Crustal-scale continental strike-slip fault systems are typically thought to be thermally segregated into an upper seismogenic expression and a lower zone of ductile deformation likely extending into the lower crust. The ability of the lower crust to sustain stress is fundamental to understanding the mechanical behavior of the lithosphere, yet significant debate exists regarding the strength of the ductile crust. The Cora Lake shear zone in the western Churchill Province of the Canadian Shield is a 4–7 km wide zone of granulite-to upper amphibolite-facies mylonite which hosts synkinematic pseudotachylyte, suggesting a locally strong lower crust. Kilometer-scale mapping suggests that the shear zone localized at a major lithological and terrane boundary, with the pseudotachylyte network centered within the ca. 1 km-wide more intensely deformed, finer-grained ultramylonitic core. In this contribution, we use optical petrography and electron-backscatter diffraction to describe microstructure across a transect of the Cora Lake shear zone. By combining these observations with grain size piezometry, experimental flow laws, and regional pressure-temperature paths, we conclude that stress amplification from a relatively common type of km-scale lithological heterogeneity and cooling history is the most reasonable explanation for the deep crustal pseudotachylyte.