A plume-triggered delamination origin for the Columbia River Basalt Group

Abstract

The Columbia River Basalt Group reveals a complete and detailed stratigraphic succession to assess the interplay of lithospheric and asthenospheric processes. This record of chemical change through time is used to evaluate genetic models for Columbia River Basalt volcanism. We recognize four primary constraints on source melting: (1) a plume component appears to be the dominant source of Imnaha Basalt; (2) Grande Ronde Basalt is best interpreted as being derived from a mafic pyroxenite or eclogite source; (3) the sequence of source melting must correspond with the stratigraphic record; and (4) working models must explain a step-function chemical change at the Imnaha–Grande Ronde stratigraphic boundary. We can envision only three potential models to satisfy these primary constraints: (1) melting of a mantle plume entrained with eclogite, (2) plume interaction with the Juan de Fuca plate, and (3) delamination triggered by plume emplacement. The first two of these are inconsistent with the time-stratigraphic sequence of melting and cannot satisfy all four primary constraints. In contrast, a model of plume-triggered delamination accurately predicts a progressive sequence of melting that satisfies each of the primary constraints. Such a model is consistent with recent numerical experiments demonstrating that delamination is the expected result of plume emplacement beneath thin Mesozoic lithosphere lying adjacent to a thick cratonic boundary. We test this model by comparing the observed history of uplift and tectonism in eastern Oregon and adjacent Washington to that predicted by the numerical models to reveal consistent stress regimes and strikingly similar topographic and structural profiles.