The hydrogeological regime of isolated sediment ponds in mid- oceanic ridges.

Abstract

The determined temperature structure, together with the measurement of a subhydrostatic shut-in pressure of 0.9-1.5bars at a sub-bottom depth of 179m, suggests that water is still flowing down the hole and into the basement 5 yr after the hole was drilled. delta 18O values in calcite taken from veins in the basement demonstrate that a similar temperature structure existed before drilling. Resistivity logs indicate that the upper 400m of the crust is characterized by high porosity, but that the porosity drops markedly in the lower part of the section. This is consistent with direct measurements of permeability. These constraints, together with measured heat flow a values in the overlying sediment pond, suggest that the upper 400-500m of crust around Hole 395A is being cooled by the lateral flow of pore water moving at a rate of about 1m/yr.-from Authors