Repeated reactivation of the Gavilgarh-Tan Shear Zone, Central India: Implications for the tectonic survival of deep-seated intra-continental fault zones

Journal Pre-proofs Repeated reactivation of the Gavilgarh-Tan Shear Zone, Central India: implications for the tectonic survival of deep-seated intra-continental fault zones Anupam Chattopadhyay, Dipanjan Bhattacharjee PII: DOI: Reference:

S1367-9120(19)30403-1 https://doi.org/10.1016/j.jseaes.2019.104051 JAES 104051

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Journal of Asian Earth Sciences

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14 December 2018 11 September 2019 28 September 2019

Please cite this article as: Chattopadhyay, A., Bhattacharjee, D., Repeated reactivation of the Gavilgarh-Tan Shear Zone, Central India: implications for the tectonic survival of deep-seated intra-continental fault zones, Journal of Asian Earth Sciences (2019), doi: http://dx.doi.org/10.1016/j.jseaes.2019.104051

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Repeated reactivation of the Gavilgarh-Tan Shear Zone, Central India: implications for the tectonic survival of deep-seated intra-continental fault zones Anupam Chattopadhyay1*, Dipanjan Bhattacharjee2 1

2

Department of Geology, University of Delhi, Delhi 110007, India

Department of Geology, Indira Gandhi National Tribal University, Amarkantak, India *Corresponding Author; Email: [email protected]

Abstract Continental crust accommodates crustal deformation through repeated reactivation of old shear/fault zones that constitute domains of long-term structural weakness. Reactivation is often triggered by mechanical and chemical (fluid-assisted) changes of the host rocks within the fault core region. However, faults can also reactivate in fluid-poor condition if they are oriented favourably with respect to the prevalent stress regime. An example comes from the Gavilgarh-Tan shear/fault zone (GTSZ) in the central Indian craton which shows clear evidences of repeated fault reactivation movements widely separated in time (in Neoproterozoic, Ordovician, Permo-Triassic, Late Cretaceous and Holocene). Presence of pseudotachylyte along some reactivated fault strands indicates seismogenic fault movements in the past. Kinematic signatures of the shear zone with contrasting slip senses indicate repeated ‘geometric reactivation’ of GTSZ in the pre-Quaternary as well as one episode of ‘kinematic reactivation’ in the Quaternary period. Under the present NNE-SSW directed compressive stress regime of the Indian craton, GTSZ is prone to further seismogenic fault movements. Key Words: Fault/shear zone, Reactivation, Frictional-viscous transition, Fault strength, Central India 1

1. Introduction: Deformation of the continental crust is characteristically heterogeneous. In contrast to the oceanic crust, where deformation-related structurally weak domains are usually restricted to the plate margin, continental crust is riddled with numerous faults/shear zones in all scales. Continents are made up of relatively weaker, quartzo-feldspathic crust which is not normally subducted and, therefore, can carry the ‘tectonic inheritance’ structures for a very long time (Sutton and Watson, 1986). These older tectonic domains constitute inherent structural weakness in the continental crust which are likely to get ‘rejuvenated’ during subsequent deformation. Moreover, many major intracontinental shear/fault zones are also known to act as channelways for the upward migration of hydrous fluids and magmas which affect their strength over a long time period. As a result, intracontinental shear/fault zones often show evidences of intermittent tectonic rejuvenation (reactivation) throughout the geologic time period (Grocott, 1977). In many sedimentary basins, contractional reactivation of extensional faults is well demonstrated (Letouzey, 1990). A large number of Phanerozoic orogens appear to coincide with the positions of ancient rifted basins, indicating a causative relationship between structurally inherited weak zones and later tectonic domains (Butler et al., 1997). In contrast, later thrusts do not reactivate the pre-existing normal faults in many collisional mountain belts (Gillcrist et al., 1987). Therefore, reactivation of pre-existing faults is not necessarily ubiquitous in nature. It is now well accepted that the main load-bearing zone of the continental lithosphere is the olivine-rich upper mantle which controls the overall deformation behaviour of the continents (Scholz 1988). This is also evidenced by the global-scale correspondence between the patterns of crustal deformation and the observed seismic anisotropy that represents deformation patterns of the continental lithospheric mantle (Silver, 1996). However, another major strength zone, although less significant than the former one, lies in the upper-middle 2

crust (at a depth of ~10–15 km) where the pressure-sensitive brittle, frictional behaviour exhibited by quartzo-feldspathic rocks at shallow crustal depth changes to temperaturecontrolled plastic (viscous) flow, typical of the deeper crust (Scholz, 1988) (Fig. 1). Shear/fault zones of the continental crust commonly transect this secondary load-bearing zone (i.e. frictional-viscous transition) and are, therefore, likely to accommodate multiple phases of subsequent crustal deformation, as demonstrated in the Outer Hebrides Fault zone (Imber et al., 2001) and the Great Glen Fault (Holdsworth et al., 2001a) in Scotland. It may be noted here that shear zones and fault zones, although classically defined as separate entities, are clubbed together in the recent fault-related literature as these two structures are usually genetically connected and grade into each other over a depth range. Tectonic rejuvenation of pre-existing weak zones in the lithosphere can occur via two related mechanisms, viz. ‘reactivation’ which involves rejuvenation of discrete structures (e.g. shear/fault zones), and ‘reworking’ which comprises repeated focussing of deformation, metamorphism and magmatism into a relatively narrow domain of the crust or lithosphere (Holdsworth et al., 2001b). Reactivation is defined as the accommodation of geologically separable (at intervals of >1 Ma) displacement events along pre-existing shear/fault zones (Holdsworth et al., 1997). The shorter interval (103 -105 years) successive displacement events on active faults, commonly related to a seismic cycle, are thereby excluded from the ambit of fault reactivation. In neotectonic settings, the timing of fault-slip events has much greater resolution, and plate motion vectors are precisely known. Therefore, in such cases reactivation is defined as accommodation of displacement along structures formed prior to the current tectonic regime (Muir Wood and Mallard, 1992). It is already mentioned that reactivation of earlier structures, although common in nature, is not ubiquitous. Some ancient fault zones may remain quiescent after their formation, while some recent faults may not necessarily reactivate any earlier structure. It is, therefore, necessary to have unambiguous 3

