Buried rhyolites within the active, high-temperature Salton Sea geothermal system

Previously unrecognized pulses of rhyolite volcanism occurred in the Salton Trough between 420 +/- 8 ka and 479 +/- 38 ka (2 sigma), based on high-spatial resolution U-Pb zircon geochronology. Presently, these rhyolite lavas, tuffs and shallow subvolcanic sills are buried to depths between similar to 1.6 and 2.7 km at ambient temperatures between 200 and 300 degrees C, and are overprinted by propylitic to potassic hydrothermal alteration mineral assemblages consisting of finely intergrown quartz, K-feldspar, chlorite, epidote, and minor pyrite. Alteration resistant geochemical indicators (whole-rock Nd-isotopes, zircon oxygen-isotopes) reveal that these rhyolites are derived from remelting of MORB-type crust that was chilled and hydrothermally altered by deep-circulating hydrothermal waters. U-Pb zircon dating confirms the presence of Bishop Tuff in well State 2-14 at similar to 1.7 km depth, approximately 5 km NE of the geothermal wells that penetrated the buried rhyolites. These results indicate accelerated subsidence towards the center of the Salton Trough, increasing from 2.2 mm/a to 3.8 mm/a. Based on these results, the present-day Salton Sea geothermal field is identified as a focus zone of episodic rhyolitic volcanism, intense heat flow and metamorphism that predates present-day geothermal activity and Holocene volcanism by at least similar to 400 ka. (C) 2008 Elsevier B.V. All rights reserved.