Blue Nile incision on the Ethiopian Plateau: Pulsed plateau growth, Pliocene uplift, and hominin evolution

Abstract

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 km 3 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-ofphase 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 >1 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.

Geology of Ethiopia: A Review and Geomorphological Perspectives

“…The morphotectonic history of the northern Ethiopian plateau has also been assessed by Gani et al (2007) using the long-term incision rate of the Blue Nile catchment. Their picture proposes that starting from a broad dome with slow rate of uplift from 29 to 10 Ma (phase I), a rapid rate of increase in the uplift occurred at 10 Ma (phase II) followed by a dramatic plateau rise at 6 Ma (phase III).…”

Geomorphology of the Lake Tana basin, Ethiopia

“…Igneous rocks comprise the Mio-Pliocene 'Trap Series' and the Quaternary 'Aden Series' (Gani, Gani, & Abdelsalam, 2007;Kazmin, 1972). The Trap series (mainly basalts) extruded from fissures and centres of flood lavas and built up a 500-1500 m thick volcanic pile (Mohr, 1964;Mohr & Zanettin, 1988).…”

The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE): Probing the transition from continental rifting to incipient seafloor spreading

“…10 Ma-present); this is postulated to be due to foundering of the plateau lithosphere by delamination or convective removal of lithospheric mantle (e.g., Duggen et al, 2003;Molnar et al, 1993) following extensive heating and weakening since the onset of fl ood basalt volcanism at ca. 30 Ma (Gani et al, 2007).…”