Shear Indicators and Strain Pattern in Quartz Mylonites of Chituad - Deori Shear Zone, Bundelkhand Massif, Central India

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1 Vol. 5(II), April, 2012, pp Shear Indicators and Strain Pattern in Quartz Mylonites of Chituad - Deori Shear Zone, Bundelkhand Massif, Central India S.C. Bhatt and A. Hussain Department of Geology Bundelkhand University, Jhansi, U.P., India geoscb@yahoo.com; hussainashiq25@yahoo.com Abstract The northcentral part of Bundelkhand massif mainly consists of Archean gneissic complex, supracrustal mafics - ultramafics and metasediments (such as the BIF). A major ~ 200 to 500 m wide, steep, north dipping E-W to ESE-WNW trending brittle-ductile shear zone extending from Kuraicha-Chituad villages, for ~25 km to the eastern border of Deori village has been traced. Protomylonites, mylonites and ultramylonites were recognized in this shear zone. Deformation pattern, shear indicators and tectonic strain imply that the Archean basement was subjected to three phases of compressional deformation (D 1 -D 3 ) and later on were affected by brittle-ductile shearing during the D 4 deformation phase. The presence of mylonitic foliations and lineations, S- fabrics, porphyroclasts of altered and kinked feldspars, deformation lamellae and undulose extinction in quartz and feldspars indicate that the Kuraicha-Deori shear zone progressively evolved in brittle-ductile environment. Occurrences of microfaults, extensional cracks, cataclastic flow also imply that ductile shearing was overprinted by brittle shearing. Stretched fabrics of elongated quartz give a magnitude of two dimensional strain ratio (R s = 3.0) and three dimensional flattening type of finite strain ( s = 0.548) developed in medium to high shear strain domains during low- grade metamorphism. Lower values of two dimensional finite strain ratio (R s =2.3) and three dimensional flattening type of finite strain ( s =0.167) were noticed in mylonitic specimens of medium to low shear strain domains. A sinistral top- to-sw sense of shear movement is revealed. Key words: Brittle- ductile shear zones, microstructural analysis, Strain pattern, Bundelkhand massif Introduction The Bundelkhand granitic massif occupying an area of ~26,000 km 2 occurs at the northern segment of the Central Indian Shield (Fig.1). The Vindyan Supergroup of rocks, at the east and the Bijawar Group at the west, surround this semi-circular batholithic massif and, is tectonically separated from the southern protocontinental block (SPB) by Central Indian Tectonic Zone (CITZ). The tectonic boundary between Aravalli and Vindhyan supergroups is demarcated by Great Boundary Fault (GBF). The northern margins of this massif are delineated by the Yamuna fault and also followed by the foredeep basin of Himalaya. The geological and structural aspects of this terrain were earlier studied by Jhingran (1958); Prakash et al. (1975); Mishra and Sharma (1975); Sharma (1982); Basu (1986); Roday et al. (1993); Roday et al. (1995); Sharma and Rahman (2000); Pati et al. (2007) and Bhatt and Hussain (2008).

