A major change in volcanic associations and their tectonic settings occurred in much of the Western United States during late Cenozoic time. Where this volcano-tectonic transition can be documented, an earlier orogenic and post-orogenic association of predominantly calc-alkalic andesitic rocks typical of circum-Pacific continental margin and island arcs was succeeded by fundamentally basaltic volcamsm which accompanied regional normal and strike-slip faulting. The igneous fields regarded here as fundamentally basaltic include: (1) basaltic fields, (2) alkalic fields in which differentiated igneous series commonly can be related to alkali-basaltic parent magmas, and (3) bimodal associations of mafic and silicic rocks, generally basalts and high-silica rhyolites. Similar igneous fields occur in other regions of the world characterized by tectonic extension. The nature and timing of the late Genozoic volcano-tectonic transition in various areas of the Western United States are documented from published references. The transition began in the southeastern part of the region in latest Oligocene time and moved northwestward through Miocene, Pliocene, and Quaternary time. The inception of basaltic, alkalic, or bimodal volcanism and associated regional extension of inland areas appears to date the times at which plate-tectonic boundaries between North America and two Pacific plates underwent drastic changes. These changes resulted from collision of the East Pacific Rise with a mid-Tertiary continental-margin trench and resulting direct contact of the American and western Pacific plates along a right-lateral transform fault system. These platetectonic interactions have evolved continuously and have determined the volcanic and tectonic evolution of the Western United States for the last 30 million years.
