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The Geysers-Clear Lake area, located about 150 km north of San Francisco, is mainly underlain by Jurassic and Cretaceous rocks of the Franciscan assemblage (composed dominantly of a melange of gray wacke and argillite with lesser amounts of altered mafic igneous rocks, radiolarian chert, serpentine, limestone, and very minor blocks of blueschist, eclogite, and amphibolite), the Great Valley sequence (mainly siltstone and argillite in the Clear Lake region), and associated ophiolitic rocks that accumulated in marine settings, and that later were deformed at an obliquely convergent subduction margin (McLaughlin, 1977,1981; McLaughlin and Ohlin, 1984; Thompson, 1989). Later strike-slip movement on northward-propagating faults of the San Andreas transform system, some of which pass through The Geysers-Clear Lake area, cut and offset the thrust sheets, and shut down subduction at the latitude of Clear Lake about 3 Ma (Atwater, 1970; Blake and others, 1978; Dickinson and Snyder, 1979). Simultaneously, behind a northward-migrating triple junction, there also has been a northward migration of volcanic centers in the Coast Ranges of California above a window of anomalously shallow asthenosphere, with the most recent volcanic activity focused in the Clear Lake region (Dickinson and Snyder, 1979; Hearn et al., 1981; McLaughlin, 1981; Fox et al., 1985).

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