Abstract

The early Tertiary Orocopia-Chocolate fault in southern California was originally interpreted as a southwest-directed thrust assisting tectonic underplating of the Orocopia Schist beneath North America during subduction. However, the predicted thrusting direction is opposite to that observed for the fault. Although this problem appeared to be resolved by interpreting the Orocopia-Chocolate fault as a normal fault, this alternative is inconsistent with the fact that neither an early Tertiary breakaway nor early Tertiary upper plate extension has been observed. I suggest that the fault was a northeast-directed passive-roof thrust that linked with a southwest-directed basal décollement (the Vincent thrust) during early Tertiary flat subduction of the Farallon plate beneath North America. This model may also explain a similar structural relationship between the Main Central thrust and the South Tibet detachment fault in the Himalaya.

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