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

Abstract

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

Linking uplift, erosion, and sedimentation using landscape evolution models: Madagascar since the Late Cretaceous

“…Based on their modeling results, both Roberts et al ( 2012 2021) and Wang et al (2021). Considering that our erosion model includes a threshold for minimum erosion, it is reasonable for our estimates to be slightly higher; we also demonstrate that the estimate of K value tends to reduce with a decreasing erosion threshold ėc .…”

“…The low erosion rate inferred from 10 Be is consistent with a model of continuous retreat of the great escarpment on the east coast since the Cretaceous rifting of the India-Madagascar margin, without the necessity for a renewed phase of uplift during the Cenozoic. The estimated K value of our best-fit model is about 6 Â 10 À6 m 0.2 /yr, lower than the calibration of Roberts et al (2012) but higher than Stephenson et al (2021) and Wang et al (2021). Considering that our erosion model includes a threshold for minimum erosion, it is reasonable for our estimates to be slightly higher; we also demonstrate that the estimate of K value tends to reduce with a decreasing erosion threshold ė c .…”

“…However, an observation emerges from our 2D planview model: if spatial variability in the uplift rates existed, dynamic reorganization of the river networks would have been active near the main drainage divide, including captures of stream channels between the west-and east-draining rivers. Therefore, as pointed out by Wang et al (2021), assumptions of stationary upstream drainage areas, upon which the inversion of river long profiles is based, could be problematic. On the other hand, as the misfit function in our inverse modeling is insensitive to the geometry of the drainage basin, our results are not suitable for negotiating between the models of Cenozoic uplift and low erosion rates on the east coast.…”

“…) andStephenson et al (2021) suggest a period of Cenozoic uplift to construct the high plateau of Madagascar. Based on basin averaged erosion rates estimated from cosmogenic 10 Be concentration,Wang et al (2021) calibrated the erosion coefficient K at 2 ⇥ 10 6 m 0.3 yr 1 (m/n=0.35) or 4 ⇥ 10 7 m 0.1 yr 1 (m/n=0.45). The low erosion rate inferred from 10 Be is consistent with a model of continuous retreat of the great escarpment on the east coast since the Cretaceous rifting of the India-Madagascar margin, without necessity for a renewed phase of uplift during the Cenozoic.…”

The Role of Weathering on Morphology and Rates of Escarpment Retreat of the Rift Margin of Madagascar

“…We test the scaling relationship of retreat rate and drainage area with the detrital cosmogenic nuclide (DCN) 10 Be concentration‐derived retreat rates of both the Madagascar escarpment and the Western Ghats using the results of Wang et al. (2021). We select escarpment‐draining basins with plateau areas larger than 3 km 2 .…”