A speculative history of the San Andreas fault in the central Transverse Ranges, California

It is generally accepted that the San Andreas fault formed between 4 and 5 Ma and that rocks west of it are now part of the Pacific plate, moving northwest relative to North America at 5 to 6 cm/yr. This model is inconsistent with the geologic record in the central Transverse Ranges.

Right-lateral shear began in the vicinity of the San Andreas fault system in early Miocene time. The San Andreas fault system in the central Transverse Ranges has since evolved through three major phases; this development has led to a generally simpler, more throughgoing main trace. Slip rates on the San Andreas system were about 1 cm/yr in the Miocene, increasing to their current level of 3.5 cm/yr between 4 and 5 Ma. The modern San Andreas fault still only accounts for just over half the current relative plate rate and retains kinematic complexities inherited from its earliest geometry.

The Early San Andreas transform system originated during early Miocene time in one of three transtensive zones that lay interior to the continent and east of the locus of transform motion between the Pacific and North American plates. The current three-fold division of motion in the plate boundary between the San Andreas fault, a coastal system, and an eastern California system dates to this time, as does the “anomalous” trend of the San Andreas fault through the Transverse Ranges. Basins and volcanic centers associated with this transtensive zone became dismembered as faults became integrated into a throughgoing system. Early motion led to juxtaposition of different rocks across faults now recognized as part of the Early San Andreas transform system, and to the development of sedimentary provincialism associated with uplift along the fault zone. Middle Miocene basins, including the Caliente, Cajon, Crowder, and Santa Ana basins that had previously received most of their sediments from sources far to the east, began to reflect local Transverse Ranges provenance. At least 100 km of slip is associated with the Early San Andreas transform system during early and middle Miocene time.

Slip across the geometrically complex late Miocene San Gabriel transform system—which includes the San Gabriel, Cajon Valley, and early Punchbowl faults—produced uplift in the proto-Transverse Ranges at a postulated restraining bend in the fault system. Compressional structures associated with this restraining bend include the Squaw Peak and Liebre Mountain thrusts, related east-striking late Miocene reverse faults and folds, and, perhaps, northeast-striking left-lateral faults in the San Gabriel Mountains. Narrow fault-controlled basins formed during this period, including the Ridge basin, Devil’s Punchbowl basin, Mill Creek basin, and part of the Santa Ana Sandstone basin. Offset of structures and relief associated with the proto-Transverse Ranges provides the best evidence for late Miocene restorations of the modern San Andreas fault. As much as 60 km of offset is associated with the late Miocene San Gabriel transform system.

Between 4 and 5 Ma, the modern San Andreas fault became the dominant member of the plate boundary system, cutting through the proto-Transverse Ranges and connecting more northerly striking traces to the north and south. The slip rate across the San Andreas fault system accelerated from 1 cm/yr to its current slip rate of 3.5 cm/yr prior to 4 Ma. The Pliocene rocks in the central Transverse Ranges do not contain evidence for relief as great as that of late Miocene or Quaternary time. The Pliocene trace of the modern San Andreas fault may have temporarily “solved” the geometric problem that led to late Miocene uplift. About 90 km of right-lateral displacement occurred on the modern San Andreas fault during Pliocene time.

During Quaternary time new regions of localized vertical deformation developed in the Transverse Ranges, apparently as the result of new geometric problems within the Pliocene solution to the restraining geometry of the fault system. Left-lateral motion on east-striking faults, probably due to a northward increase in Basin and Range extension, kinked the San Andreas fault at both ends of the Transverse Ranges, producing regions of extreme shortening and uplift. The development of young right-lateral faults through the Peninsular Ranges, including the San Jacinto and Elsinore faults, also contributed to renewed uplift in the Transverse Ranges. Sixty kilometers of right-lateral slip occurred across the San Andreas fault zone during Quaternary time.