CRUSTAL STRUCTURE OF YUKON-TANANA TERRANE IN THE NORTHERN CORDILLERA

The Yukon-Tanana terrane (YTT) is a major peri-Laurentian composite terrane, most extensively exposed in west-central Yukon, with probable exposures in eastern Alaska. It comprises a tectonic collage of internally complex but distinct structural slices. It is separated from the structurally underlying para-autochthonous North American (NA) basement by a system of thrust faults (including the Inconnu thrust on its eastern side) which are locally marked by discontinuous slivers of the oceanic Slide Mountain terrane.

In west central Yukon, the structurally highest sheet of the YTT mainly comprises the Early Mississippian volcanic-dominated Finlayson assemblage and the Simpson Range plutonic suite which locally intrudes pre-Late Devonian metasedimentary rocks of the Snowcap assemblage. Mississippian rocks of the upper structural sheet are overlain by the volcanic rocks and intruded by plutons of the Triassic Lewes River arc. The middle structural sheet includes the Snowcap assemblage that is overlain by volcanic and intruded by plutonic rocks of the Middle Permian Klondike assemblage. The upper and middle sheets are separated by the Yukon River thrust, which appears to postdate Permian metamorphism and Triassic plutonism. The amount of overthrusting on within YTT structures is difficult to determine, but is probably on the order of several 10’s of kms. Jurassic structural thickening and subsequent exhumation strongly controlled Jurassic mineral prospectivity in YTT and related intermontane terranes.

The structurally lowest sheet comprises volcanic, plutonic and sedimentary rocks of the White River assemblage, interpreted as parautochthonous NA rocks. The middle and lower sheets are separated by the now steepened Moose Creek fault, which is marked by lozenges of metamorphosed mantle peridotite; we interpret it to correlate with the Inconnu thrust. We conclude that the YTT nappes originated to the southwest of the White River assemblage (present coordinates) during pre-Early Jurassic, and were subsequently emplaced on the White River assemblage as a thick flap, that was thinned by mid-Cretaceous time. A thin flap is supported by geophysical features associated with variations in the character of the hyper-extended parautochthonous NA basement rocks imaged below the flap as far west as the Denali fault.