Abstract

The San Andreas fault zone, in the Durmid area northeast of the Salton Sea, forms the northeast boundary of the Salton trough (the landward continuation of the Gulf of California rift). The zone is characterized by subparallel faults having both right-lateral and vertical separations. Stratigraphic offsets indicate at least 1,100 m of vertical separation on the San Andreas fault, southwest side downthrown, and 850-m right-lateral strike-slip separation is indicated by offset drainage.

An anticlinal structure southwest of the San Andreas fault is probably caused by drape folding over an upfaulted basement block or a recently intruded pluton. Small folds on the southwest flank of the anticline are caused by drag along the San Andreas fault and gravity sliding. An angular discordance of 18° between the Shavers Well and Borrego Formations indicates that tilting adjacent to the San Andreas fault began prior to Borrego deposition in Pleistocene time. Folding followed deposition of the lacustrine Borrego Formation. Uplift halted, and the area was planed off by erosion. Renewed uplift followed, probably within the last few thousand years.

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