Tectonics of the Neoproterozoic Southern Granulite Terrain, South India

Geological correlations between South India and adjacent Gondwana fragments are hampered by the lack of a coherent structural framework for India's Southern Granulite Terrain (SGT). This paper is an attempt at delineating its large-scale geometry and Neoproterozoic kinematics and tectonics. The basic regional pattern is that of a high-grade gneissic layering produced during several deformation stages (grouped as D1), which became refolded during two regionally separated phases of upright folding (D2a: NNW trending; D2b: NE trending) and reworked in E- to ESE-trending zones of increased strain (D3). Of these, the Palghat Cauvery Shear Zone (PCSZ), which marks the boundary between Archaean terrains in the north and Proterozoic belts in the south, may represent a major crustal shear zone, whereas the Achankovil Zone (AZ) is better considered as a high-strain zone characterized by flattening with a reverse component. D1 structures developed close to the peak of Pan-African high-grade metamorphism. D2a and D2b occurred on the retrograde path, partly at supersolidus and partly at subsolidus conditions. D3 high-strain zones show subsolidus textures. In view of the general clockwise nature with a clear decompressional component of all published P-T paths, we propose that D2-D3 occurred during partial exhumation of the terrain. Correlations with Madagascar are weakened as the AZ is no longer considered a major crustal shear zone, whereas correlations with Sri Lanka are strengthened by the presence of a similar syntactical bend possibly produced by an indentor.