Inferred pore pressures at the Costa Rica subduction zone: implications for dewatering processes

Drilling on Ocean Drilling Program (ODP) Leg 170, offshore Costa Rica indicates that the entire incoming sedimentary section is underthrust. Thus, observed changes in the thickness of underthrust sediments as they are progressively buried beneath the margin wedge provide a direct measure of the rate and magnitude of sediment dewatering. Laboratory consolidation tests indicate that in situ excess pore-fluid pressures within the underthrust section range from 1.3 MPa at the top of the section to 3.1 MPa near the base. The inferred pore pressure profile implies that fluids escape the uppermost sediments most rapidly, whereas the basal sediments remain essentially undrained. This interpretation suggests that the sedimentary and underlying ocean crustal hydrologic systems are decoupled. We use a simple model of fluid flow to demonstrate that dewatering of the underthrust sediments can occur via lateral flow only if sediment permeability is strongly anisotropic, or if flow is focused along permeable stratigraphic layers. If significant dewatering occurs by vertical fluid flow, it must occur within closely spaced, high-permeability conduits.