Paleomagnetism of Lonar impact crater, India

Abstract

Lonar crater, India, is the best preserved terrestrial impact crater formed in basalt and is a unique terrestrial analogue for small, simple craters on terrestrial planets and the Moon. We investigated the paleomagnetic and rock-magnetic properties of the 1.88 km diameter crater in order to understand the effect of impacts on magnetization in target rocks. The magnetization in the Lonar basalts consists of an original 65 Ma Deccan magnetization and a recent overprint. We constrained the timing of magnetization acquisition at Lonar using a combination of conglomerate tests on ejecta deposit clasts and fold tests on the overturned and jumbled rim fold. In some areas, the recumbent rim fold is preserved and can be approximated as a horizontal cylindrical fold. In other areas, substantial vertical axis rotation may have occurred where tear zones developed during folding. We observed only subtle effects from the impact on the rock-magnetic properties of Lonar materials, which include a slightly elevated coercivity in shocked ejecta blocks. We show that paleomagnetism can provide a constraint on shock heating in the absence of petrographic evidence of shock (in this case, < 187 ± 15 °C). At Lonar, viscous (and/or chemical) remanent magnetization acquired in the < 50 kyr subsequent to crater formation has obscured any evidence of shock remanent magnetization. We also find no evidence of shock demagnetization or the presence of intense impact-induced or impact-amplified transient magnetic fields that have been proposed around larger impact structures.