Analyses of receiver functions recorded at two broadband seismic stations in the northern Cordillera indicate that the crust is thinner beneath Dawson (~35 km) than Whitehorse (~39 km). A simple two-dimensional gravity model, constrained by the seismic results, shows that this change in crustal thickness occurs at about 63 ¿N, in a zone approximately 35 km wide, where the Moho dips at ~8¿ to the south. The Bouguer anomaly high associated with the thinner crust can be traced westward into east-central Alaska where crustal thinning and extension are well documented; therefore we propose that west-central Yukon (north of 63 ¿N) has been extended also. On the basis of geological and other geophysical data we examine three likely time windows, in the Cretaceous, early Tertiary, and Present. We discuss data which indicate that early Tertiary and younger extension may be related to the transfer of motion from the Denali Fault System inboard to the Tintina Fault System. Earlier Cretaceous extension has been variously attributed to collision related back arc extension, syncollisional processes, or gravitational collapse of an overthickened crustal section. The relative importance of extension in these periods remains nebulus; however, the correspondence between the transition in crustal thickness determined in this study and a mapped boundary between ''lower'' plate and ''upper'' plate rocks suggests mid-Cretaceous extension exerted a significant influence on current Moho depths. ¿ American Geophysical Union 1995