The Ainslie Detachment occurs near the base of the Carboniferous Windsor Group, forming a regional flat-lying extensional fault distributed across 10 000 km2. New mapping has delineated the structure through southwestern Cape Breton Island and into central Nova Scotia. Shearing is concentrated at the top of the basal Macumber limestone along its contact with overlying evaporites and younger allochthonous units. The highly contrasting rheologies of the formations created an anisotropic zone of weakness which acted as an upper crustal stress guide, stratigraphically controlling the trajectory of the detachment through the basin. The detachment is characterized by an approximately 3–10 m thick calc-mylonite zone, with an intense planar fabric featuring alternating very fine grained shear planes and coarser annealed layers. Coarser layers are boudinaged into pinch and swell structures, locally producing segmented augen. Highly strained intraclasts, ooids, and peloids, recrystallized carbonate boudins, and carbonate vein segments are included in the calc-mylonite as semirigid inclusions and rotated porphyroclasts. Thick zones of fault breccia straddle portions of the detachment and overprint the mylonite, demonstrating an evolution to brittle conditions during progressive shear. Listric faults in the hanging wall of the detachment feature a ramp and flat geometry, with an upper detachment occurring along the upper contact of the Windsor Group with the overlying Namurian Mabou Group. Locally up to 2 km of the stratigraphic succession has been removed, with faults cutting downsection in a westerly direction producing rollover in the hanging wall.