NEOGENE BASALTIC VOLCANISM AND MANTLE EVOLUTION IN THE IDAHO-OREGON-NEVADA REGION: IMPLICATIONS FOR NORTHWESTERN USA TECTONOMAGMATIC DEVELOPMENT

Neogene tectonism and magmatism in the Oregon-Idaho-Nevada region of the northwestern United States incorporates elements of the northern Basin & Range, western Snake River Plain, eastern High Lava Plains, and CRB-Steens flood basalt provinces while providing a unique and virtually continuous window into basalt and mantle evolution over the past 17 million years. This region has long been recognized as possessing a complex lithosphere transitional between that of the Wyoming Craton and accreted terranes to the west. Basalt physical, spatial, temporal, chemical, and isotopic characteristics recently helped to define a distinctive portion of this region as the Owyhee Plateau (OP). The basalt database now includes ~500 samples allowing more complete definition of the OP, its boundaries, and basalt and mantle evolution. New information on crust-mantle structure and characteristics, mantle flow patterns, and last equilibration depths of basaltic magmas derived from a variety of seismic data and models, geodynamic simulations, and phase petrology experiments and models provide a three-dimensional view of the OP and surrounding regions to accompany the volcanological and geochemical information (NSF funded High Lava Plains Project).

Synthesis of the various data and models outlined above provides the following picture of the OP: 1) rimmed by 14-17 Ma basalt and rhyolite eruptive centers; 2) limited surface expression of <17 Ma regional extension; 3) dominated by <11 Ma basalts ranging from primitive MORB-like (HAOT) to Snake River Plain-type (SROT) that erupted from low shields and fissures often aligned ~NW-SE; 4) basalt isotope data (Sr,Nd,Pb,Os) require laterally and vertically compositionally and chronologically heterogeneous mantle sources whose contributions and characteristics have changed over time; 5) marks transition from thinner High Lava Plains-Basin and Range to thicker Snake River Plain (craton) crust and lithospheric mantle; and 6) lithospheric mantle with upper cold, fluid-bearing fertile portion and lower warmer, fluid/melt-poor barren (residuum) portion, the latter created during the mid-Miocene flood basalt episode. This picture leads us to a mantle evolution scenario that emphasizes plate margin driven processes, subduction modification, and upper mantle upwelling.