Morphology and long-term landscape evolution of the South African plateau in South Namibia
Résumé
The South Namibian Plateau, which belongs to the South African Plateau, is situated between 25°‐29°S and 15°‐20°E. This study examines the relationships between this portion of the plateau and the development of planation surfaces and scarps bounding the plateau. Geomorphological analysis through morphometric indices extracted from a DEM (90 m) and its derived maps as well as field observations supported by the literature data permit the demonstration that tectonics, climate and base level are the main factors that control the formation and evolution of the planation surfaces and scarps that separate them. From west to east, the South Namibian Plateau consists of four main morphological provinces: the coastal province and the intermediate, upper and Kalahari provinces in the interior plateau. These provinces include five Cenozoic planation surfaces that are differentiated by their elevations, regional slopes, relative reliefs and erosion processes. These surfaces have been formed by etchplanation and pediplanation. The drainage pattern and river length profiles show that the coastal province evolved differently than the interior plateau. The development of the coastal province is controlled by eustasy and tectonics, whereas climate-related processes and tectonics control that of the interior plateau. The presence of several scarps separating these surfaces, the most degraded and older of which are inside the plateau, reflects a polyphase evolution. This evolution can be highlighted by two models: (1) an eastward migration mantle dynamics causing a slight uplift and rejuvenation of the system by destabilizing the coupled weathering/erosion equilibrium and (2) an internal propagation of isostatic rebound induced by the retreat of the western scarp leading to a significant flattening in the central part of the plateau.