STRUCTURAL GEOMETRY IN A PART OF THE SOUTHERN GRANULITE BELT OF INDIA

A zone of Pan-African activity extends from the Arabian-Nubian shield through southern Madagaskar, southern India, and Sri Lanka into East Antarctica. This zone appears to be the suture between East Gondwana and various plates of West Gondwana. The study area falls within this Pan-African zone in South India. The southern Peninsular India consists of three blocks separated by several large Proterozoic shear zones. The northernmost block, the Dharwar craton, is a granite-gneiss-greenstone terrain that grades southwards into granulite facies. The southernmost block, the Kerala Khondalite Belt (KKB) is composed of amphibolite facies metasediments. In between is the Madurai Block, separated from the KKB by the Achankovil Shear Zone, and from the Dharwar craton by the Palghat-Cauvery Shear Zone (PCSZ). The present area forms a part of the Madurai block. The major event of metamorphism in the block is believed to have occurred during Pan-African time close to 0.55 Ga.

South of PCSZ there is a nearly 100 km long southwesterly closing folded belt (designated here as the Alambadi Fold) of supracrustal rocks surrounded by granite gneisses. The supracrustal belt is primarily made up of marble, calc silicate gneiss and khondalite.

Several phases of folding are recognized. The most conspicuous structure in the area is the Alambadi Fold related to the later phase of deformation. The western limb of the fold is N-S trending, which swings towards east at the hinge zone and becomes E-W trending in the eastern limb. It has a northeasterly trending steeply dipping axial plane and subvertical fold axis. Earlier than this episode of folding are isoclinal folds and sheath folds of varying dimensions on gneissosity and lithological contacts. The axial planes of a number of mappable early folds are bent by Alambadi Fold, the superposition giving rise to refolded fold patterns. The earlier folds belong to two generations, which cannot be distinguished at most places and hence are together referred to as the early folds. The late folds are coaxial with the earlier ones. The main event of metamorphism and migmatisation was coeval with the early deformation.