Geochemistry and hydromechanical interactions of fluids associated with the San Andreas fault system, California

¹⁸O values establish that waters are predominantly of meteoric origin. The chemical compositions of water and gases are controlled mainly by the ambient rock types, and chemical geothermometry gives reservoir temperatures of 80-150 degrees C indicating shallow to moderate circulation depths of up to 6 km. However, compositions and isotope abundances of noble gases and delta ¹³C values of HCO₃ indicate a significant (up to 50%) mantle component for the volatiles. The relatively high fluxes of CO₂ (C/ ³He nearly equal 10 ¹⁰) and other volatiles of mantle origin support a deep continuous flow model, especially at depths >6 km. Numerical simulations indicate that these high fluxes of CO₂ of mantle and deep crustal origin are sufficient to generate lithostatic fluid pressures, and thus a weakened fault, in time scales comparable to those of earthquake cycles.