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Long-term, deep-mantle support of the Ethiopia-Yemen Plateau
Tectonics
Abstract
Ethiopia is a key site to investigate the interactions between mantle dynamics and surface processes because of the presence of the Main Ethiopian Rift (MER), Cenozoic continental flood basalt volcanism, and plateau uplift. The role of mantle plumes in causing Ethiopia's flood basalts and tectonics has been commonly accepted. However, the location and number of plumes and their impact on surface uplift are still uncertain. Here, we present new constraints on the geological and topographic evolution of the Ethiopian Plateau (NW Ethiopia) prior to and after the emplacement of the large flood basalts (40-20 Ma). Using geological information and topographic reconstructions, we show that the large topographic dome that we see today is a long-term feature, already present prior the emplacement of the flood basalts. We also infer that large-scale doming operated even after the emplacement of the flood basalts. Using a comparison with the present-day topographic setting we show that an important component of the topography has been and is presently represented by a residual, non-isostatic, dynamic contribution. We conclude that the growth of the Ethiopian Plateau is a long-term, probably still active, dynamically supported process. Our arguments provide constraints on the processes leading to the formation of one of the largest igneous plateaus on Earth.
... The Ethiopia-Yemen Volcanic Province (EVP) formed above one or more mantle plumes, as signaled by over 40 My of primarily basaltic magmatism (Beccaluva et al., 2009;Boyce et al., 2023;Chang et al., 2020;Ebinger and Sleep, 1998;Furman et al., 2006;Furman et al., 2016;George et al., 1998;Steiner et al., 2022). The EVP comprises the Main Ethiopian Rift (MER) and the Red Sea-Gulf-of Aden-MER triple junction in the Afar Depression that transect the broad, ~2000 m-high Ethiopian-Yemen plateau that is in part dynamically supported (e.g., Ayalew et al., 2021;Kidane et al., 2006;Sembroni et al., 2016). Flood magmatism preceded initial rifting of the Red Sea and the Gulf of Aden by 31 Ma, with the MER developing much later at 18-20 Ma (e.g., Rooney et al., 2017;Wolfenden et al., 2004). ...
... The main feature emerging from all the group and phase velocity maps is an area of seismic velocities lower than the surrounding regions beneath the Northern and Central MER. Intermediate velocities characterize the area on the northwestern plateau covered by the Cenozoic flood basalts, with a discontinuity around 9 • -10 • N, where the NW-SE trending Mesozoic Tana rift basin is located (Fig. 1) (Gani et al., 2009;Hautot et al., 2006;Sembroni et al., 2016). The extent of the low-velocity anomaly, though, is much smaller than the extent of the proposed Tana-Dessie basin shown in Fig. 1. ...
... The black and cyan vectors show Nubian-fixed reference frame velocities with limits (afterBirhanu et al., 2016) and(Knappe et al., 2020) respectively. The red broken line depicts the NW-SE trending Mesozoic rift basin (afterGani et al., 2009;Sembroni et al., 2016). Major border faults (BF) are labeled as YTVL, Yerer-Tullu Wellel volcano tectonic lineament, TGD, Tendaho-Goba'ad Discontinuity. ...
... The McKenzie (1978) rift model predicts no uplift or erosion, yet both are commonly observed in rift settings, especially magma-rich ones (Sengör and Burke 1978;Franke 2013). Proposed processes responsible for synrift uplift include the following: (1) footwall rebound during extensional hanging-wall detachment (Falvey 1974;Wernicke 1992); (2) thermally induced buoyancy of the lithosphere over anomalously hot mantle (Falvey 1974;Dewey 1982); (3) thermal erosion of the base lithosphere (upper mantle thinning, Karner et al. 1992;Davies 1994); (4) magmatic addition into the extending lithosphere or underlying asthenosphere (Quirk and Rüpke 2018;Klocking et al. 2020); (5) depth-dependent extension (Driscoll and Karner 1998); (6) dynamic uplift above hot asthenosphere or plumes that exert an upward dynamic force on the base of the lithosphere (Hager and Richards 1989;Nadin et al. 1997;Lithgow-Bertelloni and Silver 1998;Quirk et al. 2013;Sembroni et al. 2016). Although most of these have been examined for their roles in rifting, the role of dynamic uplift, and especially the subsequent dissipation of that uplift in the early post-R1 rift stage (i.e. ...
... Some positive examples have apparent far-reaching sub-lithospheric 'radial fingers' or local hotspots surrounding the centre (e.g. the Iceland Plume, Schoonman et al. 2017). The magnitude of measurable positive and negative dynamic topography ranges from tens of metres as shown in the US Atlantic coastal plain (Rowley et al. 2013) to roughly 2 km as shown in the dynamically uplifted East Africa-Afar region (Fig. 2), where rifting and magmatism are thought to be topographically elevated above an upwelling mantle plume (Faccenna et al. 2013;Wilson et al. 2014;Sembroni et al. 2016;Hoggard et al. 2017). ...
... Supplementary material,Fig. A1;Sembroni et al. 2016), and by the Afar Depression where several fault-bounded magmatic rift segments with magma bodies are only now subsiding tectonically below sea-level within the more regional dynamic elevation (Fig. 2;Ebinger and Casey 2001;Wolfenden et al. 2005;Heyn et al. 2008;Acocella 2010;Ebinger et al. 2010Ebinger et al. , 2017Varet 2014;Houmed et al. 2015;Bastow et al. 2018;López-García et al. 2020). ...
Magma-rich rifting generally occurs over mantle plumes/rising convection cells; magma-poor rifting does not. Plumes produce up to 2 km dynamic elevation, outcompeting syn-rift subsidence in active and ancient magma-rich rifts, as evidenced by top-rift unconformities. Magma-rich margins show anomalously fast/large post-tectonic subsidence, evidenced by thick, short-lived little-faulted sag–salt sections. Syn-depositional thermal subsidence cannot be responsible alone, and lack of faulting argues against tectonic subsidence. We argue dissipation of dynamic topography (dynamic subsidence) is the additional component with thermal subsidence. We depict the formation of the magma-rich central South Atlantic and Gulf of Mexico salt-bearing margins relative to former plumes, showing sag–salt deposition occurs as the margins migrate off plume flanks while supra-plume rifting continues. However, the pre-sag–salt basement must first be extended/eroded during dynamic elevation to fall below its pre-uplift elevation. “Dynamo-thermal subsidence” creates “dynamo-thermal accommodation”, the sum and combined result of dynamic plus thermal subsidence, respectively. It is fast enough to negate the need for deep, sub-sea level, subaerial depressions for accumulation of thick sag/salt sections. Also, differences in dynamic elevation during salt deposition explains differences in autochthonous salt thickness.
Supplementary material: https://doi.org/10.6084/m9.figshare.c.5908896
... The chronology, lithology, and volume estimates derived from them inform the magmatic processes occurring in the underlying lithosphere. Spatial patterns across CFB provinces reflect variations in these processes (Reidel et al. 2013;Sembroni et al. 2016;Rooney 2017), and in mechanisms of crustal storage, the evolution of silicic magmas by fractional crystallization and crustal melting, and the highlevel controls that facilitate eruption. ...
... Measured stratigraphic sequences from localities on the study area (Fig. 4) emphasize three features: (1) The present-day high relief sectors of the volcanic plateau (around 3800-m high above base-level, Figs. 2a and 10) coincide with the rift shoulder (plateau-Afar rift escarpment), centered at about 11°47′25″ N; 39°26′13″ E, indicating significant uplift of the Ethiopian plateau that appeared to be most likely pre-volcanism (prior to ~ 30 Ma ago, Sembroni et al. 2016). As discussed previously, there are two lines of evidence in favor of this assumption: (a) frequent development of erosion channels (intervening sediments) within or between individual formations is indicative of lava emplacement over an elevated surface, and (b) lava flows are frequently columnar, suggesting lava ponding in depression/paleorelief developed as a result of erosion of uplifted landmass during wet environment. ...
... As discussed previously, there are two lines of evidence in favor of this assumption: (a) frequent development of erosion channels (intervening sediments) within or between individual formations is indicative of lava emplacement over an elevated surface, and (b) lava flows are frequently columnar, suggesting lava ponding in depression/paleorelief developed as a result of erosion of uplifted landmass during wet environment. There is a 2000 km-wide, dynamically supported plateau known from comparison of gravity and topography (e.g., Ebinger and Hayward 1996;Faccenna et al. 2014;Sembroni et al. 2016). (2) The thickest volcanic pile occurs towards the plateau-Afar rift margin, implying that magma productivity has been greatest under the center of the Ethiopian uplift and that the majority of the fissure-feeder loci were close to or at the Afar rift margin. ...
We present a first stratigraphy of the flood basalt succession in eastern Wollo province, northern Ethiopian plateau, based on detailed field logging. This study was initiated to resolve outstanding issues in the region regarding the timing and extent of flood volcanism, assisting a wider correlation of stratigraphic sequences across the Ethiopian plateau. Our approach involved construction of stratigraphic sections from six selected gorge localities having ~ 100% exposure. Individual flow thickness, lithology, and sequence were recorded at each locality. Features common to all localities indicate a single magmatic plumbing system for the mapped area. Three formations are identified and named, and linked to published Oligocene radiometric ages from neighboring areas. These formations are from base to top: Teri Basalt, the main phase of flood basalt emission, is conformably overlain by the Wegeltena Rhyolite, composed of silicic ignimbrites dated at ~ 30.2 Ma. Upon this rests a thin bed of conglomerate, sandstone, and mudstone representing a ~ 3 Ma episode of volcanic quiescence. At the top of the sequence lies the Kon Basalt, dated at 27.8–26.7 Ma. Comparison of this new stratigraphy with other Oligocene sequences on the northern plateau reveals significant variations in lithology and age range, with implications for how the Ethiopian flood basalt province was constructed. More work is required in seeking the fissure vents and their geometry.
... Laboratory (Griffiths et al., 1989;Griffiths and Campbell, 1991;Kiraly et al., 2015;Sembroni et al., 2017a) and numerical (Farnetani and Richards, 1994;d'Acremont et al., 2003;Burov and Guillou-Frottier, 2005;Burov and Gerya, 2014;Barnett-Moore et al., 2017;Koptev et al., 2017;Rubey et al., 2017;Cao et al., 2018) models estimated total dynamic topography ranges of 500-1500 m and 500-4000 m respectively, highlighting the strong influence of lithosphere rheology on both uplift rate and amplitude of the consequent topographic bulge. Geological and geomorphological observations in Arabia (Sengör, 2001;Daradich et al., 2003), Africa (Gurnis et al., 2000;Roberts and White, 2010;Roberts et al., 2012;Paul et al., 2014;Sembroni et al., 2016a), Australia (Czarnota et al., 2014), North America (Rowley et al., 2013;Liu, 2015;Heller and Liu, 2016), South America (Dávila and Lithgow-Bertelloni, 2013), and northern Europe (Friedrich, 2019), allowed to estimate surface uplifts of up to 2000 m. ...
... According to Cox (1989) in this geodynamic context the initial uplift of the surface in response to the dynamic effects of the plume causes the formation of a radial drainage patterns. The river network starts to erode the swell promoting rock uplift in response to The survival of a rough radial drainage pattern for 60 Ma or more in this geodynamic context can occur despite successive isostatic adjustments (because of surface erosion and/or magmatic underplating) and rifting events (Vita-Finzi, 2012;Sembroni et al., 2016a;Faccenna et al., 2019). In these cases it may be possible to recognise the first-order imprint of the mantle plume impingement on the drainage system allowing valuable estimates of dynamic topography usually difficult to obtain (Allen, 2011;Roberts et al., 2012;Vita-Finzi, 2012;Sembroni et al., 2016a). ...
... The river network starts to erode the swell promoting rock uplift in response to The survival of a rough radial drainage pattern for 60 Ma or more in this geodynamic context can occur despite successive isostatic adjustments (because of surface erosion and/or magmatic underplating) and rifting events (Vita-Finzi, 2012;Sembroni et al., 2016a;Faccenna et al., 2019). In these cases it may be possible to recognise the first-order imprint of the mantle plume impingement on the drainage system allowing valuable estimates of dynamic topography usually difficult to obtain (Allen, 2011;Roberts et al., 2012;Vita-Finzi, 2012;Sembroni et al., 2016a). In fact rivers, responding to tectonic and climatic perturbations in a time scale ranging from 10 4 to 10 6 yr (Whipple, 2001;Wegmann et al., 2007), are able to propagate their signals across the landscapes, shaping the topography (Howard, 1994;Whipple and Tucker, 1999;Snyder et al., 2000;Kirby and Whipple, 2001;Whipple, 2001;Whipple, 2004;Wobus et al., 2006;Kirby et al., 2007;Wegmann et al., 2007;Whipple et al., 2007;Kirby and Whipple, 2012). ...
Continental areas affected by mantle plume dynamics are characterised by extensive high-elevated regions drained by large radial river networks. Despite successive isostatic adjustments and rifting events, several studies demonstrated that the persistence of these drainage systems for tens of millions of years is possible. In these geodynamic contexts rivers are precious sources of knowledge because, propagating the signals of tectonic and climatic changes across landscape, they shape the topography and allow to recognise the first-order imprint imposed by mantle plume. The Horn of Africa, characterised by the coexistence of a continental rift system, a large igneous province (continental flood basalts), and a wide uplifted plateau, is an ideal test site to investigate the interrelations between surface and deep processes. Studies demonstrated the long-term persistence of some river networks draining the region and the strong influence of dome-like uplift on their evolution. However a regional-scale quantitative river network analysis is missing, as well as, a complete evolutionary scenario of the Horn of Africa drainage system. In this study we quantitatively investigated the topographic configuration of the Horn of Africa and analysed the four principal drainage systems (Blue Nile, Tekeze, Omo, Wabe Shebele basins), extracting the river longitudinal profiles and the main topographic and hydrologic parameters. In order to reconstruct the evolution of the region, we elaborated the pre−/syn- and post-flood basalts topographies and calculated the elevation gain and loss with respect to the present configuration. Finally, we delineated a possible future drainage system evolution by analysing the present drainage divides stability. The results allowed to reconstruct the evolutionary scenario of the Horn of Africa river network since Oligocene and to investigate the mutual influence between surface and deep processes in shaping the landscape, providing new constraints to understand the formation and evolution of a drainage system in a context of a topography supported by a mantle plume.
... The Afar LIP is preserved on both the Arabian and African plates and formed just prior to continental breakup and formation of the Gulf of Aden and Red Sea (Sembroni et al., 2016). A plume channel extending from the East African Rift that resulted in the Afar LIP was proposed by Sleep (1997). ...
... A majority of the Ethiopian, Yemeni, and Eritrean volcanic plateaus erupted from 29 to 33 Ma (Hofmann et al. 1997;Abbate et al., 2014;Rooney, 2017). Based on geological data and topographic reconstructions, the large topographic dome observed in Ethiopia today was shown to be a long-term feature that existed prior to the emplacement of Afar flood basalts with a nonisostatic, dynamic contribution (Sembroni et al., 2016). Ethiopian Plateau uplift probably also impacted regional climate by inhibiting moist air circulation and inducing aridification (Sepulchre et al., 2006). ...
... The mantle plume-head-induced plateaus and domes with elevation magnitudes of several kilometers related to LIP magmatism are caused by dynamic uplift induced by the buoyant asthenosphere, thermal expansion of the heated lithosphere, as well as inflation of the plume partial melts within the upper mantle and crust, all leading to uplift followed by subsidence after the plume head fades away (Nadin et al., 1997;Leng & Zhong, 2010). Note that some LIPs (e.g., Afar) have been shown to occur in regions experiencing long-term nonisostatic, dynamic uplift that both preceded and postdated LIP magmatism (Sembroni et al., 2016). The patterns of differential uplift and subsidence are related to the geometry of the plume head reacting with lithospheric irregularities and can be complicated (e.g., Sleep, 1997Sleep, , 2006. ...
This chapter summarizes geochronologic and other data for major Phanerozoic Large Igneous Provinces (LIPs), Oceanic Anoxic Events (OAEs) and organic-rich petroleum source rocks. It also evaluates the models that support or refute genetic links between the three groups. The evidence appears to favor genetic links between the three groups, however, additional high precision age and geochemical data are needed to validate several events. Furthermore, the chapter provides insights into the importance of LIPs in hydrocarbon exploration.
Plain Language Summary
During the last five hundred million years the Earth has experienced over a dozen periods each lasting about one million years when extremely massive eruptions of basalt magma erupted on the continents and in ocean basins; these are called Large Igneous Provinces (LIPs). Nearly every one of these LIP events coincided with and contributed to Earth System processes that caused periods when ocean waters experienced extremely low oxygen levels allowing preservation of organic-rich sediments on the seafloor with anomalous stable Carbon isotope ratios; the organic matter would have decomposed under higher oxygen levels. Organic-rich sediments are a key element that eventually results in the formation of buried oil and gas deposits, so understanding when they were deposited in the rock record and where they are most concentrated around the world increase the success of those exploring for oil and gas resources.
... We constrained our analysis of the evolution of the Ethiopian-Yemen highlands using the flood basalt as a reference layer (details in ref. 32 ), reconstructing the elevation over two time periods, before and after flood basalt emplacement, that is, from about 40 to 30 Ma and from about 30 to 15 Ma, respectively ( Supplementary Fig. 1 already at an average elevation of ~750 m before the main phase of flood basalt outpouring ( Fig. 2 and Supplementary Fig. 1c), increasing to ~1,250 m just after the flood basalt emplacement, at ~30 Ma ( Fig. 2). After that, a large-scale doming took place, providing an average elevation of ~2,600 m ( Fig. 2 and Supplementary Fig. 1b), decreasing during the rifting stage (<11 Ma) to ~2,250 m and reaching an average present-day average elevation of ~1,800 m ( Fig. 2 and Supplementary Fig. 1a). ...
... As noted above, we find an increase in the elevation of the Ethiopian Plateau following the main volcanic outburst (Fig. 3e), similar to what is seen in geological reconstructions ( Fig. 2 and Supplementary Fig. 1). In particular, the increase in elevation of the Ethiopian-Yemen Dome between 40 Ma and present day ( Supplementary Fig. 1a-c) shows a similar pattern to the one reconstructed from dynamic topography (Fig. 3a-c) but with a larger amplitude due to the isostatic contribution related to the emplacement of the flood basalt, to magmatic underplating and to the flexural component on the rift shoulders 32 . ...
