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ABSTRACT

The Salton Trough region of southern California records Neogene deformation and sedimentation related to both the classic San Andreas strike-slip system and Mesozoic compression and convergent margin magmatism. Besides these Mesozoic and late Tertiary tectonic features, an intervening deformational, sedimentalogical, and magmatic episode of crustal extension has profoundly affected the region. This extensional deformation is part of a much larger region of crustal extension across much of western North America, including the regions now called highly extended terranes. The Salton Trough originally opened as a half graben basin within this extensional terrane and contained multiple detachment faults within the array of domino and listric faults that offset the region west of eastern New Mexico, east of the Rio Grande Rift. This stacked array of extensional faults collectively extended the crust, offsetting Mesozoic fabrics such as the Santa Rosa Mylonite zone and the Chocolate Mountains thrust. Faults developed during this segmentation of Mesozoic features have often been mistakenly identified as faults of Mesozoic age.

Crustal attenuation and weaknesses created during regional detachment faulting appears to have localized the presence of many of the major faults within the San Andreas transform system. Strike-slip motion on these younger faults has translated the extensional terrane laterally, but has not disrupted much of the terrane except in position. The extensional terrane provides some key piercing points that can be used to help constrain motion on the San Andreas system, both in the Salton Trough and to the west in the California Continental Borderlands. The rigid beam of the Peninsular ranges appears to have acted as a boat between highly extended terranes in the current Salton Trough region and the Borderlands. Detachment faults just offshore in coastal California are thought to mimic the geometries and timing of detachment faults exposed along the margins of the Salton Trough.

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