O'Neil, et al., report that field evidence from the Pilbara craton (Australia) and Kaapvaal craton (South Africa) indicate that modern tectonic processes may have been operating at ca. 3.2 Ga, a time also associated with a high density of preserved Archaean impact indicators. Recent work has suggested a causative association between large impacts and tectonic processes for the Hadean. However, impact flux estimates and spherule bed characteristics suggest impactor diameters of <100 km at ca. 3.5 Ga, and it is unclear whether such impacts could perturb the global tectonic system.
In this paper, the authors developed numerical simulations of global tectonism with impacting effects, and simulate the evolution of these models throughout the Archaean for given impact fluxes. We demonstrate that moderate-size (∼70 km diameter) impactors are capable of initiating short-lived subduction, and that the system response is sensitive to impactor size, proximity to other impacts, and also lithospheric thickness gradients. Large lithospheric thickness gradients may have first appeared at ca. 3.5–3.2 Ga as cratonic roots, and we postulate an association between Earth’s thermal maturation, cratonic root stability, and the onset of widespread sporadic tectonism driven by the impact flux at this time.
The Archaean spherule beds offer a unique insight into impacting effects on the early Earth. The change in tectonic behavior in the Kaapvaal craton and the onset of (micro)plate motions in the Pilbara craton suggest a period of plate mobility, requiring subduction, coincident with impacting events. The long tail of large impacts into the Mesoarchean is coincident with the first observed development of significant lithospheric thickness variations, as observed in cratonic diamonds in the Kaapvaal craton at 3.5–3.2 Ga (Shirey and Richardson, 2011). The development of significant lithospheric gradients is a critical factor for subduction initiation, and the temperatures of the early Earth may have precluded these from developing naturally—intrinsically weak lithosphere may have flowed rather than developed significant gradients. The onset of plate tectonics—either through impacts or through internal forces—may have been retarded until the thermal state of the Earth permitted the generation of significant lithospheric heterogeneity.
The dynamics of the transition of the Earth from an earlier archaic regime into modern plate tectonics was likely protracted (O’Neill et al., 2018). The authors show that there is a strong association between tectonics, cratonization, and impacts, and the timing of the transition to active tectonics may have been very sensitive to the evolving solar system environment.
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