Asteroids and early precambrian crustal evolution

Abstract

Major enigmas arising from the Archaean crustal record are capable of resolution in terms of the combined role of episodic mega-impacts and of long-term mantle processes. The terrestrial impact record suggests that, for very large impact structures, the degree of preservation of proximal deformation effects decreases with the increase in the magnitude of the associated thermal effects—a consequence of extensive recrystallization and melting. This focuses attention on thermal igneous and metamorphic peaks as possible clues of impact events. Isotopic age studies define principal thermal events about 3.5, 3.0 and 2.7 (±0.1) Gyr and less well defined episodes, succeeded by protracted thermal fluctuations in infracrustal high-grade metamorphic zones. The bulk of the igneous activity associated with these episodes took place over relatively short time spans of c.20–30 × 10 6 years. Interpretations of these episodes in terms of internal dynamics of the Earth are inconsistent with the thermal behavior of silicate rheologies in a continuously convecting mantle regime. A triggering of these episodes by the mantle rebound response to intermittent extra-terrestrial asteroid impacts is considered on the basis of (1) identification of major impacts from distal ejecta horizons marked by iridium anomalies and impact/earthquake-triggered diamictite breccia and olistostromes; (2) the relationships between microtektite horizons and volcanic activity; (3) geochemical and experimental evidence for mantle upwelling from the transition zone and catastrophic adiabatic melting required to generate peridotitic komatiites. Episodic differentiation/accretion growth of sial consequent on these events is capable of resolving the volume problem which arises from comparisons between modern continental crust and sial produced by continuous two-stage mantle melting processes, taking the temporal decline in heat flow into account. Impact shock effects on the Archaean crust have been largely obscured by (1) outpouring of voluminous basic/ultrabasic lavas, inundating shock-deformed crust and extending beyond the perimeters of impact excavated basins: (2) gravity subsidence and downfaulting of terrestrial mare, accounting for the anatexis of sub-greenstones basement; (3) extensive shearing and recrystallization of impact structures, breccias and mineral deformation features beneath impact-excavated basins, relics of which may be retained in structural windows in high grade metamorphic terrains. Isostatic subsidence and anatexis of thick mare-type piles and underlying impacted crust resulted in intracrustal comagmatic plutonic and volcanic suites within periods in the order of15–30 × 10 6 years, as controlled by post-impact mantle convection cooling. Subsequent isostatic updoming of remobilized basement gneiss terrains and the rise of their anatectic products along major normal shears, bearing an analogy to metamorphic core complexes, resulted in the multiple gneiss-granite complexes amidst greenstone-dominated terrains. Recurrent post-tectonic thermal fluctuations and magmatic activity reflect a long-term existence of anomalous mantle regions beneath original mega-impact sites and thermal perturbations related to distal impacts. A broad age zonation in some Archaean shields suggests lateral migration and terrane accretion in a convection driven plate tectonic regime.