geological, geophysical, and preferably geochronological, evidences of reactivation of a preexisting structure during later tectonic activity. Only apparent parallelism of two structures may not necessarily indicate that the older one is reactivated. In the present contribution, we attempt to demonstrate the repeatedly reactivated character of a major intracontinental fault zone of the Indian cratonic landmass – the Gavilgarh-Tan Shear Zone, occurring within the southern part of the Central Indian Tectonic Zone (CITZ) (Fig. 2). On the basis of a prevalence of seismic events within the CITZ, it has been suggested that at least two major lineaments of CITZ viz. Son-Narmada South Fault (SNSF) and Gavilgarh-Tan Shear Zone (GTSZ) are prone to reactivation under the current plate tectonic regime related to the Indo-Tibetan plate collision (Acharyya and Roy, 2000; Roy and Devarajan, 2003). However, not much geological evidence of reactivation has been put forward by these authors. Recent geological investigations (especially fault zone study) and selective dating of fault-related rocks of GTSZ have unearthed a lot of information regarding repeated tectonic movements in this shear/fault zone spanning from Precambrian to Quaternary. In the forgoing sections we shall summarize the recently published (and some unpublished) data from GTSZ and shall examine whether they suffice to explain the long tectonic survival of this important intracontinental fault zone. 2. Geology of the Gavilgarh-Tan Shear/Fault Zone: The Indian craton has two major fragments. The North Indian Block (NIB) is made up of the Bundelkhand craton and its adjoining Aravalli-Delhi orogenic belt, whereas the South Indian Block (SIB) comprises Bastar, Dharwar and Singhbhum cratonic blocks (Radhakrishna and Naqvi, 1986; Roy and Prasad, 2003; Bhowmik et al., 2012). The NIB and SIB were stitched together along a crustal scale mobile belt called the Central Indian Tectonic Zone (CITZ) through multiple crustal subduction, collision and accretion events spanning

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from Palaeo- to Neoproterozoic (Roy and Prasad, 2003; Bhowmik et al., 2012). CITZ is composed of three major supracrustal fold belts (e.g. Mahakoshal, Betul and Sausar belts from north to south) occurring within a vast terrain of unclassified basement gneisses laced with linear belts of granulites (e.g. Makrohar Granulite (MG), Ramakona-Katangi Granulite (RKG) and Balaghat-Bhandara Granulite (BBG) from north to south) (Fig. 2). CITZ also comprises four major tectonic lineaments, viz. Son-Narmada North Fault (SNNF), SonNarmada South Fault (SNSF), Gavilgarh-Tan Shear Zone (GTSZ) and Central Indian Suture/Shear (CIS) (Fig. 2). Tectonics of the CITZ and the geological significance of the granulites have been described in many published works (Bhowmik et al. 1999; Roy and Prasad, 2003; Chattopadhyay and Bandyopadhyay, 2004; Roy et al., 2006; Bhowmik et al., 2012). GTSZ is a major brittle-ductile shear zone, best exposed in the Kanhan River valley between Met and Chhindvi (Fig. 3a), but traced farther beyond Seoni in the east and Gavilgarh in the west for a total length of more than 300 km (Golani et al., 2001). Intense ductile shearing of granitoids (e.g. granite, granodiorite, monzonite, aplite) has produced spectacular mylonite series rocks (proto- to ultramylonite) in the Kanhan river bed. Shear sense indicators like winged porphyroclasts, S-C-C′ fabric and asymmetric folds, among others, indicate sinistral strike-slip ductile shearing, overprinted by later discrete semi-brittle and brittle slip events (Golani et al., 2001; Chattopadhyay et al., 2008). In the western part, GTSZ is represented by a dominantly dip-slip brittle fault zone, also called Gavilgarh Fault Zone (GFZ) (Bhattacharjee et al., 2016) or Satpura Foothill Fault (SFF) (Ravi Shanker, 1987) (Fig. 3b). Along the GFZ, rocks of the Permo-Triassic age Gondwana Supergroup have been tectonically juxtaposed against the younger Deccan Trap lava flows of Upper CretaceousTertiary age (Fig. 3c-e). Kinematics and ages of the different shearing and faulting events of the GTSZ are described in detail in the next section. 3. Tectonic movements in the Gavilgarh-Tan Shear Zone: 5

As mentioned above, the Gavilgarh-Tan Shear/Fault Zone records a number of ductile and brittle shearing events of contrasting kinematics (strike/oblique-slip and/or dip-slip), possibly widely separated in time. Detailed structural/kinematic interpretations and some geochronological constraints of the tectonic movements have been published in recent years (Chattopadhyay et al., 2008; Chattopadhyay and Khasdeo, 2011; Chattopadhyay et al., 2014a, b). Below we attempt to summarize the available information, and try to interpret the data for an understanding of the reactivation history of this intracontinental fault zone. 3.1. Ductile shearing of basement rocks in the GTSZ: Ductilely sheared coarse grained porphyritic granite, medium-grained pink granite, dark coloured granodiorite and brick-red coloured, fine grained aplite are the main lithological units of the GTSZ in the Kanhan River valley, showing a variety of mylonites (Chattopadhyay et al., 2017). All the granitoids have intruded into the shear zone as tabular, sheet-like bodies and have mutually cut each other, indicating nearly simultaneous emplacement. Presence of pre-full crystallization fabrics (PFC) in the low strain domains (Fig. 4a) indicates synkinematic emplacement of these granitoids into the shear zone. Strain gradually increases towards the central part of the shear zone, but strain partitioning has occurred even at the outcrop scale, usually guided by the variable rheology of the granitoids due to their varied composition and grain size. An excellent example is the preferential concentration of shear strain in the finer grained, biotite-rich layers of granodiorite in comparison to the surrounding feldspar-rich, coarse-grained porphyritic granite (Fig. 4b). ENE-WSW striking, subvertical to steeply southerly dipping (mostly ≥65° dip) mylonitic foliation and subhorizontal to very low pitching (≤15°) stretching lineations indicate dominantly strike-slip shearing (Chattopadhyay and Khasdeo, 2011) (Fig. 3a). Spectacular shear sense indicators are found in these rocks, e.g. asymmetric winged (σ- and δ-type) porphyroclasts, asymmetric folds with axes perpendicular to the shear direction, S-C-C′ 6

