Active tectonics of the Alaotra–Ankay Graben System, Madagascar: Possible extension of Somalian–African diffusive plate boundary?

The Lake Alaotra–Ankay rift valley of Central Madagascar forms a NE–SW oriented depression filled with Neogene to recent sediments and is part of a more regional post-Miocene graben system that strikes N–S across much of the central part of the island. The region is characterized by a number of small earthquakes, active hot springs, steep fault-scarp bounded valleys, and several levels of terraces. Young, deeply incised topography related to intense tropical weathering, is largely concentrated near active fault systems. The origin and evolution of this extensional structure and its morphological expressions, however, have not been clearly documented. Mountain front sinuosity values indicate that the eastern side of the rift valley is tectonically more active than the western side, and that rapid erosion under subtropical conditions is causing rapid weathering of fault scarps. The active river network within the Alaotra–Ankay rift shows clear signs of surface re-adjustment to recent tectonic activity. The drainage network is asymmetric, with most channels on the east sides of valleys and controlled by regional eastward tilting. First-order rivers flow parallel to the basin axis. Zones of intense lavaka (erosional gully) formation are correlated with active fault systems and seismicity. Extension in the Ankay–Alaotra rift is oriented roughly the same as in the morphologically similar East African rift system, located only 500 km to the west. The eastern edge of the African continent is moving somewhat independently from the rest of Africa and the portion of the African plate east of the East African rift is regarded as a separate plate, the Somali plate. However, few plate configurations have clearly defined the southern extension of the Africa-Somali plate boundary to where it must join the Southwest Indian Ocean ridge, a necessary requirement of any true microplate bounded by plate boundaries. Instead, the southeast part of the African plate, including the Somali microplate, seems to be dominated by a number of individual rift systems, overall defining a broad diffusive extensional plate boundary. We suggest that this diffusive plate boundary includes one segment that extends off the coast of Africa through the Comores, then cuts through northern Madagascar and extends down the active Ankay–Alaotra rift to the fault-block dominated southeastern coast of Madagascar, then extends southeastward to the Southwest Indian Ocean Ridge. Thus, the southeastern part of the African plate is considerably more fragmented by plume-related uplift and diffuse extensional boundaries than previously envisaged.