The San Gabriel and San Bernardino mountain ranges of southwestern California, and associated mountains and basins westward to the Pacific Ocean, make up a unique east-trending geomorphic, stratigraphic, petrologic, and structural belt 400 km long that is offset only a few tens of kilometers right laterally by northwest-striking faults of the San Andreas type. Spot correlations across these faults, suggesting displacements of hundreds of kilometers, perhaps have other explanations. Within the transverse ranges and basins, the east trend is shown by the general petrology, the crystalline rock patterns, pre- and post-batholithic structural features, and batholithic chemical patterns. The east-west unity is especially striking west of the San Andreas fault, but it is also evident in the central and southern San Bernardino Mountains, east of that fault.

The Salton Trough and the southern boundary of the Transverse Ranges Province appear to be the most important provincial boundaries in southwestern California. West of the San Jacinto fault, which is just southwest of the San Andreas fault and nearly parallel to it, the Transverse Ranges Province is bounded on the south by the Malibu Coast–Cucamonga fault zone. At and just east of the San Jacinto fault, the southern boundary fault zone is displaced by faulting 30 to 40 km to the southeast, and is continued as the Banning fault. East of the eastern (Mission Creek) branch of the San Andreas fault, east-trending faults are present, but the San Bernardino Mountains rocks and structures merge into the southeast-trending Little San Bernardino Mountains.

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