MID-CRETACEOUS CORE COMPLEXES IN THE NORTHERN CORDILLERA: EXPOSING THE PARAUTOCHTHON THROUGH A THIN FLAP OF ALLOCHTHONOUS YUKON-TANANA TERRANE IN WESTERN YUKON

The Yukon portion of the northern Cordillera exposes highly deformed and metamorphosed parautochthonous rocks of the ancestral North American margin (PNA) and the allochthonous Intermontane terranes, dominated by Yukon-Tanana terrane (YTT). The YTT is founded on Snowcap assemblage basement that was once part of the ancestral PNA, and thus both are very similar in composition and age, making their discrimination difficult. The timing of middle to upper amphibolite facies regional metamorphism recorded in both PNA and compositionally similar YTT has not been previously well constrained, and the nature of the structural interface between them is complicated by the poly-episodic, high-strain throughout each making it difficult to distinguish contractional and extensional structures.

Technological developments and substantial increase of available high-quality geochronological and thermochronological data has helped demonstrate that the PNA is characterized by pre-late Devonian metasedimentary successions with voluminous latest Devonian to earliest Mississippian plutons and Middle Jurassic to mid-Cretaceous metamorphism. In contrast, the YTT is characterized by voluminous Mississippian to middle Permian magmatism, Mississippian to Middle Jurassic metamorphism and tectonism, which are absent in PNA. Regional geologic relationships indicate that the Intermontane terranes form a relatively thin nappe on a footwall of semi-continuous PNA that extends southwest of the nappe to at least the Denali fault. Structural windows through the thinned allochthonous upper plate expose PNA (characterized by Middle Jurassic to mid-Cretaceous metamorphism, and a lack of Mississippian to middle Permian magmatism) in west-central Yukon and eastern Alaska as mid-Cretaceous extensional core complexes. Restoration of 450 km of dextral Paleocene movement along the Tintina fault locates an offset portion of the Australia Creek core complex within a complex structural pile in the Finlayson Lake area in southeast Yukon. The presence of these core complexes beneath the YTT flap is evidence of a simple, pre-extension, thrust configuration, not requiring a complex oroclinal explanation for NAB rocks exposed on both sides of YTT.