Plio-Quaternary and Recent extensional tectonism in the Madagascar high plateaux: the Ankay-Alaotra graben

In the High Plateau basement region of Madagascar, east of Antananarivo. th e 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 g raben, from south to north, the Ankay and the Alaotra lake basins, separate d by the Andaingo topographic high. They are infilled by fluvial to lacustr ine Plio-Quaternary deposits. Tectonic and greomorphological markers, confi rmed by seismological analysis, record an active extensional deformation th at gives way to the graben development. Elsewhere in the axial part of Mada gascar, the extension is also characterized by a widespread alkaline volcan ism. These events correspond to the initiation of a N-S rift system which c ould be related to the present rift system of East Africa.