Columbia/Snake River—Yellowstone magmatism in the context of Western U.S.A. Cenozoic geodynamics

Abstract

An attempt is made to calculate the total volume of Middle Cenozoic to Recent igneous rocks in the Columbia—Snake River—Yellowstone region by adding geophysically-derived estimates of the amounts of mafic intrusion to published data on the volumes of volcanic rocks. Recently published radiometric ages allow comparison of rates of basaltic magmatism in these ensialic provinces with those along equivalent lengths of mid-oceanic ridge. The rates of total magmatic activity (extrusive and intrusive) during the genesis of the Columbia River Plateau, Western Snake River Plain and Eastern Snake River Plain—Yellowstone sub-provinces were equivalent to those of mid-oceanic ridges with spreading half-rates of 10, 3 and 0.3 cm per year, respectively. It is shown that the hypothesis that magmatism in this region is the product of a deep-mantle convective plume, situated at present beneath Yellowstone, does not explain adequately several features of the Snake River Plain and becomes untenable when applied to the province as a whole. It is proposed instead that all major post-Eocene tectonic and magmatic features of the western U.S.A., east of the Sierra Nevada and Cascades, are products of comparatively shallow-rooted diapiric upwelling in the mantle; triggered at about 40 m.y. by extraction of mafic silicate melt and volatiles from the Farallon lithospheric plate, subducted at a comparatively low angle beneath the North American plate. During the Oligocene the diapirism appears to have become concentrated beneath the present sites of the Great Basin and Columbia River Plateau. Subsequent annihilation of the Farallon plate by the northward-propagating San Andreas transform fault released the compressive stress field across the Great Basin diapir, so that its heat content could disperse largely through attenuation and melting of the sialic crust, rather than mantle partial fusion and basaltic magmatism. Conversely, constriction of the Columbia River Plateau diapir by the subduction zone to its west led to “run away” mantle fusion and massive production of basic magmas. At 10–13 m.y. the E-W zone of offset at 42–44°N between these diapirs was the sub-sialic analogue of a transform fault. Lateral shearing of uppermantle peridotite within this zone caused the partial fusion which was the source of Western Snake River Plain magmatism. Once established, this upper-mantle thermal disturbance became self-perpetuating and, as the North American plate drifted westward over it, generated the Eastern Snake River Plain—Yellowstone “hot spot track”. The constant position of this hot spot in the mantle, relative to that beneath Hawaii, during the last 10 m.y. or so may indicate that its roots now penetrate down into the mesosphere.