2 Vol. 5(II), April, 2012, pp. The marginal contacts and intra-cratonic domains of all cratonic blocks are generally characterised by high shear strain domains and fractures along which the deep crustal materials were possibly exhumed (Howell, 1995; Ramsay and Huber, 1983). The high shear strain domains are classified as ductile and brittle-ductile shear zones and define important mechanical heterogeneities. Such shear zones are deciphered both in hand specimen and in plate boundary scales, and are considered vulnerable for reactivation over very long time span (Butler et al., 1997). It is widely accepted that the geometry of mylonitic foliation, asymmetrical fabrics and stretching lineations (Bèrthe et al., 1979; Matlauer et al., 1981 and Malavielle et al., 1984; Mukherjee, 2011) and the uses of shear criteria (Simpson and Schmid 1983; Mukherjee, 2007; Mukherjee and Koyi, 2009; Mukherjee, 2010 a, b; Mukherjee and Koyi, 2010 a, b) provide strong clues for the determination of sense of shear movement in crustal scale shear zones. The progressive development of fabrics in mylonite is generally controlled by crystal plastic deformation, pressure solution and dynamic recrystallisation (Bèrthe et al., 1979). The shear zones occurring in different crustal rocks are generally characterised by different sub-types of mylonites, S-C fabrics and ultramylonites that evolved under medium to high shear strain and retrogression. This work focuses on the tectonic implications of shear structures and strain patterns developed in quartz mylonites emplaced along the Chituad-Deori brittle ductile shear zone located at the northcentral segment of Bundelkhand massif (Fig.1). Ramakrishnan and Vaidyanadhan (2008) summaried the geological aspects of Bundelkhand craton and did not mentioned any information on these E- W trending shear zones. Sharma (2009) has reviewed the work done by earlier workers on cratons of Indian shield and but no detail about this E-W trending shear zone has been provided. Fig. 1: Geological map of Bundelkhand Craton showing Location of the investigated area. (Modified after Basu, 1986) Geological Setup Detailed and comprehensive studies pertaining to geological and structural aspects of Bundelkhand massif and in the study area were carried out by Jhingran (1958), Sharma (1982) and Basu (1986), and Bhatt and Hussain (2008). Jhingran identified ten types of granites and recorded metasedimentaries within the granites. Sharma (1982) classified these 61

3 Shear Indicators and Strain Pattern in Quartz Mylonites of Chituad - Deori Shear Zone, Bundelkhand Massif, Central India: Bhatt and Hussain gneissic and granitic rocks as Kuraicha and Rajaula formations and considered them lithostratigraphically identical to each other. Granitic gneisses constituting the Bundelkhand Archean Gneissic Complex are associated with mica schists, migmatites, quartzites, amphibolites, ultramafics and mafics (Fig.2). The mafic gneisses near Kuraicha consisting of bands of melanosomes and leucosomes exhibit multiple phase foldings. The common occurrence of enclaves and xenoliths of gneissic and other supracrustal rocks within the granitoids provide strong evidences for pre-granitic origin of all metamorphics (Basu, 1982; Bhatt and Hussain, 2008). The Kuraicha gneisses yielded an age of 3297 ± 83 Ma (Mondal et al., 2002). The other associated litho-units of the gneissic complex range in age between 3.2 to 2.2 Ga. Sharma (1982) grouped these gneisses under Kuraicha Formation along with amphibolites, migmatites and schists. These gneisses show sheared contact with pink granites SE of Kuraicha and south of Chituad. These sheared gneisses occurring along the E-W to ESE- WNW trending (Fig.3a and b) major ductile to brittle-ductile shear zones are characterised by mylonitic foliations and stretching lineations (Fig 4a, b and c). The streaky (biotite) gneisses exposed S of Baragaon are characterised by bands of leucosomes and melanosomes. These gneisses also invariably strike ESE-WNW to ENE- WSW with steep ( ) northerly to northeasterly dips (Fig.3a). The lensoidal and discontinuous bodies of white to pinkish grey quartzites associated with the gneissic complex are seen NW to Roni and S to Chituad village. The migmatites occur as lensoidal bodies in the downstreams of Kamla Sagar dam and just above the hillock at Jhankri village. These migmaties are commonly characterised by leucocratic bands of quartz and feldspar with melanosomes of mafic minerals and were affected by three phases of folding and brittleductile shearing. These rocks trend ESE-WNW (Fig.3b). The hard, compact, fine grained and dark green amphibolites and metabasites also occur as lensoidal and discontinuous linear bodies in downstream of Kamla Sagar dam, NW to Roni and near Baragaon villages (Fig.2). The fine grained rock constituting bands of quartz and magnetite are widely exposed N of Mauranipur, NW of Kamla Sagar dam and Baragaon village. These lithounits dip 60 0 towards SW and rarely to NE. The quartz mylonite rocks exposed S of Jhankri village exhibit shearing and multiple phases folding. The fine grained dark green and low-grade metamorphosed mafic and ultramafics are widely exposed just behind the Panchayat Bhawan of Baragaon village (Fig. 2).The main mineral constituents of these rocks are plagioclase, pyroxenes, olivine, and hornblende. Huge outcrops of medium to coarse grained pink granites are widely exposed in the southern extremities of Deori village, and at the N and S boundaries of Baragaon (Fig. 2). The sheared contact between gneisses and granites is also marked S of Baragaon. These medium to coarse granitic rocks got mylonitised and show distinct mylonitic foliations and lineations. The porphyroclasts of quartz and feldspar exhibiting to sinistral top-to-sw sense of shear were produced in ductile regime (Fig. 5e). The shearing effects are also observed E to Deori village. These mylonitised rocks are juxtaposed with medium to coarse pink granites S to Deori village and downstream of Deori nala. These granitoids are generally compact and massive and at places intruded by two generations of quartz veins. The first generation of mega quartz veins (q 1 ) trend NE-SW while the q 2 trends NNW-SSE (Fig. 6a). Minor epidote veins are also generally seen trending NE-SW at a few places within this granitic terrain. 62

