A possible impact origin for the Bangui magnetic anomaly (Central Africa)

Abstract

Bangui is one of the most impressive magnetic anomalies on Earth and by far the largest over Africa. At Magsat altitudes, it has an amplitude of 28 nT and covers an area of 700,000 km 2. Evidence presented suggests that the anomaly might be related to a large impact structure early in the history of the Earth: north-south, west-east, and vertical derivatives of the 5-min topographic data base all reveal a double ring structure with outer ring diameter ~ 810 km and inner ring diameter ~ 490 km. The Precambrian geology shows several small ancient basins within a large basin with a basement high in the middle such as found in large impact structures. The basement complex is early Precambrian and hence the impact must be of this age. The magnetic anomaly is interpreted as being due to strong remanent magnetism in the ancient crater floor and surrounds. The anomaly can be satisfactorily modelled by a 4.5-km-thick disc, 800 km in diameter beneath the ancient basins. To model the amplitude and shape of the anomaly requires an intensity of 10 A m −1 with direction D = N 18° W, I = + 25°, i.e., significantly different from the present dipole field. The magnetisation is likely to include strong shock remanence (SRM) acquired at the time of impact, thermal remanence (TRM), partial thermal remanence (PTRM), thermochemical remanence (TCRM) and chemical remanence (CRM) acquired soon after the impact. All depend on the size of the impact and all contribute to the high intensity required to explain the anomaly.