REGIONAL CONTEXT OF THE COLUMBIA RIVER BASALT ERUPTIONS

I will present the Columbia River Basalt eruptions as an event caused by the combined effects of: (1) hot “plume” asthenosphere spreading northward beneath eastern Oregon, guided by areas of thin lithosphere. At depth, this hot mantle ascended through a well-imaged gap between the Gorda and Juan de Fuca slabs, forced up there by the weight of the slabs more than by its own buoyancy. (2) Plume-triggered delamination of the dense “eclogitic” roots to the Wallowa batholith (this batholith being the largest granitic body in an area where such bodies are uncommon). Melt production was amplified by decompression melting of asthenosphere ascending toward the evacuated volume and by melting of the sinking, fertile eclogite. (3) Establishment of an “eclogite engine” centered on the Wallowa batholith, which further amplified melt production. With this process, basaltic components added to the crust or mantle lithosphere become eclogite upon cooling, if placed deep enough. With continued basalt addition, the eclogite load will drive continued delamination, asthenosphereic return flow and decompression melting, basalt segregation and emplacement, eclogitization and delamination. This process can continue as long as the asthenosphere has enough basaltic component and temperature. Thus, while depending on anomalously hot mantle from depth, most of the Columbia River Basalt activity was structured and amplified by processes in the upper few hundred kilometers of the Earth.