RECONNAISSANCE GEOCHRONOLOGY AND THERMOCHRONOLOGY OF THE WESTERN MOJAVE BATHOLITH

Much of the Mesozoic Cordilleran batholith, including the Sierra Nevada and Peninsular Ranges batholiths, is preserved in more or less coherent structural blocks displaying regionally consistent trends in age and geochemistry. Between these coherent blocks, however, the Mojave batholith is extensively faulted by Laramide age compression and subsequent extension, Oligo-Miocene core-complex formation, and, more recently, transrotation. A combination of new zircon U-Pb SHRIMP ages, reconnaissance geologic mapping, thermobarometry, and thermochronology is now beginning to elucidate the structure of the Mojave batholith. There is a slight trend towards younger ages to the southeast, which is distinct from age trends in the adjacent Sierra Nevada batholith and may represent later tectonic modification. 81-77 Ma granitic rocks between Palmdale and the Cajon Pass are juxtaposed against 91-84 Ma granitic rocks to the north, supporting at least ~30 km of right-lateral slip on the Cajon Valley Fault north of and paralleling the San Andreas Fault. The structural history of the western Mojave is also illuminated by new biotite and K-feldspar ⁴⁰Ar/³⁹Ar ages, together with published K/Ar ages (Miller and Morton, 1980), which fall in the 70-80 Ma range throughout most of the western Mojave. These ages are substantially younger than plutonic emplacement ages and suggest that western Mojave plutonic rocks remained at depths greater than 10 km (consistent with new igneous thermobarometry results) until Laramide-age exhumation, likely related to coeval oceanic plateau subduction. Rocks northeast of Victorville, however, retain older cooling ages and low-grade Jurassic strata, suggesting an important Laramide-age structure may separate these domains. Although many structural boundaries of the western Mojave appear to be buried beneath recent alluvium, ongoing reconnaissance geologic mapping may provide additional clues about their locations and significance.