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 granite were emplaced as thrust sheets within the metasediments along the northern boundary of the Singhbhum Granite. Also 2. Geological background the mylonitised granitoid rocks (earlier designated as soda granite) may represent tectonically The Precambrian *1.6 Ga (Sarkar et al. 1986) emplaced slices of the basement rock in the Singhbhum Shear Zone (SSZ), which comprises an northern part of the SSZ (Mukhopadhyay et al. amalgamation of metasedimentary, metabasic, 1980). Sengupta and Mukhopadhyay (2000) opined metaultramaBc rocks belonging to Singhbhum that the SSZ ‘bears characters of tectonic melange’ Group (Chaibasa Formation), Dhanjhori Group, consisting of imbricate stack of tectonic horses, but Iron Ore Group(?) and granitic rocks, separates the did not elaborate in detail of the internal character Singhbhum Craton (SC) in the south from the and the nature/variation of deformation across the North Singhbhum Mobile Belt (NSMB) in the tectonic zone. north and it extends over a strike length of *200 Detailed studies on the SSZ have been carried km having a width of up to 4 km. The shear zone represents an important crustal-scale, northerly out by a large number of geoscientists in unravel- dipping, tectonic dislocation zone separating ling the fabric development and kinematics of the Palaeoarchaean to Mesoarchaean SC from the SSZ. Superposed deformation along with several Proterozoic NSMB in the north (Mukhopadhyay phases of ductile shearing marked by conspicuous 1976, 2001; Saha 1994; Mukhopadhyay and Matin shear induced foliation and near down-the-dip 2020), with top-to-south thrust movement of the mineral lineation was developed within the NSMB northern hanging wall (NSMB) over the southern close to the cratonic boundary (Ghosh and Sen- footwall (SC) (Bgure 1) (c.f., Mukhopadhyay gupta 1990; Mukhopadhyay and Deb 1995). The 1976, 2001; Saha 1994; Mukhopadhyay and Matin structures in the SSZ and its immediate footwall 2020). Since the pioneering work of Dunn and Dey and hanging wall blocks were the result of a single (1942), the overall geometry of the SSZ and its progressive non-coaxial deformation (Naha 1965; immediate footwall and hanging wall have been Mukhopadhyay et al. 1975; Mukhopadhyay 1984; thoroughly studied (Banerji 1959; Mukhopadhyay Ghosh and Sengupta 1987a; Joy and Saha 1998). It

Figure 1. Google Earth image of Precambrian belt of Singhbhum area. The position Singhbhum shear zone (SSZ), North Singhbhum mobile belt (NSMB) and Singhbhum Craton are shown. Present area of study has been boxed. 227 Page 4 of 25 J. Earth Syst. Sci. (2020) 129:227 is appropriate to note here that the layer parallel determine the deformation mechanism(s) in the shortening fabric represented by spaced to contin- SSZ (Sibson 1977; Blenkinsop 2000; Passchier and uous cleavage in the Galudih hills, about 2.5 km Trouw 2005). Stretching lineation, which typically north of the SSZ is very well-developed while in the indicate the transport direction in simple shear- Subarnarekha River section near Moubhander controlled thrust sheets, are very well-developed or (nearer to SSZ but outside the shear zone) both S1 preserved in all the mylonitised rocks. Stretching and S2 fabrics are excellently developed, here S2 is lineation in most exposures is down-the dip, sug- superposed on S1. A latter (third phase of defor- gestive of NE to SW transport of the hanging wall mation (D3) had crenulated the S2 schistosity and (NSMB) (Bgure 1). We have collected structural also give rise to regional curvature of the D2 fold orientation data and plotted them using GE-Orient axial traces (Mukhopadhyay et al. 2004). Although ver. 9.5.0 software (Holcombe 1994) to interpret not so intense in comparison to the earlier defor- large-scale structural geometry. Integrating all the mations, D1 and D2, the imprints of D3 could be data that were generated from our study, the recognized throughout the area around the shear structural evolution of the small-scale structures in zone including the SSZ rocks. The spectacular mica the backdrop of progressive development of the schist outcrops in the Tetuldanga area (*3km SSZ could be evaluated. And Bnally we have east of Ghatshila) demonstrate the successive attempted to examine whether the SSZ can be superposition of three generation structures (D1, considered as a ‘tectonic complex like’ sequence or D2 and D3) where all the three sets of folds (F1,F2 otherwise. and F3) and associated fabrics (S1,S2 and S3) are very well developed. Metamorphism in the SSZ has been considered 4. Results to have taken place under lower greenschist facies P–T conditions (Naha 1965; Sarkar 1984; Joy and 4.1 Lithotectonic units of the area Saha 1998). Sengupta et al. (2011) have demon- strated mobility of aluminium at deep crustal level SSZ is emplaced between the SC in the south and corresponding to kyanite stability Beld at the NSMB in the north (Bgure 1). The SC is one of the oldest cratonic blocks in peninsular India and inferred temperature of *480° ± 40°C and sug- gested an episode of metasomatism in SSZ and it is composed mainly of different supracrustal adjoining areas. assemblages and granitoid rocks ranging in age from *3.5 to 2.5 Ga. The NSMB, which is mainly composed of psammo-pelitic and pelitic rocks of 3. Methodology the Chaibasa Formation, Dhalbhum Formation and Dalma Volcanics and volcanogenic rocks, The work was motivated from the need to was thrusted over the SC along the arcuate SSZ constrain the internal structure of the SSZ, par- that formed close to the cratonic boundary ticularly the small-scale structures. We conBne our (Bgure 3)(Saha1994; Mukhopadhyay and Matin studies on the shear zone in the east-central part of 2020). The SSZ is constituted of a complex of the SSZ extending from west of Ghatshila in the SE dismembered blocks of strongly mylonitised to south of Jamshedpur in the NW (Bgure 1). The rocks (Bgure 3). This is denoted here as SSZ discovery of excellent exposure with small-scale complex since it comprisesofamixtureofdis- structures, although isolated in many places, membered heterogeneous lithounits ‘accreted’ within the SSZ covering the area from the imme- together in the zone. diate footwall through the central part to the immediate hanging wall of the shear zone (Bgures 1 4.1.1 Singhbhum shear zone and 2) are the catalysts for this work. We have studied the small-scale structures in outcrop- to The lithotectonic assemblage of the SSZ consists of hand sample-scale and recorded the different a structurally complex mixture of heterogeneous structures and their relative cross-cutting (geo- lithounits (Bgure 2). Rocks of diverse association metrical) relationships. We have also collected and supposedly of different ages, both from SC as around 50 samples from our study area to prepare, well as from NSMB, have been telescoped together oriented (regional transport parallel, i.e., NE–SW) to form a ‘collage’ of dismembered lithotectonic thin-sections for microstructural studies and to units, informally named here the SSZ complex J. Earth Syst. Sci. (2020) 129:227 Page 5 of 25 227

Figure 2. Geological map of the area of study (shown in Bgure 1). The outcrops of different lithotectonic units and their boundaries are fairly accurate but at places approximate due to lack of continuous exposures (see text). The area in between the lithotectonic units are covered by soil at the surface. Inset (a) shows stereo plot of structural orientation data of poles of the mylonitic foliation plane. The mean orientation of the mylonitic foliation is 312°/51°NE (number of data 31). The data show spread of the foliation poles along girdle,136°/39°SW, the b axis was computed as 51° ? 046°.

Figure 3. SimpliBed schematic sketch of regional cross-section across Singhbhum shear zone (SSZ) mark the structural contact zone between Singhbhum Craton and North Singhbhum Mobile belt with the SSZ composite tectonic zone in between. The composite tectonic zone consists of three components (see text). The position of the composite tectonic zone in deeper part is tentative.

(Bgure 2). It consists of a diverse suite of strongly between. The lensoid bodies are moderate to stee- mylonitised rocks, which include mylonite quart- ply inclined towards northeast and are subparallel zite, quartz-garnet-mica schists, metabasics, ban- with the general strike of the dominant mylonitic ded ferruginous chert/quartzite, granite mylonite, foliation (Sm) (Bgure 2). The boundaries of the quartzo-feldspathic schist and deformed conglom- lithotectonic units in the complex cannot be erate. All these rocks occur as impersistent sub- demarcated accurately due to lack of continuous elliptical blocks/lenses, trending *WNW–ESE to exposures in most of the places. Nevertheless, the NW–SE, having a strike length of tens to hundreds tentative boundaries are shown in Bgure 2. Some of of meters (Bgure 2). The area between the dis- the small exposures could not be shown in Bgure 2. membered units are now covered by soil, but we Conglomerate bands, that occur near Bisrampur suspect soil covered areas were of pervasively (22°4103400N/86°0900800E) and Dari villages deformed matrix of mylonitised zones, forming (22°3701300N/86°0903500E) (Bgure 2), are basal con- block-in-matrix arrangement. Our assumption on glomerate deBning unconformable contact with the subsurface continuity of mylonitised rocks has been basement granite (Mukhopadhyay et al. 1980). The authenticated from the bore hole cores in soil cover polymictic conglomerates are mainly of two kinds: areas. The individual lenses of the complex are clast-supported conglomerate (Bgure 4a) and exposed as disjointed blocks of mylonitised rocks matrix-supported conglomerate (Bgure 4b) with stacked together with intensely deformed matrix in arkosic to pelitic matrix. They are strongly 227 Page 6 of 25 J. Earth Syst. Sci. (2020) 129:227

