The separation of the African and Arabian plates is responsible for the opening of the Red Sea and Gulf of Aden that meet the East African Rift at the Afar triple junction. Moreover, the strike-slip movement between the African and the Arabian plates is accommodated in the northernmost part of the rift system by the Dead Sea fault and its marine extension in the Gulf of Aqaba. High volcanic and seismic activity in and around the three arms of the divergence highlights some of the key aspects of this opening system.

This complex geodynamic system is currently investigated by multiple geoscientific approaches including e.g., tectonics, volcanology, stratigraphy, geodynamics, geodesy as well as active and passive geophysical methods.

In this session, we welcome contributions that are based on (but not limited to) such methods and investigate the basins of the Gulf of Suez, Gulf of Aqaba, Red Sea, Gulf of Aden, Afar depression and their surrounding regions, from the mantle to the crust.

Co-organized by GD7/GMPV11/SM4
Convener: Laura ParisiECSECS | Co-conveners: Nico Augustin, Joël Ruch, Daniele TrippaneraECSECS
| Attendance Wed, 06 May, 08:30–10:15 (CEST)

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Chat time: Wednesday, 6 May 2020, 08:30–10:15

Chairperson: Laura Parisi, Nico Augustin, Joël Ruch, Daniele Trippanera
D1242 |
Cynthia Ebinger, Weston Harding, Christian Kakonkwe, Ellen Knappe, Ian Bastow, Rebecca Bendick, Mary Muthoni, Gladys Kianji, Nicholas Mariita, and Atalay Ayele

Lateral heterogeneities in crust and mantle structure influence the distribution of strain and magmatism in continental rift zones.  Sutures between Archaean cratons and younger orogenic belts represent some of Earth’s largest lateral heterogeneities:  > 170 km-thick, buoyant and relatively dry lithosphere juxtaposed to ~120 km-thick, more volatile-rich mantle lithosphere.  The seismically and volcanically active Turkana Depression between the Ethiopian and East African plateau magma-rich Eastern rift formed near the eastern edge of the Archaean Tanzania craton. This area was affected by rifting in Mesozoic and Paleogene time, and may have been a thin zone when magmatism started at ~40 Ma.  Several hypotheses had been proposed to explain the unusual ~300 km-breadth of the Turkana Depression.  We use new data from the Turkana Rift Arrays to Investigate Lithospheric Structure (TRAILS) is to evaluate spatial variations in the location of strain, and in the direction and magnitude of seismic anisotropy, which is strongly influenced by mantle flow patterns along lithosphere-asthenosphere topography, fluid-filled cracks (e.g., dikes), and pre-existing mantle lithosphere strain fabrics.  Complementary data sets provide a strong contextual framework.  Our results and those of regional studies show that strain is currently localized to ~100 km-wide section of the Depression, and the western sectors are inactive.  We suggest that the original location of strain and magmatism was near the eastern edge of the Tanzania craton above a steep lithosphere-asthenosphere gradient, and that rifting has migrated eastward to form a more contiguous zone between the Main Ethiopian and Eastern rift zones.    

How to cite: Ebinger, C., Harding, W., Kakonkwe, C., Knappe, E., Bastow, I., Bendick, R., Muthoni, M., Kianji, G., Mariita, N., and Ayele, A.: Constraining strain and magmatism patterns between the Ethiopian and East African plateaux from new seismic and geodetic data , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21127, https://doi.org/10.5194/egusphere-egu2020-21127, 2020

D1243 |
Frank Zwaan, Giacomo Corti, Derek Keir, Federico Sani, Ameha Muluneh, Finnigan Illsley-Kemp, and Mauro Papini

This multidisciplinary study focuses on the tectonics of the Western Afar Margin (WAM), which is situated between the Ethiopian Plateau and Afar Depression in East Africa. The WAM represents a developing passive margin in a highly volcanic setting, thus offering unique opportunities for the study of rifting and (magma-rich) continental break-up, and our results have both regional and global implications.

