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Early Neoproterozoic Deformation Kinematics in the Chottanagpur Gneiss Complex (Eastern India): Evidence from the Curvilinear Hundru Falls Shear ZoneAnalysis
Lithosphere ( IF 1.8 ) Pub Date : 2020-06-30 , DOI: 10.2113/2020/8820919 Nicole Sequeira 1 , Abhijit Bhattacharya 1
Lithosphere ( IF 1.8 ) Pub Date : 2020-06-30 , DOI: 10.2113/2020/8820919 Nicole Sequeira 1 , Abhijit Bhattacharya 1
Affiliation
Curvilinear steep shear zones originate in different tectonic environments. In the Chottanagpur Gneiss Complex (CGC), the steeply dipping, left-lateral and transpressive Early Neoproterozoic Hundru Falls Shear Zone (HFSZ) with predominantly north-down kinematics comprises two domains, e.g., an arcuate NW-striking (in the west) to W-striking (in the east) domain with gently plunging stretching lineation that curves into a W-striking straight-walled domain with down-dip lineation. The basement-piercing HFSZ truncates a carapace of flat-lying amphibolite facies paraschist and granitoid mylonites, and recumbently folded anatectic gneisses. The carapace—inferred to be a midcrustal regional-scale low-angle detachment zone—structurally overlies an older basement of Early Mesoproterozoic anatectic gneisses intruded by Mid-Mesoproterozoic/Early Neoproterozoic granitoids unaffected by the Early Neoproterozoic extensional tectonics. The mean kinematic vorticity values in the steep HFSZ-hosted granitoids computed using the porphyroclast aspect ratio method are 0.74–0.83 and 0.51–0.65 in domains with shallow and steep lineations, respectively. The granitoid mylonites show a chessboard subgrain microstructure, but lack evidence for suprasolidus deformation. The timing relationship between the two domains is unclear. If the two HFSZ domains were contemporaneous, the domain of steep lineations with greater coaxial strain relative to the curvilinear domain formed due to strain partitioning induced by variations in mineralogy and/or temperature of the cooling granitoid plutons. Alternately, the domain of gently plunging lineations in the HFSZ was a distinct shear zone that curved into a subsequent straight-walled shear zone with steeply plunging lineation due to a northward shift in the convergence direction during deformation contemporaneous with the Early Neoproterozoic accretion of the CGC and the Singhbhum Craton.Regional scale shallowly dipping foliations produced in the ductile crust are traditionally attributed to large-scale thrusting in compressional regimes [1–5]. More recently, however, extensional processes such as gravity-driven collapse [6, 7], channel flow [8], metamorphic core-complex formation ([9] and references therein), or midcrustal extension [10, 11] are also considered to be likely mechanisms for nucleation of flat-lying fabrics. Several occurrences of foreland regions in fold-and-thrust belts experiencing extensional deformation either contemporaneously [12–17] or immediately following compressional tectonics at plate margins [11, 18–21] are known. In such cases, the shear sense associated with the shallowly dipping fabric and the types of rocks juxtaposed along the fabric offer significant clues to the origin of the foliation [22].In contrast, steeply dipping foliations within ductile shear zones are well documented and their origins are explained in diverse geological settings [23–29], although curvatures in steeply dipping shear zones pose challenges. Plate boundary geometry controls the style and kinematics of structures developed along curved shear zones formed at plate margins [26, 30–33], while curvatures in intraplate shear zones are also influenced by irregularly shaped plate margins and indenters [34–38]. Other factors that influence the curvature of intraplate shear zones include the reactivation of preexisting rheologically controlled basement dislocation structures [39, 40], especially those formed at former continental plate boundaries [38, 41]; lateral rheological heterogeneities in the lithosphere [42–44]; and the interaction of synchronously active en echelon faults [45–47] as in releasing and restraining bends, pull apart basins, strike slip duplexes, and splay structures [41, 48–50]. Curvatures on steeply dipping shear zones can also be the result of reorganization of preexisting shear zones by later deformation [36, 51–54].Field studies of natural shear zones primarily use variations in the orientation of stretching lineations/foliations and the strain history of the rocks to understand the mode of formation of the shear zone. Strain modeling based on field observations of changing orientation of stretching lineations along curvilinear shear zones [55, 56] essentially attributes the changes in stretching lineation orientations to variations in finite strain and the angle between the movement direction and the strike of the shear zone.The Chottanagpur Gneiss Complex (CGC) in Eastern India (Figure 1) is an 80,000 km2 Meso- to Neoproterozoic terrain flanked in the south by an arcuate terrain boundary shear zone, the North Singhbhum Mobile Belt (NSMB) [57, 58], and in the north by the Paleoproterozoic (1.7–1.6 Ga) rocks of the Rajgir hills (Figure 1) [59, 60]. The tectonic relevance of these Paleoproterozoic rocks to the CGC is unknown. Based on detailed structural mapping, kinematic vorticity analysis, and chemical age dating in monazite, this work identifies for the first time two broadly contemporaneous Early Neoproterozoic deformation events in the CGC that produced a regional shallowly dipping foliation and a curvilinear steeply dipping shear zone, the Hundru Falls Shear Zone (HFSZ) (Figure 2(a)). The HFSZ forms a part of a network of several crustal-scale E/ENE-striking shear zones that transect the CGC, with shorter NW-striking segments curving into the E-striking shear zones (Figure 1). This study addresses the structural and kinematic significances of the shallowly dipping foliation and the shear zones/lineaments in the centrally located parts of the CGC and their implications in relation to the accretion history of the CGC with the flanking crustal block comprising the Archean Singhbhum Craton and the Early Mesoproterozoic southern part of the North Singhbhum Mobile Belt ([61]; Figure 1).The Chottanagpur Gneiss Complex (Figure 1) is dominated by granitoids. Structurally, the granitoids vary from massive, foliated to mylonitic, and gneissose. Granitoid emplacement occurred in two episodes. The older Middle Mesoproterozoic (1.4–1.2 Ga) granitoids are recognized in the central and northern parts of the CGC [59, 62, 63], whereas the younger Late Mesoproterozoic to Early Neoproterozoic (1.1–0.9 Ga) granitoids are extensive across the CGC [59, 61, 64–68]. The granitoids host outcrop to regional-scale bodies of multiply deformed anatectic quartzofeldspathic gneisses, garnet-sillimanite and calc-silicate gneisses, mafic granulites, charnockite-enderbites, alkaline complexes, and anorthosite massifs. The regional-scale bodies of granulite facies metamorphic rocks within the granitoids are displayed in Figure 1. These enclaves comprise the oldest lithodemic unit of the CGC. Age data from the Deoghar-Dumka, Saltora-Purulia, and Ranchi areas (Figure 1) indicate that the polyphase orthogneisses emplaced at 1.6 Ga [61, 69, 70] and 1.4 Ga [61, 63] were subsequently intruded by Middle Mesoproterozoic and Early Neoproterozoic granitoids and reworked extensively