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A study of mylonites from parts of the Salem-Attur shear zone (Tamil Nadu) and its tectonic implications

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The E-W running Salem-Attur shear zone demarcates the tectonic boundary between Archaean Dharwar Craton in the north and Proterozoic Southern granulite terrane in the south. This study reveals that the shear zone is a low angle thrust. The thrust
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  JOUR.GEOL.SOC.INDIA,VOL.75,JAN.2010128T. K. BISWALAND OTHERS A Study of Mylonites from parts of the Salem-Attur Shear Zone(Tamil Nadu) and its Tectonic Implications T.K. B ISWAL 1 ,V.T HIRUKUMARAN 2 , K  AMLESHWAR  R  ATRE 1 , K  RISHANU B ANDYAPADHAYA 1 ,K. S UNDARALINGAM 1 andA MIT K  UMAR  M ONDAL 1 1 Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai – 400 076 2 Department of Geology, Govt. Arts College, Salem - 636 007 Email: tkbiswal@iitb.ac.in Abstract: The E-W running Salem-Attur shear zone demarcates the tectonic boundary between Archaean Dharwar Craton in the north and Proterozoic Southern granulite terrane in the south. This study reveals that the shear zone is alow angle thrust. The thrust zone is around 10 m thick and it merges with the main shear zone along the strike. The thrustis developed on charnockite near Odyarpatti, which is retrograded into schists. Further, it is marked by gently dippingmylonitic foliation and subhorizontal lineation. The S-C fabric, mantled porphyroclasts and intragranular faults indicatenortheasterly slip along the thrust. Recumbent shear folds SF 1 are developed within the thrust zone. The thrust has beenfolded by late stage F 2 fold which has brought variation in the orientation of the mylonitic foliation from subhorizontalto vertical attitude; the mylonitic lineations have been rotated to subvertical orientation also. Additionally, the F 2 crenulations and shear cleavages and intersection lineations are superimposed on the mylonitic fabric.Thrusting alongthe Salem-Attur shear zone is probably the cause for upliftment of the charnockites to the upper crust. Post-upliftmentstage has witnessed brittle deformation in the form of development of shear fractures in NNE-SSW and E-W directions.Pseudotachylites are emplaced along these fractures. Keywords: Mylonites, Pseudotachylites, Low angle thrust, Salem-Attur shear zone.  produced from the collision between Dharwar and Africa-Antarctica continental blocks during the Pan-African period (Srikantappa et al. 1988; Raith et al. 1990; Ramakrishnan,2003; Devaraju and Janardhan, 2004).The southern and northern terranes are juxtaposed alongthe Salem-Attur shear zone. The exact nature of  juxtaposition, whether through thrusting or strike slipshearing, has been a matter of debate. Drury and Holt (1980)interpreted the Salem-Attur shear zone to be a dextral strike-slip shear zone based on apparent offset position of structuralgrains between Billirangan and Nilgiri Hill, while Naha and Srinivasan(1996) have described it as a vertical fault on the basis of predominant down dip stretching lineation. Valdiya(1998) also interpreted the shear zone to be a low anglethrust that has been locally steepened. In spite of the downdip lineations and many kinematic indicators suggesting dip-slip character, the shear zone has been interpreted in anumber of recent studies to be a transpressional-dextral strikeslip shear zone (Chetty and Bhaskar Rao, 1998; Bhadra,2000; Jain et al. 2003). Satheesh Kumar and Prasannakumar (2008) described the variation in attitude of the lineations INTRODUCTION The Salem-Attur shear zone (or Moyar-Bhavani-Salem-Attur shear zone) together with Palghat-Cauveri shear zoneform an important tectonic line in the Southern GranuliteProvince, (Fig.1). The zone runs in an E-W direction and demarcates the suture between two geologically distinctterranes namely the northern (NGT) and southern granuliteterranes (SGT) of Archaean and Neoproterozoic agerespectively (Bhaskar Rao et al. 1991, 2003; Harris et al.1994; Janardhan et al. 1996; Ghosh et al. 2004; Vijaya Raoand Rajendra Prasad, 2006).The NGT forms a part of theDharwar craton.The above mentioned terranes, apart fromtheir ages, differ remarkably in terms of structural and metamorphic evolution. The northern terrane shows atransitional margin with the schist belts of the Dharwar craton (pyroxene isograde, Fig.1), marked by an over-allanticlockwise P–T path and northeasterly structural grains;the structural grains swing into E-W direction along theSalem-Attur shear zone. The southern terrane records over-all clockwise P-T path and displays domal structures. TheSGT is also known as Pandyan Mobile Belt and has been   JOURNAL GEOLOGICAL SOCIETY OF INDIAVol.75, January 2010, pp.128-1360016-7622/2010-75-1-128/$ 1.00 © GEOL. SOC. INDIA  JOUR.GEOL.SOC.INDIA,VOL.75,JAN.2010STUDY OF MYLONITES FROM PARTS OF SALEM-ATTUR SHEAR ZONEAND ITSTECTONIC IMPLICATIONS129 as due to inhomogeneous strain. In an earlier study, Bhadra(2000) reported