Structural Analysis in the Central Sector of the North Singhbhum Fold Belt in and Around Sini, Jharkhand District, Bihar, India

The study area is dominantly made up of pelitic, semipelitic schists, amphibolites and quartzite. The metasedimentary sequence has been intruded by granitic rocks in two phases. Signatures of polyphase deformation are clearly discernible in these rocks. The first phase of deformation (D 1 ) produced schistosity (S 1 ) with downdip lineation. Mesoscopic isoclinical folds are rarely seen. The second phase deformation (D 2 ) produced asymmetric folds on S 1 and axial planar crenulations cleavage (S 2 ). In the northern part S 2 has almost completely obliterated the earlier schistosity and the regional schistosity is here combined (S 1 /S 2 ). The last deformation episode D 3 gently folded S 1 and S 2 on transverse axial planes. Mylonites, ultramylonites and phyllonites are confined to the southern part of the area which represents the western extension of the Singhbhum Shear Zone. Mylonitic foliation within the feldspathic schist and micaceous phyllonite is parallel to S 1 . Isolated late stage shear zones occur throughout the area. Evidences suggest that shearing started during D 1 and occurred during different stages in the deformation history. The deformation events comprising D 1 , D 2 , D 2A and ductile shearing are parts of the same deformation cycle with N-S compression combined with simple shear with top-to-the-south sense of movement. Late stage longitudinal compression occurs during D 3 .