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Banning Bench

Oblique aerial photograph looking northward at the Banning Bench, at the west end of San Gorgonio Pass, southern California. On the high skyline are the San Bernardino Mountains and their basement rocks of Mojave Desert-type; in the middleground are rumpled hills underlain by basement rocks of San Gabriel Mountains-type. A major strand of the San Andreas Fault separates these two basement terranes and is responsible for their juxtaposition here by on the order of 160 km of right-lateral displacement (Matti and Morton, 1993). In the foreground, Pleistocene alluvial deposits have accumulated from streams heading northward; the Pleistocene deposits are being dissected, as evidenced by ravines and arroyos incised into them, but they also are being buried by younger Pleistocene and Holocene deposits that are being shed out of canyons draining the foothills. These younger deposits form alluvial-fans that feather out on the older Pleistocene surfaces. The Banning Bench is a landform whose shape and geomorphic evolution reflect its geologic structure. The Bench has been uplifted in middle and late Pleistocene time (starting perhaps 250,000 years before present ?) by faults of the San Gorgonio Pass Fault zone: one of these bounds the south margin of the Bench, and trends roughly east-west; the other bounds the west margin of the Bench and trends northwest. Matti and others (1985, 1992) proposed that the east-trending fault zone is a contractional segment (reverse or thrust faults) while the northwest-trending fault zone is an oblique strike-slip segment. Thus, the Banning Bench appears to have been uplifted by northwest-southeast-directed compression that yielded an elevated block whose remarkable rectangular shape is being dissected and modified by erosion over the last few hundred thousand years. Although this uplift appears not to have continued into Holocene time, a similar scenario appears to be playing out with the fault relations on the Millard Canyon fan a few kilometers to the east in San Gorgonio Pass (click here for comparison). Photo by J.C. Matti, USGS, December, 1979.

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