Figure 4. Field photographs of structures in different lithotectonic units. (a) Clast-supported conglomerate on plan view. Yellow line (also the pen) marks the trace of the mylonitic foliation. Fracture (shown with arrow) seen in clast developed parallel to mylonitic foliation. (b) Matrix-supported conglomerate on plan view. Strongly developed mylonitic foliation in the matrix part (marked with line) swerve around elliptical clast which shows fracture (shown with arrow) across the clast. (c) Granite mylonite with trace of S- and C-plane on plan view. (d) Granite mylonite shows outcrop-scale side by side occurrence of ultramylonite (UI) and protomylonite (Pl). Isolated lens and pods of protomylonite (Pl) and ultramylonite (Ul) is seen. Mylonitic foliation (marked with broken yellow line) in ultramylonite zone is sheared to form C0-shear band cleavage plane (marked with continuous line C0). Tourmaline crystals (arrow with T) are seen in protomylonite zone. Island of ultramylonite lenses (shown with arrow) are present in protomylonite. deformed and show mylonitic fabric in the matrix less deformed to weakly deformed granitic rocks are part (Bgure 4b). Another small exposure of a few wrapped by mylonite–ultramylonite bands (Bgure 4d). metres thick and several metre long clast-sup- Around 500 m south of Pathargora (22°3201300N/ ported conglomerate body occurs near Tulia 86°26025.100E), the granitic protolith is strongly village. mylonitised and transformed to striped gneiss Granite mylonite is exposed near Rohinibera showing alternate dark coloured biotite-rich and light (22°3804300N/86°1902200E) and Miri (22°3204700N/ colour quartz-feldspar-rich bands. Quartzo-felds- 86°2700700E) villages. The granitic rock exposed pathic schist occurs in strike continuation of the near Rohinibera which is located close to the granite mylonite that is exposed near Miri village. Singhbhum Granite (occur in the peripheral part of Quartz-mica schist (sometimes appear as phyllonite), the SSZ and represent the basement slice), is exposed northwest of Jaduguda (22°4004100N/ moderately deformed and converted to granite 86°1801900E), is strongly mylonitised, characterised protomylonite. It appears as augen gneiss showing by millimetre-scale quartz-rich and white mica-rich augen of weakly deformed (semi-ductile) feldspar bands. Intercalated quartzite bands (up to a few tens and quartz-feldspar aggregates wrapped by a of cm thick) within the quartz–mica schist are also mylonitic foliation (Bgure 4c), On the other hand, strongly mylonitised and folded. Kyanite, which the exposure near Miri village, which is well within occurs as nodular grain aggregates in quartz schist, the SSZ, is very strongly deformed and mylonitised exposed northwest of Miri village are aligned parallel and shows evidences of conspicuous tourmaliniza- to mylonitic foliation. Kyanite also occurs as thin tion (±magnetite and apatite) and muscovitiza- veins of coarse monomineralic aggregates within the tion. Tourmaline is common and aggregates shear zone. of tourmaline crystals occur as clots or pods The Banded ferruginous quartzite (BFQ) bands wrapped by Sm of ultramylonite granite (now are exposed (Bgure 2) at two large scale outcrops, quartzo-feldspathic schist) (Bgure 4d). Metre and one near Turamdih (22°430900N/86°1102600E) and sub-metre scale isolated pods and lenses of the other near Nandup village (22°4304000N/ J. Earth Syst. Sci. (2020) 129:227 Page 7 of 25 227

86°1300800E). Quartzite bands occur near Rakha developed in different mylonitised lithounits in SSZ mines (22°37050.500N/86°22013.700E) and adjacent complex. In all rocks, an excellently developed to Rukmini Devi Temple (22°3803200N/86°1904800E) mylonitic foliation (Sm) is the main foliation that (Bgure 2). They are lens-shaped bodies that can be represents the C-plane of the mylonitic fabric. The traced over several tens of metres along strike. The foliation contains a strong stretching lineation rocks are strongly mylonitised showing a dominant throughout the area, thus, deBning a prominent mylonitic foliation and a very conspicuous down- L-S type of tectonite in SSZ. Although variable to the-dip stretching lineation in all exposures. some degrees, the pitch of the stretching lineation Whereas the original bedding lamination is either is grossly down-the-dip. The orientation of mylo- totally obliterated or not readily identiBable, the nitic foliation in the study area is measured and observed banding in the rocks represents a tectonic their poles were plotted (Bgure 2a, inset in fabric referred here as mylonitic foliation/bands. Bgure 2). The poles of the Sm show some spread on Quartz-feldspar-mica-garnet-schist (quartzo- a girdle indicating variation of their orientation feldspathic schist) is exposed in Gara Nala section from southeastern to northwestern part of the (22°4105900N/86°1605700E), north of Narwa Pahar, study area (Bgure 2a, inset in Bgure 2). This sug- and further southeast, north of Surda, Pathargora gests curviplanar nature of the mylonitic foliation; and some other places as discontinuous outcrops the b-axis orientation is 51° ? 046°. The mean (Bgure 2). In all the outcrops, the rocks show orientation of mylonitic foliation in the area is excellent development of mylonitic foliation. From *312°/51°NE (Bgure 2a, inset in Bgure 2). The microscopic studies (described later) it is under- mean orientation of the mylonitic foliation grossly stood that the rocks possibly represent a myloni- deBnes the orientation of the thrust plane/shear tised feldspathic schist derived from a zone boundary in the area of study. metasomatised granitoid protolith. The SSZ complex is characterized by the excel- Metabasics, now represented by Bne grained lent development of asymmetric structures, com- chlorite schist, occur at Rat Hole of old mine posite planar fabrics, scaly anastomosing foliations, activity (22°2505300N/86°605000E) north of Tur- C–C0 structures and abundant shear lenses in dif- amdih and south of Rakha Mine area. In the Rat ferent dimensions. All these structures are used by Hole area, the metabasic rocks show strong mylo- us as good and reliable shear sense indicators in nitic foliation and a stretching lineation and also SSZ. The oblique foliation, deBning by ISAmax, shear related features including shear lenses. The could also be used, at places, as a shear sense maBc–ultramaBc rocks, exposed far south of Rakha indicator. The different structures such as common Mines area, occur structurally below the quartzite fabric and folds are described in the following hill, show well developed mylonitic foliation. An section. early foliation predating the shear fabric could be recognised. Quartz veins, of different stages of emplace- 4.2.2 Deformation structures in conglomerate ment, are proliBc in all lithounits within the shear zone. They are commonly oriented parallel In clast-supported conglomerate, the clasts are tothemylonitefoliation.Atplaces,quartzveins Cattened and the long dimension of the deformed are aligned parallel or subparallel to the axial clasts are mostly oriented down-the-dip of the planes of the folds developed on mylonitic foli- foliation (Bgures 4a and 5a) deBning a pebble ationinbandedquartziteandBFQ.Some elongation lineation. The foliation is intensely quartz veins have participated in the shearing developed mainly in the matrix portion to deBne a process and are co-folded with the mylonitic mylonitic foliation (Sm) (Bgures 4a, b and 5a). foliation. Fractures are occasionally seen in some large clasts parallel to the mylonitic foliation in the rock (Bgure 4a). The fracture is parallel to the clasts 4.2 Description of small scale structures intermediate axis Y. It is perhaps developed during in the SSZ the early stage of deformation parallel to the XY 4.2.1 Mylonitic foliation plane of the Cattening. The clasts, particularly the smaller ones, are sub-elliptical in shape with Our studies are focussed on outcrop-scale to hand- tapering ends in section perpendicular to the foli- sample-scale and micro-scale structures that have ation and lineation (Bgures 4b and 5a). The 227 Page 8 of 25 J. Earth Syst. Sci. (2020) 129:227

