Volcanic and plutonic rocks of Archean greenstone belts in the Abitibi subprovince become younger southward; plutonic rocks are ∼10–30 m.y. younger than volcanic rocks. The Abitibi greenstone belt has an upper crust of ∼2.75–2.70 Ga oceanic supracrustal assemblages and a middle crust of ∼2.70–2.68 Ga granitic and gneissic rocks. The greenstone belt upper crust is greenschist facies with upright structures and low reflectivity, and the middle crust is amphibolite facies with moderate to shallowly dipping structures and strong reflectivity. We propose that the age progression and crustal layering resulted from accretion of oceanic assemblages followed by extensive mid-crustal magmatism linked to southward migration of an arc-trench system. Thermal softening of the middle crust during magmatism caused ductile flow now represented by strong, mid-crustal reflectivity. Upward heat flow following magmatism caused late-tectonic (2.67–2.65 Ga) low-pressure metamorphism observed in the upper crust.