Fluid accumulation and channeling along the northern Barbados Ridge decollement thrust

A volume of three-dimensional seismic reflection data, acquired in 1992, im aged 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 reflect ion by inversion for seismic impedance (the product of velocity and density ) throughout the 5 x 25 km survey area and thus map physical property varia tions. In 1997, Ocean Drilling Program Leg 171A penetrated the protodecolle ment and decollement at five sites with a logging-while-drilling (LWD) tool to log density and other physical properties of the decollement. We constr uct a regional map of density, and inferred porosity, within the decollemen t from seismic models calibrated with LWD density data. In the sediments ou t in front of the trench the protodecollement forms in a radiolarian-rich M iocene mudstone with an anomalously high porosity (70-75%) that appears as a pervasive, inherent characteristic of this interval seaward of the deform ation front. In the decollement beneath the-wedge a consolidation trend of decreasing porosity runs perpendicular to the deformation front with porosi ty decreasing from 70% at the wedge toe to 50% 4 km from the wedge toe. A s econd, 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. T his zone can be explained as an area of the decollement where fluid accumul ations develop by maintaining high fluid content. We postulate that high fl uid content is maintained by continuous recharge flowing into and along thi s channel. This porosity distribution within the decollement also strongly influences fluid migration into the overlying accretionary wedge and is dir ectly associated with fluid charging of ramps and out-of-sequence thrusts a bove the decollement.