Passive Margin Tectonics and Landscape Development: the Drakensberg Escarpment, Southern Africa

The evolution of passive margin landscapes is a controversial topic that depends on the relation between surface and deep Earth processes. These processes are linked at passive margins by erosion and isostatic uplift, the flexural response of the lithosphere, and the effects of doming over mantle upwelling. The break-up topography and the flexural rigidity of the margin are important parameters, but they are commonly poorly constrained. Escarpments are a characteristic landform of passive margins, but two quite different alternatives have been proposed for escarpment evolution. The Drakensberg escarpment of southern Africa exemplifies this debate. Over 50 years ago, King suggested that escarpment retreat followed a threshold of isostatic unloading; more recently, apatite fission track thermochronology, cosmogenic isotope data, and numeric surface process models have been used to argue that the topography of the southern African margin evolved by plateau degradation, followed by escarpment formation at an inland drainage divide, with a plateau extending to the continental margin at break-up. The Drakensberg-Maluti mountainlands are bound on three sides by marked escarpments, and not simply their seaward margin, where the Drakensberg escarpment occurs. Lithology (the escarpments are armoured by massive Karoo basalts) appears to be critical in establishing and maintaining these and other escarpments. Constraints on the pre-break up geometry of the mountainlands are provided by the identification of a dense network of dolerite dykes that supplied the Karoo basalts in the foothills of the mountainlands to a distance of at least 60 km, and by the results of drainage network analysis, which are consistent with the effects of doming. Major climatic and vegetation changes that have occurred since break-up could have played a subsidiary role in the evolution of the escarpments. The geomorphic evolution of the escarpments may have occurred by retreat of scarps on three sides from the edges of an elevated area of basalts, which may have been partly structurally controlled, over a lithosphere with moderate values of flexural rigidity. The Drakensberg example suggests that scarp retreat is still a valid mechanism of passive margin landscape evolution, and that lithology and pre-break up configuration can have dominant effects. Factors that are not directly related to break-up, including structural controls, vegetation and climate changes, need to be considered in understanding passive margin geomorphology.