Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 20-3
Presentation Time: 9:00 AM-6:00 PM


HUMPHREYS, Eugene D., Department of Earth Sciences, University of Oregon, Eugene, OR 97403, PERRY-HOUTS, Jonathan, Department of Earth Science, University of Oregon, Department of Earth Science, Eugene, OR 97403 and NICOLAYSEN, Kirsten P., Department of Geology, Whitman College, Walla Walla, WA 99362

The Yellowstone plume impacted North America in southern Oregon, initiating flood basalt magmatism ~17 Ma. From southern Oregon magmatism propagated rapidly northward, erupting its greatest volumes from central and northern Oregon during the ~400 k.y. duration of the Grande Ronde eruptions. During the waning eruptiions a topographic bullseye formed in NE Oregon, centered on the Wallowa batholith. The once-flat Imnaha and Grand Ronde flows found on top of the Wallowa Mts are elevated 2 km relative to those same flows exposed in Hells Canyon.

There are three geodynamic/tectonic processes fundamental to this evolution. Plume arrival triggered a south-to-north rollback-like delamination of oceanic lithosphere from the base of eastern Oregon; we image this dangling slab beneath NE Oregon to depths of >200 km. Delamination rollback dragged Yellowstone asthenosphere, and its related magmatism, northward. It also reduced lithospheric strength, allowing the ambient E-W minimum compressive stress to extend eastern Oregon and accommodate intense Grande Ronde diking; Grande Ronde dikes occupy ~10% of the crust across a 60-km wide zone in central eastern Oregon. Simultaneously, NE-oriented contraction occurred in central Oregon, defining a pole of rotation near the eastern end of the OR-WA border. Eastern OR extension propagated dikes north through the Wallowa batholith, concurrent with its initial uplift. We infer sills were being emplaced between the low-density batholith and its garnet-rich and high-density root at this time, thereby initiating root foundering. Although Wallowa uplift timing is not well constrained, the most significant uplift probably occurred 9-11 Ma, at the time when the Wallowa-bounding basins were forming. This coincides with eruption of the Powder River volcanics, which initiated at ~13 Ma as remarkably olivine phyric basalts having high Cr contents and other trace element and isotopic signatures consistent with their origin as high degree partial melts of a peridotitic source. Subsequent eruption of basanites at ~10 Ma requires a garnet-bearing source. One possible origin for the basanite melts is the partial melting of eclogitized and foundering lithosphere.