A Revised Tectonic Model for the Geysers-Clear Lake Geothermal Region, California

Abstract

A new program of study in The Geysers-Clear Lake geothermal region by the U.S. Geological Survey is directed toward a better understanding of the nature of heat sources in the area. Geophysical models based on gravity, earthquake tomography, seismicity and electrical geophysical data have been utilized to place new constraints on the location of possible magma bodies. In contrast to past models of a large, single magma chamber in The Geysers-Clear Lake region that generated the Clear Lake volcanic field, our model portrays a system of upper crustal plutons generated from a middle to lower crustal mafic melt zone. This mafic melt zone may be related to heating in a "slab window" resulting from passage of the Mendocino triple junction. Injection of deep crustal magmas into the upper crust appears to have been controlled by northwest-trending strike slip faults and northeast-trending, extensional, stepover features between the northwest-trending faults, The most prominent of these extensional zones trends northeast from The Geysers Production area, through the Mt. Hannah area, and across the southern end of Clear Lake. It may be related to a combination of factors including: block deformation between two key northwest trending strike-slip faults; (2) preserved extensional features from an unstable triple junction; (3) heating effects due to mafic underplating