ISOTOPIC EVIDENCE FOR PRESERVATION OF CORDILLERAN LITHOSPHERIC MANTLEDURING THE SEVIER-LARAMIDE OROGENY, WESTERN UNITED-STATES

It has been inferred that Sevier-Laramide flat subduction imparted a l arge basal shear traction force to the overriding North American plate , resulting in Laramide Rocky Mountain foreland deformation and signif icant thinning of Cordilleran lithosphere. Additional inferred consequ ences include regional refrigeration of Cordilleran crust and suppress ion of synorogenic extensional collapse. Nd isotopic data from Cenozoi c mafic volcanic rocks indicate that normal-thickness lithospheric man tle was preserved beneath the Cordilleran craton from Precambrian to l ate Cenozoic time. Therefore, Cordilleran lithosphere was not thinned significantly, and extensional collapse of orogenically thickened crus t was not suppressed by refrigeration. Analogy with the South American Cordillera suggests that flat subduction does not apply significant s hear stress to the base of overriding plate lithosphere beyond the for e-arc region and therefore does not mechanically thin overriding plate lithosphere (except in the fore-arc region). The principal result or flat subduction was enhanced mechanical coupling between the underridi ng and overriding plates along the fore-arc region. This increased the horizontal end load, due to relative plate convergence, placed on the North American plate. Horizontal end-load stress was transmitted late rally across both (1) a hinterland that had attained maximum crustal t hickness and (2) the rigid Colorado Plateau into the Laramide Rocky Mo untain foreland. Early Tertiary waning of plate-convergence rates then allowed radial extensional collapse of the southern Cordilleran regio n to drive the Colorado Plateau block northward, resulting in late Lar amide Rocky Mountain foreland deformation.