Earthquake analysis shows that the margin is still deforming under a ca. E-W extension regime (a result also obtained by analysis on fault measurements from recent field campaigns), whereas Afar itself undergoes a more SW-NE extension. Together with GPS data, we see Afar currently opening in a rotational fashion. This opening is however a relatively recent and local phenomenon, due to the rotation of the Danakil microcontinent modifying the regional stress field (since 11 Ma). Regional tectonics is otherwise dominated by the rotation of Arabia since 25 Ma and should cause SW-NE (oblique) extension along the WAM. This oblique motion is indeed recorded in the large-scale en echelon fault patterns along the margin, which were reactivated in the current E-W extension regime. We thus have good evidence of a multiphase rotational history of the WAM and Afar.

Furthermore, analysis of the margin’s structural architecture reveals large-scale flexure towards Afar, likely representing the developing seaward-dipping reflectors that are typical for magma-rich margins. Detailed fault mapping and earthquake analysis show that recent faulting is dominantly antithetic (dipping away from the rift), bounding remarkable marginal grabens, although a large but older synthetic escarpment fault system is present as well. By means of analogue modelling efforts we find that marginal flexure indeed initially develops a large escarpment, whereas the currently active structures only form after significant flexure. Moreover, these models show that marginal grabens do not develop under oblique extension conditions. Instead, the latter model boundary conditions create the large-scale en echelon fault arrangement typical of the WAM. We derive that the recent structures of the margin could have developed only after a shift to local orthogonal extension. These modeling results support the multiphase extension scenario as described above.

Altogether, our findings are highly relevant for our understanding of the structural evolution of (magma-rich) passive margins. Indeed, seismic sections of such margins show very similar structures to those of the WAM. However, the general lack of marginal grabens, which are so obvious along the WAM, can be explained by the fact that most rift systems undergo or have undergone oblique extension, often in multiple phases during which structures from older phases control subsequent deformation.

How to cite: Zwaan, F., Corti, G., Keir, D., Sani, F., Muluneh, A., Illsley-Kemp, F., and Papini, M.: Multiphase rotational extension and marginal flexure along a developing passive margin: the Western Afar Margin, East Africa, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2620, https://doi.org/10.5194/egusphere-egu2020-2620, 2020

D1244 |
Martina Raggiunti, Derek Keir, Carolina Pagli, and Aude Lavayssiere

Faults can act as preferential degassing pathways for fluids of deep origin. Their migration and consequently variation of fluid pore pressure can cause a reduction of normal stress on the fault planes and trigger earthquakes. This can generate not only microseismicity but also events with significant magnitude. To understand this phenomenon, we studied the spatial, temporal and waveform characteristics of local seismicity from the northern sector of Main Ethiopian Rift (MER) of East Africa near Fentale and Dofen volcanoes. The seismic database contains events occurred in the MER from October 2001 to January 2003, and acquired by the Ethiopia Afar Geoscientific Experiment (EAGLE Project). The recorded events have been relocated with NLLoc using a new 3D velocity model derived from a wide-angle controlled source experiment. The relocated catalog contains a total of 1543 events with magnitudes between 0 and 4. The seismicity is mainly concentrated in two areas: near the border faults of the Ethiopian plateau and within the rift. On the border faults, events mostly occur down to 20 km depth, with an average depth of ~ 12 km. Within the rift, the events mostly happen down to 15 km depth, with an average depth of ~ 9 km. The seismicity is divided into several clusters aligned parallel to the rift direction, and in profile sections the clusters are mostly dipping steeply sub-vertical and dipping consistent with Andersonian normal faults. The analysis of the temporal-spatial distribution of earthquakes shows that some of the clusters are strongly concentrated in time and in space, and therefore swarm-like. To understand if the different clusters have been conditioned by fluid migration we have also analyzed the frequency content, release of seismic moment, and b-val is cut out. The link between earthquakes and fluid migration has also been explored by interpreting the distribution of seismicity using remote sensing mapping of faults, fumaroles and hydrothermal springs. Understanding where and how the fluid migration occurs will aid geothermal exploration efforts in the region, also improved knowledge of where geothermal activity is linked to seismicity has implications for seismic hazard estimation, which is very important for this densely and economically active areas.

