Elevated fluid pressure and fault zone dilation inferred from seismic models of the northern Barbados Ridge decollement

In 1992, a large volume of three-dimensional seismic reflection data were acquired in a 5 x 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.