Spatial and Temporal Variation of Vertical Motions in the Congo River Catchment

Africa's 'basin and swell' topography is thought to be dynamically supported by mantle convection and correlates closely with long wavelength free-air gravity anomalies, which can be regarded as a proxy for the convective pattern. Seismic tomographic studies reveal a region of slow seismic velocity beneath sub-equatorial Africa. This region broadly correlates with a zone of anomalously high elevation, known as the 'African Superswell' that stretches from the South Atlantic Ocean across Africa to Afar. More recently temporal parameters such as uplift rate have been considered in dynamic topography modeling. However, theoretical predictions of dynamic topography have not been carefully tested against observations. The Congo River Catchment is of particular interest as its large catchment area drains from both negative and positive long wavelength free air gravity. The recent availability of global high-resolution topographic data (e.g. SRTM 90 meter) allows for detailed studies of geomorphological features of inaccessible areas such as the Congo Basin. DEMs show that drainage along the southern edge of the Congo River Catchment incises into a smooth, low relief surface. This surface slopes northwards from elevations of 1500m to 700m. Knickpoints on river profiles indicate that the drainage network is immature and may have experienced a recent change in base level. The low-relief surface represents the top of a group of fluviatile sediments, known as the sable ocres deposited in an aggrading environment. Topography of this surface correlates with long wavelength free-air gravity anomalies and is caused by mantle convection. This correlation is evident in an admittance analysis of gravity and topography in the frequency domain. The geology and geomorphology in this region can be used as a further test of uplift rates predicted from seismic tomographic data.