Australia’s buried asteroid impact structures: more than meets the eye

Date & time

4–5pm 18 October 2012

Location

J1 Seminar Room

Speakers

Andrew Glikson (ANU)

The discovery of large asteroid impact structures, likely and possible impact structures, onshore and offshore the Australian continent (Woodleigh [120 km; 360 Ma], Tookoonooka [55-65 km; ~125 Ma], Talundilly [84 km; ~125 Ma], Gnargoo [75 km; Lower Permian - upper Cretaceous], Mount Ashmore [>100 km; end-Eocene] and Warburton [>200 km; early Silurian - end-Carboniferous]) requires re-examination of the diagnostic criteria used for their identification. Bouguer anomalies of established impact structures (Chicxulub [170 km; 64.98 ± 0.05 Ma], Woodleigh) and possible impact structures (Gnargoo) display a unique structural architecture where pre-impact structural ridges are intersected 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 the Woodleigh impact structure and Warburton possible impact structure, suggesting deep crustal fracturing. Deformed, curved and clouded planar deformation features in quartz (Qz/PDF), displaying Miller indices ({10-11}, {10-12}, {10-13}) diagnostic of shock metamorphism, occur around exposed established impact structures (Vredefort [300 km; 2023 ± 4 Ma], Sudbury [~250 km; 1850 ± 3 Ma], Charlevoix [54 km; 342 ± 15 Ma],  Manicouagan [100 km; 214 ± 1 Ma]), Tookoonooka [Gostin and Therriault, 1997; Bron and Gostin, 2012] and Talundilly [Longley, 1989; Gorter and Glikson, 2012]). These deformed Qz/PDF allow recognition of shock metamorphism in buried impact structures which contain deformed planar elements occur. Planar sets in quartz can also occur in explosive pyroclastic units but are limited to Bohm lamella (Brazil twins) with single lamella sets {0001}. It has been suggested that a class of microstructures in quartz, termed metamorphic deformation lamella (Qz/MDL), occur in tectonic-metamorphic terrains removed from known impact sites, but no type locality has been established for Qz/MDL of non-impact origin. The expansion of the impact inventory of Earth by recently identified buried impact structures supports, in some instances, an association between impact clusters and geological epoch boundaries.

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