outer slope, a 110-km-wide upper slope forming a broad depression between two forearc highs, and a 25-km-wide steep inner slope between the landward high and forearc basin. Superimposed on these prism-wide variations are anticlinal ridges spaced ∼13 km apart; the inner and outer slopes are characterized by landward and seaward fold vergence, respectively. Between anticlines, growth strata deposited in slope basins are folded at ∼2â3 km wavelengths. These small folds deform the seafloor and increase in amplitude with depth, verging toward anticlinal hinges. We suggest that long-wavelength variations are consistent with variations in strength across the forearc. The ∼13 km anticline spacing implies deformation of a slope apron that deforms independently of a stronger wedge interior. Growth strata geometries indicate ongoing deformation within individual basins. Our model for prism architecture suggests that the wedge interior advances during great earthquakes like the 2004 Mw9.2 event, peeling up shallower and less competent trench fill, deforming the toe and the upper slope of the forearc, and producing seabottom uplift responsible for the tsunami.
Gulick, S. P. S., L. A. Lowe, T. L. Pavlis, J. V. Gardner, and L. A. Mayer, Geophysical insights into the transition fault debate: Propagating strike slip in response to stalling Yakutak block subduction in the Gulf of Alaska, Geology, 35, 763-766, 2007, 18 citations, doi:10.1130/G23585A, #1767
On the basis of faulting mapped on seismic reflection and bathymetric data, seismicity, current plate motions, and evidence that the Yakutat block may be anomalously thick, we propose a tectonic model for Yakutat-Pacific interactions, including the often-debated Transition fault. To the east, deformation associated with the Queen CharlotteâFairweather fault system is extending offshore, facilitating westward propagation of strike-slip motion along the eastern segment of the Transition fault. To the west, the oblique-slip Pamplona zone and Transition faults merge at an embayment in the continental margin, where a north-south dextral strike-slip fault within the Pacific plate, illuminated by the 1987â1992 earthquake swarm, intersects the Pacific-Yakutat tectonic boundary. These fault patterns are consistent with modern plate motions and reflect a plate boundary reorganization that may be caused by resistance to subduction by the Yakutat block, a possible moderate-sized oceanic plateau.
Bangs, N. L., S. P. S. Gulick, and T. H. Shipley, Seamount subduction erosion in the Nankai Trough and its potential impact on the seismogenic zone, Geology, 34, 701-704, 2006, 32 citations, doi:10.1130/G22451.1, #1808
Seamount subduction along subduction-zone plate boundary thrusts has long been implicated as a mechanism for abrasion and tectonic erosion of the base of the overriding plate. However, tectonic erosion processes have not been examined in detail with high-quality three-dimensional (3-D) seismic reflection imaging. In 1999 we acquired 3-D seismic reflection data from the Nankai Trough subduction zone to image the plate boundary fault and the overlying accretionary wedge structure. Fortuitously, these data reveal a small (to 1 km high) basement ridge that has subducted to 7 km subseafloor. Updip from the basement ridge, a 1-km-thick sequence of sediment from the base of the accretionary wedge appears to be missing. We interpret these data as evidence for tectonic erosion of the base of the accretionary wedge following the basement ridge subduction. Tectonic erosion has removed more than 25 km3 from the updip edge of the seismogenic zone and carried it down into the seismogenic zone. The tectonically eroded sediments are presumed to enhance fault-zone fluid content, potentially reducing fault-zone effective stress, and may temporarily inhibit earthquake rupture potential. After the passage of the ridge the boundary fault returns to its former position in a period of enhanced underplating.
