Wallowa Seismic Anomaly: Lithospheric Delamination and Northward Rollback Triggered by the Yellowstone Plume, Along the Western Edge of Precambrian North America

Date: 4/26/2019

Time: 04:15 PM

Room: Puget Sound

Three main phases of volcanism are generally associated with the 16.7 Ma Columbia River flood basalt event: Steens, Imnaha, and Grande Ronde, with the latter being the largest. Geochemical data shows a time and space progression from south to north, and a change in the geochemical signature likely related to the northward propagation of part of the Yellowstone plume beneath the lithosphere underlying the Blue Mountains terranes and Precambrian North America (Wolff and Ramos, 2013). We use new and previously acquired teleseismic data from the region of the Columbia River basalts, western Snake River Plain, and Idaho batholith, with an improved seismic tomography algorithm, to characterize the upper mantle seismic structure. Our P and S tomography models better resolve the geometries of the high-velocity anomalies beneath the source region of the Columbia River basalts. In NE Oregon, the Wallowa high-amplitude, high-velocity anomaly extends to depths of ~350 km. This anomaly is separate from the large, slab-like “Siletzia” anomaly located beneath central Idaho, and from a shallower high-velocity anomaly. We image a southward dip for the Wallowa seismic body consistent with a northward delamination triggered by the arrival of the Yellowstone plume at the base of North America at 17 Ma. The south-to-north rollback along the Precambrian margin of North America coincides with the temporal and spatial progression of the Miocene volcanism west of the major continental boundary as defined by the ⁸⁷Sr/⁸⁶Sr 0.706 isopleth. The northern end of the rollback is located today along the east-west segment of the 0.706 isopleth west of Orofino, Idaho. The separation between the Wallowa and Siletzia upper mantle anomalies and their different amplitudes and geometries are thought to result from their different tectonic histories. While the correlation between the geometries of the mantle anomalies and fundamental lithospheric boundaries is not understood, the lithospheric structure appears to be expressing significant control.

Presenting Author: A. Christian Stanciu

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