In the High Plateau basement region of Madagascar, east of Antananarivo, the late Tertiary erosional surface has been offset by a fault pattern which determines a N-S trending graben. Two basins can be distinguished in this graben, from south to north, the Ankay and the Alaotra lake basins, separated by the Andaingo topographic high. They are infilled by fluvial to lacustrine Plio-Quaternary deposits. Tectonic and geomorphological markers, confirmed by seismological analysis, record an active extensional deformation that gives way to the graben development. Elsewhere in the axial part of Madagascar, the extension is also characterized by a widespread alkaline volcanism. These events correspond to the initiation of a N-S rift system which could be related to the present rift system of East Africa.