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CRUSTAL-THICKNESS MAP OF CALIFORNIA

A contour map of crustal thicknesses in California (Figure. 8.3A) provides an overview of the geophysical setting of the San Andreas fault system. The seismic and gravity data used in compiling this map (Figure. 8.3B) were discussed by Mooney and Weaver (1989).

Crustal thickness along the San Andreas fault increases from 16-24 km in northern California to 28-32 km in southern California. Thus, the crust along the San Andreas fault system is everywhere thinner than the 36-km average for the conterminous United States (Braile and others, 1989), and in northern California it is substantially thinner than this average. To a first-order approximation, crustal thickness resembles the topography (see Jachens and Griscom, 1983, fig. 13).

Cape Mendocino in northern California marks the change from the strike-slip regime of the San Andreas fault to the subduction regime of the Cascade Range. North of Cape Mendocino, the crust thickens eastward from about 16 km at the coast to about 38 km in the southern Cascade Range (Figure. 8.3A). Near the coast, this thickness includes both the North American plate and the subducting Gorda plate. Estimates of crustal thickness in the northern Coast Ranges at Cape Mendocino lack seismic refraction or reflection control, but detailed gravity models, heat-flow observations, and teleseismic data indicate an abrupt decrease in both crustal and lithospheric thickness southward of the landward projection of the Mendocino Fracture Zone (see chaps. 9, 10; Zandt and Furlong, 1982; Jachens and Griscom, 1983).

In central California, the crust thickens eastward from about 25 km near the coast to as much as 55 km in the Sierra Nevada, but this general landward thickening is interrupted by thin crust (25 km) beneath the Great Valley (Figure. 8.3A; compare Oppenheimer and Eaton, 1984). The crust of central California represents a Mesozoic and early Cenozoic Andean-type continental margin (see chap. 3; Hamilton, 1969) that has been modified by late Cenozoic strike-slip faulting along the San Andreas fault system and by uplift of the Sierra Nevada. Andean features include a subduction complex (eastern Coast Ranges), a forearc basin (Great Valley), and a magmatic arc (Sierra Nevada). Cenozoic strike-slip faulting along the San Andreas fault system has moved a shortened Andean-marginal sequence outboard of this sequence. Southwest of the San Andreas fault, the batholithic Salinian block (western Coast Ranges) is juxtaposed, across other right/oblique-slip faults of the San Andreas fault system, against an inactive accretionary prism, or subduction complex (western Coast Ranges and offshore California).

In southern California, the crust thickens eastward from about 20 km at the western margin of the California Continental Borderland to about 32 km in the eastern Transverse Ranges (Figure. 8.3A). Over most of onshore southern California, crustal thickness is 30 +/- 2 km. Considering the complex tectonic history of this region, including the present subduction of lithospheric mantle (see below), this uniformity in crustal thickness is remarkable.