at 1.1–0.9 Ga [61, 71], coinciding with high-grade isothermal decompression [71]. The granitoids and the high-grade anatectic gneisses comprise the basement of the CGC.Amphibolite facies supracrustal rocks (Figure 1) dominated by mica schists (±garnet±sillimanite) and micaceous quartzites/quartzites, with minor proportions of carbonate rocks/marls and amphibolites, occur as east-trending belts in the CGC. These supracrustal belts dominate the topographic highs in the CGC and are closely associated with the regional-scale shear zones (Figure 1). Available dates in the supracrustal belts [61, 64, 65] suggest that the shallow-platformal foreland sediments were metamorphosed between 1.0 and 0.9 Ga. Unmetamorphosed sedimentary rocks of the Late Paleozoic to Early Cretaceous Gondwana Supergroup unconformably overlie the CGC crystalline basement (Figure 1).Structurally, very little is known about the sequence of fabric forming deformation events and the large-scale tectonic evolution of the CGC. Fold superposition structures are reported from a few areas, but a coherent understanding of the timing and relationship of the structures in the three lithodemic units is lacking. Existing information suggests that both the Early Mesoproterozoic granulite facies gneisses [58, 59, 72, 73] and the Early Neoproterozoic amphibolite facies supracrustal rocks [74–79] experienced three deformation episodes, but a disparate sequence of structures is manifested within the two lithodemic units. In the anatectic gneisses, steep axial planar fabrics to tight-isoclinal interfolial folds on mineralogical segregation banding exhibited by leucosome layers form the penetrative fabric [58, 59]. Distal from the E-striking shear zones, the composite fabric is N striking [72, 73]. Within the shear zones, the composite fabric is deflected into a set of noncylindrical E-trending upright folds with locally penetrative E-striking axial planes [58, 61]. The supracrustal rocks on the other hand record open to tight, gentle to moderately plunging, upright to steeply inclined W/WNW-trending folds [76, 78] that overprint recumbent to gently inclined folds interfolial to a penetrative schistosity. The Mid-Mesoproterozoic granitoids of the CGC share the early steeply dipping N-striking fabric of the granulite facies gneisses [62, 63] but lack the intrafolial isoclinal folds. When present, tectonic fabrics in the Early Neoproterozoic granitoids are generally E striking, i.e., subparallel with the axial planes of the upright folds in the anatectic gneisses [58, 80].The field relations discussed below are based on data collated from 300 field stations spanning ~1,200 sq. km (Figure 2(a)) in the Ramgarh-Ormanjhi area, north of the town of Ranchi in central CGC. The area comprises extensive exposures of Early Neoproterozoic (see later) blastoporphyritic granitoids, granite-granodiorite in composition, intrusive into the high-grade Early Mesoproterozoic anatectic gneisses. The Early Neoproterozoic (see later) supracrustal rocks comprising mica schists (biotite-muscovite-quartz, rare garnet), micaceous quartzites, quartzites, metacarbonates (amphibole-plagioclase-quartz±epidote±calcite), minor amphibolites (amphibole-plagioclase±quartz±epidote±sphene), and ferruginous quartzites are restricted to a tapering NW-striking belt and an E-striking belt, structurally overlying the granitoid-gneiss basement rocks (Figure 2(a)). No intrusive contact relationships between the supracrustal rocks and the granitoids were observed in the area.Four deformation events (D1-D4) are recorded in the rocks in the Ramgarh-Ormanjhi area (Figure 2(a)). The D1-D2 deformation events are associated with the Early Mesoproterozoic granulite facies metamorphism and anatexis in the basement gneisses of the CGC [61, 63, 69]. The D1 deformation is identified from the isoclinal fold hinges on D1 leucosome layers—concordant with biotite-hornblende aggregates—preserved in the interfolial domains of the penetrative steeply dipping D2 gneissic layers. The Early Neoproterozoic granitoids ([61, 64, 65]; this study, see later) intrusive into the basement gneisses truncate the D1-D2 composite layering. These Early Mesoproterozoic high temperature D1-D2 fabrics are lacking in the amphibolite facies supracrustal rocks.This study focuses on the subsequent D3 and D4 deformation events that affected the CGC during the Late Mesoproterozoic-Early Neoproterozoic. Based on the structures developed during these two deformation events, the investigated area is divided into three domains (Figure 2(a), Table 1). The rocks in large parts of the area (Domain-I) possess a shallowly dipping penetrative foliation (D3; Figure 3) prominently exhibited by mylonitic granitoids with well-developed stretching lineations (Figure 3(a)). These mylonite fabrics are coplanar with a crenulation cleavage in the overlying supracrustal rocks, and axial planes of recumbently folded D1-D2 fabric in the anatectic gneiss enclaves (Figure 3(b)). The expansive nature of the regional-scale shallowly dipping D3 foliation has not been addressed earlier in the context of tectonic evolution in the CGC. This deformation event is restricted to the higher levels of the crust and constitutes a shallowly dipping carapace over the basement. Below the carapace, the D1/D2 fabrics in the gneisses and the post-D2 granitoid intrusives are unaffected by the D3 deformation event (Figure 3(c)).The deformation event (D4) produced a ~25 km wide steeply dipping sinistral shear zone transposing all earlier fabrics into verticality within the shear zone. We identify the hitherto unreported curved shear zone as the Hundru Falls Shear Zone (HFSZ) encompassing Domain-II and Domain-III (Figure 2(a), Table 1).In Domain-I (Figure 2(a), Table 1), the blastoporphyritic granitoids occur as S>L tectonites (Figure 3(a)) with a shallowly dipping monophase mylonitic foliation manifested by the planar alignment of biotite and flattened quartz grains that wrap around winged porphyroclasts of core-mantle structured feldspar (modally microcline>plagioclase) with polycrystalline tails. The foliation dips gently to the north (Figure 2(b)), and the well-developed stretching lineations (LS3; S=stretching; Figure 3(a)) on the D3 mylonitic foliation are defined by quartz ribbons, flakes of biotite, and aggregates of recrystallized feldspar tails. LS3 plunges at low angles towards NW and SE neighboring the NW arm of the HFSZ, and E and W neighboring the eastern arm of the HFSZ (Figures 2(a) and 2(b)). These mylonitic granitoids are part of the shallowly dipping carapace, whereas the massive granitoids (determined to be Early Neoproterozoic, see later), common in the SE of Domain-I (Figure 2(a)), are inferred to be windows within the carapace, exposing the underlying basement unaffected by the D3 deformation.The penetrative and finely laminated crenulation schistosity in the supracrustal rocks (Figure 2(e)) is coplanar with the shallowly dipping granitoids in the D3 carapace overlying the basement (Figure 2(b)). The D3 schistosity in the mica schists is exhibited by shape-preferred aggregates of muscovite and biotite in the M-domains and polygonized quartz grains in the Q-domains (Figure 3(d)). The fabric in the calc-silicate rocks is defined by hornblende±epidote-rich layers alternating with plagioclase-calcite layers. Vestiges of an earlier fabric are observed in microscale as curving strands in the D3 