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The Structural Pattern of the Afro-Arabian Rift System in Relation to Plate Tectonics: Discussion
Philosophical Transactions B
Abstract
The structural pattern of the Afro-Arabian rift system suggests the influence of transcurrent faulting in the development of the main branches of the system, particularly along the Dead Sea rift, the Gulf of Suez and Red Sea, and the eastern rift of Africa. Geophysical evidence indicates that the Red Sea and Gulf of Aden formed as a result of the separation of the Arabian and African continental blocks. Previously determined rotation poles about which the blocks separated neglect some structural features of the region. A satisfactory refit of Arabia to Africa cannot be made unless some relative movements of parts of the Africa block too place. It is proposed that dextral strike-slip movements took place between Africa and Arabia along the Red Sea and that sinistral strike-slip movements occurred along the Dead Sea rift. In addition, rotation of the E. Kenya-Somalia block east of the eastern rift of Africa took place. Structural and palaeomagnetic evidence supports such movements. The structural model is compatible with the observed tectonic pattern and provides a genetic link between the formation of the Red Sea, Gulf of Aden and the African rifts.
... Numerous cycles of rifting, punctuated by a few, short-lived periods of compression and shortening/inversion, followed during the Mesozoic to early Cenozoic in West, North, Central and East Africa (Guiraud et al., 1987Schull, 1988;Fairhead and Green, 1989;Lambiase, 1989;Bosworth, 1992;Genik, 1992Genik, , 1993McHargue et al., 1992;Bosworth and Morley, 1994;Morley, 1999Morley, , 2020Fairhead, 2023). The buried Mesozoic/early Cenozoic rifts of Africa and Suess's Dead Sea to Zambezi modern riftthe Afro-Arabian rift system (Baker, 1970;Khan, 1975;Kazmin, 1977;Girdler, 1991) do not generally deform the same regions of the African plate, although Browne et al., 1985;Fairhead, 1988;Fairhead and Green, 1989;Guiraud and Maurin, 1992;Bosworth, 1992;Genik, 1992Genik, , 1993Bosworth et al., 2008). Background is from the GEBCO_2022 global terrain model (GEBCO Compilation Group, 2022). ...
... In East Africa, geologic understanding of the rift system continued to evolve, in large part due to the extensive mapping efforts of the British Geological Survey and their East African colleagues (Dixey, 1956;Baker, 1958Baker, , 1970Baker and Wohlenberg, 1971;McConnell, 1972;Baker et al., 1972). Like Northey many decades earlier, there was a recognition that the Kenya rift basins are not symmetrical in cross-section. ...
... In the WB, however, rifting progresses with limited magmatism. There are few volcanic centers, and overall higher seismic velocities in the mantle beneath the rift in the WB compared to the MER and EB, suggesting lower temperatures and less melt Baker, 1971;Baker & McConnell, 1970;Ebinger, 1989;Furman, 2007;Grijalva et al., 2018;Hodgson et al., 2017;Hopper et al., 2020). The WB of the EARS exploits pre-existing structures on a large scale, as it developed in the Proterozoic and Phanerozoic mobile belt rocks that circumvent the thick lithosphere of the Tanzanian, Congo, and Bangweulu cratons (Chorowicz et al., 1987;Corti et al., 2007;Daly, 1988;Daly & Quennell, 1986;Daly et al., 1989;Fritz et al., 2013;Katumwehe et al., 2015;Leseane et al., 2015;Njinju et al., 2019;. ...
The interplay of rapid climate change and tectonics drives landscape development, sediment routing, and deposition in early‐stage continental rift systems. The Lake Malawi Rift, in the Western Branch of the East African Rift, is an archetype of a juvenile rift and an ideal natural laboratory for evaluating lacustrine source‐to‐sink systems on orbital or shorter timescales. We examine the interplay of these processes over the past 140 kyr using observations from nested seismic reflection data sets tied to scientific drill cores, which calibrate numerical forward models of this closed sedimentary system. Fault slip rates measured from seismic data drive tectonic displacements in the model. Satellite‐derived precipitation maps constrain modern precipitation and are scaled to previous hydrologic balance studies to reconstruct past climates. Our model reproduces known sediment thicknesses across the rift and accounts for 96% of the estimated siliciclastic sediment deposited over the past 140 kyr. The results demonstrate that the onset of arid climate conditions (140–95 kyr BP) causes extreme drainage adjustments downstream and the formation of mega‐catchments that flow axially into a shallow restricted paleo‐lake. Sedimentation rates during this time are twice the present values due to increased sediment focusing via these axial systems into a much smaller, hydrologically closed lake. As the climate became wetter (95–50 kyr BP), the lake rapidly expanded, decreasing both erosion and sedimentation rates across the rift. This closed‐loop approach allows us to evaluate the role of high‐frequency climate change in modulating basin physiography as well as sediment fluxes in juvenile rift systems.
... In the WB, however, rifting appears to be less magmatic. There are few volcanic centers, and seismic imaging studies show generally higher velocities in the mantle beneath the rift in the WB compared to the MER and EB, which suggests that lower temperatures and less melt is present in the WB Baker, 1971;Baker & McConnell, 1970;C. J. Ebinger, 1989;Furman, 2007;Grijalva et al., 2018;Hodgson et al., 2017;Hopper et al., 2020). ...
Half‐graben basins bounded by border faults typify early‐stage continental rifts. Deciphering the role that intra‐rift faults play in rift basin development is challenging as patterns of early‐stage faulting are commonly overprinted by subsequent deformation; yet the characterization of these faults is crucial to understand the fundamental controls on their evolution, their contribution to rift opening, and to assess their seismic hazard. By integrating multiple offshore seismic reflection data sets with age‐dated drill core, late‐Quaternary and cumulative faulting patterns are characterized in the Central and South Basins of the Malawi (Nyasa) Rift, an active, early‐stage rift system. Almost all intra‐rift faults offset a late‐Quaternary lake lowstand surface, suggesting they are active and should be considered in hazard assessments. Fault throw profiles reveal sawtooth patterns indicating segmented slip histories. Observed extension on intra‐rift faults is approximately twice that predicted from hanging wall flexure of the border fault, suggesting that intra‐rift faults accommodate a proportion of the regional extension. Cumulative and late‐Quaternary throws on intra‐rift faults are correlated with throw measured on the border fault in the Central Basin, whereas an anticorrelation is observed in the South Basin. Viewed in a regional context, these differences do not relate solely to the proposed southward younging of the rift. Instead, it is inferred that the distribution of extension is also influenced by variations in lithospheric structure and crustal heterogeneities that are documented along the rift axis.
