Abstract

Paleomagnetic data from mainly Miocene igneous rocks in southern California suggest that large crustal regions have undergone clockwise rotation during that time. We propose a model whereby many crustal blocks presently bounded on the north and south by east-west—trending sinistral faults have undergone rotations of about 70° to 80° within the Pacific-American right-lateral shear couple. The data suggest that these crustal blocks include the western Transverse Ranges and parts of the offshore Borderland. Our model predicts that the eastern Transverse Ranges, the central Mojave Desert, and the Tehachapi Mountains region have also rotated. The rotated blocks are nested between blocks bounded by northwest-southeast—trending dextral faults. The rotations probably ceased in late Miocene time when the San Andreas fault system broke through southern California and may have begun when the Pacific plate contacted the North American plate in late Oligocene time. This geometric model for rotated blocks predicts that left-slip, right-slip, and rotation occur simultaneously; that the displacements can be calculated from the rotation (and vice versa); and that during the rotation, deep triangular basins open at the join between the rotated and unrotated blocks. It also suggests that dextral slip can occur on northwest-southeast faults without cutting the Transverse Ranges.

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