Physics of crustal fracturing and chert dike formation triggered by asteroid impact, ∼3.26 Ga, Barberton greenstone belt, South Africa

Archean asteroid impacts, reflected in the presence of spherule beds in the 3.2-3.5 Ga Barberton greenstone belt (BGB), South Africa, generated extreme seismic waves. Spherule bed S2 provides a field example. It locally lies at the contact between the Onverwacht and Fig Tree Groups in the BGB, which formed as a result of the impact of asteroid (possibly 50 km diameter). Scaling calculations indicate that very strong seismic waves traveled several crater diameters from the impact site, where they widely damaged Onverwacht rocks over much of the BGB. Lithified sediments near the top of the Onverwacht Group failed with opening-mode fractures. The underlying volcanic sequence then failed with normal faults and opening-mode fractures. Surficial unlithified sediments liquefied and behaved as a fluid. These liquefied sediments and some impact-produced spherules-filled near-surface fractures, today represented by swarms of chert dikes. Strong impact-related tsunamis then swept the seafloor. P waves and Rayleigh waves from the impact greatly exceeded the amplitudes of typical earthquake waves. The duration of extreme shaking was also far longer, probably hundreds of seconds, than that from strong earthquakes. Dynamic strains of approximate to 10(-3) occurred from the surface and downward throughout the lithosphere. Shaking weakened the Onverwacht volcanic edifice and the surface layers locally moved downhill from gravity accommodated by faults and open-mode fractures. Coast-parallel opening-mode fractures on the fore-arc coast of Chile, formed as a result of megathrust events, are the closest modern analogs. It is even conceivable that dynamic stresses throughout the lithosphere initiated subduction beneath the Onverwacht rocks.