Post-breakup evolution and palaeotopography of the North Namibian Margin during the Meso-Cenozoic
Résumé
Passive margins display various structural and morphological patterns resulting in a complex coupling between local effects.We performed an integrated analysis from the ocean to the continent at the crustal scale of the post-breakup evolution of the northern Namibian margin in terms of vertical displacements and palaeotopography. A main topographic scarp that disappears where the Palaeo-Proterozoic Damara belt crosses the coast separates the Namibian margin and the South African plateau.We analysed this transition using a synthetic crustal cross-section based on a compilation of published data as well as new field data. The thinned margin displays two domains separated by a deep sedimentary trough. This trough separates a proximal part with landward tilted blocks from the distal part with seaward tilted blocks. This structural pattern reveals a complex flowing pattern of the ductile crust controlled by magmatism. Onshore, the basalts and Cenozoic conglomerates recorded a progressive seaward tilting of the coastal zone. Post-breakup vertical motions were estimated with subsidence curves in the offshore domain and from isostatic adjustments induced by the variations of cover thickness (depth of magmatic intrusions and Apatite Fission Tracks) in onshore domain. Three domains are recognised: a subsiding offshore domain, a uplifting continental domain and a transitional domain, located on both sides of the current shoreline with successive phases of uplift and subsidence. The offshore subsidence displays variations along the margin controlled by the thinning crust generated by the continental breakup. The onshore palaeotopogaphy is approximately assessed since the end of rifting by integrating the isostatic adjustment induced by denudation: the mean elevation of the plateau has remained above 1000 m since continental breakup. No significant additional uplift was pointed out during the late Cretaceous or late Neogene. Therefore, we conclude that the South African plateau has been present in northern Namibia since 130 Ma, at least