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Study of Small-Scale Structures and Their Significance in Unravelling the Accretionary Character of Singhbhum Shear Zone, Jharkhand, India

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Study of Small-Scale Structures and Their Significance in Unravelling the Accretionary Character of Singhbhum Shear Zone, Jharkhand, India J. Earth Syst. Sci. (2020) 129:227 Ó Indian Academy of Sciences https://doi.org/10.1007/s12040-020-01496-9 (0123456789().,-volV)(0123456789().,-volV) Study of small-scale structures and their significance in unravelling the accretionary character of Singhbhum shear zone, Jharkhand, India 1 2, ABHINABA ROY and ABDUL MATIN * 1Formerly Geological Survey of India, New Delhi, India. 2Formerly Department of Geology, University of Calcutta, Kolkata, India. *Corresponding author. e-mail: [email protected] MS received 1 April 2020; revised 22 July 2020; accepted 29 August 2020 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 Bnally 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 C0, 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 C0-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 Cattening fabric (coaxial pure shear) in the northern part (outside the SSZ), to simple-shear non-coaxial type deformation producing shear fabric, dominating over the Cattening 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 Cattening: X [ Y [ 1 [ Z), may indicate the stretching nature of the SSZ. From the orientation of oblique grain shape fabric [ISAmax (h \ 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 Cuid inCux, played an important role in strain softening during the development of SSZ. Keywords. Singhbhum shear zone; small-scale structure; shear lens; tectonic complex. 227 Page 2 of 25 J. Earth Syst. Sci. (2020) 129:227 1. Introduction 1993; Mitra 1978, 1984, 1992; Newman and Mitra 1993 and references therein) and the intrinsic High strain zones or shear zones are historically material property, such as lithology, mineralogical described since the later part of the 19th century. composition, grain size, etc. (e.g., Schmid 1975; However, detailed analysis of shear zones, from Sibson 1977 and references therein). The defor- geometry to kinematics, has been a subject of mation mechanisms also vary along a shear zone active research since the later part of the last due to changing physical conditions along the zone century. Shear zones are localized planar or (Sibson et al. 1981; Wojtal and Mitra 1988). Tec- curviplanar zones with higher accumulation of tonic melange, which is often described in colli- strain, typically have a dominant component of sional tectonics, shows diverse elongate blocks and simple shear although a component of pure shear is irregularly foliated matrix (Moores and Twiss commonly present, separated from less strained or 1995). The term melange, has been used as a unstrained zones of wall rocks with a strain gradi- descriptive and non-genetic term (Wood 1974; ent across the zones (Fossen and Cavalcante 2017 Silver and Beutner 1980; Raymond 1975, 1984; and references within). Micro- and meso-scale Cowan 1986), that does not restrict to the type of structures in naturally deformed rocks are lithological units involved (e.g., sedimentary, immensely useful in overall understanding of metamorphic or igneous) and the contact rela- deformation including sequence of fabric develop- tionships between these diverse lithological units ment, state of strain, strain rate and its variation can be tectonic, stratigraphic or intrusive, and deformation history of polydeformed terrains, depending on the process of melange formation particularly of ductile shear zones. The term duc- (Festa et al. 2012). In view of similar observations, tile is restricted here to (temperature-dependent) Cowan (1978) suggested that tectonic melanges are crystal-plastic deformation (c.f., Twiss and Moores structurally equivalent to faults, along which the 2007). The total strain within a heterogeneously tectonic dislocation ‘has expanded from a plane deformed rock in a shear zone is partitioned (i.e., fault) to a zone of several meters to kilometers between the more deformed part and less deformed in width (i.e., tectonic melange)’. Tectonic events part of the shear zone. However, the investigation represent the most prominent triggering mecha- of Hudleston (1999) on this issue reveals that even nism inducing, directly or indirectly, stratal dis- the very simple and idealized shear zone networks ruption and mixing that produce a broad spectrum generate large spatial variations in strain geome- of chaotic rock bodies. try, strain magnitude and vorticity, and local In this paper, we present outcrop- to micro-scale interpretation within a large shear zone are unli- structures including fabric development in differ- kely to be representative of the bulk deformation in ent lithotectonic units of SSZ, deformation char- the macro-scale zone. In large-scale shear zone acteristics and their kinematics of development in complex, it is expected to have irregular geome- the backdrop of progressive evolution of the SSZ. tries, with less predictable variations in strain and We make an attempt to understand the type of coaxiality. shear (simple or sub-simple) in the history of Kilometre-scale shear zones usually represent deformation and to assess whether the SSZ repre- the boundaries between different tectonic terranes sents a Precambrian tectonic complex zone in (e.g., fold and thrust belt) (Boyer and Elliot 1982; continental tectonic set-up. The term progressive Boyer and Geiser 1987) with large translations deformation is used in this paper in a sense, where (e.g., Elliott and Johnson 1980; Boyer and Elliot movements took place in relatively short time 1982; McQuarrie and DeCelles 2001 and references intervals with the development of multiple sets of therein) and the complex deformation history is structures (Tobisch and Paterson 1988). The commonly depicted in small-scale structures, variation of the structures, control of material developed in response to a progressive evolution of property (lithology/rheology of the rocks), Cuids a regional-scale shear zone (Simpson and Schmid and the P–T condition in the development of dif- 1983; Choukroune et al. 1987; Hanmer and Pass- ferent kinds of structures in SSZ are elucidated. chier 1991; Druguet et al. 1997; Fossen and TikoA From the structural characteristics, it could be 1998; Piazolo and Passchier 2002; Carreras et al. established that the SSZ might represent a zone of 2005; Passchier and Trouw 2005). The mechanisms anastomosing and coalescent fault/shear zones of deformation in the evolution of large-scale shear producing an aggregate of discontinuous alloch- zones depend on physical conditions (e.g., Evans thonous lithounits. This is comparable to a large J. Earth Syst. Sci. (2020) 129:227 Page 3 of 25 227 scale lithotectonic unit developed in deeper (duc- et al. 1975; Bhattacharya 1978; Mukhopadhyay tile) level counterpart of a shallow (brittle) level 1984; Ghosh and Sengupta 1987a, b; Mukhopad- tectonic melange zone. In our view, the SSZ does hyay and Deb 1995; Joy and Saha 1998, 2000; not readily Bt into the definition of an ideal ‘tec- Matin et al. 2012; Banerjee and Matin 2013 and tonic melange’. The different stages of the devel- references therein). They emphasized that the SSZ, opment of deformational fabrics and the sense of with a protracted history of progressive deforma- shear in all the discontinuous lithounits are enu- tion, had been reactivated several times (c.f., merated in the paper. This study has important Ghosh and Sengupta 1987a, b; Mukhopadhyay and implications forming a ‘melange-like tectonic Deb 1995; Matin et al. 2012). Tectonic slices/ complex’ belt in Precambrian terrain. wedges of basement
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