Post-breakup evolution of the Namibian margin: Constraints from numerical modeling
Résumé
The Namibian margin evolution started 130 Myr ago after the breakup between South America and southern Africa. Today, it shows specific features: (i) an elevation and a bathymetry about 1000 m higher than the surrounding areas, (ii) a significant sediment supply during the Late Cretaceous following the first sedimentary peak that occurred during rifting and just afterwards, and (iii) a long-term progradation since the Mid- Cretaceous. This present configuration results in couplings between several processes and the inherited structure. We constrained this evolution using a numerical simulation applied both onshore and offshore integrating different couplings: i) a thermal adjustment of a lithosphere driven by conductivity, ii) a flexural isostasy, and iii) the loading/unloading effects of surface matter transfer. Three processeswere investigated: rock-uplift, a change in the erosion efficiency and a sea-level fall. The resultswere compared to some pertinent observationsmade on the Namibian margin such as the vertical displacements, the sediment transfers, the denudation and the stratigraphic architecture in the marginal basin. In each simulation, rift-related reliefs were rapidly relaxed resulting in an exponentially decreasing denudation and sediment accumulation. Sixty Myr after the breakup,we simulated an additional uplift, an increase in the transfer efficiency of the sediment and a sea-level fall. The subsidence increased by loading, inducing a new progradational trend. Nonetheless, the three types of events differed in the duration of this progradation trend. The larger the additional uplift, the longer the progradation. The transfer efficiency increase and sea-level fall only resulted in a transient progradation trend. From these results,we suggest that the post-rift evolution of the Namibian margin was associated, during the Upper Cretaceous, to the rejuvenation of its continental relief triggered by a climate change during the Turonian and by a slow long-term rock-uplift during the Cenozoic