Figure 5. Field photographs of structures in different lithotectonic units. (a) Matrix supported conglomerate on plan view (looking towards north) show isoclinal reclined fold (shown with arrow) with axial planar foliation trace (marked with line). Elongated clasts and fold axis of reclined fold oriented down-the-dip on mylonitic foliation. (b) Deformed clasts show strong stretching lineation (shown with arrow and marked with line) on foliation surface view in clast supported conglomerate. (c) Isoclinal reclined fold in quartzite with development of boudins in the stretched out limbs along E–W strike line in plan view. (d) Foliation surface view showing elongated clasts oriented along stretching (X) direction marking the down-the-dip lineation (see text). (e) Granite mylonite show banded structure deBned by alternate thicker protomylonite bands (white arrow) and thinner ultramylonite bands and lenses (blue arrow). (f) Striping lineation (traces of alternate light and dark colour C-plane) on C0-plane (see text) in mylonitised granite (now quartzo-feldspathic biotite gneiss), view from north on C0-plane. tapering tails of the deformed clasts are either bedded quartzite clasts are even deformed to symmetrical or near symmetrical, and, at places isoclinal reclined folds showing down-the-dip pitch asymmetrical too. In the matrix-supported con- of the fold axis (Bgure 5a). The mylonitic foliation glomerate, the intensity of deformation is much in the matrix of the matrix-supported conglomer- more pronounced compared to that in clast-sup- ate swerves round the deformed clasts (Bgure 4b). ported ones (Bgures 4b and 5a). The clasts in this Some of the elongated clasts show transverse conglomerate, irrespective of their size are strongly fracture across the long dimension (X-direction) of Cattened with very high aspect ratio in comparison the clasts indicative of extension failure in semi- to those in the clast-supported conglomerate brittle material during intense shearing. This is an (Bgures 4a, b and 5a). Clasts are aligned having evidence of strain partitioning, while the matrix is their long axis orientation along the down-the- undergoing intense ductile deformation, the clasts dip direction on mylonitic foliation. Some thinly yield to brittle failure during progressive J. Earth Syst. Sci. (2020) 129:227 Page 9 of 25 227 deformation. Spectacular stretching lineation is and ultramylonite even in scale of a single outcrop developed on the surface of the deformed clasts near Miri village (Bgure 4d) along the central part (Bgure 5b). of the SSZ. In the mylonite and ultramylonite There is an outcrop of ‘conglomerate-like’ rock zone, the rocks show strongly developed mylonitic restricted within mylonitised quartzite band south foliation (C-plane). In the adjoining less-deformed of Rakha mines area. The apparently pebble-look- pods and lenses of protomylonite granitoid rocks, ing ‘clasts’ give false impression of a conglomerate. thin isolated Blms and lenses of ultramy- From a critical observation, the rock is interpreted lonite–mylonite is developed in the ‘sea’ of weakly to be of autoclastic origin resulting from stretching deformed protomylonite granitoid (Bgure 4d). The of quartzite layers in two directions (X and Y) overall look of the deformed granitic rock shows parallel to the mylonitic foliation forming two sets ‘island-channel’ structure with ‘channels’ of Bne- of boudinage structure. Both the matrix and the grained mylonite/ultramylonite engulfed in a less clasts are made up of same quartzose material. The deformed granitic host (Bgure 4d) and vice-versa. boudinage structure can be viewed both in plan C0-shear band cleavage is developed on the and on section parallel to the Sm (Bgure 5c and d). mylonitic foliation (C-plane) (Bgure 4d). Alter- While the longest axis of the boudins as viewed on nate bands of protomylonite and ultramylonite the mylonite plane represent the X-axis and could be noticed (Bgure 4d). At places, the direction of transport (Bgure 5d), the intermediate banding is quite conspicuous deBned by thicker axis as exposed on plan view is subparallel to protomylonite bands alternating with thinner Y-axis of local strain (Bgure 5c). Often the quart- ultramylonite lens and bands (Bgure 5e). South of zite bands are isoclinally folded with stretched out Pathargora, the quartzo-feldspathic gneiss (my- limbs accompanied by the development of a set of lonitised granite) that also occur along the central fractures perpendicular to the layers/mylonitic part of the SSZ, show the development of strong foliation. The fractures may represent either mode shear fabrics (C and C0), and also the formation of I tensile fractures or extension fractures due to shear lenses due to intersection of C and C0-plane. horizontal stretching of shear zone. This is indica- Usually the C0-shear band appears as discrete tive of three-dimensional deformation. While the dark streaky lines on plan, while the C-plane is down-the-dip stretching (X-parallel) lineation is marked by light and dark colour bands. However, explained by thrust-related movement, the sub- when the C0-shear band becomes more prominent horizontal stretching (Y-parallel) may suggest and pervasive, the trace of the C-plane appears as lateral stretching characteristics of stretching shear ‘striping lineation’ on C0-shear band representing zones. This is a slight departure from a simple an intersection lineation between C- and C0 sur- shear plane strain model and may argue in favour faces (Bgure 5f). of plane strain sub-simple shear (c.f., Simpson and The granite mylonite near Rohinibera, near the DePaor 1993; Baird and Hudleston 2007; Fossen southern boundary of the SSZ, occurs as weakly and Cavalcante 2017) in some localized domains. mylonitised basement slices, is less intensely The sub-simple shear involves a combination of deformed (protomylonite to mylonite). It shows simple shear and pure shear both applied on the weak mylonitic foliation deBned by parallelism of same plane (in the present case, the mylonitic coarse-grained quartzo-feldspathic lenses bordered foliation plane Sm). The observed three-dimen- by thin bands and Blms of recrystallized Bner sional deformation feature can be explained by grained aggregates of quartz and feldspar simply applying a plane strain sub-simple shear (Bgure 4c). The rock gives an appearance of an model as opposed to non-plane strain transpression augen gneiss mylonite showing S-C fabric (Berthe type shear zone (c.f., Sanderson and Marcini 1984; et al. 1979; Lister and Snoke 1984)(Bgure 4c). The Passchier 1994, 1998; Fossen and TikoA 1998; feldspar porphyroclasts show internal crystal Passchier and Cohelho 2006; Fossen and Caval- plastic strain and are oriented parallel to S-surface. cante 2017). Recrystallized domains are thin and are developed mostly parallel to C-plane (Bgure 4c). The Bner grained thinner recrystallized mass swerve around 4.2.3 Deformation structures in granite the coarse grained lenses (Bgure 4c). The basement slice of granite near Bisrampur, where granite The granitic protolith is mylonised in different thrusted over the deformed conglomerate, is degrees to give rise to protomylonite, mylonite weakly deformed to form mylonite. 227 Page 10 of 25 J. Earth Syst. Sci. (2020) 129:227

4.2.4 Deformation structures in BFQ quartzite development of Sm and C0-type shear band cleav- age with occasional development of shear lens. The banding observed in the BFQ is in fact a Distinct shear planes/bands (C0-shear band cleav- mylonitic banding, deBned by recrystallized quartz age) are developed at an acute angle with the main and magnetite/hematite grains. The original bed- mylonitic foliation (Sm). In quartzo-feldspathic ded character of the BFQ has been modiBed by gneissic rock, that occurs adjacent to the quartz- intense deformation (c.f., Ghosh and Sengupta feldspar-mica schist, C0-cleavage is either repre- 1990). This is conBrmed from our microscopic sented by the shear band cleavage or by discrete studies. ‘Striping’ lineation is very prominent in fracture plane developed along the C0-shear band the mylonitised BFQ and banded chert. Although cleavage (Bgure 6d). morphologically the lineation appears as a striping The mylonitic foliation is, at places, weakly lineation, it has been interpreted to represent a crenulated having sub-horizontal or gently plung- transposed linear structure of an early formed ing crenulation axis while the stretching is nearly intersection lineation between bedding and axial down-the-dip showing high angularity between planar cleavage (Sengupta and Ghosh 2007). them. The crenulation may represent an initial Therefore, in theory, the early generation striping stage of deformation in a particular cycle of pro- lineation is presently oriented (either rotated or gressive shearing event, the axis of which becomes transformed) parallel to later developed stretching rotated to parallelism with the stretching linea- lineation. A strong mineral lineation is also devel- tion, i.e., the movement direction with ongoing oped parallelly to the stretching lineation, and both deformation. (early striping lineation and later shear induced stretching lineation) combined could have impar- ted a ‘striping-like’ appearance of this composite 4.2.6 Shear lens lineation (Bgure 6a and b). Since this composite lineation is restricted in the strongly mylonitised The quartz-feldspar-garnet-schist in Gara Nala BFQ, it is difBcult to resolve this issue conclu- section show excellent development of small scale sively. This lineation is deformed and folded by shear lenses. The term shear lens (of cm-scale) is progressive folding events in the sheared rock used here following Ghosh (1993). Individual (Bgure 6b), a detailed account of which was given shear lenses consist of compositely foliated by Ghosh and Sengupta (1990). The different schistose rocks appear as banded structure with lineation patterns have resulted from ongoing alternate light and dark colour bands (Bgure 6e). progressive simple shear deformation. The composite foliation is represented by a transposed schistosity (S2)(Bgure 6e) (see Matin 4.2.5 Shear related foliation and lineation et al. 2012). The earlier foliation (S1)andthe in other rocks transposed foliation (S2) represent the earlier formed Cattening type fabric pre-dating shearing In chlorite schist (metabasic protolith), Sm and in the SSZ. The S2 foliation plane is superposed stretching lineation is very well developed. C0- by the shear planes (C-plane) to form the shear plane is developed in a discrete manner on C-plane lenses (Bgure 6e). The angle between the S2 which is deCected along the C0-plane to produce plane, contained within shear lenses, and the shear band or ‘extensional crenulation cleavage’ shear bands (marked by the boundary of the (also known as shear band cleavage and C0-type shear lenses) shows a systematic variation from shear band cleavage, see Passchier and Trouw the central part to the peripheral/marginal part 2005)(Bgure 6c). Chlorite schist, occurring about 2 of the shear lenses; S2 is asymptotic and become km south of Rakha Mines close to the Singbhum near parallel to the shear lens margin (shear Granite basement, is weakly mylonitised, where it band), while in the central part the angle shows the development of S-C structure and a increases up to 35° between the S2 plane and the stretching lineation having a pitch of *50 W on shear band (Bgure 6e) imparting a sigmoidal the C-plane (Sm). With increase in intensity of shape of the S2 plane within the lenses. Individual shearing, S-plane gradually disappear and C-plane shear lenses are curved and ‘climbing’ one over deBnes the main foliation. Quartz-feldspar-mica the other to produce a stack of shear lenses schist near Miri village that occurs in strike con- (Bgure 6e and f). The boundary surface of the tinuity with granite mylonite shows excellent shear lenses (shear bands) are smooth and, at J. Earth Syst. Sci. (2020) 129:227 Page 11 of 25 227

