TECTONIC HISTORY OF THE BRISTOL AND GRANITE MOUNTAINS IN SOUTHEAST CALIFORNIA FROM THE LATE CRETACEOUS TO THE NEOGENE

Late Cretaceous synconvergent extension in the western Cordillera took place along the axis of Mesozoic crustal thickening as well as in areas along the western “Laramide deformation corridor” effected by subduction erosion and schist underplating. Demonstrating the geographic extent of these events requires looking through the extensive Cenozoic extensional overprint and common reactivation associated with Basin and Range extension. The Granite Mountains (GM) of the Mojave National Preserve are a domal plutonic complex comprised of Jurassic and Cretaceous igneous rocks with small roof pendants of middle Paleozoic marbles. Previous geobarometry and thermochronology studies in the Granite Mountains indicate mid crustal (4.5 kbar) crystallization of Late Cretaceous granites demonstrating significant tectonic burial of the Paleozoic rocks prior to intrusion. Crystallization was followed by rapid cooling to below ~150 ˚C in the Late Cretaceous, however extensional structures responsible for Late Cretaceous rapid cooling have not been identified. Along the northern base of the Granite Mountains is the Bull Canyon Fault (BCF), a low-angle Neogene normal fault with striae trending to the NW. The Bristol Mountains (BM), 5 km to the NW of the Granite Mountains and in the upper plate of the BCF, possibly records evidence for Late Cretaceous extension aiding in unroofing of the GM mid crustal rocks. Preliminary mapping of the Bristol Mountains shows plutonic distributions consistent with the Granite Mountains, overprinted by a well-developed mylonitic shear zone. Preliminary U/Pb zircon ages obtained in both ranges show consistent Jurassic ages at 157.3±1.2 Ma (BM) and 158.1±1.4 Ma (GM) and Cretaceous ages of 75.7±0.7 Ma (BM) and 80.4± 0.6 Ma (GM). Additionally, preliminary biotite ⁴⁰Ar/³⁹Ar cooling ages in the footwall of the mylonitic shear zone shows cooling below ~350˚C from 73-71 Ma. Shear indicators consistently show top-to-SW shear at medium to high temperatures. It is possible that the Neogene BCF displaced the Bristol Mountains to the NW and obscured an earlier extension event. Future work will help constrain the details of the often cryptic Late Cretaceous synconvergent extension along the southern end of the Sevier orogenic belt where Basin and Range faulting has obscured its timing and kinematics.