interfolial domains and oblique inclusion trails in pre-D3 garnets. The former fabric has the same mineral assemblage as the D3 assemblage. The intersection of the two foliations appears as a striping lineation (on D3 foliation) defined by mineral segregation banding in the mica schists and a linear alignment of prismatic minerals like hornblende in the calc-silicate rocks. The NW-trending northern supracrustal belt (Figure 2(a)) is restricted within the boundaries of the HFSZ wherein the D3 fabric becomes folded and transposed into the steep-dipping D4 shear zone fabric. However, in a few locations flanking the HFSZ where the shallowly dipping supracrustal rocks prevail, the striping lineation on the gently dipping D3 foliation surfaces plunges at shallow angles to the NW/SE (Figure 2(e)), largely collinear with the LS3 stretching lineations of the granitoids (Figure 2(b)).Shear sense indicators on the D3 foliation in both the granitoids and the supracrustal rocks are uncommon. When present, S-C fabrics and asymmetric porphyroclasts in XZ sections (perpendicular to foliation and parallel to lineation) of granitoids display a top-to-the-NW sense of movement (Figure 3(e)) and YZ sections (perpendicular to foliation and lineation) show top-to-the-NE sense of movement (Figure 3(f)). Similar YZ sections in supracrustal rocks also have top-to-NE shear sense shown by asymmetric porphyroclasts, shear band cleavage, and mica fish (Figure 3(d)).The non-anatectic amphibolite facies Early Neoproterozoic supracrustal rocks do not possess the high-grade D1-D2 fabrics in the anatectic gneisses, do not share an unconformable relationship, and are not intruded by granitoids. We therefore interpret that supracrustal unit to be an allochthonous block. Duplex structures in para-amphibolites of the supracrustal belt interleaved with slivers of shallowly dipping granitoid mylonites are observed in road-cut sections within Domain-I (Figure 3(g)). The top-to-the-NE translation inferred from the duplex structures is identical to shear sense observed in the YZ sections of the shallowly dipping granitoids and supracrustal rocks. Therefore, we infer that the D3 carapace formed due to top-to-the-NE translation of the allochthonous supracrustal rocks over the basement rocks. The boundary between the D3 carapace and the basement rocks unaffected by D3 deformation is inferred to be a shallowly dipping decollement (Figure 3(c)).The steeply dipping curvilinear HFSZ (Figure 2(a)) overprints the shallowly dipping D3 foliations in the supracrustal rocks, granitoids, and anatectic gneisses. Steeply dipping D4 shears either truncate the D3 foliation (Figures 4(a) and 4(b)) or produce open to close upright, asymmetric folds with subhorizontal fold axes on the D3 foliation (Figure 4(c)) at the boundary of the HFSZ and Domain-I and in lensoidal-shaped low-D4 strain domains within the shear zone (Figure 2(a)). Within the low-D4 strain lens along the E-trending arm of the HFSZ, the poles to the folded D3 foliation describe a well-defined girdle with a gentle W-plunging fold axis (Figure 4(c)). The LS3 stretching lineations on folded D3 surfaces generate a small circle girdle centered on the fold axis of the D3 foliation pole girdle (Figure 4(c); e.g., [81]). The negligibly small angle between the stretching lineations and the D4 fold axis [82] attests to the coaxial nature of the early LS3 lineation and the later D4 fold axis. Identical geometric relations are noted near Ormanjhi (Figure 2(a)) where the shear zone curves NW. Within the shear zone, the steeply dipping D4 mylonitic foliation becomes the penetrative fabric.The granitoid mylonites within the HFSZ are S≥L tectonites; L>S tectonites are rare. The stretching lineations on the D4 foliation, however, differ in orientation within the HFSZ. Based on the differences, the HFSZ is divided into two domains (Figure 2(a), Table 1): Domain-II curving from NW to E-W contains gently plunging stretching lineations (LS4A; Figure 4(d)), and Domain-III trending E-W contains down-dip stretching lineations (LS4B; Figure 4(e)).In Domain-II, the poles to the mylonitic D4 foliation in lower hemisphere stereoplots (Figure 2(c)) show an arc distribution demonstrating the curvilinear nature of the domain. The gently plunging LS4A stretching lineations (Figure 4(d)), like the LS3 stretching lineations in granitoids of Domain-I, plunge towards the NW/W and SE/E indicating that the stretching direction X (X>Y>Z) is collinear between Domain-I and Domain-II. The hinges of the asymmetric folds on the D3 foliation in Domain-I granitoids flanking the HFSZ are also coaxial with the stretching lineations (Figure 4(c)). Well-preserved shear sense indicators in the form of asymmetric porphyroclasts and S-C fabrics consistently display a sinistral shear sense in XZ sections (subhorizontal plane) of the Domain-II granitoid mylonites (Figure 5(a)); a north-down sense of movement is weakly developed in the YZ section (subvertical plane) (Figure 5(b)). A south-down sense of movement on S-dipping mylonite foliations is restricted to the northern margin of the HFSZ (Figure 5(c)).In Domain-III at the southern flank of the HFSZ, the subvertical S≥L granitoid tectonites possess down-dip LS4B stretching lineations (Figures 2(d) and 4(e)). The domain is juxtaposed against the southern supracrustal belt (Figure 2(a)). L>S granitoid mylonites were observed at a single location near the village of Mishirhutang (Figure 2(a)); otherwise, the LS4B stretching lineations occur on a well-defined D4 foliation plane. Rare instances of shear lenses with tight, reclined interfolial folds within the granitoid mylonites were observed within this domain (Figure 5(d)). Shear sense indicators describe a consistent sinistral (Figure 5(e)), but poorly defined N-down sense of movement in sections perpendicular to the D4 mylonitic foliation.Within the HFSZ, the penetrative D3 crenulation schistosity of the supracrustal rocks describes large-scale, E/ESE-trending, steeply inclined to upright asymmetric folds with subhorizontal (dominant) to gently plunging hinge lines (Figure 6(a)). Although at the regional scale the folds are noncylindrical, the noncylindricity is not evident in outcrop scale. The orientations of D4 axial planes and fold axes (Figure 2(f)) follow the trends of the D4 mylonitic foliation and LS4A stretching lineation in the Domain-II granitoids (Figure 2(c)). The D4 axial planar foliation is well developed in the centre of the HFSZ and describes a sinistral shear sense on plan view (Figure 6(b)). North-vergent folds are restricted to the northern flank of the HFSZ. The supracrustal rocks are poorly exposed within Domain-III of the HFSZ. Rare outcrops of weathered calc-silicate rocks exhibit subvertical E-striking penetrative foliations with very tight to isoclinal folds preserved in the intrafolial domains.Anatectic gneisses are rarely exposed in the HFSZ. The composite D1-D2 fabric in these gneisses is E/ESE striking, steeply dipping, and characterized by moderate to gently plunging fold axes with a very weak development of D4 axial planar foliation (Figure 6(c)), indicating that the HFSZ is basement piercing.The granitoid mylonites comprise quartz, microcline, biotite, and plagioclase; hornblende and garnet are rare or absent. Ilmenite, apatite, zircon, and monazite are accessory phases. Examination of a large number of thin sections cut in different