Bangs, N. L., M. J. Hornbach, and C. Berndt, The mechanics of intermittent methane venting at South Hydrate Ridge inferred from 4D seismic surveying, Earth Planet. Sci. Lett., 310, 1-5-112, 2011, doi:10.1016/j.epsl.2011.06.022, #2389 
Sea floor methane vents and seeps direct methane generated by microbial and thermal decompositions of organic matter in sediment into the oceans and atmosphere. Methane vents contribute to ocean acidification, global warming, and providing a long-term (e.g. 500-4000 years; Powell et al., 1998) life-sustaining role for unique chemosynthetic biological communities. However, the role methane vents play in both climate change and chemosynthetic life remains controversial primarily because we do not understand long-term methane flux and the mechanisms that control it ( [Milkov et al., 2004], [Shakhova et al., 2010] and [Van Dover, 2000]). Vents are inherently dynamic and flux varies greatly in magnitude and even flow direction over short time periods (hours-to-days), often tidally-driven ( [Boles et al., 2001] and [Tryon et al., 1999]). But, it remains unclear if flux changes at vents occur on the order of the life-cycle of various species within chemosynthetic communities (months, years, to decades [Leifer et al., 2004] and [Torres et al., 2001]) and thus impacts their sustainability. Here, using repeat high-resolution 3D seismic surveys acquired in 2000 and 2008, we demonstrate in 4D that Hydrate Ridge, a vent off the Oregon coast has undergone significant reduction of methane flow and complete interruption in just the past few years. In the subsurface, below a frozen methane hydrate layer, free gas appears to be migrating toward the vent, but currently there is accumulating gas that is unable to reach the seafloor through the gas hydrate layer. At the same time, abundant authigenic carbonates show that the system has been active for several thousands of years. Thus, it is likely that activity has been intermittent because gas hydrates clog the vertical flow pathways feeding the seafloor vent. Back pressure building in the subsurface will ultimately trigger hydrofracturing that will revive fluid-flow to the seafloor. The nature of this mechanism implies regular recurring flow interruptions and methane flux changes that threaten the viability of chemosynthetic life, but simultaneously and enigmatically sustains it.
Gulick, S. P. S., J. A. Austin, L. M. McNeill, N. L. Bangs, K. M. Martin, T. J. Henstock, J. M. Bull, S. M. Dean, Y. S. Djajadihardja, and H. Permana, Updip rupture of the 2004 Sumatra earthquake extended by thick indurated sediments, Nature Geoscience, 4, 453-456, 2011, doi:10.1038/ngeo1176, #2343
During subduction, weak, unlithified sediments are scraped off the down-going plate and accumulate near the subduction trench axis. The weak nature of the sediments usually impedes the propagation of fault rupture during an earthquake. However, measurements of slip during the 2004 Sumatra-Andaman Mw 9.2 earthquake show that fault rupture propagated updip, extending unusually close to the subduction trench, in the southern part of the rupture area. Here we present seismic reflection images of the southern part of the 2004 Sumatra-Andaman earthquake rupture area. We show that sedimentary strata, greater than 4 km in thickness, form coherent blocks that have been thrust onto the continental margin during subduction. The blocks form a 130-km-wide plateau overlying the seismogenic zone and the plate boundary megathrust lies near to the base of the sediments. The sediments consist of the Nicobar and Bengal Fan turbidites and exhibit strong internal cohesion. We suggest that dewatering and lithification of the sediments during burial made them unusually competent and strong, thus enabling rupture during the 2004 earthquake to propagate beneath the plateau, close to the Sunda Trench. Extending fault rupture so close to the trench, and thus further seaward, may have enhanced the tsunami hazard by displacing a greater thickness of water.
Tsuji, T., Y. Sanada, K. Yamamoto, J.-O. Park, T. No, E. Araki, N. L. Bangs, R. E. Von Huene, G. F. Moore, and M. Kinoshita, In situ stress state from walkaround VSP anisotropy in the Kimano Basin south of the Kii Peninsula, Geochem., Geophys., Geosyst., (in press), 2011, #2423
Bangs, N. L., M. J. Hornbach, G. F. Moore, and J.-O. Park, Massive methane release triggered by seafloor erosion offshore southwestern Japan, Geology, 38, 1019-1022, 2010, 1 citation, doi:10.1130/G31491.1, #2272
Vast amounts of methane hydrate exist beneath continental margins, but whether this methane releases from sediment on a large scale and affects the oceans and atmosphere remains unclear. Analysis of newly acquired three-dimensional seismic images and drilling data from a large gas hydrate province reveal a recently eroded v-shaped depression. The depression sharply cuts through a relic bottom simulating reflection (BSR) and hydrate-laden sediments. The shape of the relic BSR indicates that the seafloor depression was once a large anticline that has recently been eroded and released an estimated 1.51 x 1011 m3 of methane. We hypothesize that erosion of the seafloor via bottom-water currents unroofed buoyant hydrate-laden sediments and subhydrate overpressured free gas zones beneath the anticline. Once triggered, gas-driven erosion created a positive feedback mechanism, releasing gas and eroding hydrate-bearing sediment. We suggest that erosive currents in deep-water methane hydrate provinces act as hair triggers, destabilizing kilometer-scale swaths of the seafloor where large concentrations of underlying overpressured methane exist. Our analysis suggests that kilometer-scale degassing events are widespread, and that deep-water hydrate reservoirs can rapidly release methane in massive quantities.
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.
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.
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.
Park, J.-O., G. Fujie, L. Wiljerathne, T. Hori, S. Kodaira, Y. Fukao, G. F. Moore, N. L. Bangs, S. Kuramoto, and A. Taira, A low velocity zone with weak reflectivity along the Nankai subduction zone, Geology, 38, 283-286, 2010, 7 citations, doi:10.1130/G30205.1, #2206
Three-dimensional seismic reflection data reveal the presence of a low seismic velocity zone (LVZ) with weak reflectivity character along the Nankai accretionary prism. This LVZ is intercalated between an upper, offscraped layer and a lower, underthrusting layer in the outer accretionary wedge. Wide-angle ocean bottom seismograph data also support the presence of the LVZ, which is estimated to be a maximum of ∼2 km thick, ∼15 km wide, and ∼120 km long. The LVZ could be an underthrust package underplated in response to the lateral growth of the Nankai accretionary prism. Underplating of the underthrusting layer beneath the overlying offscraped layer would maintain a critical taper of the accretionary prism so that the offscraped layer can continue to grow seaward. The LVZ could have elevated fluid pressure, leading to rigidity reduction of the entire outer accretionary wedge. The rigidity-lowered outer wedge, containing the LVZ, may be more easily uplifted and thus eventually foster tsunami generation during a Nankai megathrust earthquake. If the fluid-rich LVZ supplies a significant amount of the fluid to the megasplay fault zone at depth, it may affect stick-slip behavior of the fault.