4 Vol. 5(II), April, 2012, pp. Fig. 2: Geological map of Kuraicha- Mauranipur area (modified after Bhatt and Hussain, 2008) 63

5 Shear Indicators and Strain Pattern in Quartz Mylonites of Chituad - Deori Shear Zone, Bundelkhand Massif, Central India: Bhatt and Hussain Deformation Pattern The older gneisses (both mafic and streaky) associated with migmatites, talc schists, amphibolite, mafic, ultramafics and supracrustal metasedimentaries of Banded Iron Formation (BIF) were affected by successive deformational phases (D 1 -D 3 ) due to regional compression (Bhatt and Hussain, 2008). The F 1 folds showing tight to isoclinal folds and plunging in NNW to NW directions predominantly occurred in various units of Archean gneisses and Banded Iron Formations (BIF). The axial planes S 1 invariably strike WNW ESE. These F 1 folds were refolded in the next compression (D 2 ) and produced asymmetrical open small and large scale F 2 folds. These F 2 folds, co-planar and co-axial to F 1 folds, are commonly recognised in gneisses, migmatites, mylonitised quartzite and at few places in the BIF. The axis of F 2 folds plunge 30 0 towards NNE and their axial planes (S 2 ) trend in NNE-SSW. The tight and isoclinal folds (F 1 ) generated during the D 1 -phase were observed predominantly within the greyish brown quartz magnetite rocks exposed NW of Kamla Sagar dam and N of Mauranipur railway station. Reclined and open folds (F 2 ) developed during the D 2 -deformation are also observed in these banded quartz- magnetite rocks. The F 3 folds exhibit tight to open geometry and developed during the next D 3 phase. The axial planes (S 3 ) of F 3 folds invariably trend NW-SE (Bhatt and Hussain, 2008). The ductile to brittle-ductile shear zones evolved in the latest D 4 phase, which were observed more prominently in gneisses, and granites. The axial planes of the F 1 fold have been displaced by the S-mylonitic surfaces, whereas the axial planes of the F 2 and the F 3 folds orthogonally disrupted by shear bands (Fig.5a). This probably took place due to back rotation of axial planes and reorientation of fold axis. It is inferred that the ductile and the brittleductile shear and the syntectonic granitic diapirism possibly took place in D 4 phase. The folds showing eye shaped elliptical geometries are resembling with type A or analogous-eyefolds class of sheath given by Alsop and Holdworth (2006). The cross sectional ellipsity of inner most ring (RY Z ) is equivalent to that of outer most ring (RY Z = RYZ) and overall ellipsity remains constant in these folds. Therefore, such folds can be referred as sheath folds. They are mainly recognised in banded mafic gneisses (Fig.5c) and were developed due to repeated rotation of hinges during progressive shearing. The basement of the sheared Archean gneissic complex along with supracrustal and granitoids were sinistrally truncated by linear quartz reefs (Basu, 1986) in the additional D 5 defomation phase. Kuraicha- Deori Ductile and Brittle-Ductile Shear Zones The major steep/subvertical, north dipping, ~ 200 to 500 m wide brittle ductile shear zones passing from gneissic terrain at Kuraicha - Chituad - Roni villages and extending up to Baragaon and Deori villages in E have been recognised (Fig.2). This brittle-ductile shear zone extends ~ 25 km up to eastern side of Deori village and is variably trends ESE-WNW to ENE to WSW (Fig.3b). It is sinistrally truncated by two major quartz reefs near Roni and Deori villages. (Fig.2).The mylonites developed along these shear zones have largely been derived from gneisses, granite gneisses and granites. Rotation of fabrics and reduction in grain size has played key roles in the evolution of proto-, blasto- and ultramylonites. The mylonitisation developed in this granitic terrain near Baragaon, southeast of Deori is mainly derived from granitic protoliths. 64

