Origin of the intensely negative magnetic anomalies in Witwatersrand strata, Vredefort impact structure, South Africa

A groundbreaking study reveals that intensely negative magnetic anomalies surrounding the Vredefort impact structure, South Africa, are primarily caused by metamorphosed banded iron formations (BIFs) at the base of the Witwatersrand Supergroup. These BIFs, rich in magnetite and garnet crystals, were rapidly exhumed during the 2019 ± 2 Ma impact event, cooling below the Curie temperature to acquire a thermal remanence aligned with Earth's 2.02 Ga paleomagnetic field. The research utilized bicycle-mounted magnetometer surveys along 340 km of transects and paleomagnetic analyses, distinguishing lightning-affected samples from those preserving the original paleodirection. Thermoremanent magnetization experiments estimated a post-impact paleofield intensity of 52 μT, providing critical insights into the structure's magnetic signature formation and the role of Bushveld-related metamorphism in magnetite crystallization.

Sleptsova, I. V., Gilder, S. A., Le Goff, M., Dellefant, F., Trepmann, C. A., Lhuillier, F., & Webb, S. J. (2025). Origin of the intensely negative magnetic anomalies in Witwatersrand strata, Vredefort impact structure, South Africa. Geophysical Journal International, 243(1). https://doi.org/10.1093/gji/ggaf305