STRUCTURAL STUDIES IN LONAR CRATER, INDIA AND LOCKNE CRATER, SWEDEN.

Abstract

Impact crater is an approximately circular depression in the surface of a solid celestial body that is formed by impact of a smaller body. Well preserved impact craters are very common on extra-terrestrial bodies like the Moon or the Mars [1,2], however on the Earth, they are constantly damaged and eliminated by the endogenic and exogenic processes such as active tectonics and erosion. Moreover, the oceanic crust covers two-third of earth’s surface, and is constantly recycled, rendering identification of younger impact structure difficult, and that of older impact structures, > 200 Ma, impossible. In consequence, only 200 impact craters have been identified on the earth till date [3]. Impact craters mark global scale catastrophic events [4], in which the deformation occurs at a remarkably high strain rate. The deformation caused by the impact, is provoked by impact generated outward radiating shock waves. Peak shock pressure exerted by the shock wave is a yard stick for the intensity of deformation. The shock pressures, therefore, provide important clues for understanding the cratering process. The most common method of estimating the shock pressures is the identification of shock indicators such as high pressure-temperature polymorphs. However, in weakly shocked rocks such shock indicators are absent and estimations become difficult. In this study, therefore, a combination of rock magnetic and microfracture investigations are used for estimation of shock pressure in the target rocks of two very different craters, the Lonar crater in India and Lockne crater in Sweden. The Lonar crater in India is a very young, 50 ka old, impact structure [5]. The crater, devoid of any tectonic overprint, can be assumed as pristine. The magnetic fabrics show a good correlation with the magmatic fabric of the impact target rock, i.e., Deccan basalt. The high coercivity component of the natural remnant magnetisation in the crater rim basalt is similar to that in the unshocked basalt, located away from the crater. The lack of any shock related magnetic overprint on the crater rim basalt is, therefore, evident in the Lonar crater. On the other hand, radial and concentric microfractures observed in basalts at the crater rim and farther away, show symmetric distribution with respect to the crater. The concentric fractures Abstract ii consistently overprint the radial microfractures. The overprinting relationships suggest that the radial and concentric microfractures were developed during propagation of the early compressional and the late decompressional (tensile) shock wave components, respectively. The results of present rock magnetic and microfracture studies, when interpreted in light of the published experimental and numerical simulation studies on the Lonar basalt [6–11], reveal that the shock pressure in the Lonar crater rim was less than 0.5 GPa but greater than 0.2 GPa. These results are in agreement with the pressures estimated through numerical modelling by Louzada et al. [12]. The Lockne crater in Sweden is a 455 Ma old impact structure that has been overthrust by Caledonian napes, obducted, and suffered eroded. The magnetic fabrics in the target basement represent a pre-impact tectonic or magmatic emplacement fabric, and are, therefore, not related to any shock related re-orientation of the magnetic axes. However, the geometry of microfractures, radial or concentric, implies their impact origin. Published reports on experimental and natural craters suggest a correlation between shock induced fractures and peak shock pressures when shock pressures exceed 0.2 GPa [8,9]. Reports of rock magnetic investigations on the target rocks, with magnetic mineralogy similar to that in Lockne basement rocks, show conspicuous shock effects on magnetic fabrics in pressure excess of 0.5 Gpa [6,7,10,12]. Based on these reports, an interpretation of the present results on magnetic fabrics and microfractures reveal that the shock pressure was in the range of 0.5 to 0.2 GPa in rocks up to about 6.5 km from the centre of the crater, and in the order of < 0.2 GPa in the rocks farther away. These estimates are slightly lower than shock pressures numerically calculated by Lindstrom et al. [13]. The shock pressures predicted here or in other published reports either represent pressures experienced by entire mineral grains or represent an average over few cubic centimetres [12–15]. The studied rocks at Lockne crater, according to present and earlier published estimates, suffered very low shock pressures. The correlation between the distinct phases of shock wave and the overprinting sets of orthogonal microfractures is strong in these rocks. This implies that, despite being weak [13], the shock waves caused perceptible deformation. Abstract iii Shock induced deformation is maximized, due to localization of stresses, as shock waves, especially weak to moderate ones, propagate across grain interfaces [16–18]. The localized stress and heat often cause melting or phase transformation in pockets along the interfaces. In the dolerite target rocks of Lockne impact structure, that according to published reports suffered shock pressure and temperature in the order of <3 GPa and <127°C [13], lingunite nano-crystals are discovered along the augite-labradorite mineral interfaces. Lingunite is a high pressure-temperature polymorph of Na-plagioclase, induced at pressures > 20 GPa and temperatures >1000°C [19,20]. Its presence suggests that although shock pressure and temperature in the bulk rocks were low, pressure-temperatures along the grain boundaries were 10 to 20 times higher due to the shock localization. The study presents a new approach for estimation of the shock pressures in weakly shocked rocks and gives new information on the relationship between shock pressure and resulting microfractures. In suggests that the shock microfractures may help in discovery of unidentified impact craters.