The Diligencia basin in the Orocopia Mountains of southeastern California has been one of the primary areas used to test the hypothesis of more than 300 km of dextral slip along the combined San Andreas/San Gabriel fault system. The Orocopia Mountains have also been the focus of research on deposition, deformation, metamorphism, uplift and exposure of the Orocopia Schist, which resulted from flat-slab subduction during the latest Cretaceous/Paleogene Laramide orogeny.
The uppermost Oligocene/Lower Miocene Diligencia Formation consists of more than 1500m of nonmarine strata, including basalt flows and intrusions dated at 24-21 Ma. The base of the Diligencia Formation sits nonconformably on Proterozoic augen gneiss and related units along the southern basin boundary, where low-gradient alluvial fans extended into playa-lacustrine environments to the northeast. The northern basal conglomerate of the Diligencia Formation, which was derived from granitic rocks in the Hayfield Mountains to the north, sits unconformably on the Eocene Maniobra Formation. The northern basal conglomerate is overlain by more than 300m of mostly red sandstone, conglomerate, mudrock and tuff. The basal conglomerate thins and fines westward; paleocurrent measurements suggest deposition on alluvial fans derived from the northeast, an interpretation consistent with a NW-SE-trending normal fault (present orientation) as the controlling structure of the half graben formed during early Diligencia deposition. This fault is hereby named the Diligencia fault, and is interpreted as a SW-dipping normal fault, antithetic to the Orocopia Mountains detachment and related faults. Deposition of the upper Diligencia Formation was influenced by a NE-dipping normal fault, synthetic with, and closer to, the exposed detachment faults. The Diligencia Formation is nonconformable on Mesozoic granitoids in the northwest part of the basin.
Palinspastic restoration of the Orocopia Mountain area includes the following phases, each of which corresponds with microplate-capture events along the southern California continental margin: 1. Reversal of 240 km of dextral slip on the San Andreas fault (including the Punchbowl and other fault strands) in order to align the San Francisquito – Fenner – Orocopia Mountains detachment-fault system at 6 Ma. 2. Reversal of N-S shortening and 90° of clockwise rotation of the Diligencia basin and Orocopia Mountains, and 40 km of dextral slip on the San Gabriel fault between 12 and 6 Ma. 3. Reversal of 40° of clockwise rotation of the San Gabriel block (including Soledad basin and Sierra Pelona) and 30 km of dextral slip on the Canton fault between 18 and 12 Ma.
These palinspastic restorations result in a coherent set of SW-NE-trending normal faults, basins (including Diligenica basin) and antiformal structures consistent with NW-SE-directed crustal extension from 24 to 18 Ma, likely resulting from the unstable configuration of the Mendocino triple junction.
Raymond V. Ingersoll is Professor of Geology in the Department of Earth, Planetary, and Space Sciences at UCLA, where he has been a faculty member since 1982. He was on the faculty at the University of New Mexico (1976-1982), after receiving his AB from Harvard University in 1969, and MS (1974) and PhD (1976) from Stanford University. His specialized interest is in the use of sand and sandstone in provenance studies; his broader interest is in the tectonics of sedimentary basins and regional paleotectonics. His primary research areas have been in the late Mesozoic forearc of northern and central California, the Cenozoic Rio Grande rift of northern New Mexico and southern Colorado, and the Cenozoic basins of southern California. He has edited books on Cenozoic Basins of Coastal California and Tectonics of Sedimentary Basins, as well as several guidebooks and volumes on regional geology. He has worked with K-12 science teachers in several capacities during the last 25 years. His primary present research interest is the systemic reconstruction of southern California back to 100 Ma.
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