Cenozoic deformation of the South African plateau, Namibia: Insights from planation surfaces

Abstract Passive margins and associated inland areas display complex vertical displacements that have been abundantly described offshore but much less so onshore due to erosion. Planation surfaces are ubiquitous markers inland that can be used as a proxy both climatic conditions and changes in vertical displacements induced by both short (faults) and long (tilting, flexure) wavelength deformations. We propose 1) a synthetic typology of the planation surfaces driven by a genetic process (weathering versus mechanical erosion), and 2) a method to use these surfaces to map short and long wavelength deformations. This methodology was applied to the Namibian margin and inland plateau to quantify the Cenozoic deformation. The results show that the Namibian margin was affected by bulging during the Oligocene and by an E-W to NW-SE extension during the Late Miocene and Pliocene. The bulge is parallel to the shoreline with a wavelength of 300 km and an amplitude up to 500 m. After investigating the available deformation processes, we propose that an increase in the spreading rate along the mid-oceanic ridge during the Oligocene generated this bulging. The vertical displacement is partially maintained afterwards via isostasy because of mass loss generated by scarp retreat. A minimum average rate of scarp retreat of 5 km/Myr was calculated, which is high compared to the rates estimated in other places in the world. We ascribe this high value to the prior intense weathering period. Indeed, the alteration largely degraded the bedrock and facilitated the formation of the subsequent scarp. This study also reveals that the high reliefs of the Damara domain existed before the Cenozoic, induced by a reactivation of the Damara structures.