The Namibian passive volcanic margin - Investigations of the South Atlantic Opening with Magnetotelluric and Gravity data.

Franz, Gesa (2017) The Namibian passive volcanic margin - Investigations of the South Atlantic Opening with Magnetotelluric and Gravity data. (Master thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 87 pp.

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Abstract

The source of magmatic features along the Namibian continental margin, and therefore the processes which lead to the opening of the South Atlantic ocean, are still debated controversially. One big question is weather hotspot volcanism was fed by a deep reaching plume or by heterogeneities of the middle and upper mantle. In an attempt to gain a better understanding of the involved magmatic processes, a 3D inversion of magnetotelluric data with an integrated seismically constrained density model was conducted. Integration was accomplished by adding a cross-gradient constraint of the density model to the inversion, which enforces model resemblance at structural boundaries. The impact of this cross-gradient constraint with a preexisting density model is limited to this model's resolution, because the cross-gradient only works at structural boundaries within the constraint model. Its benefits include enhancing of resistivity structures in the inversion model. Additionally, the density constraint does not overprint resistivity structures which are not imaged by gravimetric methods (i.e. resistivity variations due to mineral composition). An observed high resistivity anomaly below the continental margin and Walvis Ridge coincides well with seismically observed high velocity underplating. This feature is interpreted to mark magmatic intrusions from a plume source, initiating continental breakup. The eastern termination of the high resistivity structure correlates with the onset of seaward dipping reflectors in seismic data. Therefore, it marks the transition from continental to oceanic regime (continent-ocean boundary). The theory of a plume source of the magmatic features is supported by the local planar extent of roughly estimated 70 000km² and the deep reaching form of the underplating. This form with three arms at a 120° spread is suggestive for the rift arms of a hot spot impingement into the crust. However, since the aforementioned amount of intrusive material is rather small for the impact of a road plume head, the preferred model includes a plume that stopped ascending in the mid-mantel. The underplated magmatic features would then be fed by smaller plumes or hotspot-like dikes rising from this deep mantle plume. A continental breakup solely driven by plate-forces is unlikely for the South Atlantic, as lithospheric thinning and subsequent magmatism would have resulted in a larger volcanic area due to mantle heterogeneities, than the observed local magmatic underplating.

Document Type: Thesis (Master thesis)
Thesis Advisor: Grevemeyer, Ingo and Jegen, Marion
Keywords: Passive margins, Marine magnetotellurics, Magmatic intrusions, Hotspot, Walvis Ridge, Geophysical Inversion
Subjects: Course of study: MSc Geophyscis
Research affiliation: OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-JRG-B3 Seabed Resources
OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-GDY Marine Geodynamics
Date Deposited: 04 Jan 2018 11:46
Last Modified: 25 Oct 2024 13:26
URI: https://oceanrep.geomar.de/id/eprint/41180

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