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 Reflection 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 reflecting, 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 flows. Moderate to strong westward shifts of the K 3 axes are seen for the majority of the shocked basalts on the crater rim and west-southwest 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 flows on the Lonar crater rim shows a bilaterally symmetrical distribution about an E-W axial plane, which includes quaquaversal dips of the flows all around the crater rim, except to the west where overturned dips of the basalt flows 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.