THE PERMEABILITY OF YOUNG OCEANIC-CRUST EAST OF JUAN-DE-FUCA RIDGE DETERMINED USING BOREHOLE THERMAL MEASUREMENTS

Temperature measurements made in Ocean Drilling Program (ODP) Hole 102 6B, drilled into a sediment-buried basement ridge in 3.5 m.y. old crus t on the eastern flank of Juan de Fuca Ridge, revealed fluid flow from the formation and into the overlying ocean. This flow indicates that the upper basaltic crust at this site is naturally overpressured relat ive to hydrostatic. This finding is consistent with output from previo usly published numerical models of hydrothermal circulation in the upp er crust that include appropriate crustal geometry, sediment thickness ,;and basal heat flow. The fluid flowing into Hole 1026B enters the ho le through the upper 10 m of basalt below the sediment-basement contac t. The flow rate estimated from the thermal data (80-120 m/hr), in com bination with the inferred basement overpressure relative to hydrostat ic (about 20-30 kPa), was used to estimate the bulk permeability of th e shallowest oceanic crust: 5 to 9 x 10(-12) m(2). Such high bulk perm eability would allow regionally significant hydrothermal circulation, consistent with thermal homogenization of upper basement in this area.