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Heat transfer processes at the mounds area of the Galapagos Spreading Center at 86 °W are revealed by temperatures measured at 10-meter intervals in the 30 ± 10 meter sediment at each of 12 holes at Leg 70, Sites 506-509. Two hundred fourteen needle-probe values show a significant linear increase of thermal conductivity with depth in each hole. About half of the temperature-thermal resistance profiles are nonlinear and are fit to a steady-state, vertical porewater convection model. Results indicate high and variable total heat flow and localized hydrothermal discharge at < 10~ m/s, associated with individual mounds. Recharge at similar rates is indicated in the low heat-flow belt 5 km south of the mounds. Possible slow entrained recharge within 100 meters of discharging mounds is suggested. At Site 510, temperatures in the 114-meter sediment cover on 2.7 million-year old crust are linear, suggesting that the hydraulic resistance of this layer is sufficient to seal off free hydrothermal exchange between basement and bottom water. The combination of heat-flow data and physical-properties data of Karato and Becker (this volume) suggests that 50 meters of sediment may be a threshold thickness for sealing of hydrothermal circulation within basement, where the topography is smooth. We suggest that the formation of mounds may be associated with the forced localization of hydrothermal discharge through the sediment, as its thickness approaches this threshold value.

Geothermal Measurements from Drilling of Sediments near the Galapagos Spreading Center, 86°W, Deep Sea Drilling Project Leg 70