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... We therefore suggest that the DSF developed in the area of thin lithosphere. We relate this thinning as well as the shallow MEA to the presence of relatively high temperatures and decompression melting that is consistent with the upwelling of hot material from the deeper mantle, invoked previously as a cause of the Neogene rifting along the Red Sea and the associated transform faulting beneath the Dead Sea (Baker and McConnell, 1970;Khan, 1975;Wolfenden et al., 2005;Faccenna et al., 2013). The eastward thinning of the lithosphere is supported by surface heat flow measurements that vary from about 30-40 mW/m 2 beneath the western part of the Levant Basin to about 90-100 mW/m 2 along the strike of the Dead Sea Fault (Pollack et al., 1993;Förster et al., 2007Förster et al., , 2010. ...
... We therefore suggest that the DSF developed in the area of thin lithosphere. We relate this thinning as well as the shallow MEA to the presence of relatively high temperatures and decompression melting that is consistent with the upwelling of hot material from the deeper mantle, invoked previously as a cause of the Neogene rifting along the Red Sea and the associated transform faulting beneath the Dead Sea (Baker and McConnell, 1970;Khan, 1975;Wolfenden et al., 2005;Faccenna et al., 2013). The eastward thinning of the lithosphere is supported by surface heat flow measurements that vary from about 30-40 mW/m 2 beneath the western part of the Levant Basin to about 90-100 mW/m 2 along the strike of the Dead Sea Fault (Pollack et al., 1993;Förster et al., 2007Förster et al., , 2010. ...
The tectonic plate under the eastern Mediterranean Sea shows a remarkable variability as it comprises Earth's oldest oceanic lithosphere as well as the transition towards continental lithosphere beneath the Levant Basin. Its thickness and other properties offer essential information on the lithospheric evolution but have been difficult to determine seismically due to the high heterogeneity of the region and its complex crustal structure. Here, we combine a large, new surface wave dataset with published wide-angle data in order to determine lithospheric properties in the eastern Mediterranean. Our stochastic inversions of broad-band, phase-velocity dispersion measurements resolve the crust-mantle structural trade-offs and yield robust, 1-D shear-wave velocity models down to 300 km depth beneath the Ionian and Levant Basins. The thickness of the crust beneath the two locations is 16.4 ± 3 km and 22.3 ± 2 km, respectively. The Poisson's ratio (σ) of 0.32 and Vp/Vs of 1.93 in the crystalline crust confirm the presence of serpentinized oceanic crust beneath the Ionian Basin. Beneath the Levant Basin, low crustal Vp/Vs (∼ 1.7) and Poisson's (∼ 0.24) ratios indicate continental crust. Beneath the Ionian Basin, the lithosphere is about 180 km thick. By contrast, thin, 75 km thick lithosphere is found beneath the Levant Basin. S-velocity tomography based on surface wave data also shows thick, spatially variable oceanic mantle lithosphere beneath the eastern Mediterranean. Thickness of the oceanic lithosphere increases eastwards from the Triassic Ionian towards the Permo-Carboniferous lithosphere in the Central Eastern Mediterranean. These results demonstrate that oceanic lithosphere can thicken by cooling substantially beyond the limits suggested by the plate cooling model. Beneath the eastern Herodotus oceanic Basin, lithospheric thickness is decreasing to about 180 km. Thin continental lithosphere and shallow asthenosphere are present beneath the Dead Sea Fault, demonstrating that the localization of the lithospheric deformation and crustal seismicity along the fault correlates spatially with the thinning of the underlying continental lithosphere.
... Indeed, walls are widespread along the western side of Pemba Island, stretch from north to south, interspersed by several reef passes (Grimsditch et al., 2009). These walls were formed through geological processes involving tectonic movements (Baker and McConnell, 1970;Sherman et al., 2019), and over time strong physical erosion from the north flowing East African Coastal Current (EACC) has had a pronounced effect on their complexity (Klaus, 2014). The EACC is also particularly important in increasing and maintaining connectivity between coral reef ecosystems (Obura 2012;Gamoyo et al., 2019;Sekadende et al., 2020). ...
Information on the spatial distribution of habitats and vulnerable species is important for conservation planning. In particular, detailed knowledge on connectivity of marine ecosystems in relation to depth and seafloor characteristics is crucial for any proposed conservation and management actions. Yet, the bulk of the seafloor remains undersampled, unstudied and unmapped, thereby limiting our understanding of connections between shallow and deep-water communities. Recent studies on mesophotic coral ecosystems (MCEs) have highlighted the western Indian Ocean as a particularly understudied marine region. Here we utilise an autonomous underwater vehicle (AUV) to collect in-situ temperature, oxygen concentration, bathymetry, acoustic backscatter and photographic data on benthic communities from shallow (<30 m) and mesophotic (30-150 m) depths at selected sites in the Greater Pemba Channel, Tanzania . Further, we use generalised additive models (GAMs) to determine useful predictors of substratum (hard and sand) and benthic community type (coral, turf algae, fleshy algae, fish). Our results revealed the presence of a complex seafloor characterised by pockmarks, steep slopes, submarine walls, and large boulders. Photographs confirmed the presence of MCE composed of corals, algae and fishes on the eastern margins of the Pemba Channel. The GAMs on the presence and absence of benthic community explained 35% to 91% of the deviance in fish and fleshy algae assemblages, respectively. Key predictors of the distribution of hard substrata and the coral reef communities were depth, showing the upper boundary of MCEs present at 30-40 m, and seafloor slope that showed more occurrences on steep slopes. The upper 100 m of water column had stable temperatures (25-26°C) and oxygen concentrations (220- 235 μmol/l). We noted the presence of submarine walls, steeply inclined bedrock, which appeared to support a highly bio-diverse community that may be worthy of particular conservation measures. Our results also highlight the capability of using marine robotics, particularly autonomous vehicles, to fill the knowledge gap for areas not readily accessible with surface vessels, and their potential application in the initial survey and subsequent monitoring of Marine Protected Areas.
... The position of the Red Sea within a much grander, multi-continent-scale rift system was recognized more than a century ago (Suess 1891;Gregory 1896;Du Toit 1937). The "Afro-Arabian rift system" reaches from transform fault and associated pull-apart basins of Syria, Jordan and Israel, continuing along the Gulf of Aqaba to the junction with the Red Sea, joining at Afar with the Gulf of Aden and the Ethiopian rift that further links the system to the complex East African rift system (Baker 1970;Khan 1975;Kaz'min 1977;Girdler 1991). Spanning *7,500 km, this is the largest predominantly continental rift system in the world. ...
