Structure of the eastern margin of the East African Orogen in central Madagascar

Abstract

The island of Madagascar straddles the poorly understood eastern margin of the East African Orogen, which is characterised by a high-grade gneissic basement that was structurally and thermally reworked during the Neoproterozoic collision of the Dharwar craton of India with the Congo/Tanzania/Bangweulu craton of Africa. An east-west traverse across the eastern margin of the Antananarivo block of central Madagascar and the overlying Beforona belt of the Tsaratanana sheet has identified four north-south trending structural domains, which represent the easternmost units of the East African Orogen. Dominant structures in these four domains are ascribed to two main deformation events, D2 and D3, whereas a poorly preserved early foliation and late folding event are classified as D1 and D4, respectively. D2 structures developed during east-west shortening comprise relatively low-strain, upright, north-south trending folds and steeply dipping, planar, north-south trending high-strain zones, both of which are associated with a stretching lineation that plunges gently north-south. D3 structures comprise a >20 km wide mylonitic high-strain zone and smaller scale discrete shear zones that dip gently west and exhibit top-to-the-east movement, again interpreted as a result of east-west shortening. The Antananarivo block and Beforona belt were juxtaposed sometime before D2 folding, which deforms the contact between the two units, but some time after an early granulite-facies metamorphic event that is preserved only in low strain zones of the Antananarivo block. D2 coaxial strain and D3 non-coaxial strain both occurred under amphibolite-facies metamorphic conditions and, although D3 shear zones locally cut D2 structures, these two events need not be temporally distinct. Published geochronological data from rocks exposed along this traverse constrain the likely timing of D2 and D3 to 630–515 Ma. The age, style and magnitude of strain along the eastern Antananarivo block indicates that eastern Madagascar was a focus for deformation during the final stages of collision between the Dharwar and Congo/Tanzania/Bangweulu cratons.