A regional gravity study of the West African rift system in Nigeria and Cameroon and its tectonic interpretation

Abstract

This study brings together existing and new gravity data to investigate the nature and cause of the Bouguer gravity field associated with the Cretaceous West African rift system in Nigeria and Cameroon. The new gravity measurements include data collected over the basement area between the Benue Trough and the Cameroon border and fill an important gap in the gravity coverage. After removal of the long wavelength negative anomaly from the observed gravity field the remaining positive gravity anomaly, associated with the rift system, is interpreted in terms of two and three dimensional crustal models. These models are constrained by crustal thicknesses derived from a seismic refraction study carried out across and to the south of the Yola rift. The results of simple three dimensional gravity modelling indicate the crust beneath the lower and middle Benue is approximately 20 km thick while beneath the Gongola rift the crust is approximately 25 km thick relative to a normal crustal thickness of 34 km away from the rift. Assuming the thinned crust associated with the rift system is the result of simple lithospheric stretching, then the maximum possible crustal extensions of 95, 65 and 55 km have taken place perpendicular to the Benue, Gongola and Yola rifts respectively. These crustal extensions do not necessarily reflect the total crustal movement affecting these rifts since the Benue and Yola rifts have been subjected to varying amounts of shear displacement during the Cretaceous and early Tertiary times. Crustal extension estimates across the West African rift system could be as much as four times greater than the published extension estimates across the East African rift system. These rift systems have the same plate tectonic setting being the failed third arms of successful triple junctions and are the result of extensional and shear tectonics. The differences in crustal extension and the resulting isostatic response of the lithosphere beneath these rifts can explain why the West African rift system has been associated with subsidence processes throughout its development whereas the East African rift system in Northeast Africa has been strongly affected by uplift.