The Red Sea and Gulf of Aden constitute parts of the Afro-Arabian rift system that are in the most advanced stages of continental break-up. These basins have therefore received extensive scrutiny in the geoscientific literature, but several aspects of their evolution remain enigmatic. Many of their most important features lie beneath several kilometers of water, in places covered by several kilometers of evaporite deposits, and along international political boundaries. All these factors greatly complicate the acquisition and interpretation of both subsurface wellbore and geophysical datasets. Much of our understanding of the evolution of the Red Sea has therefore relied on the integration of outcrop geology and land-based analytical studies with these more difficult to obtain marine observations. While stratigraphic, radiometric and structural data indicate that extension and rifting initiated in the southern Red Sea during the Late Oligocene (~28–25 Ma), the start of rifting in the northern Red Sea is more difficult to constrain due to paucity of rift-related volcanism and reliable biostratigraphy of the oldest syn-kinematic sedimentary strata. A regional NW-SE trending alkali basalt dike swarm, with associated extensive basalt flows in the vicinity of Cairo, appears to mark the onset of crustal-scale extension and continental rifting. These dikes and scarce local flows, erupted at the Oligocene-Miocene transition (~23 Ma) and coeval with similar trending dikes along the Yemen and Saudi Arabian Red Sea margin, are interbedded with the oldest part of the paleontologically dated siliciclastic syn-rift stratigraphic section (Aquitanian Nukhul Fm.), and are associated with the oldest recognized extensional faulting in the Red Sea. Bedrock thermochronometric results from the Gulf of Suez and both margins of the Red Sea also point to a latest Oligocene onset of major normal faulting and rift flank exhumation and large-magnitude early Miocene extension along the entire length of the Red Sea rift. This early phase of rifting along the Egyptian Red Sea margin and in the Gulf of Suez resulted in the formation of a complex, discontinuous fault pattern with very high rates of fault block rotation. The rift was segmented into distinct sub-basins with alternating regional dip domains separated by well-defined accommodation zones. Sedimentary facies were laterally and vertically complex and dominated by marginal to shallow marine siliciclastics of the Abu Zenima, Nukhul and Nakheil Formations. Neotethyan faunas appeared throughout all of the sub-basins at this time. During the Early Burdigalian (~20 Ma) tectonically-driven subsidence accelerated and was accompanied by a concordant increase in the denudation and uplift of the rift shoulders. The intra-rift fault networks coalesced into through-going structures and fault movement became progressively more focused along the rift axis. This reconfiguration of the rift structure resulted in more laterally continuous depositional facies and the preponderance of moderate-to-deep marine deposits of the Rudeis, Kareem and Ranga Formations. The early part of the Middle Miocene (~14 Ma) was marked by dramatic changes in rift kinematics and sedimentary depositional environments in the Red Sea and Gulf of Suez. The onset of the left-lateral Gulf of Aqaba transform fault system, isolating the Gulf of Suez from the active northern Red Sea rift, resulted in a switch from orthogonal to oblique rifting and to hyperextension in the northern Red Sea. The open marine seaway was replaced by an extensive evaporitic basin along the entire length of the rift from the central Gulf of Suez to Yemen/Eritrea. In Egypt these evaporites are ascribed to the Belayim, South Gharib, Zeit and Abu Dabbab Formations. Evaporite deposition continued to dominate in the Red Sea until the end of the Miocene (~5 Ma) when a subaerial unconformity developed across most of the basin. With the onset of seafloor spreading in the southern Red Sea, Indian Ocean marine waters re-entered through the Bab el Mandab in the earliest Pliocene and re-established open marine conditions. During the Pleistocene, glacial-isostatic driven sea-level changes resulted in the formation of numerous coral terraces and wave-cut benches around the margins of the Red Sea, Gulf of Suez and Gulf of Aqaba. Their present elevations suggest that the Egyptian Red Sea margin has been relatively vertically stable since the Late Pleistocene. While there is general agreement that full seafloor spreading, producing well-defined magnetic stripes, has been occurring in the southern Red Sea since ~5 Ma, there is ongoing debate whether and when lithospheric break-up has occurred in the northern Red Sea. Industry wellbore and seismic data demonstrate that continental crust extends at least several tens of kilometers offshore from the present-day coastline, and that the northern Red Sea is a non-volcanic rifted margin. On the basis of integrated geophysical, petrological, geochemical and geological datasets, we contend that true, laterally integrated sea-floor spreading is not yet manifest in the northern Red Sea.
... The Pleistocene in the Arabian-Nubian shield witnessed active rifting of the Gulf of Aqaba basin (Freund et al., 1970;Eyal et al., 1981), which is concomitant with the development of drainage systems in humid climatic conditions (El-Fayumy et al., 1993;El Asmar, 1997;Abbas et al., 2016). The Gulf of Aqaba rift is one of the Afro-Arabian rifts (Baker and McConnell, 1970;Khan, 1975) that initiated rifting of the Red Sea -Gulf of Suez rift in the Oligo-Miocene. In the Middle Miocene, the Gulf of Aqaba -Dead Sea rift commenced as an active transform plate margin ( Fig. 1; Bosworth and Mc Clay, 2001); and lately attains its rift basin geometry by the Pleistocene (Basahel et al., 1982;Guiraud and Bosworth, 1999). ...
Crustal rifting of the Arabian-Nubian Shield and formation of the Afro-Arabian rifts since the Miocene resulted in uplifting and subsequent terrain evolution of Sinai landscapes; including drainage systems and fault scarps. Geomorphic evolution of these landscapes in relation to tectonic evolution of the Afro-Arabian rifts is the prime target of this study. The fracture patterns and landscape evolution of the Wadi Dahab drainage basin (WDDB), in which its landscape is modeled by the tectonic evolution of the Gulf of Aqaba-Dead Sea transform fault, are investigated as a case study of landscape modifications of tectonically-controlled drainage systems. The early developed drainage system of the WDDB was achieved when the Sinai terrain subaerially emerged in post Eocene and initiation of the Afro-Arabian rifts in the Oligo-Miocene. Conjugate shear fractures, parallel to trends of the Afro-Arabian rifts, are synthesized with tensional fracture arrays to adapt some of inland basins, which represent the early destination of the Sinai drainage systems as paleolakes trapping alluvial sediments. Once the Gulf of Aqaba rift basin attains its deeps through sinistral movements on the Gulf of Aqaba-Dead Sea transform fault in the Pleistocene and the consequent rise of the Southern Sinai mountainous peaks, relief potential energy is significantly maintained through time so that it forced the Pleistocene runoffs to flow via drainage systems externally into the Gulf of Aqaba. Hence the older alluvial sediments are (1) carved within the paleolakes by a new generation of drainage systems; followed up through an erosional surface by sandy- to silty-based younger alluvium; and (2) brought on footslopes of fault scarps reviving the early developed scarps and inselbergs. These features argue for crustal uplifting of Sinai landscapes syn-rifting of the Gulf of Aqaba rift basin. Oblique orientation of the Red Sea-Gulf of Suez rift relative to the WNW-trending Precambrian Najd faults; and extrusion of volcanic rocks in directions parallel to the rift boundaries geometrically suggest rifting on tensional fractures that mutually bridge the Najd fault-related shear fractures. These aspects might envisage reactivation of the preexisting Precambrian fracture patterns in the Arabian-Nubian shield by the Oligo-Miocene to Pleistocene rift-controlled stress field.
