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Civiero C.,Imperial College London | Hammond J.O.S.,Imperial College London | Goes S.,Imperial College London | Fishwick S.,University of Leicester | And 11 more authors.
Geochemistry, Geophysics, Geosystems | Year: 2015

Mantle 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. © 2015. The Authors.


Bellahsen N.,CNRS Paris Institute of Earth Sciences | Leroy S.,CNRS Paris Institute of Earth Sciences | Autin J.,University of Strasbourg | Razin P.,Bordeaux Montaigne University | And 5 more authors.
Tectonophysics | Year: 2013

Transfer zones are ubiquitous features in continental rifts and margins, as are transform faults in oceanic lithosphere. Here, we present a structural study of the Hadibo Transfer Zone (HTZ), located in Socotra Island (Yemen) in the southeastern Gulf of Aden. There, we interpret this continental transfer fault zone to represent a reactivated pre-existing structure. Its trend is oblique to the direction of divergence and it has been active from the early up to the latest stages of rifting. One of the main oceanic fracture zones (FZ), the Hadibo-Sharbithat FZ, is aligned with and appears to be an extension of the HTZ and is probably genetically linked to it. Comparing this setting with observations from other Afro-Arabian rifts as well as with passive margins worldwide, it appears that many continental transfer zones are reactivated pre-existing structures, oblique to divergence. We therefore establish a classification system for oceanic FZ based upon their relationship with syn-rift structures. Type 1 FZ form at syn-rift structures and are late syn-rift to early syn-OCT. Type 2 FZ form during the OCT formation and Type 3 FZ form within the oceanic domain, after the oceanic spreading onset. The latter are controlled by far-field forces, magmatic processes, spreading rates, and oceanic crust rheology. © 2013 Elsevier B.V.


Corbeau J.,CNRS Institute of Earth Sciences | Rolandone F.,CNRS Institute of Earth Sciences | Leroy S.,CNRS Institute of Earth Sciences | Al-Lazki A.,Sultan Qaboos University | And 10 more authors.
Geosphere | Year: 2014

We determine the lateral variations in seismic velocity of the lithospheric mantle beneath the Gulf of Aden and its margins by inversion of Pn (upper mantle high-frequency compressional P wave) traveltimes. Data for this study were collected by several temporary seismic networks and from the global catalogue. A least-squares tomographic algorithm is used to solve for velocity variations in the mantle lithosphere. In order to separate shallow and deeper structures, we use separate inversions for shorter and longer ray path data. High Pn velocities (8.2-8.4 km/s) are observed in the uppermost mantle beneath Yemen that may be related to the presence of magmatic underplating of the volcanic margins of Aden and the Red Sea. Zones of low velocity (7.7 km/s) are present in the shallow upper mantle beneath Sana'a, Aden, Afar, and along the Gulf of Aden that are likely related to melt transport through the lithosphere feeding active volcanism. Deeper within the upper mantle, beneath the Oman margin, a low-velocity zone (7.8 km/s) suggests a deep zone of melt accumulation. Our results provide evidence that the asthenosphere undergoes channelized flow from the Afar hotspot toward the east along the Aden and Sheba Ridges. © 2014 Geological Society of America.


Pik R.,University of Lorraine | Bellahsen N.,CNRS Paris Institute of Earth Sciences | Leroy S.,CNRS Paris Institute of Earth Sciences | Denele Y.,CNRS Paris Institute of Earth Sciences | And 5 more authors.
Tectonophysics | Year: 2013

In the Gulf of Aden, different types of fracture zones (F.Z.) have been defined and potential links with continental transfer zones have been proposed (Bellahsen et al., 2013-this volume-a). In this study, we investigate the denudation history of the south-eastern continental margin of the Gulf of Aden on the Socotra Island, in order to highlight the interplay of normal and transfer/transform tectonic structures in the course of rift evolution. Samples belong from two distinct East and West domains of the Socotra Island separated by the continental Hadibo Transfer Zone (HTZ). Tectonic denudation started during the Priabonian-Rupelian along low-angle normal faults and removed part of the overlying sedimentary formations allowing basement exhumation toward the surface (~. 1.1-1.5. km of exhumation). Forward t-T modelling of the data requires a slightly earlier date for initiation of rifting in the E-Socotra domain (~. 38. ±. 2. Ma), compared to the W-Socotra domain (~. 32. ±. 2. Ma), which suggests that the HTZ was already active at that time. A second major event of basement cooling and exhumation (additional. ~. 0.7-1. km), starting at about ~. 20. ±. 2. Ma, has only been recorded on the E-Socotra domain. This second denudation phase significantly post-dates local rifting period but appears synchronous with Ocean Continent Transition formation (OCT: 20-17.6. Ma). This late syn-OCT uplift is maximum close to the HTZ, in the wedge of footwall delimited by this transfer system and the steep north-dipping normal faults that accommodated the vertical motion. This particular pattern of uplift and denudation during the OCT reorganisation suggests that the late uplift of the margin can be strongly differential from a segment to another, depending on the amplitude of thinning experienced by each of the adjoining segments. © 2013 Elsevier B.V.


