SCLM Rifting and Regional Shearing in the N American Superior Craton − Implications for Deformation, Mineralization, and Tectonic Reconstructions
Lyal Harris, INRS-ETE, Québec
Jean Bédard, Geological Survey of Canada, Québec
3D images of S-wave seismic tomographic velocity anomalies for the Superior Province, N America, illustrate that the Abitibi Subprovince overlies a symmetrical rift in the sub-continental lithospheric mantle (SCLM) of an older 'proto-craton' (N Superior Province and Minnesota River Valley domain). Mantle plume activity led to necking, focussed thermal erosion, destruction and assimilation of ancient lithosphere, and formation of isotopically juvenile crust in a volcanic plateau-like setting. Whilst generally E-W-trending, the interpreted Archaean rift changes to a NW-SE orientation in the easternmost Abitibi and Grenville Province parautochthon. The NW-SE trending rift segment parallels the Belomorian Province separating the Kola and Karelian cratons abutting the Superior Province in tectonic reconstructions. The change in orientation by 120° suggests rifting may have occurred over a mantle plume. The NE segment of an Archaean sinistral, NE-striking shear zone (the proto-Grenville shear zone), interpreted from the offset of gravity and aeromagnetic anomalies along the SE margin of the Abitibi and Opatica subprovinces, similarly makes a 120° angle with the two rift segments and follows an aulacogen in the rifted SCLM.
Early rift structures localized ca. 2696 Ma deformation and hydrothermal fluid flow. Enhanced aeromagnetic images of the central-northern Abitibi illustrate penetrative E-W dextral ductile shearing preceded formation of discrete conjugate transcurrent and E-W reverse (± dextral) shear zones implying ca. N-S bulk shortening. The displacement history and geometry of shear zones in the Abitibi Subprovince is similar to that of structures developed during progressive lateral escape and indentation during impingement of a rigid body. We propose that southward migration of the old cratonic nucleus (N Superior Craton) in response to mantle flow acting upon its deep lithospheric keel, and not subduction-related processes, led to progressive southward accretion of crustal fragments and oceanic plateaux-like segments, shortening and inverting the initial rift. Major epigenetic gold deposits are located above rift-bounding faults in the SCLM, suggesting that early rift structures localized subsequent deformation and hydrothermal fluid flow. The 1.1 Ga Desmaraisville and 0.55 Ga Otish kimberlite clusters also coincide with interpreted SCLM rifts. Outcomes highlight the important role of ancient mantle structures on localizing deformation, hydrothermal fluid flow, emplacement of igneous bodies, and mineralization in the overlying crust.
Funding: Laurentian Goldfields, NSERC, Richmond Minerals, Fort Chimo Minerals and DIVEX. The seismic tomographic database was provided by S. Godey.