The Voltaian Basin in Ghana is interpreted as a foreland basin formed by sediments filling a flexural depression at the margin of the West African Craton. The flexure is due to northwest obduction onto the craton of younger, Pan African crust. This model is consistent with our knowledge of the Voltaian Basin from geology, seismic refraction profiles, and depths to magnetic basement. Major gravity and magnetic anomalies over the middle of the Voltaian Basin, approximately along 0o longitude, are interpreted as due to major structures in the West African Craton underlying the basin, the structures being older than the basin sediments. The anomalies could be due to either a combination of an elongate N–S striking change in crustal thickness or density in the craton and two large areas of mafic igneous rock or, preferably, to a major thrust within the West African Craton along a 300-km long boundary between crusts of different ages.