6 Vol. 5(II), April, 2012, pp. Statistical Summary Calculation Method: Frequency Class interval: 20.0 Degrees Azimuth Filtering: Deactivated Data Type: Bidirectional Population: 10 Maximum Percentage: 50.0 Percent Mean Percentage: 20.0 Percent Standard Deviation:16.3 Fig.3: a. Lower hemisphere stereographic plot showing contoured diagram of poles (N=110) to mylonitic foliation (Contour interval 1, 2, 4, 8, 12, >16% per 1% area). b. Rose diagram showing trends of shear zones. 65

7 Shear Indicators and Strain Pattern in Quartz Mylonites of Chituad - Deori Shear Zone, Bundelkhand Massif, Central India: Bhatt and Hussain Fig. 4a: Rose diagram showing trend of stretching lineation in mylonitised granite at Basari village. b. Rose diagram showing trend of stretching lineation in sheared granite gneisses near Roni village. c. Rose diagram showing trend of stretching lineation in sheared gneisses near Kamala Sagar Dam (Kuraicha). 66

8 Vol. 5(II), April, 2012, pp. W E W E W E Fig.5a. Field Photograph showing open F 2 folds in banded gneisses disrupted by sheared planes (600 m in the southwest of Chituad village). b. Field Photograph showing open minor folds displaced by extensional crenulation cleavage (C-surface) (400 m in the south of Chituad village). c. Field Photograph showing development of elliptical F 1 sheath folds in banded gneisses (200 m in the downstream of Kamala Sagar Dam). d.field Photograph showing dextrally rotated xenoliths of mafic rocks within streaky (biotite) gneisses (600 m northwest of Roni village). e. Field Photograph showing sinistral (top-to SW) rotated porphyroclast of quartz within gneisses (100 m west of Roni village). f. Field Photograph showing sinistrally rotated xenolith of mafic rocks within gneisses (100 m west of Roni village). 67

