Description
The Chocolate-Orocopia Mountains are located along the eastern side of the Salton Trough. They are comprised of a crystalline complex in which gneisses of variable age sit structurally above the Orocopia Schist. Previous studies in the Chocolate-Orocopia Mountains have suggested that this relationship is explained by the existence of a Mesozoic thrust fault, the Vincent-Orocopia-Chocolate Mountains Thrust (Crowell, 1975, 1981; Crowell and Walker, 1962). The relationships created by the thrusting were later disrupted by the San Andreas system. Similarly, other workers, e.g., Dillon (1975), Haxel and Tosdal, (1986), and Dillon et al. (1989), have undertaken intense mapping of this structure through this and adjacent ranges, and cite it as the most significant tectonic feature in this region. The purpose of this thesis is to test the hypothesis that the range constitutes a tilted crustal slab, controlled by a mid-Tertiary detachment fault system. This has been achieved by the utilization of an array of geological techniques. The extensive mapping by Dillon (1975) has been carefully reexamined and reinterpreted in the light of field investigation evidence for the existence of detachment faults. Low-altitude aerial flight and subsequent field work have confirmed the existence of the fault in some areas. Field work has also raised questions as to the validity of projecting the Chocolate Mountains thrust through the southern Chocolate Mountains. Reprocessed seismic reflection data acquired in Milpitas Wash, to the northeast of the range, have been interpreted to show evidence for tilted crustal slabs bounded by detachment faults. The data are used to construct a physical three-dimensional model designed to show the intricacies of detachment-fault geometries. The data are also used to specifically address the structural controls on Miocene basin formation related to detachment faulting. Where possible, these and other interpreted features have been physically tied to, or projected to their equivalent surface expressions. Field observations from the Chocolate Mountains and nearby ranges, combined with data from previous studies in the southwestern Cordillera suggest that the Chocolate-Orocopia Mountains may be favorably compared to metamorphic core complexes (Crittenden et al., 1980; Coney, 1980; Davis, 1980) and are merely higher crustal analogues to them. Structural relationships also compare favorably to other extensional regimes such as the South Island of New Zealand, the western continental shelf of the United Kingdom, and the North Sea tripartite graben system. Macro-, meso-, and microscopic analysis of structural relationships suggest a Mesozoic ductile deformation event, an extensionally related ductile deformation, and a mid-Tertiary brittle overprint of ductile features in the Chocolate Mountains. This is modeled as the result of the exhumation of middle-crustal rocks along crustal-scale detachment faults, resulting in the crossing of the ductile-to-brittle transition.
