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Several lines of geologic, geochemical, and geophysical evidence suggest that a northeasttrending zone of extension has provided tectonic control over magmatism and geothermal resources in The Geysers-Clear Lake region of northern California. To investigate magmatism and geothermal systems of The Geysers-Clear Lake region, we developed a new 3D seismic velocity model using finite-difference tomography based upon 3358 local earthquakes of > 1.5 M. The new tomographic images resolve the velocity structure of the crust in the region to depths of approximately 13 km. The most significant velocity contrasts in The Geysers-Clear Lake region occur in the steam production area where high velocities are associated with a Quaternary granitic pluton, and in the Mt. Hannah region where low velocities occur in a 5-7 km thick section of Mesozoic argillites. New 3D gravity models have been developed in the area of the tomographic imaging and these models demonstrate that all density contrasts can be accounted for in the upper 5-7 km of the crust; large and deeper magma bodies are not required to fit the gravity data using reasonable density contrasts. Twodimensional magnetotelluric models of data from an east-west profile indicate high-resistivities are associated with the granitic pluton in the steam production area and low resistivities in the section of Mesozoic argillites. No indication of mid-crustal magma bodies is present in the magnetotelluric data or seismic tomography. However, based upon heat flow and geologic evidence, small Recent magma bodies are thought to underlie the Northwest Geysers, Sulphur Bank Mine and perhaps other areas such as Ely Rat south of Mt. Konocti and Wilbur Springs. All of the new geophysical models and seismicity patterns indicate that a northeast-trending extensional zone within a transpressive environment is a controlling factor in the geophysical signatures. North-northeast and northeasttrending faults, fractures, lineaments and mineralized veins of Quaternary age are common in The Geysers Clear Lake region. These are apparently the result of extension within a northeast-trending zone between two active northwest-trending faults parallel to the San Andreas system, the Maacama and Bartlett Springs faults. Analysis of seismicity patterns, temperature, heat flow, geochemical, and production data for The Geysers area indicates that extension within this northeast-trending zone may control many of the active processes that are important in understanding the geothermal system, its extent, and future exploration strategies. 1 U. S. Geological Survey, MS966, Denver Federal Center, Denver, CO 80225, email: dals@bach.cr.usgs.gov. 2 1573 Manzanita Ave., Santa Rosa, CA 95404, email: mwalters@ sonic.net. INTRODUCTION The Geysers-Clear Lake area of northern California (Fig. 1) is one of the few regions of the world where dry steam is used to produce electric power. The geothermal resource of The GeysersClear Lake region encompasses an area of over 700 km2 . The developed steam reserves are the largest such development in the world and are defined by more than 600 drilled wells and an installed electrical producing capacity in 1989 of 2043 MW. However, an accelerated rate of steam pressure decline beginning in late 1984 signaled a reduction in production capability and several power plants have been decommissioned since 1989. The Geysers produced about 1230 megawatts of electrical power in July 1996 from about 420 wells. Despite, the production declines, The Geysers remains the largest producer of geothermal power in the world and exceeds the total output of any single country outside the United States. Many new data have become publicly available as the participating companies in the region attempt to evaluate the actual dynamics and geologic setting of the geothermal resource. The U.S. Geological Survey (USGS) conducted extensive geological, geophysical, and other studies in the region during the 1970's that were summarized in USGS Professional Paper 1141 (McLaughlin and Donnelly-Nolan, 1981). A publication by the Geothermal Resources Council (Stone, 1992) summarizes much of the current information available from the geothermal production area. As research continued from the 1970's to the 1990's in The Geysers-Clear Lake region, details of subsurface geology were obtained from drilling. The ability to locate the steam reservoir in fracture systems within the production area has improved, but the total region of geothermal reserves is relatively confined, and anticipated possible new production has not been developed in the broader part of the Clear Lake volcanic field to the east. One fundamental question has been the subject of much research-where is the location of the heat source for the geothermal area? Parts of the Clear Lake volcanic field are younger than 1 m.y. in age and there is widespread evidence for high subsurface temperatures over the region (Walters and Combs, 1992), so it is quite evident that there is either one broad, or several smaller, "heat sources" in the area. Many early