Southwest Spitsbergen, Wedel Jarlsberg Land, consists of two Proterozoic terranes with differing structural and metamorphic histories. The northern terrane experienced two Early Palaeozoic deformation events both accompanied by greenschist-facies metamorphism of similar grade. The southern terrane records a Neoproterozoic pervasive amphibolite-facies metamorphism and strong deformational fabric only locally retrogressed during a Caledonian greenschist-grade event. These terranes are separated by an important sinistral ductile shear zone defined as the Vimsodden–Kosibapasset zone, which comprises wrench- and contraction-dominated domains characteristic of strain partitioning in transpression zones; in this case apparently controlled by contrasting rheologies of the juxtaposed crustal domains. The northern terrane of Wedel Jarlsberg Land shares affinities with Pearya in northern Ellesmere Island of Arctic Canada whereas the southern one resembles the Timanide belt of NE Europe. A quantitative approach facilitated by a numerical plate model demonstrates that correlation with Pearya is feasible if sinistral displacement of c . 600 km occurred during the Caledonian orogeny. The correlation with the Timanides is valid if the southern terrane represents an outlier of the Timanide belt separated from Baltica by the opening of the Iapetus Ocean.

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This report presents 40 Ar/ 39 Ar step-heating geochronology results for igneous and metamorphic rocks from the Alaska Division of Geological & Geophysical Surveys' (DGGS) geologic mapping project in the Northeast Tanacross map area in the Tanacross D-1 and parts of the D-2, C-1, and C-2 quadrangles, Alaska. Field samples were collected by the DGGS Mineral Resources section during detailed geologic mapping campaigns in 2017 and 2018. The Northeast Tanacross map area lies within the Yukon-Tanana Upland and covers the boundary between the Fortymile River and Lake George assemblages (Dusel-Bacon and others, 2006). The map area encompasses documented porphyry Cu-Mo-Au deposits including Taurus (Harrington, 2010), Fishhook (also known as SW Pika), and Pika Canyon (U.S. Geological Survey, 2008), and is adjacent to the Fortymile Mining District (Yeend, 1996). Previously published geologic mapping includes a reconnaissance 1:250,000scale map (Foster, 1970) and a slightly modified map published by the USGS (Wilson and others, 2015). DGGS published 1:63,360-scale geologic maps and a report for the area (Wypych and others, 2019; Wypych and others, in press; Naibert and others, in press), which contribute to the understanding of the metamorphic and igneous map units in the area, including delineating the boundary between parautochthonous North America (Lake George assemblage) and the allochthonous Yukon-Tanana terrane (Fortymile River assemblage) in multiple places. The age data in this report constrain the crystallization ages of igneous rocks and the cooling histories of metamorphic rocks in the map area and will be used to improve upon recent mapping and further refine the uplift history of eastern Interior Alaska. DOCUMENTATION OF METHODS Sample Collection Fresh, unweathered samples from surface outcrops were collected by DGGS field geologists and were selected for dating based on the presence of sufficiently large crystals appropriate for dating. Sample location coordinates (in WGS84 datum) were obtained using GPS-enabled field tablets, with a typical reported accuracy of about 10 meters. Samples were examined under a binocular microscope and in thin section to select unaltered mineral phases before sample preparation.