Kinematics, strain pattern and geochronology of the Salem-Attur shear zone: Tectonic implications for the multiple sheared Salem-Namakkal blocks of the Southern Granulite terrane, India

Structural mapping, strain analysis, and a variety of geochronological studies were carried out to determine the tectonothermal evolution of the Salem-Attur shear zone in the Southern Granulite terrane of South India. The Salem-Namakkal blocks containing the shear zone consisted of quartzofeldspathic gneiss, charnockite and mafic granulite, and had undergone multiple phases of magmatism spanning over a period of 3.2-0.5 Ga. The rocks were deformed by four phases of deformation D-1-D-4 . The D-1 deformation was characterized by isoclinal and recumbent NE-SW trending F-1 fold with a pervasive subhorizontal axial planar granulitic fabric, S-1, and associated quartzofeldspathic leucosomes. Granulite metamorphism was dated at ca. 2.5-2.3 Ga. The F-1 fold and S-1 fabric were coaxially refolded by tight to isoclinal, upright to steeply inclined NE-SW trending F-2 folds during Dy deformation. The D-2 deformation was associated with F-2 axial planar shear zones, crenulations and leucosomes, S2 fabric. Large-scale D-2 shear zones characterized by high-temperature ductile shear fabric with a vertical flow host syntectonic syenite pluton which was dated at ca. 2.5-2.4 Ga. A P-T condition of 7 kb/600 degrees C was inferred for the D-2 deformation. The D-3 deformation was characterized by NW-SE to E-W trending F-3 folds and the Salem-Attur shear zone. The shear zone was a greenschist to amphibolite facies shear zone being characterized by mylonitic foliation and dominantly down-dip stretching lineation defined by quartz, biotite and hornblende minerals and dated at ca. 2.0 Ga. It indicated N-NNE vergence of thrusting with the mean kinematic vorticity number, Wm, as 0.7 suggesting general simple shear strain with 50% pure shear component. The D-4 deformation was manifested as NNE-SSW striking strike-slip faults and NW-SE striking extensional normal faults. Pseudotachylite veins having an age of 1.9 Ga injected during strike-slip faulting and granite-pegmatite veins showing age of 0.8-0.5 Ga intruded during normal faulting. The Salem-Namakkal blocks thus recorded a longlived shearing history. We suggest that the Salem-Attur shear zone and other shear zones such as Palghat-Cauvery, Moyar, Bhavani, Karur-Kambam-Painavu-Trichur and Achankovil shear zones, were Paleoproterozoic intraterrane shear zones which were overprinted by Meso-Neoproterozoic-Cambrian ductile and brittle deformations