40Ar/39Ar data from the Tanacross D-1 and parts of the D-2, C-1, and C-2 quadrangles, Alaska

Abstract

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.

Erosion Exposure Assessment of Infrastructure in Alaska Coastal Communities

“…For example, where sea level rise is the main driver of shoreline migration, the rising MHW datum can be mapped on an elevation model to forecast future shorelines (Crowell and others, 1997). However, sea level rise is difficult to estimate in Alaska due to limited water level and land level change data (Gorokhovich and Leiserowiz, 2012;Overbeck, 2018;DeGrandpre and Freymueller, 2019). Bull and others (2020) demonstrate the combination of environmental parameters required to model and possibly forecast permafrost erosion.…”