EARLY EOCENE VOLCANISM IN THE CORDILLERAN OROGEN OF NORTHWESTERN NORTH AMERICA AND ITS TECTONIC IMPLICATIONS

The Late Paleocene to Early Eocene was a tumultuous time in Earth’s history in terms of global tectonics including plate reorganization, mantle plumes, continental collision, and major thermogenic events. Among the expressions of this unrest are the extensive, Late Paleocene to Early Eocene graben-fill volcanic and sedimentary complexes of the northwestern Cordilleran orogen. Previously referred to as the Challis-Kamloops belt, these volcanic complexes form a broad, discontinuous belt that extends minimally from southern Yukon to Wyoming (2,500 km), overlying older accreted terranes. We conducted a regional compilation of this belt, along with new mapping, geochemistry and geochronology, because the nature, extent and timing of volcanism, along with associated structures place important constraints on the Early Paleogene evolution of the Cordilleran orogen.

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.