Depths to major density contrats beneath the West African rift system in Nigeria and Cameroon based on the spectral analysis of gravity data

Abstract

Estimates of depth to major density contrats within the lithosphere have been made, over the West African Rift System and adjacent basement areas in Nigeria and Cameroon, based on the spectral analysis of the Bouguer gravity field. The study reveals that three main density discontinuities occur in the depth ranges 7 to 12 km, 19 to 30 km and 80 to 93 km. The shallow crustal density discontinuities reflect a range of geological structures associated with the rift zones, shear zones and cratonic margins. The 19 to 30 km depth range is in good agreement with the Moho' depths determined by seismic refraction studies beneath and to the south of the Yola Rift in Cameroon and by East-West gravity modelling profiles across the Benue Trough in Nigeria. The deepest density discontinuities observed at 80 to 93 km depth are compatible with the presence of an anomalous low velocity upper mantle structure at these depths deduced from the inversion of teleseismic travel time residuals. The spectral analysis of gravity data can thus provide reliable first order estimates of depth to density discontinuities within the lithosphere beneath a rifted continental plate where such density variations would be expected.