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USGS Banner with Coachella Valley as seen from Keyes View in Joshua Tree National Park
Western Earth Surface Processes Team

San Andreas Fault System in the Inland Empire and Salton Trough

Compressional Fault Zones

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 may have no relationship kinematically to the paleotectonic Banning fault.

Southernmost scarps in the San Gregorio Pass fault zone
Southernmost scarps in the San Gregorio Pass fault zone.

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 near Banning and Cabazon, California.
Looking north at a fault scarp of the San Gorgonio Pass Fault zone near Banning and Cabazon, California. Photo by J.C. Matti, USGS.

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.

Basement thrust over sediment.
Basement thrust over sediment.

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.

Oblique aerial photograph looking northward at the Banning Bench, at the west end of San Gorgonio Pass, southern California.

Oblique aerial photograph looking northward at the Banning Bench, at the west end of San Gorgonio Pass, southern California. 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. A major strand of the San Andreas Fault separates these two basement terranes and is responsible for their juxtaposition here by on the order of 160 km of right-lateral displacement (Matti and Morton, 1993). In the foreground, Pleistocene alluvial deposits have accumulated from streams heading northward; the Pleistocene deposits are being dissected, as evidenced by ravines and arroyos incised into them, but they also are being buried by younger Pleistocene and Holocene deposits that are being shed out of canyons draining the foothills. These younger deposits form alluvial-fans that feather out on the older Pleistocene surfaces. The Banning Bench is a landform whose shape and geomorphic evolution reflect its geologic structure. The Bench has been uplifted in middle and late Pleistocene time (starting perhaps 250,000 years before present ?) by faults of the San Gorgonio Pass Fault zone: one of these bounds the south margin of the Bench, and trends roughly east-west; the other bounds the west margin of the Bench and trends northwest. Matti and others (1985, 1992) proposed that the east-trending fault zone is a contractional segment (reverse or thrust faults) while the northwest-trending fault zone is an oblique strike-slip segment. Thus, the Banning Bench appears to have been uplifted by northwest-southeast-directed compression that yielded an elevated block whose remarkable rectangular shape is being dissected and modified by erosion over the last few hundred thousand years. Although this uplift appears not to have continued into Holocene time, a similar scenario appears to be playing out with the fault relations on the Millard Canyon fan a few kilometers to the east in San Gorgonio Pass (click here for comparison). Photo by J.C. Matti, USGS, December, 1979.


Cucamonga Fault Zone

The Cucamonga Fault zone is located along the southern margin of the eastern San Gabriel Mountains, and marks the eastern end of the frontal-fault system of the San Gabriel Mountains. The Cucamonga fault is a zone of Quaternary reverse and thrust faults. The fault zone consists of numerous inter-twining, east-striking, north-dipping thrusts that separate crystalline basement rocks of the eastern San Gabriel Mountains from alluvium of upper Santa Ana valley to the south. Some thrust faults of the zone lie entirely within alluvium (Morton and Matti, 1987). Slickensides in the basement rocks are consistently oriented down-dip, indicating the most recent displacements along the Cucamonga Fault zone have been pure thrust.

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. Individual faulting events are estimated at about 6.7 M with a recurrence of about 625 years for the past 13,000 years (Matti and others, 1982; Morton and Matti, 1987). The average north-south convergence across the Cucamonga Fault zone is estimated to have been in the range of 3 mm/yr (Weldon, 1986) to 5 mm/yr (Matti and others, 1982b; Morton and Matti, 1987).

Oblique aerial photograph looking northwest at a thrust-fault scarp of the Cucamonga Fault zone along the base of the southeastern San Gabriel Mountains
Oblique aerial photograph looking northwest at a thrust-fault scarp of the Cucamonga Fault zone along the base of the southeastern San Gabriel Mountains (Cucamonga Strand C of Morton and Matti, 1987). The fault scarp trends east-west across the San Gabriel Mountain front, and disrupts sand and gravel deposits of alluvial fans that have debouched from the mouths of the mountain canyons (in this view, Day Canyon on the left and East Etiwanda Canyon on the right. The largest and best-preserved alluvial fans are latest Pleistocene and Holocene in age (15,000 to about 1,500 years), and are thought to have been deposited mainly as a result of the climate change that occurred across the Pleistocene-early Holocene boundary (Bull, 1991). Note that older strands of the Cucamonga zone form scarps north of Cucamonga Strand C (toward the mountains); these are not as well preserved as the Strand C scarp, and can be mapped only intermittently along the base of the mountains. Click here to view a trench cut by the USGS across Cucamonga Strand C. The trench showed that the Strand C scarp is formed by a low-angle thrust fault that dips north toward the San Gabriel Mountains at about 35º. Photo by D.M. Morton, USGS, July, 1973.

Photograph of trench cut by the USGS in 1979 across Strand C of the Cucamonga Fault zone
Photograph of trench cut by the USGS in 1979 across Strand C of the Cucamonga Fault zone (as used by Morton and Matti, 1987) on the Day Canyon fanhead at the base of the southeastern San Gabriel Mountains. The fault zone dips about 35º north (toward the left), and forms a zone about 2 m wide of disturbed sediment and aligned cobbles and boulders (visible behind the person). The dark-colored zone at the top of the trench wall is the thick A horizon of a soil profile that caps the alluvial-fan gravels exposed in the trench. This profile drapes down the face of the scarp and thickens at its base, where a jumbled mass of colluvial material is present just to the left of the trench spoils. This colluvial wedge has grown in two ways throughout the 12,000-year history of fault-scarp development: (1) by gravitational collapse of the over-steepened land surface that occurred during each of several successive earthquake ground ruptures that formed the scarp and contributed to its cumulative development through time, and (2) downslope movement by colluvial processes as a result of weathering and sediment transport.

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.

Oblique aerial photograph looking northeast toward the southeastern San Gabriel Mountains, with the communities of Claremont, Pomona, and Ontario in the foreground
Oblique aerial photograph looking northeast toward the southeastern San Gabriel Mountains, with the communities of Claremont, Pomona, and Ontario in the foreground. The large Canyon left of center is San Antonio Canyon, with Mt. San Antonio (Mt. Baldy) the high summit at the left edge of the photo. East of San Antonio Canyon lie Cucamonga Canyon and Day Canyon. The abrupt south front of the San Gabriel Mountains here is marked by the Cucamonga Fault zone. Although the late Quaternary history of this zone is well known, longer-term fault movements presumably responsible for uplift of the southeastern San Gabriel Mountains are not well understood. Photo by D.M. Morton, USGS, January, 1976.

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