THE OWYHEE PLATEAU AND ITS ROLE IN THE DEVELOPMENT OF THE YELLOWSTONE HOTSPOT SYSTEM

The Idaho-Oregon-Nevada border region is a desolate, sage-covered, volcanic upland that preserves a unique 17 million year record of volcanic and tectonic development. This region occupies the southeastern corner of the Oregon Plateau, approximates the inferred initial manifestation area of the Yellowstone hotspot, and is situated south of the Oregon-Idaho Graben, north of the Northern Nevada Rift, west of the Owyhee Mountains, and east of the Steens-Pueblo Mountains. We refer to this entity as the Owyhee Plateau. As one seeks a unifying explanation for late Cenozoic northwestern US flood basalt volcanism, the development of the Snake River Plain-Yellowstone and High Lava Plains volcanic trends, and the abrupt changeover from strongly differentiated lavas of the Steens-Columbia River flood basalt event to less evolved yet chemically heterogeneous basalts of the Oregon Plateau and Snake River Plain, it is essential to consider the Owyhee Plateau's role in these events. Why? The Plateau defines the geographic intersection of the two major regional volcanic trends and preserves a record of mafic magma production over the past 17 Ma. Furthermore, the isotopic diversity displayed by basalts erupted from Plateau vents requires that this region is underlain by transitional lithosphere (ancient cratonic and young accreted), and the distribution of eruptive loci and major tectonic features suggests that the Plateau has behaved as a relatively stable entity. These features are used to argue that the Owyhee Plateau represents a salient of thick, stable, transitional lithosphere that (1) preserves multiple geochemical reservoirs including those associated with a long history of subduction-related mantle enrichment and depletion, (2) acts as a lid to deflect the majority of upwelling mafic magmas and anatectic crustal melts, and the development of large magmatic systems to the edges of the Plateau, and (3) helps to focus continued mafic upwelling outward along zones of lithospheric weakness marking the developing Snake River-Yellowstone and High Lava Plains trends. In this scenario, the Owyhee Plateau occupies a central role in controlling the ultimate configuration of late Cenozoic northwestern US tectonomagmatic activity, as does slab rollback and the initiation of back-arc extensional processes.