9 Shear Indicators and Strain Pattern in Quartz Mylonites of Chituad - Deori Shear Zone, Bundelkhand Massif, Central India: Bhatt and Hussain Mesoscopic Shear Sense Indicators Three types of mylonites are well developed in the terrain. The protomylonite containing bigger phenocrysts of quartz (0.5-6cm), feldspar and higher amount of relic fragments (<50%) are exposed NW/NE of Roni village, SW of Chituad village in the gneissic terrain (Figs. 5a, b, c, d and f). This mylonite zone (protomylonite) is also recognized in pink granite at the top of hillock located SW to Baragaon and also SE to Deori village (Fig.2). The stretched and the elongated fabrics of quartz and feldspar are referred as blastomylonite. The mylonitic foliation commonly represented by preferred orientation of stretched minerals (Fig.5d, e and f) strikes E-W to ESE-WNW. The stretching mineral lineation lying sub-horizontal ( ) to main mylonitic foliation is defined by parallel alignment of elongated phenocrysts of quartz and feldspar. It invariably trends in ESE-WNW to E-W direction in sheared gneisses and mylonitised granitoids (Fig.4a, b and c). At a few places, conjugate quartz veins dominantly showing two generations (Fig. 5a and b) in granitic country displace each other. This indicates overprinting of brittle phase on ductile regime. Three types of mylonites developed in moderate to high strain zones. Both protomylonites and ultramylonite constitute bands of different composition and transition between these bands occurs in narrow zones (Fig.6b). Elongated and ribbon shaped fabrics of quartz and feldspar along with flakes of mica formed blastomylonite in high shear strain domain (Fig. 7b and c). The mylonitic foliation and stretching lineation are well developed in these mylonites. The developments of S-C fabrics, demonstrated by four stages are marked by variation in obliquity between them (Fig.5d). Based on variation in intensity of shear strain, S and C surfaces are developed at different angles during progressive shearing. The initial stage of evolution of these shear planes is manifested by high degree (45 0 ). In the second stage, the angle varies from It decreases up to 10 0 in the third stage and these sets become sub-parallel to each other during fourth stage (Bèrthe et al., 1979). Such shear planes are deformed by micro-folding during progressive stages of D 4 deformation. The axial planes of F 1 folds were slipped by S- mylonitic surfaces, while the axial planes of F 2 folds were orthogonally offset by the C- planes (Fig.5a) in sheared gneissic terrain observed S of Chituad village. These possibly indicate pre-shearing origin of these older basement rocks. The rotated porphyroclasts of quartz and feldspar ranging in size from 5 mm to 10 cm (Fig. 5e) developed during medium shear strain conditions. These asymmetrical and rotated porphyroclasts of quartz and feldspar, stretched fabrics, and S-C planes were considered strong mesoscopic shear indicators. Apart from quartz and feldspar grains, the rotated phenocrysts in mafic rocks displaying dominant antithetic (sinistral) top-to-sw shear movement are also observed within the granitic gneisses SW of Chituad village (Fig. 5d and f). Only in a few phenocrysts of rotated mafic rocks indicate a top-to-se shear movement within granitic gneisses (Fig.5d). A transition zone between protomylonite and ultramylonite containing 10-50% relics of quartz and feldspar (Hippertt and Hongen, 1998, Passchier and Trouw, 1998), is also recorded SE of Deori in sheared granitoids (Fig.5b). Under such transition stage the dynamic recrystallisation and granulation effects became more progressive and that resulted into superplastic behavior, thus producing ultramylonite. Such transition zone is well marked just beneath Deori highway over-bridge. The ultramylonite zones vary in width from a few cm to several meters (Fig.6b). These fine grained ultramylonite and coarse protomylonite were intruded by conjugate quartz veins (Fig.6a). The predominance of such extensional fractures implies that the ductile shearing event was possibly overprinted by brittle deformation in 68

10 Vol. 5(II), April, 2012, pp. later stage. However, the dextral rotation is also shown by few phenocrysts of quartz and mafic minerals and indicates a top-to-se shear also recorded within mafic gneisses exposed SE of Chituad village (Fig.5d). Microstructural Studies Microstructural analysis has been attempted by studying oriented thin sections prepared along XY, XZ and YZ planes. The microstructural analysis envisages that the protomylonite mainly constitutes large phenocrysts of quartz and K-feldspar. The porphyroclasts of quartz and feldspar ranging in size from 0.2 mm to 0.4 mm are characterized by strong undulose extinction and commonly wrapped by recrystallised quartz grains and flakes of mica (Fig. 7a, b and d). The growth of the recrystallised quartz and rarely plagioclase are observed in the tails of K-feldspar. Such phenocrysts of quartz and feldspar are designated as pressure shadow (Fig.7a). The progressive growth of mylonitic foliation is represented by preferred orientation of quartz and feldspar. These fabrics having elongated and ribbon shaped elements have an aspect ratio which varies from 5:1 to 7:1 (Fig.7b and c). Therefore, these strained elongated quartz grains were selected as kinematic indicators to estimate the finite strain. Such types of mylonitic fabrics were developed under moderate to high shear strain conditions and formed mylonite zone. The excessive shearing and plastic deformation manifested to produce dominance of ribbon structures in mylonites (Fig. 7b, and 8a). Due to excessive reduction in grain size and dynamic recrystallisation, the proto-, blasto- and S-C mylonites were progressively evolved. The initial stage of development of S- C planes is well marked in a few thin sections (Fig.7e), in which the high degree of α angle (45 0 ) is noticed. However, this angular relationship between these S-C fabrics decreases from which attributed in the second and third stages of evolution (Bèrthe et al., 1979). These newly developed shear planes (C-surface) mainly constitute elongated fabrics of quartz and feldspar. The highly sheared quartz mylonite zone of Jhankri exhibits preferred orientation of S-planes, overprinted by brittle extensional cracks (Fig.7c and 8d). The alteration and granulation effects were also observed in few phenocrysts of K- feldspar (Fig.8b). The microclinisation and twinning in plagioclase examined in a few thin sections reveal that the geometrical dimension and grain size of fabric have changed due to progressive effect of shearing. The antithetically rotated pressure shadows of quartz and feldspar (Fig. 7a) are also observed by Passchier and Simpson, (1986). The dominance of sinistral rotation were displayed by most of the asymmetrical porphyroclasts, pressure shadows and also indicated by the top-to-sw shear movement. Under moderate to high shear strain conditions, the percentage of relic fragments (quartz, feldspar) decreases up to 10-50%. Under such transition zone, the fine grain ultramylonite having lesser percentage of relic fragments and high percentage of matrix were evolved (Fig.8b and c). Strain Pattern The elliptical strained quartz grains were selected as strain markers (Kinematic indicators) to quantify finite strain. The two dimensional and three dimensional tectonic finite strains were estimated in these plastically deformed quartz grains. The length (L), width (W) and orientation angle (Ø) of these elliptical quartz markers were measured in XZ, XY and YZ sections under a polarizing microscope. The values of two dimensional finite strain (Rf) were calculated for each section of a specimen by applying methods proposed by 69

