The mechanical behaviour of the lithosphere beneath the Chad Basin and the Bangui Magnetic Anomaly. Insights from Moho depth and effective elastic thickness estimates

Abstract

We used high-resolution gravity data to analyse crustal structure beneath the Bangui Magnetic Anomaly (BMA) and the Chad Basin within the North-Central African region. Moho depths and effective elastic thickness values, calculated respectively by a regularized non-linear inversion method and by analysis of the coherence function between gravity and topography, have been interpreted with reference to seismic data, previous gravity studies and regional geology. Our aim is to determine the crustal architecture and to analyse its mechanical behaviour. Estimated Moho depths range from 30 to 43 km in the North-Central African region. Beneath the uplift zones (Adamawa-Yade plateau, Ouaddaï, Tibesti, Hoggar, Tuareg shield) and the Congo Craton, the Moho depths range between 35 and 37 km. These values can be interpreted by tectonic and seismic events recorded along the regional shear zones; but also to the remobilization of the Precambrian basement. Analysis of the coherence function between the Bouguer gravity and the topography shows two different domains of rigidity: the highest values (100 km < Te < 180 km) are observed in Central African Republic, Chad, Niger, Algeria, and Egypt; the lowest values (20 < Te < 100 km) are observed in Cameroon, Nigeria, and Sudan. The lithosphere beneath the Chad Basin has a higher rigidity, similar to that observed in Central African Republic. In Chad and the Central African Republic, there is a positive correlation between the Te and the Moho depths; this indicates that cratonic remnant (Chad craton) beneath the Chad Basin is well preserved and covered by the sediments. The low Te values in Cameroon and the constant value in the Moho depth, are a proof of the reactivation of the Precambrian basement. This reactivation is related to the tectonic, seismic and thermal events occurred during the Eburnean and Pan-African orogenies. The contrast between the Te values in Cameroon compared to others regions (Chad, Central African Republic, Niger, Egypt, Algeria) show the magnitude of these events on the crustal architecture. This being defined by the presence of cratonic cores, the Cameroon Volcanic Line (CVL) and hot spots as well as large regional shear zones, explain the fact that thermo-tectonic and seismic events that occurred at the northern edge of the Congo Craton in Cameroon, are more intense than in the Central African Republic. So according to the geodynamic process in Cameroon and Central African Republic, related to the hot spots along the CVL, thermo-tectonic and seismic events recorded along the regional shear zones (Sanaga Shear Zone and the Central Cameroon Shear Zone) and the Congo Craton, the BMA is associated to two different geotectonic contexts. The sources of the BMA are closely linked to the Congo Craton; they bottom should not exceed −37 km depth. The present results agree with previous studies which allow us to understand the evolution of the Saharan metacraton and the preservation of cratonic remnants (Murzuq, Al-Kufrah, Chad); similarly the estimated Moho depths is an important step that would serve as an input to model the sources of the Bangui Magnetic Anomaly.