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San Andreas Fault System in Southern California (SAFSOC)

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TASK 3 - Eastern California Shear Zone in Twentynine Palms Area

Task leader: Christopher Menges

Statement of Problem:
The Eastern California Shear Zone is a broad zone of strain east of the San Andreas Fault Zone that accounts for a significant fraction of the transform boundary shear between the Pacific and North America plates. In southern California, fault deformation east of the San Andreas alternates spatially between domains of northwest-trending dextral faults and east-trending sinistral faults. The Twentynine Palms area is situated along the intersection of the Mojave Desert right-lateral fault set and eastern Transverse Ranges left-lateral fault set, specifically the northwest-trending, dextral Mesquite Lake Fault and the easttrending, sinistral Pinto Mountain Fault. Detailed geologic mapping and quantitative tectonic geomorphology is needed to improve our understanding of how component faults within the Mojave Desert domain interact with each other and how strain is distributed and transmitted between the two domains. The significance of such studies is emphasized by the recent occurrence of large earthquakes, such as those at Landers and Hector Mine, nearby in the Mojave Desert. The neotectonic studies in the Twentynine Palms area can also be compared with previous mapping of Quaternary defomation across a tectonic boundary between two structural domains with geometrically similar fault sets to the north in the Eastern California Shear zone (i.e., northwest-striking dextral faults of southern Death Valley and east-trending sinistral faults of the eastern Garlock fault and northeast Mojave Desert). empty box graphic
Contorted beds of gneiss and schist of the Orocopia Schist basement complex, Red Canyon, Southern CA
Contorted beds of gneiss and schist of the Orocopia Schist basement complex, Red Canyon, Southern CA

This task will focus in particular on the following research objectives: (a) the structure, timing relations, and evolution of the numerous translational faults of the zone; (b) the interactions among these structures, particularly where the merge or intersect with each other; (c) characterization of related off-fault deformation across the zone; (d) patterns of strain transmission across the boundaries of major tectonic domains that exist in the region (eg., the dextral-slip faults of the south-central Mojave Desert lowlands and the sinistral-slip fault system of the eastern Transverse Ranges to the south).

This task will focus on geologic investigations in the south-central margin of the Mojave Desert at the transition into the northern mountain front of the eastern Transverse Ranges, centered along the area between the towns of Twentynine Palms and Joshua Tree. Specifically the project will emphasize geologic mapping and supportive geophysical and geochronologic studies on the neotectonic framework of active deformation in this part of the Eastern California Shear Zone.

Task Products:

Map, Planned: Menges, C.M., and Matti, J.C., 2011, Geologic map and digital database of the Twentynine Palms 7.5' quadrangle, San Bernardino County, California, version 1.0, USGS, Scientific Investigations Map

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