Journal of Asian Earth Sciences 34 (2009) 577–585

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Journal of Asian Earth Sciences

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Structural fabric of the Southern Indian shield as defined by gravity trends

Niraj Kumar, A.P. Singh *, Bijendra SinghNational Geophysical Research Institute, (Council of Scientific and Industrial Research), Uppal Road, Hyderabad 500 606, India

a r t i c l e i n f o

Article history:Received 19 October 2007Received in revised form 4 June 2008Accepted 28 August 2008

Keywords:Southern granulite terrainHorizontal gravity gradientGravity domainShear and sutureStructural domain

1367-9120/$ - see front matter � 2008 Elsevier Ltd. Adoi:10.1016/j.jseaes.2008.08.009

* Corresponding author. Tel.: +91 40 27204115; faxE-mail address: apsingh_ngri@yahoo.com (A.P. Sin

a b s t r a c t

Southern Indian shield represents a mosaic comprised of several smaller structural domains separated bydiscrete shear zones. Here we present a horizontal Bouguer gravity gradient map of the Indian shield,south of 14 �N, to define a continental mosaic of gravity trends domains akin to structural domains.The gravity gradient image is based on 7862 newly collected observations merged with 6359 old gravitydata. This combined dataset delineates structural boundaries of the five gravity domains related to theEastern Dharwar Craton, the Eastern Ghats Mobile Belt, the extended Eastern Ghats Mobile Belt, theSouthern Granulite Terrain, and the Western Dharwar Craton. Other belts of significant gravity gradientsare found associated with the Eastern and the Western coasts. The loci of Closepet granite and Kolarschist belts do not manifest themselves as boundary zones between two distinct gravity domains ofthe Eastern Dharwar Craton. Lack of a gravity gradient across Karur–Oddanchatram–Kodaikanal and Kar-ur–Kambam–Painavu–Trichur Shear Zones may be attributed to a lack of gravity measurements causedby difficulties in collecting data in topographically difficult terrain. The subdued gravity gradient acrossthe Palghat–Cauvery Shear Zone and a weak gradient across the Achankovil Shear Zone indicates a lith-ological and/or morphological boundary rather than a terrane boundary. Alternatively, structuraldomains encompassing Palghat–Cauvery and Achankovil Shear Zones may have been in a neighbouringposition during the Gondwana assembly, when Pan-African thermal perturbation reactivated the struc-tures and reworked partly or totally obliterating earlier crustal fabric.

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1. Introduction

Terrane boundaries typically mark major petrophysical discon-tinuities, along which discrete crustal domains were brought andwelded together during different periods of their evolutionary his-tory. Detailed information on the continuity, internal fabric andstructural development of such zones is crucial for understandingcrustal processes such as continental break-up and terrane amal-gamation (Williams et al., 2006). This is the case in Southern Indianshield which comprises highly deformed cryptic sutures; theiridentification is rather difficult due to the imprints of later tectonicevents (Drury et al., 1984; Naqvi, 1985; Krogstad et al., 1989;Chetty and Murthy, 1994; Chadwick et al., 1997, 2000; Leelanan-dam, 1990). Complementary geophysical parameters namely, dip-ping reflectors, regional pattern of heat production and offset in theMoho across these domain boundaries have provided some vitalclues to the crustal architecture and domain tectonics responsiblefor their evolution (Kaila et al., 1979; Srinagesh and Rai, 1996; Red-dy et al., 2003; Ray et al., 2003; Roy et al., 2003; Rao et al., 2006).However, the exact location and extent of the domain boundariesof the Southern Indian shield are still debated.

