Divide migration and escarpment retreat in Madagascar and the Western Ghats of India
Abstract
We present a study of the morphological features of topography of the great escarpments that characterize the passive margins of Madagascar and the Western Ghats of India. Although we confirm many of the characteristics of a great escarpment, including the steep escarpment, dominated by fluvial erosion, and a high, flat plateau with the continental water divide generally following the escarpment edge, we also find a number of important discrepancies. In particular, we find that the continental water divide frequently deviates inland of the escarpment. In this case, rivers have an upper reach on the plateau and the escarpment is represented by a large knickzone on profiles of escarpment-draining rivers. We find no spatial consistency between rivers with escarpment knickzones and rivers with their main divide on the escarpment edge; both river types are found interspersed along both margins. This suggests that the inland deviations represent river capture events where large areas of the plateau have been diverted over the escarpment. We used 1D and 2D numerical models of escarpment retreat to explore conditions needed for the occurrence, frequency and geometry of the river captures we observe on these passive margins. We find that to obtain the large number of large-area capture events we need to introduce a more erodible material to the high plateau. We argue that this is consistent with the observations of a highly-weathered regolith layer present in both Madagascar and India. With this softer surface layer, the water divide frequently advances ahead of the escarpment leading to more frequent and larger river captures. River capture provides an important feedback mechanism to escarpment retreat. The captured area increases river discharge on the escarpment and thereby increases the retreat velocity. The result is a correlation between captured area and escarpment retreat rate that we observe in both models and erosion rate data.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFMEP34B..04W