 |
Nathan L. B. BangsSenior Research ScientistPh.D., Marine Geophysics, Columbia University, New York, NY, 1990 Telephone 512-471-0424; FAX 512-471-0348
|
Nathan is interested in structural development and tectonic processes along convergent margins. He uses advanced MCS methods to acquire 3-D images of structure and stratigraphy within subduction zones in order to examine tectonic activity and deformational styles and to study fluid migration in accretionary prisms. His expertise also includes the processing, inversion, and modeling of seismic reflection data. Recently, Bangs has studied the accretionary complexes of Barbados, Chile, and the Aleutians. He is currently the U.S. co-leader of a major U.S.-Japan project to characterize changes in the physical properties of a portion of the Nankai Trough subduction zone offshore Japan. The project aims to determine how these changes relate to earthquake activity. Three-dimensional seismic data collected during summer 1999 will be used to determine at which depths sediments have sufficient strength to store the large stress energies that could be released in tsunamagenic earthquakes.
Research Projects Martin, K. M., S. P. S. Gulick, N. L. Bangs, G. F. Moore, J. Ashi, J.-O. Park, and S. Kuramoto, Possible strain partitioning structure between the Kumano forearc basin and the slope of the Nankai Trough accretionary prism, Geochem., Geophys., Geosyst., (in press), 2010 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, 24-49, 2009, doi:10.1016/j.epsl.2009.04.026, 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, 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, doi:10.1029/2008GL036974, 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, doi:10.1029/2007JB004993, 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, doi:10.1126/science.1147195, 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, doi:10.1130/G22451.1, 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, doi:10.1029/2005GL023945, 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, 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, doi:10.1130/G22008.1, 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 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, doi:10.1029/2004JB003293, 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 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, doi:10.1029/2005GL023095, 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, doi:10.1029/2004GL021861, 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, doi:10.1130/G20211.2, 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 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, doi:10.129/2003JB002654, 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 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, 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, 2004, doi:10.1029/2001JB000230, Trehu, A. M., P. B. Flemings, N. L. Bangs, J. Chevallier, E. Gracia, J. E. Johnson, C.-S. Liu, X. Liu, M. Riedel, and M. E. Torres, Feeding methane vents and gas hydrate deposits at south Hydrate ridge, Geophys. Res. Lett., 31, L23310, 2004, doi:10.1029/2004GL021286, 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, doi:10.1016/j.epsl.2004.03.035, 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, doi:10.1029/2002JB002040, 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, doi:10.1029/2002JB002243, 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, doi:10.1130/0016-7606(2002)114<0051:COISAF>2.0.CO;2, 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 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 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, 2000, doi:10.1023/A:1004782815985, 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, 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, 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, doi:10.1130/0091-7613(1999)027<0031:SACOTA>2.3.CO;2, 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 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, doi:10.1130/0091-7613(1998)026<0811:CPDIAE>2.3.CO;2, 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, 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, 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, doi:10.1016/0012-821X(95)00243-6, 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 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 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 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 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 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, 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, doi:10.1130/0091-7613(1994)022<0411:SIDDNB>2.3.CO;2, 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, doi:10.1130/0091-7613(1993)021<0905:FGATBO>2.3.CO;2, 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
Collaborative Research: A 3-D Seismic Investigation of the Nankai Trough Plate Boundary System in the Kumano Basin
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.
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.
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.
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.
Article Tools
Save to My Folders
Download Citation
Alert Me When Article is Cited
Post to CiteULike
E-mail This Page
Submit an E-Letter
Commercial Reprints and Permissions
View PubMed Citation
Related Content
Similar Articles In:
Science Magazine
Web of Science
PubMed
Search Google Scholar for:
Articles by Moore, G. F.
Articles by Tobin, H. J.
Search PubMed for:
Articles by Moore, G. F.
Articles by Tobin, H. J.
Find Citing Articles in:
Web of Science (2)
HighWire Press
CrossRef
Google Scholar
Scopus
My Science
My Folders
My Alerts
My Saved Searches
Sign In
More Information
More in Collections
Geochemistry, Geophysics
Related Jobs from ScienceCareers
Geological Sciences
Science 16 November 2007:
Vol. 318. no. 5853, pp. 1128 - 1131
DOI: 10.1126/science.1147195
Prev | Table of Contents | Next
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.
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.
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.
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 0–20° 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
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.
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 120–150 m below seafloor and exhibits typical characteristics of a gas hydrate BSR. We also imaged a second weaker BSR (BSRs), 20–40 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 1.75°–2.25° 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.
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.
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.
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 25–30 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.
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.
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.
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 (2–5.8 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 ∼25–30 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.
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.
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 (30–40% of pore space or 20–26% 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.
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.
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 25–35 km in length and ∼10 km in width, rise 1–6 km above the adjacent basement. The eastern edge of one of the ridges deepens by ∼4–6 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 ∼10–35 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 6–6.5 km/s, intersects the backstop at a depth of ∼14–15 km. The updip limit of plate boundary seismicity is located 75–100 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.
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-northeast– 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.
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 s–1 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.
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 north–south 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 north–south 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.
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 × 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 (70–75%) 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.
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.
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.
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 ¾ to 1 ¾ km of trench fill. Seismic images also reveal that a small accretionary wedge, 20–30 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 1–2 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 – 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.
In 1992, a large volume of three-dimensional seismic reflection data were acquired in a 5 × 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 12–16 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.
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.
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.
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.
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.