Korostelev F.,CNRS Paris Institute of Earth Sciences | Leroy S.,CNRS Paris Institute of Earth Sciences | Keir D.,UK National Oceanography Center | Weemstra C.,Technical University of Delft | And 8 more authors.
Terra Nova | Year: 2016

Non-volcanic continental passive margins have traditionally been considered to be tectonically and magmatically inactive once continental breakup has occurred and seafloor spreading has commenced. We use ambient-noise tomography to constrain Rayleigh-wave phase-velocity maps beneath the eastern Gulf of Aden (eastern Yemen and southern Oman). In the crust, we image low velocities beneath the Jiza-Qamar (Yemen) and Ashawq-Salalah (Oman) basins, likely caused by the presence of partial melt associated with magmatic plumbing systems beneath the rifted margin. Our results provide strong evidence that magma intrusion persists after breakup, modifying the composition and thermal structure of the continental margin. The coincidence between zones of crustal intrusion and steep gradients in lithospheric thinning, as well as with transform faults, suggests that magmatism post-breakup may be driven by small-scale convection and enhanced by edge-driven flow at the juxtaposition of lithosphere of varying thickness and thermal age. © 2016 John Wiley & Sons Ltd.


Korostelev F.,CNRS Paris Institute of Earth Sciences | Korostelev F.,CNRS Institute of Earth Sciences | Basuyau C.,CNRS Paris Institute of Global Physics | Leroy S.,CNRS Paris Institute of Earth Sciences | And 12 more authors.
Geochemistry, Geophysics, Geosystems | Year: 2014

We image the lithospheric and upper asthenospheric structure of western continental Yemen with 24 broadband stations to evaluate the role of the Afar plume on the evolution of the continental margin and its extent eastward along the Gulf of Aden. We use teleseismic tomography to compute relative P wave velocity variations in south-western Yemen down to 300 km depth. Published receiver function analysis suggest a dramatic and localized thinning of the crust in the vicinity of the Red Sea and the Gulf of Aden, consistent with the velocity structure that we retrieve in our model. The mantle part of the model is dominated by the presence of a low-velocity anomaly in which we infer partial melting just below thick Oligocene flood basalts and recent off-axis volcanic events (from 15 Ma to present). This low-velocity anomaly could correspond to an abnormally hot mantle and could be responsible for dynamic topography and recent magmatism in western Yemen. Our new P wave velocity model beneath western Yemen suggests the young rift flank volcanoes beneath margins and on the flanks of the Red Sea rift are caused by focused small-scale diapiric upwelling from a broad region of hot mantle beneath the area. Our work shows that relatively hot mantle, along with partial melting of the mantle, can persist beneath rifted margins after breakup has occurred. © 2014. American Geophysical Union. All Rights Reserved.


Ahmed A.,Seismological and Volcanological Observatory Center | Ahmed A.,University Pierre and Marie Curie | Ahmed A.,French National Center for Scientific Research | Tiberi C.,French National Center for Scientific Research | And 8 more authors.
Geophysical Journal International | Year: 2013

We analyse P-wave receiver functions across the western Gulf of Aden and southern Red Sea continental margins inWestern Yemen to constrain crustal thickness, internal crustal structure and the bulk seismic velocity characteristics in order to address the role of magmatism, faulting and mechanical crustal thinning during continental breakup.We analyse teleseismic data from 21 stations forming the temporary Young ConjugateMargins Laboratory (YOCMAL) network together with GFZ and Yemeni permanent stations. Analysis of computed receiver functions shows that (1) the thickness of unextended crust on the Yemen plateau is ~35 km; (2) this thins to ~22 km in coastal areas and reaches less than 14 km on the Red Sea coast, where presence of a high-velocity lower crust is evident. The average Vp/Vs ratio for the western Yemen Plateau is 1.79, increasing to ~1.92 near the Red Sea coast and decreasing to 1.68 for those stations located on or near the granitic rocks. Thinning of the crust, and by inference extension, occurs over a ~130-km-wide transition zone from the Red Sea and Gulf of Aden coasts to the edges of the Yemen plateau. Thinning of continental crust is particularly localized in a < 30-km-wide zone near the coastline, spatially co-incident with addition of magmatic underplate to the lower crust, above which on the surface we observe the presence of seaward dipping reflectors (SDRs) and thickened Oligo- Miocene syn-rift basaltic flows. Our results strongly suggest the presence of high-velocity mafic intrusions in the lower crust, which are likely either synrift magmatic intrusion into continental lower crust or alternatively depleted upper mantle underplated to the base of the crust during the eruption of the SDRs. Our results also point towards a regional breakup history in which the onset of rifting was synchronous along the western Gulf of Aden and southern Red Sea volcanic margins followed by a second phase of extension along the Red Sea margin. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society.