Figure 6. Field photographs of structures in different lithotectonic units. (a) Strong down-the-dip stretching lineation, appear as striping lineation, (parallel to pen) on mylonitic foliation in BFQ. (b) Mylonitc foliation with striping/stretching lineation folded to isoclinal fold (shown with arrow) in BFQ. Note hinge is curvilinear. (c) Mylonitic foliation (C-plane) and C0-plane (marked as C0-plane) in chlorite schist. Quartz veins (Q) oriented parallel to C-plane also sheared along C0-plane. C-plane folded to open dextral fold in section from west. (d) Well-developed C- and C0-plane in quartzo-feldspathic gneiss. Shear band cleavage developed along C0-plane and also discrete fracture is seen to develop along C0 plane. (e) Shear lens bounded by shear plane (boundary marked with yellow chain line). Bent S-shaped shear lens consist of schist with transposed foliation (sigmoidal trace of transposed foliation marked with blue line). (f) Stack of sigmoid-shaped shear lenses (one marked with yellow line) are ‘dragged’ along a shear plane (marked with blue line). Arrow points to riding of shear lens over other. places, shows Bne slickenline lineation suggesting 4.2.7 Shear related folds relative movement of the lenses one over the other along their boundaries. At places, as Folds on the dominant mylonitic foliation (Sm) observed in vertical sections that a later shear are common in different lithotectonic units plane, discrete in nature, was developed along including quartz veins in SSZ. They are often which the shear lens aggregates are ‘dragged’ non-cylindrical, consistently asymmetric, (Bgure 6e and f). The curved geometry (Bgure 6e S-shaped (sinistral) in section from east or and f) and riding over nature of shear lenses are Z-shaped (dextral) in section from west and the common. At places, in schistose quartzite and vergence of the folds indicating top-to-south quartzo-feldspathic schist, C-plane is over- movement. The tightness of the folds varies printed by C0-type shear band cleavage to widely from open to isoclinal, though predomi- develop shear lenses. nantly they are tight or isoclinal in nature. The 227 Page 12 of 25 J. Earth Syst. Sci. (2020) 129:227 orientation of the axial plane varies; the axial predominantly reclined in orientation and show trend usually parallel to the strike of the Sm; the plane non-cylindrical geometry. dip of axial plane varies from moderate to near Quartzite is converted to quartzite mylonite in vertical making variable angle with the which the Sm with strong stretching lineation are enveloping surface deBned by Sm. Geometry of deformed to give rise to isoclinal folds (Bgure 7c). the folds varies from upright (when non-plunging The fold hinges are gentle to moderately plunging or low plunging), inclined (when low to moder- (Bgure 7c). Stretching lineation is also developed in ately plunging) and reclined (when moderate to the quartz veins. steeply plunging). Tightness of the folds also In the mylonitised metabasics (chlorite schist) of increases from upright (open) through inclined the Turamdih area, the mylonitic foliation is folded (close to tight) to reclined (tight to isoclinal) into open to near isoclinal Z-shaped fold viewed type. Further, with increase in tightness, the in section from west, indicating top-to-south folds change from Type IB to Type 2 geometry of movement sense (Bgure 7d). Ramsay (1967). The variation of fold geometry Quartz veins are usually emplaced parallel to can be observed in a single large outcrop. They the Sm and co-folded with the mylonitic folia- are interpreted to have developed due to pro- tion of the host rock. The geometry of the folds gressive simple shear and in a number of cycles of in quartz veins, oriented parallel to the mylo- events (Ghosh and Sengupta 1987b). All the folds nitic foliation, folded along with the Sm, varies in a shear zone initiate (having viscosity contrast from tight to isoclinal asymmetric folds. The between layers) as buckle folds (Type 1B) and long limbs of the folds in quartz veins are often with progressive shear deformation they change stretched to show pinch-and-swell and boudi- gradually from Type 1B to near Type 2 geome- nage structure and the lithons of fold hinges try. The asymmetric folds can be used to obtain appear as shear lens. The shapes of the boudins the overall sense of shear. Ghosh and Sengupta are elongate ellipsoidal with stretched-out ends (1987a, b) have recognized three generations and the Sm swerves round the boudins of reclined folds including three-dimensional (Bgure 7e). At places, folds in quartz veins exposures of sheath folds. appear as Sorbey’s Bsh-hook type (Bgure 7f). In In mylonitised BFQ, the mylonitic foliation some locations, particularly near the Rat Hole along with the stretching lineation are folded into north of Turamdih the quartz veins appear as wide range of asymmetric folds varying from iso- ‘spiral-like’ structure comparable to ‘shear lens clinal (Bgure 6b) to open inclined types. The like’ geometry (Bgure 7g). The ‘conical shaped’ plunge of the folds varies widely and the hinge lines spiral like structure results from the develop- often show curvilinear pattern; from nearly sub- ment of C0-shear band cleavage over the C-plane horizontal to down-the-dip on axial plane forming (Sm); quartz veins got stretched along the C0- inclined doubly plunging and reclined folds shear band cleavage to form tapering ends (Bgure 6b). Although rarely preserved, the expo- (Bgure 7g). sures of sheath folds could be recognized in the BFQ unit near Dungridih village (Bgure 7a). Axial planar cleavage, when formed, is mainly restricted 4.3 Description of micro-scale structures in the hinge zone of the folds. Quartz veins parallel to the axial plane of the folds are common. The For study of micro-scale structures, thin sections stretching lineation on mylonitic foliation is also have been prepared from rock sections oriented folded to form deformed lineation, which is perpendicular to Sm and parallel to stretching well developed near Turamdih and Dungridih lineation (where lineation is seen in hand sample). (Bgure 6b). The geometry and kinematics of this The microstructures seen in different lithounits, deformed lineation have been discussed elaborately are characteristics of crystal-plastic deformation by Ghosh and Sengupta (1990) and Ghosh and typical of an ideal ductile shear zone (c.f., Pass- Chatterjee (1985). At places, the folds in BFQ chier 1990). Under microscope, the dominant foli- show stretched and thinned limbs and thickened ation in all lithounits is represented by C-plane hinge zones (Bgure 7b) and thin quartz vein (Sm). S-planes are mostly obliterated and occa- emplaced parallel to the axial plane of the fold sionally occurring as relicts in between C-planes. shows evidence of slip (Bgure 7b). The folds Obliquely formed C0-shear bands appear as exten- developed in banded quartzite mylonite are sional crenulation cleavage. J. Earth Syst. Sci. (2020) 129:227 Page 13 of 25 227

Figure 7. Field photographs of folds on mylonitic foliation and boudin structure in different lithotectonic units. (a) Eyed fold in BFQ on plan, outline is marked with broken line. (b) S-shaped asymmetric fold in BFQ show thinned long limbs and thickened short limb. Axial plane parallel thin quartz vein (shown with arrow) along which bands are sheared. (c) Mylonitic foliation with lineation in quartzite band in quartz-mica schist folded to isoclinal fold in section from west. Stretching lineation (Lin, marked with blue line) was folded by the fold. (d) Mylonitic foliation in chlorite schist (Sm) folded to Z-shaped fold in section from west. Axial planar cleavage (cl) is developed in the hinge area. (e) Quartz veins (Q) in mylonite quartz mica schist show boudin structure on plan, boudins are elliptical with narrow stretched out ends. (f) Sorby’s Bsh hook type fold developed in mylonitised quartz vein (Q). Mylonitic foliation in quartz vein (shown with arrow) was also developed. The yellow line marks the axial trace of the fold. (g) Spiral-shaped structure developed in quartz vein (Q) in mylonitised chlorite schist in section from west. The mylonitic foliation (Sm) is sheared along with the quartz vein by C0-shear plane (shown with broken line C0).

The quartzite mylonite adjacent to Rakha grains with tapering ends are present as relicts, Mines, is represented by mylonite to ultramylonite. which are usually wrapped by thin Blms/streaks of A few elongated elliptical porphyroclasts of quartz phyllosilicates commonly muscovite (Bgure 8a). 227 Page 14 of 25 J. Earth Syst. Sci. (2020) 129:227

Figure 8. Plane and crossed polarised light photomicrograph showing (a) Quartzite ultramylonite shows intensely developed mylonitic foliation with a few elongated porphyroclasts (C) preferentially oriented parallel to the mylonitic foliation (marked with line Sm). (b) Shape preferred oblique foliation (marked with blue line) developed in granite ultramylonite. Thin lenses and Blms of mica are at acute angle to the oblique foliation. (c) Shape preferred wavy mylonitic foliation (marked with yellow line – Sm) developed in granite ultramylonite with porphyroclasts (Cl) of partly recrystallized boudinaged feldspar and neck fold. (d) Incipiently recrystallized tourmaline porphyroclast (T) shows intragranular fractures (shown with yellow arrow) and undulose extinction. Recrystallized boundary is shown with blue arrows. (e) Quartz mica schist shows C0-plane (yellow line) developed oblique to mylonitic foliation in mica schist. (f) Mica Bsh structure (shown with blue arrow) in micaceous quartzite mylonite. Mica Bsh is near parallel to the mylonitic foliation (Sm). (g) Mylonitic foliation in quartz mica schist is folded to tight folds with sharp hinge in the mica-rich domain and rounded hinge in the quartz-rich domain. In mica-rich domain discrete C0- shear plane (marked with arrow) is developed. (h) Foliation Bsh (Sl) are stacked together in mica-rich domain of mylonitised mica schist. Mica Cakes within foliation Bsh show sigmoidal curvature. Shear planes marked with yellow arrows. Inset shows trace of foliation (Ft) and shear lens boundary (Lb) of a shear lens (marked with white arrow). (i) Mylonite quartz mica schist show shear planes (C and C0-planes) are at an acute angle (marked with arrow). At places they meet to form shear lens. (j) Mylonitic foliation (Sm, yellow line) swerves (marked with arrow) around the garnet porphyroblast (Gt) with curved internal schistosity.

