DEVELOPMENT AND DISAGGREGATION OF A PLUTONIC COMPLEX IN SE CALIFORNIA: CONSTRAINTS ON LATE CRETACEOUS COLLAPSE OF THE SEVIER OROGEN

Understanding the regional extent, kinematics and magnitude of Late Cretaceous extension in the southern Sevier orogen is challenging due to Late Cretaceous structures being commonly overprinted and reactivated during Cenozoic extension. Here we present new evidence suggesting that the Granite Mountains (GM), of the Mojave National Preserve, were unroofed in the Late Cretaceous along shear zones that were largely overprinted during Miocene extension. The Granite and Bristol Mountains (BM) lie within the Mojave Block and Laramide deformational corridor in southern California, an area greatly affected by subduction erosion, schist underplating, Basin and Range extension and dextral shearing associated with the Eastern California Shear Zone. The GM 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 GM indicate mid crustal (4.5 kbar) crystallization of Late Cretaceous granites demonstrating significant tectonic burial of the mid Paleozoic rocks prior to intrusion. Crystallization was followed by rapid cooling to below ~150 ˚C in the Late Cretaceous, however extensional structures responsible for the rapid cooling event have not been identified. The Bull Canyon Fault (BCF), a low-angle Neogene(?) normal fault, with striae trending to the NW, outcrops along the northern base of the GM. Detailed structural analysis and kinematic study indicates the BCF reactivated an older shear zone (ca. Late Cretaceous) in the Neogene. The BM, currently 5 km NW of the Granite Mountains in the upper plate of the BCF, exhibits a well-developed mylonitic shear zone, which strikes ~152˚, dips 35˚ and lineations trend ~254˚ (SW). Microstructures show consistent top-down-dip, extensional fabrics, as well as grain boundary migration (GBM) dynamic recrystallization of quartz, indicating deformation occurred at ~500 ± 50 ˚C. Preliminary U/Pb crystallization and mica cooling ages within the shear zone of the BM demonstrate Cretaceous plutons were emplaced by ~75 Ma at 750 ˚C and subsequently cooled below ~375˚C by ~72.5 Ma. GBM microstructures coupled with geochronology data indicate extensional shearing, responsible for the unroofing of mid-crustal rocks, was active from ~75 – 72.5 Ma.