The chemistry of dilute warm springs (30°-59°C) in the region traversed by the Elsinore fault zone is determined by the chemical nature of recharging groundwaters, rock type, and the depths to which these groundwaters circulate along tension faults. Compared with their groundwater recharge, the thermal waters are depleted in calcium and magnesium ions, have higher ratios of Cl/HCO3 + CO3 and Cl/SO4, and lower ratios of Cl/B and Cl/F. The anionic character and concentration of dissolved solids in the thermal waters reflect that of associated groundwaters. Reduction of SO4 to H2S at depth produces alkaline waters in granitic rocks with pH's ranging up to 9.8. This severely limits the reacting capability of the waters with the confining rock. Propylitic mineral assemblages are probably stable at depth. Estimates of temperatures at depth range from 67°-142°C. In a region of average geothermal gradient (25°-30°C/km) these temperatures suggest circulation to depths of about 2-4 kilometers. Structural relationships along the Elsinore fault zone, combined with recent interpretations of spreading configurations in the Gulf of California, suggest that the Elsinore fault zone may be an ancient transform fault, linked to the opening of the Proto-Gulf. Late Pliocene to Recent block faulting along the deactivated transform, combined with relief of regional right-lateral stress, has produced areas of tensional faulting. These faults provide access to depth.