Pecher, I. A., B. Milkereit, A. Sakai, M. K. Sen, N. L. Bangs, and J.-W. Huang, Vertical seismic profiles through gas-hydrate-bearing sediments, in Geophysical Characterization of Gas Hydrates, edited by M. Riedel, E. C. Wiloughby, and S. Chopra, SEG, 2010, doi:10.1190/1.9781560802197.ch8, #2291
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.
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.
Tsuji, T., J.-O. Park, G. F. Moore, S. Kodaira, Y Fukao, S. Kuramoto, and N. L. Bangs, Intraoceanic thrusts in the Nankai Trough off the Kii Peninsula: Implications for intraplate earthquakes, Geophys. Res. Lett., 36, L06303, 2009, 3 citations, doi:10.1029/2008GL036974, #2119
We identified intraoceanic thrusts developed as imbricate structures within the subducting Philippine Sea plate off the Kii Peninsula in central Japan manifesting as strong-amplitude reflections observed in an industry-standard three-dimensional (3D) seismic reflection data set. These imbricate intraoceanic thrusts cut through the oceanic crust as a discontinuous thrust plane striking approximately parallel to the trench. In our survey area, large intraplate earthquakes with moment magnitudes (Mw) over 7 occurred on 5 September 2004, causing strong ground motions on the islands of Japan and tsunami waves. The locations of the intraoceanic thrusts recognized in the seismic data are distributed around the estimated hypocenters of the mainshocks and aftershocks of the 2004 earthquakes. Furthermore, their geometry extracted from the 3D seismic data could explain the kind of complex rupture pattern observed during the 2004 events. Therefore we propose that the intraoceanic thrusts are seismogenically active.
Kumar, D., M. K. Sen, and N. L. Bangs, Gas hydrate concentration and characteristics within Hydrate Ridge inferred from multicomponent seismic reflection data, J. Geophys. Res., 112, B12306, 2007, 6 citations, doi:10.1029/2007JB004993, #1782
A seismic experiment composed of streamer and ocean bottom seismometer (OBS) surveys was conducted in the summer of 2002 at southern Hydrate Ridge, offshore Oregon, to map the gas hydrate distribution within the hydrate stability zone. Gas hydrate concentrations within the reservoir can be estimated with P wave velocity (V p ); however, we can further constrain gas hydrate concentrations using S wave velocity (V s ), and use V s through its relationship to V p (V p /V s ) to reveal additional details such as gas hydrate form within the matrix (i.e., hydrate cements the grains, becomes part of the matrix frame or floats in pore space). Both V p and V s can be derived simultaneously by inverting multicomponent seismic data. In this study, we use OBS data to estimate seismic velocities where both gas hydrate and free gas are present in the shallow sediments. Once V p and V s are estimated, they are simultaneously matched with modeled velocities to estimate the gas hydrate concentration. We model V p using an equation based on a modification of Wood's equation that incorporates an appropriate rock physics model and V s using an empirical relation. The gas hydrate concentration is estimated to be up to 7% of the rock volume, or 12% of the pore space. However, V p and V s do not always fit the model simultaneously. V p can vary substantially more than V s . Thus we conclude that a model, in which higher concentrations of hydrate do not affect shear stiffness, is more appropriate. Results suggest gas hydrates form within the pore space of the sediments and become part of the rock framework in our survey area.
Moore, G. F., N. L. Bangs, A. Taira, S. Kuramoto, E. M. Pangborn, and H. J. Tobin, Three-dimensional splay fault geometry and implications for tsunami generation, Science, 318, 1128-1131, 2007, 71 citations, doi:10.1126/science.1147195, #1962
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Science 16 November 2007:
Vol. 318. no. 5853, pp. 1128 - 1131
DOI: 10.1126/science.1147195
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REPORTS
Three-Dimensional Splay Fault Geometry and Implications for Tsunami Generation
G. F. Moore,1,2* N. L. Bangs,3 A. Taira,1 S. Kuramoto,1 E. Pangborn,3 H. J. Tobin4
Megasplay faults, very long thrust faults that rise from the subduction plate boundary megathrust and intersect the sea floor at the landward edge of the accretionary prism, are thought to play a role in tsunami genesis. We imaged a megasplay thrust system along the Nankai Trough in three dimensions, which allowed us to map the splay fault geometry and its lateral continuity. The megasplay is continuous from the main plate interface fault upwards to the sea floor, where it cuts older thrust slices of the frontal accretionary prism. The thrust geometry and evidence of large-scale slumping of surficial sediments show that the fault is active and that the activity has evolved toward the landward direction with time, contrary to the usual seaward progression of accretionary thrusts. The megasplay fault has progressively steepened, substantially increasing the potential for vertical uplift of the sea floor with slip. We conclude that slip on the megasplay fault most likely contributed to generating devastating historic tsunamis, such as the 1944 moment magnitude 8.1 Tonankai event, and it is this geometry that makes this margin and others like it particularly prone to tsunami genesis.
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.
Kumar, D., M. K. Sen, N. L. Bangs, C. Wang, and I. A. Pecher, Seismic anisotropy at Hydrate Ridge, Geophys. Res. Lett., 33, L01306, 2006, 9 citations, doi:10.1029/2005GL023945, #1843
P-wave velocity increases in the presence of gas hydrates and decreases in the presence of free gas in the sediments, making it an excellent means to investigate gas hydrate systems. However, seismic velocity is typically derived from surface seismic data without consideration of seismic anisotropy. The presence of anisotropy in the hydrate bearing sediments adds an additional complexity in data analysis; however anisotropy can help reveal the distribution of hydrates. Here we report on the evidence of seismic anisotropy at Hydrate Ridge along the Cascadia convergent margin. We find that the south summit is anisotropic, while the basin side (east of south summit) is isotropic. Anisotropy is likely caused by the hydrate veins. We interpret the anisotropy parameters in terms of the distribution and fabric of gas hydrates.
