SURFACE RESPONSE TO DYNAMIC TOPOGRAPHY WITHIN THE ETHIOPIAN PLATEAU, EAST AFRICAN RIFT SYSTEM

The Ethiopian Plateau, situated at the flank of the East African Rift System, is associated with the development of a prominent physiographic feature like the 1.6 km deep canyon carved by the Blue Nile river system, and has experienced a minimum of ~2 km uplift during the Cenozoic. The causes and mechanisms responsible for uplifting the plateau is poorly constrained. This study investigates spatial distribution of this uplift from quantitative geomorphology and mantle P-wave velocity anomaly, in light of teasing out the surface response to the uplift. Quantitative geomorphic data obtained from stream power law-based DEM analysis reveals meaningful pattern for determining transience of the Ethiopian Plateau landscape. Our analyses of a total of 202 convex upward and double concave tributary longitudinal profiles show the presence of distinct knickpoints and normalized steepness indices (k_sn) both upstream and downstream from knickpoints, indicating variable uplift within the plateau. The channel reaches with high k_sn values, positions of knickpoints and their upstream migration are not controlled by extensional faults or variations in bedrock strength, but by tectonic uplift that is dictated by ~500 km wide mantle low velocity region beneath the plateau with localized heterogeneity. Our statistical analysis of P-wave velocity and k_sn shows a broad zone of negative velocity anomalies where most of the k_sn values step up to >200-300. We suggest a positive correlation between elevated topography and high k_sn values corresponding to the low mantle velocity zones and that the channel reaches with these values are still adjusting by deeply incising the plateau. Our findings can bring fresh perspectives on how a rising mantle-plume contributes to the evolution of transient landscape in a non-orogenic setting.