Retreat of the Great Escarpment of Madagascar From Geomorphic Analysis and Cosmogenic 10Be Concentrations.

The eastern margin of Madagascar has a prominent relief change from the flat coastal plain to the low‐relief high plateau, characterizing a typical great escarpment topography at a passive margin. A quantification of the spatial distribution of erosion rates is necessary to understand the rate of landscape evolution. We present catchment‐averaged erosion rates from detrital cosmogenic 10Be concentrations, systematically covering distinct morphological zones of the escarpment. Erosion rates are differentiated across the escarpment, where the high plateau and the coastal plain are slowly eroding with an average rate of 9.7 m/Ma, and the escarpment basins are eroding faster with an average rate of 16.6 m/Ma. The Alaotra‐Ankay Graben related basins have the highest erosion rate with an average rate of 27 m/Ma. The spatial pattern of erosion rates indicates a retreating escarpment landscape. Retreat rates calculated from the 10Be concentrations are from 182 to 1,886 m/Ma. The rates of escarpment retreat on Madagascar are consistent with a model of a steady retreat from the coastline since the time of rifting, similar to the Western Ghats escarpment on its conjugate margin of the India Peninsula. Plain Language Summary: Eastern Madagascar is characterized by a distinct escarpment with high relief, normally indicative of high erosion rates. We investigate this by measuring erosion rates using cosmogenic isotope concentrations in sediment derived from throughout Madagascar. We calculated erosion rates and related these to distinct geomorphic zones. The pattern of erosion rates is consistent with inland retreat of the escarpment at about 1 km/Ma. This rate is consistent with geomorphic evidence and is comparable to the conjugate margin of western India. Key Points: Presentation of new detrital cosmogenic 10Be data that systematically covers the great escarpment of eastern MadagascarErosion rates inferred from 10Be concentrations are 9.7 to 27 m/Ma, systematically vary among distinct morphological zonesEscarpment retreat rates from 10Be concentrations are 182 to 1,886 m/Ma, consistent with evidences for captures and divide migration [ABSTRACT FROM AUTHOR]