Gulf Of Aden Rifts Research Articles

Seamless Map of Depth to the Moho Interface in the Afro‐Arabian Region Using Gravity Data Derived From EGM2008

AbstractThe Afro‐Arabian region is one of the few places on land, where rifting processes at divergent plate boundaries can be thoroughly investigated. One of the crucial factors in understanding rifting processes involves assessing the crustal thickness. In this study, gravity data from the Earth Gravitational Model 2008 is used to create a seamless map of the depth to the Moho interface. Unlike many previous investigations that focused on specific localized areas, within the region, results from the current study provide a comprehensive view. The depth obtained from the current investigation aligns well with findings from earlier studies, exhibiting a bias of 0.69 km and a standard deviation of 3.89 km. Within the region, maximum and minimum depths to the Moho interface are observed beneath the northwest Ethiopian Plateau and the Gulf of Aden Rift (GAR), respectively. Analyzing profiles across the Red Sea, Main Ethiopian, and GARs, the study concluded that the Southern Main Ethiopian Rift is in an earlier stage of the rifting process, while the GAR is at an advanced stage. Furthermore, the interpretation of the current findings led to the inference that there might exist two potential plume tails driving the rifting process in the East Africa Rift—one originating from the Afar region and the other from South Kenya. This inference primarily relies on the isostatic compensation stages observed in the various rift systems throughout the region.

To Rotate or to Link? The Connection Between the Red Sea and Gulf of Aden Rifts in Central Afar

AbstractCentral Afar is shaped by the interaction between the Red Sea (RS) and Gulf of Aden (GoA) rifts. While there have been several studies conducted in the region, we know surprisingly little about the mechanism of connection between these two rift branches. Here we use high‐resolution 3D lithospheric scale geodynamic modeling to capture the evolution of linkage between the RS and GoA rifts in central Afar. Our results demonstrate that the two rifts initially overlap and interact across a broad zone of faulting and vertical axis block rotation. However, through time, rift overlap is abandoned in favor of direct linkage which generates a series of localized en‐echelon basins. The present‐day direct linkage between the two rifts is supported by geodetic observations. Our study reconciles previously proposed models for the RS and GoA rift connection by considering spatial and temporal evolution of the rifts.

The significance of volcanic segments and rifts in faults characterization within the Amagmatic graben of the Afar Depression, Ethiopia

The Dobi graben is a northwestern trending, continental rift situated in the East Central Block (ECB) of the Afar Depression (AD), Ethiopia. Ongoing extensional rifting in the graben is evident from the swarm of intermediate magnitude earthquakes (5.7 < Ms < 6.3) in 1989. The graben's extension occurs on steeply dipping faults, where the maximum displacement and traced fault length spans four orders of magnitude. Using a 30 m resolution Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM), we conducted a fault population analysis in the Dobi graben. We traced 953 faults and conducted a size-frequency distribution analysis has been conducted for various spatially heterogeneous structural zones. Our results show that the frequency size distribution revealed a negative exponential fitting trend, indicating strong strain distribution within the Imbrication, the Fault Termination zone, and the active axial graben floor. However, a power law size distribution dominates most first-order border faults, suggesting that the strain is localized mainly at the graben flanks.The fault displacement-length profiles demonstrate that approximately 48% of the total fault traced lengths exhibit increasing slip rates towards the southeast, while about 40% display increasing slip rates towards the northwest. These suggest that ∼88% of the lateral propagation of the 953 faults in the Dobi graben is governed by the regional differential strain transfer of the Red Sea Rift (RSR) and Gulf of Aden Rift (GAR) to the central Afar. Most of the hmax/L aspect ratio of the Dobi graben's faults fits in Category II, which exhibits a constant hmax/L ratio, meaning that the hmax/L ratio increases with fault growth, presumably due to the graben's is in an active tectonic region. This implies that these faults are becoming more efficient at accommodating deformation as they grow. Additionally, the normalized average maximum displacement over maximum length (Dmax/Lmax) ratio for most faults is 0.03, which is in accord with the constant displacement length fault growth model.

Imaging Lithospheric Discontinuities Beneath the Northern East African Rift Using S-to-P Receiver Functions.