Kumar, D., M. K. Sen, and N. L. Bangs, Seismic characteristics of gas hydrates at Hydrate Ridge, offshore Oregon, Leading Edge, 25, 610-614, 2006, doi:10.1190/1.2202665, #1844
Gas hydrate is an ice-like substance that contains low molecular weight gases (mostly methane) in a lattice of water molecules (Sloan, 1998). Gas hydrates are widely present in permafrost and deep oceanic environments around the world. Hydrate Ridge, offshore Oregon, is one of the areas where hydrates are found in relatively high concentration. In marine environments, methane hydrates are usually stable at temperatures of 0ââ¬â20ð C, water depths greater than 500 m, and in sediments up to 300 m below the seafloor. Small amounts of free gas are often present below the gas-hydrate stability zone (GHSZ). The hydrate-to-free-gas contact gives a strong acoustic impedance contrast, which is evident on seismic sections as a bottom-simulating reflection (BSR). Offshore gas hydrate systems have received attention from the scientific community because of their potential to be
Musgrave, R. J., N. L. Bangs, J. C. Larrasoana, E. Gracia, J. A. Hollamby, and M. E. Vega, Rise of the base of the gas hydrate zone since the last glacial recorded by rock magnetism, Geology, 34, 117-120, 2006, 5 citations, doi:10.1130/G22008.1, #1807
Gas hydrate, a clathrate of methane and water widespread on continental margins, has been implicated as a trigger of climate change and submarine slides as a result of methane release when the base of its stability zone moves upward rapidly. Direct tests of these hypotheses are made difficult by the ephemeral record of gas hydrate in sediment. In places, a seismic reflector (double bottom simulating reflector, BSR) appears to mark the old base of the gas hydrate layer, but the occurrence of this feature is patchy and its interpretation is controversial. Microbial activity is stimulated in the presence of gas hydrate, and results in the production of magnetic iron sulfides; the base of the gas hydrate interval is marked by a sharp reduction in the magnetic hysteresis parameter DJH. At Hydrate Ridge on the Cascadia margin, sampled during Ocean Drilling Program Leg 204, this signature occurs between 20 and 65 m below the present-day base of the gas hydrate zone, at a depth consistent with predictions for the base of gas hydrate stability given water depths and bottom-water temperatures appropriate for the last glacial maximum. Seismic evidence for a double BSR over part of Hydrate Ridge corroborates the rock magnetic interpretation.
Trehu, A. M., N. L. Bangs, and G. Guerin, Near-offset verical seismic experiments during Leg 204, Proc. Ocean Drill. Prog., Sci. Results, 204, 1-23, 2006, #1842
Bangs, N. L., R. J. Musgrave, and A. M. Trehu, Upward shifts in the southern Hydrate Ridge gas hydrate stability zone following postglacial warming, offshore Oregon, J. Geophys. Res., 110, B03102, 2005, 11 citations, doi:10.1029/2004JB003293, #1724
High-resolution three-dimensional (3-D) seismic reflection data acquired on the R/V Thomas G. Thompson in 2000 reveal a pair of bottom simulating reflections (BSRs) across a broad region of southern Hydrate Ridge, offshore Oregon. The primary BSR (BSRp) is a regionally extensive reflection that lies 120ââ¬â150 m below seafloor and exhibits typical characteristics of a gas hydrate BSR. We also imaged a second weaker BSR (BSRs), 20ââ¬â40 m below BSRp, with similar characteristics. BSRs is interpreted as a remnant of a BSR that probably formed during the Last Glacial Maximum 18,000 years ago, when the base of the gas hydrate stability zone (GHSZ) was deeper. An increase in bottom water temperatures of 1.75ðââ¬â2.25ð and a corresponding sea level rise of 120 m could have produced the BSR shift. The preservation of BSRs for at least 5000 years, which is the time since subseafloor temperatures stabilized following ocean warming after the Last Glacial Maximum, implies very slow upward advective and diffusive flow of methane (<1 m/1000 years in the vicinity of BSRs). BSRs appears where there are no resolvable steeply dipping faults and fractures, consistent with very low advective flow rates, and has dispersed where vertical fractures are visible. Free gas released by the shift in the BSR either migrates so slowly that it remains stable beneath the GHSZ or is directed upward along fractures to reform as hydrate in the GHSZ. There is no evidence for release of this free gas into the ocean or atmosphere.
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
Tsuji, T., T. Noguchi, H. Niino, T. Matsuoka, Y. Nakamura, H. Tokuyama, S. Kuramoto, and N. L. Bangs, Two-dimensional mapping of fine structures in the Kuroshio current using seismic reflection data, Geophys. Res. Lett., 32, L14609, 2005, 20 citations, doi:10.1029/2005GL023095, #1781
Multi-channel seismic reflection data acquired in the Pacific Ocean off the Muroto peninsula of Shikoku Island, Japan reveal the two-dimensional distribution of fine structures in the Kuroshio Current. Eighty-one seismic sections, each extending 80 km perpendicular to the current and separated by 100 m, were acquired from 20 June to 15 August 1999 (57 days). The seismic data clearly show that fine structures extend over 40 km perpendicular to the current in almost all of the profiles. A simulation study using acoustic model from CTD data demonstrates that fine structure of temperature and salinity identified in CTD data acquired from the Kuroshio Current off the Ashizuri peninsula yield a synthetic seismic profile with characteristics similar to the Muroto transect profiles.
Tsuji, T., T. Matsuoka, Y. Yamada, Y. Nakamura, J. Ashi, H. Tokuyama, S. Kuramoto, and N. L. Bangs, Initiation of plate boundary slip in the Nankai Trough off the Muroto Peninsula, southwest Japan, Geophys. Res. Lett., 32, L12306, 2005, 9 citations, doi:10.1029/2004GL021861, #1784
Multi-channel seismic reflection data acquired in the Pacific Ocean off the Muroto peninsula of Shikoku Island, Japan reveal the two-dimensional distribution of fine structures in the Kuroshio Current. Eighty-one seismic sections, each extending 80 km perpendicular to the current and separated by 100 m, were acquired from 20 June to 15 August 1999 (57 days). The seismic data clearly show that fine structures extend over 40 km perpendicular to the current in almost all of the profiles. A simulation study using acoustic model from CTD data demonstrates that fine structure of temperature and salinity identified in CTD data acquired from the Kuroshio Current off the Ashizuri peninsula yield a synthetic seismic profile with characteristics similar to the Muroto transect profiles.
