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Numerous late Tertiary and Quaternary volcanic centers are scattered in the vicinity of the San Andreas Fault system across the northern California Coast Ranges (Fig. 1). These volcanic rocks are characterized by a clear age progression. The youngest volcanic rocks are found in the Clear Lake volcanic center (2.1-0.01 Ma) (Donnelly-Nolan et al., 1981). To the south, volcanic rocks become progressively older. In an area about 60-70 km north of Clear Lake, seismic tomography suggests an “unerupted magma chamber” marked by relatively low crustal velocity similar to that under Clear Lake (Benz et al., 1992).

A number of causes have been suggested for the Coast Ranges volcanism. Hearn et al. (1981) proposed that the volcanism resulted from northward motion of the North American plate over a hotspot relatively fixed in the mantle. However, the trend of the Coast Ranges volcanism is difficult to reconcile with the relative motion of the North American plate derived from global inversion of plate kinematics (Minster et al., 1974). An origin of arc-volcanism is also unlikely, since volcanic rocks in the northern Coast Ranges were erupted within 50-60 km of the plate boundary, whereas the Cascades arc-volcanism typically occurs about 300 km inboard of the plate boundary (Fox et al., 1985).

Most workers relate the Coast Ranges volcanism to thermal perturbations associated with the northward migration of the Mendocino triple junction (MTJ) (Dickinson and Snyder, 1979; McLaughlin et al., 1981; Furlong, 1984; Johnson and O’Neil, 1984; Fox et al., 1985; Liu and Furlong, 1992). Both

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