Morpho-tectonic analysis of the Tekeze River and the Blue Nile drainage systems on the Northwestern Plateau, Ethiopia
Highlights
► This study examined drainage incision and tectonic uplift of the Northwestern Plateau, Ethiopia. ► It focused on the Tekeze River and the Blue Nile drainage systems. ► Incision was moderate at 30 Ma driven by regional uplift related to the rise of the Afar mantle plume. ► Local increase of incision at 22 Ma was related to volcanoes built-up event within the plateau. ► At 11 Ma local increase of incision was related to flank uplift of rift systems to the east.
Introduction
Significant research has been conducted to understand the interplay between geomorphology and tectonics in continental collision regimes such as the Alps, the Himalayas, the Tibetan plateau, and the Colorado plateau (e.g. Pederson et al., 2002, Clark et al., 2004, Schoenbohm et al., 2004, Whipple, 2004, McMillan et al., 2006; Wobus et al., 2006; Cyr et al., 2010, Ferraris et al., 2012). However, less research effort has been devoted to understand the morpho-tectonic evolution of drainage systems in active continental extensional regimes such as the Ethiopian Plateau. The Ethiopian Plateau (Fig. 1) being one of the largest uplifted regions on Earth (the Wall of Africa) was formed during extensional tectonics associated with the Afar mantle plume and the opening of the East African Rift System which is represented in Ethiopia by the Afar Depression and the Main Ethiopian Rift (Fig. 1). This region provides the geoscientific community a unique opportunity to examine drainage evolution within continental extensional tectonic regimes and to examine the role of tectonic uplift in the evolution of regional drainage systems.
The Ethiopian Plateau is underlain by Precambrian crystalline rocks and Mesozoic sedimentary rocks, and topped by 1–3 km thick Oligocene–Quaternary volcanic rocks (Fig. 2). Through time, rivers have incised deeply into the Ethiopian Plateau forming in some places spectacular canyons that can be as deep as 1.5 km. The plateau is dissected by the Afar Depression and the Main Ethiopian Rift into the Northwestern Plateau, dominated by the Tekeze River and the Blue Nile drainage systems and the Southeastern Plateau occupied by the Wabi Shebelle and Ter Shet drainage systems (Fig. 1, Fig. 2).
Few studies have been carried out to examine drainage evolution in the Ethiopian Plateau (McDougall et al., 1975, Weissel et al., 1995, Pik et al., 2003, Gani and Abdelsalam, 2006, Gani et al., 2007). These studies have only focused on the Blue Nile and its gorge, the Gorge of the Nile (Fig. 2) which is arguably the rival of the Grand Canyon of the US. Two contrasting end-member models have emerged from these studies: (1) Steady and constant rate of incision of the Blue Nile on the Northwestern Plateau since the beginning of the incision ∼25–29 Ma ago (Pik et al., 2003); and (2) significant increase in the rate of incision of the Blue Nile at ∼10 and ∼6 Ma (Gani et al., 2007).
The Pik et al. (2003) steady–steady model relied on results obtained from Apatite/He ages of samples collected from the Precambrian crystalline rocks exposed along the lower reaches of the Blue Nile. Pik et al. (2003) concluded that the upper Blue Nile drainage system went through a long-term stability since the beginning of its incision ∼25–29 Ma. Additionally, Pik et al. (2003) suggested that the current hydrological characteristics of the Blue Nile drainage system were established early in the Oligocene time. This implies that the uplift of the Northwestern Plateau and its current physiographic features that control most of the present-day Blue Nile hydrology were established since the Oligocene.
Alternatively, Gani et al. (2007) suggested a model that involves an increased incision rate through time driven by pulsed plateau uplift. Gani et al. (2007) results were based on Geographic Information System (GIS) modeling of the Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) data and ages of volcanic rocks that occur at different elevations in the Northwestern Plateau. In this model, Gani et al. (2007) proposed three phases of incision driven by tectonic uplift with an increased rate of incision through time. The first phase of incision, which dominated the period between 29 and 10 Ma, was characterized by slow and long-term incision that gradually increased from 53 m Ma−1 to 80 m Ma−1. The second phase which occurred between 10 and 6 Ma was characterized by moderate rate of incision that increased from 80 m Ma−1 to 120 m Ma−1. The most recent incision phase occurred between 6 Ma and present where the long-term incision rate increased rapidly from 120 m Ma−1 to 320 m Ma−1. Gani et al. (2007) concluded that much of the increase in the incision rate can be attributed to tectonic uplift, possibly associated with the rise of the Afar mantle plume, rather that climate change. This conclusion is based on that the timing of the increase of the incision rate of the Blue Nile in the Northwestern Plateau having been out-of-phase with any enhanced precipitation in the region. Gani et al. (2007) based this conclusion on that during the late Miocene to Pliocene (8–3 Ma) east Africa was undergoing aridification as documented by a shift from wood-dominated vegetation to grass-dominated vegetation (Cerling et al., 1997), paleo-climatic data (Cane and Molnar, 2001) and numerical modeling on the link between the rising topography of east Africa during the late Miocene and aridification (Sepulchre et al., 2006).
