The Shallow Hydrothermal System of Long Valley Caldera, California

Abstract

Long Valley caldera produces 40 Mw of geothermal power from a shallow (<200m) reservoir in a ~165 acre area around Casa Diablo Hot Springs. Deep drilling during the 1980s established that the shallow geothermal system is sustained by outflow from a larger deep hydrothermal source in the western caldera moat. Recent exploration drilling has identified a hydrothermal outflow zone near Shady Rest that represents input to the shallow geothermal reservoir in the southern caldera moat confined predominantly to Early Rhyolite reservoir rocks at Casa Diablo. Three exploration wells in the southern moat penetrate Sierran metamorphic rocks that slid into the collapsing caldera from the southern caldera rim on a cushion of gassy, frothing Bishop Tuff late in the caldera’s collapse sequence. Fluid inclusion data from core samples indicate that the Bishop Tuff within the southern caldera was pervasively fractured by hydraulic overpressures as a post-collapse hydrothermal system developed and the Tuff began to weld within the caldera beneath the impermeable confining landslide block. The landslide effectively directs and confines the current active hydrothermal outflow into shallow Early Rhyolite units that overlie the landslide block in the southern caldera.