Deformation and dewatering of the subducting plate and evolution of the decollement zone under the northern Barbados accretionary prism: Insights from three-dimensional seismic reflection data

A 3-D seismic data set reveals a detailed structure and stratigraphy of the subducting plate and overlying sediment under the toe of the northern Barbados accretionary prism. The oceanic basement shows a preexisting horst and graben structure. The upper Cretaceous to lower Eocene sedimentary unit mostly fills in basement lows. The subducting middle to upper Eocene and Oligocene units overlie a smoothed sedimentary surface. Based on the landward thinning, we estimate that the upper two sedimentary units have lost 25% of their total initial void space within 3.5 km landward of the thrust front due to the load of the prism. This suggests that the current fluid expulsion rate under the 3.5-km prism toe is 1008 m3/yr per kilometer of strike length, much higher than previously published estimates. The fluid discharge is expected to increase to 1092 m3/yr per kilometer of strike length within 64,000 years as a thicker sedimentary 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 decollement and the underlying more permeable middle to upper Eocene turbidite-bearing section.;A constrained seismic inversion was conducted on the 3-D seismic data set to study the physical properties of the decollement/protodecollement zone (PDZ). The inversion results suggest that part of the PDZ is likely scrapped off by the prism. Fluid conduits along the decollement may originate from spatial variations of initial physical properties of the protodecollement and then be enhanced by shear-induced consolidation. There are significant differences in physical properties between the northern and southern PDZ covered by this study. The differences coincide with a change in the structure of the prism. A larger prism taper in the southern area may result from a stronger decollement. The larger prism taper coupled with less sediment supply may cause a relative retreat of the thrust front in the south, resulting in a change in structural orientation from NNW to nearly north-south. The north-south differences may ultimately be controlled by an increase in the elevation of the Tiburon Rise.