117 research outputs found

    A geological and geophysical study of the Tendaho Graben in the Afar Depression, Ethiopia: insights into transitional continental rifting

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    A detailed magnetic and gravity study across the Tendaho Graben (the Red Sea propagator within the Afar Depression, Ethiopia) revealed features that can best be interpreted as a continental rift undergoing oceanization. This NW-trending extensional structure is ~50 km wide and it is confined within well-developed NW-trending boarder faults that deform the 2 km thick and ~ 2 Ma basaltic flows of the Afar Stratoids. The age of the basaltic flows becomes progressively younger inward from the boarder faults until it reaches ~30,000 years close to the rift axis. The central part of the Tendaho Graben is characterized by a 10 km wide magnetic trough, the central part of which is dominated by a narrow zone (~3 km) of a relatively elevated magnetic anomaly that coincides with a linear region of hydrothermal activity. This magnetic geometry is similar in dimension and magnitude to that observed from magnetic stripes of typical mid-ocean ridges. Forward modeling of the magnetic data (combined with geochronological data) shows that the basaltic rocks within the magnetic trough were crystallized after 0.78 Ma under normal magnetic polarity. The width of the magnetic trough (10 km) and the age of basaltic rocks (\u3c0.78 Ma) indicate a spreading rate of ~ 0.64 cm/year. However, to achieve the ~50 km width of the Tendaho Graben which started opening ~2.0 Ma, a 1.64 cm/year spreading rate is needed between 1.6 and 0.78 Ma. This suggests that the spreading rate with Tendaho Graben is slowing down and extension within Afar is accommodated somewhere else. A new model is proposed for the evolution of the Tendaho Graben based on fieldwork, the newly acquired magnetic data and geochronology --Abstract, page iii

    Crustal structure of active deformation zones in Africa: Implications for global crustal processes

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    The Cenozoic East African rift (EAR), Cameroon Volcanic Line (CVL), and Atlas Mountains formed on the slow-moving African continent, which last experienced orogeny during the Pan-African. We synthesize primarily geophysical data to evaluate the role of magmatism in shaping Africa's crust. In young magmatic rift zones, melt and volatiles migrate from the asthenosphere to gas-rich magma reservoirs at the Moho, altering crustal composition and reducing strength. Within the southernmost Eastern rift, the crust comprises ~20% new magmatic material ponded in the lower crust sills, and intruded as sills and dikes at shallower depths. In the Main Ethiopian rift, intrusions comprise 30% of the crust below axial zones of dike-dominated extension. In the incipient rupture zones of the Afar rift, magma intrusions fed from crustal magma chambers beneath segment centers create new columns of mafic crust, as along slow-spreading ridges. Our comparisons suggest that transitional crust, including seaward-dipping sequences, is created as progressively smaller screens of continental crust are heated and weakened by magma intrusion into 15-20 km-thick crust. In the 30Ma-Recent CVL, which lacks a hotspot age-progression, extensional forces are small, inhibiting the creation and rise of magma into the crust. In the Atlas orogen, localized magmatism follows the strike of the Atlas Mountains from the Canary Islands hotspot towards the Alboran Sea. CVL and Atlas magmatism has had minimal impact on crustal structure. Our syntheses show that magma and volatiles are migrating from the asthenosphere through the plates, modifying rheology and contributing significantly to global carbon and water fluxes

    The extent of continental material in oceans: C-Blocks and the Laxmi Basin example

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    We propose a tectonic interpretation for the outer-SDRs (SDRs: Seaward-Dipping Reflectors) and Pannikar central ridge in the aborted Laxmi Basin west of India from wide-angle seismic reflection data. The outer-SDRs comprise syn-tectonic extrusives (lavas and/or volcaniclastics) emplaced above passively exhumed mid-to-lower mafic crust of continental origin. They erupted following sudden lithosphere weakening associated with isolation of a continental block (a ‘C-Block’). Continuous magmatic addition during crustal extension allowed stretching of the lower crust whilst maintaining constant or even increasing thickness. A similar process occurred at both conjugate margins allowing bulk, pure-shear plate separation and formation of linear magnetic anomalies. The Laxmi example can explain enigmatic features observed in mature oceans such as presence of distal buoyant plateaus of thick continental crust away from the margins

