SINISTRAL SHEAR ZONE AS A TERRAIN BOUNDARY IN SOUTH INDIAN CRATON

Major ductile shear zone in Precambrian provinces usually demarcates boundaries of supracrustal belts and discrete terrains. In Dharwar craton of south India a major N-S trending shear zone represented by a steep dipping mylonite belt separates two such terrains. The western terrain is characterized by Late Archaean volcano-sedimentary rocks (Dharwar Supergroup) that were deposited on the pre-2900 Ma Peninsular Gneiss. The eastern terrain is dominated by Late Archaean (2750-2550 Ma) voluminous intrusions of granites, granodiorites, monzonites, and diorites of Dharwar batholiths, with intervening volcano-sedimentary schist belts. The eastern terrain has been interpreted as accretionary arc and the western terrain is, hence, regarded as a foreland to this accretionary arc.

The present area around Javanahalli in south India forms a part within the shear zone passing along the eastern boundary of Chitradurga schist belt of western craton. The different shear zone structures within the granites and amphibolites occurring along this N-S trending shear zone are consistent with sinistral transcurrent motion. The rocks have been mylonitised to different degrees from protomylonite to ultramylonite. The shear zone is characterized by the presence of mylonites, shear bands and shear lenses. Horizontal stretching lineation is consistently present in granite and amphibolite. Assuming a simple shear model of deformation it represent the movement direction. The presence of S, C and C’ traced on horizontal surface suggest horizontal transcurrent movement with a sinistral sense of shear. In amphibolite the mylonitic foliation is folded into sinistral folds. The fold axes vary from reclined to horizontal. The folds generated at an early phase of shearing were reclined. With progressive shearing they rotate to parallel with the direction of movement. The sinistral shearing along the terrain-boundary has been interpreted as due to WNW directed oblique convergence of an oceanic lithospheric plate on the eastern side against the N-S to NNW-SSE trending margin of an overriding continental lithospheric plate in the west. The sinistral transcurrent displacements are consistent with oblique convergence analogous to Phanerozoic convergent setting.