TECTONICS OF THE COLUMBIA RIVER FLOOD BASALTS

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.