1. Proterozoic Evolution of the Western Margin of the Wyoming Craton: Implications for the Tectonic and Magmatic Evolution of the Northern Rockies Southwest Laurentia Basement Provinces The Great Falls tectonic zone is dominantly ~1.85 Ga crust deformed at ~1.77 Ga. The Selway terrane contains crust formed and accreted between 2.4 and 1.6 Ga, along with small Archean blocks. The Lewis & Clark fault zone may have developed along the southern margin of the Medicine Hat block. Basement uplifts in blue ABSTRACT Defining the extent and nature of basement provinces that lie beneath Mesoproterozoic to Phanerozoic sedimentary rocks in the region between the exposed western margin of the Archean Wyoming craton and the Neoproterozoic margin of Laurentia has been elusive because of thick cover and voluminous Cretaceous-Tertiary magmatism. Known Paleoproterozoic basement is exposed southwest of the Wyoming province in the Mojave block, but the region between this area and exposed Archean and Paleoproterozoic rocks in the Priest River complex is poorly known. U-Pb geochronological data from small exposures of pre- Belt Supergroup basement along the western side of the Wyoming craton in southwestern Montana reveal crystallization ages ranging from about 2.4 to 1.8 Ga. Rock forming events as young as 1.6 Ga are indicated by isotopic (Nd, Pb, & Sr) signatures and xenocrystic zircon populations in Cretaceous-Eocene granitoids, west of the exposed western margin of the Wyoming Province. A major component of this lithosphere was primitive, gives ages about 1.7-1.86 Ga, and occurs in a zone that appears to extend west to the Neoproterozoic rifted margin of Laurentia along the trend of the Great Falls tectonic zone (GFTZ). These data suggest that the basement in this area contains some accreted juvenile Paleoproterozoic arc-like terranes, possibly combined with a mafic underplate of similar age. This area largely underlies the Belt Basin and northern Idaho batholith and we refer to it as the Selway terrane. The Selway terrane comprises these Paleoproterozoic elements along with subordinate Archean blocks isolated from the Wyoming craton. The Selway terrane crust has been more easily reactivated structurally and much more fertile for magma production and mineralization than the thick crust and lithosphere of the Wyoming craton and is of prime importance for evaluating Neoproterozoic continental reconstructions. Major structural features like the Lewis and Clark line, Helena salient, Eocene metamorphic core complexes appear to have some basement or lithospheric control. The largely continental-sourced Idaho batholith formed mainly over Paleoproterozoic arc terranes, supporting the hypothesis that the Selway lithosphere was chemically enriched and more fertile for partial melting compared to areas underlain by Archean lithosphere. Plutons of the Cretaceous SW Montana granitic province (e.g., Boulder and Pioneer batholiths), as well as the Eocene Challis volcanics, Absaroka volcanics, and Montana alkaline province are mainly restricted to the GFTZ or parts of the Wyoming province known to contain Paleoproterozoic lower crust. Spatial (e.g., orogen-orthogonal), temporal (Cretaceous and Tertiary), and compositional (ore-bearing and non ore- bearing) variations in magmatism, therefore, were related to the compositions of mantle-derived melts, depth of melting, and the relative fertility of the lithosphere. The variations reflect characteristics established during distinct Archean and Proterozoic episodes of crustal growth that were and modified during the Mesozoic and Tertiary. Improved images of the lithosphere anticipated from EarthScope data may help define the major Precambrian provinces, their boundaries and roll in crustal evolution, although higher resolution geophysical data of the crustal structure may also be required.. The Cordilleran fold-trust belt is controlled by the basement architecture. Cretaceous - Tertiary magmatism is concentrated in areas of Paleoproterozoic crust. David Foster, Paul Mueller & Jim Vogl - University of Florida David Mogk - Montana State University, Joe Wooden - U.S. Geological Survey Eocene metamorphic core complexes are concentrated in areas underlain by Proterozoic accreted terranes and island arcs. Evidence for the age of buried Precambrian crust from inherited zircons in Cretaceous and Tertiary plutons. Geochronologic evidence for Proterozoic orogeny in the NW Wyoming Province, and for Proterozoic rocks in the Great Falls tectonic zone and Selway terrane. Aeromagnetic anomaly map of northwestern United States with interpreted Precambrian terrane boundaries and Proterozoic structures from Sims et al. (2004). CB, Cheyenne belt suture; DSZ, Dillon shear zone; GFSZ, Great Falls shear zone; LC Lewis and Clark fault zone; SF Snake River fault zone; SR Snake River Plain.