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
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
Heffernan, A. S., J. C. Moore, N. L. Bangs, G. F. Moore, and T. H. Shipley, Initial deformation in a subduction thrust system: Polygonal normal faulting in the incoming sedimentary sequence of the Nankai subduction zone, southwestern Japan, in 3D Seismic Technology: Application to the Exploration of Sedimentary Basins, edited by R. J. Davies, J. A. Cartwright, S. A. Stewart, M. Lappin and J. R. Underhill, Geol. Soc. London, Memoir, 29, 143-148, 2004, doi:10.1144/GSL.MEM.2004.029.01.14, #1723
3D seismic data from the Nankai margin provide detailed imagery documenting the onset of deformation at an active sediment-dominated accretionary prism, including a previously unmapped network of normal faults. The Nankai margin off southwest Japan is characterized by active subduction, seismogenesis, and a large accretionary prism with fold-and-thrust belt structure. Imbricate thrusting is the dominant structural style of the outer 20 km of the prism. This structural domain develops at the prism toe, where an incipient imbricate thrust displays significant along-strike variability in dip, offset, and development of hangingwall anticlines.
Trehu, A. M., P. B. Flemings, N. L. Bangs, J. Chevallier, E. Gracia, J. E. Johnson, C.-S. Liu, X. L. Liu, M. Riedel, and M. E. Torres, Feeding methane vents and gas hydrate deposits at south Hydrate ridge, Geophys. Res. Lett., 31, L23310, 2004, 31 citations, doi:10.1029/2004GL021286, #1783
Log and core data document gas saturations as high as 90% in a coarse-grained turbidite sequence beneath the gas hydrate stability zone (GHSZ) at south Hydrate Ridge, in the Cascadia accretionary complex. The geometry of this gas-saturated bed is defined by a strong, negative-polarity reflection in 3D seismic data. Because of the gas buoyancy, gas pressure equals or exceeds the overburden stress immediately beneath the GHSZ at the summit. We conclude that gas is focused into the coarse-grained sequence from a large volume of the accretionary complex and is trapped until high gas pressure forces the gas to migrate through the GHSZ to seafloor vents. This focused flow provides methane to the GHSZ in excess of its proportion in gas hydrate, thus providing a mechanism to explain the observed coexistence of massive gas hydrate, saline pore water and free gas near the summit.
Trehu, A. M., P. E. Long, M. E. Torres, G. Borhmann, F. R. Rack, T. S. Collett, D. S. Goldberg, A. V. Milkov, M. Riedel, P. Schultheiss, N. L. Bangs, S. R. Barr, W. S. Borowski, G. E. Claypool, M. E. Delwiche, G. R. Dickens, E. Gracia, G. Guerin, M. Holland, J. E. Johnson, Y.-J. Lee, C.-S. Liu, X. Su, B. Teidhert, H. Tomaru, M. Vanneste, M. Watanabe, and J. L. Weinberger, Three-dimensional distribution of gas hydrate beneath southern Hydrate Ridge: Constraints from ODP Leg 204, Earth Planet. Sci. Lett., 222, 845-862, 2004, 89 citations, doi:10.1016/j.epsl.2004.03.035, #1806
Large uncertainties about the energy resource potential and role in global climate change of gas hydrates result from uncertainty about how much hydrate is contained in marine sediments. During Leg 204 of the Ocean Drilling Program (ODP) to the accretionary complex of the Cascadia subduction zone, we sampled the gas hydrate stability zone (GHSZ) from the seafloor to its base in contrasting geological settings defined by a 3D seismic survey. By integrating results from different methods, including several new techniques developed for Leg 204, we overcome the problem of spatial under-sampling inherent in robust methods traditionally used for estimating the hydrate content of cores and obtain a high-resolution, quantitative estimate of the total amount and spatial variability of gas hydrate in this structural system. We conclude that high gas hydrate content (30â40% of pore space or 20â26% of total volume) is restricted to the upper tens of meters below the seafloor near the summit of the structure, where vigorous fluid venting occurs. Elsewhere, the average gas hydrate content of the sediments in the gas hydrate stability zone is generally <2% of the pore space, although this estimate may increase by a factor of 2 when patchy zones of locally higher gas hydrate content are included in the calculation. These patchy zones are structurally and stratigraphically controlled, contain up to 20% hydrate in the pore space when averaged over zones 10 m thick, and may occur in up to 20% of the region imaged by 3D seismic data. This heterogeneous gas hydrate distribution is an important constraint on models of gas hydrate formation in marine sediments and the response of the sediments to tectonic and environmental change.
Bangs, N. L., G. L. Christeson, and T. H. Shipley, Structure of the Lesser Antilles subduction zone backstop and its role in a large accretionary system, J. Geophys. Res., 108, 2358, 2003, 8 citations, doi:10.1029/2002JB002040, #1648
The role of a backstop in subduction zones has been the subject of numerous laboratory and numerical modeling studies; however, few field observations exist revealing how backstops control deformation in subduction zones and accretionary wedge construction. A seismic reflection and refraction survey acquired in 1998 with the R/V Maurice Ewing reveals the geometry of the forearc igneous crust, accretionary wedge, and forearc basin structure of the northern Guadeloupe area of the Lesser Antilles forearc. An accreted block of buoyant crust, accreted in the late Miocene, forms the toe of the overriding arc crust and forms the backstop. We imaged the top of this surface, beneath the forearc basin, to its seaward edge where it meets the subducting oceanic crust. The toe of the backstop was thrust upward and forms a steep buttress in contact with the lower half of the accretionary wedge. The steep buttress produces a narrow inner deformation zone with minimal backthrusting of the accretionary complex landward over the backstop, and a narrow <10 km transition between accreted and forearc basin sediment. Seismic reflections from the subducting crust and the decollement appear beneath the entire accretionary wedge and below the backstop toe. Separating the decollement and the subducting crust is an interval, usually between 500 and 750 m, of underthrust sediment carried underneath the accretionary wedge and subducted 15 km landward and beneath the toe of the backstop. We speculate that the upturned geometry of the toe of the backstop and a weak fluid-rich decollement may facilitate sediment subduction beneath the backstop and potentially into the mantle.