Nordfjord, S., J. A. Goff, J. A. Austin, and S. P. S. Gulick, Seismic facies of incised-valley fills, New Jersey continental shelf: Implications for erosion and preservation processes acting during latest Pleistocene-Holocene transgression, J. Sedimentary Res., 76, 1284-1303, 2006, 14 citations, doi:10.2110/jsr.2006.108, #1819
Incised-valley fills shallowly buried beneath the New Jersey middleâouter shelf reveal a retrogradational shift of four seismic facies, as observed in 1â4 kHz deep-towed chirp seismic data. These facies, the only preserved stratigraphic record of the latest QuaternaryâHolocene drowning and infilling of fluvial drainage systems developed on this exposed shelf at or near the Last Glacial Maximum (LGM), are interpreted as (1) fluvial lag deposits, SF1; (2) estuarine mixed sand and muds, SF2; (3) estuary central bay muds, SF3; and (4) redistributed estuary-mouth sands, SF4. These fills are truncated by a transgressive ravinement, the T horizon, which is in turn overlain by Holocene marine sand deposits. The seismic facies are bounded by reflectors marking either source diastems or unconformities: (1) the Channels horizon is the lowstand fluvial incision surface, (2) B1 is a bay flooding surface, (3) B2 is an intra-estuarine depositional surface, (4) B3 is a tidal ravinement surface, and (5) the T horizon represents erosion at or near the shoreface during Holocene transgression. The Channels horizon is generally preserved only in valley axes. Elsewhere, this sequence boundary has been modified by surfaces B1 and/or B3. Dip-oriented changes in the thickness of SF3 and SF4 suggest either a stillstand in the passage of the shoreline, which allowed such spatial variations, or that local valley shape controlled hydrodynamic conditions for sediment transport and deposition. Narrower valleys may have promoted tidally dominated, fine-grained deposition within these drowning estuaries, while broader valleys attenuated tidal flow velocities and allowed the filling of the estuary to be dominated by wave and current energy, promoting more coarse-grained deposition. Our study demonstrates that wave- and tide-dominated estuarine facies can coexist within such fill strata.
Bangs, N. L., the Nankai 3-D Working Group, T. H. Shipley, G. F. Moore, C. Moore, S. P. S. Gulick, S. Kuramoto, Y. Nakamura, and J.-O. Park, The 3-D architecture of the Nankai trough accretionary wedge and the development of the seismogenic zone: Perspectives on 3-D seismic reflection profiling in academia, Margins Newsletter, 14, 2005, #1779
Goff, J. A., J. A. Austin, S. P. S. Gulick, S. Nordfjord, B. Christensen, C. K. Sommerfield, H. C. Olson, and C. R. Alexander, Recent and modern marine erosion on the New Jersey outer shelf, Marine Geol., 216, 275-296, 2005, 22 citations, doi:10.1016/j.margeo.2005.02.015, #1756
Goldberg, D. S., A. Cheng, S. P. S. Gulick, J. Blanch, and J.-M. Byun, Velocity analysis of LWD sonic data in turbidites and hemipelagic sediments offshore Japan, ODP sites 1173 and 808, Proc. Ocean Drill. Prog., Sci. Results, 190/196, 2005, #1663
Gulick, S. P. S., J. A. Goff, J. A. Austin, C. R. Alexander, S. Nordfjord, and C. S. Fulthorpe, Basal inflection-controlled shelf-edge wedges off New Jersey track sea-level fall, Geology, 33, 429-432, 2005, 13 citations, doi:10.1130/G21310.1, #1741
A key marker for examining the latest Pleistocene history of the New Jersey margin is the high-amplitude, long-recognized R-horizon reflector. This time-transgressive surface was formed 47â 33 ka and represents the integrated topographic and bathymetric surface developed during the complicated sea-level oscillation associated with the regression that preceded the Last Glacial Maximum (LGM). From landward to seaward across the outer shelf, the R reflector changes from subhorizontal, in most locations <9 m beneath the modern seafloor, to seaward dipping, forming the base of two offlapping sediment wedges: the previously described outer-shelf wedge and a deep-shelf wedge seaward of it. This transition occurs across two inflection zones, where the dip of the R reflector steepens seaward, that can be traced for tens of kilometers along strike and mark the landward limits of these wedges. These inflection zones are possibly former wave-dominated shorefaces; these represent the primary topographic elements present during the last regression. We speculate that these inflections dictated both the locations for deposition of the two prograding, offlapping wedges that developed during the complex sea-level fall prior to the LGM and their successive erosion before and after the LGM. We suggest that such inflection zones and their associated wedges are important markers of regression in clastic-dominated outer-shelf settings along passive margins.
