Geology of the boundary area, Eagle A-1 and Tanacross D-1 quadrangles, east-central Alaska

Abstract

Structural relationships and tectonic events in the Boundary area of east-central Alaska shed light on the generally obscure tectonic history of the Yukon-Tanana Upland region. This study used detailed mapping of 160 km², with supporting petrography, geochemical analyses, and ⁴⁰Ar/³⁹Ar dating to define the geology and tectonic history of the Boundary area. A major thrust zone juxtaposes lithotectonic assemblages of metamorphic rocks in the Boundary area, with the Fortymile River assemblage (approximately middle amphibolite facies) thrust over the Nasina and Klondike series (middle to upper greenschist facies), and ultramafic rocks and metagabbro (lower greenschist facies) and epidote-amphibolite-facies gneiss and schist imbricated along the thrust faults. Final thrust imbrication postdates cooling from peak metamorphism in amphibolite-facies lithologies, which appears to be Early Jurassic age. The thrust zone is exposed partially rimming a broad, dome-shaped anticline that encompasses the entire map area. Thermally reset ⁴⁰Ar/³⁹Ar ages in the Boundary area appear to be the result of a short-lived, probably igneous, Mid-Cretaceous reheating event, with widespread and locally variable effects, contemporaneous with intrusion of the Crag Mountain pluton south of the map area. Numerous later high-angle, presumably strike-slip faults transect the Boundary area, with apparently associated latest Cretaceous to Miocene volcanic rocks.