EFFECTIVE ELASTIC THICKNESS AND CRUSTAL THICKNESS VARIATIONS IN WEST CENTRAL-AFRICA INFERRED FROM GRAVITY-DATA

The west central African region is characterized by various geological features: Cretaceous rifts (Benue), Tertiary domal uplift (Adamawa volcanic uplift), Tertiary-Recent volcanoes (Cameroon Volcanic Line or CVL), Tertiary sedimentary basins (Chad basins), and cratonic region (Congolese craton). In this study, we investigate the relationship between these tectonic features and the flexural rigidity of the lithosphere in Cameroon, in terms of effective elastic thickness (T-e), by the use of the coherence function analysis. For that purpose, we use a new dataset of similar to 32,000 gravity and topography points from Cameroon and the adjacent countries. The T-e contour map deduced from this study shows a good relationship between the tectonic provinces and the rigidity of the lithosphere, the minima (14-20 km) are beneath the active rifted and volcanic areas (Benue, CVL, and Adamawa), and the maxima (similar to 40 km) correspond to the Archean reworked unit in Congo. A spectral analysis of the gravity data is performed to determine the crust-mantle boundary in these tectonic provinces. The crustal thickness (T-c) contour map shows a variation from 14 km to about 45 km, consistent with other geophysical data. The lower values (24-20 km) are obtained in central Cameroon on the Adamawa uplift, and the highest values are found in southern Cameroon (Archean reworked Congolese craton). Comparing T-e and T-c values shows that there is generally a positive correlation between the two parameters, with an exception in Chad where this correlation is rather negative.