... Data from the Levant basin were previously published 28 and are available at the website of the subsurface research lab at Geological Society of Israel (http://www.gsi.gov.il/eng/?CategoryID=239&ArticleID=598). Data on the evolution of the Ethiopian Plateau were published in ref. 32 . ...
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 sustained by a dynamic topographic gradient.
... Tomographic studies evidenced the relative velocity anomaly several hundreds of kilometers wide beneath Ethiopia (Benoit et al., 2006;Bastow et al., 2008Bastow et al., , 2011. This event generated a domeshaped topography centered in the area of Addis Ababa with a diameter of~1200 km (Sembroni et al., 2016a). The associated volcanism caused the outpouring of a huge volume of continental flood basalts (Trap; Figs. ...
... Sections 1 and 2 trend NNW-SSE and cross the Wabi Shebele River basin from the rift shoulder to the Somalian lowlands (Fig. 2b). In the northwestern sector, section 1 evidences the tilting of the lithologies to the SE (dip angle of~30°), probably related to the flexural uplift of the rift shoulder (Weissel et al., 1995;Sembroni et al., 2016a;Fig. 3a). ...
... All the surfaces have the highest elevation (2000-3000 m) in the northwestern sector, at the eastern margin of MER. Such a configuration could be related to the flexural uplift caused by the tectonic unloading along the rift border faults (Weissel et al., 1995;Sembroni et al., 2016a), started in the Late Miocene (Wolfenden et al., 2004;Bonini et al., 2005). Usually, this local uplift ranges between 430 and 1175 m while the deformation extends up to 200 km away from the border faults (Sembroni et al., 2016a). ...
Large river systems play an important role in Earth dynamics since they exert an influence on geological, geomorphological, and geochemical processes. On the other hand, these landforms occur in a variety of topographic and plate tectonic settings and tend to persist for 10⁷–10⁸ yr, resisting variations in environmental conditions. Both modern and ancient big rivers configurations suggest that the persistence of such features is much longer on passive margins with long-lasting continental tilting and long-term rainfall without interference from continental glaciation, desertification, and volcanism. The literature on the geological evolution of river systems concentrates almost entirely on large-scale river basins. Very little research has been done to understand the evolution of smaller river networks that have a much lower persistence because they are more sensitive to climatic and tectonic changes. In this work we focused on the Wabe Shebele River basin (SE Ethiopia, SW Somalia) that drains the eastern slope of the Horn of Africa. It is a medium-scale drainage system (drainage area of ~10⁵ km²) developed on a long-lived regional slope to the SE inherited from the Early Mesozoic and influenced by tectonic structures relative to a passive continental margin and slightly affected by volcanism. By taking into consideration the present topography (swath profiles, filtered topography, slope, local relief), the river network (river longitudinal profiles, channel gradient), and the ancient landforms present in the region, we demonstrate that the Wabe Shebele River basin, despite its medium scale, is a long-lived landform persisting at least since the Oligocene. The results show that even smaller river systems can have a long-term history in favorable tectonic and topographic conditions.
... The SEEP and the Northwestern Ethiopian Plateau form a broad dome, hypothesized to be the result of the impinging of the Afar mantle plume beneath the lithosphere of the Arabian-Nubian Shield (Sembroni et al., 2016a). The highest regions in the plateau are the Bale and Ahmar mountains, reaching over 4.0 km in elevation. ...
... We therefore measured the migration distance of the knickpoint and hence calculated the celerity in Eq. (9). We followed methods outlined in previous studies Berlin and Anderson, 2007;Sembroni et al., 2016a) to calculate the best fit results of the residuals of the minimum sum of squares for seven modeled knickpoints in basins 48, 51, 68, and 74 (Fig. 4). Subsequently, we established the values for m and K in Eq. (9) to be 0.221 and 2.49 × 10 −4 ± 1.29 × 10 −4 (m 0.08 y) −1 respectively. ...
... The geomorphic proxies also show a high value at the Bale mountains and the Genale slopes, with a general decrease in the geomorphic proxy values southeastward (Fig. 15G-J). This southeastward increase in uplift may be related to the flexural uplift along the shoulder of the MER, which can be up to 200 km from the rift shoulder (Sembroni et al., 2016a). The Ahmar mountains have a relatively lower geomorphic proxy values compared to the Bale mountains adjacent to it. ...
We use morphotectonic analysis to study the tectonic uplift history of the southeastern Ethiopian Plateau (SEEP). Based on studies conducted on the Northwestern Ethiopian Plateau, steady-state and pulsed tectonic uplift models were proposed to explain the growth of the plateau since ~30 Ma. We test these two models for the largely unknown SEEP. We present the first quantitative morphotectonic study of the SEEP. First, in order to infer the spatial distribution of the tectonic uplift rates, we extract geomorphic proxies including normalized steepness index ksn, hypsometric integral HI, and chi integral χ from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) digital elevation model (DEM). Second, we compare these rates with the thickness of flood basalt that we estimated from geological maps. Third, to constrain the timing of regional tectonic uplift, we develop a knickpoint celerity model. Fourth, we compare our results to those from the Northwestern Ethiopian Plateau to suggest a possible mechanism to explain regional tectonic uplift of the entire Ethiopian Plateau. We find an increase in tectonic uplift rates from the southeastern escarpments of the Afar Depression in the northeast to that of the Main Ethiopian Rift to the southwest. We identify three regional tectonic uplift events at ~11.7, ~6.5, and ~4.5 Ma recorded by the development of regionally distributed knickpoints. This is in good agreement with ages of tectonic uplift events reported from the Northwestern Ethiopian Plateau.
... Swath profile 1 extends from the Ethiopian Plateau in the NNW to the Somalian Plateau in the SSE (Figs. 3, 4). In the NNW, the trend of mean topography shows the flexural uplift affecting the rift shoulder (2.5-3 km of elevation), as documented by Weissel et al. (1995) and Sembroni et al. (2016b). A similar flexural strain is possibly hidden by the strong fluvial erosion affecting the SSE sector where the Somalian Plateau is limited to a narrow strip of land with a mean elevation of ~2700 m (Figs. 3, 4). ...
... The study area is characterized by a general low local relief (<300 m) with the lowest values concentrated in the MER and the portions of the Ethiopian and Somalian plateaus preserved by erosion (Fig. 5a). Such features represent the ancient top surface of the Trap deposits and have been used by some authors to calculate the volume and thickness of flood basalts (Gani et al., 2007;Sembroni et al., 2016b) and to reconstruct the post-Trap topography (Sembroni et al., 2016b). Values >300 m are in the MER where volcanoes and active tectonics cause the increase of local relief up to 600 m (Figs. 4, 5a). ...
... The study area is characterized by a general low local relief (<300 m) with the lowest values concentrated in the MER and the portions of the Ethiopian and Somalian plateaus preserved by erosion (Fig. 5a). Such features represent the ancient top surface of the Trap deposits and have been used by some authors to calculate the volume and thickness of flood basalts (Gani et al., 2007;Sembroni et al., 2016b) and to reconstruct the post-Trap topography (Sembroni et al., 2016b). Values >300 m are in the MER where volcanoes and active tectonics cause the increase of local relief up to 600 m (Figs. 4, 5a). ...
We investigate the along-axis variations in architecture, segmentation and evolution of the Main Ethiopian Rift (MER), East Africa, and relate these characteristics to the regional geology, lithospheric structure and surface processes. We first illustrate significant along-axis variations in basin architecture through analysis of simplified geological cross-sections in different rift sectors. We then integrate this information with a new analysis of Ethiopian topography and hydrography to illustrate how rift architecture (basin symmetry/asymmetry) is reflected in the margin topography and has been likely amplified by a positive feedback between tectonics (flexural uplift) and surface processes (fluvial erosion, unloading). This analysis shows that ~70% of the 500 km-long MER is asymmetric, with most of the asymmetric rift sectors being characterized by a master fault system on the eastern margin. We finally relate rift architecture and segmentation to the regional geology and geophysical constraints on the lithosphere. We provide strong evidence that rift architecture is controlled by the contrasting nature of the lithosphere beneath the homogeneous, strong Somalian Plateau and the weaker, more heterogeneous Ethiopian Plateau, differences originating from the presence of pre-rift zones of weakness on the Ethiopian Plateau and likely amplified by surface processes. The data provided by this integrated analysis suggest that asymmetric rifts may directly progress to focused axial tectonic-magmatic activity, without transitioning into a symmetric rifting stage. These observations have important implications for the asymmetry of continental rifts and conjugate passive margins worldwide.
... The harmonics of the vertical stress due to mantle plume for degree n, σ zz,n are derived by performing spherical harmonic analysis of the computed stress using Equation (21). The stress values up to 2000 km from the hot spot location are considered and the stress values for a distance more than 2000 km are assumed to be zero. ...
... As it is expected, the shear stress values are correlated with the disturbing potential map presented in Figure 3a. The vertical stress due to mantle plume, computed using Equation (21) at the hotspot locations ( [38]; p. 500 Table 11.1), is also considered as an additional BV. We used the same principle as Gedamu et al. (2021) [25] to select optimal parameters for the radius of the sphere (a), the depth of the sphere (Z), and the density anomaly (δρ). ...
The Afar and Ethiopian plateaus are in a dynamic uplift due to the mantle plume, therefore,
considering the plume effect is necessary for any geophysical investigation including the estimation of
lithospheric stress in this area. The Earth gravity models of the Gravity Field and Steady-State Ocean
Circulation Explorer (GOCE) and lithospheric structure models can be applied to estimate the stress
tensor inside the Ethiopian lithosphere. To do so, the boundary-value problem of elasticity is solved
to derive a general solution for the displacement field in a thin elastic spherical shell representing
the lithosphere. After that, general solutions for the elements of the strain tensor are derived from
the displacement field, and finally the stress tensor from the strain tensor. The horizontal shear
stresses due to mantle convection and the vertical stress due to the mantle plume are taken as the
lower boundary value at the base of the lithosphere, and no stress at the upper boundary value of
the lithospheric shell. The stress tensor and maximum stress directions are computed at the Moho
boundary in three scenarios: considering horizontal shear stresses due to mantle convection, vertical
stresses due to mantle plume, and their combination. The estimated maximum horizontal shear
stresses’ locations are consistent with tectonics and seismic activities in the study area. In addition,
the maximum shear stress directions are highly correlated with the World Stress Map 2016, especially
when the effect of the mantle plume is solely considered, indicating the stress in the study area mainly
comes from the plume.
... In Ethiopia, trap volcanism occurred during the Early Oligocene 68 leading to the formation of ~1-km-thick continental flood basalts covering pre-existing topography. The Ethiopian plateau could have been as high as 2,500-3,000 m in places before the middle Miocene [69][70][71][72] . The East African Highlands (East African Rift and Afar-Yemen-Arabia Plateau) then underwent continuous uplift during the Neogene 69,72 owing to rifting processes that began in Ethiopia during the middle Miocene and then propagated southwards 68 . ...
... In the Middle East, the Iranian Plateau and Anatolian topography rose at some point after 17 Ma due to the long-term collision of the Arabian and Eurasian Plates 30 , most probably during thelate Miocene 73,74 , which is later than the settlement of high topography in East Africa [67][68][69][70][71][72] and in the Himalayan and Tibetan Plateau regions 12,62-66 . We therefore ran an experiment in which we decreased the height of the regional topography by half in the LM configuration (LM-NoAIO, Extended Data Fig. 4b). ...
In the modern northern Indian Ocean, biological productivity is intimately
linked to near-surface oceanographic dynamics forced by the South Asian, or
Indian, monsoon. In the late Pleistocene, this strong seasonal signal is
transferred to the sedimentary record in the form of strong variance in the
precession band (19–23 kyr), because precession dominates low-latitude
insolation variations and drives seasonal contrast in oceanographic
conditions. In addition, internal climate system feedbacks (e.g. ice-sheet
albedo, carbon cycle, topography) play a key role in monsoon variability.
Little is known about orbital-scale monsoon variability in the
pre-Pleistocene, when atmospheric CO2 levels and global temperatures
were higher. In addition, many questions remain open regarding the timing of the initiation and intensification of the South Asian monsoon during the
Miocene, an interval of significant global climate change that culminated in bipolar glaciation. Here, we present new high-resolution (<1 kyr)
records of export productivity and sediment accumulation from International
Ocean Discovery Program Site U1443 in the southernmost part of the Bay of Bengal
spanning the late Miocene (9 to 5 million years ago). Underpinned by a new
orbitally tuned benthic isotope stratigraphy, we use X-ray
fluorescence-derived biogenic barium variations to discern productivity
trends and rhythms. Results show strong eccentricity-modulated
precession-band productivity variations throughout the late Miocene,
interpreted to reflect insolation forcing of summer monsoon wind strength in the equatorial Indian Ocean. On long timescales, our data support the
interpretation that South Asian monsoon winds were already established by 9 Ma in the equatorial sector of the Indian Ocean, with no apparent
intensification over the latest Miocene.
... In Ethiopia, trap volcanism occurred during the Early Oligocene 68 leading to the formation of ~1-km-thick continental flood basalts covering pre-existing topography. The Ethiopian plateau could have been as high as 2,500-3,000 m in places before the middle Miocene [69][70][71][72] . The East African Highlands (East African Rift and Afar-Yemen-Arabia Plateau) then underwent continuous uplift during the Neogene 69,72 owing to rifting processes that began in Ethiopia during the middle Miocene and then propagated southwards 68 . ...
... In the Middle East, the Iranian Plateau and Anatolian topography rose at some point after 17 Ma due to the long-term collision of the Arabian and Eurasian Plates 30 , most probably during thelate Miocene 73,74 , which is later than the settlement of high topography in East Africa [67][68][69][70][71][72] and in the Himalayan and Tibetan Plateau regions 12,62-66 . We therefore ran an experiment in which we decreased the height of the regional topography by half in the LM configuration (LM-NoAIO, Extended Data Fig. 4b). ...
The drivers of the evolution of the South Asian Monsoon remain widely debated. An intensification of monsoonal rainfall recorded in terrestrial and marine sediment archives from the earliest Miocene (23–20 million years ago (Ma)) is generally attributed to Himalayan uplift. However, Indian Ocean palaeorecords place the onset of a strong monsoon around 13 Ma, linked to strengthening of the southwesterly winds of the Somali Jet that also force Arabian Sea upwelling. Here we reconcile these divergent records using Earth system model simulations to evaluate the interactions between palaeogeography and ocean–atmosphere dynamics. We show that factors forcing the South Asian Monsoon circulation versus rainfall are decoupled and diachronous. Himalayan and Tibetan Plateau topography predominantly controlled early Miocene rainfall patterns, with limited impact on ocean–atmosphere circulation. The uplift of the East African and Middle Eastern topography played a pivotal role in the establishment of the modern Somali Jet structure above the western Indian Ocean, while strong upwelling initiated as a direct consequence of the emergence of the Arabian Peninsula and the onset of modern-like atmospheric circulation. Our results emphasize that although elevated rainfall seasonality was probably a persistent feature since the India–Asia collision in the Paleogene, modern-like monsoonal atmospheric circulation only emerged in the late Neogene. A modern-like South Asian Monsoon only appeared when East African and Middle Eastern uplift led to the establishment of the Somali Jet around 13 million years ago, according to Earth system modelling using a range of regional palaeogeographies.
... In gross temporal terms, the African Surface evolved throughout Afro-Arabia during three major geotectonic episodes: (1) an initial period of rifted-margin formation consequent upon Gondwana break-up and opening of the Indian and central Atlantic Oceans at~180 Ma; (2) an intermediate phase associated with opening of the South Atlantic Ocean at~125 Ma; and (3) a terminal episode, coincident with the early rise of the African swells at Ma (Partridge and Maud, 1987;Partridge, 1998;Burke and Gunnell, 2008;Moucha and Forte, 2011;Paul et al., 2014;Burke and Wilkinson, 2016;Sembroni et al., 2016;Guillocheau et al., 2018), although others argue for an earlier termination (Decker et al., 2013). Therefore, this Afro-Arabian erosion surface is the result of~150 m.y. of continental denudation (Tardy et al., 1991;Partridge, 1998;Doucouré and de Wit, 2003;Burke and Gunnell, 2008;Guillocheau et al., 2015Guillocheau et al., , 2018Burke and Wilkinson, 2016). ...
... The first two episodes, prior to~100 Ma, were responsible for isolation of the continent and deep erosion to low elevations (Sahagian, 1988;Doucouré and de Wit, 2003;Macgregor, 2010;Hay, 2011), with large tracts of the continent remaining low-lying and with a humid climate until the Eocene (Burke, 1996;Sengör, 2001;Guiraud et al., 2005;Paul et al., 2014;Guillocheau et al., 2018;Ponte et al., 2019). The terminal stage of African Surface evolution was dictated by deformation and dissection in response to regional, pulsed processes of lithospheric buckling linked to swell and rift-flank uplift (Pik et al., 2003;Gani et al., 2007;Burke and Gunnell, 2008;Moucha and Forte, 2011;Paul et al., 2014;Burke and Wilkinson, 2016;Guillocheau et al., 2015Guillocheau et al., , 2018Sembroni et al., 2016). ...
Supergene alunite from a supergene leached capping zone in the Daero Paulos area near Asmara, Eritrea yields Thanetian (late Paleocene) K–Ar ages of 58.6 ± 2.3 and 59.4 ± 2.3 Ma. The leached capping is exposed in an erosional window through a laterite profile, with the supergene alunite forming by oxidation of pyrite contained in quartz veinlet stockworks hosted by porphyry copper-related, felsic porphyry and adjacent volcano-sedimentary country rocks. Geologic relationships show that the leached capping and contained supergene alunite formed simultaneously with the lateritic weathering event that accompanied regional planation of equatorial and subequatorial Afro-Arabia to form the African Surface. Landscape modification of the central Ethiopian-Eritrean plateau by incision of transverse valleys and canyons commenced in the Oligocene, with a principal exhumation event occurring in the Miocene in response to swell-induced rift-flank uplift during Red Sea opening.
