THE COPPER BASIN FAULT: A NEWLY DEFINED REGIONAL REVERSE FAULT IN THE LOWER COLORADO RIVER REGION, SE CALIFORNIA: IMPLICATIONS FOR THE TIMING OF INITIATION OF THE ECSZ

To the south of Picacho State Recreation Area, SE California, previous workers have mapped a continuous ‘arcuate' fault extending from the Colorado River west to Picacho Peak, a distance of ~ 8 km. Although, the structural character of the fault remained uncertain, it was commonly referred to or implied in the literature to be a high angle normal fault. We have mapped and studied this fault at a scale of 1:12000. The results of our work suggest that the fault is segmented and extends over a distance of ~12 km. We herein refer to it as the Copper Basin fault, after exposures in and around Copper Basin. At this location, the fault strikes approximately E-W and dips steeply (~58-64°) S placing probable Jurassic amphibolite grade gneiss above the Miocene (~24 Ma) Quechan Volcanics. The Copper Basin fault is characterized by an ~1 m thick fault core of brownish red clay gouge (cataclasite?). Quechan flows and flow breccias in the footwall block strike ~ N15E into the Copper Basin fault and dip moderately to steeply to the northwest. Additionally, coarse continuous foliation in the gneiss locally parallels the fault but is otherwise cut by it. The Sortan fault, which brought the gneiss in contact with the Jurassic Winterhaven Formation, ~24-28 Ma, is truncated by the Copper Basin fault. Finally, the contact between the overlying Quechan Volcanics and Winterhaven Formation is also abruptly truncated by the fault. These relationships suggest that reverse movement on the Copper Basin fault likely occurred following the eruption of the Quechan Volcanics. Given the juxtaposition of high-grade gneiss above volcanic rocks erupted at the surface, the amount of displacement along the Copper Basin fault, though currently unknown, may be significant. Evidence outlined here and in our talk suggests that the lower Colorado River region in and around the Picacho State Recreation Area was subject to significant N-S contraction following eruption and deposition of a largely volcanic and epiclastic sequence ~23-24 Ma. In addition, our work in the Indian Pass area suggests that the ~9-13 Ma basalts of Black Mountain were involved in this major strain event. Hence we speculate that the Copper Basin fault may have formed during a N-S contractional event that signaled the development and propagation of the ECZS to the latitude of Picacho State Recreation Area.