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Southern California Geology

Major Faults of Southern California
Inland Empire Region

Text from USGS Open-File Report 92-354

Summary

 

index map of faults and basement terranes Click on this thumbnail to view an index map of faults and basement terranes in the Inland Empire region of San Bernardino and Riverside Counties. The file is a 135Kb pdf graphic that requires a portable-document file reader to examine and read. Link to Adobe Acrobat Reader to download a cost-free version of a pdf reader. Depending on your browser's setup, you may read the file now or save it to your disk.

The vicinity of the central Transverse Ranges is a structurally complex region that is traversed by several major fault zones. The Banning, San Jacinto, and San Andreas faults are right-lateral strike-slip faults of the San Andreas transform system. The San Gorgonio Pass and Cucamonga fault zones are compressional thrust- and reverse-fault complexes. The Crafton Hills horst-and-graben complex, the Tokay Hill and Peters faults in the Cajon Pass region, and the Beaumont Plain fault complex are extensional fault zones. The compressional and extensional fault complexes owe their origin and kinematics to complications within the San Andreas transform fault system.

The Banning fault has had a complex history that includes both left- and right-lateral displacements. During middle Miocene time the ancestral Banning fault zone may have generated left-lateral displacements that juxtaposed the Peninsular Ranges block against the San Gabriel Mountains block along a regionally extensive fault system that included the Malibu Coast-Santa Monica fault. During late Miocene time the Banning fault was incorporated into the San Andreas transform system and generated 16 to 25 km of right-lateral displacement; during this period the Banning probably was the eastward continuation of the San Gabriel fault in the San Gabriel Mountains. The Banning fault was abandoned by the San Andreas system in earliest Pliocene time. In San Gorgonio Pass the Banning fault has been obscured and reactivated by low-angle Quaternary faulting of the San Gorgonio Pass fault zone. In the Coachella Valley the Banning fault has been reactivated by Quaternary strike-slip faulting related to the San Andreas fault, and has generated about 3 km of right-lateral displacement that largely has been absorbed by convergence within the San Gorgonio Pass fault zone.

The San Andreas fault in the vicinity of the central Transverse Ranges consists of three segments--the Mojave Desert, Coachella Valley, and San Bernardino Mountains segments. The complex San Bernardino Mountains segment consists of multiple strands that had sequential movement histories. To the northwest and southeast these strands merge to form the simpler Mojave Desert and Coachella Valley segments. The San Bernardino Mountains segment of the San Andreas fault consists of three paleotectonic strands (the Wilson Creek, Mission Creek, and Mill Creek faults) and a neotectonic strand (the San Bernardino strand). Together, these four strands have generated about 160 km of right-lateral displacement and record the total history of the San Andreas fault (sensu stricto) since its inception 4 or 5 m.y. ago. The Wilson Creek and Mission Creek faults together generated about 130 km of right-lateral displacement during Pliocene and Pleistocene time. The Wilson Creek fault, the older strand, generated about 40 km of displacement before it was deformed into a sinuous trace in the vicinity of the San Bernardino Mountains; this displacement estimate is based on our proposal that the Wilson Creek fault is the offset continuation of the Punchbowl fault in the San Gabriel Mountains--a structure that has about 40 km of right slip documented by other workers. In Pliocene time the Wilson Creek strand was succeeded by the Mission Creek strand, which generated the balance of the 130-km displacement before it was deformed and abandoned in late Pleistocene time. The Mill Creek strand subsequently evolved inboard (east) of the locked-up Mission Creek fault and generated about 8 km of right-slip during late Pleistocene time. Ultimately, the Mill Creek strand was abandoned as right-lateral activity shifted to the southwest front of the San Bernardino Mountains, where the neotectonic San Bernardino strand developed. The San Bernardino strand is aligned with the Coachella Valley segment of the Banning fault, but these two neotectonic right-lateral faults are separated by the San Gorgonio Pass fault zone and it is not clear that they ever formed a single throughgoing trace between the Coachella and San Bernardino valleys.

Two compressional fault complexes and an extensional fault system have evolved in the vicinity of the south-central Transverse Ranges in association with the San Andreas fault. The San Gorgonio Pass and Cucamonga fault zones consist of late Quaternary thrust and reverse faults that have evolved where neotectonic strands of the San Andreas and San Jacinto faults interact with the southeastern San Bernardino Mountains and San Gabriel Mountains, respectively. Northwest of the San Gorgonio Pass fault zone, crustal extension in the San Bernardino valley and vicinity has created normal dip-slip fault complexes like the Beaumont Plain fault zone, the Crafton Hills horst-and-graben complex, and the Tokay Hill and Peters faults in the Cajon Pass region.

The Quaternary tectonic framework of the central Transverse Ranges can be viewed as a regionally integrated response to an evolving left step in the San Andreas transform-fault system. The left step was initiated during Pleistocene time, and was accompanied by left-slip on the Pinto Mountain fault that gradually projected the San Bernardino Mountains westward across the path of the Mission Creek strand of the San Andreas fault and offset the Coachella Valley and Mojave Desert segments of the fault. The modern San Andreas fault has adjusted to this inherited left step, and various neotectonic fault complexes in the vicinity of the central Transverse Ranges have evolved in response to this adjustment. During latest Quaternary time, right-slip on the San Andreas fault has stepped left from the Coachella Valley segment to the Banning fault and thence into San Gorgonio Pass, where right-slip is absorbed by convergence within the San Gorgonio Pass fault zone. Some slip may step farther west onto the San Jacinto fault, where accelerated right-slip may have contributed to subsidence of the San Jacinto graben. Ultimately, slip steps back from the San Jacinto fault to the modern San Andreas fault, giving rise to the San Bernardino strand by reactivation of the Mission Creek fault. This right step has created a right-lateral shear couple and extensional strain field in the greater San Bernardino valley, with extension giving rise to normal dip-slip faults like those in the Crafton Hills horst-and-graben complex and the Tokay Hill and Peters faults.


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