TEMPORAL LINKS BETWEEN DUCTILE SHEARING, PLUTONISM, AND TECTONIC EXHUMATION NEAR THE BOUNDARY OF PARAUTOCHTHONOUS AND ALLOCHTHONOUS TERRANES IN THE NORTHERN CORDILLERA, ALASKA

Understanding the relationship of accreted terranes with pericratonic North America is critical for unraveling the complex, polydeformational history of the North American Cordillera. The Cordillera represents a multi-accretionary system that has been fundamentally active since the Jurassic. The allochthonous Yukon-Tanana Terrane is an extensive and heterogeneous accreted terrane in the northern Cordillera. The tectonic boundary separating the Yukon-Tanana Terrane from pericratonic North America is exposed in eastern Alaska and is defined by a northward-dipping and low-angle ductile shear zone. Recent bedrock mapping and sampling conducted by the Mineral Resources section of the Alaska Division of Geological & Geophysical Surveys provides an opportunity to map and characterize this tectonic boundary. The shear zone is interpreted to have exposed the structurally lower assemblages of parautochthonous North America during top-to-the-southeast directed exhumation in the Cretaceous. This interpretation is based on muscovite, biotite, and hornblende metamorphic cooling ages (ca. 100-120) of rock samples collected within the parautochthon. However, temporal relationships between shear zone formation, exhumation, and magmatism have remained incompletely understood. Our targeted petrochronology using monazite has aided these previous radiometric and kinematic interpretations. U-Th-Pb monazite petrochronology of samples within and outside the shear zone have placed better constraints on the age of shearing and exhumation. These analyses and observations support that exhumation of the parautochthonous assemblages occurred during the Cretaceous. Additionally, the ductile shear zone which facilitated juxtaposition of allochthonous and parautochthonous assemblages was active ca. 108 Ma. Widespread Cretaceous and voluminous magmatism is also found in the northern Cordillera. U-Pb zircon geochronology analyzed from 12 mid-Cretaceous plutons has better refined the crystallization history of these granitic bodies (ca. 115–100 Ma). Together, the monazite and zircon geochronology support that the shear zone and granitic plutons are linked, and that top-to-the-southeast crustal extension placed both spatial and temporal controls on the emplacement of mid-Cretaceous magma.