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Corridor C-3 (Fig. 1) traverses the transition from oceanic crust to cratonal rocks of North America along a 1,500-km transect from offshore southern California to central New Mexico (Howell and others, 1985; Gibson and others, 1985). The western end of the transect is offshore in an oceanic seamount province of the Pacific plate. Eastward, it crosses the southern California borderland and emerges onshore in the batholith-dominated Peninsular Ranges. C-3 spans the Salton trough, a transform rift floored by nascent oceanic crust, and the Basin and Range structural province, formed by extension and disruption of cratonal rocks. The eastern end of C-3 is on the stable craton.

Neogene tectonism has severely disrupted the older structural grain between the oceanic lithosphere and the craton. Much of this young tectonism is attributed to interactions along the plate edges after the spreading ridge system separating the Pacific and Farallon plates inpinged upon the North American plate in late Oligocene time. Two triple junctions—one migrating northward, the other southward relative to the Pacific plate—formed as the spreading system moved eastward relative to the North American plate. The San Andreas fault system began to develop within a broad zone of right shear as the triple junctions moved apart. The main strand of the San Andreas is but one of many northwest-trending right-lateral faults in the system. The boundary between the Pacific and North American plates is best perceived as a broad zone of right shear rather than a specific fault trace.

The continental crust of the

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