Dextral Pan-African shear along the southwestern edge of the Achankovil shear belt, south India: Constraints on Gondwana reconstructions

The Achankovil shear belt of the southern Indian Peninsular shield is prominent on Landsat images. It coincides with the boundary between charnockites of the Madurai block to the north and khondalites of the Trivandrum block to the south, and with a major change in the aeromagnetic pattern that can be traced across southern India. Field investigations reveal a major shear zone along the southwestern edge of the Achankovil shear belt, the Tenmala shear zone. Rocks in the Tenmala shear zone include charnockite, gamet-biotite gneiss, garnets-sillimanite gneiss, cordierite gneiss, and granite. Kinematic indicators include stretched and asymmetric garnet, feldspar and quartz porphyroclasts, shear bands, asymmetric folds, extensional and contractional composite structures, hook folds on rotated and deformed gash veins, and lineations. Kinematic analysis of these features along 60 km of the shear zone indicates primarily dextral shear, with a minor component of reverse shear. Textures, as well as mineral assemblages, are consistent with deformation under granulite facies conditions. Some overprint by the most recently formed charnockite postdates the shearing. Limited geochronologic data suggest a late Proterozoic (Pan-African) age of shearing. Dextral shear along the Achankovil shear belt is opposite to the sinistral shear reported for the Bongolava-Ranotsara shear zone in southern Madagascar; hence these two shear zones cannot be correlated in reconstructions of these parts of Gondwana.