Madagascar's landscape evolution: a tale of two rifts and drainage divide migration

 

The topography of Madagascar reflects a dynamic history of water divide migration, driven by rifting on both coasts, but modified by late Cenozoic volcanism and extensional tectonics. These geological events have produced distinct geomorphic landscapes and histories. We reconstruct how rifting created coastal escarpments, as well as long-wavelength tilting, that shifted the water divide, changing drainage area and erosion patterns since Cretaceous rifting. We document a westward-tilted plateau with sinuous remnant escarpments on the western margin and a linear escarpment approximately corresponding to the modern drainage divide on the eastern margin, formed during the corresponding rifting phases, separated by 80 Ma. We suggest that the western topographic remnants are part of the older, western escarpment that was destroyed during Indian Ocean rifting, which formed the younger, eastern escarpment and tilted the existing topography, causing the water divide to jump to the eastern margin. Currently, the eastern escarpment corresponds to the insular water divide in the south, but not in the central or northern regions, where the escarpment corresponds to a large, regional knickzone, several tens of kms downstream from the water divide. We argue that knickzone-type river profiles correspond to the late Cenozoic volcanic and tectonic activity that shifted the divide inland from its post-rifting position at the escarpment. These findings highlight the profound, long-term impact of drainage divide migration in shaping Madagascar’s topography and hydrology.