Fluid flow occurs in several different systems in the Costa Rica subduction zone. One is a sediment and upper plate system, which is itself composed of three subunits. The structurally lowest sediment subunit is composed of the underthrust sediments. As they thrust beneath the upper plate, they are rapidly dewatered. Flow occurs several orders of magnitude more rapidly than is expected from measured permeabilities in cores, suggesting that flow is localized along narrow, high-permeability channels and probably exits to the décollement or the seafloor through spaced, vertical conduits. Overpressures that increase with depth in the underthrust section imply little or no communication with the deeper system, discussed below.
The next regime of flow is the décollement. Structurally, this zone is marked by closely spaced shear bands, some filled with secondary smectite, which indicate collapsed former flow paths. The décollement appears structurally to develop by episodic flow and collapse, decreasing porosity, and increasing shear fabrics. Geochemically, the décollement shows a significant decrease in chlorinity, likely due to clay mineral dehydration with depth. The concentrations of K and Li suggest temperatures of 100°-150°C, equivalent to depths of 10-15 km, or distances of 40-60 km landward of the trench. Flow also occurs from the décollement to the deformed sedimentary wedge, where local fractures show very low chlorinity. In general, both the deformed sedimentary wedge and the slope apron show overall lower salinity and chlorinity than seawater values because of a combination of local formation and melting of gas hydrates and fluid influx from the décollement.
The second major flow system occurs in the upper part of the oceanic basement, inferred because of the extreme decrease in heat flow over that expected for conduction and the discovery that chemical species such as Ca, Mg, Li, Sr, K, and others show a rapid change in gradient near the basement interface, moving close to seawater values. Geochemical modeling indicates basement fluid ages of 15 to 20 k.y., and thermal models predict specific discharge rates in the range 1 to 5 m/yr. Such discrepancies represent the range of uncertainty due to lack of specific knowledge of permeabilities, layer thicknesses, driving forces, and recharge-discharge distances. Driving forces probably include temperature differences between the recharge and discharge zones, and in this location may include the addition of cyclic seismic flexure opening and closing flow pathways through a seismic cycle.