Long Valley Caldera Geothermal Area

Long Valley is a large silicic caldera located in east-central California along the frontal fault of the Sierra Nevada range within the Transitional Walker Lane and bordered and tectonically affected by the extension of the Basin and Range province. The caldera was formed 730,000 years ago following a large rhyolitic eruption (600 km3) of the Bishop Tuff; later eruptions produced the Early Rhyolite and the Moat Rhyolites. More recent activity has included basalt flows, viscous rhyolite flows, and phreatic/magmatophreatic eruptions represented by the Moat Basalts and Inyo Volcanic Chain, respectively. Increased earthquake activity and uplift of a resurgent dome in the 1980s are thought to have resulted from the injection of additional magma into the magma chamber and/or to the intrusion of shallow dikes beneath the caldera. A major ring fault bounds the structural caldera, which lies well within the topographic caldera . The resurgent dome, in the center of the caldera, is cut by numerous north–northwest striking normal faults, including a central graben 4 km in length. Faults control the locations of post-caldera magmatic and hydrothermal activity within the caldera.

The hydrothermal system is presumably driven by heat from intrusions during the last 200–300 ka beneath Mammoth Mountain. The primary hydrothermal fluids in the active hydrothermal system rise up from a deep source reservoir in the basement rocks beneath the West moat through faults and flow laterally to the southeast and discharge in the area around Hot Creek. A landslide block is also thought to control the path of the rising fluids. Hot springs occur around the south and east sides of the resurgent dome and the East moat. Steam vents occur within a few kilometers west of Casa Diablo along northwest–southeast trending faults. Hot springs occur at Casa Diablo, Hot Creek gorge, and Little Hot Creek and at other scattered sites in the South moat of the caldera. Temperature data obtained from geothermal wells drilled in the caldera show that deeper hydrothermal waters under the West moat have temperatures greater than 200°C, while shallow hydrothermal fluids to the east within the Bishop Tuff and overlying Early Rhyolite tuffs have a maximum temperature of approximately 170°C.^[2][1]

known or inferred magmatic system