Divide migration and escarpment retreat in Madagascar and the Western Ghats of India

A great escarpment is characterized with extremely asymmetrical topography with a steep and high-relief mountain range rimming a low-relief high plateau. Measured erosion rates contradict the observed high relief of the escarpments of Madagascar and India. We used cosmogenic nuclide (CN) ¹⁰Be concentrations to infer horizontal retreat rates of escarpments. Million-year scale retreat rates of Madagascar and India escarpments are ~1 km/Ma. CN ¹⁰Be-inferred retreat rates and escarpment morphology are consistent with steady retreating escarpment from modern coastlines since rifting for both margins.

The edge of the escarpment usually acts as the water divide. Previous studies conceptualize an escarpment as a migrating water divide. We studied the morphological features the escarpment and continental water divide of Madagascar and India, demonstrating that the continental water divide does not universally correspond to the steep rift escarpment due to river captures. We hypothesized that the heavily weathered plateau encourages frequent river capture and affects the morphology and rates of escarpment retreat.

We used 2D landscape evolution models to explore factors in controlling escarpment retreat. Model observations support the hypothesis that divide migration patterns control escarpment retreat patterns through the control of captured drainage area from the plateau. Through frequent river capture or divide advance into an erosional weak layer, rivers increase area and thereby increase the retreat rate. Measured escarpment retreat rates of eastern Madagascar and Western Ghats, India support this model and quantify the effect of captured area on escarpment retreat rate.