... The structural complexity of the Afar depression, particularly evident in the central and southern sectors, is the result of the interactions of geodynamic events affecting the Afro-Arabian plate since the Early Miocene. The structural history began with the anticlockwise rotation of the Danakil microplate (Baker 1970;Burek 1970;Sichler 1980) which caused it to detach from the Nubian plate with a later northward movement (Chorowicz et al. 1999;Beyene and Abdelsalam 2005). This was also facilitated by the NE drift of the Arabian plate. ...
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.
... Impingement of multiple plumes at the base of the African lithosphere may have caused or at least contributed to the stationary nature of the plate, and the slow speed of the plate allowed the effects of these plumes to be seen at the Earth's surface. Burke then noted that the great system of active continental rifts in East Africa-the western and eastern branches of East Africa, the Ethiopian rift, Gulf of Aden, Red Sea, and Gulf of Suez (the Afro-Arabian rift system of Baker 1970;Khan 1975;Kaz'min 1977)-also formed over the past 30 My. Taking a holistic view of African geology, he reasoned that this was not coincidence and that the appearance of these plumes and associated mantle circulation patterns resulted in the formation of this rift system. Burke (1996) also discussed plate boundary stresses and the evolution of the East African rift system and suggested that their role became more important after the collision of Arabia with Eurasia along the Bitlis-Zagros suture at about 15-10 Ma (Ş engör and Yilmaz 1981;Hempton 1987). ...
Throughout the greater Red Sea rift system the initial late Cenozoic syn-rift strata and extensional faulting are closely associated with alkali basaltic volcanism. Older stratigraphic units are either pre-rift or deposited during pre-rupture mechanical weakening of the lithosphere. The East African superplume appeared in northeast Africa ~46 Ma but was not accompanied by any significant extensional faulting. Continental rifting began in the eastern and central Gulf of Aden at ~31–30 Ma coeval with the onset of continental flood volcanism in northern Ethiopia, Eritrea, and western Yemen. Volcanism appeared soon after at Derudeb in southern Sudan and at Harrats Hadan and As Sirat in Saudi Arabia. From ~26.5 to 25 Ma a new phase of volcanism began with the intrusion of a dike field reaching southeast of Afar into the Ogaden. At 24–23 Ma dikes were emplaced nearly simultaneously north of Afar and reached over 2000 km into northern Egypt. The dike event linked Afar to the smaller Cairo mini-plume and corresponds to initiation of lithospheric extension and rupture in the central and northern Red Sea and Gulf of Suez. By ~21 Ma the dike intrusions along the entire length of the Red Sea were completed. Each episodic enlargement of the greater Red Sea rift system was triggered and facilitated by breakthrough of mantle-derived plumes. However, the absence of any volumetrically significant rift-related volcanism during the main phase of Miocene central and northern Red Sea – Gulf of Suez rifting supports the interpretation that plate–boundary forces likely drove overall separation of Arabia from Africa.
... Research was boosted in the 1970s by the introduction of the concept of global tectonics, considering that oceans and related accretion of oceanic lithosphere begin with an intra-continental rift stage (Baker, 1970;Girdler and Sowerbutts, 1970;Mohr, 1970;Searle, 1970;Vail, 1970;McKenzie et al., 1972;Fairhead and Girdler, 1972;. This was also the time of first acquisitions of space imagery, covering large scenes with good ground resolution, a powerful tool for structural analysis of large areas (Mohr, 1974;. ...
... The Red Sea has long been recognized as one component of a continent scale rift system that reaches from the Dead Sea to Mozambique. This lead to the popularization of the term "Afro-Arabian rift system" by geologists mapping its different segments (Baker 1970;Khan 1975;Kazmin 1977). It was suggested that the Red Sea, and similarly the Gulf of Aden, were oceanic rifts at divergent plate boundaries, with a triple junction located at Afar (Fig. 1;Gass 1970;McKenzie et al. 1970;Girdler and Darracott 1972;Burke and Dewey 1973;Le Pichon and Francheteau 1978). ...
The Red Sea is part of an extensive rift system that includes from south to north the oceanic
Sheba Ridge, the Gulf of Aden, the Afar region, the Red Sea, the Gulf of Aqaba, the Gulf of
Suez, and the Cairo basalt province. Historical interest in this area has stemmed from many
causes with diverse objectives, but it is best known as a potential model for how continental
lithosphere first ruptures and then evolves to oceanic spreading, a key segment of the Wilson
cycle and plate tectonics. Abundant and complementary datasets, from outcrop geology,
geochronologic studies, refraction and reflection seismic surveys, gravity and magnetic surveys,
to geodesy, have facilitated these studies. Magnetically striped oceanic crust is present in the
Gulf of Aden and southern Red Sea, active magma systems are observed onshore in the Afar,
highly extended continental or mixed crust submerged beneath several kilometers of seawater is
present in the northern Red Sea, and a continental rift is undergoing uplift and exposure in the
Gulf of Suez. The greater Red Sea rift system therefore provides insights into all phases of rift-todrift
histories. Many questions remain about the subsurface structure of the Red Sea and the
forces that led to its creation. However, the timing of events—both in an absolute sense and
relative to each other—is becoming increasingly well constrained. Six main steps may be
recognized: (1) plume-related basaltic trap volcanism began in Ethiopia, NE Sudan (Derudeb),
and SW Yemen at*31 Ma, followed by rhyolitic volcanism at*30 Ma. Volcanism thereafter
spread northward to Harrats Sirat, Hadan, Ishara-Khirsat, and Ar Rahat in western Saudi Arabia.
