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 Advisors: 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: 04 Jan 2018 11:46
URI: http://eprints.uni-kiel.de/id/eprint/41180

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