The Columbia River Basalts and their Triggered Delaminations
Gene Humphreys, University of Oregon
Wednesday, August 21, 2019 at 10:30 AM
Yellowstone hotspot volcanism started near the Oregon-Nevada border. But before propagating ENE, consistent with North America’s plate motion, it erupted the Columbia River Basalts (CRB) from centers in eastern Oregon. There are several strange things about these eruptions: (1) they propagated rapidly north toward Washington, (2) volcanic vigor increased as the eruptions propagated “off track” and (3) an odd topographic bullseye, centered on the Wallowa Mts, uplifted in NE Oregon. Upper mantle tomography images a slab-like high-velocity structure hanging 100s of km beneath the bullseye. The story I’ll present is this: The imaged high-velocity structure is Farallon lithosphere that was left flat beneath Oregon ~50 Ma, and then delaminated during the CRB event with a S-to-N rollback. Delamination was enabled by Yellowstone magma ponding between the Farallon slab and North America, and the northward rollback drew Yellowstone plume north with it. North America extension over this hot asthenosphere caused the increase in volcanic production. Delamination rollback passed beneath the large, pre-existing Wallowa batholith in NE Oregon, allowing its dense root to founder. This unloading caused uplift of the Wallowa Mts. Other supporting “details” will be presented during my presentation.