Tectonic delamination of the lower crust during Late Archean collision of the Abitibi—Opatica and Pontiac terranes, Superior Province, Canada

Abstract

The southeastern Superior Province of the Canadian Shield preserves a very complete record of the accretion and deformation of juvenile crust during a 100 Ma period, 2750 Ma through 2650 Ma. The region includes, from north to south, the Opatica granite-gneiss belt, the Abitibi granite-greenstone Subprovince, and the Pontiac metasedimentary Subprovince. The nature of the terrane suture and the crustal structure in the southeastern Superior Province are evaluated based on a synthesis of lithological, structural, geochronological, and geophysical data, including a reinterpretation of Lithoprobe deep seismic reflection profiles. All geological and geophysical data are consistent with the Opatica belt being contiguous with middle crust that underlies greenstones of the Abitibi Subprovince. The Opatica belt can now be considered part of the Abitibi Subprovince, which represents one large tectonic terrane, the Abitibi-Opatica terrane. The boundary between the greenstones of the Abitibi Subprovince and the metasedimentary rocks of the Pontiac Subprovince is a Late Archean terrane suture. The reinterpretation of the seismic reflection profiles suggests the Pontiac-Abitibi terrane suture involved wedging of older crust, underlying the Pontiac Subprovince, into the middle crust of the younger Abitibi Subprovince, resulting in delamination of the Abitibi lower crust. Deformation related to collision resulted in folding of the upper and middle crust of the Abitibi-Opatica plate for 250 km inboard of the terrane suture. The crustal deformation style, delamination, and thrusting of the stronger lower crust and the large-scale folding of the softer upper-middle crust are compatible with calculated strength profiles that include a lower crust composed of mafic granulite. The rheological profile of the Abitibi-Opatica plate, and the folding of the upper and middle crust during plate collision, may have resulted from a combination of radiogenic heating due to the abundance of granitoids in the middle crust and syncollisional plutonism.