Morgan, J. V., M. R. Warner, J. Urrutia-Fucugauchi, S. P. S. Gulick, G. L. Christeson, P. J. Barton, M. Rebolledo-Vieyra, and H. J. Melosh, Chicxulub crater seismic survey prepares way for future drilling, Eos, Trans. Amer. Geophys. Un., 86, 325-328, 2005, doi:10.1029/2005EO360001, #1786
Sixty-five million years ago, a large meteorite hit the Earth and formed the Ë200-km-wide Chicxulub crater in Yucatán, Mexico. The well-known, massive extinction event at the Cretaceous-Tertiary (K-T) boundary appears to have been caused, at least in part, by this impact. In the first few seconds after impact the surface of the Earth was pushed down to form a cavity Ë35 km deep, and in the next few hundred seconds this cavity collapsed to form a multi-ring basin with an inner peak ring. To examine the rings and subsurface structure of this superbly preserved impact crater, a seismic experiment was shot across the crater in January and February 2005 by a team of scientists from Mexico, the United States, and the United Kingdom (Figure 1).
Bangs, N. L., T. H. Shipley, S. P. S. Gulick, G. F. Moore, S. Kuromoto, and Y. Nakamura, Evolution of the Nankai trough decollement from the trench into the seismogenic zone: Inferences from three-dimensional seismic reflection imaging, Geology, 32, 273-276, 2004, 46 citations, doi:10.1130/G20211.1, #1680
We mapped the amplitude of the Nankai Trough subduction thrust seismic reflection from the trench into the seismogenic zone with three-dimensional seismic reflection data. The décollement thrust forms within the lithologically homogeneous Lower Shikoku Basin facies along an initially nonreflective interface. The reflection develops from a porosity contrast between accreted and underthrust sedimentary material because of accretionary wedge consolidation and rapid loading and delayed consolidation of the underthrust section. A décollement-amplitude map shows a significant decline from high amplitudes at the trench to barely detectable levels 25â30 km landward. Three other observations coincide with the amplitude decline: (1) the décollement initially steps down to deeper stratigraphic levels, (2) the wedge taper increases dramatically, and (3) the thrust becomes seismogenic. The amplitude decline and the coincident décollement and accretionary- wedge tectonic and seismogenic behavior are attributed to the loss of fluids and potentially loss of excess fluid pressures downdip along the subduction thrust.
Bangs, N. L., and S. P. S. Gulick, Physical properties along the developing décollement in the Nankai Trough: Inferences from 3-D seismic reflection data inversion and Leg 190 and 196 drilling data, Proc. Ocean Drill. Prog., Sci. Results, 190/196, 2004, #1692
Goff, J. A., B. J. Kraft, L. A. Mayer, S. G. Schock, C. K. Sommerfield, H. C. Olson, S. P. S. Gulick, and S. Nordfjord, Seabed characterization on the New Jersey middle and outer shelf: Correlatability and spatial variability of seafloor sediment properties, Marine Geol., 209, 147-172, 2004, 55 citations, doi:10.1016/j.margeo.2004.05.030, #1689
Nearly 100 collocated grab samples and in situ 65 kHz acoustic measurements were collected on the New Jersey middle and outer shelf within an area that had previously been mapped with multibeam backscatter and bathymetry data, and more recently with chirp seismic reflection profiling. Eighteen short cores were also collected and probed for resistivity-based porosity measurements. The combined data set provides a basis for empirically exploring the relationship among the remotely sensed data, such as backscatter and reflection coefficients, and directly measured seabed properties such as grain size distribution, velocity, attenuation and porosity. We also investigate the spatial variability of these properties through semi-variogram analysis to facilitate acoustic modeling of natural environmental variability.
Grain size distributions on the New Jersey shelf are commonly multi-modal, leading us to separately characterize coarse % (>4 mm), fine % (<63 μm) and mean sand grain size to quantify the distribution. We find that the backscatter is dominated by the coarse component (expressed as weight %), typically shell hash and occasionally terrigenous gravel. In sediment types where coarse material is not significant, backscatter correlates with velocity and fine weight %. Mean sand grain size and fine % are partially correlated with each other, and combined represent the primary control on velocity. The fine %, rather than mean grain size as a whole, appears to be the primary control on attenuation, although coarse % may increase attenuation marginally through scattering. Vertical-incidence seismic reflection coefficients, carefully culled of unreliable values, exhibit a strong correlation with the in situ velocity measurements, suggesting that such data may prove more reliable than backscatter at deriving sediment physical properties from remote sensing data. The velocity and mean sand grain size semi-variograms can be fitted with a von Kármán statistical model with horizontal scale 12.6 km, which provides a basis for generating synthetic realizations. The backscatter and coarse % semi-variograms exhibit two horizontal scales: one 8 km and the other too small to quantify with available data.
