GEOPHYSICAL INVESTIGATION OF THE TRANSITION ZONE BETWEEN THE CONGO CRATON AND THE KRIBI-CAMPO SEDIMENTARY BASIN (SOUTHWESTERN CAMEROON)

Abstract

Three different sets of geophysical data are used to investigate the structure of the transition zone between the north-western boundary of the Congo Craton (CC) and the Kribi-Campo sedimentary basin. These are shear wave velocity, gravity and magnetotelluric (MT) data, respectively. The combined use of these data helps to reduce the non-uniqueness of solutions inherent to forward models, leading to a better inference of the structure of the transition zone. The interpretation of shear wave velocity models enhanced the difference in composition of upper crustal layers against the similar composition (densities of ~3.0 g/cm 3 ) of the lower crust, with different thicknesses beneath both tectonic regions. The analysis of gravity maps obtained from the area shows the signature of the main geological units and particularly the north-south gradient correlating with the strike direction of the so-called Kribi-Campo fault (KCF). Using constraints from shear wave velocity models together with surface geology observations, a 2D1/2 gravity model was obtained along a profile crossing the CC margin and the Kribi-Campo sedimentary basin. The new model is consistent with the presence of a thick mafic layer (>10 km) at depth below 18 km beneath both geological units, and also with crustal thicknesses of 28 km and 45 km beneath the basin and CC, respectively. The model also suggests that KCF resulted from the intrusion of magmatic rocks during the continental collision, which were later metamorphosed into granulites. The resulting suture may be interpreted from the model as the thrusting of the Panafrican Mobile Belt rocks onto the CC. Similar conclusions are inferred from the MT model and the signature of the above mentioned presence of granulites is interpreted in this case as low resistive rocks emplaced into high resistive materials. This provides additional support for the gravity model.