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

The relationship of accreted terranes with pericratonic North America is not well constrained, but critical for unraveling the complex, polydeformational history of the North American Cordillera. The Cordillera represents a mutli accretionary system that has fundamentally been active since the Jurassic. The Yukon-Tanana Terrane (YTT) is an extensive and heterogeneous accreted terrane in the North American Cordillera. The suture zone separating the YTT from pericratonic North American is exposed in eastern Alaska and is defined by northward dipping and low-angle ductile shear zone. This shear zone is interpreted to have juxtaposed the lower plate assemblages of parautochthonous North America with the YTT during top-to-the-SE directed exhumation in the Cretaceous. This interpretation is based on cooling ages (ca. 100-120) of amphibolite-facies orthogneiss samples collected within the parautochthon. The relationship between shear zone formation, exhumation, and magmatism has remained unknown. Historically, ⁴⁰Ar/³⁹Ar thermochronology has been the primary resource in identifying the boundaries of the shear zone, even when concealed, and can be aided by a more robust mineral such as monazite. Both the upper and lower plate packages have experienced multiple phases of deformation, but the timing of lower plate deformation remains enigmatic. U-Th-Pb monazite petrochronology of targeted samples within and outside the shear zone have placed better constraints on the age of shearing and exhumation of the structurally lower parautochthonous assemblages. This analysis supports that uplift of these lower plate assemblages occurred during the Cretaceous and the ductile shear zone, which facilitated this juxtaposition, was active at 108 Ma. The Northern Cordillera is also home to widespread Cretaceous (ca. 100-115), voluminous, and metallogenically important magmatism in both Alaska and the Yukon Territory. U-Pb zircon geochronology analyzed form 15 mid-Cretaceous plutons has refined the crystallization history of these widespread granitic bodies. The monazite and zircon geochronology together show that the shear zone and granitic plutons are linked, and that top-to-the-SE crustal extension placed both spatial and temporal controls on the emplacement of mid-Cretaceous magma.