Gulick, S. P. S., N. L. Bangs, T. H. Shipley, Y. Nakamura, G. F. Moore, and S. Kuramoto, Three-dimensional architecture of the Nankai accretionary prism's imbricate thrust zone off Cape Muroto, Japan: Prism reconstruction via en echelon thrust propagation, J. Geophys. Res., 109, B02105, 2004, 15 citations, doi:10.1029/2003JB002654, #1660
A 9 km wide, 92 km long, three-dimensional (3-D) seismic reflection volume acquired off Shikoku Island, Japan, images the seaward portion of the subduction of the Philippine Sea plate at the Nankai Trough and Nankai accretionary prism. Detailed interpretation of the imbricate thrust and protothrust zones, the portions of the prism between the deformation front and the first out-of-sequence thrust, shows a high degree of variability in the thrust faults that all parallel the frontal thrust but are arranged in en echelon patterns along strike and frequently include complications such as piggyback faults and fault splays. Interestingly, the sinuous seafloor morphology of the prism does not accurately reflect the en echelon 3-D architecture of the primary prism thrusts. Seafloor morphology appears to average across several thrusts along strike and is further modified by near-surface thrust splays and backthrusts, suggesting that care must be taken in interpreting seafloor relief in terms of lateral continuity or thrust fault geometry. Subduction of the Kinan seamounts 20 km northeast of the center of the Muroto 3-D volume generated a scallop-shaped embayment in the prism; the rebuilding process appears to influence the northeastern portion of the 3-D volume where a ∼625 m landward step in the position of the frontal thrust and numerous changes in prism architecture are observed. These observations imply that accretionary prisms may reattain equilibrium following seamount subduction by lateral en echelon fault propagation into damaged zones that facilitate an increase accretion rate until a laterally continuous deformation front is reestablished.
Gulick, S. P. S., and N. L. Bangs, Negative-polarity at the frontal thrust; Is free gas the culprit?: Insights from the Nankai accretionary prism off Cape Murato using seismic-logging integration , Proc. Ocean Drill. Prog., Sci. Results, 190/196, 2004, #1691
Gulick, S. P. S., J. Jaeger, J. Freymueller, P. Koons, T. L. Pavlis, and R. Powell, Examining tectonic-climatic interactions in Alaska and the northeastern Pacific, Eos, Trans. Amer. Geophys. Un., 85, 438-439, 2004, doi:10.1029/2004EO430001, #1705
Southeastern Alaska, encompassing the glaciated Chugach-St. Elias range (Figure 1), is one of the premier locations where tectonics, orogenesis, glacial erosion, landscape modification, and continental margin sedimentation can be studied in unison, allowing for quantitative models to be developed linking this suite of processes [e.g., Jaeger et al., 2001]. This area is an exceptional natural laboratory for studying a range of geologic problems (Figures 2 and 3), including the links between orogenic processes and continental accretion, glacial landscape modification, and sedimentation. Geologic processes operate at rapid rates along the margin, which allows concurrent data collection on tectonic deformation, uplift, erosion, and sedimentation and development of comprehensive geodynamic models connecting these diverse processes. The active processes in southeastern Alaska are comparable or significantly greater than those studied in the Himalayan orogeny and include extremely high sediment yields, active faults associated with mountains and valley glaciers, and orogeny coinciding with extensive glacial cover.
