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  • 1
    ISSN: 0300-9629
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Biology , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 2014-07-22
    Description: Continental breakup is commonly preceded or accompanied by massive volcanism and deposition of flood basalts. The large volumes of magma are thought to originate from hot upwelling mantle plumes arriving at the lithosphere. The following plume conduit often leaves a trail in form of volcanic islands or aseismic ridges on the newly created oceanic crust. Due to this correlation in space and time between plume-derived structures and continental breakup, plumes are considered to have a triggering effect or even cause continental breakups. The South Atlantic is a classical example for this model including the Parana (South America) and Etendeka (Africa) flood basalts as well as the aseismic ridges Rio Grande Rise and Walvis Ridge on both conjugate margins. The Walvis Ridge connects the Etendeka flood basalts with the active volcanic islands of Tristan da Cunha, the current hotspot position. To investigate the modification of the continent ocean transition (COT) by the arriving plume head, a large geophysical on- and offshore experiment was conducted in 2011 at the intersection of Walvis Ridge with the African continent. We present two P-wave velocity models of the deep crustal structure derived from seismic refraction data. One profile crosses the ridge ~500km away from the coastline, while the other one extends along the ridge and continues onshore. 27 ocean bottom stations (OBS, spacing 13 km) were deployed for the perpendicular profile, 28 OBS, 50 land stations and 8 dynamite shots were used for the longitudinal profile. Crustal velocities beneath Walvis Ridge range between 5.5 km/s and 7.0 km/s, which are typical velocities for oceanic crust. The thickness, however, is approximately three times than normal, 17 km in the western part and increasing to 22 km towards the continent. The COT is characterized by 30 km thick crust with a high velocity lower crustal body (HVLCB) with seismic velocities up to 7.5 km/s. The western boundary of the HVLCB is at a similar longitude as similar lower crustal bodies found more south. Towards the east the HVLCB terminates against the ~40 km thick crust of the Kaoko fold belt. Here, the variation of seismic velocities indicate that hot material intruded the continental crust during the initial rifting stage. However, beyond this relatively sharp boundary (40 km wide), the remaining continental crust seems not be affected by the hot material. The second line some 500 km west of the coast indicates that the Walvis Ridge might be broader than its topographic expression. The seismic velocities are similar to those closer to the coast, but the HVLCB is thinner.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 3
    Publication Date: 2017-11-09
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
    Format: application/pdf
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  • 4
    Publication Date: 2020-02-06
    Description: Highlights • Crustal structure of Walvis Ridge reveals high seismic velocities in the lower crust intruding the African continent. • This modified crust is localized to approx. 100 × 100 km within the continent. • No indication for a large plume head observed The opening of the South Atlantic is a classical example for a plume related continental breakup. Flood basalts are present on both conjugate margins as well as aseismic ridges connecting them with the current plume location at Tristan da Cunha. To determine the effect of the proposed plume head on the continental crust, we acquired wide-angle seismic data at the junction of the Walvis Ridge with the African continent and modelled the P-wave velocity structure in a forward approach. The profile extends 430. km along the ridge and continues onshore to a length of 720. km. Crustal velocities beneath the Walvis Ridge vary between 5.5. km/s and 7.0. km/s, a typical range for oceanic crust. The crustal thickness of 22. km, however, is approximately three times larger than of normal oceanic crust. The continent-ocean transition is characterized by 30. km thick crust with strong lateral velocity variations in the upper crust and a high-velocity lower crust (HVLC), where velocities reach up to 7.5. km/s. The HVLC is 100 to 130. km wider at the Walvis Ridge than it is farther south, and impinges onto the continental crust of the Kaoko fold belt. Such high seismic velocities indicate Mg-rich igneous material intruded into the continental crust during the initial rifting stage. However, the remaining continental crust seems unaffected by intrusions and the root of the 40. km-thick crust of the Kaoko belt is not thermally abraded. We conclude that the plume head did not modify the continental crust on a large scale, but caused rather local effects. Thus, it seems unlikely that a plume drove or initiated the breakup process. We further propose that the plume already existed underneath the continent prior to the breakup, and ponded melt erupted at emerging rift structures providing the magma for continental flood basalts.
    Type: Article , PeerReviewed
    Format: text
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  • 5
    Publication Date: 2016-01-01
    Print ISSN: 0926-9851
    Electronic ISSN: 1879-1859
    Topics: Geosciences , Physics
    Published by Elsevier
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  • 6
    Publication Date: 2013-02-07
    Description: Author(s): T. Maassen, J. J. van den Berg, E. H. Huisman, H. Dijkstra, F. Fromm, T. Seyller, and B. J. van Wees We developed a spin transport model for a diffusive channel with coupled localized states that result in an effective increase of spin precession frequencies and a reduction of spin relaxation times in the system. We apply this model to Hanle spin precession measurements obtained on monolayer epitax... [Phys. Rev. Lett. 110, 067209] Published Wed Feb 06, 2013
    Keywords: Condensed Matter: Electronic Properties, etc.
    Print ISSN: 0031-9007
    Electronic ISSN: 1079-7114
    Topics: Physics
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  • 7
    Publication Date: 2015-09-15
    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.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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