... Studies of incision of the Blue Nile on the NW Plateau assume that the contemporary drainage system started forming following the eruption of the~30-Ma Oligocene flood basalt (Pik et al. 2003;Gani et al. 2007;Ismail and Abdelsalam 2012;Sembroni et al. 2016a, b). This is because the extrusion of the flood basalt must have buried any earlier drainage system. ...
... A dynamically-supported topography has been suggested for the NW Plateau based on topographic and morpho-tectonic study (Sembroni et al. 2016a). Their work implies long-term sustenance of the topography of the NW Plateau. ...
This work aimed at understanding the lithological controls on the SW-flowing segment of the Blue Nile and its ~ 1500-m deep Nile Gorge in the NW Ethiopian Plateau. The gorge exposes a ~ 1100-m-thick Mesozoic sedimentary section overlain by ~ 400-m-thick Cenozoic flood basalt. This work found no significant controls of the river channel by local structure. A NW–SE geologic cross-section was constructed to measure the valley width, the normalized valley width, and the valley asymmetry as a function of depth at 20-m intervals. Additionally, the amount of incision through time was calculated at 1-Ma interval starting 30 Ma. The Blue Nile, during its incision on the NW Plateau, (1) maintains an increase in width of ~ 30 m per 1- m, except when it incised into the upper part of flood basalts (~ 50 m per 1-m depth), a lower part of the Mesozoic sedimentary section dominated by shale–mudstone and sandstone unit (~ 70 m per 1-m depth), and a lowermost unit of the sedimentary section dominated by sandstone unit (10 m per 1- m depth); (2) maintains a NW asymmetry except in the lowermost sandstone unit where it becomes either symmetrical or asymmetrical to the NW or SE; (3) incised for ~ 700- m through the flood basalt and the upper sedimentary section between 30 and 10 Ma, an additional ~ 200- m through the middle sedimentary section between 10 and 6 Ma, and an additional ~ 600- m through the lower sedimentary section between 6 Ma and present.
... for cultural heritage tourism development African Journal of History and Culture 10:15-24 Table Table 1. Elevations of Dogu'a Tembien's caves and regional base levels, and calculation of minimum and maximum ages of the karstic galleries Sembroni et al. 2016;Sepulchre et al. 2006). Real age probably is at the lower side of this range, in line with delayed incision induced by basement rocks located just underneath the local base levels. ...
... Two geological sections allow to envisage the configuration of the geological context in which the karst systems developed. Lithology, stratigraphy and geometry of the main faults are in line with geological maps and literature ( Arkin et al. 1971;Beyth 1972;Bosellini et al. 1997;Russo et al. 1999;Sembroni et al. 2016;Sembroni et al. 2019;Sembroni et al. 2017;Tesfamichael Gebreyohannes et al. 2010;Tesfaye Chernet and Gebretsadiq Eshete 1982). Good footedness is required to access all sites; a type, L cave in limestone, T cave in tufa, † presence of church in natural cave, S large spring or resurgence; b forward trek is downslope, back trek upslope; c see Nyssen (2019a), Jacob & Nyssen (2019); d partial distance from main road to cave along rural access roads that could be covered by 4WD vehicle -the remaining distance needs to be walked; e see Nyssen (2019b) ...
Despite the high geoheritage value of caves and karsts, northern Ethiopia's largest cave at Zeyi (13.5586°N, 39.1454°E) in the Dogu'a Tembien district has received little attention so far. We have studied its geological, geomorphic, socio-cultural and historical dimensions in a holistic way. The basal member of the Antalo Limestone, in which the Zeyi cave is located, consists of grainstone and wackestone with subordinate marly interlayers. Over a length of 364 m, the oval-shaped gallery displays stalagmites, stalactites, five columns, dissolution holes ("bell-holes") following joints, stalagmitic floors and other concretions or speleothems. In absence of any dating of the cave, we contrasted its elevation above the current local base level with known average incision rates of the northern Ethiopian highlands to reconstruct its age, that was calculated as at least 2 to 4 million years. The palaeo-environmental information that is archived in the Zeyi cave sediment would hence cover the Pleistocene. The graves in the sediment at the bottom of Zeyi cave further indicate that the place could be an ancient burial site, which gives scope for archaeological research. Zeyi boosts a unique combination of abiotic, biotic and cultural components: the 19 th C. church under the overhanging cliff; the unique cave, the speleothems, cliffs and gorges; and the cave's bat colony which has been genetically confirmed to be composed of three syntopically roosting species. Accounting for a good balance between cave research, community-based geotourism, geoconservation and biodiversity conservation, the Zeyi cave has strong credentials to become a top geotouristic site in northern Ethiopia. However, major work needs to be done, including granting access for women, and organising community-based geotourism.
... Longitudinal profiles 1 and 2 in Fig. 12 show the situation for rivers stretching WNW-ESE (brown lines on the rose diagram in Figure 11). The headwaters are approximately 10 km away from the lake; however, the headward erosion has to remove a 200 m high crest (escarpment) forming the lakeshore in the west (Sembroni et al. 2016). Another situation in the SW of the lake shows, river piracy taking place in a SW to NE direction (along the green lines) and the remnant, which must be crossed, is only 3.8 km long. ...
... Generally, the neotectonic uplift of the territory can be connected with the opening of the MER (11 - 6 Ma ago according to various authors - c.f. Gani et al. 2009;Ismail and Abdelsalam 2012) but uplift of the Lake Tana sub-region probably began earlier, i.e. before the end of the mid-Tertiary flood volcanism when the asthenospheric mantle intruded the lithosphere as a result of plume activity, inducing thermal uplift (Chorowicz et al. 1998). Further an initial radial drainage network could have formed including outflow to the S (see section 4.4), which was later blocked by Late Miocene and Pliocene tectono-thermal uplift of the MER shoulders and relative subsidence of the Tana basin (Chorowicz et al. 1998;Sembroni et al. 2016). We can speculate that in this initial stage the upper part of the Blue Nile could have also drained along the ancient NW-SE (orange) or WNW-ESE (brown) structures to the area of the recent Afar depression. ...
... This site is characterized by tilted and horizontally stratified hexagonal columnar jointing and the thick weathering features of Oligocene basaltic formations. In contrast, the welded pyroclastic flows are characterized by densely welded and lithic fragments associated with rhyolitic lava flows intercalated with ash and unwelded tuffs [37,45,[48][49][50][51][52]. The flood basaltic formation and welded to partially welded pyroclastic flows show variable transmissivity and yield of the aquifers where the weathered and fractured units are characterized by good water-bearing formation. ...
The shallow volcanic aquifer is the major rural water supply source in the Ethiopian highlands. A significant number of hand pump wells in these aquifers experience a rapid decline in yield and poor performance within a short period of time after construction. Hence, reliable estimation of groundwater flow velocity is important to understand groundwater flow dynamics, aquifer responses to stresses and to optimize the sustainable management of groundwater resources. Here, we propose the geospatial technique using four essential input raster maps (groundwater elevation head, transmissivity, effective porosity and saturated thickness) to investigate groundwater flow velocity magnitude and direction in the shallow volcanic aquifers of the Ethiopian highlands. The results indicated that the high groundwater flow velocity in the Mecha site, ranging up to 47 m/day, was observed in the fractured scoraceous basalts. The Ejere site showed groundwater flow velocity not exceeding 7 m/day in the fractured basaltic aquifer and alluvial deposits. In the Sodo site, the groundwater flow velocity was observed to exceed 22 m/day in the fractured basaltic and rhyolitic aquifers affected by geological structures. The Abeshege site has a higher groundwater flow velocity of up to 195 m/day in the highly weathered and fractured basaltic aquifer. In all study sites, aquifers with less fractured basalt, trachyte, rhyolite, welded pyroclastic, and lacustrine deposits exhibited lower groundwater flow velocity values. The groundwater flow velocity directions in all study sites are similar to the groundwater elevation head, which signifies the local and regional groundwater flow directions. This work can be helpful in shallow groundwater resource development and management for rural water supply.
... This controversy perhaps indicates the relatively modest magnitude of initial uplift expected as a result of rapid thinning of depleted cratonic lithosphere. This more modest magnitude is in contrast to the kilometer-scale uplift expected to result from removing the same thickness of fertile lithosphere [e.g., (47,52,53)]. A second important observation is that many LIPs that overlie reequilibrated lithosphere (i.e., t 0 > 300 Ma) are exposed at the surface today and not buried under deep sedimentary basins. ...
Large igneous provinces (LIPs) are formed by enormous (i.e., frequently >106 km3) but short-lived magmatic events that have profound effects upon global geodynamic, tectonic, and environmental processes. Lithospheric structure is known to modulate mantle melting, yet its evolution during and after such dramatic periods of magmatism is poorly constrained. Using geochemical and seismological observations, we find that magmatism is associated with thin (i.e., ≲80 km) lithosphere and we reveal a striking positive correlation between the thickness of modern-day lithosphere beneath LIPs and time since eruption. Oceanic lithosphere rethickens to 125 km, while continental regions reach >190 km. Our results point to systematic destruction and subsequent regrowth of lithospheric mantle during and after LIP emplacement and recratonization of the continents following eruption. These insights have implications for the stability, age, and composition of ancient, thick, and chemically distinct lithospheric roots, the distribution of economic resources, and emissions of chemical species that force catastrophic environmental change.
... The region has been investigated as part of regional-scale geophysical (e.g., Bastow et al. 2008;Keranen et al. 2009) and petrological-geochemical studies of the northwestern Ethiopian plateau (e.g., Pik et al. 1998Pik et al. , 1999Ayalew et al. 2002;Ayalew and Yirgu 2003;Kieffer et al. 2004;Krans et al. 2018). In addition, some studies on the regional and local tectonic setting (e.g., Chorowicz et al. 1998;Mège and Korme 2004;Hautot et al. 2006;Sembroni et al. 2016) and the geological evolution of the Oligocene (e.g., Prave et al. 2016) and Quaternary volcanic rocks and mantle xenoliths (e.g., Abate et al. 1998;Ferrando et al. 2007) exist. ...
The Lake Tana area is located within a complex volcano-tectonic basin on the northwestern Ethiopian pla- teau. The basin is underlain by a thick succession of Oli- gocene transitional basalts and sub-alkaline rhyolites overlain in places, particularly south of the lake, by Qua- ternary alkaline to mildly transitional basalts, and dotted with Oligo-Miocene trachyte domes and plugs. This paper presents the results of integrated field, petrographic, and major and trace element geochemical studies of the Lake Tana area volcanic rocks, with particular emphasis on the Oligocene basalts and rhyolites. The studies reveal a clear petrogenetic link between the Oligocene basalts and rhy- olites. The Oligocene basalts are: (1) plagioclase, olivine, and/or pyroxene phyric; (2) show an overall decreasing trend in MgO, Fe2O3, and CaO with silica; (3) have rela- tively low Mg#, Ni and Cr contents and high Nb/La and Nb/Yb ratios; and (4) show LREE enriched and generally flat HREE patterns. All these imply the origin of the Oligocene basalts by shallow-level fractional crystallization of an enriched magma sourced at the asthenospheric mantle.The Oligocene rhyolites: (1) are enriched in incompatible while depleted in compatible trace elements, P and Ti; (2) show a strong negative Eu anomaly; (3) contain appreciable amounts of plagioclase, apatite, and Fe-Ti oxides; and (4) show clear geochemical similarity with well-constrained rhyolites from the Large Igneous Province (LIP) of the northwestern Ethiopian plateau. Low-pressure fractional crystallization of mantle-derived basaltic magma in crustal magma chambers explains the origin of these rhyolites. Our study further shows that the Oligocene basalts and rhyolites are co-genetic and the felsic rocks of the Lake Tana area are related differentiates of the flood basalt volcanism in the northwestern Ethiopian plateau.
... All rifts exhibit magmatic and volcanic features, such as dikes, sills, calderas, cones, and lava flows (Fig. 3). However, the degree of magmatism can vary drastically-from kilometer-thick flood basalts of the Ethiopian traps (Sembroni et al., 2016) to mere exhalations of volcanic gases, such as in the Central European Eger Rift (K€ ampf et al., 2013). The amount of volcanism is controlled by partial melting of mantle rocks, which is decisively governed by mantle temperature and to a lesser degree by mantle ascent rate. ...
... The topograpgy of the area, which is dominated by a thick sequence of Cenozoic volcanic rocks, is attributed to the interaction between mantle dynamics, lithosphere kinematics, magmatism, and surface processes (Davison I et al., 1998). Imaging Moho discontinuity is a key parameter to understanding the lithosphere-asthenosphere interaction, mechanisms of extensional tectonism and crustal deformation in volcanic passive margins (Sembroni A et al., 2016). ...
... The Scotian and Grand Banks Basins were the first Central Atlantic rifts to receive seawater via Tethys (Hames et al., 2002;and Schlische et al., 2003) despite the uplift or dynamic elevation that was probably present over a mantle plume (e.g., Schlische et al., 2003;Pindell and Heyn, 2022) which supplied basalts to the Central Atlantic Magmatic Province (CAMP) (Marzoli et al., 1999) (Fig. 1). The magnitude of positive dynamic topography is roughly 2 km in modern analogues such as the East Africa-Afar region, where rifting and magmatism are mostly topographically elevated above an upwelling mantle plume (Faccenna et al., 2013;Wilson et al., 2014;Sembroni et al., 2016;Hoggard et al., 2017). A few rifts of the Afar depression which have developed layered salt sequences are ~100 m below global sea level within a much broader region with an overall positive dynamically elevation. ...
This study describes the structural architecture of the central portion of the Scotian Basin and focusses on Mesozoic salt deposition and deformation linked to magma-rich rifting. BP acquired a high-quality 3D seismic survey called the Tangier survey and built a detailed velocity model with RFWI (Reflection Full Waveform Inversion) and tomography. This has allowed for high resolution seismic imaging of target sedimentary reservoirs but also for the first time, imaging of the complex crustal architecture along the central part of the Nova Scotia margin. We evaluated the regional basin architecture and salt tectonic evolution in context of Late Triassic and Early Jurassic rifting followed by passive margin gravity spreading. An exploration well (Aspy D-11) was drilled based on the 3D seismic interpretation and penetrated halite, carnallite and anhydrite salt in an allochthonous salt sheet. Basement is thought to have rifted during deposition of interlayered halite, shale, carnallite and anhydrite. Carnallite may have precipitated from enriched brines that formed at the more distal (southern) end of an elongated seaway of rifted basins which began opening first in the North in the Late Triassic and propagated southward towards Florida (e.g., Jansa, 1980; Jansa and Tucholke, 1986; Roberts and Bally, 2012). Water flowed southwards from Tethys along a chain of complexly interconnected areas of rifted basins. Flow of seawater along Nova Scotia is interpreted to have been southward along a widening and progressively subsiding region of rifts possibly as dynamic elevation associated with the CAMP (Central Atlantic Magmatic Province) dissipated southward much like what occurred in the Central South Atlantic and Gulf of Mexico (GoM) (e.g., Quirk et al., 2013; Morgan et al., 2020; Pindell et al., 2020; Pindell and Heyn, 2022). Brine was enriched along the seaway by evaporation, causing deposition of the most-soluble salt components furthest south along the sequence of basins. The presence of weathered basalt in salt and SDRs on the seismic supports magma-rich Mesozoic rifting and suggests that heating of brine pools may not have been exclusively caused by the sun, but that hydrothermal systems probably contributed to heating and the precipitation of Carnallite.
... As already outlined, between 29 and 20 Ma, this area, from the present-day continental platform to Sudan, was intermittently intruded and covered by the northernmost sprouts of the Afar plume, namely the Trap effusions (Ebinger and Casey 2001). In the open debate on what was the pre-Trap topography (e.g., Pik et al. 2003;Abbate et al. 2014;Coltorti et al. 2015;Sembroni et al. 2016), central and northern Eritrea is presumed to have been consisting of a wide low-elevation laterized peneplain covered by Trap basalts. After an early modest rifting event with basement coarse clastics emplaced before the Trap effusion, further sparse basalt basement coarse clastics supply is found along the Dogali succession. ...
The landscape of Eritrea is highly variable and reflects the complex geological history of the area, which is only partially shared with the other regions of the Horn of Africa. The structural geomorphology of Eritrea was investigated through field surveys, literature reviews and a few topographic profiles oriented east–west across the country. The information collected led to the production of a new schematic geological map. The older crustal deformations controlled the orientation of the main fluvial systems, whereas later tectonic events affected their upstream drainage networks. The geological history of Eritrea is very long as it started in the Neoproterozoic, though it was punctuated by a few, more or less long intervals of quiescence. The modern landforms derive from the combined effects of the powerful uplift, to which the whole Horn of Africa was subjected throughout the Cenozoic, and the present arid climate. Fluvial erosion resulted in the accumulation of clastic deposits, a few thousands of meters thick, which gave rise to the coastal belt. In spite of an average denudation rate of 30 mm/ka (a value similar to those inferred for the upper Blue Nile and other areas with a similar structural setting), hard rocks such as the laterites, formed on top of old peneplain surfaces, are still preserved and their present-day elevation along east–west profiles witnesses an impressive upwards dislocation of about 2500 m associated with the impingement of the Afar Plume. In Eritrea, the emplacement of Trap basalts is spatially rather limited, especially if compared with the impressive expansion all across the neighboring Ethiopia. Most of volcanic activity of Eritrea is recent (Quaternary) and associated with the very last phases of the Danakil depression formation. Presently, arid conditions and a volcanic morphology provide the Eritrean Danakil with a unique and fascinating landscape.
... In fact, the Jurassic sediments (pre-flood basalts surface) lie at an average altitude of ~2200 m (Gani et al., 2007), and the higher topography (~3700 m) of the western plateau at the latitude of the southern profile is due to the emplacement of ~1500-1800 m-thick flood-basalts deposits at the top of the Jurassic sediments. Hence, if the crust-mantle transition is a sharp boundary, then this has the implication that the western plateau is still under dynamic support process as proposed by Sembroni et al. (2016). On the other hand, if the crust-mantle is a gradational boundary of ~8.0 km thickness, the implications are that the western plateau is locally compensated with magmatic additions at the base of the lower crust as Tiberi et al. (2005) concluded from interpretation of gravity and seismic data. ...
