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Several sedimentary basins in southern California, within and south of the Transverse Ranges, display a history suggestive of a pull-apart or a tipped-wedge origin. Beginning in the Miocene, these basins apparently originated along the soft and splintered margins of the Pacific and Americas plates. Basin walls were formed by both transform faults and by crustal stretching and dip-slip faulting. Basin floors developed on stretched and attenuated marginal rocks, and some floors grew as a complex of volcanic rocks and sediments. As basins enlarged, high-standing blocks are pictured as stretching and separating laterally from terranes that were originally adjacent. Older rocks exposed around basin margins therefore cannot always be extrapolated to depth beneath the basins.

Support for such speculative models comes from accumulating understanding of the modern Salton trough. This narrow graben is now being pulled apart obliquely, with faults of the San Andreas system serving as transforms. With widening, the walls sag and stretch, and margins are inundated by sedimentation that goes on hand in hand with deformation and with volcanism within the basin. The Los Angeles basin apparently started to form as an irregular pull-apart hole in the early midMiocene, and basin-fioor volcanism accompanied subsequent voluminous sedimentation. Great thicknesses of Miocene beds and volcanic rocks in the western Santa Monica Mountains probably constitute the displaced northern part of the Los Angeles basin, and were laid down adjacent to high ground from which sediments and large detachment slabs were carried into the growing depression. Basins and intervening banks and ridges in the California Borderland may have originated in a broad right-slip regime where strike-slip faults converge and diverge in plan view to slice the terrane into wedge-shaped segments. Displacement along converging and diverging strike-slip faults bounding such wedges results in shortening and elevation, or in stretching and subsidence, respectively.

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