University of California, Santa Cruz, CA, United States

A 5X25 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 approximately 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.