Data collected since 1985 from test drilling, fluid sampling, and geologic and geophysical investigations provide a clearer definition of the hydrothermal system in Long Valley caldera than was previously available. This information confirms the existence of high-temperature (> 200°C) reservoirs within the volcanic fill in parts of the west moat. These reservoirs contain fluids which are chemically similar to thermal fluids encountered in the central and eastern parts of the caldera. The roots of the present-day hydrothermal system (the source reservoir, principal zones of upflow, and the magmatic heat source) most likely occur within metamorphic basement rocks beneath the western part of the caldera. Geothermometer-temperature estimates for the source reservoir range from 214 to 248°C. Zones of upflow of hot water could exist beneath the plateau of moat rhyolite located west of the resurgent dome or beneath Mammoth Mountain. Lateral flow of thermal water away from such upflow zones through reservoirs in the Bishop Tuff and early rhyolite accounts for temperature reversals encountered in most existing wells. Dating of hot-spring deposits from active and inactive thermal areas confirms previous interpretations of the evolution of hydrothermal activity that suggest two periods of extensive hot-spring discharge, one peaking about 300 ka and another extending from about 40 ka to the present. The onset of hydrothermal activity around 40 ka coincides with the initiation of rhyolitic volcanism along the Mono-Inyo Craters volcanic chain that extends beneath the caldera's west moat.

Collection of 80 Temperature depth logs from a geothermal well field in Long Valley, California. Spreadsheets include temperature at depth and graphs of data. Data courtesy of the US Geological Survey.

SUMMARY: This report presents the results of a cooperative study of geothermal systems in the region from Aurora, Nevada, and Bridgeport, California, south to Long Valley, California, by the Division of Mines and Geology and the Division of Earth Sciences of the University of Nevada, Las Vegas. The study is the initialreconnaissance phase of a project that was proposed to span several years. Magmatic and hydrothermal systems have been common in this region for the last 15 million years. The objectives of this study were to begin determination of the properties ~nd interactions of these systems and to develop hypotheses on the locations of undiscovered, active systems _in the region. SpeCial emphasis was given to the re~ional relationships and controls of the systems. New data presented include a Bouguer gravity map of the region, several gravity and magnetic profiles, a resistivity profile near Aurora, and a potassium-argon date on Mud Spring volcano, also near Aurora. New interpretations are presented regarding relative 8Qes and distributions of thermal fluids, gravity anomalies, d~pths to pre-Cenozoic: basement, structural controls of the geothermal systems, and the late Cenozoic tectonic-magmatic evolution of the region. The U.S. Department of Energy provided most of the funds for this project.