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Asteroid impact connections of crustal evolution

Andrew Glikson and John Vickers

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

Advances in isotopic age determinations increasingly point to an episodic nature of crustal evolution. Field and geochemical evidence indicate that at least some large impact episodes may have been associated with, and possibly triggered, magmatic and tectonic events. Corrrelations are observed between isotopic age frequency peaks and asteroid impacts records (~3.47, ~2.63, ~2.56, ~2.48, ~2.023 Ga [Vredefort] and 1.85 Ga [Sudbury]). Evidence for major dynamic and thermal effects of a large impact cluster on the early Precambrian crust is provided by ejecta fallout units associated with: (i) unconformities; (ii) tsunami boulder debris; (iii) compositional contrasts between supracrustal sequences that underlie and overlie the ejecta units; (iv) onset of episodes of iron-rich sedimentation; and (v) near-contemporaneous intrusion of granitoid magmas. An impact cluster at ~3.26–3.24 Ga, documented in the Barberton greenstone belt, South Africa, is associated with unconformities and granite activity correlated with unconformities and olistostromes in the Pilbara Craton, Western Australia. In both cratons a 300 Ma period of evolution of greenstone–granite terrains is abruptly terminated by unconformities overlain by impact ejecta, turbidite and banded iron-formation. The 3.26 and 3.24 Ga terminations involve major faulting, uplift, erosion, and the onset of high-energy sedimentation, which includes detrital components from contemporaneous granites. The onset of ferruginous sedimentation, including banded iron-formation, in the wake of the ~3.47,~3.26,~3.24,~2.63 and~2.56 Ga impacts, suggests weathering and soluble transport of ferrous oxide under low-oxidation atmosphere and hydrosphere conditions, possibly reflecting extensive mafic volcanic activity triggered by the impacts. Extensive dyke formation during 2.48–2.42 Ga (Matachewan, Scourie, Karelian, Widgiemooltha, Bangalore, Antarctica dykes) may be related to deep crust/mantle fractures triggered by the ~2.48 Dales Gorge mega-impact. Tentative observations are consistent with, but do not demonstrate, possible overlaps between Phanerozoic impacts and the onset of faulting and plate tectonics episodes.