The type of fabric in which elliptical porphyro- structure is rare and could be observed occasionally clasts are embedded in a Bne grained mylonitic in some samples. Similarly, the porphyroclasts matrix is denoted as ‘Caser type’ mylonitic fabric with r- and d-type structures are either rare or (Passchier and Trouw 2005). Core-and-mantle absent. The porphyroclasts of quartz and feldspar, J. Earth Syst. Sci. (2020) 129:227 Page 15 of 25 227

Figure 8. (Continued.) when preserved, show subgrain formation sugges- The grain shape preferred alignment of the tive of internal crystal plastic strain. In banded recrystallized grains, which is at an acute angle quartzite mylonite/ultramylonite, the matrix of with the main mylonitic foliation (Sm) deBnes the Bnely recrystallized aggregates swerves around the oblique foliation (Bgure 8b). The angle between coarser porphyroclasts (Bgure 8a). The aspect ratio the oblique foliation or ISAmax and the main shear of the elongated porphyroclasts varies widely from plane (Sm) varies from 20° to 40°, average being *2:1 to nearly 15:1. Sm is marked by thin streaks *308. This may indicate sub-simple nature of of mica-rich and quartz-rich domains where por- shear, that is, an addition of pure shear with phyroclasts are absent or completely recrystallized simple shear at least in some localised domains in (ultramylonite), while the bands and lenses are SSZ. Some sections show mica-rich seams and comparatively thick where porphyroclasts of lar- Blms in the recrystallized groundmass of quartz- ger size are present (Bgure 8a). The porphyro- feldspar and mica, aligned parallel to the Sm with clasts are recrystallized mainly along grain elongated porphyroclasts of feldspar (Bgure 8c). boundaries. The recrystallized grains are usually At places, partially recrystallized mantled feld- strain free and show mainly granoblastic texture spar porphyroclasts with asymmetric tails that (see Banerjee and Matin 2013) possibly due to appear as microboudin. The Sm in the ground- steady state grain growth overlasting dynamic mass of the feldspar porphyroclasts is bent in the recovery-recrystallization. The granite mylonite neck of the boudinaged porphyroclast of feldspar shows variation ranging from protomylonite to (in between the boudinaged feldspar porphyro- ultramylonite, the mylonite to ultramylonite clast) is seen (Bgure 8c). Folding of Sm in the being the dominant ones containing only few relict extremely thinned part of the boudinaged clasts porphyroclasts ‘Coating’ in a Bnely recrystallized (in the neck part of the boudins), is also observed matrix of quartzo-feldspathic mass and aggregates and that points to progressive rotational strain of mica Cakes. (Bgure 8c). 227 Page 16 of 25 J. Earth Syst. Sci. (2020) 129:227

Porphyroclasts of tourmaline or tourmaline other one is oblique and curved forming a network aggregates show intragranular fractures, weak of shear planes (Bgure 8i). intracrystalline ductile deformation marked by undulose extinction and only incipient recrystal- 4.4 Time relationship between deformation lization along grain boundaries (Bgure 8d). Other- and metamorphism and growth wise tourmaline grains, which occur mostly in of porphyroblasts aggregates, escape ductile deformation and major alteration. In quartz mica schist, the Sm (C-plane) Garnet and occasionally staurolite porphyroblasts is marked by alternate quartz-rich and mica-rich are common in quartzofeldspathic schists exposed domains. C0-shear band cleavage is developed in Gara Nala section and north of Pathargora. oblique to Sm (Bgure 8e). Microstructural studies on Si–Se tectonites indi- Mica Bsh structure is well-developed in mica- cate that the growth of the porphyroblasts is syn- ceous quartzite mylonite (Bgure 8f). They have tectonic with S2 foliation development (Bgure 8j). been shown empirically to be reliable shear sense The shear related mylonitic foliation (C-plane) is indicators (e.g., Lister and Snoke 1984), based on superposed on S2, which could be demonstrated their asymmetrical shape and stair stepping of the in both mesoscopic and microscopic scale of trails. Most of the mica Bsh structures observed in observation. Garnet porphyroblasts also show the area belong to Group 1 and 2 types and rarely evidences of rigid body rotation due to shearing. Group 5 type (ten Grotenhois et al. 2003; Passchier All these evidences are indicative of porphyrob- and Trouw 2005). The Sm was deformed to pro- last growth predating the shearing event. It is to duce tight folds (Bgure 8g) with the development of be noted here that porphyroblasts of garnet, sharp hinge in the mica-rich domain and rounded staurolite, andalusite, chloritoid, etc., have also hinge in the quartz-rich domain (Bgure 8g). In formed, broadly synchronous with S2 schistosity, addition to mica Bsh structure, foliation Bsh (c.f., in appropriate lithologies away from the shear Trouw et al. 2010) is abundant in mica schist and zone (e.g., Subarnarekha River section at Ghat- in the phyllonite/phyllonitic mylonite (Bgure 8h shila) in NSMB. Development of a weak down- and i). Foliation Bsh structures are polycrystalline dip mineral lineation shown by garnet and stau- lenses, exhibiting similar shapes as mica Bsh. rolite grains may again suggest that the growth Foliation Bsh shows transition to C/C0-fabric and of porphyroblasts might have continued and appear as micro-scale shear lenses stacked together transgressed the shearing event. There may not to form composite mylonite bands (Bgure 8h and be much time gap between the Cattening (S2) and i). They are conBned by anastomosing C0-shear shearing (C-plane) events in the area. The peak band cleavage planes (Bgure 8i and inset in 8h). of metamorphism might have attained at the The shear lenses are of varying sizes, stacked advanced stage of Cattening and initiation of together along distinct shear planes (Bgure 8h). In shearing in SSZ. eAect, each lens is bounded by anastomosing shear There are several stages of metasomatism in the planes (C0-cleavage). The internal fabric, deBned shear zone and adjacent rocks. Development of by mica Cakes, within the individual lenses is ori- garnet and staurolite porphyroclasts in quartzo- ented differently with respect to the adjacent len- feldspathic schist (mylonitised granite) can be ses. It shows sigmoidal character (curved foliation) explained by metasomatic activity leading to in some lenses where the orientation of mica Cakes chemical alteration of the protolith prior to or changes from high angle in the central part to synchronous with peak metamorphism and defor- become near parallel along the shear lens bound- mation. Similarly, the abundance of tourmaline, aries, i.e., asymptotic with both the boundaries apatite, magnetite as thin lenses or stringers in (Bgure 8h inset). Shear lenses are preferentially mylonitised granite may argue in favour of inCux of developed in one limb of the asymmetric micro metasomatic Cuids. Apatite and magnetite, which folds (Bgure 8g). Such features are conspicuous in also occur as discrete grains, behave as rigid por- ultramylonitic rocks at Jaduguda. The composite phyroclasts embedded in mylonitised matrix in the bands are again folded and/or traversed by shear shear zone rocks. Presence of chlorite pseudo- planes during progressive non-coaxial Cow. At morphs after garnet suggests late stage metaso- places, there are two sets of shear planes appearing matism in the shear zone. The inCux of Cuids might as anastomosing shear zone (Fossen and Caval- have greatly facilitated the strain softening process cante 2017); one set is more or less planar while the in the shear zone. J. Earth Syst. Sci. (2020) 129:227 Page 17 of 25 227

