EXTENDING THE CRB PROVINCE INTO SOUTHEASTERN OREGON: A REEVALUATION OF FLOOD-BASALT VOLUME, EFFUSION RATES, REGIONAL CORRELATIONS, AND VOLCANIC MIGRATIONS

Recent mapping and chemical fingerprinting now confirm both (1) the presence of the Imnaha and Grande Ronde Basalt Formations (CRB Group) in the Malheur Gorge of east-central Oregon, and (2) the stratigraphic relationship of these units to the Steens Basalts, which are partially older and partially interbedded with Imnaha Basalt flows south of Malheur Gorge. These correlations require a reevaluation of the extent, volume, and magma supply rates of CRB volcanism. The estimated extent of the Oregon flood basalts south of the Columbia Plateau is ~63,400 km², with an estimated volume of 50,700 km³, assuming a mean thickness of ~800 m. This added volume demonstrates that the effusion rates for CRB volcanism were much greater than previously estimated, particularly during the earliest phases (Steens/Imnaha) of eruptive activity.

            Field, petrochemical, and limited paleomagnetic data demonstrate a rapid northward migration of both volcanism and uplift, from Steens Mountain, through eastern Oregon, and across the length of the Chief Joseph dike swarm into eastern Washington. Using the recent Ar-Ar ages of Hooper et al. (2002), the lava accumulation rate for the southern eruptions of Steens, Imnaha, and Grande Ronde basalts (16.6-15.7 Ma) was ~56x10³ km³/m.y. As volcanism migrated northward into the Chief Joseph dike swarm (16.1-15.0 Ma), the lava accumulation rate for Imnaha and Grande Ronde eruptions increased to ~164x10³ km³/m.y. This migratory pattern appears to be contemporaneous with an opposing, southerly migration of magmatism along the Northern Nevada rift zone, as suggested by field considerations and paleomagnetic data (e.g., Glen and Ponce, 2002).

            Like spokes on a wheel, both migrations emanate from the McDermitt caldera, which was also the center for two subsequent, less rapid, opposing migrations, associated with eastward-advancing eruptions along the Yellowstone hotspot track and westward-advancing eruptions along the Oregon High Lava Plains. These four volcanic migrations are remarkably distinct in their eruptive styles, tectonic expressions and volcanic products. Nevertheless, the genesis of each is not only consistent with a mantle-plume origin, but also difficult to reconcile by nonplume interpretations.