Korostelev F.,CNRS Paris Institute of Earth Sciences | Korostelev F.,CNRS Institute of Earth Sciences | Weemstra C.,Technical University of Delft | Leroy S.,CNRS Paris Institute of Earth Sciences | And 19 more authors.
Geophysical Research Letters | Year: 2015

During the breakup of continents in magmatic settings, the extension of the rift valley is commonly assumed to initially occur by border faulting and progressively migrate in space and time toward the spreading axis. Magmatic processes near the rift flanks are commonly ignored. We present phase velocity maps of the crust and uppermost mantle of the conjugate margins of the southern Red Sea (Afar and Yemen) using ambient noise tomography to constrain crustal modification during breakup. Our images show that the low seismic velocities characterize not only the upper crust beneath the axial volcanic systems but also both upper and lower crust beneath the rift flanks where ongoing volcanism and hydrothermal activity occur at the surface. Magmatic modification of the crust beneath rift flanks likely occurs for a protracted period of time during the breakup process and may persist through to early seafloor spreading. Key Points Crustal and upper mantle structures are characterized in Red Sea and Afar Magmatic processes are currently modifying the crust of the Red Sea flanks Rift-flank magmatism can persist after breakup ©2015. The Authors.


Ahmed A.,CNRS Institute of Earth Sciences | Ahmed A.,Seismological and Volcanological Observatory Center | Leroy S.,CNRS Institute of Earth Sciences | Keir D.,UK National Oceanography Center | And 5 more authors.
Tectonophysics | Year: 2014

Breakup of continents in magma-poor setting occurs primarily by faulting and plate thinning. Spatial and temporal variations in these processes can be influenced by the pre-rift basement structure as well as by early syn-rift segmentation of the rift. In order to better understand crustal deformation and influence of pre-rift architecture on breakup we use receiver functions from teleseismic recordings from Socotra which is part of the subaerial Oligo-Miocene age southern margin of the Gulf of Aden. We determine variations in crustal thickness and elastic properties, from which we interpret the degree of extension related thinning and crustal composition. Our computed receiver functions show an average crustal thickness of ~28km for central Socotra, which decreases westward along the margin to an average of ~21km. In addition, the crust thins with proximity to the continent-ocean transition to ~16km in the northwest. Assuming an initial pre-rift crustal thickness of 35km (undeformed Arabian plate), we estimate a stretching factor in the range of ~2.1-2.4 beneath Socotra. Our results show considerable differences between the crustal structure of Socotra's eastern and western sides on either side of the Hadibo transfer zone; the east displays a clear intracrustal conversion phase and thick crust when compared with the western part. The majority of measurements across Socotra show Vp/Vs ratios of between 1.70 and 1.77 and are broadly consistent with the Vp/Vs values expected from the granitic and carbonate rock type exposed at the surface. Our results strongly suggest that intrusion of mafic rock is absent or minimal, providing evidence that mechanical thinning accommodated the majority of crustal extension. From our observations we interpret that the western part of Socotra corresponds to the necking zone of a classic magma-poor continental margin, while the eastern part corresponds to the proximal domain. © 2014 Elsevier B.V.


Korostelev F.,CNRS Paris Institute of Earth Sciences | Leroy S.,CNRS Paris Institute of Earth Sciences | Keir D.,UK National Oceanography Center | Ahmed A.,CNRS Paris Institute of Earth Sciences | And 7 more authors.
Geosphere | Year: 2015

We image the lithospheric and upper asthenospheric structure beneath the central and eastern parts of the northern Gulf of Aden rifted passive continental margin with 59 broadband stations to evaluate the role of transform fault zones on the evolution of magma-poor continental margins. We used teleseismic tomography to compute a relative P wave velocity model in eastern Yemen and southern Oman down to 400 km depth. Our model shows low-velocity anomalies located in the vicinities of five major fracture zones and regions of recent volcanism. These low-velocity anomalies are likely caused by localized asthenospheric upwelling and partial melting, caused by small-scale convection promoted by gradients in the lithosphere-asthenosphere boundary topography near the fracture zones. In addition, low velocities underlie regions of elevated topography between major sedimentary basins. We suggest that locally buoyant mantle creates uplift and dynamic topography on the rift margin that affects the course of seasonal rivers and the sedimentation at the mouth of those rivers. Our new P wave velocity model suggests that the dynamic topography and recent volcanism in the central and eastern Gulf of Aden could be due to small-scale convection at the edge of the Arabian plate and/or in the vicinity of fracture zones. © 2015 Geological Society of America.

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