Present State of the Hydrothermal System in Long Valley Caldera, California

Abstract

Results of test drilling to depths of 2 km and data on the chemical and isotopic content of waters from hot springs and fumaroles permit a conceptual model of the present day hydrothermal system in Long Valley caldera to be delineated. The model consists of two principal zones in which hot water flows laterally from west to east at depths less than 1 km within and around the resurgent dome. Maximum measured temperatures within these zones are near 170°C but estimates from chemical geothermometers and extrapolation of a high temperature gradient measured in a recent drill hole indicate that a source reservoir at temperatures near 240°C may exist at greater paths in the Bishop Tuff beneath the west moat. The heat source for this relatively short lived circulation system appears to be shallow magmatic intrusions beneath the west moat. This preliminary model should be updated when results of additional deep drilling planned by private industry become available.