This early magmatism occurred without significant extension or at least none that has yet been
demonstrated. It is often suggested that this “Afar” plume triggered the onset of Aden–Red Sea
rifting, or in some models, it was the main driving force. (2) Starting between *29.9 and
28.7 Ma, marine syn-tectonic sediments were deposited on continental crust in the central Gulf
of Aden. Therefore, Early Oligocene rifting is established to the east of Afar. Whether rifting
propagated from the vicinity of the Sheba Ridge toward Afar, or the opposite, or essentially
appeared synchronously throughout the Gulf of Aden is not yet known. (3) By*27.5–23.8 Ma,
a small rift basin was forming in the Eritrean Red Sea. At approximately the same time
(*25 Ma), extension and rifting commenced within Afar itself. The birth of the Red Sea as a rift
basin is therefore a Late Oligocene event. (4) At *24–23 Ma, a new phase of volcanism,
principally basaltic dikes but also layered gabbro and granophyre bodies, appeared nearly
synchronously throughout the entire Red Sea, from Afar and Yemen to northern Egypt. The
result was that the Red Sea rift briefly linked two very active volcanic centers covering 15,000–
25,000 km2 in the north and >600,000 km2 in the south. The presence of the “mini-plume” in northern Egypt may have played a role somewhat analogous to Afar vis-à-vis the triggering of
the dike event. The 24–23 Ma magmatism was accompanied by strong rift-normal extension and
deposition of syn-tectonic sediments, mostly of marine and marginal marine affinity. The area of
extension in the north was very broad, on the order of 1,000 km, and much narrower in the south,
about 200 km or less. Throughout the Red Sea, the principal phase of rift shoulder uplift and
rapid syn-rift subsidence followed shortly thereafter. Synchronous with the appearance of
extension throughout the entire Red Sea, relative convergence between Africa and Eurasia
slowed by about 50 %. (5) At *14–12 Ma, a transform boundary cut through Sinai and the
Levant continental margin, linking the northern Red Sea with the Bitlis–Zagros convergence
zone. This corresponded with collision of Arabia and Eurasia, which resulted in a new plate
geometry with different boundary forces. Red Sea extension changed from rift normal (N60°E)
to highly oblique and parallel to the Aqaba–Levant transform (N15°E). Extension across the
Gulf of Suez decreased by about a factor of 10, and convergence between Africa and Eurasia
again dropped by about 50 %. In the Afar region, Red Sea extension shifted from offshore Eritrea
to west of the Danakil horst, and activity began in the northern Ethiopian rift. (6) These early
events or phases all took place within continental lithosphere and formed a continental rift
system 4,000 km in length. When the lithosphere was sufficiently thinned, an organized oceanic
spreading center was established and the rift-to-drift transition started. Oceanic spreading
initiated first on the Sheba Ridge east of the Alula-Fartaq fracture zone at *19–18 Ma. After
stalling at this fracture zone, the ridge probably propagated west into the central Gulf of Aden by
*16 Ma. This matches the observed termination of syn-tectonic deposition along the onshore
Aden margins at approximately the same time. At*10 Ma, the Sheba Ridge rapidly propagated
west over 400 km from the central Gulf of Aden to the Shukra al Sheik discontinuity. Oceanic
spreading followed in the south-central Red Sea at *5 Ma. This spreading center was initially
not connected to the spreading center of the Gulf of Aden. By *3 to 2 Ma, oceanic spreading
moved west of the Shukra al Sheik discontinuity, and the entire Gulf of Aden was an oceanic rift.
During the last*1 My, the southern Red Sea plate boundary linked to the Aden spreading center
through the Gulf of Zula, Danakil Depression, and Gulf of Tadjoura. Presently, the Red Sea
spreading center may be propagating toward the northern Red Sea to link with the Aqaba–
Levant transform. However, important differences appear to exist between the southern and
northern Red Sea basins, both in terms of the nature of the pre- to syn-rift lithospheric properties
and the response to plate separation. If as favored here no oceanic spreading is present in the
northern Red Sea, then it is a magma-poor hyperextended basin with β factor >4 that is evolving
in many ways like the west Iberia margin. It is probable that the ultimate geometries of the
northern and southern Red Sea passive margins will be very different. The Red Sea provides an
outstanding area in which to study the rift-to-drift transition of continental disruption, but it is
unlikely to be a precise analogue for all passive continental margin histories.
... The late Cenozoic tectonic history of east Africa and Arabia has manifested itself in the well-known continental rift valleys of the East African rift system (Suess, 1891;Holmes, 1965;Baker, 1970) and the large oceanic rift of the Red Sea basin (Drake and Girdler, 1964;Lowell and Genik, 1972). There is little doubt that the voluminous outpourings of tholeiitic-toalkalic basalts associated with these areas are inherently related to rifting on such a grand scale (Cloos, 1939;Gass, 1970;Mohr, 1983). ...
Three coalesced basaltic lava fields, Harrats Khaybar, Ithnayn, and Kura, constitute the largest contiguous area of Cenozoic basalt in Saudi Arabia, similar in extent (20 564 km2) and volume (1850 km3) to Harrat Rahat, which is situated only 25 km to the south. Harrat Kura, with an age range from about 11 to 5 Ma, is the oldest of these harrats; it contains a single stratigraphic unit, the Kura basalt, primarily composed of alkali olivine basalt (AOB) with subordinant basanite and hawaiite and a single cluster of four phonolite domes. In contrast, Harrats Khaybar (5 Ma to Present) and Ithnayn (3 Ma to Present) are composed of more mildly alkaline basalt types. The two oldest units on Harrat Khaybar, the Jarad and Mukrash basalts, are dominated by dictytaxitic-to-ophitic olivine transitional basalt (OTB) flows, which were extruded primarily from major arterial lava tubes to form unusual morphological features. Whole-rock chemistry suggests that even the most primitive basalts are not primary magmas, but rather fractionated melts derived from primary magmas of variable melt fractions generated at differing mantle depths. Chemical and petrographic data suggest that open-system magma recharge was an important process in the high-level magma chamber beneath Harrat Khaybar. The north-trending volcanic axes of Harrats Rahat, Khaybar, and Ithnayn form a single, 600-km-long linear vent system, designated the Makkah-Madinah-Nafud (MMN) volcanic line. The three transitional-to-mildly alkaline harrats extruded from this volcanic line differ from most Arabian harrats, which like Harrat Kura to the west, are significantly more undersaturated. -from Authors
... (1970) have modelled the rift formation in terms of a simple N W-SE lithospheric stretching. Baker (1970) interpreted the East African Rift as corresponding to a shear zone between two plates, while Oxburgh and Turcotte (1974) took into account the distension that appears within plates moving on a non-spherical ellipsoid. Anderson (1984). ...
