The criteria for recognising the effects of impacts by large-diameter extraterrestrial projectiles (Dp »10 km) on thin, geothermally active crust must vary fundamentally from those pertaining to impacts on thick, cooler continental crust. The release of energy on the scale of -109 Mt (109 x 106 x TNT) unleashed by such events triggers seismic activity orders of magnitude higher than that induced by purely endogenic movements. Faulting at both proximal and distal foci from impact sites, and associated adiabatic melting of underlying mantle, are expected consequences. Thermal overprinting of shock-metamorphic effects (breccias, shatter cones, pseudotachylite, shock lamellae, high-pressure polymorphs) induced by both shock-induced fusion and heat transfer from impact-rebounded adiabatically melting mantle is capable of obscuring the criteria for impact. The distal effects of mega-impacts on subducted Precambrian oceanic crust are no longer preserved in the geological record. Ensuing proximal and distal phenomena may span the range between clearly recognisable extraterrestrial impact effects and endogenic igneous activity. Age correlations between Phanerozoic impacts and plateau basalts yield preliminary support for this hypothesis. Although the bulk of the terrestrial cratering record has been destroyed by both erosion of elevated terrains and plate subduction, or obscured by burial, a search for Precambrian mega-impacts is facilitated by the preservation of their likely secondary effects: mega-earthquake-triggered faults; ensuing diamictites, and the deposits of turbidity currents; microtektites; spherulitic condensates of vaporised asteroid and target materials; and distal tectonic and igneous effects. Clues to the origin of thermal events are provided by peaks on isotopic-age histograms of precise U-Pb, Ar-Ar, and Sm-Nd mineral-whole-rock ages. These peaks, spatially corroborated by detailed mapping of Precambrian terrains, support an episodic nature of at least certain major Precambrian events and some correlations with impact events, for example: (1) formation of greenstone-granite terranes at ca 3.45 Ga; (2) rifting and clastic sedimentation, including the deposition of iridium-rich spherule units in the basal Fig Tree Group (Kaapvaal Craton, South Africa) and in the Gorge Creek Group (Pilbara Block, Western Australia) at 3.2 Ga; (3) global greenstone-granite events at ca 3.0 Ga; (4) global greenstone-granite events at ca 2.7 Ga; (5) deposition of the Hamersley spherule beds, and the emplacement of global mafic dyke swarms, at ca 2.45 Ga; (6) initiation of global Proterozoic rift networks, and possibly the emplacement of the Bushveld Complex at ca 2.05 Ga; (7) the Sudbury (Canada) and Uppland (Sweden) impacts, and peak rifting in mobile belts, at ca 1.85-1.80 Ga; (8) probable impact structures in Sweden, and the GrenvilleIKeweenawan global rifting and magmatic activity, at ca 1.2- 1.05 Ga; and (9) the Beaverhead (Canada), Acraman (South Australia), and Janisjarvi (Karelia) impacts, and the Vendian-Early Cambrian global rifting, ocean-floor spreading (Iapetus Ocean), and igneous activity (Franklin Province, Canada), at 0.7-D.5 Ga. Preliminary time-series analyses of Precambrian events yield values consistent with the Phanerozoic galactic rotation period (250 ± 50 Ma), and the solar systems cross-galactic-plane oscillation period (33 ± 3 Ma). Possible correlations between mega-impacts and tectonic/thermal events are capable of being tested through isotopic-age studies of diamictites and spherule units of impact origin and of potentially coeval rifting and mafic igneous events.