    Receiver Function Constraints on Crustal Seismic Velocities and Partial Melting beneath the Red Sea Rift and Adjacent Regions, Afar Depression

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    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

    Plate kinematics of the Afro-Arabian Rift System with an emphasis on the Afar Depression

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    This work utilizes the Four-Dimensional Plates (4DPlates) software, and Differential Interferometric Synthetic Aperture Radar (DInSAR) to examine plate-scale, regional-scale and local-scale kinematics of the Afro-Arabian Rift System with emphasis on the Afar Depression in Ethiopia. First, the 4DPlates is used to restore the Red Sea, the Gulf of Aden, the Afar Depression and the Main Ethiopian Rift to development of a new model that adopts two poles of rotation for Arabia. Second, the 4DPlates is used to model regional-scale and local-scale kinematics within the Afar Depression. Most plate reconstruction models of the Afro-Arabian Rift System relies on considering the Afar Depression as a typical rift-rift-rift triple junction where the Arabian, Somali and Nubian (African) plates are separating by the Red Sea, the Gulf of Aden and the Main Ethiopian Rift suggesting the presence of sharp and rigid plate boundaries. However, at the regional-scale the Afar kinematics are more complex due to stepping of the Red Sea propagator and the Gulf of Aden propagator onto Afar as well as the presence of the Danakil, Ali Sabieh and East Central Block micro-plates . This study incorporates the motion of these micro-plates into the regional-scale model and defined the plate boundary between the Arabian and the African plates within Afar as likely a diffused zone of extensional strain within the East Central Block. Third, DInSAR technology is used to create ascending and descending differential interferograms from the Envisat Advanced Synthetic Aperture Radar (ASAR) C-Band data for the East Central Block to image active crustal deformation related to extensional tectonics and volcanism. Results of the DInSAR study indicate no strong strain localization but rather a diffused pattern of deformation across the entire East Central Block

    Birth of an ocean in the Red Sea: Initial pangs

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    We obtained areal variations of crustal thickness, magnetic intensity, and degree of melting of the sub- axial upwelling mantle at Thetis and Nereus Deeps, the two northernmost axial segments of initial oceanic crustal accretion in the Red Sea, where Arabia is separating from Africa. The initial emplacement of oceanic crust occurred at South Thetis and Central Nereus roughly 2.2and2.2 and 2 Ma, respectively, and is taking place today in the northern Thetis and southern Nereus tips. Basaltic glasses major and trace element com- position suggests a rift-to-drift transition marked by magmatic activity with typical MORB signature, with no contamination by continental lithosphere, but with slight differences in mantle source composition and/or potential temperature between Thetis and Nereus. Eruption rate, spreading rate, magnetic intensity, crustal thickness and degree of mantle melting were highest at both Thetis and Nereus in the very initial phases of oceanic crust accretion, immediately after continental breakup, probably due to fast mantle upwelling enhanced by an initially strong horizontal thermal gradient. This is consistent with a rift model where the lower continental lithosphere has been replaced by upwelling asthenosphere before continental rupturing, implying depth-dependent extension due to decoupling between the upper and lower lithosphere with man- tle-lithosphere-necking breakup before crustal-necking breakup. Independent along-axis centers of upwell- ing form at the rifting stage just before oceanic crust accretion, with buoyancy-driven convection within a hot, low viscosity asthenosphere. Each initial axial cell taps a different asthenospheric source and serves as nucleus for axial propagation of oceanic accretion, resulting in linear segments of spreading

    Surface and crustal deformation mechanism of the Dobi graben and surrounding area in the Afar Depression, Ethiopia

