Geochemical evidence for water‐rock interaction along the San Andreas and Garlock Faults of California

Mesozoic granitoid rocks adjacent to the San Andreas fault in central California have retained their radiogenic Ar for the last 70 m.y. but have, generally, the highest ¹⁸O and H₂O⁺ contents and the lowest D contents of all the granitoid rocks in California. The geographical coincidence of the D, ¹⁸O, and H₂O⁺ patterns with the present trace of the San Andreas fault leave little doubt that some kind of groundwater circulation system has operated in the vicinity of the fault in central California. Similar isotopic patterns exist in rocks along the Garlock fault. These water‐rock interactions probably took place at temperatures