Abstract: | The significance of episodicity in Earth history, including the role of extraterrestrial bombardment events, reinvokes the controversy between uniformitarianism and catastrophism of the Eighteenth century. It is suggested that some peak Precambrian tectonic thermal episodes may be genetically related to and represent the distal consequences of mega impact by large diameter ( D p10 km) projectiles. In particular, mega impacts affecting tectonically mobile and geothermally active crustal tracts, i.e. simatic crustal domains overlying relatively thin (<20 km) lithosphere, are expected to have triggered regional to global sub lithospheric perturbations, magmatic episodes and formation of large igneous provinces. The consequences of mega impacts on mobile crustal domains may be difficult to discriminate from purely endogenically triggered events due to (1) heat transfer from impact rebounded adiabatically melting mantle is capable of overprinting proximal shock metamorphic effects through extensive recrystallisation and related deformation; and (2) widespread elimination of the proximal impact effects by erosion of elevated terrains, burial and subduction. For these reasons, distal impact deposits including olistostromes, diamictites, turbidity currents, microtektites and microkrystites (spherulitic condensates of vaporised asteroid and target materials) accompanied by geochemical anomalies furnish the key for unravelling the Precambrian impact history. Clues for major impact events and bombardment periods are provided by comparisons between the isotopic ages of preserved impact signatures, dyke swarms, volcanic sequences and alkaline intrusions, and thermal peaks on isotopic age distribution histograms. In the present paper the identification of the signatures of such events in the Precambrian crustal records is attempted in two ways: (1) a regional approach, using the Pilbara Craton, Western Australia a well preserved Archaean to upper Proterozoic (3.5 1.1 Ga) c.200 000 km 2 large crustal province as an example, and (2) tests of possible correlations between impact, tectonic and magmatic events in Precambrian shields world wide. |