Abstract:
Seafloor sediments involved in subduction are subject to a rapid increase in lithostatic stress due to forced burial, and to tectonic stress. The clay rich sediments at Barbados are overpressured because of low permeability and release of bound water from clays. Drilling and logging in accretionary complexes are difficult so seismic techniques are used to map porosity and fluid pressure and deduce the likely escape paths of the porewater. At our site the toe of the Barbados accretionary complex consists of an upper accreted zone of stacked thrust slices, then a thin decollement zone, then an undisturbed zone of sediments riding down (at 20mm/year) with the subducting plate. A 3-D seismic reflection survey has been shot over this area and vertical seismic profiles and P waves from ocean bottom shots recorded in ODP borehole 949C within the 400-m thick accreted zone. We deduced velocity-porosity-overpressure relations from well logs to produce porosity and overpressure sections. These show a continuous zone of overpressure (lambda-pore pressure/overburden pressure-up to 0.9) in the basal 80 m of the accreted sediments, above the decollement. The zone has low S-wave velocity and high Poisson's ratio (up to 0.477), probably indicating loss of cohesion. Above that, the thrust faults seen in the seismic section appear to act as barriers to flow, dividing the accreted sediments into packets of distinct pore pressure. Reflections from the decollement zone are modeled by a 12.5-m thick low velocity zone indicating porosity about 55% but the anisotropy seen in cores was not resolved.