HEAT-FLUX THROUGH AN OLD (APPROXIMATE-TO-175 MA) PASSIVE MARGIN - OFFSHORE SOUTHEASTERN UNITED-STATES

New heat flow data on the southeastern United States passive margin sh ow that measured, uncorrected flux averages 49+/-11.8 mW m(-2) through old (similar to 175 Ma)oceanic crust. Nonuniform thermal gradients we re measured at about half of the 114 penetrations that comprise the da ta set and over the full range of water depths (1900 m to 4250 m) at w hich data were collected. With the simplifying assumption that the non uniform gradients were caused by a step function change in bottom wate r temperatures at some time before the heat flow cruises, concave down (decreasing dT/dz with depth) and concave up (increasing dT/dz) sedim ent thermal gradients can be explained by respective average temperatu re decreases and increases of 0.1-0.2 K amplitude occurring 28-35 days before the measurements. Thermal gradients throughput the entire regi on are strongly influenced by oceanographic phenomena and locally by t he presence of subsurface diapiric structures, while sediment thicknes s variations and lateral differences in sedimentation rate and sedimen t lithology appear to exercise relatively little control over thermal regimes. Mean reduced heat flow in the study area is estimated at simi lar to 49 mW m(-2) by decompacting and back stripping the 5-8 km of se diment deposited on the margin since the formation of the underlying o ceanic crust. This value agrees with previous measurements made on you nger crust in the Blake Ridge area but is significantly higher than th e reduced heat flow value in a similar passive margin setting at the B altimore Canyon Trough. We use the calculated average thermal gradient value and an assumption of constant conductivity to estimate temperat ures of 19.5 degrees C to 24.5 degrees C at the bottom-simulating refl ector (BSR) on the Blake Ridge slope.