phyllonites from the Attur area that showssteep foliation with near down-dip lineation. He, however,described the lineations to be intersection lineation and interpreted to be the product of dextral shearing. Chetty and Bhaskar Rao(2006) proposed a plumose structure for theshear zone. GEOLOGY OFTHE STUDYAREA In this paper we describe a structural study of the Salem– Attur shear zone between Odayarpatti and S. Nattar Mangalam villages, 7km and 16 km east of Salemrespectively along the Salem–Attur road (Figs.1, 2, 3). Theshear zone with few hundreds of meter thick on average ischaracterized by mylonites and phyllonite within thecharnockitic country rock. The mylonites are superimposed  by brittle shearing that developed pseudotachylites at ChenniMalai Hill to farther east (Fig.4). Charnockite and myloniteare the major rock types in the area. The charnockite enclosesintercalations of mafic granulites and banded magnetitequartzite at several places. The intercalations vary in sizefrom a few mm to tens of meters wide and in length they runfor up to a kilometer.The charnockite is well banded rock consisting of alternate mafic and felsic bands showingvariation in attitude dominantly in ENE-WSW direction(Fig.5a).The felsic bands show pinch and swell structuredue to layer parallel stretching. Under a microscope thefelsic bands show extremely stretched and oftenrecrystallized quartz ribbons and alkali feldspars (Fig.5b)and the mafic bands show the presence of equigranular ortho- and clino- pyroxenes, plagioclase and hornblende(Fig.5c).The mylonites include quartzofeldspathic schists(Fig.5d) and hornblende-biotite schists.The quartzo-feldspathic schists are fine grained and contain layers of quartz ribbons alternating with mica rich layers; the quartzribbons are recrystallized to smaller grains (Fig.5e). Feldspar  porphyroclasts occur along the schistosity plane. Thehornblende-biotite schists represent a phyllonite carryingfine layers of biotite, hornblende and feldspar grains (Fig.5f).We interpret these features to be the result of retrogressionassociated with shearing. SMALL SCALE STRUCTURES The charnockites are marked by penetrative ENE-WSWgneissic fabric (S 1 ) which is axial planar to a set of isoclinalrecumbent folds (F 1 ) that have folded the primary foliation(layering) (Fig.6a).Another set of recumbent folds, thoughmore open than the former, have folded the gneissic foliationwithin the shear zone and shear fractures (C-fabric) aredeveloped parallel to the axial plane of such recumbentfolds along ENE – WSW directions (Fig.6b). These foldsare identified as SF 1 folds and explained to be developed on the gneissic bandings of the charnockites during shearing.The C-fabric in the shear zone is developed parallel to suchshear fractures and remains absolutely horizontal at severalsections as in Odyarpatti (Fig.6c). The gneissic fabric (S 1 )and the C-fabric are involved in open to tight upright F 2 folding (Fig. 6d) along E-W axial plane. A crude axial planar fabric (S 2 ) is associated with F 2 folds along E-W direction.The mylonitic foliation is marked by lineations in theform of grooves and ribs which are more akin to ductileslickenside striae(Lin et al. 2007) than stretching lineation.However, the trend of the ductile slickenside striae and thestretching lineation is parallel in the study area. Hence the Fig.1. Geological map of southern India (simplified after Rama-krishnan 2003). SGT - Southern Granulite Terrane or Pandyan Mobile Belt (grey shade), NGT - NorthernGranuliteTerrane (dotted), shear zones are shown as solid lines and numbered from 1 to 14, 1 - Balehonnur, 2 - Baba- budan, 3 - Chitradurga, 4 - Mettur, 5 - Cuddapah boundary,6 - Moyar, 7 - Bhavani, 8 - Salem-Attur (Moyar-Bhavani-Salem-Attur), 9 - Swedha, 10 - Namaklal, 11 - Palghat-Cauveri/Noyil-Cauveri, 12 - Karur-Oddanchatram,13 - Karur-Oddanchatram-Kodaikanal, 14 - Achankovli.The pyroxene isograde defines the boundary between schist belt and high grade terrane. The boundary is transitional innature. Cuddapah basin represents Mesoproterozoic platform sequence of rocks and unconformably overliesthe Dharwar Craton. Inset shows the mapped localitieswithin the study area, which lies along Salem-Attur road. OP - Odyarpatti, SM - S. Nattar Mangalam, GSZ -Gangavalli Shear Zone.The distance between Salem and Attur is approximately 50 km.  JOUR.GEOL.SOC.INDIA,VOL.75,JAN.2010130T. K. BISWALAND OTHERS Fig.2. Moyar-Bhavani-Salem-Attur Shear Zone around Odyarpatti, Salem, Tamil Nadu. Fig.3. Geological map of the S. Nattar Mangalam around Godu Malai, Salem, Tamil Nadu.  