Controls on eroded rock volume, a proxy for river incision, in Africa

A database of 3023 river basins in Africa was assembled to investigate the relative effects of uplift and erosion on landscape development. The volume of rock eroded from each basin was calculated by integrating the difference between a topographic summit envelope fit across drainage divides and interfluves, and present-day topography over basin area. Africa is an excellent natural laboratory for this procedure because drainage patterns reflect the Neogene development of topographic basins and swells, themselves surficial manifestations of sub-lithospheric mantle convection. As a result, the loci of major offshore deltas and drainage divides have remained largely static, while epeirogenic (vertical) surface motions are more important than shortening. Eroded rock volume is presented as a proxy for fluvial incision and correlates strongly with long-wavelength gravity anomalies across Africa, but not with mean precipitation, which was calculated by merging satellite estimates with rain gauge data. This finding implies that spatial variations in epeirogenic uplift govern landscape evolution across the continent. Other variables that alter drainage basin geometry and the magnitude of eroded rock volumes, such as varying climate, bedrock erodibility, and drainage capture, are likely subordinate to these variations. First-order estimates of eroded rock volume onshore are potentially the most accurate indicator of offshore sedimentation because they implicitly include information pertaining to basin area and relief, which together control sediment load.