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 ch arnockites of the Madurai block to the north and khondalites of the Tr ivandrum block to the south, and with a major change in the aeromagnet ic pattern that can be traced across southern India. Field investigati ons reveal a major shear zone along the southwestern edge of the Achan kovil shear belt, the Tenmala shear zone. Rocks in the Tenmala shear z one include charnockite, gamet-biotite gneiss, garnets-sillimanite gne iss, cordierite gneiss, and granite. Kinematic indicators include stre tched and asymmetric garnet, feldspar and quartz porphyroclasts, shear bands, asymmetric folds, extensional and contractional composite stru ctures, hook folds on rotated and deformed gash veins, and lineations. Kinematic analysis of these features along 60 km of the shear zone in dicates primarily dextral shear, with a minor component of reverse she ar. Textures, as well as mineral assemblages, are consistent with defo rmation under granulite facies conditions. Some overprint by the most recently formed charnockite postdates the shearing. Limited geochronol ogic data suggest a late Proterozoic (Pan-African) age of shearing. De xtral shear along the Achankovil shear belt is opposite to the sinistr al shear reported for the Bongolava-Ranotsara shear zone in southern M adagascar; hence these two shear zones cannot be correlated in reconst ructions of these parts of Gondwana.