UNRAVELING THE BOUNDARY BETWEEN THE YUKON-TANANA TERRANE AND PARAUTOCHTHONOUS NORTH AMERICA IN EASTERN ALASKA

The boundary between the allochthonous Yukon-Tanana terrane (YTT) and parautochthonous North America (PNA) in eastern Alaska comprises a series of low-angle penetrative structures that formed and(or) were reactivated during multiple episodes of regional Permian to mid-Cretaceous deformation. The boundary is difficult to identify because 1) rocks on either side have similar compositions and early Paleozoic histories, 2) overprinting kinematic indicators are complex, 3) the geophysical character of YTT and PNA assemblages is generally similar, and 4) extensive Cretaceous and younger plutons cross-cut and obscure key relationships. Targeted geologic mapping, structural analyses, and geo- and thermochronology focused along the international border in eastern Alaska provides key constraints for delineating the terrane boundary and interpreting its kinematic evolution and tectonic history. Zircon U-Pb ages of meta-igneous rocks do not provide unique constraints, as ca. 366 to 355 Ma deformed granitoids (i.e., orthogneiss) are present in both the YTT and PNA assemblages. However, detrital zircon geochronology provides a useful discriminant, as metasedimentary rocks in the YTT typically contain early Paleozoic age populations that are absent in PNA quartzites. Allochthonous YTT rocks yield Jurassic and older ⁴⁰Ar/³⁹Ar ages, whereas PNA assemblages consistently produce mid-Cretaceous cooling ages. Together, our integrated data and observations suggest that, in eastern Alaska, the boundary between the YTT and PNA is a zone of low-angle, ductile to brittle-ductile deformation that separates thin (up to a few hundred meters) structural panels of allochthonous YTT assemblages from more deeply exhumed Late Devonian PNA assemblages. The YTT structural panels are volumetrically small and record predominately ductile deformation and amphibolite-facies metamorphism. Internally, the panels are structurally complex and contain a mix of Late Devonian to Permian rocks that were thrust onto PNA and then exhumed along low-angle detachment faults. Our findings are consistent with previously published models and provide new quantitative constraints that illuminate this under-studied region of eastern Alaska.