SURFICIAL EVIDENCE OF FLUID EXPULSION FROM THE COSTA-RICA ACCRETIONARY PRISM

The nature and distribution of authigenic carbonates, chemosynthetic b acterial mats, and unique macrobenthic chemosynthetic communities of b ivalves and tube worms are important for evaluating and reconstructing present and past fluid venting of accretionary complexes. This paper describes the authigenic carbonates, chemosynthetic fauna, and fluid v enting observed at the four tectonic regions of the costa Rica accreti onary wedge in February 1994 during an ALVIN diving program of 20 subm ersible dives. We found no surficial evidence of highly focused fluid venting at the toe of the prism (outermost 3 km), as implied by the ab sence of authigenic carbonates and chemosynthetic fauna. The absence o f vent communities on the lower 3 km of the prism and the relatively e levated heat flow with respect to the adjacent, incoming Cocos plate ( Langseth and Silver, this issue), suggests diffuse, rather than focuse d flow through the toe of the prism. Twelve active and relict vent sit es marked by small clusters of live vesicomyid clams are localized at the bases and tops of out-of-sequence-thrusts, implicating fracture pe rmeability as the fluid conduit in the lower slope region (but upslope from the toe). Vast authigenic carbonates and seven active and relict vent sites marked by large, dense clusters of chemosynthetic organism s predominate the largest mud diapir in the mid-slope region. Fluid ex pulsion appears to be more restricted on the upper slope, with only 2 small but dense vents marked by chemosynthetic fauna observed at one w all of one canyon.