Mikada, H., M. Kinoshita, K. Becker, E. E. Davis, R. D. Meldrum, P. B. Flemings, S. P. S. Gulick, O. Matsubayashi, S. Morita, S. Goto, N. Misawa, K. Fujino, and M. Toizumi, Hydrogeological and geothermal studies around Nankai trough (KR02-10 Nankai Trough cruise report), JAMSTEC J. Deep Sea Res., 22, 125-171, 2003, #1661
Burger, R. L., C. S. Fulthorpe, J. A. Austin, and S. P. S. Gulick, Lower Pleistocene to present structural deformation and sequence stratigraphy of the continental shelf, offshore Eel River Basin, northern California, Marine Geol., 185, 249-281, 2002, 15 citations, doi:10.1016/S0025-3227(02)00196-2, #1595
Seismic stratigraphic sequences and deformational features are mapped beneath the continental shelf of the offshore Eel River Basin of northern California, using a closely spaced, high-resolution multichannel seismic grid. Geometries of sequences and morphologies of bounding unconformities reflect competing tectonic and glacioeustatic influences, producing shifting sedimentation patterns in the offshore basin during the last 1 Myr. Estimated timing of unconformity formation correlates generally with a deep-ocean global δ18O curve, both before and after a 500-kyr transition from 41-kyr to 100-kyr dominated cycles. We suggest that glacioeustatic fluctuations are the dominant control on unconformity formation basinwide. However, regional tectonic influences on sequence development are also observed. Folding associated with GordaâNorth America Plate convergence ended progressively from south to north between 1.0 Ma and 500 ka, in response to northward migration of the Mendocino Triple Junction (MTJ). Since 500 ka, continued encroachment of the MTJ produces rotation of preexisting structures, uplift of the Table Bluff Anticline (TBA), related periods of channel incision south of the TBA, and generally reduced sediment preservation across the shelf. MTJ-related uplift may also have induced formation of Eel Canyon. Over the past 1.0 Myr, a dominant northern sediment source becomes progressively less important; a southern source, probably the paleo-Eel River, has become dominant since 750 ka. Seismic unconformities fall into two categories: (1) irregular surfaces of limited mappable extent, interpreted as incision/exposure surfaces forming during relative sea-level lowstands, and (2) smooth, laterally extensive regional unconformities, interpreted as ravinement surfaces that erode lowstand surfaces in all but deeply incised areas during sea-level transgressions. A sequence stratigraphic model for sediment deposition and preservation on the Eel River shelf predicts that preserved sediment on the Eel margin is dominated by fluvially derived highstand silts and muds, deposited by longshore-directed currents and waves. Lowstand sediments are preserved only in fluvial channels infilled during late lowstand/early transgression. Preserved transgressive sediments are likely limited to a thin veneer of well-sorted coarse sediment or shell debris, deposited above the transgressive ravinement. Future deep coring will be required to confirm these predictions.
Gulick, S. P. S., and A. S. Meltzer, Effect of the northward-migrating Mendocino triple junction on the Eel River forearc basin, California: Structural evolution, Geol. Soc. Amer. Bull., 114, 1505-1519, 2002, 7 citations, doi:10.1130/0016-7606(2002)114<1505:EOTNMM>2.0.CO;2, #1613
Offshore northern California, the Gorda plate is subducting obliquely beneath North America; the resulting complicated tectonic setting forms the southern end of the Cascadia subduction zone. The southern Cascadia subduction zone and overlying Eel River forearc basin lie just north of the unstable Mendocino triple junction. The Neogene strata of the Eel River basin record structural deformation caused by the underthrusting of the Gorda plate as well as deformation generated by northward migration and encroachment of the Mendocino triple junction. Three distinct deformation regimes are present in the Eel River forearc basin. (1) Along the western margin of the forearc basin and within the foreslope of the accretionary prism, thrust faults and anticlines record Plioceneââ¬â Pleistocene shortening caused by subduction of the Gorda plate. (2) The southern part of the basin rotated counterclockwise in the late Pleistocene, resulting in modern transpressional deformation offshore Humboldt Bay. The rotation and deformation are caused by north-south convergence across the boundary between the Pacific plate and the southernmost part of the forearc basin at the triple junction. (3) The northeastern margin of the Eel River basin is deformed by high-angle faults with a component of strike-slip motion that may represent the incipient northward propagation of the Pacificââ¬âNorth American transform system north of the triple junction.
