THE SUDBURY STRUCTURE (ONTARIO, CANADA) - A TECTONICALLY DEFORMED MULTIRING IMPACT BASIN

The occurrence of shock metamorphic features substantiates an impact o rigin for the 1.85 Ga old Sudbury Structure, but this has not been uni versally accepted. Recent improvements in knowledge of large-scale imp act processes, combined with new petrographic, geochemical, geophysica l (LITHOPROBE) and structural data, allow the Sudbury Structure to be interpreted as a multi-ring impact structure. The structure consists o f the following lithologies: Sudbury Breccia dike breccias occurring u p to 80 km from the Sudbury Igneous Complex (SIG); Footwall rocks and Footwall Breccia - brecciated, shocked crater floor materials, in part thermally metamorphosed by the overlying SIG; Sublayer and Offset Dik es, Main Mass of the SIC and Basal Member of the Onaping Formation (OF ) - geochemically heterogeneous coherent impact melt complex ranging f rom inclusion-rich basal unit through a dominantly inclusion-free to a capping inclusion-rich impact melt rock; Grey Member of OF - melt-ric h impact breccia (suevite); Green Member of OF - thin layer of fall ba ck ejecta; Black Member of OF - reworked and redeposited breccia mater ial; Onwatin and Chelmsford Formations - post-impact sediments. Observ ational and analytical data support an integrated step-by-step impact model for the genesis of these units. Analysis of the present spatial distribution of various impact-related lithologies and shock metamorph ic effects result in an estimated original rim-to-rim diameter of the final crater of 200 or even 280 km for the Sudbury Structure, prior to tectonic thrusting and deformation during the Penokean orogeny.