TECTONIC IMPLICATIONS OF LATE CRETACEOUS EXTENSION AND GRANITE PRODUCTION IN THE MOJAVE DESERT REGION: DELAMINATION OF MANTLE LITHOSPHERE AT THE ONSET OF THE LARAMIDE OROGENY?

Late Cretaceous geological events of the southwest Cordilleran arc to back-arc environments are consistent with the progressive shallowing of the Farallon slab, including an increase in plate convergence rates, extinction of the long-lived batholithic belt, removal of mantle lithosphere and lower crust, underplating of the Pelona-Orocopia-Rand schists beneath the extinct arc, and cooling and erosional exhumation of the North American lithosphere. Tectonic models for the Laramide, however, also need to account for the occurrence of seemingly contradictory widespread extension, anatexis and intrusion of granites to the east of the extinct arc during a restricted time interval in the Late Cretaceous (75–67 Ma), synchronous with shortening in the Rocky Mountain foreland. Extension and anatexis occurred along the axis of maximum crustal thickening of the Mojave sector of the Sevier orogen, and to a lesser extent within the interior of the Idaho-Utah-Wyoming sector to the north; extension at this time may have occurred in the northern Peninsular Range, southern Sierran, western Mojave, and Salinian segments of the Mesozoic Cordilleran arc. These processes, best exemplified in the eastern Mojave Desert, are viewed as predictable thermal, rheological, and dynamic consequences of delamination of mantle lithosphere beneath isostatically compensated mountain belts. It is proposed that delamination of lithospheric mantle, aided by decoupling of the crust from the mantle via a reduction in the viscosity of the lower crust through heating, incursion of fluids, and partial melting, triggered Laramide synconvergent extension. The proposed delamination would have occurred immediately prior to eastward propagation of low-angle subduction of the Farallon plate during the inception of the Laramide orogeny. Following delamination, extension and anatexis of the North American crust were aided locally by egress of slab-derived fluids from the low-angle Farallon slab. Lithosphere delamination may have aided in the shallowing of the slab to achieve low-angle subduction geometry, and facilitated the underplating of weak, water-rich metasedimentary protoliths for the Pelona-Orocopia-Rand schist.