For more than 1,500 km along the western Cordillera of North America, late Cenozoic extensional faulting has produced block-faulted basin-range structure characterized by alternating elongate mountain ranges and alluviated basins. The faulting follows older geologic patterns, particularly those of Mesozoic and early Tertiary deformation and of early and middle Tertiary igneous activity. Basin-range structure is commonly inferred to represent either (1) blocks tilted along downward-flattening (listric) faults in which the upslope part of an individual rotated block forms a mountain and the downslope part a valley or (2) alternating downdropped blocks (grabens) that form valleys and relatively upthrown blocks (horsts) that form mountains. Such structure has been produced by extension estimated to be from 10% to 35% of the original width of the province and as much as 100% in specific areas. The province is characterized by anomalous upper mantle, thin crust, high heat flow, and regional uplift.
Current theories on the origin of basin-range structure can be grouped loosely into four main categories. In the first, the structure is presumed to be related to oblique tensional fragmentation within a broad belt of right-lateral movement and distributed extension along the west side of the North American lithospheric plate. This motion was initiated by the collision of the East Pacific Rise with the North American plate, which brought together the North American and Pacific plates to form the right-lateral San Andreas transform fault system. The second theory relates extension to spreading caused by upwelling from the mantle behind an active subduction zone (back-arc spreading). The third theory relates the basin-range structure to spreading that resulted from presumed subduction of the East Pacific Rise beneath part of North America. The fourth theory relates the basin-range structure to plate motion caused by deep-mantle convection in the form of narrow mantle plumes. The combination of anomalous upper mantle, thin crust, high heat flow, regional uplift, and extension with a previous history of high heat generation can best be related to back-arc spreading. The spreading may have been accelerated by slackening of confining pressure after the destruction of the subduction system along western North America and may have been accompanied by right-lateral shear because of the development of the transform western margin of North America.