orientations suggests that the modal amounts of biotite in Domain-III granitoids are slightly higher relative to the Domain-II granitoids. No other systematic mineralogical variation is evident in the granitoids across the three domains. Quartz dominantly occurs as cm-scale ribbons (Figure 7(a)). Quartz grains also exist as subequant dynamically recrystallized grains anchored to the recrystallized microcline grains in the matrix (Figure 7(b)). Undulatory extinction and strongly misoriented subgrains oblique to the walls of quartz ribbons are common; the features are also observed in discrete quartz grains in the matrix (Figure 7(b)). In Domain-I and Domain-III, weakly developed chessboard subgrain structures in quartz grains, similar to those described by Blumenfeld et al. [83] and Kruhl [84], are observed (Figures 7(c) and 7(d)). But chessboard subgrain microstructures in quartz are lacking in Domain-II granitoids.Alkali feldspar, as microcline, displays core-and-mantle structures. Deformation bands, subgrains, and undulatory extinction are common in the cores of microcline grains (sub-cm sized); the polycrystalline tails of asymmetric microcline clasts are composed of aggregates of elongate (long axis: 100–200 μm) and subequant recrystallized grains. In domains where biotite flakes are closely spaced and adjacent to the quartz ribbons (Figures 7(a) and 7(b)), the microcline grains are smaller (long axis: 50–100 μm) and elongate and grain boundaries meet the micas/quartz ribbons at 90° ([85]; Figure 7(a)). The recrystallized microcline grains (<50 μm in diameter) are smaller and “pinned” within the biotite segregations and adjacent to the quartz ribbons. Commonly, discrete biotite flakes at low angle with the shear zone fabrics limit the microcline grain boundaries, and the recrystallized microcline grains in turn overgrow the biotite flakes leaving the flakes stranded within the feldspar (cf. [86, 87]). The grain boundaries of the internally strained microcline grains are invariably serrated due to progressive misorientation of subgrains (Figures 7(a) and 7(b)). The microcline-quartz contacts are serrated and share lobate-cuspate geometries; the quartz grains commonly show bulge nucleation against the microcline grains. Myrmekites are locally observed, and triple junctions are lacking in microcline and quartz.The microstructures in the three domains suggest that grain size modification during granitoid mylonitization was induced primarily by dislocation creep deformation. Subgrain rotation [88] accommodated fast grain boundary migration recrystallization was the dominant deformation mechanism; this was aided by synkinematic grain boundary sliding in the mica-rich parts. Chess-board microstructures in quartz formed by simultaneous activation of a- and c-slips indicate that the granitoids experienced subsolidus deformation in the range 650–750°C [83, 84]. Microstructural evidence does not support suprasolidus deformation—manifested by trains and imbrications of touching grains of euhedral plagioclase [89], growth of plagioclase with rationally developed faces into quartz films [90], and zoned plagioclase grains [91]—in the granitoids in any of the domains. Annealing recrystallization is extremely limited in the granitoid mylonites.The switched stretching lineations in Domain-II and Domain-III of the curvilinear, steeply dipping, sinistral HFSZ attest to differing strain regimes between the two domains. Mutually perpendicular stretching lineations due to flipping of X and Y axes of strain ellipsoids in shear zones are well documented [55, 56, 92–95]. Partitioning of strain into simple shear-dominated (subhorizontal stretching lineations) and pure shear-dominated (subvertical stretching lineations) segments is the most cited reason for switching of stretching lineations. To assess the strain conditions in the two domains, a set of five samples from Domain-II and Domain-III in the E-trending arm of the HFSZ was chosen for kinematic vorticity estimations. The mean vorticity number (Wm; [96]) was calculated from three methods based on rigid porphyroclast rotation patterns, i.e., the porphyroclast aspect ratio (PAR) method after Passchier [96] and Wallis et al. [97], the porphyroclast hyperbolic distribution (PHD) method after Simpson and De Paor [98], and the rigid grain net (RGN) method after Jessup et al. [99]. Wm values vary between zero for pure shear-dominated deformation and one for simple shear-dominated deformation. However, numerical modeling studies show that kinematic vorticity estimations from asymmetric rigid clasts generally underestimate Wm values and are associated with large uncertainties [100–103]. This study uses the Wm values in a relative sense, admitting that a larger component of the simple shear component may be present than suggested by the values obtained.The three methods assume plane strain and steady state deformation, and use the orientations of asymmetric porphyroclasts in a ductilely deformed rock to estimate the vorticity in sections perpendicular to foliation containing the maximum asymmetry [104]. Theoretically, such sections are presumed to be perpendicular to the vorticity vector and are labeled as vorticity normal sections (VNS). In monoclinically symmetric shear zones, the VNS is assumed to be either parallel (XZ plane) or perpendicular (YZ plane) to the stretching lineations, whereas in triclinic shear zones, the VNS is independent of the orientation of the stretching lineations in the rock [105, 106].The PAR method of Wallis et al. [97] uses a linear plot of the porphyroclast aspect ratio (R=long axis/short axis) versus φ (angle between the long axes of clasts and the flow plane) assuming no mechanical interaction between porphyroclasts rotating in a homogenously deforming matrix. A critical aspect ratio (Rc) separates continuously rotating porphyroclasts with no preferred alignment of their long axes from porphyroclasts that have achieved stable orientations. The Wm value is computed from the Rc value using the relationship Wm=Rc2−1/Rc2+1 [96, 97]. In the PHD method [98, 107], the aspect ratio, R, is plotted against φ of the rotated porphyroclasts with well-developed tails on a hyperbolic net. Wm is the cosine of the angle subtended by the two limbs of the hyperbola that separates back-rotated clasts from other clasts. The graphical RGN method of Jessup et al. [99] is essentially a derivative of the PAR method in which theoretically computed semihyperbolic nets for forward-rotated and back-rotated σ- and δ-type porphyroclasts (Figure 8(a) in [104]) where φ versus B∗ (B∗=Mx2−Mn2/Mx2+Mn2, where Mx and Mn are the short and long axes of clasts) plots are used to constrain the Wm value in a sample.Three granitoid mylonite samples from Domain-II and two from Domain-III were chosen, all possessing well-developed stretching lineations. The VNS was determined from each of the oriented samples by cutting several sections perpendicular to the foliation at 10° angle intervals from the horizontal and determining the section with the maximum clast asymmetry as carried out in Toy et al. [108, 109]. For the samples from Domain-II, the VNS was found to be approximately parallel to the subhorizontal stretching lineations, indicating a monoclinic symmetry for this domain. In Domain-III samples, the VNS was determined to be oblique to the down-dip stretching lineations, approximately dipping at angles between 