We used teleseismic receiver function analysis to image the crustal structure beneath 24 broadband seismic stations densely deployed along two profiles traversing different structural units across the western Afar margin. Our high-resolution receiver function results image pronounced spatial variations in the crustal structure along the profiles and provide improved insights to understand how strain is partitioned in the crust during rifting. Beneath the western plateau next to northern Afar, the crust is likely felsic-to-intermediate in composition (average Vp/Vs 1.74), with a step like thinning of the crust from an average of 38 km beneath the western plateau to an average of 22 km beneath the marginal graben. Consistently thicker crust is observed beneath the southern profile (central Afar), showing four distinct regions of uniform crustal thickness: 1) an average crustal thickness of 42 km beneath the western plateau; 2) 34 km beneath the foothills area; 3) 28 km beneath the marginal graben and the wide extensional basin and 4) 21 km beneath the central rift axis. We use crustal thickness results to estimate a stretching factor β of 2.2 and 2.7 for central Afar and northern Afar respectively. Our estimated values are lower than β > 3.0 predicted from plate reconstructions, and we interpret that the variations are best explained by 2–5 km magmatic addition into the crust. The crustal composition beneath the southern profile is more complex with elevated Vp/Vs ratios ranging between 1.79 and 1.85 beneath the western plateau and marginal graben. This is consistent with a greater mafic component and best explained by crust altered by intrusions due to significant pre and syn-rift magmatic activity. Abnormally high Vp/Vs ratios of more than 1.90 are observed beneath the axial rift zone of central Afar, which most likely suggests the localization of partial melt within the crust.
... 14 flood basalts of the Ethiopian traps(Sembroni et al., 2016) to mere exhalations of volcanic15 gases, such as in the Central European Eger Rift(Kämpf et al., 2013). The amount of volcanism16 is controlled by partial melting of mantle rocks, which is decisively governed by mantle17 temperature and to a lesser degree by mantle ascent rate. ...
Continental rifts can form when and where continents are stretched. If the driving forces can overcome lithospheric strength, a rift valley forms. Rifts are characterised by faults, sedimentary basins, earthquakes and/or volcanism. With the right set of weakening feedbacks, a rift can evolve to break a continent into conjugate rifted margins such as those found along the Atlantic and Indian Oceans. When, however, strengthening processes overtake weakening, rifting can stall and leave a failed rift, such as the North Sea or the West African Rift. A clear definition of continental break-up is still lacking because the transition from continent to ocean can be complex, with tilted continental blocks and regions of exhumed lithospheric mantle. Rifts and rifted margins not only shape the face of our planet, they also have a clear societal impact, through hazards caused by earthquakes, volcanism, landslides and CO2 release, and through their resources, such as fertile land, hydrocarbons, minerals and geothermal potential. This societal relevance makes an understanding of the many unknown aspects of rift processes as critical as ever.
... The strong contrast in velocity structure from the Eastern Plateau into the rift (Figure 9) is in sharp contrast to the conjugate side of the rift valley, with the Western Plateau showing evidence for significant magmatic modification (e.g., Mackenzie et al., 2005;Chambers et al., 2019), indicating strong asymmetry to the rifting process. The lack of evidence for magmatic modification of the crust beneath the Eastern Plateau also favours a model of dynamic uplift from a deep-seated asthenospheric anomaly (e.g., Sembroni et al., 2016), as opposed to uplift being compensated by crustal magmatic additions (e.g., Keranen et al., 2009;Chambers et al., 2019). ...
We applied the receiver function (RF) technique on high-quality teleseismic earthquake data recorded by the RiftVolc broadband network from February 2016 to October 2017. We calculate RFs at 17 stations, which are inverted to estimate Vs, and Vp/Vs structure beneath the Central Main Ethiopian Rift and the Eastern plateau. The observed slow S-wave velocity (Vs) in the uppermost crust (<6 km depth) is interpreted as sedimentary and/or volcanic layers. Beneath the rift valley, crustal Vs is heterogeneous both laterally and with depth. In particular, slow Vs (∼2–3 km/s) is localised beneath volcanic centres in the upper-mid crust but ubiquitously slow in the lower crust with Vs as low as ∼3.5 km/s common. The slow lower crust is associated with high Vp/Vs ratios of ∼1.9–2.0. The Vs and Vp are consistent with the observed seismic velocities, and interpreted the presence of the small fraction (<5%) of partial melt from previous seismic imaging studies of the lower crust. In addition, the velocity contrast is small between the lower crust and upper mantle. The results suggest that partial melt in the lower crust beneath magmatically active rifts might be more widespread than previously thought and an important component of the magma plumbing system. In contrast, Vs is far more homogeneous and faster beneath the Eastern Plateau, with a distinct velocity contrast between the crust and upper mantle suggesting less crustal deformation than what is observed beneath the central rift zone.
... The incision of the valley as a result of a lowered erosional level and highland uplift could be the driving factor for the slope instability in the case of the Mejo area. Geomorphic proxies and the thickness of flood basalts suggest that the more tectonically active southeastern escarpment of the CMER and SMER (where the Mejo site is situated) are experiencing a relatively higher rate of tectonic uplift compared to the southeastern escarpment of the northern MER and the Afar Triangle (Xue et al., 2018;Sembroni et al., 2016). This can also be noted from the Eocene-Oligocene-Miocene basalts base (35-26 My) occurring in Arba Minch at an elevation of around 1050 m a.s.l., compared to their occurrence at a much higher elevation in Mejo at around 1900 m a.s.l. ...
The Main Ethiopian Rift (MER), where active continental
rifting creates specific conditions for landslide formation, provides a
prospective area to study the influence of tectonics, lithology,
geomorphology, and climate on landslide formation. New structural and
morphotectonic data from central Main Ethiopian Rift (CMER) and southern Main Ethiopian Rift (SMER) support a model of progressive change in the regional extension from NW–SE to the recent E(ENE)–W(WSW)
direction, driven by the African and Somali plates moving apart with the
presumed contribution of the NNE(NE)–SSW(SW) extension controlled by the
Arabian Plate. The formation and polyphase reactivation of faults in the
changing regional stress field significantly increase the rocks' tectonic
anisotropy, slope, and the risk of slope instabilities forming.
According to geostatistical analysis, areas prone to landslides in the
central and southern MER occur on steep slopes, almost exclusively formed on active normal fault escarpments. Landslide areas are also influenced by
higher annual precipitation, precipitation seasonality, vegetation density,
and seasonality. Deforestation is also an important predisposition because
rockfalls and landslide areas typically occur on areas with bushland,
grassland, and cultivated land cover.
A detailed study on active rift escarpment in the Arba Minch area revealed
similar affinities as in a regional study of MER. Landslides here are
closely associated with steep, mostly faulted, slopes and a higher density
of vegetation. Active faulting forming steep slopes is the main
predisposition for landslide formation here, and the main triggers are
seismicity and seasonal precipitation. The Mejo area situated on the
uplifting Ethiopian Plateau 60 km east of the Great Rift Valley shows that
landslide occurrence is strongly influenced by steep erosional slopes and a
deeply weathered Proterozoic metamorphic basement. Regional uplift,
accompanied by rapid headward erosion forming steep slopes together with
unfavourable lithological conditions, is the main predisposition for
landslide formation; the main triggers here are intense precipitation and
higher precipitation seasonality.
... Thermochronology data also do not record significant exhumational cooling of basement rocks until the Miocene (Fig. 8), contemporaneous with southern Red Sea rifting and significantly postdating Afar plume impingement (Davison et al., 1994;Menzies et al., 1997;Balestrieri et al., 2009;Ghebreab et al., 2002). Only after the rapid extrusion of flood basalt flows greater than 2 km thick , is there evidence for significant topographic development (Sembroni et al., 2016). This is recorded by the establishment of the current Nile River course (Fielding et al., 2018;Underwood et al., 2013) and Oligocene Blue Nile canyon incision of the Ethiopian plateau inferred from AHe data (Pik et al., 2003). ...
Low-temperature thermochronology has long been utilised in the Red Sea-Gulf of Aden rift systems and adjacent hinterlands to examine exhumation cooling histories of basement blocks, particularly where datable syn-tectonic strata and/or markers are often absent. Such data have provided insights into the spatio-temporal evolution of rift basins, morphotectonic rift shoulder development and timing and rate of surface uplift. However, the relatively limited number of samples and confined areas involved in many individual case studies have precluded insights into the longer wavelength tectonic and geodynamic phenomena responsible for the latest Eocene-Oligocene to Recent separation of the Arabian plate from African and its subsequent collision with Eurasia.
We present a synthesis of a large array of titanite, zircon and apatite fission track and (U-Th-Sm)/He analyses (12 TFT, 12 THe, 32 ZFT, 392 ZHe, 465 AFT and 267 AHe) from across northeast Africa and Arabia, which provide novel insights into the Phanerozoic thermo-tectonic evolution of the Arabian plate that were previously difficult to decipher from an otherwise cumbersome and intractably large dataset. Eocene to Recent cooling-heating maps have been constructed through a regional interpolation protocol of standardised thermal history models generated from the thermochronology data coupled with burial histories produced from vitrinite reflectance and well data. The interpolations, referred to here as thermo-tectonic images, record a series of pronounced episodes of upper crustal thermal regime fluctuation related to development of the latest Eocene-Oligocene-Recent Gulf of Aden and Red Sea rift systems and Cenozoic formation of the Al Hajar Mountains of Oman and the United Arab Emirates. They also provide insights into the inherited tectono-thermal histories of these regions, which controlled the spatial and temporal distribution of subsequent strain. Integration of the thermo-tectonic images, compiled with paleogeographic reconstructions and the regional igneous and strain history, offer a fresh regional perspective allowing the interrelationship between tectonism, geodynamic activity and exhumation history of the land surface to be visualised and explored on a plate scale.
... Ethiopian Plateau uplift, often attributed to mantle plumes, is estimated to have commenced 20-30 Ma (Pik et al., 2003) with some studies postulating more rapid Late-Miocene uplift resulting from lithospheric foundering in response to extensive heating of the lithosphere (Furman et al., 2016;Gani et al., 2007). However, more recent drainage analysis argues against rapid uplift relating to lithospheric delamination processes (Sembroni et al., 2016). East African Plateau uplift is likely to have proceeded Ethiopian Plateau uplift, with some estimates around 14 Ma (e.g., Wichura et al., 2010). ...
Abstract The Turkana Depression, a topographically subdued, broadly rifted zone between the elevated East African and Ethiopian plateaus, disrupts the N–S, fault-bounded rift basin morphology that characterizes most of the East African Rift. The unusual breadth of the Turkana Depression leaves unanswered questions about the initiation and evolution of rifting between the Main Ethiopian Rift (MER) and Eastern Rift. Hypotheses explaining the unusually broad, low-lying area include superposed Mesozoic and Cenozoic rifting and a lack of mantle lithospheric thinning and dynamic support. To address these issues, we have carried out the first body-wave tomographic study of the Depression's upper mantle. Seismically derived temperatures at 100 km depth exceed petrological estimates, suggesting the presence of mantle melt, although not as voluminous as the MER, contributes to velocity anomalies. A NW–SE-trending high wavespeed band in southern Ethiopia at <200 km depth is interpreted as refractory Proterozoic lithosphere which has likely influenced the localization of both Mesozoic and Cenozoic rifting. At <100 km depth below the central Depression, a single localized low wavespeed zone is lacking. Only in the northernmost Eastern Rift and southern Lake Turkana is there evidence for focused low wavespeeds resembling the MER, that bifurcate below the Depression and broaden approaching southern Ethiopia further north. These low wavespeeds may be attributed to melt-intruded mantle lithosphere or ponded asthenospheric material below lithospheric thin-spots induced by the region's multiple rifting phases. Low wavespeeds persist to the mantle transition zone suggesting the Depression may not lack mantle dynamic support in comparison to the two plateaus.
... Reconstruction of the geometry of the lava flow provides constraints on the uplift history (Sembroni et al., 2016). After the lava flood emplacement around 30 Ma we see an increase of the average elevation by ~1000 m that ended around 10-12 Ma, prior to the rifting. ...
... The Ethiopian-Yemen basalts (related to the Afar hotspot) cover the Ethiopian, Somalian, and Yemen Plateaus, an area of ~10 6 km 2 , with an average thickness of ~1.5 km. Volume estimates for the Ethiopian-Yemen basalts range from 9 × 10 5 to 1.2 × 10 6 km 3 (Hofmann et al., 1997;Ukstins et al., 2002;Sembroni et al., 2016). The lowermost lava flow from the flood-basalt activity in Ethiopia was dated by 40 Ar/ 39 Ar methods at 30.4 ± 0.4 Ma (in the Rupelian Stage of the Oligocene). ...
... The lack of any feeder mafic dyke swarm at the present erosion level allows to exclude significant magma-assisted extension through dyking during the RBS basin development. The little strained structure of the RBS hanging wall block is thus at odds with large-offset vertical tectonics documented along the WAM fault-scarp (Sembroni et al., 2016). New insights into the uplift history of the western rift margin are supplied by specific structures displayed by the Jurassic limestone cover in the foothills, immediately west of the depression (Fig. 12A). ...
The Erta Ale rift segment, North Afar, is regarded as the most mature rift part within the entire Afar rift system. Very little is known about its deformation history because of limited exposures of geological records in its inner floor, except volcanics of the Erta Ale chain, and the poorly-known Red Beds series along the flanks of the depression. An integrated study, combining sedimentological, geochemical, ⁴⁰Ar-³⁹Ar radiometric and tectonic approaches, has been devoted to Red Beds series flanking the depression to the SW. Our new results allow to argue that (1) the >300 m-thick Red Beds exposed section comprises alluvial deposits that enclose (2) basaltic lavas and related sill intrusions that both yield ~6 Ma ⁴⁰Ar-³⁹Ar ages and display similar geochemical affinities, (3) the Red Beds series locally overlap unconformably basement bounding terrains, and (4) are involved in a limited number of low-displacement normal faults that recorded a modest amount of extension (<6%), (5) isotope contents of Red Beds volcanics indicate crustal contamination, without any contribution of the Afar plume, by contrast to the younger Erta Ale magmatism which represents the more recent Afar-plume related event in the Erta Ale segment. Combining these results leads us to regard the Red Beds series as part of an alluvial basin that post-dated a major rift event to which are attributed to (1) the present-day Ethiopian fault-scarp, (2) a concealed sedimentary depocenter at depth in its hanging wall, and (3) prominent crustal thinning. Riftward migration and axial focusing of strain during Miocene-Present times is also argued, while later flexuring of the entire Red Beds basin is assigned to magmatic loading during the axial emplacement of the Erta Ale volcanic chain. Lastly, emphasis is put on the large-scale segmentation of the Afar system into the tectonically-accreted Erta Ale rift segment to the north, and the magmatically-accreted Central segment to the south.
... At the same time, anomalous positive topography started to form in East Africa 23 , with the first evidence of doming in Ethiopia at ~30-40 Ma likely related to the Afar plume emplacement 22 . The current long-wavelength elevation of eastern and southern Africa, dynamically supported by the African Superplume, a broad low-velocity seismic anomaly imaged in the lower mantle 24,25 , is acquired at ~10 Ma 23 although the exact timing and dynamics of uplift in the Horn of Africa are still controversial [26][27][28] . The resulting lateral gradients of gravitational potential energy generate EW-directed extensional deviatoric stresses corresponding to forces on the same order as slab pull forces when integrated over the thickness of the lithosphere 29 . ...
Divergent ridge-ridge-ridge (R-R-R) triple junctions are one of the most remarkable, yet largely enigmatic, features of plate tectonics. The juncture of the Arabian, Nubian, and Somalian plates is a type-example of the early development stage of a triple junction where three active rifts meet at a ‘triple point’ in Central Afar. This structure may result from the impingement of the Afar plume into a non-uniformly stressed continental lithosphere, but this process has never been reproduced
by self-consistent plume-lithosphere interaction experiments. Here we use 3D thermo-mechanical numerical models to examine the initiation of plume-induced rift systems under variable far-field stress conditions. Whereas simple linear rift structures are preferred under uni-directional extension, we find that more complex patterns form in response to bi-directional extension, combining one or several R-R-R triple junctions. These triple junctions optimize the geometry of continental break-up by minimizing the amount of dissipative mechanical work required to accommodate multi-directional extension. Our models suggest that Afar-like triple junctions are an end-member mode of plume-induced bi-directional rifting that combines asymmetrical northward pull and symmetrical EW extension at similar rates.
... These dynamic topography contributions add to the isostatically compensated topography, especially above regions of focused upwelling. For the Ethiopian dome, independent studies attributed amplitudes of ~1.5 km to this kind of dynamic mantle support [Hoggard et al., 2016;Sembroni et al., 2016], which is why we chose this value for our initial topographic baseline. ...
Inherited rheological structures in the lithosphere are expected to have large impact on the architecture of continental rifts. The Turkana depression in the East African Rift connects the Main Ethiopian Rift to the north with the Kenya rift in the south. This region is characterized by a NW-SE trending band of thinned crust inherited from a Mesozoic rifting event, which is cutting the present-day N-S rift trend at high angle. In striking contrast to the narrow rifts in Ethiopia and Kenya, extension in the Turkana region is accommodated in sub-parallel deformation domains that are laterally distributed over several hundred kilometers. We present both analog experiments and numerical models that reproduce the along-axis transition from narrow rifting in Ethiopia and Kenya to a distributed deformation within the Turkana depression. Similarly to natural observations, our models show that the Ethiopian and Kenyan rifts bend away from each other within the Turkana region, thus forming a right-lateral stepover and avoiding a direct link to form a continuous N-S depression. The models reveal five potential types of rift linkage across the pre-existing basin: three types where rifts bend away from the inherited structure connecting via a (1) wide or (2) narrow rift or by (3) forming a rotating microplate, (4) a type where rifts bend towards it, and (5) straight rift linkage. The fact that linkage type 1 is realized in the Turkana region provides new insights on the rheological configuration of the Mesozoic rift system at the onset of the recent rift episode.