Christeson, G. L., N. L. Bangs, and T. H. Shipley, Deep structure of an island arc backstop, Lesser Antilles subduction zone, J. Geophys. Res., 108, 2327, 2003, 8 citations, doi:10.1029/2002JB002243, #1628
We present the results from a coincident seismic reflection/refraction grid conducted at the Lesser Antilles subduction zone near 16°N. This paper focuses on the seismic refraction data and constraints these data place on the three-dimensional structure of the island arc backstop. We find that the backstop in this region contains considerable topography in both the strike and dip directions. Two ridges, each 25â35 km in length and ∼10 km in width, rise 1â6 km above the adjacent basement. The eastern edge of one of the ridges deepens by ∼4â6 km over a horizontal distance of 10 km and forms the eastern edge of the backstop. In contrast to the complex nature of the backstop, the adjacent accretionary wedge displays little lateral variability at large scales. This may be a consequence of the spatial scales involved: the backstop topography is ∼10â35 km in width, while the accretionary wedge extends ∼125 km from the deformation front to the backstop. The top of the subducting oceanic crust, as identified by an increase in velocities to 6â6.5 km/s, intersects the backstop at a depth of ∼14â15 km. The updip limit of plate boundary seismicity is located 75â100 km west and downdip of the backstop. However, two earthquake clusters are observed at the intersection of the subducted Barracuda Ridge and Tiburon Ridge with the backstop, suggesting active deformation associated with the backstop edge at these locations.
DiLeonardo, C. G., J. C. Moore, S. Nisson, and N. L. Bangs, Control of internal structure and fluid-migration pathways within the Barbados ridge decollement zone by strike-slip faulting: Evidence from coherence and three-dimensional seismic amplitude imaging, Geol. Soc. Amer. Bull., 114, 51-63, 2002, 8 citations, doi:10.1130/0016-7606(2002)114<0051:COISAF>2.0.CO;2, #1804
The application of three-dimensional seismic reflection and coherence imaging to the study of the décollement zone of the Barbados Ridge accretionary complex has provided new insights into the relationships among internal structure, fluid flow, and previously unrecognized strike-slip faulting. Combined coherence and seismic amplitude imaging of the décollement zone reveal anomalous northeast-trending lineaments parallel to and abutting zones of high- amplitude, negative-polarity reflections. Analysis of these lineaments shows them to be penetrative structures dipping southeast with apparent reverse dip-slip offset. Isopach mapping of the accretionary wedge indicates significant right-lateral displacement across these structures. These faults apparently channel fluid flow within the décollement zone, and the prominent northeast- trending conduits so formed are readily visible as high-amplitude, negative-polarity reflections. Additionally, north-northeastâ trending zones of variable coherence and high positive amplitude are inferred barriers to up-structure fluid-migration pathways.
Movement along strike-slip structures probably alternates with displacement along the décollement zone. Northeast- trending strike-slip faults extend for >13 km, crossing the length of the survey area and into the incoming oceanic plate. Active arc-oblique strike-slip faulting of the décollement zone beneath the Barbados Ridge accretionary wedge implies a stress regime in that σ1 is fixed and σ2 and σ3 either transpose with time or are nearly equal. This state of stress may be a common occurrence in forearc tectonism and may have led to the formation of many, as yet unrecognized, arc-oblique strike-slip faults at convergent margins.
Lizarralde, D., W. S. Holbrook, S. McGeary, N. L. Bangs, and J. B. Diebold, Crustal construction of a volcanic arc, wide-angle seismic results from the western Alaska Peninsula, J. Geophys. Res., 107, 2164, 2002, 20 citations, doi:10.1029/2001JB000230, #1762
Results from the 1994 Aleutian Seismic experiment delineate basic oceanic arc crustal architecture, constrain magmatic flux rates and bulk arc composition, and address questions of continental crustal genesis. Here we present results from a transect across protocontinental crust of the westernmost Alaska Peninsula (line A3) and compare this structure to a purely oceanic arc transect farther west. Arc crustal structure is similar along these two transects. Magmatic accretion occurs at the top and bottom of preexisting oceanic crust as a 5- to 10-km-thick upper crustal carapace of low velocity (2â5.8 km s−1) volcaniclastics, flows and small plutons, and a mafic lower crustal underplate (∼7.0 km s−1) of variable thickness, for a maximum arc crust thickness of ∼25â30 km. Lateral lower crustal velocity gradients and high velocities (>7.5 km s−1) beneath the forearc suggest dominantly vertical lower crustal accretion above a focused melt source and a forearc underlain by little magmatic crust but rather partially intruded and/or serpentinized mantle. The ratio of upper to lower crustal volume is ∼1, and the total arc crust volume implies a magmatic flux of ∼67 km3 km−1 m.y.−1, more than twice previous estimates for this arc and global productivity. The crust is thinner and more mafic than continental crust, and it lacks a massive tonalitic upper crust characteristic of the continents. An interpreted accumulation of upper crustal carapace material at midcrustal depths on line A3 has a velocity of ∼6.4 km s−1, suggesting an intermediate composition. Accretionary complex terranes consisting of accumulations of this type material would thus have bulk compositions similar to continental crust.
Trehu, A. M., N. L. Bangs, M. A. Arsenault, G. Bohrmann, C. Goldfinger, J. E. Johnson, Y. Nakamura, and M. E. Torres, Complex subsurface plumbing beneath the southern Hydrate Ridge, Oregon continental margin, from high-resolution 3D seismic reflection and OBS data, Fourth Int. Conf. Gas Hydrates, Yokohama, Japan, 19023, 90-96, 2002, #1805
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
Zhao, Z. Y., G. F. Moore, N. L. Bangs, and T. H. Shipley, Spatial variations of the decollement/protodecollement zone and their implications: A 3-D seismic inversion study of the northern Barbados accretionary prism, Island Arc, 9, 219-236, 2000, 5 citations, #1476
Abstract We conducted a 3-D seismic inversion study to investigate spatial variations of physical properties of the décollement zone (DZ) and protodécollement zone (PDZ) under the northern Barbados accretionary prism. Significant spatial variations of physical properties were observed in the PDZ seaward of the thrust front from the inversion data. The density generally increases southward with a few localized low-density patches. A lower density commonly corresponds to a thicker PDZ, suggesting that the paleomorphology may at least partially control the variations of the physical properties. Similar low-density patches were also found in the DZ. These features may be inherited from those of the PDZ and enhanced after subduction through localized arrested consolidation. Under the prism toe, the density of the DZ increases landward. This trend may mainly result from shear-induced consolidation of the DZ but may also be related to landward increasing tectonic loading. Significant northâsouth differences in density and, thus, porosity and strength of the PDZ, are observed and these differences may continue into the DZ. A stronger DZ is likely responsible for a larger prism taper observed in the southern area of the prism toe. The larger taper, thus more horizontal shortening, coupled with a thinner sediment sheet above the PDZ in the southern area, may cause a relative retreat of the thrust front and a pronounced change in strike of the sequence thrusts south of seismic Line 690. The northâsouth differences may ultimately have originated in the approach of a structurally higher segment of the Tiburon Rise. The Tiburon Rise affects regional morphology and, thus, it controls the sedimentation and physical properties of the PDZ. It may also control sediment accumulation above the PDZ. Therefore, the sedimentational change induced by the structural high of the Tiburon Rise, in turn, resulted in structural change of the prism in the southern area.
