COMPARISON OF TIMING OF LOW ANGLE FAULTING IN THE RAND MOUNTAINS AND SOUTHERN SIERRA NEVADA RELATIVE TO THE CLASSIC VINCENT THRUST OF THE EASTERN SAN GABRIEL MOUNTAINS

In a succinct seminal paper(1968), Perry Ehlig identified many tectonic problems related to the exposures of the Pelona, Orocopia, and Chocolate Mountain schists, and to the Rand schist of the western Mojave desert. He inferred these schists might be equivalents of Catalina schist and/or Franciscan formation. Ehlig speculated that the accumulation ages of the schists might be close to the ages of nearby magmatic arc rocks. He noted the existence of profound basement mylonite zones as caprocks over large schist areas. He believed the mylonite formation and all of the schist metamorphism were probably synchronous. Our studies of several of these masses indicate that probably there are significant differences in the apparent ages of mylonite zones above each of the schists. The exposed Rand schist appears to have similar provenance to schists of the Sierra de Salinas based on James and Mattinson(1988) and our work. Our studies of populations of detrital zircons within the Pelona schist and of zircons in the youngest plutonic rocks above the basal mylonite zones indicate the Vincent mylonite zone is younger than the lowest faults of the Rand Mountain thrust stack which can be dated between 79 +/-1 and 86 +/-1 Ma. Our preliminary information suggests that the Vincent Gap mylonite zone involved granitoid rocks as young as 74 Ma or less. Recent ion microprobe analytical work reported on individual detrital zircons in the schists have suggested that the youngest detrital component may be this young. Consideration must given to partial metamorphic resetting of detrital zircon ages during the later metamorphic histories of each of the schist masses (e.g.Silver,1991). We find no support for single stage correlated deformation and metamorphism.