The Duke River fault is a terrane-bounding structure that separates the Alexander terrane from Wrangellia in southwest Yukon. Detailed geological mapping and sampling of three key areas along the fault was completed in August 2009. In these areas, the fault juxtaposes multiply folded, pervasively foliated, greenschist facies rocks of the Alexander terrane against low-grade Wrangellian rocks that record only one phase of folding. Shear bands, fold orientations, rotated grains, lineations, mica fish and fault plane orientations indicate that the Alexander terrane has been thrust over Wrangellia. Preliminary 40Ar/39Ar ages from muscovite grains that may have been reset by motions along the Duke River fault or grown during faulting range from 90-104 Ma, suggesting that movement along the fault is at least as old as Cretaceous. Miocene felsic intrusions and Miocene to Pliocene crustal tuffs of the Wrangell lavas have been deformed by the Duke River fault, suggesting movement occurred as recently as the Pliocene