5. Discussion and interpretation domains of relatively less deformed rocks, i.e., shear lenses, are observed throughout the area and Mylonites generally develop by strong ductile in particular in Gara Nala section. Here, the two crystal-plastic deformation in zones of intense non- planes (S and C) represent neither the conjugate coaxial Cow (Bell and Etheridge 1976). It is a set of movement planes, nor a strain partitioning steady-state fabric that reCects competition of two involving simultaneous development of a Cattening opposite eAects; a process of deformation tending (S-) and shear (C-) planes and hence not compa- to produce grains elongated parallel to main elon- rable to sensu stricto C/S fabric (e.g., Berthe et al. gation directions of the incremental strain ellipsoid 1979; Lister and Snoke 1984). Oblique foliation, and recrystallization tending to form new equidi- therefore, has the same kinetic significance as the mensional grains free of strain (Trouw et al. 2010). S-planes in C/S shear bands lagging behind in From our observations, it is understood that all orientation with respect to total strain ellipsoid lithotectonic units in SSZ show a consistent and (Trouw et al. 2010). On an average, the oblique prominent mylonitic foliation (Sm) representing foliation in SSZ make an angle between 308 and 43° the shear(C)-fabric. Although S- and C0-planes are with the main mylonitic foliation, possibly indi- also identiBable separately in some lithounits, cating an overall simple shear with minor pure C-surface is the dominant planar surface that shear component of deformation. represents the regional foliation in the study area In our study area, S-surface represents an earlier within the shear zone. In intensely deformed duc- developed Cattening plane (S2), which was subse- tile shear zone like the SSZ, the mylonitic foliation quently superposed by the C-plane, related to within the shear zone is nearly parallel to the shear shearing. It has been already discussed that the zone boundary, which supports an overall simple area north of the shear zone, i.e., Singhbhum shear model (c.f., Ramsay and Graham 1970). Group of rocks, have undergone pure shear defor- With increase in strain due to progressive defor- mation, while the shear zone resulted from simple mation, the angle between the C- and S-surfaces is shear deformation, both in ductile regime, there is reduced and in the high strain domain only the distinct strain compatibility (continuity of ductile C-plane is identiBable represented by the main deformation) along the northern margin of the mylonitic foliation (Sm). In low strain domains, SSZ. The southern margin of the SSZ is delimited particularly near the southern margin of SSZ near by the Singhbhum Granite basement. From our Rohinibera, the granitic rock (? basement granite) study, it is apparent that the contact zone has is converted to protomylonite showing distinct S- undergone brittle–ductile deformation causing and C-planes (Type I S-C mylonite of Lister and interleaving of mylonitised granite (basement rock) Snoke 1984) (see Bgure 4c). Eventually, a third set and the shear zone rocks (metasediments including of plane (C0-plane) which initiate at a later stage conglomerates). This is indicative of strain incom- and is seen mainly in the schists, is always discrete patibility along this southern margin of SSZ, which in nature and maintain an acute angle (*20–25°) may lead to the development of a stretching fault. with the C-surface may be formed at high angle to This is in accordance to the idea of ductile maximum extension direction of Bnite strain in thrusting as proposed by Ghosh and Sengupta stretching shear zones (Passchier 1991). Accord- (1987b). A schematic diagram is presented to show ingly, the SSZ, which shows conspicuous develop- the nature of the deformation across the SSZ, from ment of C0-plane, can be assumed to have brittle–ductile thrust imbrications in the southern undergone stretching parallel to the main C-sur- part near the basement granite, ductile thrust zone face. However, C0-plane may even form in foliated in the central part to transition zone with the mylonites at high amount of simple shear strain, Cattening type deformation of the NSMB in the when the mechanical anisotropy of the intensely northern part (Bgure 3). We call these three zones deformed mylonitic foliation is an important con- as SSZ composite tectonic zone (see Bgure 3). trolling factor under the situation. A C/S fabric All the lithotectonic units within the SSZ show may represent a conjugate set of two foliations, intense development of C- and C0-fabrics in some which were synchronously formed during a single units, strong stretching lineation and multiple progressive, non-coaxial deformation event (Hip- cycles of folding including rotation of fold hinges, pertt 1999). High strain mylonites commonly show etc., are indicating protracted nature of the subparallel sets of S- and C-planes. Apparently shearing deformation. It could thus be inferred that C/S-like geometries, which surround lenticular the different lithotectonic units in SSZ might 227 Page 18 of 25 J. Earth Syst. Sci. (2020) 129:227 represent telescoped allochthonous lenses stacked suggested several stages of recrystallization of the together as a result of ductile shearing similar to quartzite mylonite to form mylonitic banding. The the scenario of a tectonic complex zone (see Festa formation of successive generations of mylonites et al. 2012). We consider that the boundaries has also been demonstrated within the shear zone between different lithotectonic units represent (Mukhopadhyay and Deb 1995). These are further tectonic discontinuity. The overall disposition and corroborated from our present study. The rocks the inferred boundary of the units are closely show granoblastic texture, and the grains are pre- comparable to the characteristics of a possible dominantly strain free, possibly resulting from tectonic melange like complex sequence. In a large- rapid recrystallization and recovery (Passchier and scale shear zone, development of tectonic complex Trouw 2005). The complete recrystallization of the is expected when there is intensive shear associated quartz grains and near complete recrystallization with Cattening. Disintegration and incorporation of of the feldspar grains in mylonite to ultramylonite blocks of wide variety of rocks from different granite, the nature of the recrystallized grains of stratigraphic units occur during the formation of a both quartz and feldspar attesting grain boundary large-scale shear zone (Condie 1997). The different migration (GBM) recrystallization and also sub- lithotectonic blocks with their characteristic grain rotation-recrystallization (SGR); GBM is deformation signature are thus comparable to tec- dominant over the SGR and that is indicative of tonic amalgamation of ‘large-scale (tens of metres- a moderate to high deformation temperature scale) shear lenses’ in the internal part of the SSZ. (C 600°C) of deformation in SSZ (c.f., Passchier The disposition of the dismembered lithotectonic and Trouw 2005; Trouw et al. 2010). The oblique units and their deformation characteristics in the polymineralic (quartz-feldspar-mica) shape pre- present area of study is apparently comparable to ferred foliation (Bgure 8b) seen in dynamically the accretionary complex like scenario (see Sal- recrystallized ultramylonite granitoid occur mainly vador 1994; Vannucchi and Bettelli 2010), where in medium- to high-grade mylonites (Hanmer and some earlier workers equated with the ‘melange’. Passchier 1991) and that commonly develop in a We, therefore, infer that the lithotectonic blocks non-coaxial Cow. However, in domains where there are large-scale shear lenses stacked together to are alternate bands of protomylonite and mylo- form a complex of lithotectonic units in SSZ. The nite/ultramylonite, temperature might not have SSZ sequence is near similar to the tectonic sce- exceeded 450–500°C characteristics of low-(to nario, where deformation associated with large medium) grade mylonitization (Trouw et al. 2010). shear zones at the base of regional-scale thrust The overall ‘island-channel’ type structure in sheet or nappe system related to intracontinental granite mylonite, and the coexisting protomylonite deformation, which has been named as ‘tectonic and mylonite–ultramylonite bands forming anas- melanges at the base of a nappe’ by Festa et al. tomosing pattern (Bgure 4d) (c.f., Bell 1981) attest (2012). We are in favour of using the SSZ sequence to strong strain partitioning in mesoscopic-scale as a tectonic complex rather than tectonic melange within the SSZ. The small lens-shaped islands of as there is a connotation of the term melange with ultramylonite within protomylonite could result the sequence formed in convergent plate boundary from variation in rate of strain and Cow stress. scenario (see Festa et al. 2012). In our view, the Microboudinage structure in feldspar porphyro- SSZ does not readily Bt into the definition of an clast possibly suggests that the ductile Cow of the ideal ‘tectonic melange’. recrystallized matrix around elongate rigid feldspar From the characteristics of shear fabric devel- clasts produced an internal stress Beld that can opment in the SSZ, it is seen that rheologically lead to tensile fracturing and boudinage (Passchier weak units (e.g., schist of different types), sand- and Trouw 2005). The rarity of the porphyroclasts wiched between more competent units (e.g., may be due to extensive recrystallization of the quartzite, BFQ, etc.) responded in different man- granite protolith. The high degree of recrystalliza- ner to develop heterogeneous types of shear related tion is suggestive of medium- (to high) grade structures in the adjacent lithounits. Therefore, the mylonite (Passchier and Trouw 2005). role of material property in the deformation of the Tourmaline grains in mylonite granite and schist SSZ was immense. are deformed and show incipient recrystallization. In quartzite and BFQ, the quartz grains are Detailed petrological evolution of tourmaline in almost fully recrystallized to form banded ultra- mylonitised granitoids and schists of SSZ has been mylonite (Bgure 8a). Banerjee and Matin (2013) discussed by Bandyopadhyay (2003). Participation J. Earth Syst. Sci. (2020) 129:227 Page 19 of 25 227 of tourmaline crystals in the shearing process suggests their ‘inBltration’ (as Cuid) before or at least during the process of shearing deformation. The tourmaline grains, which show intragranular fracturing and intracrystalline ductile deformation including incipient recrystallization (Bgure 8d), are embedded in a host of recrystallized quartz-feld- spar aggregate matrix. Therefore, the temperature of incipient recrystallization of tourmaline may be around 600°C or more; however, it cannot be ver- iBed because recrystallization temperature of tourmaline is not worked out so far. Shear lenses are excellently developed in the schistose rock in both mesoscopic and microscopic scale (see Bgure 6e and f). Different stages of development of the shear lenses could be recog- nized. Following the theoretical postulation of Ghosh (1993), there are three cases, which are relevant in case of SSZ: (1) synchronous develop- ment of the mylonitic foliation in the shear bands and the shear lenses (e.g., Gara Nala section), (2) development of the two foliations (C and C0)in a single ongoing deformation, with the shear band Figure 9. SimpliBed schematic diagram of kinematic model cleavage (C0) initiating at a later stage of the (based on Nickelsen 1986) shows development of shear lens in deformation (e.g., Jaduguda, Bhatin areas), and cleaved mica schist rock through the mechanism of cleavage (3) the pre-existing crenulation/transposed schis- duplex. Stages of development are from top to bottom of the tosity transected by the shear bands of a later diagram. deformation which is excellently demonstrated in the shear lenses exposed in Gara Nala section, We could demonstrate the last two stages (c and d where the foliation inside the lenses is represented of Bgure 16 of Fossen and Cavalcante 2017) in our by an earlier formed crenulation/transposed study area, while the earlier two stages (a and b of schistosity (S2). However, all transition amongst Fossen and Cavalcante 2017) have either been the three types could be discernible. The shear absent or overprinted by the later ones. band cleavage and formation of shear lenses might In the central part of the shear zone, for exam- be related to cleavage duplex process. Based on ple, at Jaduguda and Bhatin areas, the relation Nickelsen (1986), a schematic diagram of the between the two sets of mylonitic foliation are kinematic model of formation of shear lenses has rather complex. In this area, it appears that the been presented (Bgure 9). It is demonstrated that shear bands and the mylonitic foliation have the shear lenses are small-scale duplexes contained developed in repetitive succession, one set super- between the subparallel Coor and roof thrust imposed on the other during a single progressive (Bgure 9). Foliation lenses may also form from deformation; the earlier stage of mylonitic foliation interconnection of C- and C0-planes, which appar- is gradually replaced by a new mylonitic foliation. ently look like anastomosing shear bands Stretching of quartz veins producing ellipsoidal (Bgure 8i). We are of the opinion that both the boudins with stretched out tapering ends in the mechanisms contributed in formation of shear micaceous matrix points to lowering of viscosity lenses. The presence of folded shear bands along contrast between the schistose mica-rich host and with shear lenses (Bgure 8g) and the discrete shear the boudinaged quartz veins during shearing bands suggest that the deformation continued in (Bgure 7e). Folding of quartz veins with Bsh-hook SSZ even after the formation of the shear lenses. geometry (Bgure 7f) suggests shearing along one Fossen and Cavalcante (2017) illustrated, with a limb of the asymmetric folds leading to its thin- schematic diagram, four stages in the formation of ning. The imprints of shear fabrics in some quartz a shear zone network evolving to an anastomosing veins indicate their emplacement prior to or shear zone with lenses of less deformed protolith. synchronous with the shearing event (Bgure 7f). 227 Page 20 of 25 J. Earth Syst. Sci. (2020) 129:227

