Origin of the Intensely Negative Magnetic Anomalies in Witwatersrand Strata, Vredefort Impact Structure, South Africa

Sleptsova, Iuliia; Gilder, Stuart; Le Goff, Maxime; Dellefant, Fabian; Trepmann, Claudia; Lhuillier, Florian; Webb, Susan (2025): Origin of the Intensely Negative Magnetic Anomalies in Witwatersrand Strata, Vredefort Impact Structure, South Africa. Geophysical Journal International, 243 (1). ISSN 0956-540X

Abstract

The central portion of the 2019 ± 2 Ma Vredefort (South Africa) impact structure comprises a 40–50 km diameter central uplift of Archean basement rocks surrounded by a 15–20 km wide collar of late Archaean to early Proterozoic Witwatersrand Supergroup sedimentary and volcanic rocks. The collar is characterized by a ring of strongly negative (up to −5 500 nT) aeromagnetic anomalies surrounding much of the structure where the strata dip steeply to overturned. To better understand the origin of this magnetic feature, we undertook a ground survey along 20 transects (340 km) in the Vredefort structure using a three-axis fluxgate magnetometer mounted on a mountain bicycle. Upward continuation of our profiles to 150 m matches the aeromagnetic data in shape and amplitude. From the bicycle measurements, we pinpointed the rocks responsible for the extremely negative anomalies. Field observations and microfabric analyses of the rocks from six outcrops substantiated that the magnetic signal correlates with 10–100 m thick metamorphosed banded iron formations (BIFs) at the base of the supergroup as the main producer of the anomalies. Paleomagnetic samples collected from the rocks at the surface that produce the most intense anomalies (up to −22 000 nT) have extremely high natural remanent magnetization intensities (up to >1000 Am−1) likely arising from lightning strikes. Stepwise demagnetization and rock magnetic experiments establish a new protocol to distinguish samples that escaped remagnetization from lightning and possess the established 2.02 Ga paleodirection at Vredefort. From a suite of thermoremanent magnetization (TRM) experiments, the best estimate for the paleofield intensity at the time of impact was 52 μT, corresponding to an average remanence of 32.5 Am−1. The results of the TRM experiments together with the paleodirection enabled us to successfully model the prominent negative anomalies in the metasediments only when accounting for the post-impact orientation of the BIFs. We interpret the strongly negative magnetic anomalies in the collar region as being formed directly after crater exhumation and uplift of the rocks. This interpretation implies that Bushveld-related metamorphism at 2.06 Ga created the up to mm-sized magnetite and garnet crystals in the BIFs, which resided at temperatures higher than the Curie temperature of magnetite (580 °C) until the impact rapidly brought the BIFs close to the surface, where magnetite cooled to acquire a thermal remanence in the 2.02 Ga field.

• Dokumententyp: Artikel (LMU)
• Organisationseinheit (Fakultäten): 20 Geowissenschaften > Department für Geo- und Umweltwissenschaften > Geophysik
• DFG-Fachsystematik der Wissenschaftsbereiche: Naturwissenschaften
• DFG: Gefördert durch die Deutsche Forschungsgemeinschaft (DFG) - 433311356