New field and detrital-zircon age data from the Selwyn basin indicate the Yukon-Tanana terrane (YTT) was a source region for Triassic sediments (Smithian Norian; conodont ages) that were deposited along the ancestral margin of North America (NAM). Triassic rocks of the NAM contain middle to late Paleozoic detrital-zircon and geochemical signatures which are unique to the YTT and absent from the NAM, demonstrating that the Triassic rocks represent the earliest observed overlap assemblage linking allochthonous terranes to the NAM in the northern Cordillera. New provenance data also defines and characterizes Jurassic assemblages. Terrane accretion in the northern Cordillera was previously thought to have commenced in Early to Middle Jurassic time; however, the presence of a 40-50 m.y. older Triassic overlap assemblage requires that Triassic rocks were deposited in a collision-related foreland basin setting rather than a stable continental terrace and rise.