PASSIVE, OFF-AXIS CONVECTION THROUGH THE SOUTHERN FLANK OF THE COSTA-RICA RIFT

Pore fluids are passively convecting through young oceanic sediments and crust around Deep Sea Drilling Project (DSDP) site 504 on the southern flank of the Costa Rica Rift, as inferred from a variety of geological, geochemical, and geothermal observations. The presence of a fluid circulation system is supported by new data collected on Ocean Drilling Program (ODP) Leg 111 and a predrilling survey cruise over the heavily sedimented, 5.9 Ma site. Deep crustal convection had been inferred near site 504 based on the geometry of surface heat flow anomalies with a wavelength of 4-7 km but appears to be precluded by low crustal permeability, as measured in DSDP hole 504B. The widely varied geothermal and hydrogeological observations near site 504 are readily explained by a model which combines 1) basement relief, 2) irregular sediment drape, 3) largely conductive heat transfer through the sediments overlying the crust, and 4) thermal and geochemical homogenization of pore fluids at the sediment/basement interface, which results from 5) topographically induced, passive hydrothermal circulation with large aspect ratio, convection cells. -from Authors