Western United States extension: How the West was widened

Cenozoic extension in the western United States presents a complex interrel ation of extension, volcanism, and plate boundary tectonics that defeats si mple notions of "active" or "passive" rifting. Forces driving extension can originate at plate boundaries, through basal traction, basal normal forces , or from buoyancy forces internal to the crust and lithospheric mantle. Th e latter two are most responsible for driving extension where it is observe d in the Basin and Range. The complex evolution of the northern Basin and R ange probably represents removal or alteration of mantle lithosphere intera cting with buoyancy stored in the crust. In contrast, crustal buoyancy forc es combined with a divergent plate boundary between about 28 and 16 Ma to d rive extension in the southern Basin and Range. The central Basin and Range most likely extended as a result of boundary forces external to itself but arising from buoyancy forces elsewhere in the western United States.