THE SIGNIFICANCE OF THE PALGHAT CAUVERY SHEAR ZONE OF SOUTHERN INDIA: NEW INSIGHTS INTO ITS TECTONOMETAMORPHIC HISTORY

The Palghat-Cauvery Shear Zone of southern India is generally believed to be a major tectonic boundary. Archean and Proterozoic basement rocks occur to the north, and Pan-African age rocks occur in addition to the south of the shear zone. Detailed structural analysis of the shear zone is lacking, and hence its existence had been postulated mostly upon sparse isotopic data. Field investigations along a transect from Namakkal (north) to Alambadi (south), indicate broadly similar structural styles throughout. An early deformation is comprised of outcrop-scale, rootless isoclinal folds. A second phase of folding produced a series of mappable upright folds with nearly E-W axial traces. Fold axes have highly variable orientations ranging from gentle easterly, through vertical, to gentle westerly plunging. Parallel alignment of platy minerals defines an axial planar foliation. The mineral lineation associated with D1 deformation is bent by D2 folds. Gneissosity associated with the shear zone can be correlated to D2 deformation.

Rock types are significantly diverse with mafic granulites, and enderbitic and quartzofeldspathic gneisses in the Namakkal area, and supracrustal rocks (principally marble-calc silicate gneiss and garnet-sillimanite gneiss) prominent near Alambadi. Metamorphism attained granulite facies conditions in both areas. P-T estimates from electron microprobe analyses of relevant phases indicate subtle differences in equilibrium metamorphic conditions. Temperatures of ~820°C at 10 kbar are estimated for the Namakkal area, and a lower peak temperature of ~700°C at Alambadi. These data, when combined with Sm-Nd and U-Pb systematics, will serve as a powerful tool to elucidate the complex histories of the two juxtaposed terranes.