... Similarly, Tiercelin et al. (2012b) noted the synchronism between the abrupt termination of Lapur Sandstone deposition and the onset of volcanism in southern Ethiopia and northern Turkana between 45 and 35 Ma, suggesting this may be related to uplift associated with the arrival of a mantle plume. The impingement of such an asthenospheric upwelling beneath the Ethiopian lithosphere is considered responsible for the late Eocene-Oligocene onset of doming of the Ethiopian Plateau (Ebinger and Sleep, 1998;Pik et al., 2003;Sembroni et al., 2016). Similarly, uplift above an asthenospheric thermal anomaly is thought to have caused the formation of the East African Dome during Miocene-Recent times (Pik, 2011;Wichura et al., 2011). ...
The Turkana Depression is a structurally complex and long-lived segment of the East African Rift System (EARS), with associated magmatism and strain nucleating there in the late Paleogene. The anomalously wide, ~N-S rift zone defines the topographic lowlands separating the Ethiopian and East African Domes. The atypical architecture and morphology of the Turkana Depression has often been attributed to the influence of an oblique, pre-existing lithospheric heterogeneity speculated to result from earlier Cretaceous-early Paleogene Anza-South Sudan rifting. However, this hypothesized period of earlier rifting is poorly constrained due to the obscuring effects of extensive Cenozoic rifting and volcanism. Similarly, the extent and timing of basin formation during the initial stages of EARS extension in Turkana is not well understood. Seismic reflection studies in Turkana have revealed the presence of older, possibly late Paleogene sub-basins, predating the Neogene onset of major faulting elsewhere in the EARS. One example, the Lothidok Basin, has previously been imaged beneath the late Miocene-Pliocene North Lokichar Basin. Its age, however, is unconstrained due to a lack of well controls, geochronological constraints and outcrop of its basal unit. Here, we present a multiple low-temperature thermochronometer [apatite fission track, apatite (U-Th-Sm)/He and zircon (U-Th)/He] study performed on Precambrian basement samples from the western margin of the overlying North Lokichar Basin. Thermal history modelling reveals a polyphase Late Cretaceous-Recent tectono-thermal evolution providing new insights into pre-EARS tectonism in Turkana and subsequent, late Paleogene ~E-W extension. Pronounced Late Cretaceous-Paleogene denudational cooling challenges the theorized linkage of the Anza-South Sudan Rifts in Turkana, perhaps suggesting later Paleogene tectonism played a more critical role in modifying the lithosphere. Subsequent Oligocene-early Miocene reheating is interpreted as resulting from burial beneath ~200–800 m of overburden, accordant with the proposed formation of the Lothidok Basin and/or coeval emplacement of thick lava flows in the region.
... The lively debate between opposite views about the recent versus ancient origin of the Nile reveals how the available information is not solid enough to develop a thorough understanding of the pre-Quaternary evolution of the Nile river system. Only a tentative piecemeal reconstruction of potential paleotectonic (Pik et al., 2003;Gani et al., 2009;Sembroni et al., 2016) and paleoclimatic scenarios (Griffin, 2002;Pickford et al., 2006;Sepulchre et al., 2006;Abdelkareem et al., 2012) is possible at present. The Quaternary, and especially the late Quaternary history of the Nile, is far better constrained . ...
This Nile Delta case study provides quantitative information on a process that we must understand and consider in full before attempting provenance interpretation of ancient clastic wedges. Petrographic and heavy-mineral data on partly lithified sand, silt, and mud samples cored from the up to 8.5 km-thick post-Eocene succession of the offshore Nile Delta document systematic unidirectional trends. With increasing age and burial depth, quartz increases at the expense of feldspars and especially of mafic volcanic rock fragments. Heavy-mineral concentration decreases drastically, transparent heavy minerals represent progressively lower percentages of the heavy fraction, and zircon, tourmaline, rutile, apatite, monazite, and Cr-spinel relatively increase at the expense mainly of amphibole in Pliocene sediments and of epidote in Miocene sediments. Recent studies have shown that the entire succession of the Nile Delta was deposited by a long drainage system connected with the Ethiopian volcanic highlands similar to the modern Nile since the lower Oligocene. The original mineralogy should thus have resembled that of modern Delta sand much more closely than the present quartzose residue containing only chemically durable heavy minerals. Stratigraphic compositional trends, although controlled by a complex interplay of different factors, document a selective exponential decay of non-durable species through the cored succession that explains up to 95% of the observed mineralogical variability. Our calculations suggest that heavy minerals may not represent >20% of the original assemblage in sediments buried less than ~1.5 km, >5% in sediments buried between 1.5 and 2.5 km, and >1% for sediments buried >4.5 km. No remarkable difference is detected in the intensity of mineral dissolution in mud, silt, and sand samples, which argues against the widely held idea that unstable minerals are prone to be preserved better in finer-grained and therefore presumably less permeable layers. Intrastratal dissolution, acting through long periods of time at the progressively higher temperatures reached during burial, can modify very drastically the relative abundance of detrital components in sedimentary rocks. Failure to recognize such a fundamental diagenetic bias leads to grossly mistaken paleogeographic reconstructions, as documented paradigmatically by previous provenance studies of ancient Nile sediments.
Afar is undergoing the final stages of continental rifting and hosts the triple junction between the Red Sea, Gulf of Aden, and Main Ethiopian rifts. To better understand the nature of the crust and continental breakup in the region, we calculate teleseismic receiver functions across northeastern Afar and the Danakil microplate, using new data from a regional deployment in Eritrea. We estimate the Moho depth and bulk crustal VP/VS ratio using the H‐κ stacking method. The heterogeneity of our crustal thickness estimates (∼19–35 km) indicates that the Danakil microplate has undergone stretching and crustal thinning. By investigating the relationship between crustal thickness and topographic elevation, we estimate the regional crustal bulk density as ρc ≈ 2,850 ± 20 kg m⁻³, which is higher than expected, given the crustal thickness of the region. We show that topography is 1.5 ± 0.4 km higher than would be expected due to crustal isostasy alone. We propose that this topography is supported by the same hot mantle upwelling suggested to be responsible for the onset of rifting in East Africa. Uplift is generated due to the presence of a hot thermal anomaly beneath the plate and by thinning of the lithospheric mantle. Our results are consistent with a number of independent constraints on the thermal structure of the asthenospheric and lithospheric mantle. Evidence of melt within the crust is provided by anomalously high VP/VS ratios of >1.9, demonstrating that magma‐assisted extension continues to be important in the final stages of continental breakup.
Modern volcanoes and volcanic centres encompass a wide variety of morphological, physical, facies and stratigraphic characteristics, and duration of primary volcanic versus sedimentary or epiclastic processes. This leads to the development of general facies models, which are relevant for modern settings, and valuable guides for making meaningful geological reconstructions of ancient volcanic successions. Our discussion covers the full scale of volcanic landforms, ranging from small individual monogenetic scoria and pumice cones and phreatomagmatic maar-type volcanoes, to larger polygenetic volcanoes. These include marine basaltic shields that from their base on the seafloor represent some of the largest volcanoes on the planet, as well as eruptive vents for flood plateau and plains basalt provinces, stratovolcanoes both found in both continental and marine settings, and the largest silicic explosive caldera volcanoes. The latter are sometimes called “supervolcanoes” which are responsible for some of the biggest explosive eruptions recorded in geological history, resulting from structural caldera collapse and in some settings contributing to regional ignimbrite “flare-up” events. Moreover, the special case of marine silicic calderas is also considered. We also now recognise a number of more complex intermediate to silicic volcanic systems; these include “multiplex volcanoes” which have changed in character through their evolution, and other multi-vent centres that appear to occur in the absence of a large central cone structure, or conversely are not controlled by caldera collapse depressions. The remaining two sections are grouped into intra- or subglacial volcanoes (formed under ice and meltwater lakes), and mafic oceanic volcanic centres, namely, spreading ridges, plateaus, seamounts, and surtseyan tuff cones. We also introduce the reader to the economic significance of potentially prospective volcano types and their successions prior to their detailed description in Chap. 18, as well as concluding with a summary of the potential hazards posed by different types of volcanoes.
Since publication of Cas and Wright (1987), the relationship between volcanism and tectonic setting has expanded into an enormous and multidisciplinary topic. In this chapter, we provide an updated review of the known tectonic settings in which volcanism occurs today and the relatively recent past, and the overall geological characteristics of these settings using the plate tectonic model as a global framework for Phanerozoic and Proterozoic times. However, there is little evidence that plate tectonics operated earlier during the Hadean and Archean, which represents almost 50% of geological time, and so we will also consider what the tectonic paradigm for volcanism was during the early history of the Earth. Initially, during the two billion years prior to when plate tectonics developed, the Earth was a magma ocean. This evolved into a single mafic–ultramafic crustal shell (stagnant lid) as the Earth cooled and was affected by mantle plume activity that influenced most volcanism. Buoyant diapiric granitoid plutonism resulting from anatexis of mafic lower crust allowed the Earth’s crust to evolve and “felsify”, leading to cratonic nuclei. Localised, limited subduction (mobile lid) commenced in the late Archean, but global plate tectonics probably didn’t develop as we know it until the Proterozoic (<2,500 Ma). In the post-Proterozoic plate tectonic framework, the first-order division of settings for volcanism is based on the overall prevailing regional tectonic regime: divergent, oblique/strike-slip, “passive or hot spot” (i.e. plume-related), or convergent settings. We describe the styles of volcanism typically found in the various plate margin and intraplate tectonic settings, incorporating information on crustal setting, magma generation, as well as the potential for the formation of Large Igneous Provinces (LIPs), providing examples from ancient terrains. In this discussion we will also introduce some of the more recent geodynamic concepts and their relationship to magmatic and volcanic activity. In particular, seismic tomography has evolved as one of the most powerful tools for studying the structure and dynamics of subduction zones and convergent plate margins. Finally, guidelines are given for the evaluation of the tectonic context in ancient volcanic and mineralised terrains. This chapter provides a dynamic framework to accompany our concluding chapter (Chap. 18) on volcanic-hosted resources.
Continental topography is dominantly controlled by a combination of crustal thickness and density variations. Nevertheless, it is clear that some additional topographic component is supported by the buoyancy structure of the underlying lithospheric and convecting mantle. Isolating these secondary sources is not straightforward, but provides valuable information about mantle dynamics. Here, we estimate and correct for the component of topographic elevation that is crustally supported to obtain residual topographic anomalies for the major continents, excluding Antarctica. Crustal thickness variations are identified by assembling a global inventory of 26,725 continental crustal thickness estimates from local seismological data sets (e.g., wide‐angle/refraction surveys, calibrated reflection profiles, receiver functions). In order to convert crustal seismic velocity into density, we develop a parametrization that is based upon a database of 1,136 laboratory measurements of seismic velocity as a function of density and pressure. In this way, 4,120 new measurements of continental residual topography are obtained. Observed residual topography mostly varies between ±1 and 2 km on wavelengths of 1,000–5,000 km. Our results are generally consistent with the pattern of residual depth anomalies observed throughout the oceanic realm, with long‐wavelength free‐air gravity anomalies, and with the distribution of upper mantle seismic velocity anomalies. They are also corroborated by spot measurements of emergent marine strata and by the global distribution of intraplate magmatism that is younger than 10 Ma. We infer that a significant component of residual topography is generated and maintained by a combination of lithospheric thickness variation and sub‐plate mantle convection. Lithospheric composition could play an important secondary role, especially within cratonic regions.
The Asian monsoons are triggered by complex interactions between the atmosphere, Asian and African orography, and the surrounding oceans, resulting in highly seasonal climate and specific regional features. It was thought that the Asian monsoon was established during the Neogene, but recent evidence for monsoon-like precipitation seasonality occurring as early as the Paleogene greenhouse period challenges this paradigm. The possible occurrence of monsoons in a climatic and paleogeographic context very different from the present-day questions our understanding of the drivers underpinning this atmospheric phenomenon, in particular with regard to its dependence on geography. In this study, we first take advantage of the wealth of new studies to tentatively draw an up-to-date picture of Asian tectonic and paleoenvironmental evolution throughout the Cenozoic. We then analyse a set of 20 paleoclimate simulations spanning the late Eocene to latest Miocene (~40-8 Ma) in order to better understand the evolution of the distinct Asian monsoon subsystems. At odds with the traditional view of a monsoonal evolution driven mainly by Himalayan-Tibetan uplift, our work emphasizes the importance of peripheral mountain ranges in driving the evolution of Asian climate. In particular, the uplift of East African and Anatolian-Iranian mountain ranges, as well as the emergence of the Arabian Peninsula, contribute to shaping the modern South Asian summer monsoon. We also suggest that East Asian monsoon establishment and the aridification of inland Asia are driven by a combination of factors including increasing continentality, the orographic evolution of the Tibetan Plateau, Mongolia, Tian Shan and Pamir, and pCO2 decrease during the Cenozoic.
Rifted-passive margins extend over a distance of 105,000 km of the Earth’s surface and account for 30% of the world’s giant oil and gas fields. Rifted-passive margins are largely known from subsurface mapping using various geophysical methods because they are largely covered by water and thick sedimentary and magmatic deposits. Most rifted-passive margins have been studied by academic groups and explored by the oil industry. The objective of this chapter is to summarize advances in the academic understanding of rifted-passive margins based on recent studies from the last 10–15 years and then apply this information to practical exploration challenges such as: (1) Does a productive rifted-passive margin mean that its conjugate is a “look-alike” basin with similar hydrocarbon as its hydrocarbon-rich twin? (2) Where should the deepwater limit of deepwater data acquisition and exploration be drawn in the deep area of a rifted-passive margin—at the continent-ocean transition or extending into even deeper water in the area underlain by oceanic crust? (3) Should volcanic rifted-passive margins whose accommodation is largely filled by volcanic and volcaniclastic rocks be considered a lower priority for deepwater exploration than a non-volcanic, rifted-passive margin whose accommodation is largely filled by more porous and permeable sedimentary rocks with greater source and reservoir potential?
Northern Malawi's Nyika Plateau is a 3,700 km² large, highly elevated (∼2,500 m) plateau located at the western margin of the Miocene‐Recent Malawi rift and the confluence of multiple Proterozoic orogenic belts. Neighboring asthenospheric upwelling in the Rungwe Volcanic Province, associated with the active East African Rift, has created similar topographic highs, leading some to speculate that the formation of Nyika could be related. Here, we present new low‐temperature data using apatite fission track, apatite (U‐Th‐Sm)/He and zircon (U‐Th)/He thermochronology to constrain the upper crustal thermal history of the Nyika region since the Devonian. The data suggest that Nyika was an isolated feature since at least the Permo‐Triassic, well before more recent rifting in Malawi, and may have developed as a horst between two large Karoo grabens, the Henga‐Ruhuhu and the North Rukuru to the southeast and northwest, respectively. Similarities between the thermal histories of Nyika and the currently separated Livingstone Plateau to the east allow for the possibility that these may have been connected in a contiguous highland prior to the formation of the intervening Neogene Malawi rift. Thermal history models for exposed Precambrian basement samples adjacent to Nyika, and once buried beneath the neighboring Karoo basins, indicate that up to 3.4 km of Permo‐Triassic section has since been eroded, with samples along the plateau not indicating burial of Karoo‐type sediment at this time. Most recent cooling histories suggest that the plateau surface continued to denude at varying degrees from the Cretaceous and reached near‐surface temperatures in the Late Paleogene‐Neogene.
Diverse lines of geological and geochemical evidence indicate that the Eocene-Oligocene transition (EOT) marked the onset of a global cooling phase, rapid growth of the Antarctic ice sheet, and a worldwide drop in sea level. Paleontologists have established that shifts in mammalian community structure in Europe and Asia were broadly coincident with these events, but the potential impact of early Oligocene climate change on the mammalian communities of Afro-Arabia has long been unclear. Here we employ dated phylogenies of multiple endemic Afro-Arabian mammal clades (anomaluroid and hystricognath rodents, anthropoid and strepsirrhine primates, and carnivorous hyaenodonts) to investigate lineage diversification and loss since the early Eocene. These analyses provide evidence for widespread mammalian extinction in the early Oligocene of Afro-Arabia, with almost two-thirds of peak late Eocene diversity lost in these clades by ~30 Ma. Using homology-free dental topographic metrics, we further demonstrate that the loss of Afro-Arabian rodent and primate lineages was associated with a major reduction in molar occlusal topographic disparity, suggesting a correlated loss of dietary diversity. These results raise new questions about the relative importance of global versus local influences in shaping the evolutionary trajectories of Afro-Arabia’s endemic mammals during the Oligocene.
The understanding of the lateral variations in the lithospheric strength can help in identifying the distribution pattern of surface deformation and its response to long-term forces that are related to deep Earth processes. Former studies suggest that Afar and the Ethiopian Plateaus are not in isostatic equilibrium indicating the presence of deep compensation processes. We estimated the effective elastic thickness of the lithosphere (Te) over Ethiopia and its surrounding region using a method that combines the Vening Meinesz-Mortiz and flexural theory of isostasy in a spherical harmonics domain. Our effective elastic thickness model is different from those presented in the previous similar studies, because of considering the effects of mantle dynamics. We incorporated the uplift forces due to mantle plume in additions to surface and sub-surfaces loads, like topographic/bathymetric masses, crustal crystalline and sediments in our method so that it becomes more appropriate for the regions. In addition, laterally-variable models of upper mantle density, Young's modulus and Poisson's ratio have been considered from the CRUST1.0 model. We found a significant improvement of the solution in the Te estimate with consideration of the dynamic effects when we compared it with other studies especially in Afar, the Ethiopian plateaus, and the Main Ethiopian Rift valley. Our results are also consistent with significant lithospheric strength present in cratonic formations, with maximum of Te in Sudan and Tanzanian Cratons. Low Te was found over different tectonically active places such as, Afar and the Main Ethiopian Rift valley.
