Geological evolution and incision history of the Gorge of the Nile on Ethiopian Plateau from remote sensing and geographic information system analysis, and field studies

Integrated remote sensing and Geographic System Analysis (GIS) analysis has proven to be an effective approach in geological studies of vast, inaccessible areas with dispersed field information. Such an approach has been undertaken to study the 1.6 km deep Gorge of the Nile formed by the Blue Nile, situated on the Northwestern Ethiopian Plateau. The analysis of Digital Elevation Models (DEMs) extracted from the Shuttle Radar Topography Mission (SRTM) and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) for river geomorphic information shows a dramatic decrease in average gradient and an increase in valley width, depth and asymmetry from headwater to downstream along the Gorge of the Nile. The integration of orbital-optical and radar remote sensing data including ASTER, Landsat Thematic Mapper (TM), RADARSAT, and DEMs with field data to trace lithological units and structural features of the study area has helped in delineating geologic history of the area as well as the geologic controls on the formation of the Gorge of the Nile.; This study has revealed that the Blue Nile Basin evolved in three main phases: (1) Pre-sedimentation phase involving pre-rift peneplanation of the Neoproterozoic basement rocks; (2) Post-rift, Mesozoic sedimentation with repeated marine transgressions and regressions, including fluvial sandstones, and marine limestone and glauconitic mudstones; (3) Post-sedimentation phase which involve extensive volcanism from Middle Oligocene to Quaternary related to the Afar mantle plume. These stratigraphic units are deformed by normal faults of NW and NE-trends with a series of horsts and grabens that are thought to be formed by polyphase deformation and/or stress compartmentalization, indicating that the Blue Nile Basin is located within an extensional regime.; Analysis of DEMs with GIS has been used to study the timing, pattern, depth and rate of incision in the evolution of the Gorge of the Nile as well as the Blue Nile drainage system within the active epiorogenic regime. This work has shown that the incision initiated no less than 30 Ma and experienced three major tectonic-driven incision phases: (1) Slow and steady incision of phase I (30-10 Ma); (2) An intermediate rate of incision of phase II (10-6 Ma); and (3) Very rapid incision of phase III (6 Ma-present) with a drastic increase in incision rate around 6 Ma. These incisions are thought to be driven by the Afar Mantle plume related volcanism, rift-flank uplift associated with the Main Ethiopian Rift and the Afar Depression, and to an increased tectonic movements associated with the Messinian Salinity Crisis. Therefore, it is likely that the Ethiopian plateau has undergone a pulsed growth that is also supported in the long-profile analysis of the tributaries of the Blue Nile. This study also suggests that headward incision of the Blue Nile drainage system largely controlled by the spatial and temporal distribution of shield volcanoes.