SOUTHERN CALIFORNIA

TRANSVERSE RANGES AND THE SALTON TROUGH [c3, p70-71]

Where the San Andreas fault bends eastward, near the south end of the Great Valley, it forms the boundary between the southern "tail" of the Sierra Nevada and rocks of the Transverse Ranges (Figure 3.3). Continuing southeastward along the Transverse Ranges, the fault trends along the north slope of the San Gabriel Mountains and the edge of the Mojave Desert for about 100 km, and then cuts diagonally between the San Gabriel and San Bernardino Mountains, where a major strand (San Jacinto fault) splits off to the south. The rocks traversed by the San Andreas fault in the Transverse Ranges are mainly Mesozoic plutons and their Precambrian metamorphic and plutonic host rocks.

South of the Transverse Ranges, the bedrock along the fault is concealed by Quaternary deposits of the Salton Trough for a length of more than 200 km. The Salton Trough, which includes the Coachella and Imperial Valleys, widens toward the southeast, and the number of faults and complexity of the zone increase. The east wall of the trough is at the San Andreas fault, where it consists of Precambrian rocks and Mesozoic plutons and of schists like those exposed in the Orocopia and Chocolate Mountains ( Figure. 3.8). The west wall consists of Cretaceous plutonic rocks of the Southern California batholith and their metamorphic host rocks, similar to rocks exposed in the nearby San Jacinto Mountains. As shown by Fuis and others (1982), the trough is a gap in the crystalline basement that is filled with Quaternary and older Cenozoic sedimentary rocks. The gap increases irregularly in width from 20 km at the north end of the Salton Sea to 60 km at the United States-Mexican border. The enormous thickness of the sedimentary fill is indicated by a drill hole that bottoms in Pleistocene(?) sedimentary rocks at a depth of about 4 km (Muffler and Doe, 1968). Seismic-refraction studies show an interface with "basement" rocks at a depth of 5 to 6 km, and the "basement" rocks below the 5- to 6-km-deep interface are thought actually to be metamorphosed Cenozoic fill (Fuis and others, 1982). Near the south end of the Salton Sea, the San Andreas appears to terminate as a transform fault at a spreading center, or pullapart zone, that is the most northerly in a series of spreading centers distributed along the length of the Gulf of California which form part of the East Pacific Rift. The proximity of this pullapart zone accounts for the abundant young volcanic and geothermal features in the area (Elders and others, 1972).

The Precambrian rocks and associated Mesozoic plutons that constitute much of the crystalline basement cut by the San Andreas in southern California are locally seen to lie in thrust-fault contact on relatively younger metamorphic rocks. In the San Gabriel Mountains, on the southwest side of the San Andreas fault, the principal country rocks are divided into two plates by the Vincent thrust (lat 34°19' N.,long 117°45' W.). As described by Ehlig (1981), the upper plate of this thrust is a Precambrian gneiss-amphibolite-granite complex (U-Pb age, approx 1,700 Ma; Silver, 1966) intruded by a Precambrian anorthosite-syenite-gabbro complex (U-Pb age, 1,220 Ma; Silver, 1971), all of which are intruded by the Late Triassic Lowe Granodiorite (U-Pb age, 220 Ma; Silver, 1971), by mid-Mesozoic rhyolitic to basaltic dikes, and, finally, by granitic plutons of probable Late Cretaceous age (U-Pb age, 80 Ma; Carter and Silver, 1971). The oldest rocks in the upper plate are thought to be a remnant of a Precambrian craton. Northeast of the fault, in the Mojave Desert and San Bernardino Mountains, some Precambrian rocks are unconformably overlain by lower Paleozoic miogeoclinal strata that are thought to represent an essentially autochthonous part of the North American craton (Burchfiel and Davis, 1981; Ehlig, 1981).

The lower plate of the Vincent thrust consists of the Pelona Schist, which is largely a sedimentary section of arkosic sandstone, siltstone, and shale that has been metamorphosed to white micaquartz-albite schist and locally includes metavolcanic rocks, metachert, marble, and serpentinite (Ehlig, 1981). The section in the San Gabriel Mountains has an exposed thickness of 3.5 km. The metamorphism decreases downward, away from the fault, and so sedimentary structures, including graded bedding, are well preserved in the lowest 1 km of section. This "upside down" metamorphism and other features indicate that the metamorphism of the lower-plate rocks occurred during thrusting. Pelona-type schist also occurs beneath Precambrian rocks of the Barrett Ridge slice west of the Tejon Pass and crops out in places along the Garlock fault ( Figure. 3.9). On the northeast side of the San Andreas fault in the Orocopia-Chocolate Mountains area, just east of the Salton Sea, the schist, there known as the Orocopia Schist, lies below the Orocopia thrust. Scattered exposures of Pelona-type schist and the Vincent-Orocopia thrust continue into southwesternmost Arizona (Haxel and Dillon, 1978).

The protoliths of the Pelona-type schist are thought to be deep-marine sedimentary rocks deposited on oceanic crust, possibly representing mostly the distal parts of turbidite fans. They probably were Jurassic or Cretaceous in age and are thought to have been metamorphosed by the thrusting that probably occurred no later than Late Cretaceous time (Haxel and others, 1985). The tectonics of the Pelona and Orocopia Schists is controversial. The Vincent-Orocopia thrust may have dipped southwest, and the Precambrian and other rocks of the upper plate been thrust northeastward over backarcbasin protoliths of the Pelona and Orocopia Schists (Haxel and Dillon, 1978). Conversely, the thrust relation may represent a gently north-northeast dipping subduction zone in which the protoliths of the schists were trench deposits similar to the Franciscan rocks and were thrust north-northeastward under the sialic North American plate (Burchflel and Davis, 1981; Crowell, 1981). The Pelona-type schist is reminiscent of the Franciscan-derived South Fork Mountain Schist and related semischists of northern California that form a narrow, virtually continuous selvage for hundreds of kilometers along the west edge of the Klamath Mountains and the west side of the Great Valley, where they form the lower plate of the Coast Range thrust (Blake and others, 1967).