How to cite: Raggiunti, M., Keir, D., Pagli, C., and Lavayssiere, A.: Seismicity induced by fluid migration in the Main Ethiopian Rift, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-539, https://doi.org/10.5194/egusphere-egu2020-539, 2019

D1245 |
Emma L. Chambers, Nicholas Harmon, Derek Keir, Catherine Rychert, and Ryan Gallacher

Within the melt-rich northern East African Rift system, extension progresses from continental rifting in the Ethiopian rift to near continental breakup in Afar. Multiple models have been proposed to understand the evolution of lithospheric stretching and magmatism, but previous studies do not provide a single absolute seismic velocity model of the crust and upper mantle for all stages of the rift. Here we jointly invert surface waves from ambient noise and teleseismic Rayleigh waves to obtain shear velocity maps from 10 to 210 km depth, enabling us to analyse variations in crustal and upper mantle shear wave velocity structure spatially and in depth. Using one model allows us to interpret and understand the pattern of crustal and lithospheric thinning from the rift flanks into the rift, the depth and locus of melt generation, and how these processes vary as a rift evolves towards incipient seafloor spreading.

We observe in areas unaffected by rifting, a fast lid (>0.1 km/s faster than surroundings) at lithosphere-asthenosphere-boundary depths (~60 - 80 km). The fast-lid is not visible directly beneath the rift and we instead observe slow velocities (slow enough to contain partial melt (3.95 – 4.10 ± 0.03 km/s)), which we interpret as evidence for melt infiltration into the uppermost mantle beneath the rift. In addition, the fast lid thins into the rift, until it is no longer observed, suggesting the rift is more stretched than the surrounding plate (~18% thinner). The slow velocities in the asthenosphere beneath the rift are segmented, ~110 km wide, ~60 – 120 km deep with ~70 km spacing between segments.  The shallowest and slowest anomalies occur beneath Afar, which is at later stage rifting. At crustal depths we observe a broadening in the slow velocity zones along the length of the Main Ethiopian Rift. Furthermore, the slow crustal velocities laterally spread to beneath areas of the Ethiopian Plateau that were affected by flood basalt volcanism (velocities of 3.30 – 3.80 ± 0.04 km/s). We interpret the broadening of the slow velocity as the Moho acting as a barrier causing lateral migration of melt into areas of pre-existing weakness. Our model provides the first comprehensive seismic model of the northern East African Rift allowing us to interpret rift structure. The segmented slow velocities in the asthenosphere suggest discrete melt-rich upwelling may drive the early the breakup process, with shallowing of the top of the melt-rich zone as the rift evolves and the lithosphere is modified by melt infiltration, with the Moho and lithosphere thinning later in the rifting process.

How to cite: Chambers, E. L., Harmon, N., Keir, D., Rychert, C., and Gallacher, R.: Imaging segmentation in early stage rifting (prior to breakup) using a joint inversion of Rayleigh waves from teleseisms and ambient noise tomography in the northern East African Rift, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9219, https://doi.org/10.5194/egusphere-egu2020-9219, 2020

D1246 |
Valentin Rime, Anneleen Foubert, Robin Fentimen, Haileyesus Negga, Afifé El Korh, Thierry Adatte, Irka Hajdas, Balemwal Atnafu, and Tesfaye Kidane

The Danakil depression (Afar, Ethiopia) is a rift valley forming the southernmost part of the Red Sea rift. It is situated between the Ethiopian plateau and the Danakil block and is thought to represent an advanced stage of rifting, characterized by important tectonic and volcanic activity. Its floor is situated 120 meters below sea level and is covered by salt pans.

This study focuses on a 625 m deep borehole drilled in the central part of the basin. It mainly consists of evaporites dominated by halite along with clastic and carbonate sediments. Lithostratigraphy and facies description were completed by micropaleontological, geochemical, mineralogical and organic matter analysis. They reveal the complex history of this rift basin. Two marine Red Sea incursions are identified. Strong water stratification during the older marine incursion led to the formation of sapropel layers and magnesite. The restriction of the basin and the strong aridity led to the formation of evaporites, culminating in the deposition of potash salts. Between the two marine events, continental evaporites contributed to several hundreds of meters of basin fill.  The younger marine incursion was probably characterized by wetter environments, resulting in the deposition of smaller volumes of evaporites. Since then, hypersaline lakes and salt pans filled the basin. Ongoing radiocarbon and U/Th datings will constrain further the Pleistocene stratigraphy and timing of the marine incursions.

