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Several sedimentary basins in southern California, within and south of the Transverse Ranges, display a history suggestive of a rhombochasmic origin. Beginning in the early Miocene, segments of the continental margin at the soft and splintered border between the Pacific and Americas plates were apparently fragmented so that basins originated as irregular pull-aparts. Basin walls were formed by both transform faults and by crustal stretching and dip-slip faulting. Deep basin floors grew as a complex of volcanic rocks and sediments. As basins enlarged, high-standing blocks are pictured as separating laterally from terranes that were originally adjacent. Older rocks exposed around margins therefore cannot be extrapolated to depth within the basins.
Support for such a speculative model comes from accumulating understanding of the Salton trough. This narrow graben is 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 occurs simultaneously with deformation and volcanism in the basin floor. The Los Angeles basin apparently started to form as a rhombic hole in the middle Miocene, with basin-floor volcanism accompanied and followed by voluminous sedimentation. The Miocene "Topanga basin" in the western Santa Monica Mountains contains vast thicknesses of volcanic and sedimentary rocks that were laid down adjacent to high ground, from which sediments and huge detachment slabs were carried into a spreading hol . Other basins that perhaps reveal stages in the history of crustal stretching, culminating in pull-aparts and rhombochasms, are parts of Ventura basin, Ridge basin, and several offshore depressions, including the Santa Barbara Channel.
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