Geothermal state of DSDP Holes 333A, 395A AND 534A: Results from the Dianaut Program

Heat-flow values determined from the DIANAUT program provide insight into the geothermal state of three DSDP holes situated on the western flank of the Mid-Atlantic Ridge. Very low values (20 mW/m2 max.) observed in 3.5-m.y.-old crust at Hole 333A decrease with depth and indicate persistent basement cooling due to cold water circulation in the porous level of the upper oceanic crust. Cold ocean bottomwater continues to invade the upper level of fractured 7.3-m.y.-old basement at Hole 395A, but temperature v. time records show large-scale temperature fluctuations, probably associated with borehole convection, at 500 mbsf and stable temperatures, probably indicating a slow return to thermal equilibrium, at 600 mbsf. The 154-m.y.-old crust at Hole 534A exhibits a relatively steady thermal state with a heat flow close to theoretical values for a conductively cooling plate; small variations in the thermal gradient recorded in this well probably represent very slow and subtle movement of borehole fluids. The close comparison of these latest results with those obtained several years previously, demonstrates the presence of relatively stable thermal regimes.