Imaging the lithosphere is key to understand mechanisms of extension as rifting progresses. Continental rifting results in a combination of mechanical stretching and thinning of the lithosphere, decompression upwelling, heating, sometimes partial melting of the asthenosphere, and potentially partial melting of the mantle lithosphere. The northern East African Rift system is an ideal locale to study these processes as it exposes the transition from tectonically active continental rifting to incipient seafloor spreading. Here we use S‐to‐P receiver functions to image the lithospheric structure beneath the northernmost East African Rift system where it forms a triple junction between the Main Ethiopian rift, the Red Sea rift, and the Gulf of Aden rift. We image the Moho at 31 ± 6 km beneath the Ethiopian plateau. The crust is 28 ± 3 km thick beneath the Main Ethiopian rift and thins to 23 ± 2 km in northern Afar. We identify a negative phase, a velocity decrease with depth, at 67 ± 3 km depth beneath the Ethiopian plateau, likely associated with the lithosphere‐asthenosphere boundary (LAB), and a lack of a LAB phase beneath the rift. Using observations and waveform modeling, we show that the LAB phase beneath the plateau is likely defined by a small amount of partial melt. The lack of a LAB phase beneath the rift suggests melt percolation through the base of the lithosphere beneath the northernmost East African Rift system.

Scaling properties of normal faults in the central Afar, Ethiopia and Djibouti: Implications for strain partitioning during the final stages of continental breakup

The central Afar depression offers an opportunity for quantitative analysis of the final stages of continental breakup at a divergent triple junction. Here, Quaternary faulting dissects Plio-Pleistocene flood basalts, providing a regionally pervasive datum to quantify the regional finite strain and fault scaling laws. To this end, we mapped and measured more than 8500 normal faults using remote sensing products. Fault length generally follows a power law distribution, but exponential distributions are found in areas of magmatic extension. Fault throw and fault length scale following a power law relationship where T=0.031L0.96, similar to previous studies in Afar and elsewhere in the world. Combining the power law distribution with this throw scaling enables a comprehensive assessment of Quaternary finite strain throughout the region. Strain is greatest near Manda Inakir, Lake Asal, and the Dobe-Hanle accommodation zone. The Makarassou fault zone shows low strain, suggesting it does not kinematically link the Manda Inakir and Asal rift segments. The presence of moderate to high strain in the amagmatic portions of the central Afar rift suggests that those areas accommodate a significant proportion of plate motion. This may provide a future path for propagation between the Gulf of Aden Rift and the Red Sea Rift.

Upper lithospheric structure of the Dobi graben, Afar Depression from magnetics and gravity data

We employed high-resolution ground magnetic, aeromagnetic and satellite gravity data to image the upper lithospheric structure beneath the Dobi graben and surroundings. This graben is located in the East Central Block (ECB) of the Afar Depression (AD) which is enclosed between the Red Sea and the Gulf of Aden rifts. Our aim is to understand the styles of crustal extension beneath the Dobi graben. We used two-dimensional (2D) radially-averaged power spectral analysis of both the aeromagnetic and the satellite gravity data to image deeper magnetic and density sources. We used the spectral analysis to estimate the depth to the boundary between the upper and lower crust, and the depth to Moho. Subsequently, we performed 2D forward modeling using the high-resolution ground magnetic and the satellite gravity data to produce a NE-SW trending model for the upper lithosphere beneath the Dobi graben and surroundings. We found the crust to be thinner beneath the Dobi graben reaching a thickness of only ∼23 km. We also found the boundary between the upper and lower crust to be at depth between 10 and 12 km. Additionally, we found two ∼5 km wide zones where melt and mafic dikes are possibly present within the lower crust. These zones are centered beneath a relay zone on the southwestern side of the Dobi graben and beneath a narrow (∼2 km wide) graben just to the northeast of the Dobi graben. Our model agrees with previous findings, especially Vp/Vs ratios of 1.85 or higher calculated from broadband passive seismic receiver function studies, which indicate the presence of melt in the lower crust beneath the graben. Because of this and the occurrence of earthquakes above ∼15 km deep seismogenic zone, we propose that, while the upper crust beneath the Dobi graben is mechanically stretching, the lower crust is ductily stretching aided by the presence of melt and the intrusion of mafic dikes.

