Blue Nile Incision on the Ethiopian Plateau: Pulsed Plateau Growth, Pliocene Uplift, and Hominin Evolution

The 1.6-km-deep Gorge of the Nile, a rival of the Grand Canyon, resulted from the deep incision of the Blue Nile drainage into the uplifted Ethiopian Plateau. Understanding the incision history of the plateau is crucial to unraveling the Cenozoic tectonoclimatic evolution of the region, particularly because the region has long been used as a natural laboratory to understand the geodynamics of continental rifting and the evolution of hominins. We undertake a quantitative geomorphologic approach integrating field, geographic information system (GIS), and digital elevation model (DEM) data to analyze incision (volume, long-term rates, and spatiotemporal variability) and river longitudinal profiles of the Blue Nile drainage. Previously published isotopic ages of the Cenozoic volcanic rocks are used to constrain long-term incision rates through geologic time. Our data argue that (1) the Blue Nile drainage has removed at least 93,200 km3 of rocks from the northwestern Ethiopian Plateau since ca. 29 Ma (early Oligocene) through a three-phase (ca. 29-10 Ma, ca. 10-6 Ma, and ca. 6 Ma to present) incision, where long-term incision rates increased rapidly and episodically in the late Miocene (ca. 10 Ma and ca. 6 Ma); (2) being out-of-phase with the past climatic events and in-phase with the main volcanic episodes of the region, this episodic increase of incision rate is suggestive of episodic growth of the plateau; (3) of the ~2-km rock uplift of the plateau since ca. 30 Ma, 0.3 km was due to isostatic uplift related to erosional unloading, and the rest was due to other tectonic activities; (4) the extremely rapid long-term incision rate increase, thus a rapid uplift of the plateau, ca. 6 Ma might be related to lithospheric foundering, caused by ponded plume material beneath the Ethiopian Plateau and aided by huge tectonic stresses related to the Messinian salinity crisis of the Mediterranean Sea. These events could have caused the plateau to rise \u3e1 km within a few m.y. in the early Pliocene. This uplift history of the Ethiopian Plateau can shed critical light on the geodynamics of the Afar mantle plume and the evolution of the East African hominins via climate change

Remote Sensing Analysis of the Gorge of the Nile, Ethiopia with Emphasis on Dejen-Gohatsion Region

Digital Elevation Models (DEMs) extracted from the Shuttle Radar Topography Mission (SRTM) with 90 m x-y resolution, and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) with 15 m x-y resolution have been used in conjunction with ASTER and RADARSAT images, and field studies to extract geological and geomorphological information in order to understand the geological controls on the Gorge of the Nile in Ethiopia. The Blue Nile on the NW Ethiopian Plateau forms a ~150 km bend and carves the ~1.6 km deep Gorge of the Nile. The river shows a dramatic drop in gradient from ~4 m/km to ~0.42 m/km as it spirals around Tertiary to Quaternary shield volcanoes. A ~1200 m thick section of Mesozoic sedimentary rocks bounded between Tertiary and Quaternary volcanic rocks and Neoproterozoic basement rocks is exposed within the Gorge of the Nile. Our work shows that: (1) SRTM DEMs are effective for the characterization of 3D spatial relationships between the river\u27s course and regional geomorphological features such as Tertiary to Quaternary shield volcanoes, Afar Depression and the Main Ethiopian Rift. These DEMs are also useful in extracting the river\u27s geometric properties and the analysis of drainage network. (2) ASTER band (7-3-1) and band-ratio (4/5-3/1-3/4) images better resolve lithological units and lithologically defined structures. (3) The side-looking geometry of the Standard Beam RADARSAT data is effective in mapping morphologically defined structures due to radar shadow-illumination effect. (4) Fusion of ASTER and RADARSAT data using Color Normalization Technique (CNT) enhances the mapping ability because the fused image preserves the spectral information of ASTER data and incorporates terrain morphological characteristics from RADARSAT data. (5) Three dimensional (3D) perspective views generated by draping ASTER images over ASTER DEMs are effective in mapping sub-horizontal lithological units such as those dominating the Gorge of the Nile. These perspective views are also effective in highlighting lithologically defined structures. This study also shows a number of possible geologic controls in the evolution of the Gorge of the Nile. Base-level adjustment due to regional uplift, spatial distribution of shield volcanoes, obstruction and diversion due to bed rock structures, and differential incision due to varying lithology have significant roles in deep carving of the Gorge of the Nile, deflection of the course of the Blue Nile, and in producing a diverse drainage network

Stratigraphic and Structural Evolution of the Blue Nile Basin, Northwestern Ethiopian Plateau

The Blue Nile Basin, situated in the Northwestern Ethiopian Plateau, contains ~1400 m thick Mesozoic sedimentary section underlain by Neoproterozoic basement rocks and overlain by Early-Late Oligocene and Quaternary volcanic rocks. This study outlines the stratigraphic and structural evolution of the Blue Nile Basin based on field and remote sensing studies along the Gorge of the Nile. The Blue Nile Basin has evolved in three main phases: (1) pre-sedimentation phase, include pre-rift peneplanation of the Neoproterozoic basement rocks, possibly during Palaeozoic time; (2) sedimentation phase from Triassic to Early Cretaceous, including: (a) Triassic-Early Jurassic fluvial sedimentation (Lower Sandstone, ~300 m thick); (b) Early Jurassic marine transgression (glauconitic sandy mudstone, ~30 m thick); (c) Early-Middle Jurassic deepening of the basin (Lower Limestone, ~450 m thick); (d) desiccation of the basin and deposition of Early-Middle Jurassic gypsum; (e) Middle-Late Jurassic marine transgression (Upper Limestone, ~400 m thick); (f) Late Jurassic-Early Cretaceous basin-uplift and marine regression (alluvial/fluvial Upper Sandstone, ~280 m thick); (3) the post-sedimentation phase, including Early-Late Oligocene eruption of 500-2000 m thick Lower volcanic rocks, related to the Afar Mantle Plume and emplacement of ~300 m thick Quaternary Upper volcanic rocks. The Mesozoic to Cenozoic units were deposited during extension attributed to Triassic-Cretaceous NE-SW-directed extension related to the Mesozoic rifting of Gondwana. The Blue Nile Basin was formed as a NW-trending rift, within which much of the Mesozoic clastic and marine sediments were deposited. This was followed by Late Miocene NW-SE-directed extension related to the Main Ethiopian Rift that formed NE-trending faults, affecting Lower volcanic rocks and the upper part of the Mesozoic section. The region was subsequently affected by Quaternary E-W and NNE-SSW-directed extensions related to oblique opening of the Main Ethiopian Rift and development of E-trending transverse faults, as well as NE-SW-directed extension in southern Afar (related to northeastward separation of the Arabian Plate from the African Plate) and E-W-directed extensions in western Afar (related to the stepping of the Red Sea axis into Afar). These Quaternary stress regimes resulted in the development of N-, ESE- and NW-trending extensional structures within the Blue Nile Basin