K-PG TERMINAL SUTURING OF THE INSULAR BELT DURING CONTRACTIONAL UPLIFT OF MELT-WEAKENED NORTH AMERICA, CLEARWATER AND COAST MOUNTAINS, AK

Accretion of the Insular belt to the North American margin was the most profound continental crustal growth event in the Phanerozoic record of the northern Cordillera and is heralded as an archetypal example of continental growth. Identifying the terminal suture has been hampered by Cenozoic margin parallel translation, which obfuscates much of this accretionary history. Vestiges of inverted metamorphism spatially associated with the Insular-North America boundary and distributed along the Cordilleran margin from north to south are the: 1) Valdez Creek region, 2) Kluane Lake region, and 3) Coast Mountains. In each instance, inverted metamorphic gradients overprint the boundary between Insular-derived metasedimentary rocks and North American-affinity lithologies. Isograds parallel a regional foliation that dips towards North-America and exhibits kinematics compatible with west-vergent thrusting of North America over the Insular belt. Detailed traverses within the Valdez Creek and Coast Mountains inverted metamorphic belts were performed in conjunction with microstructural analysis and in-situ monazite petrochronology.

Inverted metamorphic gradients in the Valdez Creek region lie within a ~5 km thick ductile shear zone juxtaposing the North American-derived Maclaren schist over Insular-affinity Clearwater metasedimentary rocks. Footwall affinity rocks contain ca. 90 Ma plutonism and ca. 60 Ma gold mineralization, while the top of the shear zone is occupied by a thick ca. 70 Ma tonalite sill. Inverted metamorphism in the Juneau area overprints several lithotectonic packages including Gravina overlap assemblages and North American-affinity Taku and Yukon Tanana Terranes. Footwall rocks exhibit ca. 90 Ma plutonism and ca. 58 Ma gold mineralization with the top of the inverted metamorphic sequence delineated by ca. 70 Ma tonalite rocks. Monazite petrochronology performed across both demonstrates that deformation accompanied the inversion of metamorphic gradients from 68-56 Ma during contractional uplift of melt-weakened North America-affinity rocks over the Insular belt, similar to recent work in the Kluane Lake region. We propose that these three localities represent a single contractional structure responsible for the terminal Insular-North American suture followed by dissection and dismemberment by Denali and Gastineau Channel faults.