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

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Abstract
Summary: 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.