SEISMIC STRUCTURE BENEATH THE COLUMBIA RIVER FLOOD BASALT EVENT, AND ITS SIGNIFICANCE

The Columbia River flood basalt (CRB) story, as based largely on geochemistry and geochronology, is well known to most. I’ll show results of four crust and upper mantle seismic images, and my interpretation of their significance with respect to the CRB event. My main point: arrival of a Yellowstone plume triggered the delamination of gravitationally unstable eastern Oregon lithosphere, which then amplified and organized the CRB event.

The four seismic structures are:

(1) A large, 100-300 km deep, seismically fast, nearly vertical, cylinder-shaped body lies beneath NE Oregon (termed the Wallowa anomaly). This is thought to be a piece of oceanic slab that, during the CRB event, delaminated from the base of eastern Oregon, propagating from south to north and driving CRB volcanism northward with it.

(2) A local and pronounced Moho down warp (55 km deep; adjacent depths of 30 km) beneath the Wallowa anomaly. This depression is thought to be pulled down by oceanic slab left at the base of North America that did not delaminate. The northern continuation of this flat slab is imaged beneath eastern Washington.

(3) Regional crustal anisotropy fast-axis orientation points toward NE Oregon. This is a result of crust flowing toward the area of thickened crust and (as part of another story) the Wallowa Mts.

(4) Seismically slow lithospheric “holes” located beneath the major CRB crustal chambers: Steens (NE Nevada and adjacent Oregon), Imnaha/Grande Ronde (westernmost Snake River Plain), Wanapum/Saddle Mts (SE Washington). These are thought to be the main CRB mantle melt source volumes.