11 Shear Indicators and Strain Pattern in Quartz Mylonites of Chituad - Deori Shear Zone, Bundelkhand Massif, Central India: Bhatt and Hussain Lisle (1985), Ramsay (1967), Dunnet, (1969), Siddans, (1980), Owens (1984). About 100 to 150 elongated grains (Figs. 7b, 8c) were measured for Rf and θ parameters in each thin section. Subsequently, these values were plotted on logarithmic graphs and were matched with reference curve of Lisle (1985). The two dimensional finite strain ratios (Rs) were determined from these curves (Fig. 9) for XZ and YZ planes of each section. The values of two dimensional strain ratio imply that the mylonitic specimens, belonging to a highly sheared domains located near Chituad (Rs= 3.0), Kuraicha (2.85) and Deori (Rs= 2.7) villages witnessed higher values of two dimensional finite strain (Fig.9a and b). By contrast, the specimens collected from Tejpura (Rs=2.5), Singharwara (Rs=2.3) and Pahari (Rs=2.4) villages show lower magnitudes of two dimensional finite strain (Fig.9c and Table-1). The estimated values of two dimensional strain ratio (Rs) for XY and YZ sections were plotted on a Flinn plot (Fig. 10). This Flinn plot (Fig. 10) is represented by fields of apparent flattening, apparent constriction separated by plane strain (k=1). The values of k which express the degree of prolateness and oblateness were determined from the equation proposed by Ramsay and Huber (1983). The values of principal natural or logarithmic strain parameter 1, 2 and 3 were deduced from the equation given by Ramsay and Huber (1983). The shapes of the strain ellipsoid at each locality is also shown on Hossack s (1969, Fig.11) of natural strain, plotted against the Lode shape parameter, v. Subsequently, the amount of three dimensional finite strain (Nadai, 1963, in Ramsay and Huber, 1983) was quantified for each specimen by applying the formula given by Ramsay and Huber (1983). PROTOMYLONITE Fig. 6.a. Field photograph showing two generation (q 1, q 2 ) of quartz veins in migmatites near Jhankri village. b. Field photograph showing a transition zone between coarse grained protomylonite and fine grained ultramylonite, also displaying brittle fractures and two generations of quartz veins displaced by a microfault (Location: below bridge near Deori dam). b 70

12 Vol. 5(II), April, 2012, pp. Fig.7: a. Photomicrograph showing sinistrally rotated porphyroclast of quartz forming pressure shadow and surrounded by recrystallised quartz (XXL). b. Photomicrograph showing elliptical and ribbon shaped quartz grains (E and R) surrounded by recrystallised quartz grains (XXL). c. Photomicrograph showing folded S planes, overprinted by brittle extensional cracks in quartz mylonites. d. Photomicrograph of ultramylonite showing groundmass of dynamically recrystallised and equant quartz 71

