Seismically inferred dilatancy distribution, northern Barbados Ridge decollement; implications for fluid migration and fault strength

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doi: 10.1130/0091-7613(1994)022<0411:SIDDNB>2.3.CO;2
Author(s): Shipley, Thomas H.; Moore, Gregory F.; Bangs, Nathan L.; Moore, J. Casey; Stoffa, Paul L.
Author Affiliation(s): Primary:
University of Texas, Institute for Geophysics, Austin, TX, United States
University of Hawaii, Honolulu, HI, United States
University of California, Santa Cruz, CA, United States
Volume Title: Geology (Boulder)
Source: Geology (Boulder), 22(5), p.411-414. Publisher: Geological Society of America (GSA), Boulder, CO, United States. ISSN: 0091-7613 CODEN: GLGYBA
Note: In English. Univ. Hawaii, Sch. Ocean and Earth Sci. and Technol., Contrib. No. 3512; Univ. Tex., Inst. Geophys., Contrib. No. 1025. 21 refs.; sects., sketch map
Summary: A 5×25 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.
Year of Publication: 1994
Research Program: ODP Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; 18 Geophysics, Solid-Earth; 20 Geophysics, Applied; Accretionary wedges; Atlantic Ocean; Barbados Ridge; Decollement; Dilatancy; Faults; Fluid phase; Geophysical methods; Geophysical surveys; Leg 110; Marine sediments; Mechanical properties; Mid-ocean ridges; North American Atlantic; North Atlantic; ODP Site 671; Ocean Drilling Program; Ocean floors; Pore pressure; Pressure; Reflection methods; Sediments; Seismic methods; Surveys; Three-dimensional models
Coordinates: N153000 N153500 W0584000 W0590200
Record ID: 1994026281
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