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

Abstract

Detailed structural and stratigraphic analysis of a three‐dimensional seismic 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 with relief of about 300 m and spacing of 5–6 km. The small‐scale structure is characterized by a series of NNE trending normal faults, resulting in horsts and graben with relief of 50–200 m and spacing of 0.2–2 km. The structural fabric at both scales is preexisting. Structures in the underthrust sediment are mostly secondary, having formed by differential compaction. Three distinctive seismic stratigraphic units are defined in the underthrust sedimentary section. The upper Cretaceous to lower Eocene unit mostly fills in basement lows, resulting in great thickness variations. The thickness of the 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 rate of 1008 m3/yr per kilometer of strike length from the toe. The fluid discharge is expected to increase to 1092 m3/yr within 64,000 years as a thicker sediment section is subducted. Our results also suggest that the basement indirectly controls fluid movement in the underthrust Oligocene unit by creating secondary normal faults that act as major fluid conduits between the overlying décollement and the underlying, more permeable middle to upper Eocene turbidite‐bearing section. Ponded sediments between basement horst blocks are not faulted and may compact more slowly.