Interpretation of satellite magnetic anomalies over the West African Craton

Abstract

Satellite magnetic anomaly maps of west Africa display persistent anomalies that are spatially related to the geological structure of the West African Craton. The Reguibat and Man shields to the north and south of the Taoudeni Basin have positive magnetization contrasts whereas the basin, which was disrupted by rifting and volcanism in the Late Proterozoic and Mesozoic, has a negative magnetization contrast. A forward model of the craton is developed, based on the surface geology and including aspects of the evolution of deep-seated thermal, petrological, and magnetization contrasts. Anomalies calculated from the model correspond fairly well to the most reliable observed anomalies. The anomaly associated with the basin is due partly to the anomalies arising from the shields and partly to intra-basin deep-seated demagnetization resulting from Mesozoic thermal and metasomatic demagnetization. Another similar demagnetized zone probably contributes to the Reguibat Shield anomaly. The Man Shield anomaly results primarily from the Archean segment of this shield. The largest magnetization contrast required, for a 70-km-thick shield slab, is + 0.3 A m −1.