The Selwyn fold belt (SFB) in the eastern Yukon and southwest NWT is a zone of NE-SW oriented crustal shortening that developed in clastic sequences of the Neoproterozoic to Silurian Selwyn basin and subsequent Devonian to Triassic back-arc basins. The SFB developed in the foreland of the Cordilleran orogeny and is located between the Cordilleran metamorphic hinterland of the Yukon-Tanana terrane to its SW and the classic fold and thrust belt in carbonate dominated shelf sediments of the Mackenzie platform to the NE. The SFB is generally described as comprising spaced thrusts that separate panels of regionally extensive, closed to tight, upright folding and associated axial planar foliation development. Rocks of the SFB in the Nahanni region of southwest NWT and eastern Yukon preserve multiple generations of folding and overprinting penetrative foliation development indicating that the kinematic evolution of the SFB was, at least locally, quite complex. This study presents results of a detailed structural analysis of penetrative deformation of the southern SFB in the Nahanni region.
Structural data collected along SW-NE sections across the SFB from the NWT and into the Yukon. The northern section, from Howard’s Pass to Summit Lake, traverses the Selwyn Basin and the overlying Earn Group. The southern section, from the Fork Creek range to Little Owls Lake, traverses the Hyland Group. North of Little Owls Lake, a regionally consistent, moderately to steeply dipping, penetrative foliation is observed in most outcrops, and it is axial planar to folding at all scales. At Little Owls Lake, three fabrics are consistently observed: a moderately steep dipping crenulation foliation (S1) axial planar to NW-plunging and NW-dipping inclined folds; an overprinting subvertical crenulation cleavage (S2) axial planar to upright folds (F2); and a late northeast-striking, steep crenulation cleavage (S3) with no associated mesoscopic or macroscopic folding (S3). Based on similar attitudes and fold geometries, the S2 fabric observed at Little Owls is correlated with the regional foliation observed north of Little Owls along both sections. Chlorite-zone metamorphism extends across both sections, and both grade into biotite-zone metamorphism along the Fork Creek range. At the southern end of the Fork Creek range syn-kinematic cordierite, andalusite, and rare staurolite porphyroblasts are developed in coarse-grained biotite-quartz schists that preserve additional foliation generations not observed elsewhere. This zone of higher metamorphic grade is spatially associated with the Nahanni pegmatite suite, and is likely a function of heat conducted from a source intrusion at depth.
The geometry of the penetrative foliations and the overall fold continuity indicate that there is no geometric requirement for significant displacement along thrust faults within the study area. Fewer deformation events are recorded along the Summit Lake section than in the Little Owls Lake or the southern Fork Range areas. This may indicate that deformation propagated from south to north (present coordinates) along the section, and/or from deeper to shallower stratigraphic levels in the crust.