Multiple mantle upwellings in the transition zone beneath the northern East‐African Rift system from relative P‐wave travel‐time tomography

AbstractMantle plumes and consequent plate extension have been invoked as the likely cause of East African Rift volcanism. However, the nature of mantle upwelling is debated, with proposed configurations ranging from a single broad plume connected to the large low‐shear‐velocity province beneath Southern Africa, the so‐called African Superplume, to multiple lower‐mantle sources along the rift. We present a new P‐wave travel‐time tomography model below the northern East‐African, Red Sea, and Gulf of Aden rifts and surrounding areas. Data are from stations that span an area from Madagascar to Saudi Arabia. The aperture of the integrated data set allows us to image structures of ∼100 km length‐scale down to depths of 700–800 km beneath the study region. Our images provide evidence of two clusters of low‐velocity structures consisting of features with diameter of 100–200 km that extend through the transition zone, the first beneath Afar and a second just west of the Main Ethiopian Rift, a region with off‐rift volcanism. Considering seismic sensitivity to temperature, we interpret these features as upwellings with excess temperatures of 100 ± 50 K. The scale of the upwellings is smaller than expected for lower mantle plume sources. This, together with the change in pattern of the low‐velocity anomalies across the base of the transition zone, suggests that ponding or flow of deep‐plume material below the transition zone may be spawning these upper mantle upwellings.

Upper mantle temperature and the onset of extension and break-up in Afar, Africa

It is debated to what extent mantle plumes play a role in continental rifting and eventual break-up. Afar lies at the northern end of the largest and most active present-day continental rift, where the East African Rift forms a triple junction with the Red Sea and Gulf of Aden rifts. It has a history of plume activity yet recent studies have reached conflicting conclusions on whether a plume still contributes to current Afar tectonics. A geochemical study concluded that Afar is a mature hot rift with 80 km thick lithosphere, while seismic data have been interpreted to reflect the structure of a young, oceanic rift basin above mantle of normal temperature. We develop a self-consistent forward model of mantle flow that incorporates melt generation and retention to test whether predictions of melt chemistry, melt volume and lithosphere–asthenosphere seismic structure can be reconciled with observations. The rare-earth element composition of mafic samples at the Erta Ale, Dabbahu and Asal magmatic segments can be used as both a thermometer and chronometer of the rifting process. Low seismic velocities require a lithosphere thinned to 50 km or less. A strong positive impedance contrast at 50 to 70 km below the rift seems linked to the melt zone, but is not reproduced by isotropic seismic velocity alone. Combined, the simplest interpretation is that mantle temperature below Afar is still elevated at 1450 °C, rifting started around 22–23 Ma, and the lithosphere has thinned from 100 to 50 km to allow significant decompressional melting.

Receiver function constraints on crustal seismic velocities and partial melting beneath the Red Sea rift and adjacent regions, Afar Depression

The Afar Depression is an ideal locale for the investigation of crustal processes involved in the transition from continental rifting to oceanic spreading. To provide relatively high resolution images of the crust beneath the Red Sea rift (RSR) represented by the Tendaho graben in the Afar Depression, we deployed an array of 18 broadband seismic stations in 2010 and 2011. Stacking of about 2300 receiver functions from the 18 and several nearby stations along the ~200 km long array reveals an average crustal thickness of 22±4 km, ranging from ~17 km near the RSR axis to 30 km within the overlap zone between the Red Sea and Gulf of Aden rifts. The resulting anomalously high Vp/Vs ratios decrease from 2.40 in the southwest to 1.85 within the overlap zone. We utilize theoretical Vp and melt fraction relationships to obtain an overall highly reduced average crustal Vp of ~5.1 km/s. The melt percentage is about 10% beneath the RSR while the overlap zone contains minor quantities of partial melt. The observed high Vp/Vs values beneath most of the study area indicate widespread partial melting beneath the southwest half of the profile, probably as a result of gradual eastward migration of the RSR axis. Our results also suggest that the current extensional strain in the lower crust beneath the region is diffuse, while the strain field in the upper crust is localized along narrow volcanic segments. These disparate styles of deformation imply a high degree of decoupling between the upper and lower crust.