Our studies are in conformity with the Bndings movement direction and that the deformation of Lister and Snoke (1984), who claimed that continued for a long time. Although rare, the out- quartzites with mica Bsh are a special type of S-C crops of sheath folds are usually identiBable on plan mylonite, a structural setting in which two folia- view, that is, perpendicular to stretching lineation, tions are developed: C-surfaces as trails of mica occasional presence of close outcrop pattern of folds fragments forming the main Sm representing the in BFQ in Ramchandrapahar, exposed in vertical microscopically thin displacement discontinuity, section parallel to the dominant down-the-dip while the S-surface is deBned by an oblique folia- stretching lineation and perpendicular to axial pla- tion of quartz in the matrix showing grain-shape nar/mylonitic foliation, indicate that the fold hinge preferred orientation. This oblique foliation is on mylonitic foliation is also stretched at high angle formed when the matrix is dynamically recrystal- to the shear direction. This is indicative of a sub- lized during deformation. According to Ten sidiary component of Cattening with concomitant Grotenhuis et al. (2003), the principal mechanisms stretching parallel to the shear zone across the shear of formation of mineral Bsh, especially muscovite direction. Such an observation may argue in favour Bsh are intracrystalline deformation with con- of at least minor stretching of the shear plane along comitant selective grain size reduction combined the Y-direction of the bulk strain. This is charac- with rigid body rotation. In our study area, mainly teristic of stretching shear zone. It is therefore, the Groups 1, 2, 5 and rarely 3 of mineral Bsh could apparent that the shear zone rocks are, at places, be recognised, the Group 1 being the predominant stretched along both dip and strike directions of the one (c.f., Ten Grotenhuis et al. 2003; Passchier and predominant shear fabric (Sm). Stretching in both Trouw 2005). In our study area, the Bsh structures along down-the-dip (X-direction) and strike (Y- do not appear to show evidences of appreciable direction) (Bgure 5c and d) has also been demon- rotation, and result mainly from brittle (micro- strated by the ‘clasts’ in autoclastic ‘conglomerate’ faulting) processes. from south of Rakha Mines indicating three- Conglomerates show highly variable deformation dimensional nature of deformation (X[Y[1[Z). characteristics. Mylonitic foliation (Sm) in the This is characteristic of sub-simple nature of shear in matrix part and strong elongated down-the-dip a stretching shear zone (Fossen and Cavalcante lineation deBned by the clasts along with stretching 2017). These type of features, which represent 3D lineation on the pebble surface are common deformation, can be explained localised domains (Bgure 5b). The material property of the protoliths of simple shear and Cattening (c.f., Baird and played an important role in the deformation of the Hudleston 2007). conglomerate. The higher state of strain, particu- There is gradual change in geometry of folds larly of shear strain in the matrix-supported con- from upright near Ghatshila–Galudih area, away glomerate could possibly be explained as due to from the shear zone in the north, to inclined and high proportion of matrix, platy/tabular nature of reclined folds towards south within the main shear quartzite clasts and presence of abundant Cuids zone (see Bgure 3). In the shear zone, the folds are during deformation (see Gay and Fripp 1976). consistently asymmetric and their vergence ubiq- Presence of occasional isoclinal reclined fold in uitously points to top-to-south movement attesting some deformed quartzite clasts (Bgure 5a) in to reverse sense of movement in the SSZ. Ghosh matrix-supported conglomerate is possibly sugges- and Sengupta (1987b) have demonstrated repeated tive of an early fold perhaps formed by Cattening development of sub-horizontal folds and rotation of whose axis has been later rotated into parallelism their hinges towards the direction of maximum with the direction of maximum elongation (see stretching (down-the-dip) to produce reclined folds Mukhopadhyay et al. 1980) due to later shearing in of successive generations. They have demonstrated the rocks (c.f., Escher and Watterson 1974). change in orientation and tightness of folds from In BFQ and quartzite, the Sm and stretching open nearly non-plunging to steep down-the-dip lineation are interpreted to have developed in the plunging reclined folds with progressive deforma- early stages of the shearing due to the combined tion in SSZ and also suggested several cycles of eAects of pure shear and simple shear, and that is events in fold formation and deformed lineation. followed by folding of the Sm and stretching lin- We support their contention that the group of open eation during the ongoing progressive deformation folds with sub-horizontal axis must have developed in the SSZ. The development of sheath folds on Sm towards a late stage of the same progressive form surface suggests rotation of hinge lines towards deformation. As a result, the earlier generation J. Earth Syst. Sci. (2020) 129:227 Page 21 of 25 227 reclined folds are refolded by the later ones due to progressive simple shearing, and also the upright folds of near parallel (Type IB) geometry is tight- ened to produce near similar folds of Type 2 geometry. Mukhopadhyay and Deb (1995) have described folds with variable geometry and shape as well as sheath folds and interpreted them to be the result of progressive simple shear in the SSZ. Thereispresenceofabundanttourmaline, apatite–magnetite veins and stringers, kyanite nodules and veins, secondary chlorite (after bio- tite and garnet) and muscovite (mainly after feldspar) and economically significant sulphide and uranium mineralization in the mylonitised Figure 10. Schematic diagram of generalized 3D model of SSZ and adjoing areas marking the simple shear and Cattening rocks of the SSZ. This is certainly indicative of deformation in the SSZ. profuse infux of Cuid(s) of different composition in the shear zone, resulted in metasomatism that might have played an important and eAective involves an orthogonal combination of simple and role in the shear zone formation. They span in pure shear, as opposed to the case of sub-simple time from pre- to syn-shearing events. From their shear, where the two components are applied in the study, Sengupta et al. (2004) have demonstrated same plane (see Fossen and Cavalcante 2017, their that several rocks of SSZ and adjoining area in Bgure 12). The consequence of having a pure shear the north have undergone complex metasomatic component within the shear zone is that the shear alteration at mid-crustal level corresponding zone material is extruded in the X direction of the to kyanite stability at the inferred temperature strain ellipsoid (Fossen and Cavalcante 2017). of *480° ± 40 °C (Bandyopadhyay 2003). We According to them, if the pure shear component could recognize imprints of several pulses of also aAects the shear zone walls while the shear metasomatism with respect to the shearing his- zone component does not, the eAect is that both the tory of the rocks of the SSZ in the study area. The walls and the shear zone are lengthened during inCux of Cuids and their eAects on subsequent deformation, and thus either a stretching fault metasomatism might have played an important (Means et al. 1980; Means 1995) or a sub-simple role in the strain softening process responsible for stretching shear zone is formed. We have presented the concentration of strain in the evolution and a schematic diagram of generalized 3D model of the progression of the SSZ. From our study, it is SSZ depicting the simple shear with Cattening evident that (1) presence of Cuid associated with deformation (pure shear) conceived from our study metasomatic alteration of different rock types, (Bgure 10). For the conservation of continuity (2) dynamic recrystallization leading to grain (completely ductile deformation) as at the north- reBnement and superplasticity in rocks, (3) ern margin of the SSZ, a stretching shear zone chemical alteration of silicates to phyllosilicates occurred while a ductile–brittle stretching (muscovite, chlorite, etc.), and (4) moderate fault/ductile thrust formed along the southern temperature (medium grade metamorphism) margin of SSZ (see Bgure 3). greatly facilitated the development of the shear In our study, we have not made any attempt to zone. estimate quantitatively the Wk values. So we are Our detailed study on small-scale structures may not in a position to decipher the precise nature of highlight the nature of strain in the SSZ; whether it strain in SSZ. The localized occurrences of three- represents (a) simple shear, (b) sub-simple shear dimensional deformation features, noted and dis- (c) transpression, or (d) simple shear with Catten- cussed by us (the extension structure along the X ing (c.f., Baird and Hudleston 2007), while (a) and and Y directions), seen in autoclastic ‘conglomer- (b) are plain strain, (c) and (d) are non-plane 3D ate’ south of Rakha mines area and fold in BFQ strain. Sanderson and Marchini (1984) outlined a near Nandup village are rather exceptions in SSZ. simple three-dimensional (3D) strain model, known Also, the absence of back rotated porphyroclasts, as transpression/transtension, to explain many which is emphasized as a good indicator of sub- shear zones. Sanderson and Marcini’s model simple shear (Simpson and DePaor 1993), and 227 Page 22 of 25 J. Earth Syst. Sci. (2020) 129:227 overall consistency of stretching lineation without Bisrampur village, wedges/slices of basement rotation on the mylonite plane (Fossen and TikoA Singhbhum granite over the cover sediments 1998) suggest that the dominant nature of strain is were emplaced as thrust slices (Mukhopadhyay simple shear in SSZ. All the reliable shear sense et al. 1980). indicators, particularly the C- and C0-structures, asymmetry of folds, oblique foliation, etc., strongly argue in favour of up-dip movement of the hanging 6. Conclusions wall showing vergence top-to-south, where the stretching lineation is consistently parallel to 1. The SSZ complex marks the tectonic bound- shear/movement direction. These are conclusive ary between two tectonic blocks, the SC in evidences in support of simple shear strain in SSZ. the south and NSMB in the north. The In terms of strain, transpression/transtension micro- to meso-scale structures reveal the develops non-plane strain and that for Sanderson evolution of internal deformation history of and Marchini (1984) model is constrictional for the SSZ (c.f., Feinstein et al. 1999). Our transtension and Cattening for transpression. The study establishes that the central part of the lineation is less pronounced in the case of trans- SSZ represents a tectonic complex zone, pression and produces S-tectonites. In contrast, the consists of a collage of dismembered litho- SSZ is characterized by excellent development of tectonic units in a deformed matrix, each LS tectonites. In view of that the SSZ cannot be having different structural features, devel- considered as a transpressional shear zone. In oped in response to the convergence of the summary, we emphasize that the strain pattern of NSMB on the Singhbhum Craton. SSZ is mainly a plane strain simple shear type with 2. Our study puts forward that in response to localized domains of 3D strain. We also conclude progressive deformation, there is gradual tran- that the SSZ may be called a shear zone complex sition from predominantly Cattening type fab- rather than a melange zone and the small-scale ric in the northern part (outside the SSZ) to structures attest a protracted, evolved shear zone shear-related fabrics in the central part dom- with variation of strain pattern and degree of inated by simple shear with subordinate pure deformation across the shear zone. shear (sub-simple shear) of the SSZ. The The deformation characteristics across the basement granite near the southern boundary terrain from Ghatshila–Galudih area in the north of the SSZ, did escape intense ductile defor- (outside the shear zone) to the main shear zone mation, but yielded to brittle–ductile deforma- to the south varies from co-axial pure shear tion producing basement slices/wedges. The deformation outside the shear zone to non- Singhbhum Granite of the SC form the but- coaxial simple shear deformation within the tressing mass for the south directed compres- shear zone with a transition zone in between (see sive deformation front. The conglomerates, Bgure 3). Our study is in agreement with the quartzites, BFQ, maBc–ultramaBcs and other earlier models, that the direction of compressive lithounits, which directly overlie the basement force had been from north to south with top-to- granite, are sheared and disjointed resulting in south sense of shear (c.f., Ghosh and Sengupta tectonic interleaving of different lithological 1990; Sengupta and Mukhopadhyay 2000;Sen- units to form ‘tectonic complex like’ assem- gupta and Chattopadhyay 2004). While the blage. The northern margin of the granite granite basement in the south remains least basement also participated in shearing event aAected by ductile shearing, the adjacent sedi- producing protomylonite and augen gneiss mentary and the maBcultramaBc rocks including mylonite. Our study suggests overall sub-sim- the granites within the shear zone are strongly ple shear of the SSZ complex. deformed due to shearing. It is important to note 3. The dominant mylonitic foliation is represented that adjacent to the basement granitoid, the SSZ by the C-fabric, with the occasional relict of shows interleaving of weakly mylonitised granite earlier S-surface and later developed discrete C0- basement and basal conglomerate in the shear band cleavage observed mainly in schis- Rohinibera area, where basement slices or wed- tose rocks. Shear lenses are formed by different gesofgranitoidsemplacedoverthebasalcon- processes at different locations. Shear fabrics glomerate. Near the northern border of the (C- and C0) postdates the development of Singhbhum granite, *500 m south of the crenulation/transposed schistosity (S2) and the J. Earth Syst. Sci. (2020) 129:227 Page 23 of 25 227