Views on when the Nile System (henceforth the Nile) connected from its sources in the Ethiopian and East Africa plateaus to its sink in the eastern Mediterranean have been vastly different. These include the ones advocating for it to be as old as early Oligocene (~30 Ma) to those which consider it to be as young as Pleistocene (~2.5 Ma). This work presents geoscientific observations supporting the notion that the emergence of the Nile System as a major trans-continental drainage system with its present-day drainage pattern, as it was assembled from several drainage sub-systems, is as recent as Pleistocene in age. Geomorphological observations show that the two sources of the Nile are separated by the ~300 km wide Turkana Depression, a feature that pre-dates the development of the dynamic topography of the two sources of the Nile. This indicates that there was no connectivity between drainage systems of the two sources of the Nile other than in its downstream in Khartoum, Sudan. Geochronological studies of samples from Precambrian crystalline basement rocks and morpho-tectonic analyses using Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) indicate that the evolutionary paths of the dynamic topography of the two sources of the Nile were independent and have evolved through separate volcanic and tectonic uplift histories since the early Oligocene. These studies also indicate that the major tectonic uplift phase in the two sources of the Nile is much younger than early Oligocene, and that it has occurred during the Pliocene (~5.4 Ma). Geomorphological observations from eastern and northern Sudan (Gezira alluvium fan and the Great Bend of the Nile) and southern Egypt (the Tortonian (~11 Ma – ~7 Ma) rivers of Egypt) do not support the existence of an integrated Nile drainage system since the early Oligocene. Rather, these observations show the youthful nature of the Nile, and that the integrated Nile was only established after the Messinian (~5.6 Ma). Records of the sedimentary rocks that fill the Messinian Eo-Nile canyon show that only the top part of the canyon is filled with sediment sourced from the Ethiopian plateau. Sediment budget considerations point to that building of the Oligocene to Pliocene sedimentary section of the submerged Nile delta cone requires sediment sources other than the Ethiopian plateau. Mineralogical, paleontological, and geochemical provenance studies show that the contribution of sediment transported from the Ethiopian plateau by the Blue Nile and the Tekeze River to the submerged Nile delta cone might have only commenced at the beginning of the Pleistocene.
The Ethiopian Highlands, with up to 1,500-m-deep canyons surrounded by low relief plateau surfaces, are one of the most spectacular examples of transient fluvial landscapes on Earth. We analyze river profiles extracted from a 90 mdigital elevation model of the upper Blue Nile catchment and identify 116 major knickpoints on 137 river profiles. We use 1-D river profile models to simulate three potential mechanisms for knickpoint formation: plateau uplift, capture of large lakes or internal drainage on the plateau surface, and sediment-flux-dependent river incision with a low sediment flux from the plateau surface. We define a normalized upstream knickpoint propagation distance, χkpj, and demonstrate that the erosion models predict different distributions of this metric in transient profiles following plateau uplift. Model knickpoints resulting from the scenario of plateau uplift or common base-level fall using the stream-power model display similar upstream propagation distance in χ space. The results of the same scenario modeled with the sediment-flux-dependent incision model show upstream knickpoint propagation distance proportional to catchment area. Perturbations to these trends result from drainage capture. Comparing the model results with observed χkpj values of knickpoints and field observations, we recognize effects characteristic of the sediment-flux-dependent incision model. However, most profiles are best explained by the systematic transfer of drainage area from the plateau to the surrounding rivers. We propose a new model of landscape evolution for the upper Blue Nile catchment dominated by discrete events of capture of drainage area from the plateau.
The Ethiopian region records about one billion years of geological history. The first event was the closure of the Mozambique ocean between West and East Gondwana with the development of the Ethiopian basement ranging in age from 880 to 550 Ma. This folded and tilted Proterozoic basement underwent intense erosion, which lasted one hundred million years, and destroyed any relief of the Precambrian orogen. Ordovician to Silurian fluviatile sediments and Late Carboniferous to Early Permian glacial deposits were laid down above an Early Paleozoic planation surface. The beginning of the breakup of Gondwana gave rise to the Jurassic flooding of the Horn of Africa with a marine transgression from the Paleotethys and the Indian/Madagascar nascent ocean. After this Jurassic transgression and deposition of Cretaceous continental deposits, the Ethiopian region was an exposed land for a period of about seventy million years during which a new important peneplanation surface developed. Concomitant with the first phase of the rifting of the Afro/Arabian plate, a prolific outpouring of the trap flood basalts took place predominantly during the Oligocene over a peneplained land surface of modest elevation. In the northern Ethiopian plateau, huge Miocene shield volcanoes were superimposed on the flood basalts. Following the end of the Oligocene, the volcanism shifted toward the Afar depression, which was experiencing a progressive stretching, and successively moved between the southern Ethiopian plateau and the Somali plateau in correspondence with the formation of the Main Ethiopian Rift (MER). The detachment of the Danakil block and Arabian subcontinent from the Nubian plate resulted in steep marginal escarpments marked by flexure and elongated sedimentary basins. Additional basins developed in the Afar depression and MER in connection with new phases of stretching. Many of these basins have yielded human remains crucial for reconstructing the first stages of human evolution. A full triple junction was achieved in the Early Pliocene when the MER penetrated into the Afar region, where the Gulf of Aden and the Red Sea rifts were already moving toward a connection via the volcanic ranges of northern Afar. The present-day morphology of Ethiopia is linked to the formation of the Afar depression, MER, and Ethiopian plateaus. These events are linked to the impingement of one or more mantle plumes under the Afro-Arabian plate. The elevated topography of the Ethiopian plateaus is the result of profuse volcanic accumulation and successive uplift. This new highland structure brought about a reorganization of the East Africa river network and a drastic change in the atmospheric circulation.
BETWEEN Lake Tana and the Sudan border the Blue Nile has cut a deep
gorge across the Ethiopian Highlands. The first 150 km of its course
lies on lavas of the Tertiary Trap Series, after which the river flows
through horizontal Mesozoic and Palaeozoic sediments until the
Precambrian is reached 330 km from Lake Tana1. About 280 km
below the lake the Addis Ababa-Debre Markos road crosses the gorge at
10° 05'N, 38° 10'E, where 250 m of Trap Series basalts overlie
at least 1,150 m of sediments the base of which is not exposed. The
gorge is 1,400 m deep and 20 km wide and is incised into a plateau 2,600
m high.
This outline of the topographic evolution of Africa tied to the history of the African Surface illustrates how a unique geomorphic history over the past 180 million years refl ects the continent's distinctive tectonics. The African Surface is a composite surface of continental extent that developed as a result of erosion following two episodes of the initiation of ocean fl oor accretion around Afro-Arabia ca. 180 Ma and 125 Ma, respectively. The distinctive tectonic history of the African continent since 180 Ma has been dominated by (1) roughly concentric accretion of ocean fl oor following those two episodes; (2) slow movement of the continent during the past 200 m.y. over one of Earth's two major large low shear wave velocity provinces (LLSVPs) immediately above the core-mantle boundary; (3) the eruption during the past 200 m.y. of deep mantle plumes that have generated large igneous provinces (LIPs) from the coremantle boundary only at the edge of the African LLSVP; and (4) two episodes during which basin-and-swell topography developed and abundant intracontinental rifts and much intra-plate volcanism occurred. Those episodes can be attributed to shallow convection resulting from plate pinning, i.e., arrested continental motion, induced by the successive eruption of the Karroo and Afar plumes. Shallow convection during the second plate-pinning episode generated the basins and swells that dominate Africa's present relief. By the early Oligocene, Afro-Arabia was a low-elevation, low-relief land surface largely mantled by deeply weathered rock. When the Afar plume erupted ca. 31 Ma, this Oligocene land surface, defi ned here as the African Surface, started to be fl exed upward on newly forming swells and to be buried in sedimentary basins both in the continental interior and at the continental margins. Today the African Surface has been stripped of its weathered cover and partly or completely eroded from some swells, but it also survives extensively in many areas where a lateritic or bauxitic cover has accordingly been preserved. Great Escarpments, which are best developed in the southern part of the continent, have formed on some swell fl anks since the swells began to rise during the past 30 m.y. They separate the high ground on the new swells from low lying areas, and because they face the ocean at some distance from the African coastline, they mimic rift fl ank escarpments at younger passive margins. The youthful Great Escarpments have developed in places where the original rift fl ank uplifts formed at the time of continental breakup. Their appearance is therefore deceptive. The African Surface and its overlying bauxites and laterites embody a record of tectonic and environmental change, including episodes of partial fl ooding by the sea, during a 150-million-year long interval between 180 Ma and 30 Ma. Parts of African Surface history are well known for some areas and for some intervals. Analysis here attempts to integrate local histories and to work out how the surface of Afro-Arabia has evolved on the continental scale over the past ∼180 m.y. We hope that because major landscape development theories have been spawned in Africa, a review that embodies modern tectonic ideas may prove useful in re-evaluation of theory both for Africa itself and for other continents. We recognize that in a continental-scale synthesis such as this, smoothing of local disparities is inevitable. Our expectation is that the ambitious model constructed on the basis of our review will serve as a lightning rod for elaborating alternative views and stimulating future research.
Rifts feature highly in the geological history of the Earth, as well as on the surfaces of other solar system bodies such as Mars. On Earth, continental rifts can develop into new plate boundaries where oceanic lithosphere starts to form. This paper examines a seismically and volcanically active rift system, the East African Rift, in which all stages of this complex evolution can be identified and modelled. Using evidence from geological, geophysical and geochemical studies, I outline the deformation of a continental plate from rift initiation to continental break-up, citing examples from the length of the East African Rift.
Rift formation is a crucial topic in global tectonics. The Yemen rift-related area is one of the most important provinces, being connected to the rifting processes of the Gulf of Aden, the Red Sea and Afar Triangle. In this paper, a review of the Yemen volcanic province and its relations with the Red Sea rifting are presented. Tertiary continental extension in Yemen resulted in the extrusion of large volumes of effusive rocks. This magmatism is divided in the Oligo-Miocene Yemen Trap Series (YTS) separated by an unconformity from the Miocene-Recent Yemen Volcanic Series (YVS). Magmas of the YTS were erupted during the synrift phase and correlate with the first stage of sea-floor spreading of the Red Sea and the Gulf of Aden (30 - 15 Ma), whereas the magmas of the YVS were emplaced during the post rift phase (10 - 0 Ma). A continental within plate character is recognized for both the YTS and YVS basalts. The YTS volcanic rocks are contemporaneous with, and geochemically similar to, the Ethiopian rift volcanism, just as the volcanic fields of the YVS are geochemically alike to most of the Saudi Arabian volcanics. YTS and YVS have analogous SiO2 ranges, but YVS tend to have, on average, higher alkalis and MgO contents than YTS. Fractional crystallization processes dominate geochemical variations of both series. Primitive magmas (MgO > 7.0%) are enriched in incompatible ele- ments and LREEs with respect to primitive mantle, but YVS are more enriched than YTS. To first order, the different geochemical patterns agree with different degrees of partial melting of an astenospheric mantle source: 25% - 30% of partial melting for YTS and 10% - 3% for YVS. Secondly, the higher degree of enrichment in incompatible elements of YVS reflects also greater contribution of a lithospheric mantle component in their source region.
The role of mantle–lithosphere interactions in shaping surface topography
has been long debated1–3. In general3,4, it is supposed that
mantle plumes and vertical mantle flows result in axisymmetric, longwavelength
topography, which strongly differs from the generally
asymmetric short-wavelength topography created by intraplate tectonic
forces. However, identification of mantle-induced topography
is difficult3, especially in the continents5. It can be argued therefore
that complex brittle–ductile rheology and stratification of the continental
lithosphere result inshort-wavelengthmodulationandlocalization
of deformation induced by mantle flow6. This deformation
should also be affected by far-field stresses and, hence, interplay with
the ‘tectonic’ topography (for example, in the ‘active/passive’ rifting
scenario7,8). Testing these ideas requires fully coupled three-dimensional
numerical modelling of mantle–lithosphere interactions, which so
far has not been possible owing to conceptual and technical limitations
of earlier approaches. Here we present new, ultra-high-resolution, threedimensional
numerical experimentsontopographyovermantle plumes,
incorporating a weakly pre-stressed (ultra-slow spreading), rheologically
realistic lithosphere. The results show complex surface evolution, which
is very different fromthe smooth, radially symmetric patterns usually
assumed as the canonical surface signature ofmantle upwellings9. In
particular, the topography exhibits strongly asymmetric, small-scale,
three-dimensional features, which include narrow and wide rifts,
flexural flank uplifts and fault structures. This suggests a dominant
role for continental rheological structure and intra-plate stresses in
controlling dynamic topography, mantle–lithosphere interactions,
and continental break-up processes above mantle plumes.
The major and trace element and radiogenic isotope compositions of basalts from throughout the East African rift system are reviewed in the context of constraints from previous geophysical studies. The data indicate the presence of two mantle plumes, the East African and Afar plumes, which dynamically support the East African and Ethiopian plateaus. Rifting across the plateaus is accompanied by the generation of large volumes of basaltic magma and associated evolved derivatives. Relatively few matic magmas have an unambiguous Afar mantle plume signature, notably the MgO-rich picrites and ankaramites from the 29-31 Ma Ethiopian traps, and the most recent basalts (< 5 Ma) from Afar. The Eocene Amaro basalts from southern Ethiopia also have a plume source but their lower source temperatures and isotopic characteristics are distinct from those of Afar. The remaining basalts from the Ethiopian rift, and throughout the Kenya and Western rifts, have a lithospheric source region as reflected in both radiogenic isotope and trace element characteristics. The Amaro basalts are suggested as the first manifestations of magmatism from the East African plume; subsequent magmatic activity being represented by progressively younger episodes further south through Turkana, Kenya and into Northern. Tanzania, as the African plate migrated north. Despite their clear lithospheric characteristics, U-series data on geologically recent basalts from the axis of the Kenya rift show that they were generated in a dynamic melting regime. Melting is effected when lithospheric mantle heats up and becomes incorporated into the convecting mantle, hence leading to greater degrees, of lithospheric thinning than are indicated by extension across individual rift basins.
The Miocene–Holocene East African Rift in Ethiopia is unique worldwide because it subaerially exposes the transition between continental rifting and seafl oor spread-ing within a young continental fl ood basalt province. As such, it is an ideal study locale for continental breakup processes and hotspot tectonism. Here, we review the results of a recent multidisciplinary, multi-institutional effort to understand geologi-cal processes in the region: The Ethiopia Afar Geoscientifi c Lithospheric Experiment (EAGLE). In 2001–2003, dense broadband seismological networks probed the struc-ture of the upper mantle, while controlled-source wide-angle profi les illuminated both along-axis and across-rift crustal structure of the Main Ethiopian Rift. These seismic experiments, complemented by gravity and magnetotelluric surveys, provide important constraints on variations in rift structure, deformation mechanisms, and melt distribution prior to breakup. Quaternary magmatic zones at the surface within the rift are underlain by high-velocity, dense gabbroic intrusions that accommodate extension without marked crustal thinning. A magnetotelluric study illuminated par-tial melt in the Ethiopian crust, consistent with an overarching hypothesis of magma-assisted rifting. Mantle tomographic images reveal an ~500-km-wide low-velocity zone at ≥ ≥75 km depth in the upper mantle that extends from close to the eastern edge of the Main Ethiopian Rift westward beneath the uplifted and fl ood basalt– capped NW Ethiopian Plateau. The low-velocity zone does not interact simply with the Miocene–Holocene (rifting-related) base of lithosphere topography, but it also provides an abundant source of partially molten material that assists extension of the seismically and volcanically active Main Ethiopian Rift to the present day.
The rapid expansion of broad-band seismic networks over the last decade
has paved the way for a new generation of global tomographic models.
Significantly improved resolution of global upper-mantle and crustal
structure can now be achieved, provided that structural information is
extracted effectively from both surface and body waves and that the
effects of errors in the data are controlled and minimized. Here, we
present a new global, vertically polarized shear speed model that yields
considerable improvements in resolution, compared to previous ones, for
a variety of features in the upper mantle and crust. The model,
SL2013sv, is constrained by an unprecedentedly large set of waveform
fits (˜3/4 of a million broad-band seismograms), computed in
seismogram-dependent frequency bands, up to a maximum period range of
11-450 s. Automated multimode inversion of surface and S-wave forms was
used to extract a set of linear equations with uncorrelated
uncertainties from each seismogram. The equations described
perturbations in elastic structure within approximate sensitivity
volumes between sources and receivers. Going beyond ray theory, we
calculated the phase of every mode at every frequency and its derivative
with respect to S- and P-velocity perturbations by integration over a
sensitivity area in a 3-D reference model; the (normally small)
perturbations of the 3-D model required to fit the waveforms were then
linearized using these accurate derivatives. The equations yielded by
the waveform inversion of all the seismograms were simultaneously
inverted for a 3-D model of shear and compressional speeds and azimuthal
anisotropy within the crust and upper mantle. Elaborate outlier analysis
was used to control the propagation of errors in the data (source
parameters, timing at the stations, etc.). The selection of only the
most mutually consistent equations exploited the data redundancy
provided by our data set and strongly reduced the effect of the errors,
increasing the resolution of the imaging.
Our new shear speed model is parametrized on a triangular grid with a
˜280 km spacing. In well-sampled continental domains, lateral
resolution approaches or exceeds that of regional-scale studies. The
close match of known surface expressions of deep structure with the
distribution of anomalies in the model provides a useful benchmark. In
oceanic regions, spreading ridges are very well resolved, with narrow
anomalies in the shallow mantle closely confined near the ridge axis,
and those deeper, down to 100-120 km, showing variability in their width
and location with respect to the ridge. Major subduction zones worldwide
are well captured, extending from shallow depths down to the transition
zone. The large size of our waveform fit data set also provides a strong
statistical foundation to re-examine the validity field of the JWKB
approximation and surface wave ray theory. Our analysis shows that the
approximations are likely to be valid within certain time-frequency
portions of most seismograms with high signal-to-noise ratios, and these
portions can be identified using a set of consistent criteria that we
apply in the course of waveform fitting.
