Tectonic Amalgamation of Crustal Blocks along Gadag-Mandya Shear Zone in Dharwar Craton of Southern India

Abstract

Abstract The crustal scale Shear Zone that can be traced from Gadag in the north to Mandya in the south in Dharwar Craton of southern India is considered as the boundary between two subcratonic blocks namely the Eastern Dharwar Craton (EDC) and the Western Dharwar Craton (WDC) in published literature. The present study on the Gadag-Mandya Shear Zone (GMSZ) in the Javanahalli-Hagalvadi sector has brought out a detailed account on the disposition, geometry and kinematics of the shear zone, and also the distinctive structural patterns of the two adjacent supracrustal belts, namely the Chitradurga schist belt (CSB) in the west and Javanahalli schist belt (JSB) in the east. The JSB has an overall N-S striking and gentle easterly dipping geometry, the structural features of which are indicative of a predominant non-coaxial deformation and westward transportation of the supracrustal assemblage. In contrast, deformation in the CSB, which is defined mainly by a flattening type of strain, has produced an overall verticality of the structures (dominant foliation, axial planes of regional folds). Difference in metamorphic grade is apparent from the mineral assemblages in the rocks of these two belts, while the JSB showing an amphibolite facies CSB is restricted to green schist facies mineral assemblages. The 2 to 3 km wide, N-S striking, prominent ductile shear zone, GMSZ, has affected both CSB and JSB rocks and also the syntectonically emplaced Bukkapatna granite. The geometric and kinematic evidences from both the belts and the GMSZ point to an E-W horizontal compressional regime. The gentle dipping, amphibolite grade JSB and its underlying gneisses acted as an allochthon which got amalgamated with the subvertical, green schist grade CSB by a top-W thrusting along the GMSZ. The subvertical CSB impeded further up-dip movement of the eastern block, resulting in the subsequent development of N-S trending sinistral strike slip shear along the contact of the two contrasting belts to accommodate the continued E-W compression. The linear Bukkapatna Granite, synkinematically emplaced along GMSZ shows evidences of both magmatic stage and solid state deformation in the shear zone.