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Density contrasts caused by the juxtaposition of domains withdifferent petrophysical properties, at locations such as sutures,can cause paired (positive and negative) gravity anomalies. Suchpaired gravity anomalies, encountered across dissimilar structuralprovinces worldwide (Gibb and Thomas, 1976; Fountain and Salis-bury, 1981), were used as a signature to define the ancient suturezones of the Peninsular India (Subrahmanyam and Verma, 1986;Mishra et al., 2000; Singh and Mishra, 2002; Singh et al., 2003,2004, 2006). However, some paired gravity anomalies were not rec-ognized because they are sometimes overprinted by later events ormasked by the presence of long wavelength anomalies related toisostatic roots compensating for topographic loads (Subrahman-yam, 1978; Narain and Subrahmanyam, 1986; Rao and Prasad,2000). Paired gravity anomalies may be easily recognised in residualgravity anomaly maps as they are no longer masked by often largerregional geophysical responses. A standard separation filter for thepotential field maps is, however, not yet given by elementary func-tion that is numerically stable and is also physically comprehensiblewhen applied to real, random anomalies (Jacobsen, 1987). Horizon-tal-gradient maps on the other hand are vivid, simple and intuitivederivative products which reveal the anomaly texture and highlightthe anomaly-pattern discontinuities. Because long wavelength fea-tures are eliminated and gradients serve to enhance edges betweencrustal blocks of differing density (Thomas et al., 1992) horizontalgravity gradient are found extremely useful in delineating the

578 N. Kumar et al. / Journal of Asian Earth Sciences 34 (2009) 577–585

boundaries of crustal domains (Cordell and Grauch, 1985; Sharptonet al., 1987; Thomas et al., 1987). The horizontal gradient simplyindicates the presence of a measure of the lateral change in densityand requires no assumptions about the sources. Its magnitude isdependent on the density contrast across the domain boundary,the vertical extent of the contrast, the dip of the boundary and itsdepth of burial (Thomas et al., 1992). The steepest horizontal gradi-ent of a gravity anomaly will be located directly over the edge of thebody if the edge is vertical and far removed from all other edges orsources (Thomas et al., 1992). The horizontal gradient of Bouguergravity is used here to define a gravity trend map covering Peninsu-lar India 14 �N, from which is derived a map of first order gravity do-mains. The map is then used to demonstrate that structural featuresare traceable from the exposed shield for hundreds of kilometres. Abrief examination is also made of the relationship of the evolution-ary history of underlying structural fabrics of the Precambrian crust,as defined by the gravity gradient.

2. Major crustal boundaries

Based on the lithological variations, differences in volcano-sed-imentary environment, magmatism and grade of metamorphismthe Southern Indian shield is divided into four major crustal prov-inces (Fig. 1) namely, the Eastern Dharwar Craton (EDC), the Wes-tern Dharwar Craton (WDC), the Eastern Ghats Mobile Belt (EGMB)and the Southern Granulite Terrain (SGT; Rogers, 1986; Naqvi andRogers, 1987). The SGT was further subdivided into (1) NorthernMarginal Zone, (2) Central Zone, and (3) Southern Zone. The South-ern Zone was again subdivided into Madurai Granulite Block andKerala Khondalite Belt (Ramakrishnan, 1993).

An approximately 1–1.5 km wide mylonitic shear zone linked toan easterly dipping reflector interpreted as a thrust fault along theeastern margin of the Chitradurga schist belt divides the Dharwarcraton into the WDC and the EDC (Kaila et al., 1979; Drury et al.,1984; Naqvi, 1985; Chadwick et al., 1989, 1997; Singh et al.,2004). Another crustal scale shear zone marked by eastward dip-ping discontinuous thrust fault separates the Cuddapah Basin, onthe eastern fringe of the EDC, from the EGMB (Kaila and Bhatia,1981; Leelanandam, 1990; Chetty and Murthy, 1994; Singh et al.,2004). The N–S directed narrow belt of Closepet granite was alsobelieved to mark the geo-suture joining the two Dharwar cratons.According to Swamy Nath et al. (1976) collision of the two Dhar-war domains and intracrustal melting at the end of the Archaeanhas resulted in the emplacement of granites along their line ofjunction. Similarly, the narrow linear greenstones belts of theEDC was reported to represent pieces of oceanic crust that weldedtogether to form a composite schist belt (Krogstad et al., 1989,1995). The gneisses to the west and the east of the Kolar schist beltare different and that their contacts are tectonic, the belt in allprobability is another site of a Late Archaean or Early Proterozoicsuture, along which the eastern part of the EDC accreted to the Ar-chaean nucleus around 2550 Ma ago.

