We present a new three-dimensional model of the major fault systems in southern California. The model describes the San Andreas fault and associated strikeslip fault systems in the eastern California shear zone and Peninsular Ranges, as well as active blind-thrust and reverse faults in the Los Angeles basin and Transverse Ranges. The model consists of triangulated surface representations (t-surfs) of more than 140 active faults that are defined based on surfaces traces, seismicity, seismic reflection profiles, wells, and geologic cross sections and models. The majority of earthquakes, and more than 95% of the regional seismic moment release, occur along faults represented in the model. This suggests that the model describes a comprehensive set of major earthquake sources in the region. The model serves the Southern California Earthquake Center (SCEC) as a unified resource for physics-based fault systems modeling, strong ground-motion prediction, and probabilistic seismic hazards assessment.

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We have studied the paleomagnetism of various Neogene age rocks in southern California as a means of determining the amounts of Oligocene and younger tectonic rotation and translation which has occurred in this region. Our results suggest that fully 25% of this area, in particular the Transverse Ranges, has undergone extreme clockwise rotation. Work in southeastern California implies that 40° of clockwise rotation has occurred here, although paleomagnetic declinations adjacent to a major right lateral fault are apparently rotated over 200°. The crustal block bounded by the San Gabriel and San Andreas fault has undergone a net clockwise rotation of 35° although the data here can be interpreted to show that an original early Miocene rotation of about 50° was followed by a late Miocene or Pliocene counterclockwise rotation of 15°. Paleomagnetic results from the offshore islands suggests that San Clemente, Santa Barbara, and San Nicolas islands have not rotated but Santa Catalina has undergone about 90° of clockwise rotation. All of the northern Channel Islands, including Anacapa, Santa Cruz, Santa Rosa, and San Miguel, are implied to have rotated 70° or 80° clockwise. This result is also found for the Santa Monica Mountains east of Anacapa Island. In the Santa Ynez Range north of the Channel Islands, paleomagnetic study of the Monterey Formation also indicates large clockwise rotations of near 90°. These data also suggest that the Santa Maria Basin is not rotated and that the north boundary of the rotated region is the Santa Ynez River fault. Stratigraphic control on the paleomagnetic data from the Monterey Formation implies that the rotation began about 16 m.y. ago and may be continuing today in the western region. Paleomagnetic inclination data from our study show that the northern Channel Islands, in particular, may have translated 15° northward since middle Miocene time. However, equally valid interpretations of these same data are that the low inclinations are due to the combined effects of erroneous structural corrections, non dipole magnetic field behavior and right offset on the San Andreas fault system. Palinspastic reconstruction of southern California regions for the early Miocene implies that parts of the Transverse Ranges structures were once aligned with north trending extensional structures in the southwestern United States. We propose that Pacific‐American plate interactions both rifted the continental crust to create this pattern and rotated the western‐most structures within a dextral simple shear zone which had a half width of about 400 km.

Exceptional examples of restraining and releasing bend structures along major strikeslip fault zones are found in the California continental Borderland. Erosion in the deep sea is diminished, thereby preserving the morphology of active oblique fault deformation. Long-lived deposition of turbidites and other marine sediments preserve a high-resolution geological record of fault zone deformation and regional tectonic evolution. Two large restraining bends with varied structural styles are compared to derive a typical morphology of Borderland restraining bends. A 60-km-long, 158 left bend in the dextral San Clemente Fault creates two primary deformation zones. The southeastern uplift involves 'soft' turbidite sediments and is expressed as a broad asymmetrical ridge with right-stepping en echelon anticlines and local pull-apart basins at minor releasing stepovers along the fault. The northwest uplift involves more rigid sedimentary and possibly igneous or metamorphic basement rocks creating a steep-sided, narrow and more symmetrical pop-up. The restraining bend terminates in a releasing stepover basin at the NW end, but curves gently into a transtensional releasing bend to the SE. Seismic stratigraphy indicates that the uplift and transpression along this bend occurred within Quaternary times. The 80-kmlong, 30-408 left

Geologic observations and previous paleomagnetic studies have suggested that the western Transverse Ranges arrived at their anomalous east‐west orientation by clockwise tectonic rotation. A paleomagnetic study of the northern Channel Islands was undertaken, in order to test the extent of rotated areas and to develop constraints on tectonic models concerning the formation of the southern California Borderland. The islands of Anacapa, Santa Cruz, Santa Rosa, and San Miguel form an east‐west trending chain and are considered part of the Transverse Range physiographic province. Oligocene through Miocene volcanic and intrusive rocks occur on these islands, and were sampled for this paleomagnetic study. Eocene sandstones were sampled on San Miguel Island, and Eocene sandstones and Miocene siltstones were sampled on Santa Cruz Island. Paleomagnetic results from igneous units are characterized by declinations deflected clockwise by 69° to 81° from expected directions and inclinations which are too shallow by 10° to 25°. Normal and reversely magnetized units yield antipodal mean directions. The mean result is concordant to the directions obtained from individual units which passed fold and baked contact tests. These results suggest that the northern Channel Islands have been tectonically rotated into place since early(?) Miocene time as the outer borderland area translated northwestward within a large shear zone between the Pacific and North American Plates. The data from the northern Channel Islands when averaged with the data from the Santa Monica Mountains yield a result of I = 36.1° ± 5.1°, D = 72.6° ± 6.3° for the southern part of the western Transverse Ranges (52 units, 405 samples). The shallow paleomagnetic inclinations suggest a northward latitude translation of 14.0°+3.7° or −3.9° for the western Transverse Ranges. However, most plate tectonic reconstructions suggest only 4°±3° of northward translation. The discrepancy may be due to initial dips of the volcanic flows which would cause an artificial shallowing of inclination when the structural dip of the units was corrected to an assumed original horizontal. Other causes for the shallow inclinations could be the effects of an offset dipole field or nondipole field components, or motions of small tectonic plates between the North American and Pacific plates as the Farallan plate broke up and was subducted.

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