Study of small-scale structures and their significance in unravelling the accretionary character of Singhbhum shear zone, Jharkhand, India

Abstract

Localized strain within tabular ductile shear zones is developed from micro- to meso- to even large scales to form complex structures. They grow in width and length through linkage of segments with progressive accumulation of strain and displacement, and finally produce shear zone networks characterized by anastomosing patterns. Singhbhum shear zone (SSZ) represents a large composite zone characterized by a collage of different dismembered lithotectonic segments, with heterogeneous structural features, within a matrix typical of a shear zone. Structural features indicate that the material properties of protoliths have a great role in controlling the mechanics of deformation. Meso- and micro-scale structural studies of the east-central part of the SSZ reveal ‘tectonic complex like’ (? deeper level equivalent of melange type complex) assemblage of dismembered lithoteconic units. Shear-induced foliations, S, C and C′, were developed while the main mylonitic foliation is represented by C-plane. Apart from that, shear lenses are exceptionally well developed in both meso- and micro-scale in most of the units, particularly in schistose rocks. They were formed from different processes during progressive simple shear, which includes (1) anastomosing C-planes, (2) intersection between C- and C′-planes, (3) disruption of stretched out longer limbs of asymmetric folds, and (4) cleavage duplex. Fabrics recorded in rocks indicate that there was a progressive change in the development of predominantly flattening fabric (coaxial pure shear) in the northern part (outside the SSZ), to simple-shear non-coaxial type deformation producing shear fabric, dominating over the flattening fabric, in the southern part (within the SSZ) that is in close proximity with the Singhbhum Craton. Although an overall plane strain simple shear model is apparent, occasional presence of extensional features along two directions of the mylonitic foliation, demonstrative of three-dimensional deformation (simple shear and flattening: X > Y > 1 > Z), may indicate the stretching nature of the SSZ. From the orientation of oblique grain shape fabric [ISAmax (θ < 45°)], there is slight deviation from simple shear, i.e., a sub-simple nature of plane strain shear could be inferred. However, in conformity with simple shear model the ubiquitously developed stretching lineation shows consistency in orientation being parallel to the movement direction. There is no evidence of transpression. Shear sense indicators invariably indicate up-dip ductile thrust movement with vergence top-to-the south. Microstructural deformational characteristics indicate that peak temperature attained during the deformation in shear zone was ~600 °C. Prolonged period of metasomatism, induced by fluid influx, played an important role in strain softening during the development of SSZ.