Impact bombardment and its role in proto-continental growth on the early earth

Abstract

There is evidence that during 4.3-3.9 Ga the Earth experienced a period of intense bombardment similar to that recorded on the moon. From cratering mechanics and by comparison with more recent terrestrial impact structures the effects of large impact structures, D ⩾ 100 km, have been modelled for an early terrestrial proto-crust with an assumed thickness of 15 km. The direct effects were analogous to those on the Moon: the formation of multi-ring basins with a topography of ∼ 3 km, the uplift of deep-seated material to the surface, the fracturing of the crustal column and the generation of surface impact lithologies. However, unlike the Moon, the highly active nature of the Earth resulted in more long-lived indirect effects: impact-induced volcanism due to distortion of the geothermal gradient by uplift and the addition of post-shock heat, intra-basin sedimentation by volcaniclastics and reworked impact lithologies and ultimately subsidence of the basin due to loading by volcanic and sedimentary products. Very large basins, D > 1000 km, excavated the local lithosphere and produced the equivalent of mantle plumes below the impact sites. These conclusions are incorporated into a model of early crustal evolution of the Earth in which the net effect of large impact events was to localize and accelerate endogenic activity. In this scenario, the intense bombardment culminated at 3.9 ± 0.1 Ga with a period of heavy bombardment by a residual population of 10–100-km-sized bodies. The impact of these bodies represented a first-order event in the progressive evolution of the proto-crust. The resultant mare basin-sized structures overlapped in space and their effects overlapped in time to give rise to large areas, 10 5–10 4 km 2, of vigorous endogenic activity and thermally and geologically anomalous crust. Subsidence of these major basins resulted in the reprocessing of large volumes of basaltic volcanics and impact melt leading to the production of sialic partial melts. Large impacts thus may have been a triggering mechanism for the generation of extensive concentrations of differentiated crust and for a proto-crust of basaltic composition would have been the loci of protocontinental nuclei production. Further reworking of these nuclei and subsequent cratonization by strictly internal mechanisms would have produced the stable shield areas during the Late Archean.