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Gneissic and granitic rocks sit structurally above the Mesozoic Orocopia Schist in the Orocopia and Chocolate Mountains of southeastern California (fig. 1). The gneissic and granitic units were originally emplaced above the schist along a Late Mesozoic thrust as is generally believed. In the Orocopias, the east-dipping system of faults that currently juxtapose the units, however, is a major system of anastamosing normal faults, which records a continuum of deformation from ductile mylonitic textures to brittle fault gouge. The middle to upper-crustal deformation by large-scale extension is a continuation of the highly extended terrane along the Colorado River and that to the west in the California Continental Borderlands. This crustal extension in the Orocopia-Chocolate Mountains region helped localize the San Andreas fault system and is itself offset along the San Andreas to numerous exposures west of the main strand of the San Andreas, including many oil-bearing basins.

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