We analyse P-wave receiver functions across the western Gulf of Aden and
southern Red Sea continental margins in Western Yemen to constrain
crustal thickness, internal crustal structure, and bulk seismic velocity
characteristics in order to address the role of magmatism, faulting and
mechanical crustal thinning during continental breakup. We analyse
teleseismic data from 21 stations forming the temporary Young Conjugate
Margins Laboratory (YOCMAL) network together with GFZ and Yemeni
permanent stations. Analysis of computed receiver functions shows that
(1) the thickness of unextended crust on the Yemen plateau is ~35 km;
(2) this thins to ~22 km in coastal areas and reaches less than 14 km on
the Red Sea coast, where presence of a high velocity lower crust (HVLC)
is evident. The average Vp/Vs ratio for the western Yemen Plateau is
1.79, increasing to ~1.92 near the Red Sea coast and decreasing to 1.68
for those stations located on or near the granitic rocks. Thinning of
the crust, and by inference extension, occurs over a ~130 km wide
transition zone from the Red Sea and Gulf of Aden coasts to the edges of
the Yemen plateau. Thinning of continental crust is particularly
localized in a <30-km-wide zone near the coastline, spatially
co-incident with addition of magmatic underplate to the lower crust,
above which at the surface we observe the presence of seaward dipping
reflectors (SDRs)_and thickened Oligo-Miocene syn-rift basaltic flows.
Our results strongly suggest the presence of high velocity mafic
intrusions in the lower crust, which are likely either synrift magmatic
intrusion into continental lower-crust or alternatively depleted upper
mantle underplated to the base of the crust during the eruption of the
SDRs. Our results also point toward a regional breakup history in which
the onset of rifting was synchronous along the western Gulf of Aden and
southern Red Sea volcanic margins followed by a second phase of
extension along the Red Sea margin.
Continental breakup is caused by some combination of heating and
stretching. The Afar Rift system in Africa is an example of active
continental rifting, where a mantle plume probably weakened the
lithosphere through thermal erosion and magma infiltration. However, the
location and degree of plume influence today are debated. Here we use
seismic S-to-P receiver functions to image the mantle structure beneath
Afar. We identify the transition between the lithosphere and underlying
asthenosphere at about 75km depth beneath the flanks of the continental
rift. However, this boundary is absent beneath the rift itself and we
instead observe a strong increase in seismic velocities with depth, at
about 75km. We use geodynamic modelling to show that the velocity
increase at this depth is best explained by decompression melting of the
mantle in the absence of a strong thermal plume. So, although the
absence of mantle lithosphere beneath the rift implies a plume may have
once been active, we conclude that the influence of a thermal plume
directly beneath Afar today is minimal.
Calabria represents an ideal site to analyze the topography of a
subduction zone as it is located on top of a narrow active
Wadati-Benioff zone and shows evidence of rapid uplift. We analyzed a
pattern of surface deformation using elevation data with different
filters and showed the existence of a long wavelength (>100 km)
relatively positive topographic signal at the slab edges. The elevation
of MIS 5.5 stage marine terraces supports this pattern, although the
record is incomplete and partly masked by the variable denudation rate.
We performed structural analyses along the major active or recently
reactivated normal faults showing that the extensional direction varies
along the Calabrian Arc and laterally switches from arc-normal, within
the active portion of the slab, to arc-oblique or even arc-parallel,
along the northern and southern slab edges. This surface deformation
pattern was compared with a recent high resolution P wave tomographic
model showing that the high seismic velocity anomaly is continuous only
within the active Wadati-Benioff zone, whereas the northern and
southwestern sides are marked by low velocity anomalies, suggesting that
large-scale topographic bulges, volcanism, and uplift could have been
produced by mantle upwelling. We present numerical simulations to
visualize the three-dimensional mantle circulation around a narrow
retreating slab, ideally similar to the one presently subducting beneath
Calabria. We emphasize that mantle upwelling and surface deformation are
expected at the edges of the slab, where return flows may eventually
drive decompression melting and the Mount Etna volcanism.
The Afar depression is an ideal locale to study the role of extension
and magmatism as rifting progresses to seafloor spreading. Here we
present receiver function results from new and legacy experiments.
Crustal thickness ranges from ˜45 km beneath the highlands to
˜16 km beneath an incipient oceanic spreading center in northern
Afar. The crust beneath Afar has a thickness of 20-26 km outside the
currently active rift segments and thins northward. It is bounded by
thick crust beneath the highlands of the western plateau (˜40 km)
and southeastern plateau (˜35 km). The western plateau shows
VP/VS ranging between 1.7-1.9, suggesting a mafic
altered crust, likely associated with Cenozoic flood basalts, or current
magmatism. The southeastern plateau shows VP/VS
more typical of silicic continental crust (˜1.78). For crustal
thicknesses <26 km, high VP/VS (>2.0) can
only be explained by significant amounts of magmatic intrusions in the
lower crust. This suggests that melt emplacement plays an important role
in late stage rifting, and melt in the lower crust likely feeds magmatic
activity. The crust between the location of the Miocene Red Sea rift
axis and the current rift axis is thinner (<22 km) with higher
VP/VS (>2.0) than beneath the eastern part of
Afar (>26 km, VP/VS < 1.9). This suggests
that the eastern region contains less partial melt, has undergone less
stretching/extension and has preserved a more continental crustal
signature than west of the current rift axis. The Red Sea rift axis
appears to have migrated eastward through time to accommodate the
migration of the Afar triple junction.
The topography of the African continent is characterized by large-scale extensional features such as the East African Rift, widespread volcanic activity, and anomalously subsided basins and uplifted domes. These enigmatic surface features have long suggested that the African continent is shaped by significant dynamic forcing originating in the underlying mantle. Here we simulate mantle convection backwards in time to reconstruct the evolution of dynamic topography of Africa over the past 30 million years. We show that the current high topography of the East African Rift system is due to the southward propagation of a topographic swell that encompassed the western margin of Arabia and the Afar region before 30 million years ago. We suggest that this dominant swell formed in response to the upwelling of the African superplume and the relative northward motion of the African tectonic plate over it. We also find that the adjacent Congo Basin has gradually subsided over the same time period in response to convective drawdown in the mantle. We conclude that much of Africa's recent geological history is driven by buoyancy forces in the mantle. Our findings have important implications for African volcanism, erosion, sediment transport and river-basin drainage patterns.
We suggest that the uplift of rift flanks results from mechanical unloading of the lithosphere during extension and consequent isostatic rebound. This mechanism is presented as an alternative to explanations for rift flank uplift involving thermal or dynamic processes, and magmatic thickening of the crust. Our hypothesis is based on two critical concepts. First, the lithosphere retains finite mechanical strength or flexural rigidity during extension. Second, isostatic rebound (uplift) of the lithosphere follows when the kinematics of extension produces a surface topographic depression that is deeper than the level to which the surface of the extended lithosphere would subside assuming local isostatic compensation. We develop and analyze two kinematic models for instantaneous extension of the lithosphere to show that flexural rebound is a viable explanation for the uplift of rift flanks. -from Authors
The rare-earth-element concentrations and Nd, Sr, and Pb isotopic compositions of the basalts in the Gulf of Aden are described and related to asthenospheric and lithospheric interactions with a thermal toruslike plume. Specific attention is given to the spatial and temporal traits of the mantle sources, and isotopic and geochemical data are used to determine the extent to which basaltic volcanism is derived from a mantle plume, the mantle lithosphere, and upwelling of the depleted atmosphere. The impingement and dispersion of a plume head is confirmed beneath the Afar region, and the geological record shows continental stretching and rifting prior to the impingement in the outskirts of the Horn of Africa. The data suggest that the isotopic variations along the Gulf of Aden/Red Sea/Ethiopia Rift system can be explained by the interaction of a thermal toruslike plume with the depleted asthenosphere and the overlying continental mantle lithosphere.
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 >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.
We present analysis of new gravity data to produce a 2D crustal and upper mantle density model across the northern Main Ethiopian Rift (NMER). The magmatic NMER is believed to represent the transitional stage between continental and oceanic rifting. We conclude that beneath our profile, magma emplacement into the upper crust occurs in the form of a 20 km-wide body beneath the axis of the rift, and a 12 km-wide off-axis body beneath the NW margin of the rift. These are coincident with Quaternary volcanic chains, anomalies in seismic velocity and conductivity identified by the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) along the same profile. We also identify a shallow, high-density body beneath the axial Boset volcano interpreted as either a dyke zone or a magma reservoir that may have fed Quaternary felsic volcanism. Our results provide supporting evidence for a c. 15 km-thick mafic underplate layer beneath the northwestern rift flank, imaged by the EAGLE controlled-and passive-source seismic data. A relatively low-density upper mantle is required beneath the underplate and the rift to produce the long wavelength features of the gravity anomaly. The resulting model suggests that the lithosphere to the SE of the rift is unaffected by rifting processes. Our results combined with those from other EAGLE studies show that magmatic processes dominate rifting in the NMER.
The northern Main Ethiopian Rift captures the crustal response to the transition from continental rifting in the East African rift to the south, to incipient seafloor spreading in the Afar depression to the north. The region has also undergone plume-related uplift and flood basalt volcanism. Receiver functions from the EAGLE broadband network have been used to determine crustal thickness and average V p / V s for the northern Main Ethiopian Rift and its flanking plateaus.
On the flanks of the rift, the crust on the Somalian plate to the east is 38 to 40 km thick. On the western plateau, there is thicker crust to the NW (41–43 km) than to the SW (<40 km); the thinning taking place over an off-rift upper mantle low-velocity structure previously imaged by travel-time tomography. The crust is slightly more mafic ( V p / V s ∼ 1.85) on the western plateau on the Nubian Plate than on the Somalian Plate ( V p / V s ∼ 1.80). This could either be due to magmatic activity or different pre-rift crustal compositions. The Quaternary Butajira and Bishoftu volcanic chains, on the side of the rift, are characterized by thinned crust and a V p / V s > 2.0, indicative of partial melt within the crust.
Within the rift, the V p / V s ratio increases to greater than 2.0 (Poisson’s ratio, σ > 0.33) northwards towards the Afar depression. Such high values are indicative of partial melt in the crust and corroborate other geophysical evidence for increased magmatic activity as continental rifting evolves to oceanic spreading in Afar. Along the axis of the rift, crustal thickness varies from around 38 km in the south to 30 km in the north, with most of the change in Moho depth occurring just south of the Boset magmatic segment where the rift changes orientation. Segmentation of crustal structure both between the continental and transitional part of the rift and on the western plateau may be controlled by previous structural inheritances. Both the amount of crustal thinning and the mafic composition of the crust as shown by the observed V p / V s ratio suggest that the magma-assisted rifting hypothesis is an appropriate model for this transitional rift.
Few constraints on the timing, amount and distribution of lithospheric extension associated with flood-basalt magmatism were available from the southern Main Ethiopian rift system, where the base of the Cenozoic volcanic succession is exposed by faulting. New structural observations, together with K-Ar and 40Ar/39Ar geochronology data from a transect of the Chamo basin-Amaro horst-Galana basin, show that basins are bounded by faults with steep dips at the surface, and the stratal dips of Eocene-Recent volcanic and sedimentary units are generally less than 20°. The small amounts of lithospheric extension and the large volumes of magma estimated in this study of the southern Main Ethiopian rift suggest a very hot plume and/or efficient thinning of the mantle lithosphere from below by mantle plume processes during two discrete episodes of flood-basalt volcanism. -from Authors
Cenozoic uplift of the East African Plateau has been associated with fundamental climatic and environmental changes in East Africa and adjacent regions. While this influence is widely accepted, the timing and the magnitude of plateau uplift have remained unclear. This uncertainty stems from the lack of datable, geomorphically meaningful reference horizons that could record surface uplift. Here, we document the existence of significant relief along the East African Plateau prior to rifting, as inferred from modeling the emplacement history of one of the longest terrestrial lava flows, the similar to 300-km-long Yatta phonolite flow in Kenya. This 13.5 Ma lava flow originated on the present-day eastern Kenya Rift flank, and utilized a riverbed that once routed runoff from the eastern rim of the plateau. Combining an empirical viscosity model with subsequent cooling and using the Yatta lava flow geometry and underlying paleotopography (slope angle), we found that the prerift slope was at least 0.2 degrees, suggesting that the lava flow originated at a minimum elevation of 1400 m. Hence, high paleotopography in the Kenya Rift region must have existed by at least 13.5 Ma. We infer from this that middle Miocene uplift occurred, which coincides with the two-step expansion of grasslands, as well as important radiation and speciation events in tropical Africa.
The Main Ethiopian Rift (MER) has a complex structural pattern composed of southern, central, and northern segments. Ages of onset of faulting and volcanism apparently indicate a heterogeneous time-space evolution of the segments, generally referred to as a northward progression of the rifting process. New structural, petrological, and geochronological data have been used to attempt reconciling the evolution of the distinct MER segments into a volcanotectonic scenario accounting for the propagation of the Afar and the Kenya Rifts. In this evolutionary model, extension affected the Southern MER in the early Miocene (20 - 21 Ma) due to the northward propagation of the Kenya Rift-related deformation. This event lasted until 11 Ma, then deformation decreased radically and was resumed in Quaternary times. In the late Miocene (11 Ma), deformation focused in the Northern MER forming a proto-rift that we consider as the southernmost propagation of Afar. No major extensional deformation affected the Central MER in this period, as testified by the emplacement at 12 - 8 Ma of extensive plateau basalts currently outcropping on both rift margins. Significant rift opening occurred in the Central MER during the Pliocene (similar to 5 - 3 Ma) with the eruption of voluminous ignimbritic covers (Nazret sequence) exposed both on the rift shoulders and on the rift floor. The apparent discrepancy between the heterogeneous propagation of the three MER segments could be reconciled by considering the opening of Central MER and the later reactivation of the Southern MER as due to a southward propagation of rifting triggered by counterclockwise rotation of the Somalian plate starting around 10 Ma.
The equivalent elastic thickness (EET) is used to estimate lithospheric strength expressed in response to loading by topography and subsurface loads. The data on EET allow comparisons between different plates and detection of thermal events. In oceans, the EET corresponds to the mechanical "core" of the lithosphere, i.e., a geotherm (400-600°C). In continents, the EET has no relation to any depth. This has led to doubts in applicability of a unique approach to the continents and oceans, and in the utility of estimates of EET for continents. Rheological data suggest that most rocks are inelastic in the long term (>0.1 m.y.). This requires interpretation of the EET in terms of real rheology. We propose an analytical model that gives rheological interpretation of both the oceanic and continental EET. It also allows estimates of the mechanical thickness of the lithosphere. The EET depends upon three parameters: geotherm age, crustal thickness, and flexural plate curvature. Any one of these values can be estimated if the others are known. Comparisons of model predictions with the observed EET suggest that most continental plates have a weak lower crust, allowing mechanical decoupling between the upper crust and the mantle lithosphere. Such decoupling leads to strong reduction in the EET and thus can be easily detected. Flow of rocks in the weak lower crust may have a significant influence on the temporal evolution of relief (mountain building, erosion). Differences in the mechanical behavior of oceans and continents can be explained by domination of different parameters: geotherm age has a major control in the oceans, whereas in the continents crustal thickness is equally important. Additional local variations of the EET result from weakening by flexural stresses.
Sculpting of the Earth’s surface by flowing water is apparent even to a casual observer, yet we know surprisingly little in a quantitative way about how the process of erosion actually occurs over a wide range of length and time scales. Earth’s surface topography at any particular time represents the combined effects of tectonism on one hand, and erosion/depositional processes on the other. Given this situation, it makes sense to study erosion in settings where we can estimate the tectonic component of the topography.
The eastern rift of Africa is a zone of normal faults separating the Horn of Africa from the remainder of the continent. The zone is typically troughlike, 40 to 65 km wide, and traverses two broad, elongated domal uplifts in Ethiopia and Kenya.
The foundation rocks of the region are metasediments and intrusives of the late Precambrian orogenic belt, which has a meridional trend. The Paleozoic was dominantly an era of denudation in eastern Africa, but late Paleozoic continental sediments (Karroo System) are locally preserved. Mesozoic marine sediments represent an epicontinental marine transgression and regression. Severe coastal warping occurred along the Indian Ocean margin, and in the early Tertiary such warping initiated the Red Sea, Gulf of Aden, and Afar depressions.
Uplift of the Ethiopian and Kenyan domes has been synchronous in three major pulses of late Eocene, mid-Miocene, and Plio-Pleistocene age. Volcanism of intermediate and silicic type shows some relation to uplift in time and space and to the onset of graben faulting, but major flood basalt extrusions in the early Tertiary in Ethiopia were related to massive crustal warping along the future rift margins. The volcanism associated with the eastern rift is overwhelmingly alkaline, and at some volcanoes a strongly alkaline fractionation series is distinguished from a more mildly alkaline series. The flood phonolites, trachytes, rhyolites, and ignimbrites of Kenya, and the pantelleritic ignimbrites of Ethiopia, could have resulted from anatexis of a mantle-derived accreted layer at the base of the crust.
The eastern rift began as a chain of marginally warped depressions which were accentuated as domal uplift proceeded, until, in mid-Miocene to early Pliocene times, faulting produced asymmetrical grabens. The final uplift phase in the early Pleistocene was accompanied by major graben faulting, and subsequent faulting has intensely fractured the floor of the rift along an axial zone marked by caldera volcanoes. The evolution and nature of the faulting, the evidence from the distribution and ages of volcanoes, and seismic and gravity data all indicate that the eastern rift lies along a zone of progressive crustal thinning with local crustal disruption.
The eastern rift can be considered as a plate boundary which meets the Red Sea and Gulf of Aden spreading axes at the Afar triple junction. Plate analysis suggests that the eastern rift marks a line of very slow crustal spreading, which helps account for many of the peculiar or unique features of this continental rift.
