Introduction to the Special Section on Fluid Flow in the Costa Rica Accretionary Prism

Abstract

Previous 2D and 3D seismic reflection studies off the Nicoya Peninsula, on the Pacific margin of Costa Rica indicated a reversed polarity décollement beneath the toe of the prism (Shipley et al., 1990), consistent with elevated fluid pressures. In addition, Shipley and Moore (1986) reported a decrease in thickness of underthrust sediment beneath the toe of the wedge of up to 50% over the outer 3–5 km, implying massive dewatering through the toe. Because such dewatering must pass through the toe of the wedge, we suspected the presence of significant fluid venting. During February 1994, we carried out 20 ALVIN dives in search of indicators of fluid venting on the Costa Rica prism. As the trench off Costa Rica reaches depths of 4500m, the ALVIN went through a process of refit and Navy clearance to dive to depths of up to 4500m, and our deepest dive reached that depth. In addition to the dives, we mapped a broad area of the slope with SeaBeam, obtained a series of “pogo” heat flow transects, and recovered a series of piston cores and giant gravity cores for the study of pore water geochemistry. Here we report on the structures observed in the dive program (McAdoo et al.), the nature and locations of fluid vents (Kahn et al.), the heat flow results (Langseth and Silver), the pore water studies (Zuleger et al.), and a reinterpretation of surface amplitude data from 3D seismic records, finding a remarkable association between amplitude anomalies and regions of fluid vents (Mclntosh and Silver).