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Southern California Areal Mapping Project

Geologic Framework of San Gorgonio Pass

View looking east through San Gorgonio Pass View looking east through San Gorgonio Pass. Photo by J.C. Matti, USGS. Click for more info and enlarged image.

| Selected References |

SCAMP currently is performing geologic-mapping investigations that will clarify the geologic setting and geologic history of the San Gorgonio Pass region. This region is the most complex part of the San Andreas fault system in southern California, and for this reason the area is referred to as a structural knot in the San Andreas. Geologic features that characterize the knot include:

  • it has the deepest seismicity in southern California (22 km);
  • focal-mechanism depths and types vary from deep to shallow and include virtually every mechanism type (right-lateral and left-lateral strike slip, contractional, and extensional);
  • the region is traversed by a complex zone of young north-dipping thrust faults that interact in complicated ways with right-lateral strike-slip faults.

The San Gorgonio Pass knot intervenes between two modern strands of the San Andreas: the San Bernardino strand to the northwest and the Garnet Hill strand to the southeast. Although aligned with each other, these San Andreas strands do not connect straightforwardly in the San Gorgonio Pass region. Geologic-mapping by SCAMP and seismic-reflection and refraction studies by the USGS Earthquake Hazards Program should provide information about how late Quaternary right-slip strain is passed through the San Gorgonio Pass knot. These geologic and geophysical studies will have implications for rupture-initiation or rupture-termination scenarios on the San Andreas, and will allow questions to be posed such as (1) can ground ruptures triggered by large earthquakes on the southern San Andreas fault propagate through San Gorgonio Pass and extend northwestward onto the San Bernardino strand? and (2) can large earthquakes in the San Gorgonio Pass region trigger ruptures that unzip the San Andreas to the northwest?

Geologic-mapping and associated studies in the San Gorgonio Pass region in FY 2000 will clarify the structural setting and history of the tectonic knot as it has affected the region for the last 1.5 million years. These investigations will assist in evaluating seismic potential for the San Andreas fault system, particularly in evaluating whether the San Gorgonio Pass knot is a sticking point blocking future fault-rupture propagation from the San Andreas to the southeast or an earthquake-nucleation point triggering fault rupture on the San Andreas to the northwest.

Initial releases in FY 2000 will be the Beaumont, Cabazon, and Whitewater 7.5' quadrangles, followed with the Forest Falls and San Gorgonio Mtn 7.5'quadrangles.

View looking west toward San Gorgonio Pass View looking west toward San Gorgonio Pass. Photo by J.C. Matti, USGS. Click here for more info and enlarged image.

Recent SCAMP Products

Matti, J.C., and Morton, D.M., 1993, Paleogeographic evolution of the San Andreas fault in southern California: a reconstruction based on a new cross-fault correlation, in Powell, R.E., Weldon, R.J., II, and Matti, J.C., eds., The San Andreas fault system: displacement, palinspastic reconstruction, and geologic evolution: Geological Society of America Memoir 178, p. 107-159.

Jachens, R.C., 1992, Aeromagnetic map of the Palm Springs 1:100,000 scale quadrangle, California: U.S. Geological Survey Open-File Report 92-549, scale 1:100,000.

Jachens, R.C., and Langenheim, V.E.,1996, Aeromagnetic map of the San Bernardino 1:100,000-scale quadrangle, California: U.S. Geological Survey Open-File Report 96-284.

Sieh, K.E., and Matti, J.C., 1992, The San Andreas fault system between Palm Springs and Palmdale, southern CaLifornia: field-trip guidebook, in Sieh, K.E., and Matti, J.C., eds., Earthquake Geology San Andreas fault system Palm Springs to Palmdale: Los Angeles, Association of Engineering Geologists, southern California Section, Guidebook and reprint volume, p. 1-12.


Cooperative interactions

SCAMP geologic investigations in the San Gorgonio Pass region are coordinated with several USGS projects, and with the San Gorgonio Pass Water Agency. These cooperative investigations include:

Geologic framework for earthquake studies:

The USGS Earthquake Hazards Program sponsors earthquake investigations in southern California, including investigations of ground-shaking potential, earthquake monitoring, and earthquake history. The California Division of Mines and Geology has a program of earthquake-fault mapping and zonation designed to recognize fault lines and to outline setback guidelines. Geologic-mapping studies by SCAMP are coordinated with these earthquake investigations, and are designed to clarify the distribution, origin, and history of faulting within the San Gorgonio Pass region.

Geologic framework for ground-water studies:

Water Resources Division (WRD) of the USGS is actively conducting ground-water investigations in the San Gorgonio Pass region in support of water-supply issues. These studies include ground-water flow modeling and drilling investigations to obtain information about subsurface geology. Ground-water investigations are conducted out of the WRD San Diego office. SCAMP geologic-mapping studies are coordinated with the ground-water investigations, with the joint objective of developing a refined hydrogeologic model for the San Gorgonio Pass Basin. The hydrologic and geologic/geophysical investigations are funded cooperatively by the San Gorgonio Pass Water Agency.

| Selected References | Return to "Where is SCAMP Currently Working?" |

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