The Axum volcanic rocks constitute important deposits of trap (Oligocene) and post-trap (Miocene- to Pliocene) volcanism at the northern end of the great Ethiopian flood basalt sequence. The petrologic diversity of lavas erupted in this area is significant, ranging from basanites to tephrites and phonolites to trachytes. The variation in the concentration of major elements (Fe 2O 3, TiO 2, CaO), rare earth elements (REE), and incompatible element ratios (Zr/Nb, Nb/Y) in the volcanic rocks of Axum demonstrates the heterogeneous character of their source region. Such heterogeneity can be interpreted by the involvement of a mantle reservoir to different degrees and mechanisms of partial melting. The geochemical data show that the Axum volcanic rocks represent two magma series, which we designate as the flood basalt sequence and the post-trap basalt sequence. The flood basalt sequence, which erupted contemporaneously with the Oligocene Ethiopian flood basalts, exhibit high TiO 2 (2.6 - 4.4 wt%), Fe 2O 3 (13.4 - 17.4 wt%), and high Zr/Nb ratio (9 - 18). In contrast, the post-trap basalt sequence, which has a slight tendency towards the composition of the north-central Ethiopian shield volcanoes (Guguftu shield volcano) and the central and southeastern Eritrean volcanic suites, exhibit low TiO 2 (2.0 - 2.6 wt%), Fe 2O 3(10.5 - 14.6 wt%), Zr/Nb ratio (2.8 - 3.1), and high Nb (60 - 84 ppm), Th (3.9 - 7.2 ppm), and Nb/Y ratio (2.2 - 2.7). The acidic rocks, on the other hand, are indistinguishable from each other and values of their trace element ratio are comparable with the trace element ratios of the post-trap basalt sequence. The acid volcanics (phonolites and trachytes) might, therefore, have formed mostly through fractional crystallization of the post-trap basalt sequence magmas.
Presents a description of the stratigraphy, feeder sites, structural controls on volcanism, eruptive rates, petrography and geochemistry of the Ethiopian flood basalt province. The description is complicated by gaps in knowledge resulting from the rugged terrain, inconsistencies in data, complex geological evolution, and an unusually large variation in eruptive lithology. -A.W.Hall
The behaviour of the lithosphere prior to rifting and rift- related volcanism in the southern Red Sea area is recorded in the stratigraphy underlying the Yemen Volcanic Group. The Medj-zir Formation is dated as Paleocene to Oligocene in age and is the upper part of the Cretaceous to Paleogene Tawilah Group of Yemen. The Medj-zir Formation is well exposed over a large area of western Yemen, forming a unit which varies in thickness from 30–60 m in the west to 60–75 m in the east. The Yemen Volcanic Group overlies the Medj-zir with a sharp, but conformable boundary. The lowest volcanics are dated as 30–32 Ma. The Medj-zir Formation consists of clastic and subordinate carbonate facies which indicate deposition in fluvial, shallow marine and lacustrine-lagoonal environments. Three members are recognized: the basal Zijan Member comprises fluvial facies and shallow marine sandstones and mudstones; the Kura Member is made up of fluvial channel and overbank facies, the latter including well-developed ferruginous paleosols; the Lahima Member, at the top, is fine-grained, including gastropod-rich limestones deposited in a lacustrine or brackish lagoonal environment. These facies and transitions between them represent deposition in a coastal plain to shallow marine shelf setting which was subjected to minor fluctuations in sea level. It is noteworthy that there are no angular unconformities or erosional hiatuses within the Medj- zir Formation anywhere in western Yemen. Deposition- al hiatuses of unknown duration are represented by well-developed paleosols indicating long periods of tectonic stability. The contact with the overlying volcanics is conformable in all examined sections and there are no volcanic horizons or clasts in the Medj-zir Formation.
This book provides a comprehensive description of groundwater resources in Ethiopia and its various dimensions (groundwater as resource, environmental functions, and socioeconomics). The prevailing knowledge of groundwater resources in Ethiopia (or elsewhere in Sub Saharan Africa) was based on geological and stratigraphic framework known nearly four decades ago (mainly 1960‘s and 70‘s). Thanks to the substantial geoscientific research since the 70‘s a new set of relevant geological/stratigrahpic data has been created that helps to re-define our understanding of groundwater resources in Africa as a whole and in Ethiopia in particular: a) For the first time the basement aquifer of Ethiopia has been described hydrogeologically based on genesis of regoliths (deep weathering and striping history); clear regional difference in groundwater potential is shown for the first time; comparative accounty has been given regarding groundwater occurrence in the generally low grade basement rocks of Ethiopia (Arabian Nubian shield) and high grade basement rocks of the rest of Africa. b) For the first time groundwater occurrence in multilayred sedimentary rocks account for spatial variation in degree of karstification; deformation history, and stratigraphy. c) The vast volcanic aquifers of Ethiopia which have previously classified based on their ages are now reclassified based on age, morphology (eg. groundwater in plateau volcanics, groundwater in shield volcanics) and aquifer structure. d) The loose alluvio lacustrine sediments which were known as least extensive in previous works based on areal cover are in fact shown to host the most voluminous groundwater resources in Ethiopia. These aquifers have now been described based on their geomorphology, extent, and genesis. The aim of this book is to use these newly created knowledge to redefine the understanding of groundwater resources in Ethiopia.
To further advance our understanding of the way in which a portion of the Afri-can superswell in eastern Africa formed, and also to draw attention to the importance of eastern Africa for the plume versus plate debate about mantle dynamics, upper-mantle structure beneath eastern Africa is reviewed by synthesizing published results from three types of analyses applied to broadband seismic data recorded in Tanza-nia, Kenya, and Ethiopia. (1) Joint inversions of receiver functions and surface wave dispersion measurements show that the lithospheric mantle of the Ethiopian Plateau has been signifi cantly perturbed, much more so than the lithospheric mantle of the East African Plateau. (2) Body wave tomography reveals a broad (≥ ≥300 km wide) and deep (≥ ≥400 km) low-velocity anomaly beneath the Ethiopian Plateau and the eastern branch of the rift system in Kenya and Tanzania. (3) Receiver function stacks showing Ps conversions from the 410 km discontinuity beneath the eastern branch in Kenya and Tanzania reveal that this discontinuity is depressed by 20–40 km in the same location as the low-velocity anomaly. The coincidence of the depressed 410 km discontinuity and the low-velocity anomaly indicates that the low-velocity anomaly is caused primarily by temperatures several hundred degrees higher than ambient mantle temperatures. These fi ndings cannot be explained easily by models invoking a plume head and tail, unless there are a suffi cient number of plume tails presently under eastern Africa side-by-side to create a broad and deep thermal structure. These fi ndings also cannot be easily explained by the plate model. In contrast, the breadth and depth of the upper-mantle thermal structure can be explained by the African superplume, which in some tomographic models extends into the upper mantle beneath eastern Africa. Consequently, a superplume origin for the anomalous topog-raphy of the African superswell in eastern Africa, in addition to the Cenozoic rifting and volcanism found there, is favored.
My contention-not universally agreed upon-in this paper is that hotspots are related to plumes, which are mantle phenomena and are known by their manifestations at Earth's surface. The most unequivocal of these manifestations is uplift. There is no other process on this planet that creates domes of lithospheric flexure (i.e., falcogenic [large-wavelength] domes) of similar to1000 km diameter and 1 to 2 km amplitude within several million years. Such domes rise rapidly following arrival of a plume under the lithosphere. Uplift probably occurs by detaching pieces of the lithosphere and sinking them into the plume head. Domes do not generate enough extension to form rifts, but they do generate sufficient gravitational potential to lead to rifting. Giant radiating-dike swarms probably form as a result of spatial and temporal proximity of a plume with a pole of extensional rotation. The most active region of plume activity today is in Africa. In the Afar region, a dome of almost 1000 km radius began rising after the early Eocene, and it had probably reached an elevation of more than 1 km by early Oligocene time. Both basalt extrusion and rifting began afterward. A similar sequence of events occurred at the Kenya dome. Rifting and volcanicity at the Kenya dome progressed from north to south, giving the impression of a progressive tearing of Africa from the Afar southward. The uplift argument shows that the widespread late Devonian rifting and volcanism in eastern Europe were not plume-related phenomena, whereas the disruption of Gondwanaland in the Mesozoic was.
Geological observations of mantle flow-driven dynamic topography are
numerous, especially in the stratigraphy of sedimentary basins; on the
contrary, when it leads to subaerial exposure of rocks, dynamic
topography must be substantially eroded to leave a noticeable trace in
the geological record. Here, we demonstrate that despite its low
amplitude and long wavelength and thus very low slopes, dynamic
topography is efficiently eroded by fluvial erosion, providing that
drainage is strongly perturbed by the mantle flow driven surface uplift.
Using simple scaling arguments, as well as a very efficient surface
processes model, we show that dynamic topography erodes in direct
proportion to its wavelength. We demonstrate that the recent deep
erosion experienced in the Colorado Plateau and in central Patagonia is
likely to be related to the passage of a wave of dynamic topography
generated by mantle upwelling.
The Ethiopia/Afar hotspot has been frequently explained as an upper
mantle continuation of the African superplume, with anomalous material
in the lower mantle under southern Africa, rising through the transition
zone beneath eastern Africa. However, the significantly larger amplitude
low velocity anomaly in the upper mantle beneath Ethiopia/Afar, compared
to the anomalies beneath neighboring regions, has led to questions about
whether or not along-strike differences in the seismic structure beneath
eastern Africa and western Arabia are consistent with the superplume
interpretation. Here we present a new P-wave model of the hotspot's deep
structure and use it to evaluate the superplume model. At shallow (<
˜400 km) depths, the slowest velocities are centered beneath the
Main Ethiopian Rift, and we attribute these low velocities to
decompression melting beneath young, thin lithosphere. At deeper depths,
the low velocity structure trends to the northeast, and the locus of the
low velocity anomaly is found beneath Afar. The northeast-trending
structure with depth is best modeled by northeastward flow of warm
superplume material beneath eastern Africa. The combined effects of
shallow decompression melting and northeastward flow of superplume
material explain why upper mantle velocities beneath Ethiopia/Afar are
significantly slower than those beneath neighboring East Africa and
western Arabia. The superplume interpretation can thus explain the deep
seismic structure of the hotspot if the effects of both decompression
melting and mantle flow are considered.
Our new 1-by-1 degree global crustal model, CRUST1.0, was introduced
last year and serves as starting model in a comprehensive effort to
compile a global model of Earth's crust and lithosphere, LITHO1.0
(Pasyanos et al., 2012). The Moho depth in CRUST1.0 is based on 1-degree
averages of a recently updated database of crustal thickness data from
active source seismic studies as well as from receiver function studies.
In areas where such constraints are still missing, for example in
Antarctica, crustal thicknesses are estimated using gravity constraints.
The compilation of the new crustal model initially followed the
philosophy of the widely used crustal model CRUST2.0 (Bassin et al.,
2000; http://igppweb.ucsd.edu/~gabi/crust2.html) to assign elastic
properties in the crystalline crust according to basement age or
tectonic setting (loosely following an updated map by Artemieva and
Mooney (2001; http://www.lithosphere.info). For cells with no local
seismic or gravity constraints, statistical averages of crustal
properties, including crustal thickness, were extrapolated. However, in
places with constraints the depth to basement and mantle are given
explicitly and no longer assigned by crustal type. This allows for much
smaller errors in both. In each 1-degree cell, boundary depth,
compressional and shear velocity as well as density is given for 8
layers: water, ice, 3 sediment layers and upper, middle and lower
crystalline crust. Topography, bathymetry and ice cover are taken from
ETOPO1. The sediment cover is based on our sediment model (Laske and
Masters, 1997; http://igppweb.ucsd.edu/~sediment.html), with some
near-coastal updates. In an initial step toward LITHO1.0, the model is
then validated against new global surface wave disperison maps and
adjusted in areas of extreme misfit. This poster presents the next
validation step: compare the new Moho depths with in-situ active source
and receiver function results. We also present comparisons with
CRUST2.0. CRUST1.0 is available for download. References:
Pasyanos, M.E., Masters, G., Laske, G. and Ma, Z., LITHO1.0 - An Updated
Crust and Lithospheric Model of the Earth Developed Using Multiple Data
Constraints, Abstract T11D-09 presented at 2012 Fall Meeting, AGU, San
Francisco, Calif., 3-7 Dec, 2012. Artemieva, I.M. and Mooney, W.D.,
Thermal thickness and evolution of Precambrian lithosphere: A global
study, J. Geophys. Res., 106, 16,387-16,414, 2001. Bassin, C., Laske,
G. and Masters, G., The Current Limits of Resolution for Surface Wave
Tomography in North America, EOS Trans AGU, 81, F897, 2000. Laske, G.
and Masters, G., A Global Digital Map of Sediment Thickness, EOS Trans.
AGU, 78, F483, 1997. URL: http://igppweb.ucsd.edu/~gabi/crust1.html
Following the release of global continental effective elastic thickness
(Te) maps obtained using different approaches, we now have the
opportunity to provide better constraints on Te. We improve previous
estimates of Te derived from thermo-rheological models of lithospheric
strength (or Ter) using new equations that consider
variations of the Young's Modulus in the lithosphere. These new values
are quantitatively compared with those obtained from an inverse approach
(or Tei) based on a comparison of the spectral coherence
between topography and gravity anomalies with the flexural response of
an equivalent elastic plate to loading. The two models show in general a
good agreement, having equal means (at the 95% significance level) in
about half of the continental areas. In other regions Tei
exceeds Ter in about 65% of the data points, showing that
Tei provides an upper bound on Te. The two data sets have a
similar range, but demonstrate different distributions. Ter
has a bimodal distribution, with the two peaks representative of the
cratons and of the areas outside of them. In contrast, Tei
has more uniform distribution without predominant peaks. Our models show
higher similarities in the Meso-Cenozoic orogens than in the Archaean
and Proterozoic shields and platforms, due to the methods employed. For
the regions with the most robust determinations of Ter and
Tei, the relationship between them is close to linear. The
results of this work can be used for further studies on the mechanical
properties of the lithosphere.
While the Cenozoic Afro-Arabian Rift System (AARS) has been the focus of numerous studies, it has long been questioned if low-velocity anomalies in the upper mantle beneath eastern Africa and western Arabia are connected, forming one large anomaly, and if any parts of the anomalous upper mantle structure extend into the lower mantle. To address these questions, we have developed a new image of P-wave velocity variations in the Afro-Arabian mantle using an adaptively parameterized tomography approach and an expanded dataset containing travel-times from earthquakes recorded on many new temporary and permanent seismic networks. Our model shows a laterally continuous, low-velocity region in the upper mantle beneath all of eastern Africa and western Arabia, extending to depths of ~ 500–700 km, as well as a lower mantle anomaly beneath southern Africa that rises from the core-mantle boundary to at least ~ 1100 km depth and possibly connects to the upper mantle anomaly across the transition zone. Geodynamic models which invoke one or more discrete plumes to explain the origin of the AARS are difficult to reconcile with the lateral and depth extent of the upper mantle low-velocity region, as are non-plume models invoking small-scale convection passively induced by lithospheric extension or by edge-flow around thick cratonic lithosphere. Instead, the low-velocity anomaly beneath the AARS can be explained by the African superplume model, where the anomalous upper mantle structure is a continuation of a large, thermo-chemical upwelling in the lower mantle beneath southern Africa. These findings provide further support for a geodynamic connection between processes in Earth's lower mantle and continental break-up within the AARS.
The flexural rigidity of the lithosphere is often estimated from the linear transfer function (admittance) between gravity anomalies and topography. Admittance estimates strongly weight provinces with large topographic relief, which will tend to be those provinces with low flexural rigidity if the cause of the topography is subsurface variations in density. If the observed admittance of continents is modeled in terms of surface loading of an elastic plate, there is a strong bias toward low flexural rigidities. Models incorporating both surface and subsurface loading yield much higher rigidities. Although the two models may match the observed admittance equally well, only those with both surface and subsurface loading are consistent with the observed pattern of coherence between gravity and topography as a function of wavelength. A new method of analysis shows that surface and subsurface loading are approximately equal in importance in the vicinity of the Kenya rift valley. The flexural rigidity of the East African lithosphere is about 2×1023 N m, or an effective elastic thickness of the plate of 25–30 km. Data from the conterminous United States are consistent with the presence of provinces with a wide range of flexural rigidities, averaging tens of kilometers in effective elastic thickness.
We study the contribution of mantle flow to surface deformation within the Mediterranean Basin. Flow is modeled numerically based on lateral changes in mantle temperature estimated from tomography models. We find that modeling results are significantly affected by the properties of the selected tomography models. Shear-velocity models based on surface-wave observations achieve the highest resolution of upper-mantle structure, and, as a result, are most successful in predicting microplate motion and dynamic topography.
New K-Ar dates are presented for areas in W and SE Ethiopia. In the west, the dates distinguish the Geba Basalts of 40 to 32 Ma (previously determined by Merla et al. 1979) from the Welega Shield Volcanics which are shown to range from 11.2 ± 2.2 to 7.8 ± 1.6 Ma. In SE Ethiopia, the Lower Stratoid flood basalts range from 30 ± 4.5 to 23.5 ± 4.5 Ma and are unconformably overlain by the Reira-Sanete shield volcanics which range from c. 15 to c. 2 Ma. The unconformity is marked by a palaeosol as are several of the intervals between the major volcanic stages of Ethiopia.
Using new field observations, together with previously published results and unpublished data from the Ethiopian Institute of Geological Surveys, it is suggested that the Ethiopian Flood Basalts were erupted in three major stages. Stage 1, which is mainly older than 40 Ma is separated from Stage 2, 34 to 30 Ma for NW Ethiopia and 40 to 30 Ma for SW Ethiopia, by erosional unconformities. Stage 3 spans 30 to 26 Ma in NW Ethiopia, and 30 to 21 Ma in SW Ethiopia and both are marked by the incoming of silicic volcanism. In W Ethiopia, Stages 1 and 3 are not developed, whilst in SE Ethiopia the Tertiary volcanism commences with Stage 3 flood basalts. The overlying shield volcanics; (25 to 13 Ma in NW Ethiopia and 15 to 7 Ma in W Ethiopia) represent a localized terminal episode built on the Plateau and are considered a fourth stage.
The earliest volcanism is restricted to two areas: in SW Ethiopia, where the Akobo Basalts give ages as old as 49.4 Ma (Davidson & Rex 1980), and in NW Ethiopia where the Ashange Basalts underlie the Aiba Basalts which are dated at 34 to 30 Ma. The Ashange volcanism may have begun in the Palaeocene—Eocene but radiometric dates are ambiguous and inconsistent. These two volcanic centres show lithological and geochemical differences, and can be related to two separate rift zones north of Lake Turkana and further north in the Tana Graben. Volcanism appears to have spread by the expansion of these areas, and to have migrated eastwards with time towards the Main Ethiopian Rift which begun to form about 14 Ma ago. There is a complex pattern of domal uplifts within these two volcanic centres with volcanism either preceding doming, or the two being contemporaneous.