This map view image of North America demonstrates that following the Laramide orogeny, Arc volcanism migrated southwestward and northwestward as the slab steepened and swept back westward. You can see that arc volcanism was reestablished along the western margin of North America by ~20ma. During the slab roll back and the migration of arc volcanism, large magnitude extension occurred in western NA, creating numerous metamorphic core complex structures (orange blobs). The thinning and heating during slab roll back and gravitational potential energy stored in a thickened crust during Mesozoic shortening are considered much of the driving mechanism for the later basin and range extension. During later Cenozoic time right oblique shear between the Farallon and North American plates also contributed to the extension that created the Basin and Range Province. Courtesy of Prof. Burchfiel.