Gravity Field Modelling for Lithospheric Thickness and Flexural Rigidity Estimation in West Africa

This study applies forward and inverse gravity modelling to estimate lithospheric thickness and effective elastic thickness, a proxy for flexural rigidity, across the West African region with particular focus on the Nigerian shield and marginal basins. Lithospheric mechanical strength controls the isostatic response to surface and subsurface loads, affecting the amplitude and wavelength of gravity anomalies, the geometry of sedimentary basins, and the location of ore-forming fluid migration pathways. Current estimates of West African lithospheric rigidity are based on global models at inadequate resolution for the complex geological structure of Nigeria. This study utilises the GOCE satellite gravity field model (combined with terrestrial and shipborne observations using the least squares collocation method to produce an improved Bouguer gravity anomaly grid at 5-kilometre resolution for the study region. Isostatic residual anomaly computation using Airy and Pratt isostatic models is performed at varying crustal compensation depths. Spectral admittance and coherence methods applied to Bouguer anomaly and topographic data estimate effective elastic thickness as a function of position. Forward lithospheric flexure modelling reproduces observed basin geometry and isostatic anomaly patterns. Findings reveal effective elastic thickness values ranging from 5 to 15 kilometres over the Benue Trough and Chad Basin, consistent with Mesozoic-Tertiary rifting, compared to 35 to 60 kilometres over the Archean basement of the Jos Plateau, confirming the contrast between thermally weakened basin lithosphere and cold cratonic mantle lithosphere. The isostatic residual anomalies identify uncompensated loads consistent with late Cenozoic volcanic underplating beneath the Jos Plateau. The study contributes an original high-resolution lithospheric rigidity model for Nigeria.

Keywords: gravity modelling, lithospheric thickness, effective elastic thickness, West Africa, isostasy.