Abstract

A great escarpment at a rift margin can correspond directly to a major water divide but at many margins, including in Madagascar, the escarpment often appears as a steep knickzone on rivers that have their main water divides in the interior of the high plateau. We hypothesize that this variability in morphology is a reflection of the frequency and size of drainage area captured from the high plateau over the escarpment. To test this hypothesis, we document morphological features and weathering conditions from river sediment of Madagascar. We propose that the existence of a weathered weak surface layer of crystalline bedrock encourages large river captures from the upper plateau, leading to a dominance of knickpoint type rivers. We demonstrate that this is feasible, using 2-D landscape evolution models and show that an easily-eroded surface layer is prone to fast divide migration through frequent river capture and reversal. A positive scaling relationship between captured area and escarpment retreat rate is found from models. We demonstrate that this scaling is also observed in the great escarpments of Madagascar and India.