Direction of Impact at Lonar Crater, India: Results from Microfracture Density and X-Ray Diffraction Analysis

ABSTRACT Oblique impacts significantly influence the cratering processes. It causes an asymmetrical crater morphology. In the downrange, it leads to farther distribution of the ejecta blanket and higher shock pressures, which causes more intense deformation and concentric deviation in linear structures. Based on the fact that attenuation of the shock wave and corresponding strain is not uniform in an oblique impact, this study uses the microfracture density and X-ray diffraction (XRD) spectra of the basalts from the crater wall and rim of Lonar crater to determine the possible impact direction. The density of intergranular microfractures in plagioclase and the full width at half maxima (FWHM) of pyroxene (2¯21), plagioclase (040) and magnetite (311) peaks are determined. The microfracture density represents the intensity of brittle deformation, while the changes in XRD spectra relate to the lattice deformation. Both these techniques have proven sensitivity even at low dynamic stress. Our XRD analysis does not present any perceptible trend of FWHM. They are probably affected by post-impact hydrothermal alteration. Notably, the microfracture density is higher in the samples from the southern part of the crater, thus indicating that the damage is higher in the southern part of the crater rim. The study concludes that the probable direction of impact for the Lonar crater is north to south.