Jurassic thrusting of Precambrian basement over Paleozoic cover in the Clipper Mountains, southeastern California

The Clipper Mountains in the eastern Mojave Desert expose evidence of Jurassic plutonic intrusion along what was an active thrust at the east fringe of the exposed Cordilleran Jurassic magmatic arc. This event occurred during a period of widespread arc magmatism and intra-arc thrusting in the Cordillera related to subduction under the west edge of North America. Jurassic plutons in the eastern Mojave Desert are compositionally more diverse and more K (sub 2) O-rich than Cretaceous plutons. Late-kinematic intrusion of the Jurassic Goldhammer pluton, exposed in the Clipper Mountains, was along an active ductile thrust fault that put Proterozoic basement gneiss over Paleozoic strata by the time of intrusion. U-Pb geochronology and hornblende geobarometry are interpreted to indicate that the pluton was emplaced at 161 + or - 10 Ma at a pressure approximately 0.46 GPa or more. This pressure corresponds to approximately a 17-km depth or more, at least 13 km greater than inferred stratigraphic overburden (2-4 km) at the time of intrusion. The excess we attribute to pre-intrusion tectonic burial from overthrusting of the observed allochthon of Proterozoic basement and (or) from earlier Mesozoic overthrusting. Ductile deformation continued along the observed thrust system during intrusion of the Goldhammer pluton. Fabrics in the pluton and country-rock record ductile shearing that was partly top westward but mostly top-eastward; the shearing began before or during the intrusion and continued during and after intrusion. The Jurassic burial history in the Clipper Mountains parallels that in adjacent ranges to the east, but contrasts with that in ranges to the south and west where exposed Paleozoic rocks were at colder and shallower crustal levels in Jurassic time. The tectonic record in the Clipper Mountains suggests large crustal thickening and topographic uplift that would be expected to leave a sedimentary record in Jurassic basins.