Abstract

The Santa Clara trough, part of the Ventura basin, California, originated during late Miocene time and then deepened rapidly, probably as the result of crustal extension. Inasmuch as the crustal stretching amounted to about 50 per cent it is inferred that the floor of the trough was greatly attenuated. This stretching was associated with irregular transform movements between the North American and Pacific lithospheric plates. In Pleistocene time, the Transverse Ranges were compressed, and the Santa Clara trough was much constricted. Concomitantly, the modern Gulf of California opened and several major faults originated or were reactivated. During late Cenozoic time, beginning about 30 m.y. ago, coastal California and its Borderland have been part of a broad transform zone with a pliant and yieldable crust.

Sedimentary basins within this yieldable belt have been formed by at least five processes: 1) Crustal megarifting, in which a major transform fault meets a spreading center as in the Gulf of California today, 2) Fault-zone rifting, where smaller basins originate within a braided strike-slip fault zone, 3) Fault divergence, where strike-slip faults in map view form long lenses between them with down-tipped triangular basins, 4) Fault-margin sagging, where restraining curvature of a strike-slip fault carries one wall up upon the other to depress the lower wall under its weight. Fault-margin basins may also form where a wall sags as it is stretched in moving around a gentle curve, and 5) Fault-margin folding, where large synclines and depressions oblique to major strike-slip faults originate along fault margins in response to regional simple shear between lithospheric plates. Each of these basin types can be recognized in the region that includes central and southern California and the Borderland.

The structure of the region before about 30 m.y. ago, and back in time to well within the Mesozoic Era, was primarily the result of complex plate convergence with only short intervals of divergence and transform movements. Plate-convergence tectonic models need to be reconstructed and depicted for pre-Miocene rocks as new data from coastal and Borderland exploration become available.