A tectonic survey has been completed in Eastern Africa, using remote sensing as well as structural analysis in the field. New data have been collected in the eastern and western branches of the East African Rift system.The geomorphology suggests that the main deformation occurred in the two principal branches of the East African Rift system, but compilations of earthquake data show that recent movements are widespread over a large area in Eastern Africa. Many ancient structures have been reactivated during Neogene times, especially large Precambrian lineaments which are evident on satellite imagery. The Tanganyika–Rukwa–Malawi lineament, along its segment between Lakes Tanganyika and Malawi, now serves as an intracontinental transform zone, with folds and transcurrent faults. The Aswa lineament is, at the present time, only partly reactivated between Lake Mobutu and the Gregory Rift, and also along its southeastern continuation to the Indian Ocean.In many areas, tension gashes or striations on the slickensides have given the local palaeostress orientation. It appears to be changing with time, but the most typical orientation indicates horizontal extension striking NW–SE or WNW–ESE. At the earliest stage of rifting compression was horizontal, then it later became vertical. Compilation of earthquake foci from publications, confirms these results.All these observations together with data concerning the geometry of the deformation, as well as the mechanisms of the Cenozoic intracontinental deformation, implies a near NW–SE movement of the Somalian block, relative to the African continent. A dynamic tectonic model for the rift opening and propagation of the main fracturing across Africa is proposed. It suggests that opening is more important in the north than in the south. Four stages can be identified in the evolution of the East African Rift. The pre-rift stage corresponds to a dense fracturing, characterized by strike-slip faults, while a shallow but wide depression formed, and open tension gashes gave way to tholeiitic volcanism. The initial rifting stage is recognized by oblique-slip faults, bounding tilted blocks, but the uplift of the rift shoulders is not very important. The following ‘typical’ rift formation stage is associated with normal faults bordering the main tilted blocks, while subsidence of the rift floor and uplift of the shoulders are important. The advanced rifting stage, shown by important magmatic intrusions along the rift axis, corresponds to the initial formation of oceanic crust. These stages correspond to different present day aspects of various rift segments in Eastern Africa.
... geological and geophysical evidence now exist to suggest that the Red Sea depression has been formed by extensional tectonics, and is connected to the global rift system through the Gulf of Aden and Carlsberg ridge systems (e.g., Girdler, 1969;Baker, 1970;McKenzie et al., 1970;Le Pichon et al., 1973). In the central and southern Red Sea, a median spreading center has been mapped by bathymetric, gravity and magnetic data, and by an active seismic belt {Cirdler, 1969; Fairhead and Girdler, 1970;Le Pichon et al., 1973;Girdler and Styles, 1974). ...
An active median spreading center has been identified in the Red Sea south of 22°N, but from published reports, the northern Red Sea appears to be essentially aseismic. We have used microearthquake monitoring techniques along the Egyptian Red Sea margin to investigate the active tectonics in the region and have found that the northern Red Sea is not aseismic. Events recorded with epicenters in the Red Sea define an active zone extending south-southeast from the Gulf of Suez into the axial region of the Red Sea down to 25.75°N, with additional microseismicity between 24° and 25°N, suggesting active median spreading in the northern Red Sea. Two areas of intense microearthquake activity have also been identified; the first at the southern end of the Gulf of Suez and the second in the Egyptian Red Sea Hills, clustered at approximately 25.28°N, 34.52°E. The Gulf of Suez seismicity results from the adjustments in motion at the triple junction between the African and Arabian plates and the Sinai subplate. The source of the seismicity in the Egyptian Red Sea Hills is unknown, but a uniform fault plane mechanism is not indicated by first motion studies or the spatial distribution of the hypocenters.
... Research was boosted in the 70s by the introduction of the concept of global tectonics, considering that oceans and related accretion of oceanic lithosphere begin with an intra- continental rift stage (Baker, 1970;Girdler and Sowerbutts, 1970;Mohr, 1970;Searle, 1970;Vail, 1970;McKenzie et al., 1972;Fairhead and Girdler, 1972;. This was also the time of first acquisitions of space imagery, covering large scenes with good ground resolution, a powerful tool for structural analysis of large areas (Mohr, 1974;). ...
This overview paper considers the East African rift system (EARS) as an intra-continental ridge system, comprising an axial rift. It describes the structural organization in three branches, the overall morphology, lithospheric cross-sections, the morphotectonics, the main tectonic features—with emphasis on the tension fractures—and volcanism in its relationships with the tectonics. The most characteristic features in the EARS are narrow elongate zones of thinned continental lithosphere related to asthenospheric intrusions in the upper mantle. This hidden part of the rift structure is expressed on the surface by thermal uplift of the rift shoulders. The graben valleys and basins are organized over a major failure in the lithospheric mantle, and in the crust comprise a major border fault, linked in depth to a low angle detachment fault, inducing asymmetric roll-over pattern, eventually accompanied by smaller normal faulting and tilted blocks. Considering the kinematics, divergent movements caused the continent to split along lines of preexisting lithospheric weaknesses marked by ancient tectonic patterns that focus the extensional strain. The hypothesis favored here is SE-ward relative divergent drifting of a not yet well individualized Somalian plate, a model in agreement with the existence of NW-striking transform and transfer zones. The East African rift system comprises a unique succession of graben basins linked and segmented by intracontinental transform, transfer and accommodation zones. In an attempt to make a point on the rift system evolution through time and space, it is clear that the role of plume impacts is determinant. The main phenomenon is formation of domes related to plume effect, weakening the lithosphere and, long after, failure inducing focused upper mantle thinning, asthenospheric intrusion and related thermal uplift of shoulders. The plume that had formed first at around 30 Ma was not in the Afar but likely in Lake Tana region (Ethiopia), its almost 1000 km diameter panache weakening the lithosphere and preparing the later first rifting episode along a preexisting weak zone, a Pan-African suture zone bordering the future Afar region. From the Afar, the rift propagated afterward from north to south on the whole, with steps of local lithospheric failure nucleations along preexisting weak zones. These predisposed lines are mainly suture zones, in which partial activation of low angle detachment faults reworked former thrust faults verging in opposite directions, belonging to double verging ancient belts. This is responsible for eventual reversal in rift asymmetry from one basin to the next. Supposing the plume migrated southward, or other plumes emplaced, the rift could propagate following former weaknesses, even outside areas influenced by plumes. This view of rift formation reconciles the classical models: active plume effect triggered the first ruptures; passive propagations of failure along lithospheric scale weak zones were responsible for the onset of the main rift segments. Various other aspects are shortly considered, such as tectonics and sedimentation, and relationships of the ‘cradle of Mankind’ with human evolution. By its size, structure and occurrence of oceanic lithosphere in the Afar, the EARS can be taken as a model of the prelude of oceanic opening inside a continent.
... About 585 m.y. 1ater, the same area began to be a spreading zone of three p1ates: Arabia, Ethiopia and 50ma1ia (Baker 1970). ...
Part of illustrative matter is folded in pocket. Summary in Hebrew and German. Vita. Thesis--Bonn. Bibliography: p. 149-155.