These findings shed a new light on the basin history. The successive flooding and desiccation events are a consequence of sea-level variations but also important tectonic activity. Rift margin uplift prevented flooding during the Holocene sea-level highstand and contributed to the restriction of the depression. Significant basin subsidence at very short time scales created accommodation space for the voluminous sediment infill. This implies very active rifting during the last 250 ka.

How to cite: Rime, V., Foubert, A., Fentimen, R., Negga, H., El Korh, A., Adatte, T., Hajdas, I., Atnafu, B., and Kidane, T.: Evaporites reveal Pleistocene basin dynamics in the Danakil depression (northern Afar, Ethiopia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14917, https://doi.org/10.5194/egusphere-egu2020-14917, 2020

D1247 |
Zachary Molitor, Oliver Jagoutz, Leigh Royden, Stephanie Brown, Guido Port, Ali Dogru, and Joao Keller

As a young, mid ocean ridge, the Red Sea is a unique natural laboratory for studying the processes that drive continental rifting and breakup. The role of hot spots, frequently attributed to mantle plumes, in triggering or driving breakup and their impact on crustal structure and topography is not well understood. We have found that the Red Sea ridge bears a resemblance to the Reykjanes ridge in terms of bathymetry, morphology, geophysical properties, basalt chemistry, and modelled melting temperature and pressure of primary basalts. The results of modelling basalt melting temperature call into question the role of mantle temperature on generating excess melt beneath the Red Sea and Reykjanes ridges. Within 300 kilometers of a hotspot center, determined by seismic tomography, mantle excess temperatures are as high as 300 degrees relative to an ambient mantle temperature of about 1300 C. Outside of this radius excess temperatures are not significant (less than 50 C), and unlikely to cause significant melting anomalies. It is likely that the southern Red Sea and northern Reykjanes ridge are directly affected by hot, buoyant upwelling from the Afar and Iceland mantle plumes, and the central Red Sea and southern Reykjanes ridge may be affected by dynamic pressure related to actively upwelling mantle around the mantle plumes.

How to cite: Molitor, Z., Jagoutz, O., Royden, L., Brown, S., Port, G., Dogru, A., and Keller, J.: Mantle Plume – Spreading Ridge Interaction: A Comparative Study of the Red Sea and Reykjanes Ridges, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11138, https://doi.org/10.5194/egusphere-egu2020-11138, 2020

D1248 |
Mohamed Saleh, Frederic Masson, Nadia Abou-Aly, and Abdel-Monem Mohamed

We use combined GPS velocities covering Sinai peninsula to estimate the current rates across the Sinai micro-plate boundaries. New GPS velocities were estimated for 67 sites located within and around Sinai (Arabia, Eurasia and Nubia plates) covering the time span 1999-2018 using GAMIT/GLOBK 10.6 (Herring et al., 2015). We have combined our velocity field with two other recent solutions of GPS sites located around Sinai area. We used the VELROT tool from GAMIT/GLOBK package to combine all solutions resulting in a velocity field of 265 GPS sites in ITRF2018. Then, we selected 61 sites that represent the Sinai plate interior to estimate the Euler pole of Sinai micro-plate. Our computed the Euler pole parameters, latitude, longitude, and angular velocity for Sinai are 53.3±1.8°, -7.8±2.2°, and 0.451±0.014°/Ma, respectively, which are comparable to previous estimates, but with better uncertainties. The relative block motions at the Sinai plate boundaries are estimated using the DEFNODE code (McCaffrey, 2002) by minimizing the GPS residual motions within the blocks in a least squares sense. Our block motion model for Sinai sub-plate shows a fault-parallel velocity at the Gulf of Aqaba of 4.7-4.5 mm/yr, associated with negligible fault-normal component, which decreased toward the north direction along the Dead Sea Transform Fault. On the other hand, an opening rate of 3 mm/yr is estimated at the southern part of the Gulf of Suez with negligible fault-parallel component. At central and northern parts of the Gulf of Suez, the opening rate decreases until it vanished at the northern part of the Suez Canal while the fault-parallel component increases.

