MANTLE FLUIDS IN THE SAN-ANDREAS FAULT SYSTEM, CALIFORNIA

Fluids associated with the San Andreas and companion faults in central and southcentral California have high He-3/He-4 ratios. The lack of c orrelation between helium isotopes and fluid chemistry or local geolog y requires that fluids enter the fault system from the mantle. Mantle fluids passing through the ductile lower crust must enter the brittle fault zone at or near lithostatic pressures; estimates of fluid flux b ased on helium isotopes suggest that they may thus contribute directly to fault-weakening high-fluid pressures at seismogenic depths.