Gulick, S. P. S., A. S. Meltzer, and S. H. Clarke, Effect of the northward-migrating Mendocino triple junction on the Eel River forearc basin, California: Stratigraphic development, Geol. Soc. Amer. Bull., 114, 178-191, 2002, 8 citations, doi:10.1130/0016-7606(2002)114<0178:EOTNMM>2.0.CO;2, #1614
The Eel River forearc basin, northern California, lies at the southern end of the Cascadia subduction zone and at the leading edge of the migrating Mendocino triple junction. Stratigraphic relationships within the Eel River forearc basin suggest that the current outer-arc high formed between ca. 3 and 2 Ma when the margin switched from a nonaccretionary to an accretionary phase and then uplifted to attain critical taper. Between ca. 2 and 1 Ma, an influx of sedimentation from the ancestral Klamath and Eel River systems increased the width of the northern California margin and caused continued uplift followed by widespread erosion of the western margin of the basin at ca. 1 Ma. In the northeastern part of the forearc basin, localized erosion of the shelf occurred at ca. 500 ka. The arrival of the northward-migrating Mendocino triple junction at ca. 500 ka is documented by uplift, northward tilting, erosion of the margin as much as 20 km north of Cape Mendocino, and reduced deposition within the forearc basin as much as 80 km north of the current position of the triple junction. Terrestrial sediments delivered to the continental margin and eroded sediments near the triple junction largely bypassed the southern part of the basin and were likely deposited in northern areas of the basin or flowed down the Eel Canyon to be deposited within the Gorda Fan.
Gulick, S. P. S., A. S. Meltzer, T. Henstock, and A. R. Levander, Internal deformation of the southern Gorda plate: Fragmentation of a weak plate near the Mendocino triple junction, Geology, 29, 691-694, 2001, 9 citations, doi:10.1130/0091-7613(2001)029<0691:IDOTSG>2.0.CO;2, #1662
North-south compression across the Gorda-Pacific plate boundary caused by the northward-migrating Mendocino triple junction appears to reactivate Gorda plate normal faults, originally formed at the spreading ridge, as left-lateral strike-slip faults. Both seismically imaged faults and magnetic anomalies fan eastward from ∼N20°E near the Gorda ridge to ∼N75°E near the triple junction. Near the triple junction, the Gorda plate is faulted pervasively and appears to be extending east-southeast as it subducts beneath North America. Continuation of northeast-southwestâoriented deformation in the southern Gorda plate beneath the continental margin contrasts with the northwest-southeastâtrending structures in the overlying accretionary prism, suggesting partial GordaâNorth American plate decoupling. Southeast of the triple junction, a slabless window is generated by removal of the subducting Gorda plate. Southwest of the triple junction, the Pacific plate acts as a rigid barrier forcing southern Gorda crust to rotate clockwise, fragment, and flow into the slabless window. Net clockwise rotation of the southern Gorda crust forms a boundary with the nonrotating northern Gorda plate, which is observed as a bend in the magnetic anomalies. This boundary, which is compressional on the western end and extensional to the east, may separate the stress regime of the southern Gorda plate from the remainder of the Cascadia subduction zone.
Kuramoto, S., J. Ashi, J. Greinert, S. P. S. Gulick, T. Ishimura, S. Morita, K. Nakamura, M. Okada, D. Rickert, S. Saito, E. Suess, U. Tsunogai, and T. Tomosugi, Surface observations of subduction related mud volcanoes and large thrust sheets in the Nankai subduction margin: Report on YK00-10 and YK01-04 cruises, JAMSTEC J. Deep Sea Res., 19, 131-139, 2001, #1632
Moore, G. F., A. Taira, N. L. Bangs, S. Kuramoto, T. H. Shipley, C. M. Alex, S. P. S. Gulick, D. J. Hills, T. Ike, S. Ito, S. C. Leslie, A. J. McCutcheon, K. Mochizuki, S. Morita, Y. Nakamura, J.-O. Park, B. L. Taylor, G. Toyama, H. Yagi, and Z. Y. Zhao, Structural setting of the ODP Leg 190 Muroto transect, Proc. Ocean Drilling Prog., Init. Rept., 190, 1-14, 2001, 18 citations, #1541