Recent denudation rates of southwestern Madagascar from a 10Be analysis of river sand samples

Many regions, though tectonically inactive, present significant reliefs of elusive origins. In these areas, it is critical to constrain denudation rates to assess the long-term evolution of these reliefs, but data and thus information on what controls their spatiotemporal evolution are scarce, especially in the Southern Hemisphere.

In this study, we present in-situ cosmogenic ¹⁰Be data from 14 new sand samples of main rivers and their tributaries of southwestern Madagascar, a subtropical island of southeastern Africa (Indian Ocean). This island presents in its central part a low relief high plateau composed of Mesoarchean to Neoproterozoic crystalline basement, a narrow coastal plain in its eastern part, separated from the central plateau by a great escarpment, and two large sedimentary basins of Carboniferous to Neogene ages in its western part. Its recent (i.e., past 15 Ma) tectonic activity and associated uplift is mainly attributed to mantle upwelling, probably related to the East African Rift System, creating long-wave uplift of 1 to 2 km. Limited seismicity associated with extensive settings is measured on the island. In terms of climate, Madagascar undergoes a monsoon type of climate with a strong gradient in humidity from northeast to southwest.

Our new cosmogenic ¹⁰Be data comes in complement of an important dataset of 99 samples previously published in three different studies. This brings the total dataset to 116 samples, covering over 50% of the total island surface. Our results are in good agreement with the previously published data with overall low denudation rates (4 ± 1 to 30 ± 6 mm/ka). This dataset allows exploring how the island erodes and calculating and comparing sediment fluxes from the eastern and western sides. Our results show that, although the eastern great escarpment is retreating at rates of 182 to 1886 m/Ma, average denudation rates of the basins draining it (16.6 mm/ka) are comparable, though slightly lower than the average denudation rates we measured in the southwestern basins (17.2 mm/ka).