Abstract
Oceanic heat flow surveys were conducted in the Guaymas Basin, Gulf of California and on the southern flank of the Costa Rica Rift, eastern equatorial Pacific. Thermal data from both field areas reflect the passage of fluids of varying temperatures through the sediments and crust, and help to constrain numerical models of coupled mass and energy transfer.Fluids are passively convecting through oceanic sediments and crust around DSDP Site 501/504 on the southern flank of the Costa Rica Rift. The presence of fluid circulation is supported by new data collected during ODP Leg 111 and an earlier survey of the heavily sedimented, 5.9 Ma site. While deep crustal convection appeared to be required to explain the broadly spaced heat flow anomalies, this process seemed to be precluded by measurements of low permeability in the oceanic crust beneath the upper 200 meters.The seemingly incompatible observations near Site 501/504 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 driven, passive fluid circulation. The major observation not explained with this model is an apparent reduction in heat flow with depth in DSDP Hole 504B, which probably results from borehole convection.There is vigorous, axial hydrothermal circulation in the Guaymas Basin, a thickly sedimented, 2 km-deep rift in the Gulf of California. The basin is composed of sub-parallel northern and southern troughs which are the physiographic expressions of seafloor spreading. Enormous sediment input in the basin precludes the formation of 'normal' oceanic crust.Heat flow data delineate complicated thermal processes in the basin. Heat flow patterns are largely governed by the geometry of heat sources, shallow sills and structural trends. Broad heat flow highs, several square kilometers in area, result from a combination of deep, conductive heat sources; slow, intergranular fluid circulation; and high-temperature venting. Heat flow lows in between the highs and near normal faults which bound the basin may indicate hydrothermal recharge. (Abstract shortened with permission of author.)