Finite strain pattern in Andriamena unit (north-central Madagascar): evidence for late Neoproterozoic–Cambrian thrusting during continental convergence

This paper deals with the late Neoproterozoic–Cambrian tectonic evolution of a part of north-central Madagascar, which is characterized by the occurrence of a mafic-ultramafic sequence (the Andriamena unit) overlying a gneissic-granitic basement. The finite strain pattern has been determined by carrying out a SPOT satellite image analysis, structural mapping of specific areas and kinematic analyses of shear zones. Structural investigations reveal the presence of two superposed finite strain patterns, D1 and D2. The D1 event is related to the emplacement of the Andriamena unit on the top of the gneissic-granitic basement. The western contact between these units is a major mylonitic zone characterized by a non-coaxial strain regime consistent with a top-to-east displacement. We suggest that the Andriamena unit originated as a lower crustal fragment of a middle Neoproterozoic continental magmatic arc related to the closure of the Mozambique Ocean. This fragment was thrusted onto the gneissic-granitic basement after 630 Ma, i.e. the age of emplacement of characteristic stratoid granites found only in the lower unit. The D2 event is related to east-west horizontal shortening mainly accommodated by F2 upright folds. In-situ electron microprobe dating of monazites from the Andriamena unit constrains the age of the D1 and D2 events between 530 and 500 Ma under amphibolite to granulite-facies conditions (5–7 kbar, 650–700 °C). The eastward thrust emplacement of the Andriamena unit (D1) followed by the horizontal shortening (D2) are ascribed to the same Cambrian tectonic regime (i.e. east-west convergence). Such D1–D2 bulk strain pattern has been recognized throughout Madagascar and at various structural levels of the crust: in the lower crust in Southern Madagascar and in the uppermost crustal level in the SQC unit (central Madagascar). The D1–D2 event is interpreted to result from the continental convergence of the Australia–Antarctica block and the Madagascar, India, Sri Lanka block during the final amalgamation of Gondwana.