Fluid expulsion and overpressure development during initial subduction at the Costa Rica convergent margin

Abstract

A high convergence rate and almost complete subduction of incoming sediments make the Costa Rica convergent margin an extremely dynamic environment in which to examine the interrelationship of tectonically driven loading, compaction, and fluid expulsion. In this study, we investigate overpressure development and fluid expulsion using a numerical modeling method that allows fluid pressures, porosities, and permeabilities to evolve during subduction. Incoming sediments at the Costa Rica margin consist of ∼180 m of hemipelagic sediments overlying 200 m of pelagic carbonates. Results of the modeling suggest that a permeability–porosity relationship for the hemipelagic sediments derived from laboratory tests (log( k) = −22 + 7.8 × porosity) is compatible with pore pressures inferred from consolidation tests and compaction rates estimated from sediment porosity and thickness changes, in contrast to results of previous simplified calculations. Comparison of one and two-dimensional modeling indicates that lateral fluid flow modifies the pore pressure profile only moderately, unless the permeability of the pelagic carbonate sediment was an order of magnitude or more higher than indicated by laboratory tests (5 × 10 −16 m 2). Due to complete subduction of the incoming sediment column, high porosity, and thus high permeability sediments are directly below the decollement zone. Rapid dewatering of these sediments can support high rates of fluid flow along the decollement or upward through the wedge. However, within the first 1.5 km of subduction, drainage leads to a rapid decrease in simulated porosity (0.8 to 0.5) of the uppermost sediments, which would decrease permeabilities to 0.5% of initial values and inhibit further dewatering.