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Determination of the rate and distribution of heat flow through the Earth's crust provides one of the most direct sources of information on the temperature distribution within the earth. Many thousands of heat flow measurements have been made in the ocean basins by measuring the thermal gradient in the uppermost 10 meters of the sea floor using conventional oceanographic techniques (e.g., Langseth, 1965). The Deep Sea Drilling Project has provided the opportunity to measure in situ sediment temperatures to depths of hundreds of meters beneath the sea floor using the equipment and techniques described by Erickson et al. (1975). Heat flow values computed in the same region using both deep borehole and shallow sediment temperature data are in generally good agreement, confirming that the conventional measurements have not been systematically affected by variations in bottom water temperature having durations less than a few thousand years or by other near surface disturbances in the sediments. Within the last few years, increasing attention has been paid to the question of the nature of the physical mechanisms by which heat is transmitted through the oceanic crust beneath the sediments. The discrepancy between heat flow predicted theoretically through young ocean crust using cooling lithosphere models (Parsons and Sclater, 1977) and that measured, the very high variability of the heat flow measured near the axes of active mid-ocean ridges (e.g., Sclater et al., 1974-, Williams et al., 1974) and, more recently, direct observation of active hydrothermal vents on the Galapagos Ridge (Von Herzen, personal communication, 1977), has made it clear that hydrothermal circulation in the ocean crust is an important, if not the dominant factor, in the transmission of heat through the sea floor near spreading mid-ocean ridges. Lister (1972) proposed a model in which the circulation pattern is controlled by a combination of sea floor topography and variations in the permeability of the sea floor owing to impermeable sediment accumulation and sealing of hydrothermal vents by precipitation of secondary minerals. Heat flow measurements precisely located with respect to sea floor topography (Williams et al., 1974) support Lister's idea that elevated areas with little or no

Downhole Temperature Measurements and Thermal Conductivities of Samples, Site 396 Deep Sea Drilling Project Leg 46