Simple models for the Beattie Magnetic Anomaly in South Africa

The origin of the approximately 1000 km-long Beattie Magnetic Anomaly (BMA) in South Africa remains unclear and contentious. Key issues include the width, depth and magnetization of its source. In this study, we use uniformly magnetized spheres, prisms and cylinders to provide the simplest possible models which predict the 1 km-altitude aeromagnetic measurements along a profile across the BMA. The source parameters are adjusted by forward modeling. In case of a sphere, an inversion technique is applied to refine the parameters. Our results suggest that two similarly magnetized and adjacent sources, with a vertical offset, can explain the observed magnetic anomaly. The best fitting model corresponds to two highly-magnetized (> 5 A m− 1) sheet-like prisms, extending from 9 to 12 km depth, and from 13 to 18 km depth, respectively, and with a total width reaching 80 km. Other less-preferred models show thicker and deeper magnetized volumes. Associated magnetizations seem to be mostly induced, although a weak remanent component is required to improve the fit. We also compare our results with the interpretation of independent magnetotelluric and seismic experiments along the same profile. It suggests that the geological sources for the BMA are mostly located in the middle crust and may be displaced by a shear zone or a fault. Contrary to previous models suggesting a serpentinized sliver of paleo-oceanic crust within the Natal–Namaqua Mobile Belt, we propose that granulite-facies mid-crustal rocks within this belt may cause the BMA.