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Ocean Drilling Program Leg 139 drilled at two sites located in an elongate zone of exceptionally high heat flow. Site 858 was located over an active vent field where heat flows exceeding 20W/m have been measured. Site 857 was drilled 1.5 km to the south in an area where the local heat flow is about 1 W/m. Drilling and logging revealed strong contrasts in igneous basement structures and thermal regimes between the two sites. The lower 469 m of Hole 857D, which penetrated to a total depth of 936 m, consists of at least 27 distinct basaltic sills intercalated with turbidite sediments. Deep-sea drilling in Guaymas Basin, Yamato Basin, and now Middle Valley suggest that this layer-cake basement structure is typical of spreading centers located near copious sources of sediment. Temperatures are greater than 250°C at 500 mbsf at Site 857, but the temperature gradient to that depth is nearly uniform, suggesting that heat flow in the vertical direction is predominantly conductive. However, pore-water and sediment chemistry indicate lateral flow of pore water within the sediments. After Hole 857D was drilled into the sill-and-sediment complex, water flowed down the hole at an enormous rate and entered basement at two or more discrete and thin zones, one of which is believed to be associated with a normal fault. Basalt was encountered at a shallower depth (258 mbsf) in Hole 858G. Logging and coring in the basaltic section indicate that basement below this site was constructed of extrusives. Seafloor heat flow, downhole temperature measurements, and open-hole temperature logs suggest the presence of a broad upwelling plume of high temperature water ( 280°C) comparable to that flowing from the vents. The results indicate that the active vent field is located over an extinct volcanic center which in the past may have stood as a small seamount above the floor of Middle Valley. Correlation of logs taken at Sites 857 and 858 sets constraints on the magmatic and tectonic evolution of the area encompassing the two sites.

Structure of Igneous Basement at Sites 857 and 858 Based on Leg 139 Downhole Logging