Gravity Anomaly Modelling for Crustal Structure Determination in the Southwestern Nigeria

This study models Bouguer gravity anomalies to determine crustal structure along a north-south transect through southwestern Nigeria, from the basement complex into the Dahomey Basin. Knowledge of crustal structure and Moho depth is fundamental for understanding the tectonic evolution of the West African craton and for assessing the hydrocarbon potential of the marginal Dahomey Basin. This study compiles and reprocesses available Bouguer gravity data for a 400-kilometre transect spanning from Ogbomoso, Oyo State to Badagry, Lagos State, supplemented by 22 new gravity station measurements at specific geological boundaries. Standard corrections are applied and the Bouguer anomaly is computed. Regional trend analysis and upward continuation separate deep crustal signals from shallow anomalies. Two-dimensional forward modelling with the Talwani method is performed using GMSYS software, constrained by seismic refraction velocity models from the West African literature. Layer densities are assigned from published petrophysical data for West African rocks. Findings reveal a Moho depth ranging from 36 kilometres beneath the basement complex to 24 kilometres at the Dahomey Basin margin, consistent with continental thinning at the passive margin. A mid-crustal density boundary at 18 to 22 kilometres depth is interpreted as the transition between upper and lower crust. The Bouguer anomaly increases southward from minus 60 milligals over the basement complex to minus 10 milligals over the coastal basin, tracking the crustal thinning and sedimentary loading effects. The study contributes a quantitative crustal model for southwestern Nigeria with implications for geothermal resource assessment and tectonic interpretation.

Keywords: Bouguer gravity, crustal structure, Moho depth, Dahomey Basin, southwestern Nigeria.