The deep crustal structure of the Mojave Desert, California, from Cocorp seismic reflection data

Abstract

COCORP seismic reflection profiling in the western and northern Mojave Desert of southern California has revealed the presence of numerous major low‐angle reflecting horizons within the crust. These complex, though laterally continuous, horizons are interpreted to represent major southwesterly dipping crustal fault zones, and as such they place important constraints on the tectonic evolution of the region. The upper‐most horizon is interpreted to be the Rand thrust, which, where exposed, places Precambrian and Late Cretaceous crystalline rocks over possibly younger Pelona‐Rand‐Orocopia Schist. This reflecting horizon extends for 25 km southwest of the Rand Mountains where it appears to be truncated at a depth of about 7.4 km by another horizon, which may be a later low‐angle normal fault. The other reflecting horizons are not traceable to the surface, and so greater ambiguity remains in their interpretation. The most prominent of these horizons occurs at midcrustal depths (15±6 km), exhibits a “ramp and flat” geometry, and extends over the northern area of the Mojave survey into the Basin and Range Province. A lower horizon, at depths of 20–30 km in the northern part of the survey area, is antiformal and appears to terminate above a flat and relatively continuous Moho‐depth horizon. The crustmantle transition appears to be represented by a continuous series of reflections which occur at about 10 s (33 km) in the north of the survey and at about 8–9 s (26–29 km) in the south. These reflections are offset in the vicinity of the town of Mojave. The deep intracrustal fault zones inferred from the COCORP survey may represent (1) the deep crustal continuation of the system of Mesozoic thrusts which crops out in southern Nevada and southeastern California, (2) Late Cretaceous to Early Cenozoic, northeast‐vergent thrusts related to the uplift of the Pelona‐Orocopia‐Rand Schist, or (3) low‐angle normal faults related to Early Miocene, northeast‐southwest directed crustal extension. The COCORP survey also traversed the major strike‐slip faults that bound the Mojave block. The San Andreas fault zone appears to truncate reflectors at depths of 6, 8 and 20 km within the Mojave basement, suggesting that it is a major vertical feature which extends to at least 20 km depth. Conversely, the Garlock fault does not offset an underlying reflecting horizon which occurs at 9 km depth and therefore appears to be a relatively shallow crustal feature.