Retreat of the Great Escarpment of Madagascar from Geomorphic Analysis
and Cosmogenic 10Be Concentrations
Abstract
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 m/Ma to 1886
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.