Contrasted styles of rifting in the eastern Gulf of Aden: A combined wide‐angle, multichannel seismic, and heat flow survey

Continental rifts and passive continental margins show fundamental along‐axis segmentation patterns that have been attributed to one or a number of different processes: extensional fault geometry, variable stretching along strike, preexisting lithospheric compositional and structural heterogeneities, oblique rifting, and the presence or absence of eruptive volcanic centers. The length and width scales of the rift stage fault‐bounded basin systems change during the late evolution of the new plate boundary, and the role of magmatism may increase as rifting progresses to continental rupture. Along obliquely spreading ridges, first‐order mid‐ocean ridge geometries originate during the synrift stage, indicating an intimate relationship between magma production and transform fault spacing and location. The Gulf of Aden rift is a young ocean basin in which the earliest synrift to breakup structures are well exposed onshore and covered by thin sediment layers offshore. This obliquely spreading rift is considered magma‐poor and has several large‐offset transforms that originated during late stage rifting and control the first‐order axial segmentation of the spreading ridge. Widely spaced geophysical transects of passive margins that produce only isolated 2‐D images of crust and uppermost mantle structure are inadequate for evaluation of competing rift evolution models. Using closely spaced new geophysical and geological observations from the Gulf of Aden we show that rift sectors between transforms have a large internal variability over short distances (∼10 km): the ocean‐continent transition (OCT) evolves from a narrow magmatic transition to wider zones where continental mantle is probably exhumed. We suggest that this small‐scale variability may be explained (1) by the distribution of volcanism and (2) by the along‐strike differences in time‐averaged extension rate of the oblique rift system. The volcanism may be associated with (1) the long‐offset Alula‐Fartak Fracture Zone, which may enhance magma production on its younger side, or (2) channeled flow from the Afar plume material along the newly formed OCT and the spreading ridge. Oblique extension and/or hot spot interactions may thereby have a significant control on the styles of rifting and continental breakup and on the evolution of many magma‐poor margins.

Geodetic constraints on present‐day motion of the Arabian Plate: Implications for Red Sea and Gulf of Aden rifting

Five years of continuously recording GPS observations in the Kingdom of Saudi Arabia together with new continuous and survey‐mode GPS observations broadly distributed across the Arabian Peninsula provide the basis for substantially improved estimates of present‐day motion and internal deformation of the Arabian plate. We derive the following relative, geodetic Euler vectors (latitude (°N), longitude (°E), rate (°/Myr, counterclockwise)) for Arabia‐Nubia (31.7 ± 0.2, 24.6 ± 0.3, 0.37 ± 0.01), Arabia‐Somalia (22.0 ± 0.5, 26.2 ± 0.5, 0.40 ± 0.01), Arabia‐India (18.0 ± 3.8, 87.6 ± 3.3, 0.07 ± 0.01), Arabia‐Sinai (35.7 ± 0.8, 17.1 ± 5.0, 0.15 ± 0.04), and Arabia‐Eurasia (27.5 ± 0.1, 17.6 ± 0.3, 0.404 ± 0.004). We use these Euler vectors to estimate present‐day stability of the Arabian plate, the rate and direction of extension across the Red Sea and Gulf of Aden, and slip rates along the southern Dead Sea fault south of the Lebanon restraining bend (4.5–4.7 ± 0.2 mm/yr, left lateral; 0.8–1.1 ± 0.3 mm/yr extension) and the Owens fracture zone (3.2–2.5 ± 0.5 mm/yr, right lateral, increasing from north to south; 1–2 mm/yr extension). On a broad scale, the Arabian plate has no resolvable internal deformation (weighted root mean square of residual motions for Arabia equals 0.6 mm/yr), although there is marginally significant evidence for N‐S shortening in the Palmyride Mountains, Syria at ≤ 1.5 mm/yr. We show that present‐day Arabia plate motion with respect to Eurasia is consistent within uncertainties (i.e., ±10%) with plate tectonic estimates since the early Miocene when Arabia separated from Nubia. We estimate the time of Red Sea and Gulf of Aden rifting from present‐day Arabia motion, plate tectonic evidence for a 70% increase in Arabia‐Nubia relative motion at 13 Ma, and the width of the Red Sea and Gulf of Aden and find that rifting initiated roughly simultaneously (±2.2 Myr) along the strike of the Red Sea from the Gulf of Suez to the Afar Triple Junction, as well as along the West Gulf of Aden at 24 ± 2.2 Ma. Based on the present kinematics, we hypothesize that the negative buoyancy of the subducted ocean lithosphere beneath the Makran and the Zagros fold‐thrust belt is the principle driver of Arabia‐Eurasia convergence and that resisting forces associated with Arabia‐Eurasia continental collision have had little impact on plate motion.

