Late Paleogene - Early Neogene Abandoned Rift along the River Nile, Egypt
by Ali Abdelkhalek
Date of Examination:2021-02-23
Date of issue:2021-04-30
Advisor:Prof. Dr. Jonas Kley
Referee:Prof. Dr. Jonas Kley
Referee:Prof. Dr. Mohamed Saleh Hammed
Referee:Dr. Reinhard, Gast
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EnglishThe River Nile is one of the major ﬂuvial systems in the world. The Egyptian Nile has been the most studied part of the river. However, the origin, tectonic style and geodynamic evolution of the Nile are still widely questionable. Several studies have interpreted the river as an erosional feature, while few others believe that its origin might be connected or related with intracontinental rifting processes. It was also suggested that the Nile and its drainage systems were originated by gradual uplift, volcanism and dynamic topography. These processes were, in turn, rolled by the geodynamics of deep mantle plumes. This study suggests a new rift model for the River Nile in Egypt, in which the results of multiple surface and subsurface datasets were used. The results of interpretation and analysis of gravity, magnetic, and recently acquired high-resolution 2D and 3D seismic reflection datasets were integrated with ﬁeldwork studies and processing algorithm techniques of multispectral satellite images and radar data (e.g. Landsat-8, Sentinel-2, ASTER and ALOS-PALSAR) as well as interpretation of Digital Elevation and Terrain Models (DEMs & DTMs) of different spatial resolutions (~225, 90, 30, 12.5 m) (e.g. SRTM, AW3D30 and TanDEM-X). The onset of the main rifting phase that controlled the evolution of the Nile Valley and its drainage was contemporaneous with the initiation of the Red Sea-Gulf of Suez and East African rift systems during the Oligocene ~30-28 Ma, and the concomitant regional uplifting processes, terrain exhumation and volcanism of the eastern Red Sea-Gulf of Suez hills. Both the ancestral and present-day drainage systems of the River Nile in Egypt have probably evolved along the Late Paleogene-Early Neogene rift system of NW to NNW orientation. The proposed rift segments and their bounding faults might have been aborted at a very early “embryonic” stage. Interpretation of gravity and magnetic anomaly models combined with reflection seismic data revealed structures and crustal thickness variations suggesting that several rift basins occur beneath and around the modern Nile Valley. Three main Late Paleogene-Early Neogene rift segments-trending NW and NNW can be characterized along the eastern and western shoulders of the river. These rift segments and their bounding faults were rejuvenated along NW- to NNW-oriented intracontinental half-graben basins that formed earlier during the Early Cretaceous rifting. The rift segments are separated and terminated by oblique/orthogonal E-ENE to NE-trending transfer/accommodation zones. The central and northern segments are geometrically well defined and semi-parallel to the Late Oligocene Red Sea- Gulf of Suez rift system with a dominant NNW trend, while the southern segment has a prevalent NW orientation with an oblique complex-pattern as a result of the interference between pre-existing structures of E-W, NE, N-S and NW trends. An extensive network of widespread normal faults, related folds, fractures, linear narrow grabens and volcanism accommodated the deformation along the Nile Valley. The NW- to NNW-oriented drainage systems of the ancestral Nile in Egypt have probably originally evolved along these intracontinental rift segments. Evidences were found of the “reactivated-rift” along numerous grabens within the eastern and western Eocene limestone plateaux of the modern Nile Valley; among them are the early syn-rift gravels, fluvial clastics and red beds of Oligocene age, hundreds of NW and NNW-striking extensional faults, fractures, extensional fault-related folding, and Oligo-Miocene age rift-related basalt dikes and flows at ~25-23 Ma. In addition, NW elongated “forced anticlinal folding” could be mapped within the faulted shoulders of the rift segments. These are made up of steeply dipping and rotated pre-rift Eocene and Cretaceous rocks, and were formed by local uplift, which in turn was driven by vertical uprising of the volcanic intrusions. Furthermore, analysis of high-resolution DEMs revealed a network of km-scale inverted Oligocene gravel-filled channels within the Eocene carbonates of the Nile Valley. Several channel outcrops are covered with a thick layer of dust and other debris where it is quite hard to characterize them, even on high-resolution Google Earth imagery. However, the results of applying spectral reflectance analysis, bands rationing and principal components on satellite data were utilized to distinguish the Oligocene gravel and red sandstone bodies from the hosting carbonate rocks. Present- day inverted channels are useful to reconstruct the fluvial depositional system and possible flow direction(s) of the paleo-rivers that filled the Nile grabens and fault blocks during the very early phase of Oligocene intracontinental rifting. The control of pre-existing structures on the evolution of the proposed Nile rift and the related volcanism is shown by the inherited Precambrian basement structures and morphotectonic features The RN in Egypt offers a good opportunity to study this approach. Analysis of gravity, magnetic, seismic and field data shows a prominent impact of the structural inheritance and morphology of former rifts on the development and geometry of the abandoned Oligocene rift along the RN. It is proposed that the initiation of the river and its drainage was controlled by intracontinental rifting during the Oligocene, whereas its evolution, segmentation and termination were influenced by basement structures and pre- existing rift-related faults/folds of the former Cretaceous basins. The locations of both rift segments and transfer/accommodation zones were strongly influenced by older Mesozoic rift basins. The Late Oligocene Nile rift segments were rejuvenated along intracontinental rift basins and their bounding faults that formed originally during the Early-Late Cretaceous rifting (from south to north; Kharit, Nuqra, KomOmbo, Assiut and Beni Suef basins). Reactivation of NW-trending Precambrian basement shears (Najd-Kharit Trend) controlled the regional geometry of the southern rift segment. Highly oblique E-W to ENE-trending dextral shear zones along Wadi Assiuty and Qena dome represent two main transfer zones; across them the rift segments and their marginal faults show flipping of their positions and polarities. Further to the north, NE-ENE- oriented uplifted ridges, folds, dextral shears, and thickening of the pre-rift Mesozoic sediments terminated the northward propagation of the rift. Most of these features were nucleated during the Early- Late Cretaceous extension phases in addition to the control of the successive convergence, basin inversion and Syrian Arc wrench tectonics in the Late Cretaceous. The Neogene Nile in Egypt consequently evolved along a NW to NNW-trending short-lived, intracontinental rift that was initiated by NE-SW extension during the Oligocene ~25-23 Ma, and was abandoned at an early stage. Possible reasons for the rift abandonment are: (1) Progressive concentration of the extension and strain/stress localization onto the Red Sea-Gulf of Suez rift in the east. The transfer of movement on the latter rift system resulted in abortion of the evolving structures in central Egypt, and consequently led to the abandonment of an embryonic rift. (2) Strain/stress dissipation over wide rift shoulders >300 km. (3) Obliquity of many reactivated pre-existing faults to the ~23 Ma regional (N60˚E) extension, and subsequent termination of newly developed rift-parallel faults. (4) Rifting confined to thicker (more brittle?) crust along the central part of the Nile. (5) Further structural control is that the northern rift termination and abortion coincide with oblique ENE to NE-trending “Syrian Arc” structures of the Bahariya-Fayium fold-fault bet, major basalt flows, and northward thickening of the Mesozoic pre-rift sediments of the ENE and NE half-graben basins of the north Western Desert along the Tethyan shelf. Moreover, an increase in the strength of the lithosphere across the Tethyan continental margin could have acted as a barrier to the northward propagation of the rift.
Keywords: River Nile; Continental Rifts; Structural Modelling; Extensional Tectonics; Abandoned Rifts; Tectonic Evolution; Gulf of Suez; Red Sea; East African Rift; Dynamic Topography