Crustal Deformation and Lower Crustal Delamination During Late Archean Indentation Tectonics in the SE Superior Province, Canada

The SE Superior Province of the Canadian Shield preserves a record of the Late Archean accretion and deformation of juvenile crust during a 100 Ma period, 2750 Ma through 2650 Ma. The region includes the amphibolite-grade Opatica granite-gneiss belt, the Abitibi granite-greenstone Subprovince, and the amphibolite-grade Pontiac metasedimentary Subprovince. The nature of the terrane suture and the crustal structure in the region are evaluated based on a synthesis and interpretation of structural and geophysical data. The data are consistent with the Opatica belt being contiguous with the middle crust that underlies greenstones of the northern Abitibi Subprovince. The Opatica belt can now be considered part of the Abitibi Subprovince which represents one tectonic terrane, the Abitibi-Opatica terrane, that is more than 250 km wide, N-S. The boundary between the southern Abitibi Subprovince and the metasedimentary rocks of the Pontiac Subprovince is identified as a Late Archean terrane suture that can be traced to the Moho on Lithoprobe deep-seismic reflection profiles. This new interpretation of the seismic reflection profiles suggests that the terrane collision involved wedging of older crust (ca. 3000 Ma), underlying the Pontiac Subprovince, into the younger middle crust of the Abitibi Subprovince. The Pontiac Subprovince represents a tectonic indenter that caused the development of shear zones which traverse the Abitibi-Opatica terrane. Deformation related to collision resulted in folding of the upper and middle crust of the overriding Abitibi-Opatica plate for 250 km inboard of the terrane suture. The crustal deformation style, with delamination and thrusting of the lower crust and large-scale folding of the upper and middle crust of the Abitibi-Opatica terrane indicates that, at the time of collision, its rheological profile included a weak middle crust overlying a relatively strong lower crust, the latter thought to be composed largely of mafic granulite. That interpretation is supported by calculated strength profiles for the Abitibi and Pontiac lithospheres. The rheological profile of the Abitibi-Opatica plate ca. 2700 Ma may have resulted from a combination of radiogenic heating due to the abundance of precollisional granitoids in the middle crust and syncollisional granitic plutonism.