20° and 30° towards the west, generating a triclinic symmetry for this domain. Once the VNS was determined, several parallel rock slabs were cut, and measurements were made on images of these polished rock slabs. In a couple of locations of Domain-II granitoids, measurements were made manually on well-exposed horizontal surfaces (Figure 8(a)); these data were coupled with those obtained from the polished rock slabs. Only feldspar clasts were chosen for the analyses. The samples were chosen with wide ranges of size and shape of clasts; the long axis of the clasts ranged between 3 and 53 mm, and the aspect ratios of the clasts ranged between 1 and 3.5, although the aspect ratios of symmetric clasts were as high as 6. Mica-dominated ultramylonites and mylonites with <25% clasts were chosen to ensure that the clasts experienced free rotation in the deforming matrix. In four samples, the number of clasts measured per sample is between 144 and 215; in RR-250, fewer clasts (n=66) were measured because of the coarse size of the clasts.The three methods were applied to the Domain-II sample RR-350 (Figures 8(b)–8(d)). The PHD method (Figure 8(b)) yielded a Wm value of 0.69, smaller than that estimated using the other two methods which yielded internally consistent values, e.g., 0.74–0.80 (PAR) (Figure 8(c)) and 0.72–0.75 (RGN) (Figure 8(d)). This discrepancy may reflect the observations of Xypolias [104] in that the PHD method overestimates the pure shear components relative to the other methods that produce comparable results. For consistency, we employed the PAR method to estimate Wm values for all the five samples (Figures 8(c) and 8(e)–8(h)). The results of the PAR method estimate Wm values for Domain-II in the range between 0.74 and 0.83 (Figures 8(b), 8(e), and 8(f)), whereas the two samples from Domain-III yield Wm=0.51–0.65 (Figures 8(g) and 8(h)). It follows that Domain-III accommodates a higher pure shear component relative to Domain-II within the HFSZ. These values lie outside the range of estimated Wk values in natural samples commonly between 0.65 and 0.75 in rocks [103]. Stahr III and Law [103] suggest that for rigid grain methods applied to natural samples, vorticity values are overestimated for pure shear-dominated flows, while lower values are obtained for simple shear-dominated flows. For such a scenario, the difference between the vorticity values we estimate for Domain-II and Domain-III would in reality be larger, allowing for Domain-II to be completely simple shear dominated (Wk≥0.83) and Domain-III to be pure shear dominated (possibly Wk≤0.59). This is consistent with the fact that simple shear-dominated deformation produces subhorizontal stretching lineations for monoclinic shear zones [25, 56, 110].To constrain the age of deformation in the HFSZ, we obtained chemical dates in monazites in granitoids and mica schists in and neighboring HFSZ. Following Schoene et al. [111], we use the term “date” to define the Th-U-total Pb date calculated from the measured element abundances using decay equations following Montel et al. [112], whereas the term “age” is the geologic interpretation of a date in a tectono-metamorphic context.Age determination using the Th-U-total Pb (total) chemical age technique in monazite has several unique advantages. The technique has high spatial resolution (beam diameter~1 μm) and can be used to date texturally constrained in situ monazite [113–116]. Also, monazites tend to grow readily, unlike zircons, by fluid-induced dissolution-precipitation processes in a wide range of temperature, and especially at low temperatures [114, 117–121] well below the blocking temperature of intracrystalline U, Pb diffusion at T<850°C [122–125]. As a consequence, the ages of the deformation fabric at greenschist/amphibolite facies can be readily obtained from structurally and texturally constrained monazite grains; also, older magmatic and high-T metamorphic dates may be preserved especially in dry assemblages where fluid activity is limited as in weakly strained rocks, and in monazites sequestered within robust grains lodged in strain shadow zones of subsequent low-T events.Th-U-total Pb dates in monazites were determined in the supracrustal rocks and granitoid mylonites within and outside the HFSZ (locations in Figure 2(a), Table 2). Sample-wise summary of rock types and mineralogy, compositional variations in monazites, age range, and mean±2σ of age populations are summarized in Table 2. The analyses of monazites were performed using a 4-WDS Cameca SX-100 electron probe microanalyzer in the National Facility, Indian Institute of Technology, Kharagpur. The crystal configurations, the analytical conditions, and the standards used for analysis are identical to the protocol-I of Prabhakar [126]. The monazites are strongly zoned in Th and Y (Figures 9(a)–9(d); Table 2); variations in Pb and U concentrations are negligible in the analyzed monazites (Table 2). Element abundances, monazite spot dates (±2σ), and error % (error %=100×{2σ error in Ma/age in Ma}; Prabhakar [126]) are provided in Supplementary Data1 (available here). Spot dates with error%<8 were considered and statistically resolved for mean ages of populations using Isoplot 3.0 [127]. The spot dates were monitored against the standard Moacyr monazite; the date for the Moacyr monazite is determined to be 497±10 Ma (EMP age, [128]), 487±0.5 Ma (TIMS age, [129]), and 509.3±0.5 Ma (TIMS age [128]). For the duration of analyses, the Moacyr monazite was analyzed 12 times, and the date (±2σ) varied between 487±29 and 504±32, with a mean value of 496±8 Ma.There is no existing information on metamorphic temperature in the supracrustal rocks specifically from the Ramagarh-Ormanjhi area. But P-T path reconstructions from the supracrustal rocks elsewhere in the CGC such as in the Rourkela-Rajgangpur [64, 65] and Gridih-Dumka-Deogarh-Chakai areas [130] suggest that the rocks were metamorphosed between 550 and 670°C at midcrustal depth. The T estimates are consistent with those estimated in this study from deformation microstructures in the granitoid mylonite. These T values are ~200°C below the blocking temperature of U, Pb intracrystalline diffusion in monazites (>850°C; [122–124]). We suggest therefore that the sharply defined Th and Y zonations in monazites lineated parallel to and overgrowing the metamorphic fabrics in the mica schists (Figures 9(c) and 9(d)) are of metamorphic origin. These monazites were produced by fluid-induced dissolution-precipitation processes [131, 132], and not due to intracrystalline diffusion. Hence, the chemical dates retrieved from these chemical domains correspond to deformation-metamorphic events. By the above reasoning, monazites hosted in biotite defining the mylonitic fabric in granitoids are of metamorphic origin. But monazites within clasts of former magmatic feldspar grains in the granitoid mylonites and in granitoids with no mesoscale fabric are likely to be of magmatic origin.The spot dates in all rocks taken together vary between 874±60 Ma (RR-6) and 1066±64 Ma (RR-35) (Table 2). Only 8 out of 120 spot ages are >1000 Ma, out of which one spot date 1066±64 Ma (RR-35) is >1050 Ma (Table 2). This data is taken to be an outlier. Seven of the >1000 Ma spot dates are obtained in nebulously zoned cores in mica schists that are mantled by chemical domains with younger dates. A mean date of 960±60 Ma is obtained for all the 120 