How to cite: Saleh, M., Masson, F., Abou-Aly, N., and Mohamed, A.-M.: An up-to-date Mapping for the Movement and the Deformation of the Sinai Micro-plate from a Combined GPS Velocity Field , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19491, https://doi.org/10.5194/egusphere-egu2020-19491, 2020

D1249 |
Hany Khalil, Fabio Capitanio, Peter Betts, and Alexander Cruden

Rifting in the Afar region is considered to be the only known example of the formation of an incipient divergent triple junction. Taking the Afar region as an example, we use three-dimensional (3D) laboratory experiments to test hypotheses for the formation and evolution of divergent triple junctions. We systematically evaluate the role of mechanical weakening due to plume impingement versus inherited weak linear structures in lithospheric mantle under both far-field orthogonal and rotational extensional boundary conditions. The interaction between far-field boundary forces and inherited rheological heterogeneities results in a range of complex rift propagation geometries and structural features, such as rift segmentation and ridge jumps, which are comparable to those observed in the Afar region. The combination of rotational boundary conditions and inherited linear heterogeneities favours the formation of rifts that connect at high-angles. Lithospheric weakening associated with a mantle plume triggers different rifting styles but has little influence on large-scale continental breakup. When compared to the Afar region, our results suggest that the rotation of the Arabian plate since the Oligocene led to rifting of the Red Sea and the Gulf of Aden, which are distinct from the formation of the Main Ethiopian Rift. We suggest that rifting in the Afar region is not consistent with the incipient divergent triple junction hypothesis. Rather, the Afar triple junction formed as a result of complex multi-phase rifting events driven by far-field tectonic forces.

How to cite: Khalil, H., Capitanio, F., Betts, P., and Cruden, A.: Modelling constraints on rifting in the Afar region: the birth of a triple junction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1935, https://doi.org/10.5194/egusphere-egu2020-1935, 2020

D1250 |
Mohammad Bagherbandi and Nureldin A. A. Gido

The principle of isostasy plays an important role to understand the relation between different geodynamic processes. Although, it is difficult to find an exact method that delivers a complete image of the Earth structure. However, gravimetric methods are alternative to provide images of the interior of the Earth. The Earth’s crust parameters, i.e. crustal depth and crust-mantle density contrast, can reveal adequate information about the solid Earth system such as volcanic activity, earthquake and continental rifting. Hence, in this study, a combine Moho model using seismic and gravity data is determined to investigate the relationship between the isostatic state of the lithosphere and seismic activities in East Africa. Our results show that isostatic equilibrium and compensation states are closely correlated to the seismicity patterns in the study area. For example, several studies suggest that African superplume causes the rift valley, and consequently differences in crustal and mantle densities occur. This paper presents a method to determine the crustal thickness and crust-mantle density contrast and consequently one can observe low-density contrast (about 200 kg/m3 ) and thin crust (about 30 km) near the triple junction plate tectonics in East Africa (Afar Triangle), which confirms the state of overcompensation in the rift valley areas. Furthermore, the density structure of the lithosphere shows a large correlation with the earthquake activity, sub-crustal stress and volcanic distribution across East Africa.

How to cite: Bagherbandi, M. and Gido, N. A. A.: How isostasy explains continental rifting in East Africa?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-149, https://doi.org/10.5194/egusphere-egu2020-149, 2019

D1251 |
Xiyuan Li, Wangshui Hu, Zhongying Lei, Chijun Huang, and Silin Yin

In the process of plate tectonic movement, extensional faults and conversion faults occur.In the process of studying the rift system of central and west Africa, by comparing the basin types and fault plane distribution characteristics of Africa and South America on both sides of the Atlantic ocean, it can be seen that the main continental fault on both sides of the Atlantic ocean and the fault developed at the mid-ocean ridge on the bottom of the Atlantic ocean belong to the conversion fault.The function of conversion faults is to regulate the difference in the moving speed between blocks in the contemporaneous structure. Therefore, the conversion faults developed in these three regions are similar and interrelated in terms of structure type, structure style, block movement mode and direction.The main transference faults in various regions play a role in regulating the differences of continental extension and inversion tectonic rates in the Atlantic ocean, Africa and South America.

There are two transition fault systems in the rift system of central Africa and west Africa. Under the joint action of these two transition fault systems, extensional basins and transition basins are mainly developed in the rift system of central and west Africa. Moreover, these two transition fault systems play different roles in different stages of the tectonic movement of the whole African plate.

After detailed interpretation of seismic data, it can be found that there are mainly fault-controlled inversion structures in Doseo basin and Doba basin.

