Active sedimentation along the present San Andreas transform fault occurs primarily at the head of the Gulf of California, including the Salton Trough. Here a vertical thickness of about 6,000 m (20,000 ft) of young sediments, mainly derived from ancestral Colorado rivers, has accumulated on spreading quasi-oceanic floor as the gulf has opened. Northwestward along the trace of the San Andreas, nonmarine sedimentary wedges are more closely related to regional uplift and depression than to the San Andreas itself. Only northwest of San Francisco does the San Andreas mark the shoreline, where presumably shallow marine beds pinch out and change facies at the fault scarp.
In the past, the fault zone has demarked highlands from adjacent depressions, some of which have been deep and narrow. In the Ridge Basin, for example, 12,000 m (40,000 ft) of marine and nonmarine beds accumulated during late Cenozoic time in a moving depression associated with major strike slip. Marked facies changes from shale and sandstone along the basin axis to conglomerate and sedimentary breccias at the margins document fault movements with both strike-slip and dip-slip components occurring concurrently with sedimentation. Other Cenozoic sedimentary units in California probably have similar origins.
Basins and sites of deposition along transform faults are related mainly to bends and irregularities in fault-zone traces; tectonic plates do not slide simply by each other without deforming along their edges. Gapping or pulling apart of the crust results with motion away from bends; the holes formed are as large as giant rhombochasms. With motion of plate margins toward bends, overlapping, thickening, and elevation of the crust on one side may be associated with the sagging and depression of the other plate to make a shallow depositional site.
Where bends of plate margins arc gentle, one side may stretch and sag to make a long narrow basin, while the other is squeezed and elevated to form an upland source area, In addition, complex braiding and slicing, and splaying of faults take place in major transform zones, at places on a huge scale, to form crustal blocks that tip and rise and fall. In this regime, several sedimentation sites evolve. Such origins probably can be recognized also for basins in Venezuela, New Zealand, maritime Canada, and the Levant.