The Great Basin of the western United States corresponds closely to a well-defined zone of high heat flow, thin crust, and anomalously high-attenuation low-velocity upper mantle. Seismicity, of predominantly normal faulting type, occurs in two marginal zones above the lateral transitions in the upper mantle, which can be correlated also with the most recent volcanism. Petrological studies have indicated that the region was the site of calc-alkaline andesitic volcanism during the middle to late Cenozoic which changed abruptly to fundamentally basaltic volcanism in the late Cenozoic, accompanied by the beginning of major Basin-Range crustal extension. This is interpreted as a change from the island-arc-type volcanism to active interarc spreading. The latter was triggered by the termination of the early to middle Cenozoic West Coast subduction zone. The release of the compressive stress field of the sub-duction zone is considered necessary for interarc spreading. We suggest that the spreading is caused by a mantle diapir mobilized by the descending lithospheric slab. The diapir is trapped beneath the sialic crust, and flattens and spreads out laterally as it rises. This mechanism can explain the extension and the outward migration of volcanism in the Great Basin and the marginal seismic zones. The anomalous upper mantle now present beneath the Great Basin is interpreted as the remnant of this diapir.