Generation of high pore pressures in accretionary prisms: Inferences from the Barbados Subduction Complex

Abstract

Recent geological and geophysical exploration of several accretionary complexes reveals that pore pressures inside them are much higher than the hydrostatic values. Indirect inferences further suggest that such high pore pressures may be present inside most accretionary complexes. In this study the mechanisms for generating excess pore pressures is investigated by focusing on the Barbados Ridge Complex which is an end‐member of sediment‐rich accretionary prisms. Modeling is carried out using a two‐dimensional finite element procedure, with constraints provided by the following data: Deep Sea Drilling Project results on stratigraphy and lithology, seismic results on sediment structure and thickness, regional geology on tectonic history, and laboratory measurements on mechanical properties of sediments. Coupling among the equations for fluid flow, heat transfer, and sediment deformation is made throughout the computation. The results of the modeling predict that the principal decollement beneath the Barbados Ridge Complex is a zone of high pore pressure, high porosity, and reversal in seismic velocity. These characteristics may be responsible for the mechanical weakness of the decollement, which allows continuous subduction of sediments below it, and for its seismic signatures. Results of the modeling further suggest that tectonic overburden may be the most important mechanism for the generation of high pore pressures inside accretionary prisms. The direct effect of tectonic compression is of secondary importance, and the effect due to temperature increase with subduction is of little significance. Thrust faulting near the toe of accretionary prisms can disturb pore pressure patterns and produce localized highs. Rich sediment supply, low permeability, high subduction angle, and high subduction velocity are all favorable factors in creating high pore pressures inside accretionary prisms.