This work aims at understanding the morpho-tectonic evolution of the drainage systems in the Northwestern Plateau by focusing on the Tekeze River and the Blue Nile. It relies on GIS analysis of DEMs extracted from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and SRTM data, and published geological information. Specifically, this work: (1) Presents the geological context of the Tekeze River and the Blue Nile drainage systems; (2) Use morpho-tectonic analysis including: (A) The delineation of sub-basins of the two rivers and the calculation of their Normalized Steepness Index (Ksn) to evaluate the effect of tectonic uplift on the evolution of these sub-basins; (B) the extraction of longitudinal profiles and the computation of the Concavity (θ) to evaluate the relative effect of tectonic uplift on the incision rates of these tributaries. (C) The calculation of the Regression Fit (r2) between the observed and predicted channel profiles of the Tekeze River and the Blue Nile tributaries from the longitudinal profiles and the θ values to evaluate the maturity of these channels. (3) Use these results to discuss the evolution of the Tekeze River and the Blue Nile drainage systems on the Northwestern Plateau.
Section snippets
Overview of the Tekeze River and the Blue Nile
The Tekeze Rive and the Blue Nile drainage systems are located between latitudes ∼8°30′ and ∼15°00′N and longitudes ∼36°00′ and ∼40°00′E (Fig. 3). The length of Tekeze River in Ethiopia is ∼608 km and its average elevation is ∼1850 m above sea level (asl). The river flows in a north direction from its southernmost reaches, then circles Mount Ras Dashen in a semi-circular loop by flowing northeast, then north, then northwest before descending towards the low lands of the Sudan, (Fig. 3). The
Tectonic and uplift history
Three tectonic and geological events have shaped the present morphology of the Northwestern Plateau. The oldest of these events was the eruption of the ∼30 Ma volcanic rocks which covered an area of ∼500,000 km2 and it is inferred to mark the appearance of the Afar mantle plume (Mohr and Zanettin, 1988, Hofmann et al., 1997, Sengor, 2001, Kieffer et al., 2004). This event results in the burial of whatever drainage system that dominated the region prior to the Oligocene. It is likely that the
Geology
Fig. 5A and B are a generalized geological map of the Tekeze River and the Blue Nile (Tefera et al., 1996) and a generalized geological section across the two rivers. The Northwestern Plateau, the focus of this study, is underlain by Precambrian crystalline rocks which are regionally overlain by a Mesozoic sedimentary section with various thicknesses constituting the Jurassic Adigrad sandstone, the Late Jurassic Abay and Antalo limestone, and the Cretaceous Amba Aradom sandstone. However, in
Methods and results
This study focuses on the extraction of morpho-tectonic parameters of the Tekeze River and the Blue Nile including: (1) the Normalized Steepness Index (Ksn); (2) the Concavity (θ) values of the longitudinal profiles; and (3) the Regression Fit (r2) between the observed and predicted channel longitudinal profiles.
Discussion
Morpho-tectonic analysis of the Tekeze River and the Blue Nile suggests that the incision history of these drainage systems on the Northwestern Plateau is much more complicated than can be explained by either the steady-state incision model of Pik et al. (2003) or the increased incision rate through time model that is suggested by Gani et al. (2007). It is suggested here that the incision history of these drainage systems was influenced by a number of tectonic and geological events including:
Conclusions
Analysis of ASTER and SRTM DEMs to extract morpho-tectonic parameters of tributaries of the Tekeze River and the Blue Nile including the Normalized Steepness Index (Ksn), Concavity (θ), and Regression Fit (r2) between the observed and predicted channel profiles suggests that the evolution of these rivers drainage systems on the Northwestern Plateau was influenced by three tectonic and geological events. (1) The first event which is characterized low to moderate incision rate on the entire
Acknowledgements
This research was made possible through funding provided by the National Science Foundation, Office of International Science and Engineering, International Research Experience for Students (NSF-OISE-IRES) program Grant Number 0927906. We thank S. Drury and an anonymous reviewer for detailed and constructive reviews.
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