The Northern Namibian Margin: Crustal Structure and Post-Breakup Evolution

Olivier Dauteuil¹, Francois Deschamps¹, Olivier Bourgeois², Francois Guillocheau¹, Antoine Mocquet², and Delphine Rouby^1
1Université de Rennes 1 -CNRS/INSU, Geosciences Rennes, Rennes, France
²Universite de Nantes - CNRS/INSU, Planetologie et Geodynamique, Nantes, France

The passive margins display several patterns resulting from a complex coupling between local effects and global processes. Several studies were done on continental or on oceanic domains and at the surface. However, few of them integrate the two domains with a crustal and a mantle scale. We propose such an approach about the northern Namibian margin.

The Namibian margin belongs to the southern African margins that limit the high African plateau. Its eastern side displays a high topography with an average elevation around 1200 m. A coastal scarp that reaches locally 2200 m borders this high plateau from the coastal plain and offshore domain. This scarp is discontinuous and disappears when the Damara belt crosses the coast. Thus, the transition between the high plateau and the deep part of the margin displays various patterns poorly constrained. We analyse this transition through a crustal cross-section based on data synthesis and field trips.

The reconstruction of the onshore evolution indicates a denudation of 5 km, since 130 Ma including 3 km during the Cenozoic. It allows us to propose a landscape at the end of the breakup. The land erosion is compared to the sedimentary records: it displays a maximum of sediment transfert during the upper Creataceous with two main deposite centers: one is located on the upper part of the margin where it exists landward-tilted blocks and the second one above the transition between ocean and continent above seaward-tilted blocks. These two deposite centers accommodated differential subsidence between upper and lower domains of the margin that is controlled by deep structures of the margin. The correlation between geomorphic markers located onshore and offshore suggests that the margin underwent a large-scale flexure after the rifting. The origin of this flexure is debated, however it strongly contrained the sedimentary transport.

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