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    Our Study used the Advanced Synthetic Aperture Radar (ASAR), C- Band (h = 5.6 cm) of the ENVISAT satellite ASAR data, Landsat Operational Land Imager (OLI), and Shuttle Radar Topography Mission (SRTM) digital elevation models, as well as the integration of geophysical ground base magnetic survey data, aeromagnetic, and Satellite gravity data, to investigate the time series surface deformation and crustal structure of the Dobi graben and surrounding area. Results from our fault population analysis using SRTM DEM aided by satellite imageries suggest that the direction of the faults' lateral propagations are highly influenced by the two regional volcanic rifts, the Red Sea Rift (RSR) and Gulf of Aden Rift (GAR). Additionally, the Dmax/Lmax ratio of the faults is calculated as ~0.03, which indicates the normal faults in the region can be characterized by the constant displacement fault growth model. Our fault population analysis also indicates that the possible presence of melt material in the lower crust likely acts as a barrier for lateral propagations of the faults. Results from our InSAR analysis suggest that an extension process with a creeping mechanism associated mainly with normal faulting presumably causes subsidence within the graben and uplifting in the rift shoulder. The abnormal, continuous uplifting in the horst area might be associated with the temporary reactivation of normal faulting in the region. Finally, our 2D magnetic and gravity forward modeling revealed 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 crusts to be at depth of between 10 and 12 km. Additionally, we found two ~5 km wide zones where melt and mafic dike intrusions 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. We propose that, while the upper crust beneath the Dobi graben is stretching mechanically, the lower crust is stretching ductily, aided by the presence of melt and the intrusion of mafic dikes

    The role of pre-existing Precambrian structures in the development of Rukwa Rift Basin, southwest Tanzania

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    This study has been supported by BG Group Tanzania (now Shell) under the initiative of the University of Dar es Salaam (Tanzania) - University of Aberdeen (United Kingdom) - BG Group Tanzania (now Shell). Tanzania Petroleum Development Corporation (TPDC) provided aeromagnetic data used in this study at no cost. We are grateful to the editors and anonymous reviewers for detailed and constructive reviews that improved the manuscript.Peer reviewedPostprin

    Role of subaerial volcanic rocks and mantle plumes in creation of South Atlantic margins: implications for salt tectonics and source rocks

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    Seaward-dipping re¯ectors (SDRs) represent ¯ood basalts rapidly extruded during either rifting or initially subaerial sea-¯oor spreading. Evaporites can form on this basaltic proto-oceanic crust, as in the Afar Triangle today. Evidence for SDRs in South Atlantic deep-water regions comes from proximity to the uniquely large Paranaà ±Etendeka volcanic province onshore, the Tristan and Gough hot spots, drilled volcanic rocks, and seismic pro®les showing SDR provinces more than 100 km wide, as much as 7 km thick, and thousands of kilometers long. SDRs are clearest adjoining the Aptian salt basins. However, we speculate that SDRs are also present but seismically obscured below the salt basins. We argue that the conjugate Aptian salt basins are post-breakup, not pre-breakup; they were separated from the start by a mid-oceanic ridge; distal salt accumulated on proto-oceanic crust, not rift basins. This hypothesis is supported by: seismic stratigraphy and structure; magnetic anomalies; plate reconstructions; and hydrothermal potash evaporites. An important implication for exploration is that thick basalts, rather than rift-age source rocks, may underlie distal parts of the salt basins

    Layered crustal and mantle structure and anisotropy beneath the Afar Depression and Malawi Rift Zone

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    Although a wealth of geophysical data sets have been acquired within the vicinity of continental rift zones, the mechanisms responsible for the breakup of stable continental lithosphere are ambiguous. Eastern Africa is host to the largest contemporary rift zone on Earth, and is thus the most prominent site with which to investigate the processes which govern the rupture of continental lithosphere. The studies herein represent teleseismic analyses of the velocity and thermomechanical structure of the crust and mantle beneath the Afar Depression and Malawi Rift Zone (MRZ) of the East African Rift System. Within the Afar Depression, the first densely-spaced receiver function investigation of crustal thickness and inferred velocity attenuation across the Tendaho Graben is conducted, and the largest to-date study of the topography of the mantle transition zone (MTZ) beneath NE Africa is provided, which reveals low upper-mantle velocities beneath the Afar concordant with a probable mantle plume traversing the MTZ beneath the western Ethiopian Plateau. In the vicinity of the MRZ, a data set comprised of 35 seismic stations is employed that was deployed over a two year period from mid-2012 to mid-2014, belonging to the SAFARI (Seismic Arrays For African Rift Initiation) experiment. Accordingly, the first MTZ topography and shear wave splitting analyses were conducted in the region. The latter reveals largely plate motion-parallel anisotropy that is locally modulated by lithospheric thickness abnormalities adjacent to the MRZ, while the former reveals normal MTZ thicknesses and shallow discontinuities that support the presence of a thick lithospheric keel within the MRZ region. These evidences strongly argue for the evolution of the MRZ via passive rifting mechanisms absent lower-mantle influences --Abstract, page iv
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