The Transverse Ranges and the east-trending folds and reverse faults that elevate them began forming in mid-Pleistocene time by regional north-south crustal shortening. The adjacent Mojave Desert and Basin and Range provinces continue to respond to this regional strain by east-west crustal extension. Before ∼5 Ma the regional structure was characterized by conjugate northwest-trending right-slip faults (San Andreas set) and northeast-trending left-slip faults (Garlock set). Thereafter, the San Andreas set of faults became simple shears separating the North American and Pacific plates. With the mid-Pleistocene inception of the Transverse Ranges, the San Andreas fault deviated from its N40°-45°W trend in short N75°W-trending segments on the north and south sides of these mountains in response to the new, and local, strain system of upward crustal extension.

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