13 Shear Indicators and Strain Pattern in Quartz Mylonites of Chituad - Deori Shear Zone, Bundelkhand Massif, Central India: Bhatt and Hussain grains in mylonitised gneisses. e. Photomicrograph showing S-C planes in intensively mylonitised granite. Fig. 8: a. Photomicrograph showing deformed lamellae and undulose extinction in quartz grains with dynamically recrystallised quartz grains. b. Photomicrograph showing alteration of orthoclase into microcline surrounded by elongated quartz and few plagioclase grains. c. Photomicrograph showing few relicts of broken and polycrystalline quartz grains in a higher fraction of dynamically recrystallised quartz matrix. d. Photomicrograph showing k-feldspar porphyroclast with undulose extinction, antithetic microfaults, healed vein and surrounded by recrystallised quartz fabrics. The mylonite specimens collected from Deori, Baragaon, Kuraicha and Singharwara exhibit three dimensional finite strain ratio ranging from Rs= 2.7 to 3.0 in XZ sections and Rs= 2.10 to 2.70 in YZ sections (Table-1 and Fig. 9a and b) with a bulk finite strain ( s) ranging from s= to (Table-2 and Fig. 11). The values of k in these specimens also vary from 0.75 to 1.4 and display the ellipsoids to have oblate shapes (Fig. 10). The highest octahedral shear strain (Yoct) have been recorded in specimen 2 collected from Kuraicha (Yoct =0.633) which is decreasing up to in specimen (4) collected from Chituad village (Table- 2). The mylonites exposed in low shear domains near Singharwara (3), Pahari (7) and Tejpura (1) are characterized by low two dimensional finite strain ratio (Rs) ranging from 2.30 to 2.7 in XZ sections and 2.2 to 2.30 in YZ sections (Table-1 and Fig.9) with principal logarithmic strain ( s) varying from to (Table-2 and Fig. 72

14 Vol. 5(II), April, 2012, pp. 11). The k values computed for specimen (3) and (7) of respectively indicate they are oblate due flattening strain (Fig.10). The maximum value of k (k=1.4) represents a prolate ellipsoid (5) due to constriction, rotation or extension (Fig. 10). The octahedral shear strain (Yoct) also varies from to (Table-2). Kinematic Interpretation and Tectonic Significance The observations on deformation pattern, shear indicators and tectonic strain imply that the older basement rocks underwent compressional tectonism in the earlier phases of deformation (D 1 -D 3 ). This was before they were subjected to later ductile to brittle-ductile shearing. The roof of the granitic massif did not preserve any imprints of folding but were intensely affected by brittle-ductile shearing in pink granitoids exposed near Baragaon and Deori villages in the D 4 phase of deformation, which indicates the syn-tectonic origin of these granitoids. The shearing effects were witnessed by the presence of rotated porphyroclasts of quartz, feldspar and mafic rocks (Figs. 5d, e and f) in sheared gneisses and mylonitised granite. The kinematics of shear indicators examined in various types of mylonites implies that the shearing effects, dynamic recrystallisation and tectonic strain transition played significant roles in transformation of shear planes and rotation of fabrics. The evolution of proto-, S-C and ultramylonites was influenced by sub-phases of D 4 deformation in retrogressive low grade metamorphism. The geometrical style and attitudes of asymmetrical fabrics /rotated porphyroclasts of quartz and feldspar, mylonitic foliation, stretching lineation sheath folds, S-C fabrics and optical characters of quartz and feldspar (undulose extinction, kinking, deformation lamellae, twining, alteration, etc.) infer that the brittle-ductile Kuraicha-Deori shear zone was progressively developed in a non-coaxial (simple shear) regime in a ductile environment. The extensional and microcracks observed in a few phenocrysts of quartz and feldspar imply that the ductile shearing was possibly overprinted by brittle shearing. It is also inferred that the progressive deformation of quartz in protomylonite to mylonite was accommodated by crystal plastic deformational processes. The quartz and K- feldspar were also deformed by fracturing, in a cataclastic and brittle regime of later deformational phases. The K-feldspar examined under microscope in a protomylonite are mainly characterized by micro fractures, healed veins, kinks and recrystallised aggregates which are suggestive brittle-ductile transitional behavior of the shear zone. The stretched fabrics, ribbons of quartz and feldspar, strain and pressure shadows having relic fragments (20-70%) were also generated in moderate to high shear strain domains. Such shear domains are characterized by higher two (Rs =3.0) and three dimensional finite strain ( s =0.548). The larger phenocryst of quartz and feldspar with relic fragments (10-50%) were developed in low to moderate shear domains and are represented by low two dimensional (Rs =2.10) and three ( s= 0.167) dimensional strain. The maximum oblateness shown by specimen 2 (Table- 1 and 2) which belong to Kuraicha gneiss is represented by flattening type of strain. The prolate ellipsoid (specimens) falling in constriction field and showing higher value of k (1.4) belongs to mylonitised pink granite of Baragaon. The majority of mylonitic specimens falling in flattening field exhibit moderate to high shear strain values. It can be summarized that the mylonite zone evolved in moderate to high shear strain domains are represented by flattening strain. 73

