Madagascar is one of the world's largest islands, separated from the Africa continent by the Mozambique Channel (Figure 1a). The main water divide of Madagascar follows the longitudinal axis of the island and separates drainages of the Indian Ocean from those of the Mozambique Channel (Figure 1b), although the topography is strongly asymmetric with eastward-flowing rivers shorter (<150 km) than westward flowing rivers (>300 km). Madagascar was at the center of the Gondwana supercontinent and was surrounded by the ancient Africa continent and the continent of the Seychelles-India (Wit, 2003). The rifting between Madagascar and Seychelles-India started between 120 and 92 Ma based on ages of basaltic intrusions and dikes at the eastern margin of Madagascar (Melluso et al., 2005;Torsvik et al., 1998). Final separation is dated to the late Cretaceous from volcanic provinces and the oldest seafloor magnetic anomaly in Indian Ocean, Chron 34 (∼84 Ma) (Eagles & Hoang, 2014), and is limited to being older than the Deccan volcanic province eruption at the western margin of the India peninsula (∼65 Ma) (Collier et al., 2008). Rifting between Madagascar and Seychelles-India has formed the paired mountain ranges along the coast at the conjugate margins of eastern Madagascar and western India (Gunnell & Harbor, 2008).Unlike the west margin where the central high plateau gradually flattens into the coastal plain, the east margin is characterized by an escarpment where the topography abruptly increases from the coastal plain, rapidly rising to the high flat plateau, with the water divide sitting at or near the eastern margin of the highlands. The topography of the conjugate margin of western India, the Western Ghats, is similar and is well-recognized as a great escarpment (Gunnell & Harbor, 2008Mandal et al., 2015), but Madagascar has not received the same attention, even though the topography exhibits the same strong asymmetry.Although there is no major post-rift tectonics on Madagascar, there is evidence for Cenozoic uplift. A major erosional unconformity from Oligocene to Miocene (30-16 Ma) is revealed in offshore wells on the western continental margin (Delaunay, 2018). This unconformity is likely regional given that Oligocene sediment is also

Abstract. High-relief great escarpments at passive margins present a paradoxical combination of high-relief topography but low erosion rates suggesting low rates of landscape change. However, vertical erosion rates do not offer a straightforward metric of horizontal escarpment retreat rates, so we attempt to address this problem in this paper. We show that detrital cosmogenic nuclide concentrations can be interpreted as a directionally dependent mass flux to characterize patterns of non-vertical landscape evolution, e.g., an escarpment characterized by horizontal retreat. We present two methods for converting cosmogenic nuclide concentrations into escarpment retreat rates and calculate the retreat rates of escarpments with published cosmogenic 10Be concentrations from the Western Ghats of India. Escarpment retreat rates of the Western Ghats inferred from this study vary within a range of hundreds to thousands of meters per Myr. We show that the current position and morphology of the Western Ghats are consistent with an escarpment retreating at a near-constant rate from the coastline since rifting.

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Madagascar, also known as the "red island," is famous for its reddish-colored soil-covered high plateau and a great escarpment mountain range rimming the plateau. We show here that the red soils mantling the high plateau are the product of chemically degraded crystalline rock. The same crystalline rock of the great escarpment of Madagascar is, however, contrastingly less degraded. We find that the surficial chemically degraded rock layer on the plateau encourages plateau river redirections through river piracy by the steep escarpment rivers, and this process accelerates the lateral migration rate of the great escarpment mountains.

Abstract. High-relief great escarpments at passive margins present a paradoxical combination of high relief topography, but low erosion rates suggesting low rates of landscape change. However, vertical erosion rates do not offer a straightforward metric of horizontal escarpment retreat rates, so we attempt to address this problem in this paper. We show that detrital cosmogenic nuclide concentrations can be interpreted as a directionally-dependent mass flux to characterize patterns of non-vertical landscape evolution, e.g. an escarpment characterized by horizontal retreat. We present two methods for converting cosmogenic nuclide concentrations into escarpment retreat rates and calculate the retreat rates of escarpments with published cosmogenic 10Be concentrations from the western Ghats of India. Escarpment retreat rates of the Western Ghats inferred from this study vary within a range of 100s m/Ma to 1000s m/Ma. We show that the current position and morphology of the Western Ghats are consistent with an escarpment retreating at a near constant rate from the coastline since rifting.