Numerical Modeling of Dewatering of Underthrust Sediments, Costa Rica Subduction Zone

Drilling-related investigations at the Costa Rica subduction zone have provided constraints on dewatering processes and overpressure generation during initial subduction. The 380 m thick incoming sediment section, which is composed of 160 m of hemipelagic mudstones overlying 220 m of pelagic carbonates, is nearly completely underthrust beneath the margin wedge. Within the first 1.6 km of subduction, drilling results indicated that the upper, hemipelagic unit and the lower, pelagic carbonate unit thin to 67% and 80%, respectively, of their original thickness, presumably due to dewatering. Despite the observed thinning, laboratory consolidation tests suggested that the underthrust sediments are only partly drained in response to rapid loading beneath the margin wedge. In this investigation, we simulated the progressive loading, fluid migration, and compaction of sediments during their initial subduction by combining a loading model with a transient fluid flow model. Results of the modeling indicate that previously measured permeabilities of the hemipelagic sediments are sufficiently high to allow the observed dewatering while being low enough to maintain elevated pore pressures. In contrast, the modeling suggests that the laboratory-determined permeabilities of the pelagic carbonates are too high to be representative of large-scale in situ permeabilities, because the laboratory-derived permeabilities would allow much greater dewatering and compaction than observed.