Title: "Plume interaction with the Farallon slab reflected in volcanic perturbations along the Yellowstone adakite hotspot track."
The Yellowstone hotspot has been described as a long-lived feature that that resided offshore to produce the Siletzia oceanic plateau in Paleocene to Eocene time. Wells et al. (2014) suggested that Farallon-plate subduction resulted in the accretion of Siletzia at 50 Ma followed by overriding of the hotspot by about 42 Ma. In this talk, I will discuss how the Yellowstone hotspot was manifested in eastern Oregon in late Oligocene to early Miocene time, after accretion of Siletzia but before generation of the Columbia River flood basalts and the Snake River Plain hotspot track. I will present data that reveal a broadening of the southern segment of the ancestral Cascades arc into the Oregon back-arc region from 30-20 Ma. This period of volcanism generated rocks with elevated LILE/HFSE values and depleted Nb concentrations, consistent with a hydrated and metasomatized mantle source enriched in the water-soluble elements Rb, Ba, K and Pb. This broadening event is attributed to thermal uplift of the Farallon slab by the underlying Yellowstone mantle plume, with heat diffusion, decompression and the release of volatiles promoting high-K calc-alkaline volcanism throughout the Oregon back-arc region. Several of the 30-20 Ma outcrops in Oregon-Nevada border region are dominated by andesitic lavas with high Sr/Y values consistent with adakites derived from the melting of oceanic crust, or from combined oceanic crust and mantle sources. These define a broad area of northeast trend parallel to plate motion which we consider to be an older extension of the Yellowstone-Snake River Plain hotspot track, where melting of oceanic crust was a consequence of slab breakup. Slab uplift and the cessation of corner flow resulted in a volcanic hiatus that began at 22-20 Ma but ended at 16.7 Ma with coeval rupture of the uplifted slab in two separate places. The eastern rupture resulted in the eruption of Steens Basalt derived from the ascent of a dry mantle-plume source contaminated with depleted mantle. The western rupture resulted in the contemporaneous rejuvenation of high-K calc-alkaline volcanism due to renewed subduction and melting of a wet mantle source restricted to the Nevada-California border region. The southern segment of the ancestral Cascades arc has since migrated to the west at 7.8 km/m.y. Destruction of the slab is evident today in the seismic resolution of a slab hole beneath southern Oregon bound on the south by the Yellowstone adakite hotspot track. Its demise is the combined result of thermal erosion, rapid tearing, and the foundering of slab segments from 30-10 Ma.