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Southern California Areal Mapping Project

Geologic Framework of the Yucca Valley Region

| Selected References |

  In 1994 SCAMP conducted geologic-mapping investigations in the Yucca Valley region in support of ground-water issues in the Morongo Basin. The work was coordinated with USGS Water Resources Division (San Diego office), and provided geologic information useful to the siting of ground-water recharge fields in the Yucca Valley region. An administrative report displaying and interpreting mapping in the Yucca Valley North and South and Joshua Tree North and South 7.5' quads was provided to the Mojave Water Agency in June, 1994. As part of these investigations, SCAMP sponsored a graduate-research study at California State University, Los Angeles, which resulted in an MS thesis (Hopson, 1996) and several publications that refine our understanding of the Pinto Mountain fault zone and its Quaternary movement history (Hopson, 1994a, b).


  • Detailed mapping of Quaternary surficial materials led to the recognition of an extensional dip-slip fault complex previously unrecognized in the Yucca Valley region. These faults probably are part of the Pinto Mountain fault zone, an east-trending left-lateral strike-slip fault that extends from the Morongo Valley region to beyond Twentynine Palms. The dip-slip faults may reflect a left step in the Pinto Mountain left-lateral zone, thus leading to extension in the Yucca Valley region. Some of these faults ruptured in the 1992 Landers M=7.5 earthquake. The extensional faults have partitioned the Yucca Valley region into structural packages that coincide with known ground-water basins, and thus provide a hydrogeologic framework for siting future recharge fields.
  • Old, tilted sedimentary sequences north and south of Yucca Valley have been interpreted as Quaternary in age by previous workers. By contrast, we interpret these deposits as late Miocene in age. These sedimentary materials contain basement clasts, paleocurrent indicators, and catastrophic debris sheets that may provide insight into the long-term late Cenozoic history of the left-lateral Pinto Mountain fault.

Cooperative Interactions and Products

Hopson, R.F., 1994a, Tectonic geomorphology along the Pinto Mountain fault, Mojave Desert, southern California: Geological Society of America Abstracts with Programs, v. 26, p. 60.

Hopson, R.F., 1994b, Geomorphic evidence for Quaternary activity along the Pinto Mountain fault, Mojave Desert, southern California, in Murbach, D. and Baldwin, J., eds., Mojave Desert Annual Field Trip Guidebook #22-Martin Stout volume: Santa Ana, California, South Coast Geological Society, p. 197-207.

Hopson, R.F., 1996, Basement rock geology and tectonics of the Pinto Mountain Fault, San Bernardino County, southern California: Los Angeles, California State University, unpublished M.S. thesis, 132 p.

Hopson, R.F., 1998, Quaternary geology and neotectonics of the Pinto Mountain fault, Mojave Desert, southern California: California Geology, v. 51, no. 6, p. 3-13.

Matti, J.C., 1994., Preliminary geologic map of the Yucca Valley region: U.S. Geological Survey unpublished administrative report provided to the Mojave Water Agency.

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