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Crustal architecture and deep structure of the Namibian continental shelf and adjacent oceanic basins around the landfall of Walvis Ridge from wide-angle seismic and marine magnetotelluric data

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Planert, L. , Behrmann, J. , Jegen, M. , Heincke, B. , Jokat, W. and Bialas, J. (2012): Crustal architecture and deep structure of the Namibian continental shelf and adjacent oceanic basins around the landfall of Walvis Ridge from wide-angle seismic and marine magnetotelluric data , AGU Fall Meeting 2012, San Francisco, CA, USA, 3 December 2012 - 7 December 2012 .
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Abstract:

The opening of the South Atlantic ocean basin resulted in voluminous magmatism on the conjugate continental margins of Namibia and Brazil, including the formation of the Parana and Entendeka large igneous provinces (LIPs), the formation of up to 100 km wide volcanic wedges characterized by seaward dipping reflector sequences (SDRs) near the continent-ocean transition, 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. Hence, the passive margins bordering the South Atlantic are today considered as type examples for models involving hotspot related continental break-up. However,the presence of volcanic features (SDRs, LIPs) appears to be limited south of the hotspot trails. The resulting 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 to be adressed here is whether magmatism is the driving force for continental break-up, or whether even rifting with abundant hotspot related magmatism is in principle in response to crustal and lithospheric stretching. In 2010/11, a combination of on-/offshore wide-angle seismic, marine magnetotelluric and on-/offshore seismological data were acquired around the landfall of Walvis Ridge at the Namibian passive continental margin. The set of experiments was designed to provide crustal velocity and conductivity information and to investigate the structure of the upper mantle. In particular, we aimed at identifying deep fault zones and variations in Moho depth, the presence of interleaved sediment layers in SDR sequences as well as magmatic intrusions and underplated material near the continent-ocean transition. The sedimentary portions down to the igneous basement were additionally constrained by coincident single-channel reflection seismic data. Here, we present preliminary results for two wide-angle seismic transects and first results for a marine magnetotelluric profile. Tomographic analysis of the seismic data reveals the velocity structure of the crust down into the uppermost mantle. The probably most striking feature of our models is the sharp lateral transition in crustal structure and thickness associated with the northern boundary zone of Walvis Ridge towards the Angola Basin. Here, the rather thin oceanic crust in the basin lies opposite to the ~35 km thick igneous crustal root founding the highest elevated northern portions of Walvis Ridge. In contrast, the southern termination of Walvis Ridge and the corresponding transition towards the adjacent 25-30 km thick crustal portions further south is much more subdued. Due to the presence of a high-velocity (6.5-7.2 km/s) lower crust we argue that the Namibian shelf south of Walvis Ridge comprises a transitional igneous origin. We suggest that the northern boundary zone close to the landfall of Walvis Ridge represents an important transtensional tectonic feature which may have provoked the preferential extraction of melts into the footwall of this structure.

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