Les anomalies gravimétriques de la boucle du Niger. Leur signification dans le cadre de l'orogenèse panafricaine

Abstract

The eastern margin of the west African craton along the Niger River in Mali is characterized by long wavelength, positive gravity anomalies. One of these anomalies, the Gourma anomaly, corresponds to the Gourma basin, a gulf of northeast–southwest subsidence, perpendicular to the Panafrican belt which presents all the characteristics of an aulacogen. The gravity anomalies show, under the upper Precambrian sediments, a thick layer of dense rocks, probably associated with pre-Panafrican rifting that occurred ca. 800–850 Ma ago along the eastern margin of the West African craton with a triple point in Mali.The extension of this structure inferred from Bouguer anomaly maps can be traced southward along more than 200 km north-northeast–south-southwest (Mopti anomaly). The gravity pattern shows that the shape of the subsiding basins is linked to preexisting fractures which have been rejuvenated during geological time. The subsidence, in part due to the weight of effusive and intrusive rocks, probably caused a downward deflection of the crust, explaining the negative anomalies that edge the Gourma and Mopti positive anomalies.