CENOZOIC MAGMATISM IN THE CORDILLERAN: CAUSES AND CONSEQUENCES

Among the many features that distinguish the Cordilleran is a magmatic record well over 100 Ma long that includes both convergent margin volcanism, as expected along a plate boundary, but also wide spread, and extremely voluminous, volcanism well inboard of the plate boundary. Geophysical imaging of western North America now provides an opportunity to connect the surface magmatic record with the structure and dynamics of the underlying mantle. The spread of convergent margin volcanism well into North America between about 100-50 Ma is generally attributed to shallowing of the dip of Farallon subduction. Beginning around 50 Ma, volcanism rapidly moved west likely due to the accretion of the Siletzia oceanic plateau into Oregon and Washington causing the Farallon to break and reinitiate subduction west of Siletzia. The sinking of the stranded portion of the Farallon then initiated the massive ignimbrite sweep SW from Montana and NW from Mexico to converge in an E-W line across central Nevada and Utah by approximately 30 Ma. Diffuse calc-alkaline volcanism continued in the Pacific Northwest in eastern Oregon-Washington and westernmost Idaho until 20 Ma. Perhaps initiated either by a slab tear or further steepening of the dip of Juan de Fuca subduction, the pattern of diffuse volcanism was replaced at 16.5 Ma by eruption of the large-volume Columbia River – Steens flood basalts. While often attributed to the initial stage of the Yellowstone Plume, the Columbia River/Steens basalts have few, if any, geochemical features suggestive of a plume origin, but several features of a source strongly influenced by subducted components. Back-arc driven flow in the mantle along with minor extension explains the wide-spread bimodal volcanism across eastern Oregon in the late Cenozoic. Snake River Plain/Yellowstone volcanism shows many features expected for plume-induced volcanism, with other features that possibly reflect deflection of the plume by a flat-lying segment of the Farallon slab imaged in the transition zone beneath the Snake River Plain. While not all magmatism in the Cordillera is the direct result of convergent margin processes, interaction of North America with the various subducted oceanic plates may have established the conditions that created the tectonically and magmatically active Cordillera.