peak metamorphism in the area. The successive Author statement fabric development attests to progressive defor- mation history of the shear zone. Material Abhinaba Roy: Conceptualization, data collection, properties (lithological character) played an investigation, validation, writing review and edit- important role in the formation of shear ing. Abdul Matin: Conceptualization, data collec- induced structures in the shear zone. It is tion, writing original draft, formal analysis, and presumed that although the activation energy methodology. was high during the shearing, the high strain rate prevented the growth of new minerals in References the shear zone. 4. Consistent mylonitic foliation and lineation and Baird G B and Hudleston P 2007 Modeling the inCuence of small-scale folds indicate top-to-south move- tectonic extrusion and volume loss on the geometry, ment of the northern hanging wall block over displacement, vorticity, and strain compatibility of ductile the Singhbhum Craton in the south. shear zones; J. Struct. Geol. 29 1665–1678, http://dx.doi. 5. Metasomatism leading to chemical alteration org/10.1016/j.jsg.2007.06.012. of rocks might have played an important role Bandyopadhyay N 2003 Metamorphic history of the rocks in in the shearing mechanism and perhaps trig- the southern sector of the Proterozoic Singhbhum Shear Zone and its environs; Unpublished Ph.D. thesis, University gered the shear zone nucleation and of Calcutta. progression. Banerji A K 1959 Cross-folding and thrust tectonics from the 6. The microstructural characteristics, particu- Singhbhum shear zone; Quart. J. Geol. Min. Metall. Soc. larly the recrystallization texture, in mylonite India 31 59–60. and ultramylonite indicate a wide range of Banerjee S and Matin A 2013 Evolution of microstructures in Precambrian shear zones: An example from eastern India; deformation temperature from low- to medium- J. Struct. Geol. 50 199–208. or high-grade (*600 °C). In that case, the Bell T H 1981 Foliation development – the contribution, maximum temperature estimated from the geometry and significance of progressive bulk inhomoge- deformation characteristics in SSZ may be neous shortening; Tectonophys. 75 273–296. slightly higher than that inferred by Sengupta Bell T H and Etheridge M A 1976 The deformation and et al. (2004). recrystallization of quartz in a mylonite zone, Central Australia; Tectonophys. 32 235–267. Berthe D, Choukroune P and Jegouzo P 1979 Orthogneiss, mylonite and non-coaxial deformation of granites: The Acknowledgements example of the South Armoricain shear zone; J. Struct. Geol. 1 31–42. We take this opportunity to acknowledge Atomic Blenkinsop T G 2000 Deformation microstructures and mechanisms in minerals and rocks; Kluwer Academic Mineral Division, Govt. of India and their Beld Publishers, Dordrecht. geologists for their support during our Beld work. Bhattacharya D S 1978 Contrasts across the shear zone in the We are indebted to Dr S Dasgupta, formerly of Precambrian rocks of western Singhbhum; Geologie en Geological Survey of India for his association with Mijnbouw 57 59–63. Beldwork, constructive discussions and critical Boyer S E and Elliot D 1982 Thrust systems; AAPG Bull. 66 comments in improving the quality of the work. 1196–1230. Boyer S E and Geiser P A 1987 Sequential development of We are grateful to Prof. D Mukhopadhyay for thrust belts. Implications for mechanisms and cross section discussion and suggestions in course of this study. balancing; Geol. Soc. Am. Abstr. Progr. 19 597. Petrologists of Central Petrological Laboratory Carreras J, Druguet E and Griera A 2005 Shear zone-related and Eastern Region of Geological Survey of India folds; J. Struct. Geol. 27 1229–1251. are thanked for their support in preparation of Choukroune P, Gapais D and Merle O 1987 Shear criteria and structural symmetry; J. Struct. Geol. 9 525–530. micro-sections and photomicrography. Discussion Condie K C 1997 Plate Tectonics and Crustal Evolution; 4th with Smt. Arya Ghosh of Central Petrology Divi- edn, Buttterworth Heinemann, Oxford. sion on the kinematics of shear zone is thankfully Cowan D S 1978 Origin of blueschist-bearing chaotic rocks in acknowledged. We are beneBted from comments the Franciscan Complex, San Simeon, California; Bull. and suggestions by two anonymous reviewers to Geol. Soc. Am. 89 1415–1423. improve the quality of the manuscript. We also Cowan D S 1986 The origin of some common types of melange in the westem Cordillera of North America. In: Formation acknowledge editorial handling and constant sup- of Active Ocean Margins (eds) Nasu N, Kobayashi K, port from Associate Editor, Saibal Gupta and his Nyeda S, Kushiro I and Kagami H, Dordrecht, D. Reidel comments. Publ., pp. 257–272. 227 Page 24 of 25 J. Earth Syst. Sci. (2020) 129:227

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