Asteroid impact connections of crustal evolution*

Abstract

Advances in isotopic age determinations increasingly point to the episodic nature of the evolution of lithosphere and crust, and in this paper well-documented field and geochemical evidence is to be reviewed, which indicates that at least some of these episodes were associated with large asteroid impacts. Observed overlaps between early Precambrian impacts [∼3.47, ∼2.63, ∼2.56, ∼2.48, ∼2.023 Ga (Vredefort) and 1.85 Ga (Sudbury)] and isotopic age frequency peaks, defined within age errors, while not constituting proof of cause–effect relationships, invite tests of field evidence relevant to this question. Evidence for major dynamic and thermal effects of large impact clusters 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 greenstone–granite evolution is terminated abruptly 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.