An elastic finite-element analysis of the African intraplate stress field is used to determine constraints on the stress state resulting from variations in the gravitational potential energy of the lithosphere (U_l) produced by lateral density variations. The modeling is constrained by 150 stress indicators extracted from the World Stress Map Project data set (Zoback, 1992). Lateral variations in U_l are calculated using a simple lithospheric density model that is consistent with observed geoid anomalies across mid-ocean ridge and continental margins. Predicted tectonic stresses in the oceanic regions of the African plate range from tension along the mid-ocean ridges (9 MPa) to compression in the ocean basins (10 MPa). Continental regions near sea level are in a near-neutral state of stress, with large extensional stresses present in the Ethiopian highlands (15 MPa), the East Africa rift (9 MPa), and southern Africa (8 MPa). The general agreement between the predicted and the observed stress fields suggests that the principal long-wavelength features of the intraplate stress field, including the observed extension in eastern and southern Africa can be explained in terms of stresses arising from lithospheric density variations without appeal to poorly determined sublithospheric processes. The state of stress in continental regions with elevation greater than 70 m is predicted to be extensional, providing an alternative source of continental tension which has important implications for the dynamics of continental break-up.
Predicted tectonic stresses for African plate at selected element locations. Solid bars indicate compression, open arrows indicate tension. Maximum extension in Ethiopian highlands is 17 MPa (averaged over a 125 km--thick lithosphere), maximum compression in oceanic basins is 10 MPa. Dashed lines show location where Delta U_l is zero for three continental crustal density ($\rho_{c}$) values as indicated. Mid-ocean ridges and the continental regions have potential energy in excess of plate mean for $\rho_{c}$ in this range.