Evolution and thickness of the lithosphere beneath coastal California

Many of the tectonic features of the California Coast Ranges are directly related to the northward migration of the Mendocino triple junction along the western edge of North America during the past 20 to 30 m.y. At the triple junction, the western edge of the North American plate slides off the (relative) northward-moving and subducting Gorda plate, leaving the thin western edge in direct contact with asthenosphere upwelling to fill the space vacated by the underlying Gorda plate. Two-dimensional, time-dependent thermal modeling of this process, constrained by teleseismic delay studies, is used to construct a map of lithospheric thickness of coastal California. Among the implications of this map are that (1) the high heat flow in the Coast Ranges can be almost entirely accounted for by the asthenospheric upwelling associated with the migrating Mendocino triple junction; (2) the general elevation and the late Cenozoic volcanism in the California Coast Ranges are responses to a zone of unusually thin (20 to 45 km) lithosphere that extends southward behind the Mendocino triple junction; and (3) the course of some segments of the San Andreas fault (in both central and southern California) appear to deviate from a deeper, more fundamental, transform boundary separating the Pacific and North American plates. *Present address: Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071