JOUR.GEOL.SOC.INDIA,VOL.75,JAN.2010STUDY OF MYLONITES FROM PARTS OF SALEM-ATTUR SHEAR ZONEAND ITSTECTONIC IMPLICATIONS131 term mylonitic lineation is used hereafter to describe theselinear features. The mylonitic lineations have a low plungetowards NE or are subhorizontal in that direction onsubhorizontal C-surfaces or parting planes (Fig.6c).However, where the linear features are folded by F 2 folds,the lineations plunge down dip on F 2 limbs (Fig.6e); in thesesituations, the F 2 fold axis and intersection lineation produced from the intersection between S 2 and S 1, and S 2 andCfabric are subhorizontal (Fig.6f). Thus two types of lineation namely mylonitic lineation and intersectionlineation, coexists at several places. F 2 folds show plungereversal and at places they are vertical. In such localities,the intersection lineation and fold axis become vertical whilemylonitic lineation remains horizontal.Therefore it is quitedifficult to distinguish these two sets of lineations purely based on orientation.At several localities, pseudotachylites occur as millimeter to centimeter scale dark coloured bands or veins alongfractures in the charnockite.At places they occur as angular  patches and are cut by close spaced fractures (Fig. 7a). Inhand specimen and at the outcrop scale angular clastfragments of charnockitic composition set in a dark colored matrix could be identified within the pseudotachylyte. Under the microscope microlites are identified (Fig.7b). From the bulk chemistry determined by XRF (data available with firstauthor), the pseudotachylites are inferred to have beenformed as a result of nearly complete melting of the former quart, feldspar, pyroxene and mica dominated rock i.e.,from the charnockite. MICROFABRIC STUDY OF MYLONITES The microfabric analysis of the mylonites was carried out on thin sections oriented parallel to the myloniticlineation and perpendicular to the foliation, which is referred as the “ Vorticity Profile Plane ” (Passchier and Coelho,2006).The clasts are dominantly alkali feldspars, which haveundergone both plastic and plastic–cataclastic deformation.Hence many feldspar porphyroclasts are observed along themylonitic foliation (Fig.7c). The porphyroclasts showrotation indicative of dextral top-to-the-NE sense of shear.Microfaults inside the feldspar porphyroclast, as they are athigh angle to C planes, show sinistral shearing antithetic tomain shearing (Fig.7d). In phyllonites, hornblende fish areobserved showing distinct tails that also suggests dextraltop-to-the-NE sense of shear (Fig.7e). In addition to these,S-C fabric is observed in phyllonite where the C-fabric isdefined by shear bands marked by growth of mica and  polygonized thin quartz ribbons while the S-fabric is defined  by an oblique growth of quartz and biotite to C-fabric(Fig.7f).The angularity between S- and C- suggests a thrustslip shearing. This is further substantiated by asymmetricfolds developed in the quartz ribbons where the S-fabricremains axial planar to the folds. LARGE SCALE STRUCTURES The Salem-Attur shear zone has been mapped in detailin two areas, namely Odyarpatti (Fig.2) and S. Nattar Mangalam (Fig.3). The Odyarpatti area shows thecharnockitic rocks forming very high hills.Along the hillslopes the mylonitic rocks are exposed at several places,more distinctly at one locality where they occur as a lowdipping zone. The variations in orientation are due to F 2 folding of the foliation. This is clearly seen in a stereogramof poles to foliation (Fig.2a).The plot shows a girdledistribution with E-W axial plane and westerly plunging E -axis (fold axis). The lineations collected from the shear zoneas well as the country rock and plotted in the Fig.2bshow diverse orientation; this is partly due to F 2 folding and  partly to the association of intersection with mylonitic Fig.4. Geological map of the Gangavalli shear zone around ChenniMalai Hill, Salem, Tamil Nadu.  JOUR.GEOL.SOC.INDIA,VOL.75,JAN.2010132T. K. BISWALAND OTHERS lineation.At Odayarpatti, the shear zone appears to be bifurcating (Fig.2); in fact this is due to gently dipping shear zone intersecting steep topography.The area around S Nattar Mangalam (Fig. 3), farther east of Odyarpatti, is dominated by charnockite. The Salem-Attur shear zone passes within the charnockite and trendsin an E-W direction. Due to extremely weathered character,the shear zone occupies low lying topography.A stereogramof foliation poles (Fig. 3a) shows the E-W stike distributed on a girdle. This is because of F 2 folding which has an axial plane striking E-W and â-axis plunging to WSW.Thelineations plunge due E or W (Fig. 3b). This suggests that inthis area, the lineations are more uniform than in theOdyarpatti area. Farther towards the east, near Chenni MalaiHill, the Gangavalli shear zone is exposed (Fig.4). It is a NNE-SSW trending brittle shear zone where the charnockiteis extremely fractured and shows various sets of shear fractures. Rose diagram reveals that the approximately adbc e f  4.5 mm4.5 mm 4.5 mm 4.5 mm Fig.5.(a) Charnockite showing banding. The felsic bands are boudinaged due to layer parallel stretching. (b) Sheared charnockite under microscope. (c) Photomicrograph of mafic bands in charnockite. (d) Field occurrence of quartz feldspathic mylonite. (e) Photo-micrograph of mylonite showing recrystallized quartz. (f) Photomicrograph showing phyllonites within mafic granulites.
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