Abstract

The late Cenozoic Great Basin is the widest and most extended part of the Basin and Range province of western North America. Any hypothesis for Great Basin evolution must explain its extraordinary east-west extension, north-striking fault blocks, east-striking transverse zones, bimodal (basalt and rhyolite) magmatism, bilateral symmetry, outward-younging magmatism and tectonism, and north- to northwest- striking rift zones and geophysical anomalies. Although the Great Basin formed during basin-range tectonism that began at about 20 Ma, some of its features are inherited from a pre-basin-range (middle Cenozoic; 45 to 20 Ma) episode and older tectonic episodes. During and predating both the pre-basin-range and basin-range episodes, east-striking transverse zones allowed the crust north and south of them to extend at different amounts, styles, and rates of strain. The pre-basin-range episode formed during subduction of Pacific plates beneath western North America. Like the basin-range episode, it was characterized by extension, although in most places that was expressed more by emplacement of voluminous calc-alkaline, shallow intrusions (partial melts of crustal rocks) in east-trending igneous belts and by metamorphic core complexes than by faults. When subduction ceased beneath the Great Basin at about 20 Ma, most of the relative motion between western North America and the northeast Pacific basin was instead taken up by the San Andreas/Walker Lane transform. Oblique extension or boundary traction forces caused by the torque between this transform and the stable western North America interior led to east-west extension along north-striking basin-range structure. The brittle-ductile transition deepened and the continental lithosphere became mechanically coupled, allowing fractures to penetrate the entire lithosphere and tap basaltic magmas from the asthenosphere. Their rise caused low-volume partial melts of crustal rocks, resulting in high-silica rhyolites that locally accompanied eruption of the basalts. During this time, a large mantle plume rose beneath the entire Great Basin area, causing its bilateral symmetry and outward-younging features. The north- to northwest-striking rifts and magnetic belts may represent zones of extension of older structures above the crest of the plume head that was bulging and spreading outward at the base of the lithosphere.