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SUMMARY

The crust along the San Andreas fault system thickens from about 16 km at Cape Mendocino, in northern California, to about 30 km in southern California and thus is significantly thinner than the average thickness (36 km) for the conterminous United States. Lithospheric thickness (20-60 km) is also substantially less along most of the San Andreas fault system than is typical for continental areas (60-170 km). The lithosphere is thinnest at both ends of the fault system, at the Mendocino triple junction on the north, where the North American plate is sliding off the edge of the Gorda plate as it moves northward, and in the Salton Trough on the south, where onshore spreading centers of the East Pacific Rise are generating new crust in a rift between the North American and Pacific plates. In contrast, the lithosphere is abnormally thick (250 km) in the Tranverse Ranges, where "subduction" of lithospheric mantle is occurring.

The crust of central California was formed at an Andean-type continental margin and has been modified by large offsets along strike-slip faults of the San Andreas fault system. East of the San Andreas fault, the Andean-marginal sequence includes a subduction complex (Franciscan rocks), a forearc basin (Great Valley sequence), and a magmatic arc (plutons of the Sierra Nevada). The subduction complex appears to have been emplaced as a tectonic wedge beneath sedimentary rocks of the forearc basin. West of the fault, displaced blocks constitute an Andean-marginal sequence that has been shortened by strike-slip faulting.

The tectonic wedge of Franciscan rocks east of the fault is reinterpreted to extend from its tip beneath the Great Valley all the way to the San Andreas fault. This interpretation is motivated by the apparent continuity between crystalline basement rocks beneath the Great Valley and mafic rocks at midcrustal depths in the Diablo Range, beneath the Franciscan rocks. The presence of extended crust atop the tectonic wedge (outliers of Coast Range ophiolite and Great Valley sequence) has led us to propose the following tectonic evolution for the wedge. (1) Franciscan rocks were uplifted and upper-plate rocks (those above the subduction zone) were extended during the Cretaceous (or, possibly, early Tertiary) well west of their current position in the Coast Ranges. (2) The Franciscan rocks and overlying extended crust were subsequently forced landward during one or more episodes in the form of a wedge that largely followed the contact between Great Valley basement and the Great Valley sequence. (3) Wedge movement began during the subduction of the Farallon plate (or its derivative) beneath central California; however, it apparently is also occurring at present, in the San Andreas transform regime. Present movement is interpreted to result from compression across the San Andreas fault system coupled with differential motion between the upper and lower crust; this differential motion is interpreted to occur on thrust fault(s) at the base of the wedge that sole into the brittle-ductile transition zone.

The crustal structure in southern California shares several features in common with central California, including, west of the fault, an Andean-marginal sequence that has been shortened or, at least, shuffled by strike-slip faulting, and, east of the fault, subduction-complex rocks that are inferred to have moved landward as a tectonic wedge into the continental rocks. However, major differences are apparent in southern California. First, east of the San Andreas fault, the Andean-marginal sequence is incomplete: A forearc basin is absent, and the magmatic arc is diffuse. Second, new continental crust has formed in the Salton Trough, an active crustal pullapart basin, by a combination of rapid sedimentation, metamorphism, and magmatic intrusion at the onshore spreading centers. In addition, the motions of the crust and lithospheric mantle differ in southern California: The crust is moving as a collage of blocks, with only minor interblock convergence, whereas the lithospheric mantle is converging and "subducting" beneath the Transverse Ranges.

The interpretations of (1) a midcrustal detachment in the brittle-ductile transition zone in central California and (2) a crust-mantle detachment in the Transverse Ranges of southern California would appear to require that the deformational style and (or) location of the San Andreas fault system change with depth in these regions.

The properties of the lithosphere along the San Andreas fault are not at all typical of continental areas, and further characterization of these properties presents a significant scientific challenge.