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The crustal structure of southern California is complicated by the Big Bend in the San Andreas fault, situated between the Coast Ranges and Transverse Ranges, and by onshore spreading centers of the East Pacific Rise, situated in the Salton Trough (Figures. 8.2, 8.3). The Big Bend is thought to result from westward movement of the Sierra Nevada relative to the Mojave Desert, along the Garlock fault (Hill and Dibblee, 1953). The San Andreas fault crosses the Transverse Ranges, between the Big Bend and Salton Trough, at an angle oblique to relative plate motion, while somehow remaining a largely vertical, strike-slip fault.

The onshore spreading centers in the Salton Trough are situated at echelon offsets between the San Andreas, Imperial, and Cerro Prieto faults (see Figure. 3.8; Lomnitz and others, 1970). These three faults are interpreted as transform faults; the San Andreas links the northernmost spreading center in the Salton Trough with the Mendocino triple junction. A progressive decrease in spreading rate northward along the East Pacific Rise is inferred to give rise to movement on the San Jacinto, Elsinore, San Miguel/Newport-Inglewood, and other faults in southern California and Mexico (Lomnitz and others, 1970; Elders and others, 1972).

First, we discuss a transect across southern California, Centennial Continental-Ocean Transect C3 (Howell and others, 1985). Second, because of the three-dimensionality of the geology and tectonics in southern California, we include a discussion of block motions, largely from Weldon and Humphreys (1986).