GEOPHYSICAL EVIDENCE FOR CRUSTAL THICKNESS VARIATIONS BETWEEN THE DENALI AND TINTINA FAULT SYSTEMS IN WEST-CENTRAL YUKON

Analyses of receiver functions recorded at two broadband seismic stati ons in the northern Cordillera indicate that the crust is thinner bene ath Dawson (similar to 35 km) than Whitehorse (similar to 39 km). A si mple two-dimensional gravity model, constrained by the seismic results , shows that this change in crustal thickness occurs at about 63 degre es N, in a zone approximately 35 km wide, where the Moho dips at simil ar to 8 degrees to the south. The Bouguer anomaly high associated with the thinner crust can be traced westward into east-central Alaska whe re crustal thinning and extension are well documented; therefore we pr opose that west-central Yukon (north of 63 degrees N) has been extende d also. On the basis of geological and other geophysical data we exami ne three likely time windows, in the Cretaceous, early Tertiary, and P resent. We discuss data which indicate that early Tertiary and younger extension may be related to the transfer of motion from the Denali Fa ult System inboard to the Tintina Fault System. Earlier Cretaceous ext ension has been variously attributed to collision related back are ext ension, syncollisional processes, or gravitational collapse of an over thickened crustal section. The relative importance of extension in the se periods remains nebulus; however, the correspondence between the tr ansition in crustal thickness determined in this study and a mapped bo undary between ''lower'' plate and ''upper'' plate rocks suggests mid- Cretaceous extension exerted a significant influence on current Moho d epths.