Erosion rates and landscape evolution of the conjugate margins of Madagascar and India

Abstract

A great escarpment at a passive margin is the mountainous area that separates the low-relief high plateau and the low-relief but low-lying coastal plain. River incision models, such as the streampower incision model, are often used to quantify the relationship between erosion rates and topographic metrics. Stream-power incision models predict higher erosion rates for steeper river reaches, while accounting for vertical rock motion due to uplift or denudation. Escarpment rivers are steep compared to rivers from active-tectonic regions, suggesting high erosion rates on the escarpment. But slow erosion rates interpreted from cosmogenic nuclide concentrations contradict the morphology-inferred denudation pattern. This thesis addresses this central issue by examining denudation rates and topographic characteristics of the conjugate margins of Madagascar and India which both preserve the great escarpment topography. The Western Ghats escarpment in India exhibits a morphology consistent with a steady state retreat. Channel profiles are consistent with the concept of steady retreat in which the lower coastal plain reach is graded to a low slope sufficient to transport sediment from the eroding escarpment reach, and the steeper escarpment reach is adjusted to erode the escarpment; denudation is concentrated at the escarpment. New cosmogenic 10Be-derived erosion rates of the Madagascar escarpment and river steepness indices are also consistent with concentrated denudation at the escarpment. To quantify the retreat rate, a model for interpreting cosmogenic 10Be concentrations as a directional mass flux to characterize non-vertical landscape changes, e.g. the laterally retreating escarpment is proposed. Provided that secular equilibrium of cosmogenic nuclides within a catchment is maintained between newly produced nuclides per unit time and nuclide exportation by erosion, a detrital cosmogenic 10Be concentration can be used to quantify the mass flux, as a function of its direction. Escarpment retreat rates of the Western Ghats and Madagascar escarpments inferred from this model vary within a range of 100s m/Ma to 1000s m/Ma. Retreat rates of two escarpments from conjugate margins are consistent with, or slightly lower than, a steady retreat rate from the coastline since rifting. This thesis also explores the coupling between divide migration and escarpment retreat. The escarpment retreat is typically modelled as a migrating water divide, where the water divide coincides with the upper end of the escarpment. However, the continental water divide of Madagascar does not generally coincide with the escarpment, although it runs in parallel. Morphological features of the escarpment and the inferred stability of the continental water divide imply that captures of highland rivers are prevalent and are the main driver of divide migration along the Madagascar continental divide. A 1D numerical model of escarpment river is utilized to explore factors in controlling the escarpment retreat under various generic divide migration patterns. Observations from the model support the hypothesis that divide migration patterns control escarpment retreat patterns through the control of upstream drainage area at the escarpment edge. A power law dependence of the escarpment retreat rate on upstream drainage area is demonstrated for the Madagascar-type escarpment.