INTERPRETING THE TECTONIC HISTORY OF CRUSTAL-SCALE HIGH STRAIN ZONES: INSIGHTS FROM THE CORA LAKE SHEAR ZONE, WESTERN CHURCHILL PROVINCE

The Athabasca Granulite Terrane in the western Churchill Province, Canada, is a >20,000 km² exposure of deep crustal polycyclic granulites. The region is cut by numerous steeply-dipping mylonite to ultramylonite zones that are critical for interpreting the geometry and tectonic history of the terrane. The P-T conditions and timing of shearing provide critical information about the relative depth of adjacent domains, and the time at which they were juxtaposed. The Cora Lake shear zone (CLsz) is a classic example. It bisects the granulite facies region, juxtaposing two compositionally contrasting domains: the Northwestern domain and Chipman domain. The Northwestern domain is dominated by ~2.6 Ga anhydrous orthogneisses emplaced at lower crustal depths (0.9 GPa). They contain an early subhorizontal fabric, interpreted to reflect ~2.6 Ga lower crustal flow. The Chipman domain is underlain by rheologically strong ~3.2 Ga tonalitic gneiss, with no evidence for 2.6 Ga flow.To investigate crustal flow and rheologic heterogeneity in the lower crust, a P-T-t-D transect has been studied across the CLsz into the NW domain. Although the charnockitic gneisses of the NW domain contain suitable assemblages for P-T calculations, they lack monazite, which is critical for timing deformation. Felsic granulites, interlayered with the charnockites, typically contain abundant monazite but tend to lack index minerals for thermobarometry. Paired sampling of both lithologies along the transect across the CLsz, and correlation of microstructures allows construction of a high resolution P-T-t-D gradient. Northwest of the CLsz, the S₂ fabric is steeply dipping, dextral on a shallow to the SW lineation, and defines the axial planes of large amplitude folds (F₂). Preliminary results suggest that it formed at 1.9 Ga and ca. 0.9 GPa. Intense ultramylonitic fabric within the CLsz formed soon after S₂ (ca. 1.88 Ga), and at slightly lower pressure (ca. 0.85 GPa), with sinistral strike slip motion. This shearing reflects the final juxtaposition of the Northwestern and Chipman domains. It was followed by regional exhumation at ca. 1.85 Ga. These data suggests that the CLsz formed in response to the waning phase of granulite deformation, in an extensional setting, and was the fundamental structure juxtaposing two rheologically contrasting domains.