Geometry and evolution of a mid-crustal extensional fault system: Chemehuevi Mountains, southeastern California

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Abstract
Summary: The extensional fault system exposed in the Chemehuevi Mountains area of the southern Cordillera provides data on the mode of mid-crustal accommodation to continental extension. A stacked sequence of three mid-Tertiary low-angle normal faults cut gently down-section through deformed Proterozoic and Mesozoic crystalline basement below Cenozoic strata. Hanging wall blocks are consistently displaced relatively NE across these three detachment faults, recording unidirectional extension of quartzofeldspathic crust at palaeodepths of 6–10 km. The two structurally deepest faults in the sequence are exposed over 22 km in a down-dip direction, across a total area in excess of 350 km 2 , and were initiated with a regional dip of less than 15° NE. Both of the structurally deepest faults are corrugated parallel to the direction of transport; wavelengths of the corrugations range between 200 m and 10 km, and amplitudes range from 30 to 400 m. These undulations are broad mullion structures that developed coeval with fault slip. Amplitude and wavelength vary with footwall rock type and pre-existing structural grain. Slip on the faults at the present level of exposure was accomplished by brittle deformation, with the generation of gouge, breccias, rocks of the cataclasite series, and rare pseudotachylites. Major mylonite zones in the Chemehuevi Mountains are older and unrelated to the extensional faulting. These data support the conclusion that mid-crustal extension in the Chemehuevi Mountains area was accommodated by an asymmetrical normal-slip shear system. Extension occurred along seismically active, gently NE-dipping, undulating surfaces. During their evolution they rose from middle- to upper-crustal depths.