MORPHO-TECTONIC ANALYSIS OF DRAINAGE SYSTEMS ON SOUTHEASTERN ETHIOPIAN PLATEAU: EVIDENCE FOR DIFFERENT UPLIFT HISTORIES ON THE FLANKS OF THE AFAR DEPRESSION AND MAIN ETHIOPIAN RIFT

We used morpho-tectonic analysis utilizing the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Digital Elevation Model (DEM) to: (1) examine the incision history of the Shebelle (originating from the southern escarpment of the Afar Depression) and Genela (originating from the southeastern escarpment of the Main Ethiopian Rift) drainage systems in the Southeastern Ethiopian plateau; (2) use the incision history as a proxy for the uplift rates of the plateau in general and on the southern flank of the Afar Depression and the southeastern flank of the Main Ethiopian Rift; and (3) compare our findings with the incision history of the Blue Nile and the Tekeze river in the northwestern Ethiopian plateau and the uplift rates of the plateau. For this we have used the normalized steepness index, concavity and regression fit extracted from longitudinal profiles of major channels of the Shebelle and Genela drainage systems in addition to knickpoint analysis. We assumed that the incision of the two drainage systems on the Southeastern plateau started ~30 Ma because of the presence of volcanic rocks related to the Afar mantle plume on top of Mesozoic clastic and carbonate sedimentary rocks or Precambrian crystalline rocks. Hence, the outpouring of these volcanic rocks has buried drainage systems older than ~30 Ma. The presence of major knick faces along the longitudinal profiles suggest three regional tectonic uplifting events in the Southeastern plateau since ~30 Ma similar to what has been observed in the Northwestern plateau. The morpho-tectonic indices suggest an increase of incision rate from north to south towards the Afar Depression southern escarpment and from east to west towards the southeastern escarpment of the Main Ethiopian rift. Furthermore, the Shebelle and Genela drainage systems show different geomorphologic signatures, which mark a higher uplift on the southern escarpment of the Afar Depression compared to the southeastern escarpment of the Main Ethiopian Rift. Our results provide new insight for understanding uplifting history in the Southeastern Ethiopia plateau indicating the superposition of rift-flank uplift on the southern escarpment of the Afar Depression and the southeastern escarpment of the Main Ethiopian Rift on three-phase regional uplift of the Southeastern Plateau.