Role of Dynamic Topography in Sustaining the Nile River Over 30 Million Years
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ARTICLES https://doi.org/10.1038/s41561-019-0472-x Role of dynamic topography in sustaining the Nile River over 30 million years Claudio Faccenna 1,2,3*, Petar Glišović2,4, Alessandro Forte 5, Thorsten W. Becker 2,3, Eduardo Garzanti6, Andrea Sembroni1 and Zohar Gvirtzman7,8 The Nile is the longest river on Earth and has persisted for millions of years. It has been suggested that the Nile in its present path is ~6 million years old, whereas others argue that it may have formed much earlier in geological history. Here we present geological evidence and geodynamic model results that suggest that the Nile drainage has been stable for ~30 million years. We suggest that the Nile’s longevity in essentially the same path is sustained by the persistence of a stable topographic gradient, which in turn is controlled by deeper mantle processes. We propose that a large mantle convection cell beneath the Nile region has controlled topography over the last 30 million years, inducing uplift in the Ethiopian–Yemen Dome and subsidence in the Levant Sea and northern Egypt. We conclude that the drainage system of large rivers and their evolution over time can be sus- tained by a dynamic topographic gradient. he drainage systems of major rivers and their evolution over Messinian salinity crisis (∼5.8 Ma) (refs. 16–19), and that at earlier time represent fundamental, yet poorly understood, problems times detritus at the delta was derived only locally, from the west- Tin geomorphology and hydrology1–4. This is because the laws ern margin of the Red Sea (Red Sea Hills)19. In equatorial Africa, that govern landscape evolution and the landscape’s response to tec- the fluvial network was believed to be directed westward as part tonics and climate are still poorly known. Among transcontinental of a palaeo-Congo continental-scale drainage system until the late drainage systems, many rivers are sourced from orogenic belts and Miocene onset of the East African Rift (∼11–5 Ma), when river flow high plateaux (for example, the Amazon, Tigris–Euphrates, Ganga– was diverted northward20. In this context, the uplift of Ethiopia Brahmaputra and Yangtze). Others, including the Nile, the Orange would be a recent feature21. and the Yenisei, flow instead from intraplate swells generated by Here, we present geological and geophysical arguments support- plume-related upwellings3. Such rivers drain large igneous prov- ing the idea that the Nile has been sustained by a mantle ‘conveyor inces in their upper reaches and are thus characterized by basaltic belt’ operating through most of the Tertiary, with a convective detritus with distinct petrographic, mineralogical and geochemical upwelling centred under the Ethiopian highlands and a downwelling signatures5. The origins of some of these long-lived rivers have been under the eastern Mediterranean, creating a topographic gradi- discussed and, for the Amazon River6 and rivers of Australia7–9 and ent that supported the Nile’s course over ~30 Myr. Such a course, Africa9,10, a link between dynamic topography, uplift history and which is similar to the present-day one, was likely established in river profiles has been explored. the early Oligocene (∼30 Ma). Before that, our modelling shows that Here, we propose that the drainage of one of the longest rivers on the drainage pattern was probably directed northwestward and con- Earth, the Nile, is indeed controlled by topography related to mantle trolled by the rifting process occurring in the Gulf of Sirte. dynamics (that is, dynamic topography). The Nile flows for more than 6,800 km; it crosses the largest desert on Earth, and it served Source-to-sink geological evidence as a cradle of civilization in the Stone Age (Fig. 1). The Blue Nile As Herodotus inferred in the fifth century bc22, Nile sediments and Atbara branches start on the volcanic Ethiopian Plateau and chiefly derive from the Ethiopian highlands. Before the closure join into the White Nile before discharging into the Mediterranean of the Aswan High Dam in 1964, the Nile Delta was receiving Sea, where a large delta is present. The Nile cuts through a topo- 230 ± 20 × 106 tons yr-1 of sediment, ~95% of which derived from graphic swell forming six cataracts and has a gentle gradient in its Ethiopian highlands tributaries (Blue Nile, 140 ± 20 × 106 tons yr-1, lowest portion. and Atbara, 82 ± 10 × 106 tons yr-1) (refs. 5,23). Extrapolating the Long-running debates about the birth and life of the Nile have present-day sediment flux to the past is not straightforward2,24. For focused on the question of whether the present-day Nile represents example, the dramatic sea-level fall during the Messinian salinity the evolution