As a representative of transition basins, fault-controlled inversion structures are widely developed in the Doseo basin, but they have different distribution characteristics.Among them, fault-controlled inversion structures with large inversion ranges are distributed near large faults in the basin, while fault-controlled inversion structures with small inversion ranges are far away from the structural units of the main controlled faults, the inversion structures have a small amplitude, and the stratigraphic reconstruction fragmentation degree is relatively weak. The inversion structures with weak inversion are mainly developed in the middle, western depression and southern uplift of Doseo basin.And as the representative of the extensional basin. In Doba basin, fault-controlled inversion structures are mainly developed, and the structures with high inversion rate are distributed in the central depression zone of the basin. The low inversion rate structures are distributed in the uplift and slope areas in the western part of the basin. By studying the development types and distribution locations of inversion structures in basins, it can be known that different types of basins have different transformation conditions during inversion.

Therefore, by comparing the differences in the plane and vertical characteristics of the inversion tectonic development of Doseo and Doba basins, as well as the studies on the eastern and western and non-other basins, it can be concluded that during the tectonic evolution of the rift system in central and west Africa, especially during the transition inversion stage, there were significant differences between the transition basin and the extensional basin.

How to cite: Li, X., Hu, W., Lei, Z., Huang, C., and Yin, S.: Study on the inversion structure of rift system in central and west Africa, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1658, https://doi.org/10.5194/egusphere-egu2020-1658, 2019

D1252 |
Zara Franceschini, Stéphane Scaillet, Raffaello Cioni, Giacomo Corti, Federico Sani, Gaëlle Prouteau, Bruno Scaillet, and Abate Assen Melaku

The volcano-tectonic evolution of the Main Ethiopian Rift (MER) is punctuated with periods of intense silicic volcanism, characterized by large explosive caldera-forming eruptions and the production of several ignimbrite deposits. These volcanic paroxysms require large volume of evolved silicic magma accumulated in shallow chambers into the continental crust; however, the relations between magmatism and tectonics during rifting, and the influence of the distribution and timing of regional tectonics on the ascent of magma and its stalling in large magmatic reservoirs remain poorly defined.

We present new geochronological data (40Ar/39Ar dataset of 29 samples) providing new constraints on the timing, evolution and characteristics of volcanism in the Central sector of the MER, where large ignimbrite deposits and remnants of several calderas testify the recurrence of silicic flare-ups. Specifically, we investigate in detail the eastern margin of the rift, where a voluminous, widespread, crystal-rich ignimbrite (Munesa Crystal Tuff, MCT) has been described. This deposit has been correlated to a thick ignimbrite occurring at the bottom of geothermal wells in the rift, pointing to a giant eruptive event attributed to a huge caldera structure, presumably buried beneath the rift floor. At least other two widespread ignimbrite units are present along the same margin for several tens of kilometres, testifying the high volcanicity of this sector of the MER.

Our survey and analyses suggest that, at least in the eastern margin of the rift, activity was clustered in periods of large magma production and emission, resulting in the recurrence of intense volcanic phases interspersed with periods of rest of volcanism. Ignimbrites and other volcanic deposits occur in the investigated area, spanning an age interval from 3.5 Ma to as recent as 150 ka. The MCT emission, around 3.5 Ma, was followed, after a long quiescence, by an important phase with the emplacement of both mafic (lava flows and scoria cone) and evolved (ignimbrites) products between 1.9-1.6 Ma. After that, volcanism occurred more frequently, possibly with a lower amount of erupted magma and still alternating with quiescent periods, with volcanism clusters at ~ 1.3-1.2 Ma, ~ 0.8-0.7 Ma and ~ 0.3-0.2 Ma. This clustered volcanic activity will be compared with the episodic rifting of this sector of the Main Ethiopian Rift.

