Thermo‐mechanical interaction of a large impact melt sheet with adjacent target rock, Sudbury impact structure, Canada

Paul L. G€OLLNER1, Torben W€USTEMANN1, Lisa BENDSCHNEIDER1, Sebastian REIMERS1, Martin D. CLARK1,4, Lisa GIBSON2, Peter C. LIGHTFOOT3, and Ulrich RILLER1
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13268]
1Institut für Geologie, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany
2Vale, North American Exploration, 337 Power St., Copper Cliff, Ontario, Canada
3Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street N., London, Ontario N6A 5B7, Canada
4Present address: Department of Geology, University of the Free State, 205 Nelson Mandela Drive, Bloemfontein, South Africa
Published by arrangement with John Wiley & Sons

The 1.85 Ga Sudbury Igneous Complex (SIC) and its thermal aureole are unique on Earth with regard to unraveling the effects of a large impact melt sheet on adjacent target rocks. Notably, the formation of Footwall Breccia, lining the basal SIC, remains controversial and has been attributed to impact, cratering, and postcratering processes. Based on detailed field mapping and microstructural analysis of thermal aureole rocks, we identified three distinct zones characterized by static recrystallization, incipient melting, and crystallization textures. The temperature gradient in the thermal aureole increases toward the SIC and culminates in a zone of partial melting, which correlates spatially with the Footwall Breccia. We therefore conclude that assimilation of target rock into initially superheated impact melt and simultaneous deformation after cratering strongly contributed to breccia formation. Estimated melt fractions of the Footwall Breccia amount to 80 vol% and attest to an extreme loss in mechanical strength and, thus, high mobility of the Breccia during assimilation. Transport of highly mobile Footwall Breccia material into the overlying Sublayer Norite of the SIC and vice versa can be attributed to Raleigh–Taylor instability of both units, long‐term crater modification caused by viscous relaxation of crust underlying the Sudbury impact structure, or both.

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