fabrics, and mineral (mica and titanite) fish, to name a few. Examined in proper sections (normal to the rotation axis of shearing), all these kinematic indicators show a sinistral shear sense (Figs. 4b-d). Temperature-sensitive microstructures, e.g. high-temperature grain boundary migration recrystallization (i.e. Regime 3 GBM of Hirth and Tullis, 1992) in quartz, core-and-mantle structure in K-feldspar, and asymmetric development of myrmekite lobes around microcline grains indicate high temperature (T>500°C) shearing (e.g. Stipp et al., 2002), possibly below the frictional-viscous transition zone i.e. at more than 15 km depth (e.g. Chattopadhyay et al., 2008). However, in the marginal part of the shear zone (especially at the northern margin, exposed in the Nakta Nala river: Fig. 3a), foliation in porphyritic granite shows elongated and fractured feldspar grains in both vertical and horizontal sections, indicating a biaxial stretching in response to a flattening strain. Moreover, these deformed granites show a strong foliation, but no discernible stretching lineation – indicating nearly ‘pure flattening’ type strain. On the basis of kinematic vorticity analysis of mylonites, GTSZ was proposed as a ‘partitioned transpression’ system where the simple shear strain was dominantly accommodated in the central part while the marginal parts accommodated the contraction i.e. ‘flattening’ component (for details of the strain analysis, see Chattopadhyay and Khasdeo, 2011). 3.2. Brittle-ductile and brittle overprinting of the mylonites: The ductile mylonitic fabrics are overprinted by discrete foliation-parallel slip zones, commonly laced with layers of dark coloured, aphanitic pseudotachylyte (Fig. 5a). Chattopadhyay et al. (2008) have identified two different types of pseudotachylytes in this terrain, viz. cataclastic pseudotachylyte (Pt-C) and mylonitized pseudotachylyte (Pt-M) (Fig. 5a, b). Pt-M layers show a strong internal foliation defined by stretched quartz grains and mylonitic rock fragments, with profuse development of chlorite. Asymmetry of the matrix foliation and of the strain shadow fringes around relict quartz grains indicate dextral strike7

slip shearing of the Pt-M layers (Fig. 5c). Ductile stretching of quartz grains, brittle fracturing of feldspar, and extensive chloritization indicates that this dextral shearing took place at 300°–400°C temperature, possibly with some fluid ingress along the discrete shear bands. Formation of pseudotachylyte (frictional slip) and its mylonitization (viscous shearing) indicates that the dextral shearing occurred within the frictional-viscous transition zone (~10– 15 km depth) (Chattopadhyay et al., 2008). It may be mentioned here that the dextral shear sense has been deduced exclusively from the asymmetric grain-shape fabric of the mylonitic foliation, which is a post-emplacement deformation feature of the Pt-M layers. The sense of slip/shearing during the formation of pseudotachylyte (Pt-M) is difficult to constrain as the original pseudotachylyte fabric is strongly overprinted and transposed by the subsequent dextral-sense ductile shearing. The Pt-C layers formed even later as they overprint the mylonite fabric as a wholly brittle wrench-type deformation with crushing of both quartz and feldspar, indicating <300°C temperature of deformation, corresponding to a shallower depth. Rotation of crushed blocks within the brittle sheared zones, and the sense of slip along meltfilled Pt-C layers at the outcrop scale (Fig. 5a: bottom right side) indicate sinistral shear sense during this brittle slip. At a few places, Pt-C layers form along the margin of older Pt-M layers which indicates that later brittle slip was focussed along the boundary between Pt-M layers and host mylonites due to their rheological difference (Fig. 5b). Fracture patterns related to the Pt-C layers indicate that they were formed by a seismogenic rupture with earthquake magnitude of approximately Mw ~ 5.17 (Chattopadhyay et al., 2014a). On the basis of the available evidence, Chattopadhyay et al. (2008) inferred that GTSZ was repeatedly reactivated by wrench shearing, often seismically, over a long period of time as the shear zone exhumed from depth to shallower level across the frictional-viscous transition. Timing of shearing/faulting events in GTSZ was constrained through laserprobe 40

Ar/39Ar dating of mylonites and pseudotachylytes (Chattopadhyay et al., 2014b). 8

Neoproterozoic age (ca. 880-900 Ma) was reported for the initial ductile shearing, followed by dextral shearing (related to Pt-M) at ca. 672 Ma and a later brittle slip (related to Pt-C) at ca. 459 Ma. Although Ar-dating of pseudotachylytes is in general fraught with some uncertainties, mainly because of the resetting of Ar-isotopic clock due to Ar-loss (e.g. Sherlock et al. 2009), the reported ages match very well with the independently worked out sequence of geological events in GTSZ, as outlined above. The ca. 672 Ma age of Pt-M probably reflects the age of dextral shearing and mylonitization after the formation of the pseudotachylyte, as discussed above. It should be taken as the minimum age of Pt-M formation. Chattopadhyay et al. (2017) have recently dated these granitoids using U-Pb zircon and U-Th-total Pb monazite methods and determined a more precise age bracket of syntectonic intrusion and transpressional deformation of the granitoids within the GTSZ (ca.1000— 950 Ma). The basement granitoid and gneisses of GTSZ therefore, record at least three wrench-type slip events between Neoproterozoic and Ordovician. 3.3. Late brittle faulting involving the cover rocks: Large scale brittle faulting is best observed in the western part of GTSZ, often referred to as the Gavilgarh Fault Zone (GFZ) (Fig. 3b). Here the GTSZ lineament runs through the Deccan Trap country, and demarcates the southern limit of the Satpura mountain range. The GFZ also demarcates the northern limit of the Tapti-Purna river basin which contains a thick sequence (>400 m thick) of Quaternary sediment (Ravi Shanker, 1987). Along the fault line, cross-bedded sandstones of Gondwana Supergroup are exposed in direct contact with Deccan Trap basalt lying south of the fault zone (Fig. 3c). In the northern side of the GFZ, the Gondwana sequence is overlain successively by calcareous beds of Lameta Formation and basaltic lava flows of the Deccan Trap. Evidently, the northern side of the GFZ has been uplifted, possibly along a reverse fault, which has juxtaposed the deeper level Gondwana rocks against the younger Deccan basalt flows. Crushing of sandstone units at the 9

