Archean continental assembly in the southeastern Superior Province of Canada

Between 1988 and 1993, seismic reflection and refraction surveys were acqui red across the medium- to high-grade Opatica plutonic gneiss belt, the low- grade Abitibi greenstone belt, and the Pontiac metasedimentary belt, all of which form part of the late Archean Superior Province. Shallowly north dip ping reflections define a structural style consistent with the northward un derthrusting and accretion over about 30 Ma of various exotic terranes agai nst a backstop provided by the Opatica belt. This rapid southward growth of the Archean protocraton was driven by at least one north dipping subductio n zone as revealed by north dipping reflections that extend to 65-km depth in the upper mantle below the Opatica belt. In contrast to the mainly ortho gneissic Opatica and Pontiac belts, the midcrust of the Abitibi belt compri ses metasedimentary and igneous rocks, plus imbricated units of unknown aff inity. Relict midcrustal accretionary complexes of substantial size, which are indicative of primary suture zones, are interpreted near the northern a nd southern limits of the Abitibi belt. An interpreted basal decollement an d significantly older ages in the north suggest that the upper crustal gree nstone rocks are allochthonous. Evidence of large-scale extension appears t o be confined to the Southern Volcanic Zone of the Abitibi, which developed into a half graben as the original suture zone was reactivated in extensio n. Unusually high seismic P wave velocities, 7.5-8.2 kms(-1), are present i n the lower 8 km of the Abitibi crust, and they correlate well with a downw ard reduction in seismic reflectivity attributable to late modification of the deepest part of the crust. Crustal xenolith studies suggest that this p rocess may be linked to early Proterozoic magmatism.