CRUSTAL ARCHITECTURE AND PROCESSES IN THE NEOARCHEAN: AN INTEGRATED GEOPHYSICAL-GEOLOGIC CASE STUDY IN THE WESTERN SUPERIOR CRATON, CANADA

Neoarchean is considered as a turning point during the Earth’s evolution in terms of cratonization and development of plate tectonics. The supporting crustal processes, however, remain elusive due to limited comprehensive imaging of crustal structures underlying Neoarchean terranes. This study uses new seismic, magnetotelluric, gravity, and geologic data from the Metal Earth Sturgeon transect (70 km) to investigate the Neoarchean crustal architecture underlying the greenstone-dominated western Wabigoon terrane (WWT) and the tonalite-trondhjemite-granodiorite-dominated Winnipeg River terrane (WRT) of the Superior craton. The results suggest that (1) the Sturgeon Lake greenstone belt is 5‒10 km thick and separated by a thrust fault from the underlying basement, (2) the basement is characterized by gneissic fabrics from the mid- to lower-crust of probably Paleo to Mesoarchean WRT rocks, and that (3) the crust was extensively overprinted by Neoarchean posttectonic magmatism that was characterized by hydrogen/carbon-rich silicates and likely caused by partial melting of mantle/crustal rocks triggered by subduction-related fluids/melts. With the reprocessed Lithoprobe seismic data from the same region, a three-dimensional crustal architecture is reconstructed and reveals a convergent belt between the main WRT to the north and a continental margin promontory of the WRT to the south. This Neoarchean convergent belt is characterized by allochthonous greenstone belt as thrust sheets in the upper crust, a frontal collision zone in the mid- to lower-crust, an apparent crustal root of 3‒5 km negative relief into the mantle, and subcreted crustal rocks beneath a mantle wedge. Regional geology suggests that an outboard, north-dipping subduction zone associated with the Wawa terrane provided the fluids/melts for the posttectonic magmatism in the WRT-WWT crust. This study favors a model of subduction followed by collision over a collision-only model, which indicates that multiple subduction zones were operative synchronously in a period during the assembly of the western Superior. Cratonization by accretionary orogenesis due to plate tectonics in the Neoarchean, therefore, is unraveled and characterized in this study using a three-dimensional crustal architecture.