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

• Main Author: Franz, Gesa
• Format: Thesis
• Language: English
• Published: 2017
• Subjects: Course of study: MSc Geophyscis; Three Arms; South Atlantic Ocean
• Online Access: https://oceanrep.geomar.de/id/eprint/41180/; https://oceanrep.geomar.de/id/eprint/41180/1/Franz,Gesa_Masterthesis_2017_FINAL.pdf

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, ...