DEFORMATION HISTORY OF SOUTHERN GRANULITE TERRANE; A STUDY OF THE MYLONITE AND PSEUDOTACHYLYTE OF THE SALEM ATTUR AREA. TAMIL NADU, INDIA

Granulite terranes exhibit prolonged history of deformation and exhumation. Neoarchaean-Paleoproterozoic Salem-Namakkal tract of Southern Granulite Terrane (SGT) of India exposes large outcrop of tonalite-trondhjemite gneiss with batholic intrusion of charnockites. These are overlain by allochthonous sheet of basic granulites. The TTG–charnockitic basement has undergone multiple generation of folding comprising isoclinal recumbent F₁ fold superimposed by co-axial F₂ fold in NE-SW direction, producing hook pattern. F3 folds are in NW-SE direction and have produced domes and basin structures. Though basic granulites have undergone similar folding history, evidence of such folding is seen only in development of gneissosity fabric. Folds are rarely developed due to lack of competence contrast. Ductile shearing along the basal thrust has retrograded the granulite to amphibolite. The amphibolite layer varies in thickness from few meters to tens of meters and acts as decollement. Forward and backward thrusts splay from the basal thrust. The thrust has given rise to fault propagation fold that has produced large scale warping of the overlying basic granulites. However the pre-existing gneissosity in the basic granulite show different attitude from the underlying thrust. Thrusting has led to the exhumation of granulites to upper crust. Subsequently, conjugate brittle strike slip shear zones in 15^o and 310⁰ direction indicating a compression in N-S direction have been developed. These are emplaced by pseudotachylite veins that vary in width from few mm to few meters and length varies from few cm to 100 of meter. Gangavalli shear zone shows large emplacement of such pseudotachylite material in the charnockitic host rock suggesting a paleo-seismic zone. Assimilation of wall rock fragments is a common feature observed in the pseudotachylyte vein. Reaction of melt with nearby low temperature rocks give a distinct shape to pseudotachylyte and associated clasts; thus revealing melting process. Invaded clasts are mostly subrounded produced by decrepitating of grain boundary due to ultrahigh heat generated during friction event. Clasts form in this process follow a distinct pattern i.e. “Power Law Distribution”. This suggests prolonged ductile-brittle deformation history of the Southern Granulite Terrane.