The Orocopia Mountains border the Salton Sea northeast of the San Andreas fault in Riverside County, California. The range core, composed of Orocopia schist, is separated on the southwest from

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deformed late Cenozoic non-marine strata by the Hidden Spring fault zone, a branch of the nearby San Andreas.

Northeast of the Orocopia schist a mile-wide wedge of gabbro, diorite, anorthosite, gneiss, and alaskite lies between a northeast-dipping fault, at places folded, and a high-angle major fault marked by great crushing. These rocks, intruded by volcanics and highly deformed, resemble rocks in the western San Gabriel Mountains, about 150 miles northwest.

Northeast of the high-angle fault, augen gneiss with migmatite on the southeast and granite on the north underlies unconformably about 4,800 feet of newly discovered fossiliferous marine Eocene strata which are probably correlative with Coast Range middle Eocene. Unconformably overlying this sequence is a 5,000-foot thick variable series of undated non-marine conglomerate, sandstone, shale, and tuff, with volcanic flows and intrusions. In this series, lenses of granitic breccia characterize the northwest, and platy tuffaceous sandstone with gypsum-bearing interbeds the southeast.

Major faults separate the area into tectonic blocks of different geologic history, and local correlation across the faults is not possible. Understanding of the significance of these faults and others in the vicinity, like the San Andreas, awaits regional study such as that now underway. Strike separations dominate over dip separations on minor faults associated with complex folds.

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