Deformation history of the northwestern Selwyn Basin, Yukon, Canada: Implications for orogen evolution and mid-Cretaceous magmatism

Abstract

Neoproterozoic to Paleozoic slope-to-basin facies continental margin strata underlie area 700 x 200 km across central Yukon Territory, Canada, and collectively define the Selwyn Basin. In a Cordilleran framework, Selwyn Basin strata form a strongly deformed and thrust-faulted package located between the Mackenzie foreland fold-and-thrust belt, and accreted terranes and displaced elements of the ancient North American continental margin. Orogeny commenced in the Jurassic as exotic elements of the composite Yukon-Tanana terrane overrode the ancient continental margin. Collision-related deformation had ceased by ca. 100 Ma, and was followed by a Late Cretaceous (post–85 Ma) dextral transcurrent regime, which laterally displaced elements of the newly assembled continental margin along the orogen-parallel Tintina fault. In western Selwyn Basin, more than 100 km of structural overlap was accommodated on two main detachments, the Robert Service and underlying Tombstone thrust faults. Internal deformation within the thrust sheets is intense, characterized by shear-related folds and fabrics. Metamorphic grade reaches lower to middle greenschist facies at the deepest structural levels exposed, and is characterized by chlorite-muscovite schists. The onset of deformation is constrained by the Late Jurassic age of the youngest units deformed during orogeny. The end of ductile deformation is constrained by new 40Ar/39Ar ages for metamorphic muscovite that range from 104 to 100 Ma. Due to the low metamorphic grade, these ages are interpreted to closely follow the waning of deformation. At ca. 93 ± 3 Ma, isolated granitic intrusions of the Tombstone-Tungsten magmatic belt were emplaced across the western Selwyn Basin in a tensional, postcollisional regime. Restoration of displacement on the Tintina fault places the western Selwyn Basin adjacent to the Yukon-Tanana terrane uplands of east-central Alaska in the Early to mid-Cretaceous. Despite their adjacent positioning in cross-orogen section during orogenesis, the two elements feature some significant differences in Jurassic-Cretaceous deformation. Most notably, the Yukon-Tanana terrane uplands record a significant extensional event at 120–105 Ma, which resulted in NW-SE–oriented extension, exhumation of deep structural levels, and voluminous felsic plutonism. In contrast, western Selwyn Basin did not undergo equivalent uplift and extension, and features temporally and spatially restricted plutonism. Within an orogenic framework, the Yukon-Tanana terrane uplands can therefore be considered to represent an exhumed core characterized by high heat flow, whereas the western Selwyn Basin represents an immediate northeastern salient to the exhumed core. These differences have important implications for the geodynamic setting of mid-Cretaceous plutonism across these two major lithologic-tectonic entities of the northern Cordillera.