The upper mantle seismic velocity structure of South-Central Africa and the seismic architecture of Precambrian lithosphere beneath the Congo Basin

The seismic architecture of the lithosphere beneath the Congo Basin is investigated using a new shear wave velocity model of the upper mantle for central and southern Africa derived from an inversion of Rayleigh wave group velocity measurements. The model is similar to other tomographic models derived from Rayleigh wave phase velocities, revealing a region of fast upper mantle velocities in the 50-100 km depth interval beneath the northwestern, central and southern portions of the basin, and slower upper mantle velocities beneath the northeastern part of the basin, as well as beneath Proterozoic mobile belts to the east and south of the basin. The upper mantle velocity pattern indicates that Proterozoic lithosphere may lie beneath the northeastern side of the basin, but it does not support the presence of Proterozoic lithosphere beneath the entire northern portion of the basin. This finding suggests that lithospheric structure beneath the basin is not uniform, as is commonly assumed in geodynamic models explaining how the basin formed. A second geodynamic implication concerns the Neoproterozoic rifting event that may have initiated basin subsidence. The proposed locations of the rifts are in the region of the velocity model where the velocities begin to change from faster to slower going from the center of the basin toward the northeast. Thus, the rifts may have formed along the border between two separate, smaller Archean blocks, as opposed to within the middle of a single, larger Archean block, alleviating the need to explain how a Neoproterozoic rift might form within the interior of a large Archean craton.