Deformation and dewatering of the subducting plate beneath the lower slopeof the northern Barbados accretionary prism

Detailed structural and stratigraphic analysis of a three-dimensional seism ic data set reveals two scales of structural fabric in the westward dipping oceanic basement under the toe of the northern Barbados accretionary prism . The large-scale fabric consists of three NNE trending structural highs wi th relief of about 300 m and spacing of 5-6 km. The small-scale structure i s characterized by a series of NNE trending normal faults, resulting in hor sts and graben with relief of 50-200 m and spacing of 0.2-2 km. The structu ral fabric at both scales is preexisting. Structures in the underthrust sed iment are mostly secondary, having formed by differential compaction. Three distinctive seismic stratigraphic units are defined in the underthrust sed imentary section. The upper Cretaceous to lower Eocene unit mostly fills in basement lows, resulting in great thickness variations. The thickness of t he Oligocene and middle to upper Eocene units is less variable. We estimate that the upper two sedimentary units have lost 25% of their total initial void space owing to the load of the accretionary prism within the first 3.5 km landward of the thrust front. This suggests a current fluid expulsion r ate of 1008 m(3)/yr per kilometer of strike length from the toe. The fluid discharge is expected to increase to 1092 m(3)/yr within 64,000 years as a thicker sediment section is subducted. Our results also suggest that the ba sement indirectly controls fluid movement in the underthrust Oligocene unit by creating secondary normal faults that act as major fluid conduits betwe en the overlying decollement and the underlying, more permeable middle to u pper Eocene turbidite-bearing section. Ponded sediments between basement ho rst blocks are not faulted and may compact more slowly.