The causal relationship between the activity of mantle plumes and continental break-up is still elusive. The Afro-Arabian rift system offers an opportunity to examine these relationship, in which an ongoing continental break-up intersects a large Cenozoic plume related flood basalt series. In the Afar region, the Gulf of Aden, the Red-Sea and the Main Ethiopian Rift form an R-R-R triple junction and separate the Ethiopian and Yemen Traps by ~600 km. We provide an up-to-date synthesis of the available geophysical and geological data from this region. We map the rift architecture in the intersection region of the rifts and review the spatio-temporal constraints in the development of the different features of the plume‐rift system. We infer two spatial constraints in the development of the rifts: (1) the connection of the Main Ethiopian Rift to the Gulf of Aden and to the Red Sea by its northeastward propagation; (2) the abandonment of an early tectonic connection between the Red Sea and the Gulf of Aden. Additionally, chronological evidence suggests that regional uplift and flood basalt eruptions sufficiently preceded rifting. By this, we infer a progressive development in which the onset of the triple junction marks a tectonic reorganization and was the last feature to develop, after all rift arms were thoroughly developed. We argue that the classical active and passive rifting mechanisms cannot simply explain the progressive development of the Afro-Arabian rift and propose a scenario of plume-induced plate rotation that includes an interaction between active and passive mechanisms. In this scenario, the arrival of the Afar plume provided a push-force that promoted the rotation of Arabia around a nearby pole, enabling rifting and ultimately the break-up of Arabia from Africa.
The structural setting and Cenozoic origins of the East African Rift System (EARS) and its southward propagation are described with an emphasis on the eastern branch – the Kenya Rift. The possible role of mantle plumes, lithospheric thinning, pre-rifting structures (basement cratons, mobile belts, earlier rifts, basement foliations), domes, active and passive rifting are outlined. The chapter also describes the major features of accommodation zones, border faults, half-grabens, axial rift faults, monoclines and the major characteristics of Kenya rift basins and their role as sediment traps through time. Volcanics are largely absent in the western branch of the EARS whereas Neogene sodic volcanics dominate in the eastern branch. Volcanic lithologies include alkali basalts, comendites, rhyolite, trachytes, trachyphonolites and phonolites that erupted as either flood lavas or from central volcanoes located at regular intervals along the Kenya Rift. The spatial and temporal distributions of past eruptions are described. The geological evolution of the rift is presented along with schematic 3D models of past lakes and their palaeogeographic setting.
The geomagnetic field over Lake Kivu, an East African rift valley lake, is generally characterized by gentle gradients, and small, isolated anomalies of 50-100 gammas amplitude. Lineated anomalies do not occur except in association with Cenozoic volcanics. In the Northern Basin, which is probably Pliocene in age, the well-bedded sediments average about 300 m in thickness, exceeding 500 m in places. Elsewhere in the lake, the sediment thickness averages only about 100 m. Sonobuoy refraction studies show that the basement underlying the sedimentary column is composed of material having a compressional wave velocity of 5.3–5.4 km s⁻¹, probably representative of sialic plutonic or metamorphic rock.
Gravity measurements show that the western branch of the East African rift is approximately the surface trace of a transition in deep structure across Africa. Bouguer gravity anomalies in the immediate vicinity of Lake Kivu show a strong negative correlation with elevation. The Bouguer anomaly versus elevation relationship, however, is rather different from that for the Gregory Rift or from East Africa in general, thus implying differences in structure.
The geophysical data in Lake Kivu is consistent with the idea that it is at a stage of volcanic and hydrothermal activity following graben formation. Evidence for sea-floor spreading is not observed, but this tectonic process may be in a nascent stage of development.
The East African Rift may be considered as a model to understand the structural evolution of basins prelude to the opening of an ocean. The main grabens are divided into three (western, eastern and southeastern) branches; they all are asymmetrical and bordered by a major normal fault on one side, a faulted flexure on the other side. NW-SE transverse structures are large intracontinental transform zones or, when smaller, transfer faults. Subsidiary basins, sometimes located far from the main breaking line, are interpreted as large open tension gashes or as juvenile grabens with southwards progression of the rift opening. Three types of paleostress orientation are distinguished: a typical early strike-slip regime, (extension NW-SE compression NE-SW); a regional extensive regime (extension NW-SE, compression vertical); and a local extensive regime (extension at right angle to the main scarps, compression vertical) due to local vertical movements. The dynamics of the Rift correspond to a NW-SE trending divergent movement of large continental blocks. Four stages are evident during the Rift emplacement: 1. prerift, characterized by early tholeiitic volcanism and small topographic depressions, but without initial uplift; 2. initial rifts, its dense fracturing affecting large areas made of oblique-slip normal faults; 3. typical rift, bordered by high uplifted shoulders; its deformation is concentrated along the main faults and scarps; 4. advanced rift, with high volcanic activity which preludes formation of an oceanic crust. Each segment of the Rift is likely to be at a different stage at present time. Ancient Precambrian structures such as old plate boundaries resulting from collision or transcurrent movements are reactivated and may determine the Rift emplacement.
In the absence of well defined transform faults in the East African rift system for constraining the plate kinematic reconstruction, the pole of relative motion for the African (Nubian) and Somalian plates has been determined from residual motion.If Africa and Somalia are to continue to drift apart along the East African rift system (which would then evolve into a series of ridges offset by transform faults) then incipient transform faults that may reflect the direction of relative motion should already be in place along the East African rift system. The incipient transforms along the East African rift system are characterized by shear zones, such as the Zambezi shear zone in the south and the Aswa and Hamer shear zones in the north. Some of these shear zones have been associated with recent strike-slip faulting in the NW-SE direction during periods of earthquakes. Provided that these, consistently NW-SE oriented, strike-slip movements in the shear zones give the direction of relative motion of the adjacent plates, then they can be used to constrain the position of the Africa-Somalia Euler pole.Due to the fact that identifying transform faults in the East African rift system is difficult and because the genesis of transform faults characterizing a plate boundary at an inception stage is not well known, the discussion here is limited to the northern segment of the East African rift system where shear zones are better characterized by the existing geophysical data. The characterizing features vary with latitude, indicating the complexity of the problem of the genesis of transform faults. I believe, however, that the relatively well defined intra-continental transform fault in the northern East African rift system, which is characterized by strike-slip faulting and earthquakes, constrains the pole of relative motion for the African and Somalian plates to a position near 1.5°S and 29.0°E.
The Afar depression results from the separation of the Arabian and
Nubian plates, with generation of new oceanic crust. Volcano-tectonic
spreading axes form part of broad uplifted structures. The Gulf of Aden
and Red Sea rifts constitute one and the same tectonic megastructure;
but the distinct characteristics of the East African rift system suggest
that it does not pertain to the megastructure.