Mapping the Crust-Mantle Boundary Beneath Afar Depression

The funnel-shaped Afar depression in the Horn of Africa situated at the separation zones of the Nubian, Somalian and Arabian plates, represents a triple junction between the Main Ethiopian Rift, the Red Sea and Gulf of Aden rifts. This depression occupies an important place in that it is the only emerged triple junction in the world. Assuming that the mechanism of compensation is one in which the topography is regionally compensated by the flexuring of the elastic plate and that linear relationship exists between the gravity field of the compensation and the topography isostatic correction, isostatic anomaly and Moho depth maps are computed for the depression. The results show that nonlinear elements are small in Afar and hence the crust-mantle interface beneath Afar modelled using admittance technique compares very well with the existing seismic results. The results also show that there is crustal attenuation to the north and east and that there is an under compensation in the north that may be due to the upwelling of mantle material along the active axis.

Comparative K–Ar and Ar/Ar dating of Ethiopian and Yemenite Oligocene volcanism: implications for timing and duration of the Ethiopian traps

It is now generally accepted that continental flood basalt (CFB) volcanism bears a strong relationship with continental breakup. The Ethiopian Afar plume has been linked to the opening of the Afar depression. Propagation of the Red Sea and Gulf of Aden rifts within the depression, still an ongoing process, has rifted away the Ethiopian and Yemenite trap sequences. They are in some locations more than 2 km thick and comprise a wide range of volcanic products, from tholeiitic basalts, in the lower part, to more acidic material in the upper part. Recent studies have established that the bulk of trap volcanism erupted about 30 Ma ago over a period of 1 Myr in the Ethiopian sections, while ages obtained on the Yemenite sections seem more distributed through time. Here, for the first time in a single study, we present geochronological results obtained for basalts and more evolved products for both Ethiopian and Yemenite traps. This approach eliminates inter-laboratory biases and discrepancies in the ages of standards, and imposes better constraints on the eruptive chronology of this CFB province. In addition, both the K–Ar and 40Ar/ 39Ar techniques have been applied simultaneously, in order to demonstrate that similar ages are indeed obtained for undisturbed samples. The two dating techniques used here yield concordant ages for most samples. On both sides of the Afar depression, our results support that the onset of basaltic volcanism is coeval, with undistinguishable ages of 30.6±0.4 and 30.2±0.4 Ma obtained from Ethiopia and Yemen, respectively. Most of the basaltic lava pile has been erupted in less than 1 Myr, but acidic volcanism seems more spread out through time. It is coeval with basalts in northern Ethiopia but extends to about 26 Ma in Yemen, as already recognized. A younger rhyolitic episode, probably related to the major 20 Ma phase of opening of the Red Sea and Gulf of Aden, as expressed in the Afar depression, is also observed in Yemen and central Ethiopia.

New seismic model of the upper mantle beneath Africa

We present a seismic model of the upper 400 km of the mantle beneath Africa and surrounding regions. This model is constructed by inverse modeling of fundamental mode Rayleigh wave phase velocities (40–200 s) for about 8000 propagation paths. Among the most pronounced anomalies are high shear velocity structures (as much as 6% higher than in the Preliminary Reference Earth model) beneath the West African, Congo, and Kalahari cratons that extend to about 250 km depth. These structures have near-vertical margins across which the shear velocity changes by as much as 6% over 500 km distance. Anomalous low shear velocities (3%–4% lower than in the Preliminary Reference Earth Model) structures are observed beneath the East African, Red Sea, and Gulf of Aden rifts, and beneath the northwestern Indian Ocean. These structures extend to a depth of at least 250 km. Our model cannot be reconciled with models that invoke a large number of plumes that have impinged on the base of the lithosphere, nor does our seismic model indicate that high-temperature, low-density material beneath the lithosphere is responsible for the uplift of southern Africa.

