Cenozoic Volcanism and Plate-Tectonic Evolution of the Western United States. II. Late Cenozoic

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