Indenter tectonics in central Madagascar
Giese, Jorg; Schreurs, Guido; Berger, Alfons; Herwegh, Marco; Gnos, Edwin
It is widely accepted that Madagascar occupied a central position within the East African Orogen (EAO after Stem 1994) between eastern Africa and southern India before breakup of Gondwana. Tight-fit reconstructions of its exact position are mostly based on similarities of structural features such as high-strain zones which can be traced on formerly adjacent, but nowadays dispersed continental fragments. The Ranotsara Zone, an area in southern Madagascar where the general N-S trend of geological units and structures is deflected into a NW-SE orientation is frequently used to make correlations with shear zones in southern India and eastern Africa. However, detailed field studies and interpretation of remote sensing data suggest that the Ranotsara Zone can not be used for Gondwana reconstructions. It rather represents a regional scale flexure of mainly metasedimentary units of the SW-Madagascar Block. The flexure is thought to be caused by (probably oblique) indentation of the more rigid Antananarivo Block during amalgamation of Gondwana. The contact between the Antananarivo Block and the Itremo - and Ikalamavony Units (both part of the SW - Madagascar Block) can be traced for more than ~750 km across the island and is referred to by us as the Itremo - Ikalamavony Thrust. Quartzites generally constitute the basis of the SW-Madagascar Block and show mylonitic structures. Although a predominantly top-to-the-east tectonic transport can be inferred, detailed field observations as well as quartz microstructures and textures indicate a complex polyphase deformation history under varying metamorphic conditions. Indentation of the Antananarivo Block into the units of the SW Madagascar Block and thrusting of these units onto the Antananarivo Block seems to be partly coeval with regional Panafrican deformation.