Structural and anisotropy of magnetic susceptibility (AMS) evidence for oblique impact on terrestrial basalt flows: Lonar crater, India

Abstract

The anisotropy of magnetic susceptibility (AMS) technique, combined with the analyses of satellite images and the geological structure, are examined on Lonar crater , a small, ~1.8-km-diameter impact crater in India, to evaluate the direction and obliquity of asteroid impact. The Advanced Spaceborne Thermal Emission and Refl ection Radiometer (ASTER) image (15-m resolution) of the ca. 52 ka Lonar crater in the subhorizontal Deccan basalt (ca. 65 Ma) shows that this simple, bowl-shaped impact crater has a near-circular rim with a circularity of ~0.95. Most of the highly refl ecting, continuous-ejecta blanket around the crater rim can be enveloped with an ellipse whose major E-W axis is coincident with the diameter of the crater rim and minor N-S axis is relatively displaced toward the west by ~200 m. The present ejecta distribution, which appears to be close to its pristine shape, extends to a distance of ~700 m in all the directions from the crater rim except to the west where it extends to a distance of a little more than 1 km. The circular shape of the crater rim, the E-W bilateral symmetry of the enveloping ellipse on the ejecta, and the greater extension of the ejecta toward the west appear to be the result of an oblique impact from the east with an angle of incidence of 30°-45° when compared with experiments. The AMS data suggest that the target basalts occurring at ~2 km west-southwest of the crater rim are highly shocked, as indicated by the random orientation of their K 3 susceptibility axes in comparison to the unshocked basalts at ~2 km east-southeast of the crater; the unshocked basalts show a bimodal distribution of susceptibility axes typical of lava fl ows. Moderate to strong westward shifts of the K 3 axes are seen for the majority of the shocked basalts on the crater rim and westsouthwest of the crater; the shocked basalts also indicate an oblique impact from the east when compared with modeling and experiments. A general lowering of degree of anisotropy of the Lonar shocked basalts (~1.01) compared to the surrounding unshocked basalts (~1.03) is found to be a characteristic feature of impact crater target rocks. Variation in attitudes of the basalt fl ows on the Lonar crater rim shows a bilaterally symmetrical distribution about an E-W axial plane, which includes quaquaversal dips of the fl ows all around the crater rim, except to the west where overturned dips of the basalt fl ows are seen. It appears that oblique impact and the symmetry in structural variations around the crater rim have a relationship for a small crater such as Lonar.

Shock pressure estimates in target basalts of a pristine crater: A case study in the Lonar crater, India

“…First, the rocks at the crater rim have experienced shock pressures high enough to reorient the AMS and NRM vectors (Misra et al, 2010;Arif et al, 2012). Second, the shock pressure at the crater rim was too low to reorient the rock magnetic vectors (Louzada et al, 2008).…”

“…570 ka in age. This simple crater was formed by the impact of an east-to-west-directed hypervelocity projectile (Misra et al, 2010). The cra-ter is ~1.88 km wide and 150 m deep from the rim, with an ~7-10-m-deep shallow saline lake at its bottom.…”

“…However, with the discovery of shocked material, such as impact glasses in the impact breccia and shatter cones in basaltic fragments, the impact origin is now well established (Nayak, 1972(Nayak, , 1993Fredriksson et al, 1973;Murali et al, 1987). Misra et al (2010) and Arif et al (2012) studied the magnetic fabrics of the target basalts around the Lonar crater. They suggested that the shock waves overprinted the primary magnetic fabrics such that the minimum susceptibility axis (k 3 ) was reoriented subparallel with the east-to-west downrange direction of the impact.…”

Influence of bottom water anoxia on nitrogen isotopic ratios and amino acid contributions of recent sediments from small eutrophic Lonar Lake, central India

“…1A). Origin and formation age of the crater were subject to various investigations (Misra et al 2010). The crater is almost circular and its rimto-rim diameter averages 1880 m (Fig.…”

The Lonar Crater: The Best Preserved Impact Crater in the Basaltic Terrain

“…However, till date, fragments of the impactor (meteor) have not been retrieved from the site and the geochemical evidence too has not yet been successful in unequivocally characterizing the nature of the impactor. Although the characterization of the impactor is still elusive, based on the circularity of the crater, it is likely that the angle of incidence of the impactor was[30°from the horizon and the greater extension of the ejecta in the west probably indicates that the impact was from the east (Misra et al 2010).…”