15 Shear Indicators and Strain Pattern in Quartz Mylonites of Chituad - Deori Shear Zone, Bundelkhand Massif, Central India: Bhatt and Hussain The development of S-C fabrics is demonstrated by sinistrally rotated porphyroclasts of quartz, feldspar and mafic minerals (Fig.5 d, e and f). These asymmetrical fabrics showing dominant sinistral top-to-sw sense of shear movement. The sense of shear movement exhibited by shear indicators and preferred orientation represented by aligned fabrics of S and C surfaces reveal that the porphyroclasts rotated due to variable tectonic stresses under moderate to high shear strain domain in a ductile regime. The intensity of deformation became more pronounced in the high shear domains which were characterized by ubiquitous presence of mylonitic foliation, stretching lineation, asymmetrical and rotated porphyroclasts, S-C fabrics and eye shaped sheath folds. These fabrics were attributed to plastic deformation along major ESE-WNW brittle-ductile shear zones. The geometry of C-surfaces and extensional crenulation cleavage (Figs.7c and 5b) provide strong clues for predominance of extensional tectonism in the development of ductile to brittle-ductile shear zones. Fig. 9: a, b, c. Rf/Ø values plotted on Lisle curves for elongated quartz grains in XZ and YZ sections. 74

16 Vol. 5(II), April, 2012, pp. Fig. 10. Flinn plot showing relationship between estimated finite strain and k-values (Oblateness and Prolateness). Table-1: showing various parameters to estimate strain in two dimensions (Rs) and k values. Sl. No. Specimen RXZ no. Ri Rs RYZ Ri Rs RXY Ri Rs k

17 Shear Indicators and Strain Pattern in Quartz Mylonites of Chituad - Deori Shear Zone, Bundelkhand Massif, Central India: Bhatt and Hussain Fig. 11: Hossacks plot showing calculated finite strain ( s) and oblate and prolate ellipsoids using Lode s parameter (v). Conclusions 1. The detail structural studies envisage that the Son Narmada Lineament (SNL) located in the south of this massif was considered to be oldest source of thermo-tectonic and tectonic activities and possibly has been activated during Archean to Palaeoproterozoic periods (Mondal et al., 2000). It was assumed that the oldest crust consisting of TTG gneisses (3270 Ma) mafic and streaky gneisses (3297 Ma) along with migmatites, amphibolites and schist were actively deformed and metamorphosed in the three episodes of compressive tectonism (D 1 -D 3 ). The oldest and youngest events of granitic magmatism were syntectonically took place before 2500 Ma and 3297 Ma respectively, which were elegantly erupted along major fractures and brittleductile shear zones in D 4 phase. The linear quartz reefs were evolved along NE-SW trending major shear zones in the later phase of deformation (D 5 ). 2. It is inferred that this brittle-ductile to ductile crustal shear zones evolved at shallow depth and at the contact of gneissic and granitic complex under the influence of moderate to high non-coaxial shear strain. 3. This shear zone dominantly showing top-to-sw (sinistral) sense of shear movement evolved under high magnitude of flattening type of finite strain. 4. The microstructural studies imply that the fabrics in mylonite zones were deformed by crystal plastic and strain softening processes under low to moderate temperature conditions at intermediate to shallow depth. 76

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