spot dates taken together; further subsets of population could not be statistically resolved using the software of Ludwig [127] for all of the data taken together (Figure 9(e)) and for the individual samples (Table 2). Overall, the grain margins yielded younger dates relative to the interiors of the nebulously zoned monazites in both mica schists and the granitoids, but given the 2σ errors of spot dates, the difference in dates did not translate into statistically resolvable populations. No systematic age difference could be deciphered between the HFSZ, the shallowly dipping Domain-I mylonites, and the massive granitoid, i.e., both D3 and D4 deformations are Early Neoproterozoic in age, and immediately followed large-scale granitoid emplacement in the CGC. The dates from the present study area (Figure 9(e)) are identical to monazite chemical dates obtained by Bhattacharya et al. [65] and Chowdhury and Lentz [133] in the Gangpur Schist Belt (Figure 9(f)) which is part of the accretionary zone at the southern flank of the CGC (Figure 1). It is evident therefore that the D3 and D4 deformations in central CGC are contemporaneous with the accretion of the CGC with the Archean Singhbhum Craton in the south.The age ranges and the mean ages of Early Neoproterozoic metamorphic monazites in the mica schists overlap with the chemical ages in monazites hosted in magmatic feldspar clasts and in biotite aggregates in the Domain-I and Domain-II granitoid mylonites (Table 2). Several lines of evidence point towards episodic granitoid emplacement in the area, broadly contemporaneous with the nucleation of the shallowly dipping D3 carapace and the HFSZ (D4): (1) The shallowly dipping Domain-I granitoids in the carapace over the basement are intensely mylonitized, and microcline porphyroclasts larger than 2 mm diameter are uncommon. But microcline porphyroclasts in Domain-II and Domain-III granitoid mylonites are profuse and are commonly greater than 5 mm diameter, and the largest clasts measure 5 cm or more in length. (2) The bulk of the HFSZ granitoids possess monophase steeply dipping mylonitic foliation and lack evidence of an overprinted D3 deformation fabric. Thus, if all the granitoids were pre-D3, feldspar clasts in the D4 shear zone should be similar sized or smaller, and the D3 fabric is likely to be preserved at least locally. The occurrence of larger-sized K-feldspar clasts in granitoids forming the bulk of D4 shear zones, and the lack of D3 structures, suggest that these granitoids were unaffected by the D3 deformation. Taken together, this implies that a broad contemporaneity existed between granitoid emplacement and D3-D4 deformations. (3) Weakly developed chessboard subgrain microstructures in quartz in Domain-I and Domain-III indicate that the Early Neoproterozoic D3 and D4 deformations occurred at high temperature (T>650°C; [84]). But suprasolidus deformation microstructures such as minerals with euhedral faces piercing neighboring quartz and K-feldspar grains [91], trains and imbrications among euhedral feldspar aggregates [89], and optically homogenous quartz grains impinging against feldspars [134, 135] are lacking in the granitoids in the three domains. Combining the evidences, we suggest that the HFSZ nucleation was contemporaneous with postemplacement cooling of Early Neoproterozoic felsic granitoids. Similar situations have been invoked to explain microstructural development and strain partitioning in shear zones and shear bands in a large number of terrains, e.g., the Iberian Arc [136], the Hermitage Massif, France [137], the Papoose Flat pluton, California [138], the Central Alps [139], and the leucogranites of the Higher Himalayan crystallites [140].This study for the first time documents the existence of expansive domains of horizontal/shallowly dipping carapaces in the CGC that reoriented structurally higher parts of older (Early Mesoproterozoic) anatectic grade D1-D2 steeply dipping deformation fabrics (Figure 3(c)) in the basement gneisses [61, 63, 69, 70]. The D3 deformation is inferred to be associated with the transport of the allochthonous amphibolite facies supracrustal unit over the basement granitoids and gneisses along a gently dipping midcrustal decollement (Figure 10(a)). The NE-directed transport of the supracrustal unit produced the penetrative shallowly dipping schistosity in the supracrustal rocks, the mylonitic foliation in the granitoids, and recumbent folding of the D1-D2 structures in the upper parts of the high-grade anatectic gneisses. The deformation is reminiscent of a basement-involved thin-skinned tectonic regime [141–143]. The NW/SE-trending LS3 stretching lineations on the mylonitic D3 foliation in the granitoids (Figures 2(b), 3(a), and 3(f)) suggest lengthening orthogonal to the transport direction (cf. [144], Figure 10(a)).Several tectonic explanations can be suggested for the formation of the shallowly dipping carapace. The broad contemporaneity of the D3 structures with the accretion of the CGC with the Singhbhum Craton (Figures 1, 9(e), and 9(f)) suggests that the deformation occurred in an overall compression regime. However, the absence of older-on-younger thrust sequences, i.e., the Early/Middle Mesoproterozoic gneisses overlying the Early Neoproterozoic supracrustal rocks, undermines thrusting as the causal mechanism for the shallowly dipping D3 carapace. Normal shear sense observed on the gentle N-dipping D3 foliation planes (Figures 2(a), 2(b), and 2(e)) in the granitoids and in the supracrustal rocks (Figures 3(d) and 3(e)) preserved in outcrops largely unaffected by the D4 deformation implies that the D3 foliation is akin to a midcrustal regional-scale low-angle normal fault. Such low-angle normal faults that produce shallowly dipping mylonitic fabrics are genetically associated with extensional tectonics [145–152] and are widely reported in detachment zones of metamorphic core complexes ([9] and references therein). As in these detachment zones, the D3 shallow-dipping carapace separates rock units of differing metamorphic grade and origin, but exhumation of the high-grade basement gneisses along the detachment needs to be investigated further before definite conclusions can be made. Several studies have nonetheless reported near-isothermal decompression paths in the high-grade basement rocks during the Early Neoproterozoic deformation of the CGC [63, 71, 153].Steeply dipping to vertical shear zones with subhorizontal stretching lineation form due to transpression or transtension [24, 26, 154–158]. Numerical strain modeling [156] suggests that folds on an initially horizontal layer (in this case, the shallow-dipping D3 foliation) in a transtension regime initiate at angles greater than 45° to the shear zone wall, but these fold hinges do not rotate into parallelism with the shear zone wall, irrespective of the amount of pure shear or simple shear acting on the shear zone. In contrast, folds in a transpressive regime originate at angles less than 45° to the shear zone wall, and rotate into parallelism with the shear zone wall with progressive deformation [156]. Within Domain-II in HFSZ, the D4 fold axes and axial planes of close, asymmetric, steeply inclined to upright, gently plunging folds on the earlier shallowly dipping D3 foliation in the supracrustal rocks (Figures 2(e) and 6(a)) and granitoid mylonites (Figure 4(c)) are collinear and subparallel to the shear zone