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  • 1
    Publication Date: 1999
    Keywords: reflection seismics, refraction seismics, tectonics/crustal structure
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  • 2
    Publication Date: 2019-07-16
    Description: Petroleum systems located at passive continental margins received increasing attention in the last decade mainly because of deep- and ultra‐deep-water hydrocarbon exploration and production. The high risks associated with these settings originate mainly from the poor understanding of inherent geodynamic processes. The new priority program SAMPLE (South Atlantic Margin Processes and Links with onshore Evolution), established by the German Science Foundation in 2009 for a total duration of 6 years, addresses a number of open questions related to continental breakup and post‐breakup evolution of passive continental margins. 27 sub‐projects take advantage of the exceptional conditions of the South Atlantic as a prime “Geo‐archive.” The regional focus is set on the conjugate margins located east of Brazil and Argentina on one side and west of Angola, Namibia and South Africa on the other (Figure 1) as well as on the Walvis Ridge and the present‐day hotspot of Tristan da Cunha. The economic relevance of the program is demonstrated by support from several petroleum companies, but the main goal is research on fundamental processes behind the evolution of passive continental margins.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Book , NonPeerReviewed
    Format: application/pdf
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  • 3
    Publication Date: 2019-07-16
    Description: Southern Africa and its southern continental margin offer an unrivalled region, where continental accretion, continental breakup and magmatic processes over a period of more than 3.5 billion years can be studied. The Agulhas-Karoo Geoscience Transect is part of the South African - German cooperative research project Inkaba yeAfrica, which aims to investigate this part of the continent and ocean in a cone-shaped sector from core to space. Geophysical and geological data and samples were collected along this transect which spans from the Agulhas Plateau across the Agulhas-Falkland fracture zone, the Outeniqua Basin, the Cape Fold Belt, the Namaqua-Natal Belt onto the Karoo Craton. A combined onshore-offshore deep crustal seismic reflection and refraction survey as well as several magnetotelluric surveys provide information about tectonic and magmatic structures and constraints for physical parameters from sedimentary sequences to the upper mantle. The main objectives include an understanding of the crustal nature of the Agulhas Plateau, the processes accompanying and succeeding the crustal shearing process along the Agulhas-Falkland Transform/Fracture Zone, the offshore basin formation in relation to breakup, the deep-seated tectonics of the Cape Fold Belt, and the geometries and sources of the Beattie Magnetic Anomaly and the Southern Cape Conductivity Belt. The geophysical data are integrated with geological, petrological and geochemical analysis on rock composition, age and alteration history to form an overarching geodynamic model of the evolution of this region and its sedimentary, tectonic and magmatic units.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 4
    Publication Date: 2019-07-16
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 5
    Publication Date: 2019-07-16
    Description: The opening of the South Atlantic ocean basin was accompanied by voluminous magmatism on the conjugate continental margins of Africa and South America, including formation of the Parana and Entendeka large igneous provinces (LIP), the build-up of up to 100 km wide volcanic wedges characterized by seaward dipping reflector sequences (SDR), as well as the formation of paired hotspot tracks on the rifted African and South American plates, the Walvis Ridge and the Rio Grande Rise. The area is considered as type example for hotspot or plume-related continental break-up. However, SDR, and LIP features on land are concentrated south of the hotspot tracks. The segmentation of the margins offers a prime opportunity to study the magmatic signal in space and time, and investigate the interrelation with rift-related deformation. A globally significant question we address here is whether magmatism is the drives continental break-up, or whether even rifting accompanied by abundant magmatism is in response to crustal and lithospheric stretching governed by large scale plate kinematics. In 2010/11, an amphibious set of wide-angle seismic data was acquired around the landfall of Walvis Ridge at the Namibian passive continental margin. The experiments were designed to provide crustal velocity information and to investigate the structure of the upper mantle. In particular, we aimed at identifying deep fault zones and variations in Moho depth, constrain the velocity signature of SDR sequences, as well as the extent of magmatic addition to the lower crust near the continent-ocean transition. Sediment cover down to the igneous basement was additionally constrained by reflection seismic data. Here, we present tomographic analysis of the seismic data of one long NNW oriented profile parallel to the continental margin across Walvis Ridge, and a second amphibious profile from the Angola Basin across Walvis Ridge and into the continental interior, crossing the area of the Etendeka Plateau basalts. The most striking feature is the sharp transition in crustal structure and thickness across the northern boundary of Walvis Ridge. Thin oceanic crust (5-7 km) of the Angola Basin lies next to the 35 km thick igneous crustal root founding the highest elevated northern portions of Walvis Ridge. Both structures are separated by a very large transform fault zone. The velocity structure of Walvis Ridge lower crust is indicative of gabbro, and, in the lowest parts, of cumulate sequences. On the southern side of Walvis Ridge there is a smooth gradation into the adjacent 25-30 km thick crust underlying the ocean-continent boundary, with a velocity structure resembling that of Walvis Ridge The second profile shows a sharp transition from oceanic to rifted continental crust. The transition zone may be underlain by hydrated uppermost mantle. Below the Etendaka Plateau, an extensive high-velocity body, likely representing gabbros and their cumulates at the base of the crust, indicates magmatic underplating. We summarize by stating that rift-related lithospheric stretching and associated transform faulting play an overriding role in locating magmatism, dividing the margin in a magmatic-dominated segment to the south, and an amagmatic segment north of Walvis Ridge.