LATE PALEOZOIC EVOLUTION OF YUKON-TANANA AND SLIDE MOUNTAIN TERRANES, FINLAYSON LAKE DISTRICT, SOUTHEASTERN YUKON, CANADA

The Finlayson Lake massive sulphide district of SE Yukon is underlain by variably deformed, metamorphosed and imbricated pre-Late Devonian to Permian rocks of Yukon-Tanana and Slide Mountain terranes and affiliated syn- to post-amalgamation overlap assemblages. Rocks of Yukon-Tanana terrane are the thrust-imbricated remnants of a predominantly ensialic, Late Devonian to Early Permian convergent margin system and its pre-Late Devonian basement. Three regional thrust sheets formed during Early Permian shortening: the Cleaver Lake (upper), Money Creek and Big Campbell (lower). Geological and geochemical data indicate that rocks of the Cleaver Lake thrust sheet formed in fore-arc and arc settings, rocks of the Money Creek thrust sheet represent primarily an arc setting and rocks of the Big Campbell sheet represent primarily an extensional back-arc realm. As Early Permian thrust faulting was NE-vergent, the arc and fore-arc parts of the system would restore SW of the back-arc, implying NE-dipping subduction under the SW margin of the terrane. Slide Mountain terrane occurs primarily north and east of the Jules Creek fault, the primary boundary between the two terranes. It comprises Carboniferous to Lower Permian basinal clastic rocks, chert, and basalt that are inferred to have been deposited on oceanic crust in the (trans?) extensional back-arc region behind the west-facing Yukon-Tanana terrane arc. Lower Permian basalt and chert were deposited on both sides of the Jules Creek fault, narrowly distributed with comagmatic plutonic rocks along it . This distribution suggests that the Jules Creek may have been a ‘leaky' transform fault, one that initiated soon after Early Permian shortening in Yukon-Tanana terrane. Upper Permian and Upper Triassic conglomerates are also narrowly distributed near, and offset by, the Jules Creek and equivalent faults, implying strike-slip(?) reactivation at those times.

Various geological considerations indicate a Middle Permian shift to subduction under the northeastern margin of the amalgamated terranes. This Middle Permian shift marked the onset of the closure of the Slide Mountain ocean which culminated with the Jurassic emplacement of the amalgamated terranes onto the North American continental margin along the Inconnu thrust fault.