Pe. Sacks et al., DEXTRAL PAN-AFRICAN SHEAR ALONG THE SOUTHWESTERN EDGE OF THE ACHANKOVIL SHEAR BELT, SOUTH-INDIA - CONSTRAINTS ON GONDWANA RECONSTRUCTIONS, The Journal of geology, 105(2), 1997, pp. 275-284
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.