geophysical-geological interpretations involved a large magma chamber at depths starting at 7 km, but there was little published evidence for intrusive activity until subsurface information was released (Schriener and Suemnicht, 1980; Thompson, 1992; Hulen and Walters, 1994) about a concealed, Quaternary granitic pluton in The Geysers production area locally known as "the felsite" (Figs. 2,3). The age of the intrusion was determined at 0.9 Ma-2.4 Ma by Schreiner and Suemnicht (1980) and more recently at 1.32 Ma by Dalrymple (1992) and 0.57 Ma by Pulka (1991), which makes the pluton too old to be directly responsible for heat in the production area. The pluton is a composite body composed of rhyolite porphyry, granite, and granodiorite (Hulen and Nielson, 1993; Nielson and Hulen, 1996). Younger magma bodies, not yet sampled and dated, may have been emplaced close to, or within the "felsite" of the steam production area. Such unsampled intrusions are assumed to be the source of heat for the present geothermal system. In the Clear Lake volcanic field (Figs. 2,3), measured temperatures in deep exploration wells have been as high as 326» C at depths of less than 3.5 km ( Walters and Combs, 1992). Extrapolation of measured temperatures and fluid inclusion data suggests a temperature of close to 400° C at a depth of 3.7 km in the Wilson No. 1 well (Fournier, 1991) near Mt. Hannah (Figs. 2, 3). In the Borax Lake area (Figs. 2,3), temperatures >200°C have been measured in wells at depths of 2 km (Goff and Decker, 1983; Beall, 1985). Several geophysical anomalies seem to be centered over the Mount Hannah area (Fig. 2) and a residual Bouguer gravity low of about -30 mgal amplitude was interpreted by Isherwood (1975) to be caused by a spherical magma chamber centered at 13.5 km depth. Stanley and Blakely (1995) presented an updated review of the constraints provided by drilling, geophysical, geochemical, and geological data on heat sources. This review preferred a magmatic model that involves a lower-crustal magma chamber and discrete, isolated upper-cmstal plutons located along tectonic features. The geophysical interpretations indicated that a large magma chamber in the upper and mid-crust is not required or supported by the geophysical data. Stanley and Rodriguez (1995) outlined a revised tectonic model based upon the premise that features of extension and magma injection in the region could be related to details of Gorda Plate motion. In this paper we present evidence to support our view that the effects of a northeast-trending zone of extension explain many of the enigmatic aspects of thermal, geologic, and geophysical data from the region. This zone of extension can be mapped from The Geysers production area across the south end of Clear Lake and is related to transpressive stress which aligns extensional features along the direction of maximum compression. However, primary magma injection in this zone of extension probably has its roots in details of plate interactions in the interval 1 -2 Ma when the Gorda-North America-Pacific plate triple junction passed through the area. GEOLOGICAL AND GEOPHYSICAL SETTING The Geysers-Clear Lake region is in the northern California Coast Ranges (Fig. 1) between the San Andreas fault system and the Coast Range thrust (Bailey et al., 1964). The Coast Range thrust fault separates accrectionary wedge rocks of the Franciscan assemblage from the Great Valley Sequence, both of Jurassic and Cretaceous age (Fig. 1). The Franciscan assemblage consists of melange units and less disturbed sedimentary, metasedimentary, and metavolcanic rocks that were scraped off the subducting plate in Jurassic to Cretaceous time. The Great Valley Sequence is interpreted to represent arc-trench gap or forearc basin deposits that were derived from the Klamath and Sierran magmatic-arc terranes (Dickinson, 1970; Ingersoll et al., 1977). The Great Valley sequence consists largely of sandstones and shales, underlain by the Coast Range ophiolite (McLaughlin and Pessagno, 1978). At the scale of The Geysers-Clear Lake region, McLaughlin and Ohlin (1984) described tectonostratigraphic subterranes of the Franciscan assemblage and the lower part of the Great Valley Sequence. These terranes are bounded by segments of the northwest-trending San Andreas fault system that include the Maacama, Healdsburg, and Bartlett Springs fault zones (Figs. 1,2,3). The Collayomi fault zone (Figs. 2,3) occurs primarily in ophiolitic serpentinites that separate central Franciscan assemblage units from Great Valley Sequence and eastern Franciscan assemblage terrane of the Mt. Hannah and Clear Lake region. The Collayomi fault zone is not seismically active. The rocks of the Clear Lake volcanic field are primarily Pleistocene and 2 m.y., or less, in age (Donnelly-Nolan and Hearn, 1981). The nearest similar volcanic rocks are Pliocene age i

u.s. geological survey open file report/open-file report

Tectonic controls on magmatism and geothermal resources in the Geysers-Clear Lake region, California: Integration of new geologic, earthquake tomography, seismicity, gravity, and magnetotelluric data