Explaining the Intensely Negative Magnetic Anomalies in the Vredefort Impact Structure, South Africa

The 2024 Ma Vredefort (South Africa) impact structure comprises a 40-50 km central region 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 prominent, negative (<-5,500 nT) aeromagnetic anomalies surrounding much of the structure where the strata dip near-vertical to overturned. To better understand the origin of this magnetic feature, we undertook a ground magnetic survey throughout the Vredefort structure using a three-axis fluxgate magnetometer mounted on a mountain bicycle. The upward continuation of our profiles to 150 m matches the aeromagnetic data in shape and amplitude. The near-ground magnetic measurements allowed us to pinpoint the rocks responsible for the intensely negative anomalies. Field observations and microfabric analyses of six outcrops confirmed that the magnetic signal correlates with specific metamorphosed banded iron formations (BIFs) at the base of the supergroup, 10 to 100 m thick, as the main producer of the strongly negative magnetic 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 A/m) likely arising from lightning strikes. Stepwise demagnetization and rock magnetic experiments distinguish one site as likely having escaped remagnetization from lightning that possesses the established 2.02 Ga paleodirection at Vredefort. Thermoremanent magnetizations (TRM) imparted on 41 samples using a 40 μT field yielded an average intensity of 25 A/m. Using the results of TRM experiments and 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 anomaly in the collar region as being formed directly after crater exhumation and uplift of the rocks. This interpretation implies that Bushveld-related metamorphism 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.