outcrop scale, and northward tilting of the Gondwana-Lameta boundary near Salbardi supports the idea of reverse faulting and uplift of the northern block. Thickness of the exposed Gondwana sequence is maximum in the central part (near Salbardi and Dharul) and diminishes towards east and west, indicating variable fault throw along the strike (Fig. 3c). Down-dip plunging slickenside lineation in Gondwana sandstone near the fault line indicates dip-slip movement along the fault (Fig. 6a). A tectonic sliver of mylonitic granite gneiss, very similar to that observed in the type GTSZ in the Kanhan valley, is exposed just along the fault line, bound on all sides by crush zones against Gondwana sandstone and/or Deccan basalt (Fig. 7a). This confirms the presence of the ductile GTSZ at depth along the brittle (GFZ) fault zone. Recently, detailed geomorphic analyses of Quaternary rivers (e.g. sinuosity index, valley floor width/depth ratio, longitudinal profile, hypsometric index etc.) have been carried out which suggests that the northern side of the GFZ was uplifted with respect to the southern side (Bhattacharjee et al., 2016). Sandstone units in the hangingwall (northern side) show asymmetric folding with southerly vergence near the fault plane, but the folds die out farther north, away from the fault line (Figs 3d, 6b). This suggests that the northern block was thrusted up the fault plane under a compressional stress regime. Near Khatijapur (Fig. 3c), a low northerly dipping (dip ≈ 30°) reverse fault has displaced Deccan Trap flows in a ‘toptowards-south’ sense (Fig. 6c). The vertical columnar joints in the lava flows have tilted towards north by block rotation of the hanging wall. A gouge zone, comprising angular clasts of basalt within a finely crushed basaltic groundmass, occurs along one of the exposed fault planes (Fig. 6c, inset) which has possibly followed the contact between two lava flows. An antithetic normal-sense displacement of the gouge zone is seen in this outcrop (Fig. 6c). This small scale normal fault was possibly formed by slip along the columnar joint planes when the columns tilted towards north by body rotation. All these evidences indicate that GTSZ (GFZ) in this area manifested in a reverse fault with ‘top-towards-south’ displacement. In

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response to the tectonic upliftment, all the major rivers flowing across GFZ from the north have developed tectonic terraces and bed rock knick-points in the northern side of GFZ, whereas no such terrace or knick-point is found in the southern side of the fault (Bhattacharjee, 2015). Optically Stimulated Luminescence (OSL) and/or Infra-Red Stimulated Luminescence (IRSL) dating of quartz and feldspar grains derived from the terrace sediments of different rivers have indicated burial ages of the sediments, and thereby constrained the minimum age of terrace formation in these rivers by tectonic movements. At least four phases of reverse slip along the GFZ were interpreted from the calculated knickpoint migration rate and the minimum ages of terraces – ca. 65-80 ka, ca. 50 ka, ca. 30-35 ka, and ca. 14 ka (Bhattacharjee et al., 2016). The observed reverse faulting along the GFZ challenges the earlier proposition that the GFZ (or SFF) is a post-Cretaceous normal fault flanking the ‘Satpura Horst’ (Ravi Shanker, 1987) formed under the prevalent tectonic stress regime. The total fault-slip across the GFZ in Salbardi area, calculated from the vertical displacement of horizontal time marker beds (e.g. the Lameta Formation) from borehole and surface data, was found to be about 650 meters (Ravi Shanker 1987), of which only about 1520 meters can be directly accounted for by the observed displacement in the Quaternary strath terraces in the field. The rest (>600 m) is interpreted as Tertiary (post-Deccan Trap) but preQuaternary displacement (Bhattacharjee, 2015). Another recent work (Copley et al., 2014) has estimated >1000 m post Deccan Trap reverse displacement along Tapti North Fault, lying parallel to the GTSZ about 50 km to the west-northwest of the present study area, of which only 15-20 m is directly calculated as Quaternary slip, quite similar to the above mentioned slip estimate from the GFZ. In the type area of the ductile GTSZ (i.e. Kanhan River valley), brittle faults are observed along Bamnia Nala river between Chhindvi and Nonchhapar villages (Fig. 3a, 6d). Here the base of Lameta Formation is displaced up by at least 300 m in 11

the northern side of the fault about 3 km northeast of Chhindvi village (Fig. 3a). Nakta Nala river, flowing southward across the fault, carves out deep gorges in the basement gneiss about 500m north of this fault line (Figs. 3d,e), but shows no observable down-cutting of the bedrock in the south of the fault. This clearly suggests uplift of the northern side of this fault. However, absolute ages of the faulting events are not known from this area. Presence of hot springs all along the brittle faults in the GTSZ and GFZ terrains attest to their neotectonic nature. The available data indicate that Tertiary and Quaternary brittle faulting in the GTSZ/GFZ manifested in reverse-slip movement, and do not match the kinematics of earlier (Neoproterozoic-Ordovician) ductile or brittle-ductile wrench shearing events. Some earlier workers (Roy and Devarajan, 2003; Seva Dass, et al., 2005) have argued that the Tertiary-Quaternary movements on the GTSZ have actually reactivated an older basin-bounding normal fault which formed along the old GTSZ during the opening of Gondwana basins in the Permian. In fact, almost the whole Satpura Mountain is underlain by Gondwana sequence below the Deccan Trap flows, and the older Gondwana rocks are exposed along the GFZ near Salbardi. Therefore, it is likely that a normal fault, marking the southern limit of Satpura Gondwana basin, formed along the GTSZ in the Permo-Triassic, exploiting the already existing weakness of the ductile (or brittle-ductile) mylonitic fabrics of GTSZ, and in turn it was reversely reactivated during Tertiary-Quaternary faulting. The mylonitic gneiss sliver along the GFZ, just below the Gondwana sequence, can be interpreted as a ‘footwall cut-off’ formed and transported up by the reverse fault during reactivation of the old, steep dipping normal fault (Fig. 7b) (e.g. Bhattacharjee, 2015). However, convincing, unequivocal geological evidence in favour of reverse reactivation of a pre-existing normal fault at the outcrop scale is not available in the GTSZ at present. 4. Discussion:

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From the descriptions presented above, it is clear that the GTSZ has moved repeatedly in different modes throughout the geological time period. There were two phases of movement in the Neoproterozoic, one in Ordovician, one possible (?) activity in PermoTriassic, one in Tertiary and a number of movements in the Quaternary (Holocene) (Fig. 8). Can we term all these movements as ‘reactivation’? As mentioned in Section 1, for very old faults, reactivation is defined as repeated slip movements on a fault, separated by >1 Ma time interval. By that definition, the pseudotachylyte-generating strike-slip fault movements in the GTSZ (ca. 672 Ma and ca. 459 Ma) are ‘reactivation’ of the original ductile shear zone (ca. 950 Ma) as they represent strikingly different deformation conditions represented by different metamorphic grades, and also different senses of slip. The 40Ar/39Ar ages indicate that the slip events were separated by more than 200 Ma in each case (Fig. 8i-iii). These Pt-M and Pt-C related reactivation events were ‘geometric reactivation’ but not ‘kinematic reactivation’ of the ductile shear zone (sensu Holdsworth et al., 1997), as the later fault slip in both cases followed the mechanical anisotropy created by the mylonitic foliation oriented parallel to the shear plane, but the slip sense in each case was opposite to the immediately preceding event (i.e. sinistral ductile shearing, followed by dextral semi-brittle shearing and at last sinistral, wholly brittle shearing, as documented by Chattopadhyay et al., 2008). The Permo-Triassic normal faulting is not yet well constrained geologically. If it is established beyond doubt, it should be considered a major reactivation event of GTSZ, with a completely different kinematics (normal-sense slip). The Tertiary compressional reverse fault slip represents another major reactivation, again with a different (i.e. reverse-slip) kinematics (Fig. 8-iv). Multiple fault-slip events in the Holocene (Bhattacharjee et al., 2016) were separated by <105 year intervals and occurred under the same tectonic stress regime, i.e. the NNE-SSW oriented far field compressive stress arising out of the Indo-Tibetan plate collision. These are, therefore, not considered as ‘reactivation’ events. Rather they are successive slip events on a

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neotectonic active fault. The Holocene events as a whole constitute an episode of ‘reactivation’ of GTSZ after the major Tertiary (or pre-Quaternary) movement (Fig. 8-v). The Tertiary and Holocene reactivation events took place in the same sense (reverse faulting), and, therefore, can be termed as ‘kinematic reactivation’ of the GTSZ/GFZ (sensu Holdsworth et al., 1997). Geodetic GPS measurement has indicated that there is crustal contraction of about 2 mm/year across the Son-Narmada Fault zone (Banerjee et al., 2008) in response to the far field plate boundary stresses caused by India-Tibet continental collision along the Himalayas. This contraction is mostly accommodated by reverse fault movement along GTSZ and SNSF, which together account for about 95% of the total seismic moment released within CITZ (Pimprikar and Rao, 2000). GTSZ is, therefore, stressed at present and is prone to further earthquakes. One question that naturally comes up at this point is what makes the GTSZ so much prone to reactivation over such a long time. The answer possibly lies in the long term strength of deep-seated intracontinental fault zones (Holdsworth et al., 2001a; Imber et al., 2001). Most commonly, the fault cores in long-lived fault zones show fluid-induced phase changes e.g. breakdown of feldspar to white mica or clay minerals, development of chlorite from biotite etc., leading to an interconnected network of mechanically weak layers (‘framework collapse’). This permanently reduces the shear strength of the fault zone rocks and brings the frictional-viscous transition to a shallower level where brittle (frictional) behaviour should otherwise dominate. Crushing of rocks and mineral grains along the fault zone often results in fine grain size of fault rocks, which may trigger grain-boundary sliding and/or the onset of diffusion creep with the aid of fluids infiltrating the fault zone. This further weakens the fault zone with respect to the surrounding intact rock that deforms mainly by frictional processes at the shallow depth. If the fault is oriented favourably for reactivation in the prevailing stress regime, then subsequent crustal deformation will be focussed along these weak fault zones, 14

leading to preferential reactivation of faults over creation of new fractures. Most of the intracontinental faults cut the frictional-viscous transition zone (~10–15 km depth) and are, therefore, affected by the weakening processes mentioned above. In Gavilgarh-Tan Shear Zone, we do not see extensive fluid ingress within the sheared granitic rocks, but fluid related breakdown of feldspar to mica has been documented from thin sections (Chattopadhyay and Khasdeo, 2011). Chlorite is developed profusely along the Pt-M layers, which represent one phase of movement (dextral strike-slip). Therefore, fluid activity has certainly played an important role in at least one phase of fault reactivation. Strong mechanical anisotropy imparted by the steeply dipping mylonitic foliation in the GTSZ rocks was likely to be much prone to later reactivation in a strike-slip mode, even in the absence of large-scale fluid activity. In the Tertiary and Quaternary, dip-slip reverse faulting occurred along GTSZ (GFZ) in response to the compressive stresses emanating from Indo-Tibetan collision. Favourable orientation of the GTSZ with respect to the plate tectonic stress regime in Tertiary and Quaternary can be safely assumed, as Indian plate has a dominantly NNE-directed compression against the Tibetan plate since Eocene (Gowd et al., 1996) and the major compressive stress has since been at a high angle to the dominantly east-west trending GTSZ. The dip of the brittle fault in western part of GTSZ (GFZ) is difficult to constrain, as the fault plane is rarely exposed in the field. The reverse fault shows quite low dip (≈30°) near Khatijapur (Fig. 6c), but at other places (e.g. Salbardi) dip of the fault plane is possibly steeper (Fig. 6a). In the eastern part of GTSZ, the late faults (e.g. Bamniya Nala fault), affecting the ductile shear zone, are certainly much steeper (dip>70° northerly), which suggests that the late reverse faults observed in the western part (in the GFZ) have not necessarily followed the steep dipping mylonitic foliation, which controlled the strike-slip events in the eastern part of the GTSZ. One point of interest here is: how could a steeply dipping fault/shear zone like GTSZ actually take up dip-slip reverse faulting (Tertiary-

