REGIONAL GEOLOGY OF FLOOD BASALTS AND BASIN-AND-RANGE VOLCANIC ROCKS NORTH OF STEENS MOUNTAIN, OREGON

Mapping along the middle and south forks of the Malheur River has established a physical connection between two mid-Miocene (~16.6-15.3 Ma) flood-basalt successions – Steens basalt to the south and the basalt of Malheur Gorge (BMG) to the north. The lowermost flows of Steens basalt are chemically and stratigraphically equivalent to the lowermost flows of the BMG. The uppermost flows of Steens basalt are chemically distinct and pinch out to the north; however, they are partially interbedded with the middle and uppermost flows of the BMG, which continue to thicken northward. The upper portion of the tholeiitic succession is interbedded with a group previously unrecognized lavas – the Venator Ranch basalts. The Birch Creek and Hunter Creek basalts, the youngest lavas of the flood-basalt succession, also thicken to the north.

Subsequent (<15.3 Ma) eruptions were more localized and dominated by calc-alkaline to mildly alkaline lavas associated with Basin-and-Range extension. Deep canyons, generated by local uplift, were filled with andesitic lavas of the Keeney sequence (~13-10 Ma). The final eruptive products include the Devine Canyon tuff (~9.7 Ma), the Drinkwater basalt (~6.9 Ma), and the Voltage flow (>32,000 yrs.b.p.).

Regional correlations suggest that flood-basalt volcanism began over a broad region of southeastern Oregon, but then migrated rapidly to the north with advancing time. The rapid accumulation of ~220,500 km³ of basaltic lava over an interval of ~1.3 m.y. appears to be contemporaneous with a markedly consistent, northward propagation of regional uplift, basalt regression, and vent migration across the breadth of eastern Oregon and into southeastern Washington. The high magma supply rate requires a source of anomalously hot mantle. The migratory patterns of thermal uplift and coeval eruption are consistent with the northward advance of a mantle-plume head against the thick cratonic margin of North America.