Bangs, N. L., T. H. Shipley, J. C. Moore, and G. F. Moore, Fluid accumulation and channeling along the northern Barbados Ridge decollement thrust, J. Geophys. Res., 104, 20399-20414, 1999, 48 citations, #1428
A volume of three-dimensional seismic reflection data, acquired in 1992, imaged the decollement beneath the northern Barbados Ridge accretionary prism revealing reflection amplitude and waveform variations attributed to fluid accumulations along the plate boundary fault. We model the seismic reflection by inversion for seismic impedance (the product of velocity and density) throughout the 5 Ã 25 km survey area and thus map physical property variations. In 1997, Ocean Drilling Program Leg 171A penetrated the protodecollement and decollement at five sites with a logging-while-drilling (LWD) tool to log density and other physical properties of the decollement. We construct a regional map of density, and inferred porosity, within the decollement from seismic models calibrated with LWD density data. In the sediments out in front of the trench the protodecollement forms in a radiolarian-rich Miocene mudstone with an anomalously high porosity (70â75%) that appears as a pervasive, inherent characteristic of this interval seaward of the deformation front. In the decollement beneath the wedge a consolidation trend of decreasing porosity runs perpendicular to the deformation front with porosity decreasing from 70% at the wedge toe to 50% 4 km from the wedge toe. A second, distinct trend also forms along a 10-km-long, 1- to 2-km-wide, NE-SW zone in which porosity is 70%, as high as it is in the protodecollement. This zone can be explained as an area of the decollement where fluid accumulations develop by maintaining high fluid content. We postulate that high fluid content is maintained by continuous recharge flowing into and along this channel. This porosity distribution within the decollement also strongly influences fluid migration into the overlying accretionary wedge and is directly associated with fluid charging of ramps and out-of-sequence thrusts above the decollement.
Holbrook, W. S., D. Lizarralde, S. McGeary, N. L. Bangs, and J. B. Diebold, Structure and composition of the Aleutian Island arc and implications for continental crustal growth, Geology, 27, 31-34, 1999, 118 citations, doi:10.1130/0091-7613(1999)027<0031:SACOTA>2.3.CO;2, #1443
We present results of a seismic reflection and refraction investigation of the Aleutian island arc, designed to test the hypothesis that volcanic arcs constitute the building blocks of continental crust. The Aleutian arc has the requisite thickness (30 km) to build continental crust, but it differs strongly from continental crust in its composition and reflectivity structure. Seismic velocities and the compositions of erupted lavas suggest that the Aleutian crust has a mafic bulk composition, in contrast to the andesitic bulk composition of continents. The silicic upper crust and reflective lower crust that are characteristic of continental crust are conspicuously lacking in the Aleutian intraoceanic arc. Therefore, if island arcs form a significant source of continental crust, the bulk properties of arc crust must be substantially modified during or after accretion to a continental margin. The pervasive deformation, intracrustal melting, and delamination of mafic to ultramafic residuum necessary to transform arc crust into mature continental crust probably occur during arc-continent collision or through subsequent establishment of a continental arc. The volume of crust created along the arc exceeds that estimated by previous workers by about a factor of two.
Shipley, T. H., N. L. Bangs, and G. F. Moore, Shallow aseismic portion of the Barbados plate boundary, Proc., Workshop on Recurrence of Great Interplate Earthquakes and Its Mechanism, Sci. and Tech. Agency, Kochi, Japan, 91-96, 1999, #1468
Moore, J. C., A. Klaus, N. L. Bangs, B. A. Bekins, C. J. Bucker, W. Bruckmann, S. N. Erickson, O. Hansen, T. Horton, P. Ireland, C. O. Major, G. F. Moore, S. Peacock, S. Saito, E. J. Screaton, J. W. Shimeld, P. H. Stauffer, T. Taymaz, P. A. Teas, and T. Tokunaga, Consolidation patterns during initiation and evolution of a plate-boundary decollement zone; northern Barbados accretionary prism, Geology, 26, 811-814, 1998, 44 citations, doi:10.1130/0091-7613(1998)026<0811:CPDIAE>2.3.CO;2, #1803
Borehole logs from the northern Barbados accretionary prism show that the plate-boundary decollement initiates in a low-density radiolarian claystone. With continued thrusting, the decollement zone consolidates, but in a patchy manner. The logs calibrate a three-dimensional seismic reflection image of the decollement zone and indicate which portions are of low density and enriched in fluid, and which portions have consolidated. The seismic image demonstrates that an underconsolidated patch of the decollement zone connects to a fluid-rich conduit extending down the decollement surface. Fluid migration up this conduit probably supports the open pore structure in the underconsolidated patch.
