(Table 2) Bottom-hole temperature measurements at DSDP Sites 68-501, 69-504, and 69-505

Drilling durin Deep Sea Drilling Project Legs 68, 69, and 70 on the southern limb of the Costa Rica Rift was used to study geothermal processes in the ocean crust. Two areas were drilled. One was a geothermally hot site on 6.2-m.y.-old crust, where topography is smooth, heat flow is close to that predicted by conductive cooling of the lithosphere (200 mWm**-2), and hydrothermal circulation may be sealed within the crust. The other was on 3.9-m.y.-old crust, where rough topography is associated with low heat flow (15 to 50 mWm**-2) and possible open convection of sea water. At both sites, about 250 m of siliceous-calcareous sediments overlies igneous basement. In the hot area, it blankets the topography, whereas in the cold area, basement outcrops still occur. Operations included numerous down-hole experiments in both areas and hydraulic piston coring of a 230-m sediment section in the hot area. […]