Deformation and Metamorphism in the Western Part of Singhbhum Shear Belt Around Lapsa Buru and Kharsawan, Bihar

The rocks record four phases of deformation, of which the imprints of the first two are not readily recognisable. The deformation during the third phase was the most intense and is manifested in the northern part of the area by a set of E-W folds on an earlier schistosity and its transposition into a later schistosity. Towards the south, the transposition increases and culminates in the formation of the shear zone. The fourth deformation, restricted within the shear zone, indicates a relative downsliding movement of the northern block over the southern block. Two phases of regional metamorphism are recognised: (i) a progressive cycle, synchronous with the first deformation, and (ii) a progressive-retrogressive cycle, syn-to post-kinematic with respect to the third deformation. The later cycle shows a general increase of grade towards tbe north. The study records two highly contrasting, but grossly simultaneous, tectonometamorphic environments during the third deformation: (i) low stresses, low strain-rates, more of grain rotation and progressive metamorphism in the areas of folding in the north, and (ii) high stresses, high strain-rates, more of flowage and slip and retrogressive metamorphism in the shear zone in the south.