The Role of Weathering on Morphology and Rates of Escarpment Retreat of the Rift Margin of Madagascar

Abstract

AbstractA 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 into the interior of the high plateau. We hypothesize that this variability in morphology is a reflection of the frequency and size of the drainage area captured from the high plateau over the escarpment. To test this hypothesis, we document morphological features and weathering conditions of river sediment in Madagascar. We propose that the existence of a weathered, weak surface layer of crystalline bedrock encourages river reversal and large river captures from the upper plateau, leading to a high frequency of knickpoint‐type rivers. We demonstrate that this is feasible using 2D 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 the captured area and escarpment retreat rate is found from the models. We demonstrate that this scaling is also observed in the great escarpments of Madagascar and India. This divide migration mechanism provides a new explanation for the morphology of river profiles and great escarpments.