walls along the entire length of curvilinear HFSZ. This is strong evidence for the HFSZ forming D4 deformation to have developed in a transpressive regime. Therefore, even if the shallow D3 fabrics formed due to extension, the HFSZ and possibly other steeply dipping regional-scale shear zones traversing the CGC (Figure 1) accommodated the compressional strain within the foreland region during the contemporaneous accretion of the CGC with the Singhbhum Craton in the south.Theoretical models provide insights into dynamics of formation of transpressional shear zones [24, 26, 105, 106, 110, 154, 158–161]. Fossen and Tikoff [26] propose five transpressional models that encompass the entire range of possible strain variations in monoclinic transpressional shear zones. Domain-II in HFSZ with horizontal stretching lineations compares favorably with Types D and E of Fossen and Tikoff [26]. Type D exhibits a plane strain-dominated deformation with horizontal shortening normal to the vertical shear zone wall compensated by shear zone parallel extension in the horizontal plane, and Type E has an additional component of vertical pure shear for similar parameters as in Type D. But, highly constrictional strains are produced in Type E conditions resulting in subhorizontal L tectonites within steeply dipping shear zones [26, 154]. Such L tectonites are lacking in Domain-II, making Type D strain conditions (Figure 10(b)) more probable than Type E. Notably, in the Type D model, the stretching lineation is always subhorizontal in orientation for a vertical shear zone irrespective of the amount of finite strain [26]. The triclinic symmetry of Domain-III hinders direct comparisons with theoretical models. A better constraint on shear zone parameters such as obliquity angle, extrusion angle, and vorticity number is required before this domain can be modeled to produce meaningful results.Therefore, we suggest that Domain-II of the HFSZ tended towards a monoclinic symmetry, the strain was not constrictional but accommodated a lateral stretch parallel to the shear zone walls, and an increase in the amount of strain would not cause a switch in the orientation of the subhorizontal stretching lineations into down-dip orientations. Domain-III experienced a higher proportion of a pure shear component, possibly including small amounts of constrictional strain (presence of L>S tectonite at Mishirhutang, Figure 2(a)), and the symmetry was triclinic accommodating a subvertical stretch component.Flipped lineations are attributed to variation in the magnitude of strain, strain partitioning, or change in the angle between the shear zone wall and the orientation of the far field stress [55, 56, 93, 95, 110, 161–163]. Simple shear-dominated strain produces subhorizontal stretching lineations, while pure shear-dominated strain produces subvertical stretching lineations on steeply dipping transpressional shear zones [24–26, 56, 161]. Vorticity estimations (Figure 8) demonstrate that Domain-II developed with a considerably higher simple shear component of the bulk strain as compared to Domain-III. A cut-off value of Wk=0.81 has been proposed to separate pure shear- (Wk<0.81) from simple shear-(Wk≥0.81) dominated flows in monoclinic shear zones [25]. The range of Wm values determined for the monoclinic Domain-II straddles Wk=0.81, and consequently the flow regime would be intermediate between pure and simple shear dominated. For the cut-off value of Wk=0.74 proposed by Law et al. [164] and the observation of underestimating vorticity values in simple shear-dominated flows [101–103], Domain-II is totally simple shear dominated. The lower range of Wm values estimated for Domain-III granitoids is indicative of a larger coaxial shear component, but due to uncertainties associated with Wm estimations from rigid clasts and the triclinicity in Domain-III (oblique VNS), these results are only speculative. Even so, because of the differing strike orientations and symmetries in the two domains, the flipped stretching lineations are unlikely to have simply formed from an increase in the coaxial strain component [25, 26, 161].Strain partitioning is the fundamental mechanism for producing variations in structures along or across shear zones. Strain partitioning occurs predominantly along structural weaknesses [165] such as rheological heterogeneities, lithological contacts, and preexisting deformation structures. Anisotropies produced during progressive deformation can further allow subsequent strain to be partitioned. Partitioned strain is also a cause for primary curvatures on transpressional shear zones [56, 162, 163, 165, 166].In transpressional shear zones, rheologically stronger units accommodate the pure shear component, while the simple shear component is partitioned into the weaker units. The orientation of the left-lateral transpressive HFSZ implies that the far field compressional paleostress axes were oriented in the arc between NNE and ENE. For a uniform orientation of the far field stress and if Domain-II and Domain-III were contemporaneous, the discordant kinematics and the shear zone curvature could have resulted from strain partitioning due to rheological contrasts induced by subtle variations in mineralogy of the plutons or variations in rheology and deformation behavior in minerals in cooling plutons [137]. Biotite-richer plutons theoretically allow greater partitioning of simple shear (cf. [167]); but modal amounts of biotite appear to be somewhat greater in the Domain-III granitoids than in the Domain-II granitoids of the HFSZ. Evidently, strain partitioning in the HFSZ would have had to overcome the mineralogical differences in the two domains to attain the present structure. Differential cooling in the granitoid plutons in the two domains is more likely to have played a role in partitioning strain.The southern boundary of Domain-III is juxtaposed with the southern supracrustal belt (Figure 2(a)) dominated by amphibolites, calc-silicate gneisses, and quartzites. The domain extends westward as far as the E-trending supracrustal band continues. Thus, the close association of Domain-III with the southern supracrustal belt (Figure 2(a)) dominated by weak-to-shear minerals (such as biotite and muscovite) suggests that partitioning of the bulk strain at the junction of the contrasting rheology might have helped to nucleate the straight-walled E-trending Domain-III with triclinic symmetry and down-dip lineations (Table 1). The juxtaposition of these two lithologies would lead to competency contrasts resulting from material heterogeneity, promoting strain partitioning with further deformation [168]. Nucleation of shear bands along domain boundaries due to strain incompatibilities and mechanical anisotropies has been noted [168–170]. Such rheological contrasts could have induced partitioning of the bulk strain into pure shear- and simple shear-dominated domains, a possibility for developing the structures and symmetry observed in Domain-III (Table 1).If strain partitioning was indeed the major factor controlling the simultaneous formation of the two domains and producing the curvature of the HFSZ, the two domains can be compared to a large scale S-C fabric [171–173] within a sinistral setting, with Domain-II and Domain-III corresponding to the S and C fabrics, respectively (Figure 2(a)). The two domains also have geometric settings comparable to primary curvatures