WALLOWA BATHOLITH ROOT FOUNDERING, REGIONAL CRUSTAL FLOW AND TOPOGRAPHIC EVOLUTION

A major cause and consequence of the Columbia River flood basalt (CRB) event are triggered lithospheric instabilities. The most fascinating is the foundering of the Wallowa batholith root and the resulting regional restructuring of the crust and topography.

In NE Oregon, the Wallowa Mt uplift elevated an isolated Cretaceous batholith ~2 km shortly after the CRB event. This uplift is the center of a topographic bullseye 150 km wide. With no evidence of significant regional horizontal strain, the vertical motions indicate the evolving density structure. By integrating a host of observations, we account for the bullseye structure.

When CRB magmatism enabled the S-to-N delamination of older ocean lithosphere that was left beneath eastern Oregon, the region elevated ~½ km. Uplift propagated north with the delamination, ending at the northern end of what will become the bullseye; this is where the dangling delaminated oceanic slab is tomographically imaged in the upper mantle. Delamination removed the root’s basal support, and emplacement of a basaltic sill (inferred from surface-wave imaging) above the root further reduced the root’s support. Root foundering caused lower crust flow into the space evacuated by the root. The batholith rose isostatically, and a depression formed around the Wallowa uplift as lower crust flowed beneath the Wallowa batholith. The bullseye’s outer ring of high elevation is thought to be (mostly) the uplift created during the ocean lithosphere delamination.

Lower-crustal seismic anisotropy forms a clear radial pattern centered on the Wallowa Mt region and extending to distances of 500 km(!), indicating an apparent flow of crust toward the Wallowa Mts. Complicating this interpretation is a Moho depression of ~10 km (imaged with receiver functions) local to where the oceanic slab is apparently left at the base of North America, i.e., near the Oregon-Washington border. Each of these crustal sinks of inflow involved a large volume of crust.

Delamination and root foundering are popular subjects. In eastern Oregon we have a localized case of delamination and an isolated “Green’s function” instance of root foundering. Wallowa root foundering excited regional crustal flow and modification which is reflected in the topography.