EVOLUTION OF JURA-CRETACEOUS BASINS IN SOUTHWEST YUKON, WITH IMPLICATIONS FOR MESOZOIC ACCRETIONARY PROCESSES IN THE WESTERN NORTH AMERICAN CORDILLERA

A series of Jura-Cretaceous basinal assemblages are juxtaposed between the Insular (Wrangellia and Alexander terrane) and Intermontane terranes (Yukon-Tanana terrane) in southwest Yukon. These include: the Bear Creek assemblage (204 Ma), Dezadeash Formation (164-150 Ma), Kluane Schist (92 Ma) and the Blanchard River assemblage (126 Ma), which is the focus of this study. Basins of similar tectonic affinity are found from southern British Columbia into southern Alaska, which include the Methow-Tyaughton, Gravina, Nutzotin and Kahiltna assemblages. The deposition, deformation and metamorphism of these assemblages record complicated interactions that occurred between the Insular and Intermontane terranes. Investigating these basinal assemblages offers insight into the reorganization of the Northern Cordilleran margin during the Mesozoic.

The Blanchard River assemblage is a strongly metamorphosed Early Cretaceous marine clastic sequence structurally overlain by Proterozoic to Devonian meta-siliciclastic rocks of the Yukon-Tanana terrane. Detrital zircon (DZ) signatures from the Blanchard River assemblage indicate primary sediment sourcing from the Yukon-Tanana terrane and plutons within it (199-190 Ma peak). Evidence for minor sediment sourcing from Insular terrane arc intrusions (based on a DZ age peak that includes 125-120 Ma zircon) suggests proximity of the Insular terranes to the western margin, and the Intermontane terranes, during deposition of the Blanchard River assemblage.

Data from the Blanchard River assemblage and other Jura-Cretaceous basins in southwest Yukon supports tectonic models proposing sediment deposition within pull apart basins formed during sinistral transtension along the boundary between the Insular and Intermontane terranes. Postdating deposition of the Blanchard River assemblage and the Kluane Schist, a tectonic transition into dextral transpression caused northeastward movement of the Insular terranes, which initiated closure of the Jura-Cretaceous basins. In southwest Yukon, this compressional tectonism is constrained by the Late Cretaceous burial of the Blanchard River assemblage and the Kluane Schist to amphibolite facies metamorphic conditions beneath the Yukon-Tanana terrane.