Shipley, T. H., N. L. Bangs, and A. T. Henning, Sediment velocity estimation using iterative 3-D migrations of short offset seismic reflection data in deep water, Marine Geophysical Researches, 20, 479-494, 1998, 3 citations, doi:10.1023/A:1004782815985, #1442
In deep ocean settings where water depth greatly exceeds the source-to-receiver length, the geometry is insufficient for accurate determinations of velocity from reflection-moveout. However, velocities are crucial for estimates of physical properties and image processing. Focusing analyses with conventional post-stack two-dimensional migration improves images, but does not produce geologically meaningful velocities except in the special case of a two-dimensional earth. For the more general case of the three-dimensional earth there is no a priori method to determine the degree of geometrical complexity. We present a technique using a short-offset three-dimensional (3-D) data set over the 5 km deep trench west of the Lesser Antilles. These data illustrate highly sensitive post-stack 3-D focusing analyses (± 20 m sâ1 interval velocities), and the relationship of these seismically derived velocities to rock velocities. In our Barbados example we were able to establish the presence of a widespread 80-160 m thick low-velocity zone at and above the main low-angle fault. This observation suggests the water-rich décollement leaks water into the overlying sections. Also evident is a low-velocity section associated with turbidite sands. These results are confirmed with sparse logging data and well samples. Deep-water short offset 3-D experiments provide a potentially effective approach for velocity estimation, replacing the operational complexity of long-offsets with simpler short-offset techniques. In areas of structural complications and abundant diffracted energy, it is a surprisingly accurate method, utilizing the high fidelity 3-D wavefield and the information carried in zero-offset diffraction ellipsoids. The velocity used to properly collapse a diffraction ellipsoid is explicitly the velocity of propagation in the media since the travel path is known exactly. Thus, the derived velocities should closely represent rock velocities, unlike the 2-D case where the propagation geometry is not known.
Bangs, N. L., and S. C. Cande, Episodic development of a convergent margin inferred from structures and processes along the southern Chile margin, Tectonics, 16, 489-503, 1997, 92 citations, #1276
Seismic reflection data acquired in the vicinity of Isla Mocha across the southern coast of Chile image structures formed along the continental margin and reveal an episodic history of accretion, nonaccretion, and possibly erosion. Structures formed at the toe of the continental slope suggest frontal accretion of þ to 1 þ km of trench fill. Seismic images also reveal that a small accretionary wedge, 20ââ¬â30 km wide, abuts the truncated continental metamorphic basement that extends seaward from beneath the shelf. The small size of the accretionary wedge on three profiles examined here is not consistent with a long history of accretion with the current deformational style, as current rates of frontal accretion could have accumulated all of the existing accretionary wedge in less than 1ââ¬â2 m.y. This is a small fraction of convergence history along this margin, and the current accretionary mode has not been consistently maintained in the past. The Isla Mocha region is located between the temperate climate of central Chile and the glacial climate of southern Chile, and climatic conditions in this region have likely fluctuated sufficiently to cause significant variation in trench sediment supply. Accretionary and nonaccretionary or erosional episodes are probably linked to temporal variations in trench sediment thickness, as suggested by observations along the Chile margin. Currently, thick trench sediment correlates with accretion along the southern Chile margin, and thin trench sediment correlates with nonaccretion/tectonic erosion as near the Chile Ridge and from the Juan Fernandez Ridge to northern Chile. The Isla Mocha region also lies 900 ââ¬â 1000 km north of the Chile triple junction, and the Chile Ridge lies approximately 2000 km to the west and has not yet collided and affected the margin near Isla Mocha. This part of the precollision zone provides an excellent reference to examine the effects of Chile Ridge collision in the development of the Chile margin. The most apparent effect of subduction of the buoyant, young crust of the Chile Ridge is a shallow trench that is nearly devoid of sediment. Consequently, the triple junction is undergoing nonaccretion or erosion, and the accretionary complex near the triple junction remains smaller than to the north or south because the current phase of rapid accretion elsewhere in the trench has bypassed the triple junction region. The interplay of subduction zone processes, such as trench sedimentation and ridge collision, has resulted in an episodic development of the margin and produced a discontinuous record of convergence history within the accretionary wedge.
Bangs, N. L., T. H. Shipley, and G. F. Moore, Elevated fluid pressure and fault zone dilation inferred from seismic models of the northern Barbados Ridge decollement, J. Geophys. Res., 101, 627-642, 1996, 25 citations, #1179
In 1992, a large volume of three-dimensional seismic reflection data were acquired in a 5 Ã 25 km area across the toe of the Barbados accretionary complex that covers the Deep Sea Drilling Project leg 78A and Ocean Drilling Program legs 110 and 156 drilling sites. These data are used to examine the acoustic character of the decollement seismic reflection and to qualitatively and quantitatively characterize fluid pressures within the fault zone. Seismic models have been constructed across a 6-km region of the decollement where it has been mapped as a moderate to bright polarity-reversed reflection. The models show that this segment of the decollement reflection is caused by a low-velocity interval, usually 12â16 m thick. The top of the low-velocity interval appears to be a sharp boundary that requires a decrease in velocity from 1.8 km/s to between 1.7 and 1.65 km/s, with some localized bright reflections with an even lower velocity of 1.6 km/s. The base of the low-velocity layer is less certain from modeling. The base consists of either a velocity increase that is usually approximately half the velocity contrast at the top of the layer, or the velocity increase is equal to the contrast at the top of the layer but distributed over a 10-m-thick interval. Comparison of these results to laboratory experiments on the relationship between fluid pressure and seismic velocity indicates that in this interval of the decollement, fluid pressure is at or near lithostatic. Furthermore, the reflection coefficients of the decollement are sufficiently large that some dilation of the fault zone is required. The dilation should lead to high fracture zone permeability and explain the observation of a laterally consistent decollement reflection along a 5-km segment of the decollement. It is within these segments of the fault that fluid pressure approaches lithostatic and significantly reduces fault strength.
Brown, K. M., N. L. Bangs, P. N. Froelich, and K. A. Kvenvolden, The nature, distribution and origin of gas hydrate in the Chile triple junction region, Earth Planet. Sci. Lett., 139, 471-483, 1996, 42 citations, doi:10.1016/0012-821X(95)00243-6, #1248
A bottom simulating reflector (BSR) is regionally distributed throughout much of the Chile Triple Junction (CTJ) region. Downhole temperature and logging data collected during Ocean Drilling Program (ODP) Leg 141 suggest that the seismic BSR is generated by low seismic velocities associated with the presence of a few percent free gas in a 10 m thick zone just beneath the hydrate-bearing zone. The data also indicate that the temperature and pressure at the BSR best corresponds to the seawater/methane hydrate stability field. The origin of the large amounts of methane required to generate the hydrates is, however, problematic. Low total organic carbon contents and low alkalinities argue against significant in situ biogenic methanogenesis, but additional input from thermogenic sources also appears to be precluded. Increasing thermal gradients, associated with the approach of the spreading ridge system, may have caused the base of the hydrate stability field to migrate 300 m upwards in the sediments. We propose that the upward migration of the base of the stability field has concentrated originally widely dispersed hydrate patches into the more continuous hydrate body we see today. The methane can be concentrated if the gas hydrates can form from dissolved methane, transported into the hydrate zone via diffusion or fluid advection. A strong gradient may exist in dissolved methane concentration across the BSR leading to the steady reabsorbtion of the free gas zone during the upward migration of the BSR even in the absence of fluid advection.