Stratigraphic Correlation by Integrating Photostratigraphy and Remote Sensing Multispectral Data: An Example from Jurassic-Eocene Strata, Northern Somalia

Integrated analyses of aerial photographs and multispectral remote sensing images were used for stratigraphic correlation in mainly carbonate and evaporitic rocks. These rocks crop out in an area of northern Somalia characterized by an arid climate. By the aerial photo analysis, we recognized photostratigraphic logs and stratal patterns and established correlations based on the tracing of physical surfaces with chronostratigraphic significance, such as photohorizons and photostratigraphic discontinuities. A limited number of field sections provided the lithological interpretation of the packages of strata delineated in aerial photos. By satellite multispectral (Landsat Thematic MapperTM) data analysis we identified image facies that represent packages of strata with different lithological characteristics. To interpret the image facies, we compared the responses in the thematic mapper (TM) bands with the laboratory spectroscopic properties of rock samples from the study area, and interpreted the absorption features by p trographic analysis. The Mesozoic and Tertiary strata analyzed herein are part of several formations deposited on a passive margin preceding the Oligocene-Miocene Gulf of Aden rifting and initial drifting. Following this approach, a number of stratigraphic units were recognized and mapped on aerial photos, and a framework of photostratigraphic correlation surfaces was delineated over significantly wide areas. These surfaces approximate time surfaces and are traced both within and across the lithostratigraphic units, improving existing maps. This method represents a mapping tool preliminary to more detailed field work, and is particularly useful in areas of difficult access.

Geochemistry of Ordovician Keli Group basalts associated with Besshi-type Cu-Zn deposits from the southern Trondheim and Sulitjelma mining districts of Norway

Besshi-type volcanogenic Cu-Zn deposits in the Scandinavian Caledonides are hosted by Ordovician metabasalts and clastic sediments of the Storen, Fundsjo and Sulitjelma groups. The basalts are transitional between T-MORB and marginal basin tholeiites in composition and are characterised by Nd and Pb isotopic compositions which overlap the more radiogenic values of Lower Palaeozoic MORB. These features, along with the intercalation of the basalts with tuffs and continentally derived sediments, indicate an epicontinental rift or marginal basin origin, possibly analogous to the present Red Sea and Gulf of Aden rifts. This implies the development of a restricted ocean basin in the north of Iapetus between the Laurentian and Baltoscandian microcontinents during the Cambrian and Early Ordovician.

Gravity study of the Central African Rift system: a model of continental disruption 2. The Darfur domal uplift and associated Cainozoic volcanism

Gravity studies of the Darfur uplift, Western Sudan, show it to be associated with a circular negative Bouguer anomaly, 50 mGal in amplitude and 700 km across. A three-dimensional model interpretation of the Darfur anomaly, using constraints deduced from geophysical studies of similar but more evolved Kenya and Ethiopia domes, suggests either a low-density laccolithic body at mid-lithospheric depth (~ 60 km) or a thinned lithosphere with emplacement at high level of low-density asthenospheric material. The regional setting of the Darfur uplift is described in terms of it being an integral part of the Central African Rift System which is shown to be broadly equivalent to the early to middle Miocene stage in the development of the Afro-Arabian Rift System. Comparisons between these rift systems suggest that extensional tectonics and passive rifting, resulting in the subsiding sedimentary rift basins associated with the Ngaoundere, Abu Gabra, Red Sea and Gulf of Aden rifts, are more typical of the early stage development of passive continental margins than the active domal uplift and development of rifted features associated with the Darfur, Kenya and Ethiopia domes.

Age of the lower flood basalts of the Ethiopian plateau

VARIOUS stages of continental fracturing and crustal separation are displayed in Ethiopia at present. Separation of the African and Arabian plates has taken place along the Red Sea and Gulf of Aden rifts and in several areas of Afar where axial volcanic ranges are composed of low-potassium oceanic tholeiites1. The onset of spreading was accompanied by deep faulting and intense fissural activity, with the eruption of very fluid flood basalts over Mesozoic sediments and Precambrian basement. The flood basalts outcrop on the plateaux flanking the rift system (Fig. 1).

Volcanism in the Afar depression: Its tectonic and magmatic significance

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.

Volcanism in the Afar depression: Its tectonic and magmatic significance

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.