Impact fragmentation of Lonar Crater, India: Implications for impact cratering processes in basalt

Abstract

Impact fragmentation is an energetic process that has affected all planetary bodies. To understand its effects in basalt, we studied Lonar Crater, which is a rare terrestrial simple impact crater in basalt and analogues to kilometer-scale simple craters on Mars. The Lonar ejecta consists of basaltic fragments with sizes ranging from silt to boulder. The cumulative size and mass frequency distributions of these fragments show variation of power index for different size ranges, suggesting simple and complex fragmentation. The general shape of the fragments is compact, platy, bladed, and elongated with an average edge angle of 100°. The size distribution of cobble-to boulder-sized fragments is similar to the fracture spacing distribution in the upper crater wall, indicating the provenance of those large fragments. Its consistency with a theoretical spallation model suggests that the large fragments were ejected from near surface of the target, whereas the small fragments from deeper level. The petrophysical properties of the ejecta fragments reflect the geophysical heterogeneity in the target basalt that significantly reduced the original size of spall fragments. The presence of Fe/Mg phyllosilicates (smectites) both in the ejecta and wall indicates the role of impact in excavating the phyllosilicates from the interior of basaltic target affected by aqueous alteration. The seismic images reveal a thickness variation in the ejecta blanket, segregation of boulders, fractures, and faults in the bedrock beneath the crater rim. The fracturing, fragmentation, and fluvial degradation of Lonar Crater have important implications for Mars.

Impact spallation processes on the Moon: A case study from the size and shape analysis of ejecta boulders and secondary craters of Censorinus crater

“…14. A power-law fit to each CSFD in the radial sectors provides a b-value, which is an indicator of impact fragmentation process, if it involves a simple or complex fragmentation. The smaller b-values suggest simple fragmentation, while the larger b values indicate complex fragmentation (Hartmann, 1969;Kumar et al, 2014). In order to assess whether the b-values would also reflect the oblique impact, we compare the b-values of the 16 radial sectors.…”

“…A large scatter in b-values is observed for the boulder densities below 750. Above this value, The shape of the ejecta boulders provides important insights into the impact fragmentation processes (e.g., Melosh, 1984;Kumar et al, 2014). We measured the shape of 241,989 ejecta boulders as shown in Fig.…”

“…In addition, the edge angle would reflect the angles of fracture intersections in the source regions (near-surface bedrock in the crater interior), from which the ejecta boulders were originated (e.g., Kumar et al, 2014). The observed shapes of the ejecta fragments may not fully reflect the original shapes of the ejected fragments because the collision between the ejected fragments during the ballistic transport and deposition would significantly modify their original shapes (Kumar et al, 2014). Therefore, the observed shapes are an outcome of the entire ejecta emplacement process.…”

“…The laboratory experiments (e.g., Gault et al, 1963;Fujiwara et al, 1977Fujiwara et al, , 1978Capaccioni et al, 1984;Schultz and Gault, 1985;Polanskey and Ahrens, 1990;Housen et al, 1991;Sagy et al, 2001) and numerical modelling (e.g., Melosh, 1984;Grady and Kipp, 1987;Melosh et al, 1992;O'Keefe and Ahrens, 1985;Ryan and Melosh, 1998) have provided important insights into the impact fragmentation processes. For impact craters on Earth, the size distribution of ejecta fragments has been used to model the impact spallation processes, for example at Lonar crater (e.g., Kumar et al, 2014). The impact fragments on the lunar surface were also characterized by many workers (e.g., Shoemaker et al, 1968;Hartmann, 1969;Moore, 1971;Butler and King, 1974;Cintala and McBride, 1995;Bart and Melosh, 2007, 2010a, 2010b.…”

Volcanism on farside of the Moon: New evidence from Antoniadi in South Pole Aitken basin

“…The multiple fragmentation tends to increase the power index value, as multiple fragmentation results in more number of smaller fragments repeatedly (Kumar et al, 2014). Additionally, the magnitude of explosive eruption may also control the size distribution of the fragments.…”