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 6
    Publication Date: 2019-07-16
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 7
    Publication Date: 2019-07-16
    Description: Passive continental margins offer the unique opportunity to study the processes involved in continental extension and break up and the role of hot-spot related magmatism. Several geophysical experiments (Seismics, Magnetotellurics and Seismology) were therefore carried out 2011 and 2012 in northern Namibia to image crust and upper mantle at the landfall of the Walvis Ridge. The aim of these studies is to shed light on the present-day structure of this area to understand the dynamics of the breakup of Pangaea and the processes involved. First results of the seismic part of the project show anomalous velocity structures and reflective properties in the mid- and lower crust. The lower crust (onshore) is characterized by an unusual high velocity body which might be associated with magmatic processes of the plume continent interaction. The distribution of the upper mantle wave propagation velocities shows a rather complex, very inhomogeneous pattern. Onshore magnetotelluric (MT) data were acquired at 167 sites in a ~140 km wide and ~260 km long corridor from the Atlantic Ocean through the Kaoko Mobile Belt onto the Congo Craton. The data are generally of excellent quality. A first inspection of magnetotelluric and vertical magnetic transfer functions indicates significant three-dimensional (3-D) structures in the crust and upper mantle, particularly in the Western Kaoko Zone in the vicinity of prominent shear zones. The seismological team operates a passive-source seismic experiment for two years onshore/offshore NW Namibia. The seismic network consists of 28 land-based and 12 ocean-bottom stations covering an area of 400km x 800km. Different seismic methods, such as body wave and surface wave tomography, receiver function, shear wave splitting, etc, will be used to image the seismic anomalies in the upper mantle and to map the thickness of the crust and mantle lithosphere in this ocean-continental transition area. The aim is to find the mantle deformation styles related to the plume-lithosphere interaction along the Walvis Ridge.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 8
    Publication Date: 2015-12-18
    Description: Upwelling hot mantle plumes are thought to disintegrate continental lithosphere and are considered to be drivers of active continental breakup. The formation of the Walvis Ridge during the opening of the South Atlantic is related to a putative plume-induced breakup. We investigated the crustal structure of the Walvis Ridge (southeast Atlantic Ocean) at its intersection with the continental margin and searched for anomalies related to the possible plume head. The overall structure we identify suggests that no broad plume head existed during opening of the South Atlantic and anomalous mantle melting occurred only locally. We therefore question the importance of a plume head as a driver of continental breakup and further speculate that the hotspot was present before the rifting, leaving a track of kimberlites in the African craton.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 9
    Publication Date: 2019-07-17
    Description: A number of geophysical on-shore and off-shore experiments were carried out in a profile across the southern margin of the African continent in the framework of the Inkaba yeAfrica project. Refraction seismic experiments have shown that the crustal thickness decreases rapidly from over 40 to around 30 km well inland of the present coast, before gently thinning out towards the Agulhas Falkland Fracture Zone, which marks the transition zone between continental and oceanic crust. This is consistent with a non-volcanic mode of breakup due to shear along the Agulhas-Flakland Transform Fault. In region of the abruptly decreasing Moho depth inland from the coast, lower crustal P-wave velocities up to 7.4 km/s are observed. We interpret these to represent metabasic lithologies of the Mesoproterozoic Namaqua-Natal Metamorphic Complex, or intrusions of gabbroic material added to the base of the crust by younger magmatism. This magmatism could be the result of the mid-Jurassic Karoo-Ferrar-Chon Aike event. The velocity model for the upper crust has excellent resolution, and is consistent with the known geological record. A comparison of the velocity model with electrical conductivity models shows that a zone of high seismic velocities north of the centre of the Beattie Magnetic Anomaly, one of the largest magnetic anomalies, correlates well with a resistive zone. Contrary to existing interpretation, the Beattie Magnetic Anomaly does not origin from an area which is electrically conductive but seems to correlate with a source region which is resistive and has high seismic velocities.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 10
    Publication Date: 2019-07-17
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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