CRUSTAL BOUNDARIES AND PLUTONIC IMAGING ALONG THE NORTHERN ROCKY MOUNTAINS SEGMENT OF THE WESTERN GEOSWATH

The proposed western Geoswath transects major and minor Archean, Proterozoic, and Phanerozoic crustal provinces and boundaries. Major Precambrian provinces and boundaries along the Geoswath in the northern Rocky Mountains include the Neoproterozoic passive margin and rifts, the Paleoproterozoic Selway composite terrane (2.4-1.6 Ga), the Paleoproterozoic Great Falls tectonic zone (1.86-1.77 Ga), the Mesoproterozoic Belt basin, and the Archean Wyoming craton. The northern Archean Wyoming craton is divided into two crustal provinces along the Geoswath – the Montana metasedimentary terrane and Beartooth-Bighorn magmatic terrane. The GeoSwath traverse also cuts across Cretaceous through Eocene igneous provinces including calc-alkaline mesozonal plutons (Idaho Batholith), epizonal plutons and volcanic rocks (e.g. Boulder Batholith and Elkhorn Volcanics, Challis-Absaroka Volcanics and associated plutons), and the central Montana Alkalic Province, as well as the Cordilleran thrust belt and Rocky Mountain Basin and Range. Current geophysical, geochemical and isotopic data indicate that the crust is vertically stratified. The lower crust of the Precambrian provinces is more mafic and apparently younger than the surface rocks, due to underplated mafic material added during the late Archean, Paleoproterozoic, and/or Mesoprotoerozoic. Hf-isotopic compositions and ages of inherited zircons, along with whole-rock Nd, Sr, and Pb isotopic compositions of Cretaceous metaluminous granitoids (plutonic imaging) show that the basaltic lower crustal layer is Paleoproterozoic in age. The northern Rockies GeoSwath traverse provides an unprecedented opportunity to image at depth the lithospheric structure across these fundamental crustal boundaries, the source areas and plumbing systems of a variety of magmatic and metallogenic systems, and to interpret contemporary geological phenomena (reactivation of ancient structures, seismicity, geohazards, metallogenesis, hot spot magmatism) with respect to the lithospheric structure of this region.