How to cite: Franceschini, Z., Scaillet, S., Cioni, R., Corti, G., Sani, F., Prouteau, G., Scaillet, B., and Assen Melaku, A.: High intensity ignimbritic activity in the Central sector of the Main Ethiopian Rift, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-676, https://doi.org/10.5194/egusphere-egu2020-676, 2019

D1253 |
| Highlight
Ali Abdelkhalek, Jonas Kley, Mohamed Hammed, and Ahmed Saied Ali

The origin and intricate history of the River Nile are still widely disputed. Some studies have claimed that the present course of the Nile has formed at ~7-5 Ma, while others have suggested a much longer evolution. We proposed earlier that the southern and central segments of the River Nile in Egypt have originally evolved along a NW trending short-lived rift that was formed by NE-SW extension at ~25-23 Ma, and abandoned at an early stage. Here we focus on the development of the northern segment of the river, which we interpret as having both a relatively young age (~7-6 Ma) and different tectonic evolution. Gravity models and 3D seismic and well data show the presence of a deeply buried canyon west of the northern modern River Nile, 120 km southwest of Cairo, and approximately parallel to its present-day valley with a predominant NNE-NE course. The U-shaped canyon is up to 13 km wide and attains a maximum depth of around 1,900 meters, about as deep as the Grand Canyon of the Colorado River in Arizona, USA. The canyon was cut into a rising plateau along deep-seated NNE to NE-striking faults that formed at ~90-80 Ma as secondary shears to the main structures of the WNW oriented Cretaceous Beni-Suef rift and possibly have been reactivated at ~14 Ma with the origin of the Gulf of Aqaba-Dead Sea NNE sinistral transform plate boundary. The deeply incised canyon formed as a result of severe erosion due to significant sea level drop and desiccation of the Mediterranean in late Miocene time (Messinian Crisis ~7-6 Ma), which was accompanied by continued progressive uplift of the north-eastern Egyptian terrain[KJ1] . [2] The ancestral river excavated and widened a vast braided channel[3]  that cut deeply into Turonian-Campanian sediments in the Beni-Suef basin. The canyon attained its maximum depth by ~5 Ma, and subsequently it was filled by six successive clastic-dominated units of different fluvial facies.[4] 

How to cite: Abdelkhalek, A., Kley, J., Hammed, M., and Ali, A. S.: The Buried Grand Canyon in Egypt: Structural controls on the Neogene River Nile, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-821, https://doi.org/10.5194/egusphere-egu2020-821, 2019

D1254 |
David Fernández-Blanco, Gino de Gelder, and Christopher A-L. Jackson

Intra-continental abandoned rifts can fail for many reasons and are typically considered to be tectonically inactive. It is widely thought that the Oligo-Miocene Suez Rift, Egypt, which is located at the propagating northern end of the Red Sea spreading ridge, was abandoned in the Pliocene when motion between the African and Arabian plates was accommodated instead by the sinistral Dead Sea transform fault. However, local evidence for Plio-Quaternary normal faulting, the presence of uplifted Quaternary shorelines along the rift margins, and low-magnitude but widespread seismicity, together suggest the Suez Rift is tectonically active. Here, we present the first detailed analysis of this post-“abandonment” tectonic activity. We analyze the fluvial and tectonic geomorphology of the rift using freely available, 30 m-horizontal resolution digital elevation models (DEMs). These data reveal widespread normal fault offsets of Plio-Quaternary rocks at outcrop-to-basin scale, even in rift sectors >250 km north of the southern terminus of the rift. River morphology, tectonic knickpoints, normalized steepness indexes (ksn), and chi (χ) maps also provide evidence for relatively young faulting. Uplifted Quaternary shorelines show that active normal faults have footwall uplift rates of up to 0.125 mm/yr, even in locations >200 km north of the rift terminus, with these rates being relatively consistent for both rift margins. Our preliminary results provide clear evidence for young and ongoing tectonic activity in the Suez Rift and thus question the notion that this evolving plate boundary is currently in a state of complete tectonic quiescence. We speculate that the present tectonic activity in the Suez Rift results from the translation of far-field stresses imposed by the Afar plume, or by a recent change in the Eulerian pole of rotation between the African and the Arabian plates. Our results call for further analyses of the recent rifting in the Suez Rift and the exploration of recent activity in other “abandoned” rifts.

