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Geologic history of the Clear Lake region in the Northern California Coast Ranges

The Geysers-Clear Lake area is located in the northern California Coast Ranges about 150 km (93 mi) north of San Francisco and about 50 km (31 mi) east of the San Andreas Fault. The region is cut by many northwest- to north-trending faults, which are part of the broad San Andreas transform fault system that separates the North American and Pacific plates.

Subduction ended at the Clear Lake latitude about 3 Ma as the San Andreas transform fault propagated northward behind the Mendocino triple junction, leaving behind a “slab window,” a ductile opening into the mantle through which mantle material rises then melts crustal material. The resulting crustal-silicic melt began erupting as the Clear Lake Volcanics approximately a million years after subduction ceased in this region. Volcanism was initially widespread but in the past million years has focused near Clear Lake. Volcanic vent alignment and basin development in the Clear Lake region has been controlled by tectonic extension within the San Andreas transform fault system. Movement between the faults is the source for the pull-apart basin at Clear Lake.

Clear Lake Volcanics

The Clear Lake Volcanics are the northernmost and youngest of several volcanic fields in the California Coast Ranges that are progressively older to the south. They date from 2.1 Ma to about 10 ka and have an estimated erupted volume of 100 km3. Silicic lavas are the dominant type and basalts are rare. The most voluminous rock type in the volcanic field is rhyodacite. No ash-flow tuffs have been recognized. Volcanism has moved progressively northward within the volcanic field following an early phase when predominantly basaltic andesite lava erupted over a wide area. Since about 1 Ma, volcanism has been localized south and east of Clear Lake, a long-lived lake that occupies a volcano-tectonic basin. Geophysical data suggest the presence of a large silicic magma body under the main Clear Lake volcanic field.