EARLY MANIFESTATION OF YELLOWSTONE HOTSPOT VOLCANISM ALONG A NORTHEAST-TRENDING, 280-KM-LONG LITHOSPHERIC DISCONTINUITY

In north-central Nevada, the Yellowstone hotspot generated bimodal and rhyolitic eruptions from ~16.5-15.0 Ma, along the NW-trending northern Nevada rift system and within the McDermitt and Santa Rosa volcanic centers. These early eruptions occurred above transitional to cratonic lithosphere lying east of the 0.704 Sr isopleth. Farther west, contemporaneous eruptions from ~16.8-15.5 Ma were generated above oceanic lithosphere along a distinct, NE-trending belt that parallels the 0.704 Sr isopleth for ~280 km, from southeastern Oregon to northeastern California. This linear belt is defined in part by the initial flood-basalt eruptions of the Columbia River Basalt Group from NE-trending feeder dikes exposed at Steens Mountain (~16.7 Ma) and by massive rhyolite eruptions from an alignment of NE-trending calderas of the High Rock caldera complex in northwestern Nevada (~16.5-15.5 Ma). We present new Ar-Ar and zircon age data to show that the SW extent of this belt includes contemporaneous fissure-feeding eruptions of mildly alkaline to calc-alkaline lavas from the Smoke Creek dike swarm along the Nevada-California border region. Recent seismic studies suggest that the volcanism along the belt resulted from the abrupt generation of magma exploiting a major lithospheric weakness partly defined by the 0.704 Sr line. In SE Oregon, this discontinuity is delineated by the transition from thin crust (~25-35 km) and lithosphere (~55-60 km) underlying the High Lava Plains to much thicker crust (>35 km) and lithosphere (~80 km) underlying the Owyhee Plateau (Eager et al., 2011; Hopper et al., 2014). Whereas the fissure-fed tholeiitic lavas at Steens Mountain appear to be derived from a mixture of plume and depleted mantle sources, the calc-alkaline to mildly alkaline lavas of the Smoke Creek dike swarm appear to be the earliest lavas of the middle Miocene ancestral Cascades arc of southern Oregon, northeast California and northwestern Nevada, having followed a hiatus in arc volcanism of 5-6 m.y. The temporal and spatial coincidence of these arc lavas with hotspot volcanism suggests that rejuvenation of the volcanic arc was directly related to plume emplacement.