The Central Metasedimentary Belt boundary thrust zone is a 10 km thick, 200+ km long, stack of crystalline thrust sheets, enclosed by an anastomosing network of ductile thrust zones, formed at mid- to deep-crustal depths in the southwest Grenville orogen, Ontario. It has behaved as a coherent upper amphibolite facies thrust zone, accommodating northwestward transport of the Central Metasedimentary Belt, the largest lithotectonic entity in this part of the orogen, by coherent and contemporaneous displacements. The earliest thrusting was well under way by ca. 1.19–1.18 Ga and the boundary thrust zone was reactivated at ca. 1.08–1.05 Ga. The early thrusting records the closure of a back-arc basin within the Central Metasedimentary Belt, which closed at ca. 1.19–1.18 Ga. The younger thrusting may reflect continental collision to the southeast of the exposed Grenville and represent intraplate reactivation of the boundary thrust zone, which acted as an older, crustal-scale zone of weakness. Transverse mid- to deep-crustal thrusting was apparently contemporaneous with longitudinal (orogen-parallel) shearing at higher structural levels. The rheological behaviour of the deforming media may have influenced the localization of both the upper and lower limits of the boundary thrust zone at the time of its initiation. The upper limit coincides with a chain of relatively stiff metagabbro bodies, which may have acted as a barrier to the upward migration of fluids responsible for syntectonic nephelinization at the top of the thrust zone.