How to cite: Fernández-Blanco, D., de Gelder, G., and Jackson, C. A.-L.: Are abandoned rifts tectonically active? Morphotectonic evidence from the Gulf of Suez , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18075, https://doi.org/10.5194/egusphere-egu2020-18075, 2020

D1255 |
Daniele Trippanera, Margherita Fittipaldi, Nico Augustin, Froukje M. van der Zwan, and Sigurjón Jónsson

The Red Sea is a unique place to study the birth of an oceanic rift basin and the interplay between magma and tectonics at a young divergent plate boundary. The Red Sea is a NNW-SSE oriented and 2000 km long rift system with a spreading rate decreasing from ~16 mm/yr in the south to ~7 mm/yr in the north. The morphology also changes along the rift axis: the south portion is a continuous and well-developed rift, clearly exposing oceanic crust, the central portion is characterized by deeps made by oceanic crust separated by shallower inter-trough zones, and the northern part contains more widely spaced and less obvious deeps with the transition to the continental crust not well defined. While the central Red Sea morphology has been extensively studied, the structure of the northern Red Sea and its link to the central Red Sea are still unclear. Indeed, the northern Red Sea rift is offset by 100 km to the central Red Sea axis by the still poorly understood Zabargad fracture zone.

Here we aim at improving the understanding of the volcano-tectonic structure of the axial part of the southern tip of the northern Red Sea that corresponds to the Mabahiss Deep. To this aim, we carried out multiple multibeam surveys with R/V Thuwal and R/V Kobi Ruegg to map the sea bottom to add to what had been done in earlier surveys. In addition, we obtained several sub-bottom profiling lines across and along the deep to better constrain the shallow sedimentary structure.

Our results show that the 15 km long, 9 km wide and 2250 m deep Mabahiss Deep along with the 800 m high and 5 km wide central volcano are the key prominent structures of the area. The deep is bordered by a series of Red Sea parallel normal faults on two sides forming a graben-like structure and thus suggesting a rift-like morphology. The central volcano is well preserved and has a 2 km wide summit caldera containing several volcanic cones and thus suggesting a permanent magmatic source underneath of a relatively young age. The ocean floor outside the deep and the volcanic edifice is mostly covered by salt flows, limiting structural analysis of the surrounding areas.

A comparison between the northern and central Red Sea suggests, although in both areas thick salt covers most of the ocean floor, that the axes have similar rift-like structures with stable axial volcanism. However, in the central Red Sea larger portions of the oceanic crust are free of salt and the deformation seems larger with more prominent faults that also affect the floor of the deeps and split apart volcanic edifices, enhancing the occurrence of diffused monogenic volcanic cones. Therefore, this might suggest, despite the central and northern Red Sea sharing the same structure and evolution, that the less volcanic and tectonic activity in the north probably reflects the decreasing spreading rate from south to north along the Red Sea.

How to cite: Trippanera, D., Fittipaldi, M., Augustin, N., van der Zwan, F. M., and Jónsson, S.: Tectonics of the Northern Red Sea, insights from multibeam bathymetric mapping of Mabahiss Deep., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9401, https://doi.org/10.5194/egusphere-egu2020-9401, 2020

D1256 |
Neil Mitchell, Wen Shi, Ay Izzeldin, and Ian Stewart

Thick evaporites ("salt") were deposited in the South and North Atlantic, and Gulf of Mexico basins, in some parts deposited onto the flanks of nascent oceanic spreading centres.  Unfortunately, knowledge of the history of evaporite movements is complicated in such places by their inaccessibility and subsequent diapirism.  This is less of a problem in the Red Sea, a young rift basin that is transitioning to an ocean basin and where the evaporites are less affected by diapirism.  In this study, we explore the vertical movements of the evaporite surface imaged with deep seismic profiling.  The evaporites have moved towards the spreading axis of the basin during and after their deposition, which ended at the 5.3 Ma Miocene-Pliocene boundary.  We quantify the evaporite surface deflation needed to balance the volume of evaporites overflowing oceanic crust of 5.3 Ma age, thermal subsidence of the lithosphere and loss of halite through pore water diffusion, allowing for isostatic effects.  The reconstructed evaporite surface lies within the range of estimated global sea level towards the end of the Miocene.  Therefore, the evaporites appear to have filled the basin almost completely at the end of the Miocene.  Effects of shunting by terrigenous sediments and carbonates near the coast and contributions of hydrothermal salt are too small to be resolved by this reconstruction.

How to cite: Mitchell, N., Shi, W., Izzeldin, A., and Stewart, I.: Movements of thick evaporites on the flanks of a mid-ocean ridge: the central Red Sea Miocene evaporites, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4641, https://doi.org/10.5194/egusphere-egu2020-4641, 2020