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Quaternary) under fault-normal compression unless there is marked weakening of materials along the fault plane? There is no clear answer to this question from the available field evidences at present. It is obvious that this phase of reverse reactivation did not follow the steeply dipping foliation planes, as was the case for the older wrench-type reactivations of GTSZ. One possibility is that in the western part, the Permo-Triassic normal fault, bordering the GTSZ (GFZ) was listric in nature, with steep dip near the surface level but shallow dip at depth. Such a listric fault is likely to be partly reactivated (at depth) and partly cut-off (at shallower level) by the later dip-slip reverse fault. In the GTSZ, the mylonitic sliver observed near Salbardi is actually interpreted as a small horse of the ductile mylonites cut by the reverse fault at depth and brought to the shallower level by the up-dip movement as a ‘footwall cut-off’ (e.g. Fig. 7b). Some workers have suggested that reverse reactivation of a steeply foliated fault zone is not unusual as most fault zones are constituted of multiple anastomosing fault strands in three dimension, and selective reactivation of a few lowdipping strands can result in a steeply inclined fault zone accommodating contractional movement by dip-slip reverse faulting, as has been observed in the Alpine fault, New Zealand, and in Darling Mobile Zone, Western Australia (White et al., 1986). Such selective reverse reactivation of interconnected fault segments at depth is another possible explanation for large-scale dip-slip reactivation of GTSZ, as the fault zone is quite thick (about 2 km wide at surface) and comprises multiple slip zones. But direct evidence in favour of this is lacking at present as we know very little about the fault/shear zone geometry of the GTSZ/GFZ at depth. A reactivated fault zone in the continental crust should be recognized on the basis of multiple criteria – stratigraphic, structural (kinematic), geochronological and neotectonic/ seismic signatures (Holdsworth et al., 1997). It is clear from the above discussion that GTSZ satisfies majority of these criteria. Steeply dipping fault zones like GTSZ are usually more 16

prone to reactivation as they cut deep into the lithosphere and transect the load bearing zones i.e. the frictional-viscous transition zones in the mid-crust and in the upper mantle. Although geophysical (Deep Seismic Sounding) studies in CITZ (Kaila and Krishna, 1992) has interpreted that GTSZ transects the crust-mantle boundary, the role of mantle rheology in the reactivation of GTSZ is yet to be addressed. However, presence of overprinting ductile, semibrittle and brittle deformation fabrics in the mylonites of the GTSZ clearly suggests that this fault zone has reactivated repeatedly in the frictional-viscous transition zone of the quartzofeldspathic rocks (10-15 km depth) and the fault behaviour was controlled by the deformation behaviour of different minerals (e.g. quartz, feldspar and mica) in this secondary load-bearing zone in the middle crust. With such a protracted history of repeated reactivation, GTSZ stands out as a long-lived fundamental weak zone within the Indian craton. It is also interesting to note that the major transpressional deformation (i.e. sinistral ductile shearing) in the GTSZ (ca. 950 Ma) has been interpreted as a result of oblique collision between continental (or continental and arc) fragments within CITZ during the assembly of supercontinent Rodinia (Chattopadhyay and Khasdeo, 2011; Chattopadhyay et al., 2017). Subsequent dextral brittle-ductile (ca. 672 Ma) reactivation of the GTSZ temporally coincide with the large-scale continental rifting and the breakup of Rodinia between ca. 860 and ca. 570 Ma. Reactivation of major faults/shear zones around this time (ca. 650—700 Ma) has been reported from the Eastern Indian craton also (Crowe et al., 2001). The ca. 459 Ma brittle reactivation (sinistral-sense slip) provides the first record of Pan-African tectonic activity within the Central Indian Tectonic Zone. In southern India, Sri Lanka and southern Madagascar similar ages have been interpreted as the timing of extensional collapse of the Pan-African orogens and onset of the assembly of Gondwana (Rajesh et al. 2006 and references therein). The Tertiary-Quaternary reverse reactivation events of GTSZ (GFZ) have evidently taken place in response to the crustal stresses related to 17

Indo-Tibetan plate collision. The repeated fault movements in the GTSZ can, therefore, be tentatively correlated with major crustal collision/accretion events, which demonstrate that far-field plate tectonic forces can affect fundamental weak zones within the deep interior parts of an apparently ‘stable’ craton and may cause intraplate earthquakes.

Acknowledgement: The first author appreciates numerous discussions with Prof. Dhrubajyoti Mukhopadhyay and his research students regarding the issue of reactivation of shear/fault zones in the Indian craton in different academic forums. Research funding from Department of Science and Technology (DST), Govt. of India (Grant No. SR/S4/ES-470/2009 to AC) and from the University of Delhi (Faculty R&D grant no. RC/2015/9677 to AC) are thankfully acknowledged.

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References Acharyya, S., Roy, A., 2000. Tectonothermal history of the Central Indian Tectonic Zone and reactivation of major faults/shear zones. Journal of the Geological Society of India 55, 239-256. Banerjee, P., Bürgmann, R., Nagarajan, B., Apel, E., 2008. Intraplate deformation of the Indian subcontinent. Geophysical Research Letters 35. Bhattacharjee, D., 2015. Tectonics of Gavilgarh-Tan Fault Zone, Central India: A Multidisciplinary Approach, Department of Geology. University of Delhi, Delhi. Bhattacharjee, D., Jain, V., Chattopadhyay, A., Biswas, R.H., Singhvi, A.K., 2016. Geomorphic evidences and chronology of multiple neotectonic events in a cratonic area: Results from the Gavilgarh Fault Zone, central India. Tectonophysics 677, 199-217. Bhowmik, S.K., Pal, T., Roy, A., Pant N.C. 1999. Evidence of pre-Grevillian high-pressure granulite metamorphism from the northern margin of Sausar mobile belt in Central India. Journal of the Geological Society of India 53, 385-399. Bhowmik, S.K., Wilde, S.A., Bhandari, A., Pal, T., Pant, N.C., 2012. Growth of the Greater Indian Landmass and its assembly in Rodinia: Geochronological evidence from the Central Indian Tectonic Zone. Gondwana Research 22, 54-72. Blumenfeld, P., Bouchez, J.-L., 1988. Shear criteria in granite and migmatite deformed in the magmatic and solid states. Journal of Structural Geology 10, 361-372. Butler, R., Holdsworth, R., Lloyd, G., 1997. The role of basement reactivation in continental deformation. Journal of the Geological Society 154, 69-71.