FOLLOWING deposition of the Mesozoic sediments and extrusion of the dominantly basaltic lower-mid Tertiary Trap Series over the Precambrian rocks of the Ethio-Arabian region, significant rifting commenced during the early Miocene period, about 20 to 50 m.y. ago1"3. This rifting has continued up to the present time to form the Gulf of Aden, the Red Sea, and the Ethiopian Rift Valley; these intersect in the Afar Depression which is largely covered by later volcanics and sediments.
A crustal attenuation model, initially proposed to explain the evolution of the Canadian margin of the Labrador Sea, is applicable to both intra-cratonic rift zones and passive continental margins. Examples examined are the East African rifts and the New Zealand Plateau, S. W. Pacific. In the model continental lithosphere is stretched and rifted over thermally and chemically expanding asthenosphere blisters. Magmas, abundantly generated in the zone of partial melting at the lithosphere-asthenosphere boundary, ascend and intrude into the fracturing crust. Upon cooling, these intrusives produce low amplitude magnetic anomalies. Sea-floor spreading begins when the asthenosphere finally breaks through en masse and it is thus only a late phase in a much longer cycle. Attenuated continental crust is often underlain by low velocity, low density, upper mantle, which is indicative of the blending of the crust and mantle. It is suggested that substantial parts of marginal quiet magnetic zones are continent-ocean transitions that formed by crustal attenuation. The attenuation model is used to explain the evolution of the Arctic region. It should help bridge the gap between fixists and mobilists.
From essential gravimetrical and petrological data, we propose a new model of geodynamic evolution for the volcanic Hoggar swell and its nearby regions. The Eastern Hoggar, the Tenere troughs, and the North Hoggar troughs correspond to a single distensive structure which developed during the Late Mesozoic, by reactivation of N-S- and NW-SE-trending Pan African faults, in response to the stress field probably associated with the Central Atlantic opening. Contemporaneously, as suggested by the xenoliths entrained by the Cenozoic alkali basaltic eruptions of the Eastern Hoggar, an extensive lower crustal basic magmatism, (of tholeiitic affinity and probably associated with crustal stretching) may have developed beneath the Hoggarian part of this distensive structure. On the other hand, at the Hoggar level, a transverse lineament, oriented NE-SW (Oued Amded), cross-cuts the distensive structure. This lineament subsequently controlled the localization of the Mio-Plio-Quaternary volcanic districts, and possibly upper mantle modifications (through reheating, partial melting, metasomatism and magmatic veining) produced by astenospheric material transfer (gas, fluid, kimberlitic to carbonatitic magmas), resulting in a density reduction. On the other hand, the high deformation of the peridotitic inclusions of the Cenozoic alkali basalts emplaced along the Oued Amded lineament may indicate that this lineament has also controlled the emplacement of small-sized asthenospheric diapirs into the upper mantle. Thus, in the Hoggar, it appears that doming was probably preceded by a rifting phase, and that doming and Cenozoic volcanism were possibly controlled by the intersection of this rift by a transverse wrench fault.
Three main types of volcanism, viz. oceanic, continental and stratoid, reflect the tectonic evolution of Afar.Stratoid volcanism (flood basalts and ignimbrites) is believed to be an early manifestation of continental rifting, as exemplified by the Ethiopian Rift. It probably developed during the early stages of Red Sea and Gulf of Aden rifting.Continental volcanism is located on either side of the rift's axis; it is mainly silicic with minor proportions of basalts. The large, essentially silicic, central volcanoes present along the margins of the depression north from 11° 00 'N are believed to be due to interactions between subcrustal magma and sialic crust.Volcanism, of oceanic type, essentially basaltic with differentiates reaching to peralkaline silicics, is related to crustal separation. It occurs only along the rift-in-rift axes of the Red Sea-Gulf of Aden megastructures and is partly superimposed on the stratoid volcanics. The southern edge of this oceanic megastructure marks the northern limit of the continental rift (main Ethiopian rift). Tectonic and thermal evidence suggests the rise of upper mantle material beneath the active axes of the Afar depression.
Changes in the topography of rifts as they evolve toward aulacogens and passive margins control river drainage and, therefore, sedimentation. The African continent offers an opportunity to assess the effect of rifting on river drainage and sedimentation at several stages of development. The East African system is young and sediment-starved as a result of both the regional doming that accompanies early phases of rifting and the back-tilting of footwall fault blocks. Each of these factors diverts rivers away from subsiding basins. The longitudinally segmented nature of an early rift discourages axial drainage and major river inputs originate on the hanging-wall platform or roll-over of half graben sub-basins. During later stages of development, regional subsidence may enlarge the drainage basin, while sedimentary infilling of the trough may facilitate axial drainage. A good example of this occurs in the West African system where the Benue River flows for 1200km along a rift trough and the drainage basin incorporates the catchment of the River Niger. Sedimentary sequences accumulating in rift basins are characterised by strong cyclicity of depositional style. This can be taken to reflect the sporadic nature of the pulses associated with faulting. However, it can also arise from the changes in base-level that are brought about by fluctuations in climate in closed basins and by eustatic adjustments in sea-level where marine inundation has occurred. Taking examples from the East African system and from the Gulf of Suez, changes in sedimentation that are consequent upon marine or lacustrine transgression and regression are shown to be almost indistinguishable from those triggered by boundary fault activity. Cyclical patterns of sedimentation can only be attributed to tectonic activity if corroborative evidence is available.
Based on field studies supplemented by remote sensing and aeromagnetic data from central Tanzania, a Phanerozoic structural history for the region can be developed and placed in a broader rift context. The major contribution of this work is the recognition of rift morphology over an area lying 400 km beyond the southern termination of the Eastern, or Kenya, Rift. The most prominent rift structures occur in the Kilombero region and consist of a wide range of uplifted basement blocks fringed to the west by an east-facing half-graben that may contain 6-8 km of sedimentary strata. Physiographic features and river drainage anomalies suggest that Holocene/Neogene deformation occurs along both rift-parallel and transverse faults, in agreement with the seismogenic character of a number of oblique faults. The present-day rift pattern of the Kilombero extensional province results from the complete overprinting of an earlier (Karoo) rift basin by Neogene-Holocene faults. The Kilombero rift zone is assumed to connect northward into the central rift arm (Manyara) of the Eastern Rift via an active transverse fault zone. The proposed rift model implies that incipient rifting propagates throughout the cold and strong lithosphere of central Tanzania following Proterozoic basement weakness zones (N140°E) and earlier Karoo rift structures (north-south). An eventual structural connection of the Kilombero rift zone with the Lake Malawi rift further south is also envisaged and should imply the spatial link of the eastern and western branches of the East African Rift System south of the Tanzanian craton.
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