Erosional history and topographic evolution of Madagascar rifted margins

Topographies along passive continental margins are shaped by escarpment erosion and dissection of ancient plateau surfaces. Thermochronology and cosmogenic nuclide studies from these settings indicate a near steady process of denudation and inland escarpment propagation since break-up. Yet, a discrepancy exists between measured erosion rates and the scale of observed topographic features, often implying a more complex denudation history. The Madagascar landscape preserves the imprint of two major rifting events, expressed in a remnant escarpment in the west, a deeply dissected central plateau, and a coherent steep escarpment on the east. While steady inland retreat explains the kinematics and position of the great eastern escarpment, the western margin records a more complex morphology, inconsistent with the model of stable retreat of a coastal escarpment. Here, we reconstruct Madagascar’s Cenozoic denudation history and landscape dynamics by combining topographic analysis, ¹⁰Be cosmogenic nuclide concentrations in sediment and numerical modelling of landscape evolution. Catchments draining the plateau–escarpment reveal escarpment retreat rates of several hundred to over a thousand m/Myr on the wet eastern margin, and a few hundred m/Myr along the smaller, drier remnants on the western margin. Retreat rates scale with plateau extent and divide position, which control stream power at the escarpment, while low-erodibility lithologies (granites, basalts) locally inhibit retreat and preserve inland relicts. Numerical models constrained by erosion rates and bedrock erodibilities from Madagascar reproduce observed patterns and demonstrate that eastward migration of the main divide after ~90 Ma triggered large-scale drainage reorganization and pulses of rapid retreat, up to 4 km/Myr, across the western margin. Our findings highlight the inherently dynamic nature of passive margin landscapes, where divide migration and spatial variations in fluvial erosional efficiency govern the long-term evolution of passive margins, with fundamental implications for hydrology, landscape transience, and biodiversity.