MID-MIOCENE EMPLACEMENT OF THE YELLOWSTONE MANTLE PLUME IN THE CASCADIA BACK-ARC REGION

The southern Cascades back-arc region was subject to an extraordinary mantle-melting event that generated ~200,000 km³ of basaltic lava over an very short interval of geologic time, from ~16.6-15.0 Ma. Feeder dikes for this event occur along the length of the northern Nevada rift, along the Steens Mountain escarpment in southeastern Oregon, in the Monument dike swarm of north-central Oregon, and in the Chief Joseph dike swarm of northeastern Oregon and adjacent Washington. Eruption volume and effusion rates increase from south to north, with the Chief Joseph dike swarm generating >85% of the total volume over a sustained effusion rate of at least 500-600 km³/km/my (Hooper et al., 2007). In contrast, the degree of crustal extension in the back-arc area decreases from south to north. This inverse relationship alone suggests that back-arc extension was not the controlling factor in flood-basalt initiation. The abrupt onset of volcanism and the high effusion rate, over an exceedingly short period of time, in a region of minimal extension, is better explained by the abrupt arrival of a thermal and/or fertile anomaly at mantle depths. High ³He/⁴He ratios in Imnaha Basalt suggest a deep-mantle source, consistent with a plume origin. The only contender for such a source is the mantle plume that currently resides beneath the Yellowstone caldera to a depth of at least 500 km. Projecting back in time, the Yellowstone plume can be traced along the Snake River Plain hotspot track to the very site of flood-basalt volcanism at ~16.6 Ma. This spatial and temporal connection of a flood-basalt province to a hotspot track is consistent with a plume head/tail pair, as predicted by traditional models. Plume emplacement was near the Nevada-Oregon-Idaho tri-state area, where Steens Basalt erupted ~16.6 Ma, followed by a burst of rhyolitic volcanism from ~16.5-15.5 Ma. Outward spreading of the plume head is marked by the rapid migration of basaltic volcanism and dike intrusion away from this region, along three radial trends: northward into the Chief Joseph and Monument dike swarms, and southward into the northern Nevada rift. Northward migration into the Chief Joseph dike swarm was accompanied by plume-triggered delamination and the spontaneous melting of lower crust to generate the voluminous basaltic andesites of the Grande Ronde formation.