The regional tectonic setting and possible causes of Cenozoic extension in the North American Cordillera

Abstract

Timing and tectonic setting of middle Cenozoic crustal extension in the North American Cordillera supports the concept that an overthickened crustal welt formed behind or astride the thrust belts as a result of compression during the Mesozoic to early Cenozoic. At the end of the Laramide Orogeny the gravitationally unstable welt collapsed by deep-seated crustal extension. The extension was aided by a lowering of crustal viscosity resulting from a complex pattern of volcanism and a reduction in intraplate compressive stress. As plate regimens evolved along the Pacific margin during the late Cenozoic, subduction progressively ceased as did compressive stress also. An evolving transform boundary and a massive Cordilleran-wide lithospheric uplift allowed a second phase of extension to develop across the already thinned and thermally weakened crust to form the Basin and Range Province, being active up to the present time.