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DISPLACEMENT OF BASEMENT ROCKS BY THE SAN ANDREAS FAULT [c3, p72-73]

The distribution of the Pelona-type schist (Figure. 3.9) has played an important role in many attempts to measure the slip on the San Andreas fault in southern California. In their classic report, Hill and Dibblee (1953) recognized the similarity of the Pelona Schist of the San Gabriel Mountains area to the Orocopia Schist east of the Salton Sea and postulated that these schists are offset 257 km from one another by the San Andreas fault. Crowell (1962) noted that the Precambrian rocks and Pelona-type schist of the Tejon area are separated from those of the San Gabriel Mountains area by the San Gabriel fault and that the Tejon, San Gabriel Mountains (Soledad), and Orocopia Mountains areas contain not only similar Precambrian rocks and Pelona-type schist but also similar Oligocene and other Tertiary strata. The similar lithologies and geologic histories of the rocks of these three areas indicated to him that these rocks once formed an east-west-trending belt which has been segmented and displaced by right-lateral slip of approximately 50 km on the San Gabriel fault and 210 km on the San Andreas fault. However, the validity of the concept that the Tertiary rocks of the San Gabriel (Soledad) and Orocopia Mountains areas once were parts of a single basin was questioned by Spittler and Arthur (1973), who believed that the Tertiary strata of these two areas were deposited in separate basins, consist of distinctly different flow rocks, and are dissimilar in age.

More recent schemes for measuring offset along the San Andreas fault in southern California have been proposed. Among them, Powell (1981) postulated that all major exposures of the Pelona-type schist are in the core of an antiformal fold and that they once formed a nearly continuous structure which subsequently has been disrupted by Cenozoic strike-slip faults ( Figure. 3.10). An Important part of Powell's palinspastic reconstruction of the antiform is an old east-west-trending fault that is a composite of the San Francisquito, Fenner, and Clemens Well faults. This fault is thought to have had 80 km of right-lateral slip, cutting the antiform, and later was cut diagonally by the San Andreas fault. The axis of the antiformal structure is a linear feature that can be used as a "piercing point" in measuring offset where the axis appears on opposite sides of a fault. On this basis, the axis in the San Gabriel Mountains (Sierra Pelona) area is offset 220 km along the San Andreas fault from the axis in the Orocopia Mountains area (Powell, 1981). The Transverse Ranges segment of the San Andreas fault was described by Matti and others (1985) as consisting of several old strands (Wilson Creek, Mission Creek, and Mill Creek faults) and a young strand (San Bernardino strand). The total displacement across all of these strands is thought to be 160+10 km (Matti and others, 1986), on the basis of exposures of a distinctive Triassic megaphyric monzogranite. The exposures of this monzogranite, which are on the northeast side of the San Andreas fault in the San Bernardino Mountains but on the opposite side of the fault in Liebre Mountain (lat 3443' N., long 11840' W.), are thought to represent displaced parts of the same pluton.