THE GEYSERS-CLEAR LAKE GEOTHERMAL AREA, CALIFORNIA - AN UPDATED GEOPHYSICAL PERSPECTIVE OF HEAT-SOURCES

The Geysers-Clear Lake geothermal area encompasses a large dry-steam p roduction area in The Geysers field and a documented high-temperature, high-pressure, water-dominated system in the area largely south of Cl ear Lake, which has not been developed. Both systems have been extensi vely studied with geophysical techniques, drilling, and geological map ping during the past 20 years. An updated view is presented of the geo logical/geophysical complexities of the crust in The Geysers-Clear Lak e region in order to address key unanswered questions about the heat s ource and tectonics. Early geophysical interpretations used a gravity low centered in the area between Clear Lake and The Geysers to suggest that a large magma chamber existed at depths starting at about 7 km. This first-order assumption of a large magma chamber expressed in the gravity data was used as a guide in subsequent geophysical and geologi cal interpretations. Drill-hole temperature evidence is strongly sugge stive of a shallow, hot-intrusive body, but in this paper the complexi ties are documented of the geological and geophysical data sets that m ake it difficult to pinpoint the location of ''magma'' or hot, solidif ied intrusive material, Forward modeling, multidimensional inversions, and ideal body analysis of the gravity data, new electromagnetic soun ding models, and arguments made from other geophysical data sets sugge st that many of the geophysical anomalies have significant contributio ns from rock property and physical state variations in the upper 7 km and not from ''magma'' at greater depths. Regional tectonic and magmat ic processes are analyzed to develop an updated scenario for pluton em placement that differs substantially from earlier interpretations, In addition, a rationale is outlined for future exploration for geotherma l resources in The Geysers-Clear Lake area.