EARLY EOCENE VOLCANISM IN THE CORDILLERAN OROGEN OF NORTHWESTERN NORTH AMERICA AND ITS TECTONIC IMPLICATIONS
The volcanic complexes currently cover an aerial extent of ca. 35,000 km2. Thicknesses range from 1-4 km, but because of erosion, most recently by Pleistocene glaciation, the original volume would have been much greater. Volcanism took place within pull-apart grabens and half-grabens associated with dextral faulting on a high-standing plateau, concurrently and spatially associated with the exhumation of metamorphic core complexes, and extension-related plutonism of the Coast Belt. Though styles of volcanism vary locally, shallow-dipping thick sequences of fissure-fed, plateau forming flows are common. The rocks are basaltic andesites to dacites and are mostly calc-alkaline to locally alkaline. Trace element signatures are typical of lithospheric melts with a crustal influence, lacking geochemical evidence for asthenosphere input along the length of the belt. Radiogenic isotopes match the basement terranes. Compiled and new radiometric ages show that this voluminous volcanism had a rapid onset at ca. 57 Ma, continuing to an abrupt offset at ca. 47 Ma, with volcanism becoming younger to the south. The age progression, nature and extent of volcanism, and structures are consistent with volcanism occurring in rift-basins atop their underlying basement terranes, during dextral transiting along the margin of western North America.