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Chattopadhyay, A., Bandyopadhyay, B., 2004. Fold-thrust tectonics of the Sausar Fold belt and its bearing on the adjacent high pressure metamorphic rocks, Uniformi-tarianism Revisited: Comparison between Ancient and Modern Orogens of India (IGCP-453). Geological Survey of India Special Publication, 319-330. Chattopadhyay, A., Chatterjee, A., Das, K., Sarkar, A., 2017. Neoproterozoic transpression and granite magmatism in the Gavilgarh-Tan Shear Zone, central India: Tectonic significance of U-Pb zircon and U-Th-total Pb monazite ages. Journal of Asian Earth Sciences 147, 485-501. Chattopadhyay, A., Bhattacharjee, D., Mukherjee, S., 2014a. Structure of pseudotachylyte vein systems as a key to co-seismic rupture dynamics: the case of Gavilgarh–Tan Shear Zone, central India. International Journal of Earth Sciences 103(3), 953-965 Chattopadhyay, A., Holdsworth, R.E., Sherlok, S.C., Widdowson, M., 2014b. Constraining the ages of polyphase fault reactivation of the Gavilgarh-Tan Shear Zone, central India using laserprobe40Ar-39Ar dating of pseudotachylytes (Abstract). Tectonics Studies Group Meeting, Cardiff, UK. Chattopadhyay, A., Khasdeo, L., 2011. Structural evolution of Gavilgarh-Tan Shear Zone, central India: A possible case of partitioned transpression during Mesoproterozoic oblique collision within Central Indian Tectonic Zone. Precambrian Research 186, 70-88. Chattopadhyay, A., Khasdeo, L., Holdsworth, R.E., Smith, S.A.F., 2008. Fault reactivation and pseudotachylite generation in the semi-brittle and brittle regimes: examples from the Gavilgarh–Tan Shear Zone, central India. Geological Magazine 145.

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Copley, A., Mitra, S., Sloan, R.A., Gaonkar, S., Reynolds, K., 2014. Active faulting in apparently stable peninsular India: Rift inversion and a HoloceneǦ age great earthquake on the Tapti Fault. Journal of Geophysical Research: Solid Earth 119, 6650-6666. Crowe, W.A., Cosca, M.A., Harris, L.B., 2001. 40Ar/39Ar geochronolgy and Neoproterozoic tectonics along the northern margin of the Eastern Ghats Belt in north Orissa, India. Precambrian Research 108, 237-266. Gillcrist, R., Coward, M., Mugnier, J.-L., 1987. Structural inversion and its controls: examples from the Alpine foreland and the French Alps. Geodinamica acta 1, 5-34. Golani, P.R., Bandyopadhyay, B.K., Gupta, A., 2001. Gavilgarh-Tan Shear: A Prominent Ductile Shear Zone in Central India with Multiple Reactivation History. Geological Survey of India Special Publication 64, 265-272. Gowd, T., Rao, S.S., Chary, K., 1996. Stress field and seismicity in the Indian shield: effects of the collision between India and Eurasia. Pure and Applied Geophysics 146, 503-531. Grocott, J., 1977. The relationship between Precambrian shear belts and modern fault systems. Journal of the Geological Society 133, 257-261. Hirth, G., Tullis, J., 1992. Dislocation creep regimes in quartz aggregates. Journal of Structural Geology 14, 145-159. Holdsworth, R., Butler, C., Roberts, A., 1997. The recognition of reactivation during continental deformation. Journal of the Geological Society (London) 154, 73-78. Holdsworth, R., Stewart, M., Imber, J., Strachan, R., 2001a. The structure and rheological evolution of reactivated continental fault zones: a review and case study. Geological Society, London Special Publications 184, 115-137.

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Holdsworth, R., Handa, M., Miller, J., Buick, I., 2001b. Continental reactivation and reworking: an introduction. Geological Society, London Special Publications 184, 1-12. Imber, J., Holdsworth, R., Butler, C., Strachan, R., 2001. A reappraisal of the SibsonǦ Scholz fault zone model: The nature of the frictional to viscous (“brittleǦ ductile”) transition along a longǦ lived, crustalǦ scale fault, Outer Hebrides, Scotland. Tectonics 20, 601-624. Kaila, K., Krishna, V., 1992. Deep seismic sounding studies in India and major discoveries. Current Science 62, 117-154. Letouzey, J., 1990. Fault reactivation, inversion and fold-thrust belt. Petroleum and Tectonics in mobile belts, 101-128. Muir Wood, R., Mallard, D., 1992. When is a fault ‘extinct’? Journal of the Geological Society 149, 251-254. Pimprikar, S.D., Rao, P.R., 2000. Interim report on analysis and interpretation of seismic data from Jabalpur Observatory. Geological Survey of India (Unpublished Report). Radhakrishna, B., Naqvi, S., 1986. Precambrian continental crust of India and its evolution. The Journal of Geology, 145-166. Rajesh, V., Yokoyama, K., Santosh, M., Arai, S., Oh, C., Kim, S., 2006. Zirconolite and baddeleyite in an ultramafic suite from southern India: Early Ordovician carbonatite-type melts associated with extensional collapse of the Gondwana crust. The Journal of geology 114, 171-188. Ravishanker, 1987a. Neotectonic Activity along the Tapti-satpura lineament in the Central India. Indian Minerals 41, 19-30.

