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The San Andreas fault zone in the Salton trough is characterized by subparallel, high-angle faults, en-echelon folds, and systematically arranged arrays of reverse and normal faults which are typical of major wrench faults elsewhere in the world. Detailed field mapping in the Mecca Hills shows that the fault zone is delineated by two northeast-striking high-angle faults which subdivide pre-Cenozoic crystalline and schistose basement and overlying late Cenozoic and Quaternary nonmarine sedimentary rocks into three structural domains: (1) a high-standing, relatively undeformed block northeast of the fault zone; (2) a 1.5 km-wide zone of folded sedimentary rocks between the two faults; and (3) a sparsely exposed, deeply down-faulted block on the southwest. Locally the two fa lts branch upward and flatten abruptly outward into low-angle thrusts which carry thrust slices and gravity slides of sedimentary rocks from the central block short distances on to the adjacent blocks. Between the two faults the sedimentary succession is folded into broad west-northwest-trending open folds which tighten and are overturned and truncated against the faults. Where exposed in the fault zone the basement-sedimentary rock contact is a buttress unconformity which also is folded and overturned locally. Field observations reveal that the basement in the core of the fold is fractured and sheared pervasively, indicating that the mechanism of basement deformation was one of cataclastic flow by piecemeal slip on the closely spaced fracture and shear planes, whereas the folding of the overlying sedimentary rocks was largely a passive consequence of deformation at the basement level. Although horizontal shear strain predominated, local vertical uplift of the basement occurred as a result of compression between two convergently and laterally slipping rigid crustal blocks.
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