Thermochronology of the Yukon-Tanana Terrane, West-Central Yukon: Evidence for Jurassic Extension and Exhumation in the Northern Canadian Cordillera

Abstract

Within the McQuesten area of the central Yukon region of the Canadian Cordillera, the >100-km-long Willow Lake fault juxtaposes two crustal domains: (i) on the northeast, the Reid Lakes complex, an unmetamorphosed and predominantly undeformed Devono-Mississippian assemblage of plutonic and volcanic rocks; and (ii) on the southwest, typical Yukon-Tanana terrane (YTT) with deformed greenschist to amphibolite facies metamorphosed pre–late Devonian to late Permian rocks. In this study we investigate the tectonic history of these two crustal domains and present sensitive high-resolution ion microprobe U-Pb ages that delineate middle to late Paleozoic arc magmatism and fit key units into the YTT’s regional lithotectonic framework. Notably, magmatic zircons possess ca. 193 Ma overgrowths, and patches of recrystallized zircon with low Th/U ratios occur in samples proximal to the fault. Mississippian (ca. 349−325 Ma) biotite and hornblende 40Ar/39Ar cooling ages of the Reid Lakes complex suggest cooling immediately following volcanism and batholith emplacement. Plutonic samples near the fault trace, exhibiting weak to high strain, yield younger, ca. 193−191 Ma biotite 40Ar/39Ar ages. Zircon (U-Th)/He ages from the magmatic complex reflect ca. 216−214 Ma cooling through the uppermost crust of structurally higher volcanogenic rocks, followed by ca. 159–130 Ma cooling of the deeper-seated plutonic phase. Across the Willow Lake fault, YTT metamorphic rocks yielded ca. 197−165 Ma biotite and hornblende 40Ar/39Ar ages and ca. 113−90 Ma zircon (U-Th)/He ages. The marked step in cooling ages across the fault and the juxtaposition of the deformed rocks against undeformed rocks strongly indicates the Willow Lake fault is an extensional structure with top-down-to-the-northeast displacement, which exhumed the middle crust commencing in the early Jurassic. Aishihik suite magmatic rocks of the Stikinia/Quesnellia arc in the McQuesten area yielded a zircon age of ca. 205 Ma and 205−196 Ma40Ar/39Ar cooling ages. These ages and the presence of magmatic epidote suggest that the Aishihik suite cooled rapidly and was exhumed from midcrustal levels, further illustrating the significance of the Jurassic event that exhumed the central belt of YTT rocks. A similar tectonothermal evolution in the nearby Stewart River area, along strike to the northwest from McQuesten, was attributed to transtensional exhumation and cooling during slab rollback, leading to unroofing of the outboard regions of the terrane. We suggest that early Jurassic extensional exhumation of the YTT domain was mechanically linked to the Teslin fault system and more widespread that previously considered, likely accommodating crustal thinning from the Yukon-Tanana upland of eastern Alaska to the Glenlyon region of central Yukon. Parts of this system may have been episodically reactivated and overprinted, perhaps even as part of the Eocene Tintina fault, masking the early history.