Geophysical and structural criteria for the identification of buried impact structures, with reference to Australia.

The discovery of large asteroid impact structures, likely and possible impact structures, onshore and offshore the Australian continent (Woodleigh [120km; ~360Ma], Gnargoo [75km; Lower Permian — upper Cretaceous], Tookoonooka [55–65km; ~125Ma], Talundilly [~84km; ~125Ma], Mount Ashmore [>100km; end-Eocene] and Warburton twin structures [>400km; pre-end Carboniferous]) requires re-examination of the diagnostic criteria used for their identification. Bouguer anomalies of established impact structures (Chicxulub [170km; 64.98±0.05Ma], Woodleigh impact structure and Gnargoo probable impact structure display a unique structural architecture where pre-impact structural ridges are intersected and truncated by the outer ring of the circular structure. Seismic reflection data outline circular central uplift domes, basement plugs and rim synclines. Sharp circular seismic tomography anomalies indicate low velocity columns under both the Woodleigh impact structure and Warburton probable impact, hinting at deep crustal fracturing. Deformed, curved and clouded intra-crystalline planar deformation features in quartz (Qz/PDFs), displaying Miller indices ({10–11}, {10–12}, {10–13}) diagnostic of shock metamorphism, abound around exposed established impact structures (Vredefort [298km; 2023±4Ma], Sudbury [~250km; 1850±3Ma], Charlevoix [54km; 342±15Ma], Manicouagan [100km; 214±1Ma]), Tookoonooka and Talundilly). Deformed Qz/PDFs allow recognition of shock metamorphism in buried impact structures, where original Qz/PDFs were bent, recrystallized and/or clouded during formation of the central uplift and hydrothermal activity triggered by the impact. Planar deformation in quartz can also occur in explosive pyroclastic units but are limited to Boehm lamella (Brazil twins) with single lamella sets {0001}. It has been suggested that a class of microstructures in quartz, referred to as metamorphic deformation lamella (Qz/MDL), occur in endogenic tectonic–metamorphic terrains. However, no type locality has been established for Qz/MDL of non-impact origin. [ABSTRACT FROM AUTHOR]