Defining the extent and age of basement provinces west of the exposed western margin of the Archean Wyoming craton has been elusive because of thick sedimentary cover and voluminous Cretaceous–Tertiary magmatism. U–Pb zircon 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 ∼2.4 to ∼1.8 Ga. Rock-forming events in the area as young as ∼1.6 Ga are also indicated by isotopic (Nd, Pb, Sr) signatures and xenocrystic zircon populations in Cretaceous–Eocene granitoids. Most of this lithosphere is primitive, gives ages ∼1.7–1.86 Ga, and occurs in a zone that extends west to the Neoproterozoic rifted margin of Laurentia. These data suggest that the basement west of the exposed Archean Wyoming craton contains accreted juvenile Paleoproterozoic arc-like terranes, along with a possible mafic underplate of similar age. This area is largely under the Mesoproterozoic Belt basin and intruded by the Idaho batholith. We refer to this Paleoproterozoic crust herein as the Selway terrane. The Selway terrane has been more easily reactivated and much more fertile for magma production and mineralization than the thick lithosphere of the Wyoming craton, and is of prime importance for evaluating Neoproterozoic continental reconstructions.