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Roy, A., Devarajan, M.K., 2003. Tectonics and Seismicity of Central India: A Review. In: Abhinaba Roy and D.M Mohabey (Eds), Seismicity of Central India, Gondwana Geological Magazine Special Publication 5, 23-44. Roy, A., Prasad, M.H., 2003. Tectonothermal events in Central Indian Tectonic Zone (CITZ) and its implications in Rodinian crustal assembly. Journal of Asian Earth Sciences 22, 115-129. Roy, A., Kagami, H., Yoshida, M., Roy, A., Bandyopadhyay, B., Chattopadhyay, A., Khan, A., Huin, A., Pal, T., 2006. Rb–Sr and Sm–Nd dating of different metamorphic events from the Sausar Mobile Belt, central India: implications for Proterozoic crustal evolution. Journal of Asian Earth Sciences 26, 61-76. Scholz, C., 1988. The brittle-plastic transition and the depth of seismic faulting. Geologische Rundschau 77, 319-328. SevaDass, Devarajan, M.K., Roy, A., 2005. Tectonic inversion and Intraplate seismicity in Central India: A consequence to India-Asia collision. In: Contributions to Kangra Earthquake Centenary Seminar-2005. Geological Survey of India Special Publication 85, 137-143. Sherlock, S.C., Strachan, R.A., Jones, K.A., 2009. High spatial resolution 40Ar/39Ar dating of pseudotachylites: geochronological evidence for multiple phases of faulting within basement gneisses of the Outer Hebrides (UK). Journal of the Geological Society 166, 1049-1059. Silver, P.G., 1996. Seismic anisotropy beneath the continents: Probing the depths of geology. Annual review of Earth and Planetary Sciences 24, 385-432.

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Stipp, M., StuÈnitz, H., Heilbronner, R., Schmid, S.M., 2002. The eastern Tonale fault zone: a ‘natural laboratory’ for crystal plastic deformation of quartz over a temperature range from 250 to 700 C. Journal of Structural Geology 24, 1861-1884. Sutton, J., Watson, J.V., 1986. Architecture of the continental lithosphere. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences 317, 5-12. White, S., Bretan, P., Rutter, E., 1986. Fault-zone reactivation: kinematics and mechanisms. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences 317, 81-97.

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Explanation of Figures: Fig. 1: Depth-wise distribution of fault zone rocks shown against the lithospheric strength profile (modified after Holdsworth et al. 2001b). Fig. 2: Simplified geological map of the Central Indian Tectonic Zone (CITZ) showing different supracrustal belts, major lineaments and the granulite belts (modified after Roy and Prasad 2003, Bhattacharjee 2015). Fig. 3: a) Simplified geological map of Gavilgarh-Tan Shear Zone (GTSZ) exposed in Kanhan River valley, showing major lithological and structural variations. b) GoogleEarthTM image showing the GTSZ-GFZ lineament. Part of GTSZ (in Kanhan valley is seen in the eastern part of the figure. GTSZ extends much farther to the east. c) Simplified geological map of GFZ showing juxtaposition of Gondwana rocks against Deccan Trap basalts of the south along the GFZ line. d) and e) Lowplunging asymmetric (south-vergent) folds in Gondwana sequence adjacent to the fault line in Dharul and Kkairali sectors. Fig. 4: a) Pre-full Crystallization (PFC) fabric (sensu Blumenfeld and Bouchez 1983) defined by parallel orientation of euhedral K-feldspar crystals in porphyritic granite; b) strong deformation shown by prominent foliation and sinistrally rotated (δ-type) winged porphyroclast in a biotite-rich granodiorite apophysis within a relatively less deformed K-feldspar rich porphyritic granite, indicating rheologically controlled strain partitioning; c) K-feldspar porphyroclast tails showing sinistral shear sense in mylonite; d) S-C structure in photomicrograph confirms sinistral shear sense; Fig. 5: a) Multiple veins of pseudotachylyte (Pt-C) in partly brecciated granitic mylonite in Kanhan River – note the sinistral-sense slip along the foliation, opening up jogs

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filled with Pt-melt (slip sense shown by arrows in the right bottom side of the photograph); b) the later generation pseudotachylyte veins (Pt-C) originate at the contact between Pt-M and the host mylonite; c) fabric asymmetry and strain fringe structures (chlorite beard around relict quartz: near small white arrow) in Pt-M indicate dextral shearing; Fig. 6: a) Down-dip slickenside lineations on a sub-vertical slip plane in Gondwana sandstone; b) Asymmetric fold in Gondwana sandstone indicating a southward transport along GFZ, c) North-dipping reverse fault has cut up the basalt flows and rotated the columnar joints near Khatijapur Lake; the inset figure shows close-up view of the gouge layer in basalt; d) outcrop of brittle fault near Bamniya/Markadoda nala; e) deep gorge formed by down-cutting of basement by Nakta Nala river, about 200m north of the Bamniya Nala fault; f) position of the gorge with respect to the fault shown in Google Earth image. See text for details. Fig. 7: a) Map of GFZ line in Salbardi area showing a sliver of granitic mylonite gneiss bound by crush zones at the contact between Deccan Trap basalt and Gondwana sandstone; b) a schematic diagram explaining the possible origin of the mylonite sliver as a ‘footwall cutoff’ horse during structural inversion of a normal fault in the basement. Fig. 8: Different phases of fault movement along GTSZ through the geological time: i) Sinistral-sense ductile shearing of granitoids at >15 km depth; ii) dextral sense brittle-ductile slip along discrete planes (Pt-M) at 10-15 km depth; iii) sinistral sense brittle movement (Pt-C) at <10 km depth; iv) reverse-slip affecting Lameta and Deccan Trap flows; v) fault-slip recorded in Quaternary sediments and strath terraces (data after Chattopadhyay et al. 2014b, Bhattacharjee et al. 2016).

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Highlights: x

Gavilgarh-Tan Shear Zone (GTSZ) is a prominent lineament in central Indian craton

x

GTSZ records multiple phases of tectonic movement from Neoproterozoic to Holocene

x

Ductile, brittle-ductile and brittle shearing occurred with contrasting kinematics

x

Structural evidences show at least five major reactivation episodes of GTSZ

x

GTSZ is proved to be a very long-lived fundamental weak zone of Indian craton

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Declaration of interests

√‫ ܆‬The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Signed as under: Prof. A. Chattopadhyay, University of Delhi, India

Dr. Dipanjan Bhattacharjee, Indira Gandhi National Tribal University, Amarkantak, India

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