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Southern California Geology

Major Faults of Southern California
Inland Empire Region

Text from USGS Open-File Report 92-354

Compressional Fault Zones

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.

San Gorgonio Pass Fault Zone
Cucamonga Fault Zone

San Gorgonio Pass Fault Zone

We apply the name San Gorgonio Pass fault zone to a series of Quaternary reverse, thrust, and wrench faults that extends from the Whitewater area westward to the Calimesa area. This system is associated spatially with the Banning fault, but the evolution of the San Gorgonio Pass fault zone has no relationship kinematically to the paleotectonic Banning fault. The following discussion is based on unpublished mapping still in progress. For more information and a map see the website: http://www.scecdc.scec.org/sanandre.html

Southernmost scarps in the SGP fault zone Southernmost scarps in the SGP fault zone Click here for more info and enlarged image.

In map view, the San Gorgonio Pass fault zone has a distinctive zig-zag character caused by repetition of a distinctive fault geometry--an L-shaped fault distribution in which the elongate staff of the L is oriented northwestward and the shorter base of the L eastward to northeastward. The east-oriented segments are reverse and thrust faults, with moderately dipping reverse faults in the west half of the fault zone and shallowly dipping thrust faults in the east half. The northwest-oriented segments appear to be vertical wrench faults having oblique right-lateral displacements. These segments have approximately the same orientation as active right-lateral faults in the region.

Looking north at a fault scarp of the San Gorgonio Pass Fault zone Looking north at a fault scarp of the San Gorgonio Pass Fault zone near Banning and Cabazon, California. Photo by J.C. Matti, USGS. Click here for more info and enlarged image.

On the east, faults of the San Gorgonio Pass zone first appear a few km west of Whitewater River, where the Coachella Valley segment of the Banning fault splays into multiple north-dipping thrust sheets (Morton and others, 1987). Traced westward, faults of the San Gorgonio Pass zone disappear in the Calimesa area--a region where we identify normal faults of the Crafton Hills horst-and-graben complex and where the San Bernardino strand of the San Andreas fault changes to a more northerly strike. These spatial relations between neotectonic fault complexes having three different kinematic styles (right-lateral strike slip, extension, and contraction) suggest that the fault systems are mechanically interrelated.

Lobe-shaped hills fronted by inferred faults Lobe-shaped hills fronted by inferred faults. Click here for more info and enlarged image.
Basement thrust over sediment

Basement thrust over sediment . Click here for more info and enlarged image.

Faults of the San Gorgonio Pass zone all are late Quaternary in age. Some faults in the complex may have been active only in late Pleistocene time; others have been active throughout the late Pleistocene and Holocene and have generated ground ruptures as recently as a few thousand years ago (J.C. Tinsley and J.C. Matti, unpubl. trench data, 1986). Faults with confirmed Holocene displacements have been identified only in the eastern part of the San Gorgonio Pass zone between Beaumont and Whitewater; faults in the western part of the zone between Beaumont and Calimesa appear to have been active only in late Pleistocene time (J.C. Matti and D.M. Morton, unpubl. data). However, future ground ruptures throughout the entire extent of the San Gorgonio Pass fault zone cannot be ruled out.

View looking northward at the Banning Bench

View looking northward at the Banning Bench, at the west end of San Gorgonio Pass. On the high skyline are the San Bernardino Mountains and their basement rocks of Mojave Desert-type; in the middleground are rumpled hills underlain by basement rocks of San Gabriel Mountains-type.(click here for comparison). Photo by J.C. Matti, USGS. Click here for more info and enlarged image.


Cucamonga Fault Zone

index map Click on this thumbnail to view an index map of faults and basement terranes in the Cucamonga Fault Zone. 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.
View looking northwest at a thrust-fault scarp View looking northwest at a thrust-fault scarp of the Cucamonga Fault zone along the base of the southeastern San Gabriel Mountains. Click here to view a trench cut by the USGS across the Cucamonga Fault zone. Photo by D.M. Morton, USGS. Click here for more info and enlarged image.

The Cucamonga fault is a zone of Quaternary reverse and thrust faults that separates crystalline rocks of the San Gabriel Mountains from alluviated lowlands of the upper Santa Ana River valley (Morton, 1975a,b, 1976; Morton and Matti, 1987). The pre-Quaternary history of the Cucamonga fault is obscure, but its latest Pleistocene and Holocene history reflects convergence between the Perris block and the San Gabriel Mountains (Morton and Yerkes, 1974; Morton and others, 1982; Morton and Matti, 1987).

Trench cut Trench cut by the USGS in 1979 across Strand C of the Cucamonga Fault zone on the Day Canyon fanhead at the base of the southeastern San Gabriel Mountains. The fault zone dips about 35 north (toward the left), and forms a zone about 2 m wide of disturbed sediment and aligned cobbles and boulders (visible behind the person). Click here for more info and enlarged image.

Faulting within the Cucamonga fault zone has recurred episodically during Quaternary time. The oldest faults occur within the north part of the fault zone, where some faults cut crystalline basement rock but do not break even the oldest Quaternary alluvial units. Younger faults occur farther south at the mountain front and form conspicuous scarps in young Pleistocene and Holocene alluvial fans (Morton and Matti, 1987). These relations suggest that during late Pleistocene and Holocene time, faulting within the Cucamonga fault zone migrated southward. This southward-younging pattern is complicated by merging of individual strands locally and by apparent merging of all strands in the western part of the fault zone. Latest episodes of strain release may have occurred mainly in the eastern 15 km of the fault zone and not throughout its entire 25-km length. The more complicated fault pattern in the eastern part of the zone may reflect interaction between the Cucamonga and San Jacinto faults. We speculate that northwestward migration of the Perris block by right-lateral strike-slip on the San Jacinto fault during the Quaternary has been taken up partly by reverse and thrust-fault displacements within the Cucamonga fault zone.

Looking northeast toward the southeastern San Gabriel Mountains Looking northeast toward the southeastern San Gabriel Mountains, with Claremont, Pomona, and Ontario in the foreground. The abrupt south front of the San Gabriel Mountains here is marked by the Cucamonga Fault zone. Photo by D.M. Morton, USGS. Click here for more info and enlarged image.
Continue to Extensional Faults

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