GEOTHERMAL SOURCE CHARACTERISTICS OF MONO HOT SPRINGS, CALIFORNIA

Mono Hot Springs and other nearby geothermal springs occur in an area distant from known active tectonic structures and volcanoes, within California’s Sierra Nevada mountain range. Volcanism and tectonism at the eastern edge of the Sierra Nevada lie ≥30 km away, similar to the thickness of local continental crust. Silica and cation geothermometry (DeGraff et al., 2018) suggests that Mono Hot Springs’ geothermal water equilibrated at pH 5-6 with rock at 74-79°C. Published measurements tens of kilometers from, but surrounding Mono Hot Springs yield geothermal gradients from ~15°C/km in the center of the Sierra to ~50°C/km near volcanically-active Mammoth Mountain. From these, we infer that Mono Hot Spring waters equilibrated at 3.5±1.5 km depth. Given the lack of nearby Quaternary volcanism and the geothermometer temperatures, a magmatic heat source appears unlikely. Instead, the most plausible source for Mono Hot Springs is deep circulation of meteoric water driven by the local 1.2 km of relief, which produces topography-induced hydraulic head. Unlike most of the Sierra Nevada batholith, infrequent small earthquakes do occur around Mono Hot Springs, supporting the proposition that fracture porosity is being seismically maintained, allowing geothermal flow through regional joint systems and/or minor unmapped faults.

These inferences depend upon the assumption of water-rock equilibrium. Failure of this assumption would imply that the geothermal waters experienced higher temperatures and flowed to deeper depths. In any case, if the water source is meteoric, driven as proposed here, sustained drought could be expected to impact flows and temperatures of Mono Hot Springs.