Lithospheric tectonic structures developed under high-grade metamorphism in the Southern part of Madagascar

Abstract

This paper describes the tectono-metamorphic evolution of a segment of the Precambrian deep crust, in the southern Madagascar island. This crust corresponds to an Archaean basement reworked by a widespread, late panAfrican event (550–580 Ma) during the formation of the Mozambican belt. The finite geometry and associated metamorphism are depicted by satellite imaging, field mapping and P-T estimations using both conventional thermobarometric methods and TWEEQ software program with internally consistent thermodynamic data and uniform set of solution models. The structural pattern developed during high-grade metamorphism shows the juxtaposition of domains with complex fold geometries separated by a 15 km wide ductile shear zone. Within the folded domains, kilometre scale interference patterns associated with strongly dipping metamorphic stretching lineations can be described as superposed folding (F1 and F2 folds). The tight and upright F2 folds result from East-West horizontal shortening. The shear zone is defined by homogeneous orientations of steep foliations, sub-horizontal stretching lineations, and kilometre scale strain gradient. Within the shear zone, we observe dominant non-coaxial criteria at various scales that are consistent with a sinistral strike-slip system during D2 deformation stage. Nevertheless, we have also found in the shear zone, geometries typical of a horizontal shortening. Such a strain pattern is characteristic of transpression tectonics.The synkinematic metamorphic conditions are estimated on mafic garnetiferous metabasites. Results show that regional transpression tectonics has developed under very high and constant thermal regime (about 800°C). A pressure gap, of about 3 kbar between the domains separated by the shear zone is identified. This implies tectonic coupling of two different structural levels during tranpressive tectonic.