on shear zones such as a right-stepping restraining bend or a splay structure ([41, 48, 50]. These possibilities would however require the curving segment of the shear zone (Domain-II) to accommodate a greater amount of pure shear producing down-dip lineations, reverse faults, positive flower structures, and vertical extrusion of material [41, 50, 174, 175] as compared to the straight segment (Domain-III) [56]. This is opposite to the fact that Domain-III rather than Domain-II accommodated a greater pure shear component as determined from kinematic vorticity estimations (Figure 8).Alternatively, the two domains within the HFSZ were not contemporaneous, but formed during progressive D4 deformation as separate shear zones with distinct characteristics (Table 1), i.e., the early-formed Domain-II shear zone curved into the subsequent Domain-III segment (Figures 10(b) and 10(c)). The rare shear lens with tight reclined folds interfolial to the steep-dipping mylonitic foliation of Domain-III (Figure 5(b)) could be a result of overprinting relationships between the two domain shear zones. Reorganization of early-formed deformation structures during progressive deformation is widely reported [36, 51–54]. Emerging data suggests that changing convergence vectors during plate motions can play a major part in the development of new structures that reorient earlier fabrics in the rocks [176–180].Considering that the two domains formed sequential shear zones, the protocol of Diaz-Azpiroz et al. [181]) was used to equate the Wm values estimated for the two domains to the vorticity number, Wk values [182], which translate to convergence angles (α) equal to 19–25° for Domain-II and 30–40° for Domain-III (Figure 11). α is the angle between the plate motion vector and the strike of the shear zone, where α=90° represents orthogonal convergence and α=0° represents strike slip. The convergence direction in the E-trending Domain-III was computed to lie between 50°N and 60°N (Figure 10(c)). By contrast, the convergence direction for the curved Domain-II is computed to be 87°N–94°N. Since the vorticity measurements were conducted on the E-striking segment of Domain-II (Figures 8(b)–8(f)), and the Domain-II shear zone would have been drawn into parallelism due to superposition of the Domain-III shear zone, the actual strike orientation of Domain-II prior to reorientation was possibly NW-SE (~135°N). If so, α values of 19–25° translate to a convergence orientation of 110°N–116°N (Figure 10(b)). Although the values are only suggestive, the HFSZ could have formed because of a shift in the convergence vector for transpressional shortening from E/ESE (Domain-II) to NE/ENE (Domain-III). The change in the orientations of the strongly developed subhorizontal stretching lineations in Domain-II to the subvertical orientation in Domain-III may have resulted from a switch in the orientation of the convergence direction, causing the earlier formed NW fabrics to sigmoidally curve into the E-striking shear zone of Domain-III. A similar switch in convergence vector has been proposed to explain the structural framework in Costa Rica due to the change in the convergence direction of the Cocos plate [177].In the Early Neoproterozoic, the Chottanagpur Gneiss Complex experienced a major episode of felsic granitoid emplacement. This was followed by the translation of allochthonous supracrustal rocks which formed a shallowly dipping carapace (Figure 10(a)), and finally nucleation of several E-striking crustal-scale steeply dipping transpressional shear zones (Figure 1). The mesoscale structures and the deformation kinematics of the shallowly dipping D3 carapace and the shear zones (D4) especially in the interior of CGC were hitherto undocumented. This study addresses these issues using structural mapping (Figure 2), microstructural studies (Figure 7), vorticity analyses (Figure 8), and monazite geochronology (Figure 9) in central CGC.The D3 deformation that produced recumbent folding of older steep-dipping basement gneisses, duplex structures in the granitoids/supracrustals, and asymmetry microstructures in granitoids/supracrustal rocks suggests top-to-the-NE translation of the allochthonous supracrustal unit orthogonal to NW-SE lineations in shallowly dipping mylonites (Figure 10(a)). The translation of the supracrustal unit is attributed to midcrustal extension occurring along a shallowly dipping decollement that extended across large parts of central CGC, similar to midcrustal low-angle normal faults, but the implications of the finding are yet to be fully understood.A steep-dipping curvilinear D4 Hundru Falls Shear Zone (HFSZ) is SE striking in the NW and E striking in the east and exhibits sinistral kinematics and dominantly north-down sense of movement along bounding inward dipping margins (Figure 10(d)). The HFSZ truncates the shallowly dipping granitoid mylonites (Domain-I). Gently plunging stretching lineations in the NW-striking arm of the HFSZ (Domain-II) are collinear with the axes of E/SE-trending asymmetric folds on shallowly dipping mylonites, and steeply inclined/upright folds on originally shallowly dipping crenulation foliation in supracrustal rocks. Along the southern flank of the E-striking arm of the HFSZ, steep mylonite foliations are associated with steeply plunging stretching lineations (Domain-III). Kinematic vorticity analyses for Domain-II (Wm=0.73–0.83) and Domain-III (Wm=0.51–0.65) suggest a plane strain, sinistral monoclinic kinematic model for Domain-II and a sinistral, pure shear-dominated, triclinic deformation regime for Domain-III.Left-lateral simple shear-dominated transpression is suggested to have nucleated the NW arm of the HFSZ (Figure 10(b)). Continued transpression with changing convergence direction caused existing structures to curve into the progressively formed E-trending HFSZ with steeply plunging stretching lineations (Figure 10(c)) producing the structures observed in the area (Figure 10(d)). Early Neoproterozoic metamorphic monazites in supracrustal rocks and metamorphic rims around magmatic monazite cores in granitoid mylonites suggest that the D3 and D4 deformation events were contemporaneous with accretion of the Archean Singhbhum Craton along the southern margin of CGC. This study adds to the growing body of emerging data that suggests strain field instabilities induced by variations in plate convergence angle cause reorientation of mesoscale structures in a regional scale.The authors declare that they have no conflicts of interest.The work forms a part of the doctoral dissertation of NS. Financial support for the work was provided by University Grants Commission (New Delhi) through a Junior Research Fellowship to NS. AB acknowledges the financial support for fieldwork and chemical analyses provided by the host institute through the CPDA funding scheme for the block years 2017–2020. We thank C. Fernandez and M. Diaz-Azpiroz for their insightful comments on shear zone formation in an earlier draft of the manuscript. Editorial handling of the manuscript by Damien Nance is greatly appreciated.Supplementary Material1: electron probe microanalytical data in wt% oxides on monazites from the present study area, chemical ages, and ±2σ errors of spot ages in Ma determined using the formulation of Montel et al. [112]. Sample locations are shown in Figure 2(a).
更新日期：2020-08-20
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