Channelized fluid flow in oceanic crust reconciles heat-flow and permeability data

Hydrothermal fluid circulation within the sea floor profoundly influences t he physical, chemical and biological state of the crust and the oceans. Cir culation within ridge flanks (in crust more than 1 Myr old) results in grea ter heat loss(1-3) and fluid flux(4) than that at ridge crests and persists for millions of years, thereby altering the composition of the crust and o verlying ocean(5,6). Fluid flow in oceanic crust is, however, limited by th e extent and nature of the rock's permeability(7). Here we demonstrate that the global data set of borehole permeability measurements in uppermost oce anic crust(7-9) defines a trend with age that is consistent with changes in seismic velocity(10,11). This trend-which indicates that fluid flow should be greatly reduced in crust older than a few million years-would appear to be inconsistent with heat-flow observations, which on average indicate sig nificant advective heat loss in crust up to 65 Myr old(3). But our calculat ions, based on a lateral flow model, suggest that regional-scale permeabili ties are much higher than have been measured in boreholes. These results ca n be reconciled if most of the fluid flow in the upper crust is channelized through a small volume of rock, influencing the geometry of convection and the nature of fluid-rock interaction.