Abstract

It has been inferred that Sevier-Laramide flat subduction imparted a large basal shear traction force to the overriding North American plate, resulting in Laramide Rocky Mountain foreland deformation and significant thinning of Cordilleran lithosphere. Additional inferred consequences include regional refrigeration of Cordilleran crust and suppression of synorogenic extensional collapse. Nd isotopic data from Cenozoic mafic volcanic rocks indicate that normal-thickness lithospheric mantle was preserved beneath the Cordilleran craton from Precambrian to late Cenozoic time. Therefore, Cordilleran lithosphere was not thinned significantly, and extensional collapse of orogenically thickened crust was not suppressed by refrigeration. Analogy with the South American Cordillera suggests that flat subduction does not apply significant shear stress to the base of overriding plate lithosphere beyond the fore-arc region and therefore does not mechanically thin overriding plate lithosphere (except in the fore-arc region). The principal result of flat subduction was enhanced mechanical coupling between the underriding 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 laterally across both (1) a hinterland that had attained maximum crustal thickness and (2) the rigid Colorado Plateau into the Laramide Rocky Mountain foreland. Early Tertiary waning of plate-convergence rates then allowed radial extensional collapse of the southern Cordilleran region to drive the Colorado Plateau block northward, resulting in late Laramide Rocky Mountain foreland deformation.

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