Paleomagnetic study of impactites from the Karla impact structure suggests protracted postimpact hydrothermalism

Abstract

We present a paleomagnetic study of the ~10 km diameter Karla impact structure in Russia. We sampled the target carbonate rocks, and a yet undocumented fragmental melt-bearing lithic breccia layer. This impact breccia, which contains carbonate melt, is enriched in stoichiometric magnetite by a factor of ~15 compared to the target lithologies, and carries a stable natural remanent magnetization. The weak remanent magnetization and the presence of both normal and reverse polarities down to the centimeter scale indicate that the breccia does not carry a thermoremanent magnetization (TRM), but rather a chemical remanent magnetization (CRM). The presence of stoichiometric magnetite and the absence of TRM suggest that the magnetite was formed during relatively low-temperature postimpact hydrothermalism that affected the porous impact breccia layer. During this process, the breccia acquired a CRM. The paleomagnetic direction is compatible with a Cenozoic age for the impact event, but cannot bring more precise constraint on the age because of the stable position of the Eurasian plate over the last 60 Myr. However, the presence of both polarities indicates that mild hydrothermalism took place over a period of time long enough to span at least one reversal of the geomagnetic field, that is, over a time scale of the order of 100 kyr. This confirms that protracted hydrothermal systems associated with impact craters are long lived, even in relatively small craters such as Karla, and are key features of the geologic and environmental effects of impacts on Earth.