REMOVAL OF MANTLE LITHOSPHERE, SYNCONVERGENT EXTENSION AND CRUSTAL ANATEXIS IN THE LATE CRETACEOUS CORDILLERAN INTERIOR

Synconvergent extension within orogenic belts is widely recognized, however the driving mechanisms remain controversial. The interior of the retroarc Cordilleran fold-thrust belt provides an opportunity to better understand the triggers for synconvergent extension. Extension at 75-68 Ma is widespread and importantly, is associated with heating followed by rapid cooling, anatexis and magmatism, and perhaps rock uplift. We see two potentially contemporaneous events as compatible with structural, metamorphic, petrologic, and thermochronologic observations, and the evolving plate geometry and velocity: (1) removal of mantle lithosphere, thereby increasing the lateral contrast in gravitational potential energy, and (2) decoupling of the middle to upper crust from the mantle by reducing lower crustal viscosity during anatexis, allowing lateral contrasts in potential energy to relax by extensional flow. The proposed delamination (or convective removal) occurred immediately prior to eastward propagation of low-angle subduction during the inception of the Laramide orogeny, and may have aided in the shallowing of the slab. This removal is ~coeval with inferred subduction-erosion beneath the Peninsular Ranges-Salinian-Sierran arc to the west but differs in its magmatic and locally tectonic response. Removal of mantle lithosphere may have provided the additional heat necessary for anatexis of tectonically thickened crust and production of Cordilleran peraluminous granites; thinning of the lithosphere would result in an increase in Moho temperature and transfer of heat would be rapid if by intrusion of adiabatic basaltic asthenospheric melts into the lower crust. Mass balance considerations permit the lithospheric mantle beneath the Cordilleran interior and Mesozoic arc to have been substantially thickened and subsequently removed. Removal may have been piecemeal or catastrophic, and was broadly coeval for several 100’s of km along strike. In the eastern Mojave Desert region, the recognition of a thin and fertile Archean mantle lithosphere is compatible with prior density-driven foundering. Delamination of lithospheric mantle explains many enigmatic yet prevalent aspects of the Late Cretaceous metamorphic, magmatic, and kinematic history of the Cordilleran interior.