Bangs, N. L., and K. M. Brown, Regional heat flow in the vicinity of the Chile triple junction constrained by the depth of the bottom-simulating reflection, Proc. Ocean Drill. Prog., Sci. Results, 141, 253-258, 1995, #1073
Bangs, N. L., D. S. Sawyer, and X. Golovchenko, The cause of the bottom-simulating-reflection in the vicinity of the Chile triple junction, Proc. Ocean Drill. Prog., Sci. Results, 141, 243-252, 1995, #1102
Brown, K. M., and N. L. Bangs, Thermal regime of the Chile triple junction: Constraints provided by downhole temperature measurements and distribution of gas hydrate, Proc. Ocean Drill. Prog., Sci. Results, 141, 259-275, 1995, #1101
Brown, K. M., N. L. Bangs, K. M. Marsaglia, P. N. Froelich, Y. Zheng, B. M. Didyk, D. Prior, E. L. Richford, M. E. Torres, V. B. Kurnosov, N. Lindsley-Griffin, S. Osozawa, and A. Waseda, A summary of ODP Leg 141 hydrogeologic, geochemical, and thermal results, Proc. Ocean Drill. Prog., Sci. Results, 141, 363-372, 1995, #1103
Moore, G. F., Z. Y. Zhao, T. H. Shipley, N. L. Bangs, and J. C. Moore, Structural setting of the Leg 156 area, northern Barbados Ridge accretionary prism, Proc. Ocean Drilling Prog., Init. Rept., 156, 13-27, 1995, #1074
Sawyer, D. S., N. L. Bangs, and X. Golovchenko, Deconvolving Ocean Drilling Program temperature logging tool data to improve borehole temperature estimates: Chile triple junction, J. Geophys. Res., 99, 11995-12003, 1994, #1055
We present a technique for correcting borehole fluid temperature observations made by the Ocean Drilling Program (ODP) with the Lamont temperature logging tool (TLT), for the effects of the slow temperature response of one of its sensors. TLT data have been recorded in many ODP boreholes, but, perhaps partly because of tool response effects, the data have only rarely been used. It has been shown that a continuous temperature log is the convolution of a tool response function and the temperature history experienced by the tool. We use temperature data from ODP Leg 141 to estimate the tool response function of the TLT. We then use Wiener filter theory to design a decon volution operator to remove the effect of the tool response from the recorded data. We apply the deconvolution operator to the data from Leg 141, assess the effectiveness of the deconvolution technique, and extrapolate the resulting borehole fluid temperatures to estimate the equilibrium geotherm at the two sites considered. The geothermal gradient in the accretionary wedge near the Chile Triple Junction increases with depth. This suggests that the thermal environment is not steady state, that fluid flow is transporting heat, or, most likely, both. The average heat flow in the accretionary wedge near the Chile Triple Junction is higher over the subducting Chile Ridge axis than over subducting young oceanic crust near the ridge axis.
Shipley, T. H., G. F. Moore, N. L. Bangs, J. C. Moore, and P. L. Stoffa, Seismically inferred dilatancy distribution, northern Barbados Ridge decollement: Implications for fluid migration and fault strength, Geology, 22, 411-414, 1994, 92 citations, doi:10.1130/0091-7613(1994)022<0411:SIDDNB>2.3.CO;2, #1025
A 5 x 25 km, three-dimensional seismic survey of the lower part of the northern Barbados Ridge accretionary prism creates a three-dimensional image of a major active decollement fault. The fault is usually a compound negative-polarity reflection modeled as a low-velocity, high-porosity zone less than ∼14 m thick. This thickness is significantly less than that defined by drilling of a >40 m zone of deformation at Ocean Drilling Program (ODP) Site 671B, located within the surveyed area. We infer that the seismically defined fault is a thin, high-porosity zone and is thus an undercompacted, high-fluid-pressure dilatant section. If these inferences are correct, then map-view variations in seismic-reflection waveform and amplitude illustrate complex patterns of fault-zone fluid content and fluid migration paths. The amplitude map suggests kilometre-wide channels of locally high porosity and thus focused fluid flow. These paths are only subparallel to the expected minimum head, as inferred from the shape of the overlying sediment wedge; other factors must modify fluid concentrations and ultimately migration. Several areas of positive-polarity fault reflections define square-kilometre-sized regions inferred to be lower porosity sections producing strong asperities in an otherwise weak fault. One, coincident with Site 671B, may explain the success of drilling through the fault here. All other holes drilled in the area were within the negative-polarity regions and were unsuccessful in penetrating through the entire fault zone, possibly because of instability associated with high fluid pressures and a weak fault. ODP Leg 156 planned for 1994 will test inferences related to fault permeability and fluid pressures.
Bangs, N. L., D. S. Sawyer, and X. Golovchenko, Free gas at the base of the gas hydrate zone in the vicinity of the Chile triple junction, Geology, 21, 905-908, 1993, 88 citations, doi:10.1130/0091-7613(1993)021<0905:FGATBO>2.3.CO;2, #1039
At Ocean Drilling Program Site 859 in the vicinity of the Chile triple junction, the source of the bottom simulating reflection (BSR) at the base of the gas hydrate layer has, for the first time, been logged to reveal the nature of the impedance contrasts producing the reflection. We estimate from the P-wave velocity (Vp) that hydrate occupies no more than 18% of the pore space just above the BSR and is not concentrated enough to cause the reflections. The BSR is caused by a sharp drop in Vp, and presumably density, from ∼1950 to 1600 m/s (on average) within an 8 m interval. Seismic modeling of wave form and amplitude vs. offset of the BSR at Site 860 indicates that the BSR is produced by a 12 m interval with low Vp and shear-wave velocities that are consistent with small quantities of free gas (∼1% of pore space) in the interval.
Bangs, N. L., S. C. Cande, S. D. Lewis, and J. Miller, Structural framework of the Chile margin at the Chile Ridge collision zone, Proc. Ocean Drilling Prog., Init. Rept., 141, 11-21, 1992, #966