SEISMIC MODELING OF THE DECOLLEMENT ZONE AT THE BASE OF THE BARBADOS RIDGE ACCRETIONARY COMPLEX

Wide-aperture seismic reflection profiling of the northern Barbados Ridge accretionary complex at 16-degrees-12'N has imaged the decollement horizon across more than 100 km of the accretionary complex revealing prominent changes in the amplitude of the decollement reflection. Synthetic seismograms from models of the variation of velocity and density with depth were compared with selected seismic traces between 2.5 km seaward of the toe of the accretionary complex and 20 km landward of the toe to determine the changes in velocity-density properties along the decollement thrust zone. Modeling requires a thin, low-velocity layer to develop landward along the decollement beneath the complex, while no such layer exists at the corresponding stratigraphic boundary in the undeformed sediments seaward of the complex. Two-dimensional modeling that incorporated lateral variations in topography, velocity, and density was performed on a 7-km section of the decollement 20 km landward of the deformation front. The best fitting model includes a 20-m-thick low-velocity layer at the decollement horizon with a relative velocity decrease in the layer of 0.2 km/s. Changes in the reflection amplitude along this 7-km section could be produced by either a change in the velocity anomaly within the layer or a thinning of the decollement layer to less than 4 m. The shape of the reflected wavelet from the decollement indicates that the principal cause of the amplitude variation is a change in the velocity contrast rather than a change in thickness. The low-velocity layer can be explained by a decollement shear zone that has high porosity maintained by elevated pore fluid pressure. High fracture permeability within the decollement permits dewatering of sediment attached to the oceanic crust and thrust beneath the accretionary complex.