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Flat subduction of the Farallon plate beneath the western United States during the Laramide orogeny was caused by the combined effects of oceanic plateau subduction and unusually great suction in the mantle wedge, the latter of which was caused by the shallowing slab approaching the North American craton root. Once in contact with basal North America, the slab cooled and hydrated the lithosphere. Upon removal, asthenospheric contact with lithosphere resulted in magma production that was especially intense where the basal lithosphere was fertile (in what now is the Basin and Range Province), and this heating weakened the lithosphere and made it convectively unstable. Small-scale convection has since affected many areas. With slab sinking and unloading, the western United States elevated into a broad plateau, and the weak portion gravitationally collapsed. With development of a transform plate boundary, the western part of the weak zone became entrained with the Pacific plate, and deformation there is dominated by shear.

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