Origin of the Ring-shaped Magnetic Anomaly in the Sedimentary Strata of 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 Group sedimentary and volcanic rocks. The collar is characterized by a ring of prominent, negative (<-5500 nT) aeromagnetic anomalies in the northeastern, northern, and western parts, where the sedimentary strata dip near-vertical to overturned. In order to better understand the origin of this magnetic signature, we undertook a ground magnetic survey throughout the Vredefort Dome using a three-axis fluxgate magnetometer mounted on a mountain bike. Upward continuation of our profiles to 150 m matches the aeromagnetic data (150 m altitude). We confirmed by direct field observation at 7 surface outcrops that the strong magnetic signal at all measured profiles correlates with a particular banded iron formation, called the Contorted Bed, as the producer of the distinct ring-shaped negative magnetic feature. Paleomagnetic samples collected from the sedimentary rocks that produce the most intense anomalies (up to -22,000 nT) from the near-surface profiles have extremely high natural remanent magnetization (NRM) intensities (up to 2438 A/m) that likely arise from lightning strikes. However, metamorphism and remagnetization of the Witwatersrand strata are attributed to the Bushveld magmatic event at 2050-2060 Ma when the Earth's magnetic field was directed at D = 175°; I = -62°, as constrained from a site 200 km away. Using this direction, in combination with the tilted to overturned strata in the northern part of the structure related to the 2024 Ma Vredefort event explains the negative anomalies. To the south, where the sediments are reportedly not overturned, magnetic anomalies are strongly (up to +1,500 nT from the aeromagnetic data) positive. The same is true in the Johannesburg area where the non-overturned Contorted Bed also forms positive magnetic anomalies. Hence, we interpret the strongly negative magnetic anomaly in the collar sediments at Vredefort as being due to a Bushveld-aged remagnetization component that was structurally inverted during meteorite impact some 30 Ma later.