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Joerg Giese, University of Bern (Switzerland)
Guido Schreurs, University of Bern (Switzerland)
Alfons Berger, University of Bern (Switzerland)
Marco Herwegh, University of Bern (Switzerland)
Edwin Gnos, Museum of Natural History (Switzerland)
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During amalgamation of Gondwana the crystalline basement of Madagascar was incorporated into the East African Orogen and occupied a central position between eastern Africa and India. It was deformed at metamorphic conditions varying from upper greenschist to granulite facies. Several wide zones with (sub)vertical, high-grade gneissic foliation planes can be traced for hundreds of kilometres and have been interpreted as major vertical ductile shear zones by various authors. In contrast to the general N-S trending structural pattern, the Ranotsara Zone in southern Madagascar trends NW-SE. It has not only been regarded as a Proterozoic major intra-crustal sinistral strike-slip shear zone, but also as a terrane boundary separating a region with Archean crust to the north from a region with Neoproterozoic crust to the south. Therefore, various studies have used the Ranotsara Zone for correlations with shear zones in southern India and eastern Africa to reconstruct the relative paleopositions of the now dispersed continental fragments. However, detailed field studies and interpretation of remote sensing data indicates that the Ranotsara Zone can not be used for Gondwana reconstructions. The regional pattern of steeply dipping N-S trending high grade composite foliation and late stage subvertical axial planes of upright folds, as well as similarily oriented coherent geological units are all slightly deflected into a NW-SE orientation in the area around Ihosy. There is neither evidence for a newly developed, discordant mylonitic foliation along the Ranotsara Zone nor is there a gradient in shear deformation across it. The Ranotsara Zone rather represents a regional scale flexure of metasedimentary units of the SW-Madagascar Block. The flexure is thought to be caused by (probably oblique) westward indentation of the rigid Antananarivo and adjacent Antongil Block as outer parts of the Indian Dharwar craton.
North of the Ranotsara Zone, a major ductile shear zone can be identified separating mainly metasedimentary rocks of the Ikalamavony and Itremo units from predominantly magmatic rocks of the Antananarivo Block to the east. This contact can be traced for more than 750 km from NW to SE across Madagascar and is what we refer to as the "Itremo - Ikalamavony Thrust". Large-scale recumbent folding and internal imbrication of metasedimentary rocks in the hanging-wall are associated with the shearing. Stretching lineations within the mylonitic foliation in this shear zone generally trend E-W to NE-SW. Microstructures and textures of quartz mylonites from different locations along strike of the thrust were analysed in order to constrain kinematics as well as the metamorphic conditions during deformation. We suggest that the Itremo-Ikalamavony Thrust and the flexure of the geological units of the SW-Madagascar Block in the Ranotsara Zone are both the result of indenter tectonics.
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