of the proto-Nile that has flowed from the Ethiopian crisis (∼5.8 Ma) produced canyons along the course of the early highlands since the early Tertiary period. Two competing models ‘Eonile’ incised to a depth of ∼1.5 km under Cairo and ∼170 m have been proposed: the first envisions a long-term river discharge below the present sea level at Aswan25. from Ethiopia since the Oligocene epoch (∼30 million years ago Sediment analysis from Nile Delta cores documents the con- (Ma)) (refs. 11–15); the second suggests that the Mediterranean was nection from the Ethiopian highlands to the Nile Delta extending connected with Ethiopia only in the late Miocene epoch after the back to the Oligocene26. Those sediments include the occurrence of 1Laboratory of Experimental Tectonics, Department of Sciences, Roma Tre University, Rome, Italy. 2Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX, USA. 3Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX, USA. 4Geotop, Université du Québec à Montréal, Montréal, Quebec, Canada. 5Department of Geological Sciences, University of Florida, Gainesville, FL, USA. 6Department of Earth and Environmental Sciences, Università di Milano-Bicocca Milano, Milan, Italy. 7Geological Survey of Israel, Jerusalem, Israel. 8The Hebrew University of Jerusalem, Jerusalem, Israel. *e-mail: [email protected] 1012 NatURE GEOscIENCE | VOL 12 | December 2019 | 1012–1017 | www.nature.com/naturegeoscience NATURE GEOSCIENCE ARTICLES a b Longitude Longitude 20° E 30° E 40° E 50° E 20° E 30° E 40° E 50° E 40° N 40° N 0 250 500 0 250 500 Turkey B km km Italy Greece Cyprus Syria Iran 30° N Mediterranean Sea Lebanon Iraq Palestine Jordan 30° N Israel Sirte Persian N Kuwait Latitude 20° N Rift Gulf Libya Nile River A Egypt Aswan Saudi Arabia Dam BN 10° N WN Red Sea Latitude 20° N A Chad Residual topography Elevation Atbara River –2 –1 012 (km) (m a.s.l.) Sudan Blue Nile River Eritrea Yemen c 4,790 AB 3 Filtered topography (50 km) 2,500 Gulf of Aden Filtered topography (300 km) 2 Djibouti Residual topography 10° N 0 Ethiopian Somalia 1 Plateau 0 –2,500 CAR White Nile River Ethiopia –1 –5,348 Congo Topography (km) –2 Delta sediments –3 0.5 246 thickness (m) 0 100 200300 Distance along profile (km) Fig. 1 | The East Africa–eastern Mediterranean sediment transport system. a, Topography of the Nile drainage showing thickness of upper Tertiary sediments in the Nile Delta basins including the Levantine basin (eastern lobe) and Herodotus basin (western lobe) (data from ref. 27). Elevation in m above sea level (a.s.l.). CAR, Central African Republic. b, Residual topography map estimated considering crustal thickness and variable crustal density from crustal model CRUST1.0 (refs. 38,47). River labels are WN, White Nile; BN, Blue Nile; N, Nile; and A, Atbara River. Points A and B indicate start and end points for river profile graphed in c. c, Comparison between topography and its residual component along a profile approximately following the Nile (topography is filtered in frequency domain by 50 and 300 km low pass filters). detrital zircons yielding young uranium–lead ages (30–20 Ma), Thermochronology indicates that erosion and exhumation reflecting the detrital trace of bimodal Ethiopian volcanic products in the Blue Nile basement units started as early as ~28–30 Ma since the Rupelian age (∼31 Ma) (ref. 26). (ref. 33). Local estimates of the incision rate along the Blue Nile indi- An Oligocene age of the East African–Mediterranean transport cate three pulses of uplift since ~10 Ma (ref. 21). However, systematic system is also supported by sediment thickness data27,28. Figure river analyses indicate an early doming episode (Oligocene) main- 2 shows the total volume of fully compacted sediment in the tained for a long time30. The incisional history of the Blue Nile and Herodotus and Levant basins, which is estimated to be ~580,000 km3, Tekeze rivers evidences three base level changes superimposed on of which more than 30% was deposited after the Messinian age the uplifting bulge: the oldest one, between 20 and 10 Ma, related (~390,000 km3 in the Oligocene–Miocene and 190,000 km3 in the to capture of plateau internal drainage; the second one, between 10 Pliocene–Pleistocene)27,28. The volume increase may in part be only and 5 Ma, related to a local tectonic event; and the last one, in the an apparent increase29, or it may be related to the gain of drainage Quaternary period, related to a climatic and/or tectonic event30. area by river capture30,31. Inversion of river profiles has been used to infer uplift rate and The total volume of sediment may be compared with the total hence continental-scale landscape evolution over Africa, indicating amount of eroded material. This has been defined by subtracting a main phase of uplift starting at ~30 Ma (refs. 9,10).