LATE BRITTLE TECTONICS IN A PRECAMBRIAN DUCTILE BELT - EVIDENCE FROM BRITTLE STRUCTURES IN THE SINGHBHUM SHEAR ZONE, EASTERN INDIA

The Singhbhum Shear Zone, a 200 km long tectonic lineament, passes clo se to the boundary between the Archaean 'Singhbhum craton' and the Pro terozoic 'Singhbhum mobile belt'. Evidence from meso-structures reveal s development of this shear zone in a ductile tectonic regime (1.6-2.2 Ga.) by thrusting of the northern block over the southern block along the N-dipping shear surface. A hitherto unrecognized, late brittle te ctonic regime (less than or equal to 1.6 Ga.) comprising two discrete phases is established from dynamic analysis of the fractures and veins in the shear zone. Episodic extension parallel and normal to the shea r zone was a major consequence of this middle Proterozoic (less than o r equal to 1.6 Ga.) brittle tectonic episode, and was associated with the intrusion of a swarm of basic dykes.