Occurrence of pseudotachylite along the Moyar–Bhavani ShearZone (MBSZ) and the Mettur Shear Zone (MeSZ) portray their devel-opment in zones of extensive faulting related to rapid upliftment ofthe deeper crustal rocks and are together (Moyar–Bhavani–MetturShear Zone: MBMeSZ) interpreted as a domain boundary (Raithet al., 1999; Chetty et al., 2003; Chetty and Bhaskar Rao, 2006). Asouthward dipping reflector (Reddy et al., 2003; Rao et al., 2006),paired gravity anomaly (Singh et al., 2003, 2006), faulted contact(Harinarayana et al., 2006), northward verging northern block ofthe SGT (Chetty et al., 2003; Chetty and Bhaskar Rao, 2006), andhigh pressure–temperature granulites exposed on the surface(Chetty and Bhaskar Rao, 2004) indicate crustal thrusting throughcontinental collision along the MBMeSZ, which took place around

the time of the Archaean-Proterozoic boundary (Bhaskar Raoet al., 1996), but not later than 1.6 Ga (Radhakrishna et al., 1999).The Palghat–Cauvery Shear Zone (PCSZ) shows a discontinuity inlithological units, structural geology, metamorphic pressure–tem-perature conditions, Nd model ages and Rb/Sr mineral ages (Druryet al., 1984; Harris et al., 1994; Bhaskar Rao et al., 1996; Meissneret al., 2002). Consequently, the PCSZ was proposed as anotherprominent domain boundary (Drury et al., 1984; Radhakrishna,1989; Harris et al., 1994; Chetty, 1996; Chetty et al., 2003; Gopala-krishnan, 2003; Collins et al., 2007). Contesting this contention,Ghosh et al. (2004) rather recognized on similar grounds the Kar-ur–Kambam–Painavu–Trichur Shear Zone (KKPTSZ) as a domainboundary in the SGT. According to Bhaskar Rao et al. (2003) Kar-ur–Oddanchatram–Kodaikanal Shear Zone (KOKSZ) represents adécollement zone with contrasting thermal histories, and thus pos-sibly demarcates a domain boundary in the SGT. Similarly, Ach-ankovil Shear Zone (ACSZ) is considered to mark the domainboundary between Madurai Granulite Block towards the northand Kerala Khondalite Belt towards the south (Drury et al., 1984;Chetty, 1996; Guru Rajesh and Chetty, 2006).

3. The data and horizontal gravity gradient

The gravity map used for the present investigation is based on7862 newly collected observations merged with 6359 data col-lected over years and maintained at the National Geophysical Re-search Institute (NGRI, 1978; Krishna Brahmam et al., 1986). Theentire data set has been homogenized and linked to the IGSN 71gravity datum and processed following the GRS 80 formula (Mor-elli et al., 1974; Moritz, 1980). The density used for the completeBouguer reduction was 2670 kg/m3, and terrain correction wasachieved by approximating topographic masses with polyhedronswithin a radius of 167 km using SRTM 90 M digital elevation data(ftp://edcsgs9.cr.usgs.gov/pub/data/srtm). The gravity data werefurther subjected to indirect effect correction to account for theCentral Indian Ocean Geoid Low (Chapman and Bordine, 1979),which otherwise would cause very large negative bias in the Pen-insular Indian gravity field. The resultant gravity anomaly (Fig. 2),is referred to hereafter as the Bouguer anomaly and this has beensubjected to the derivative procedures (Cordell and Grauch, 1985).

The details of the derivative procedures have been described byCordell and Grauch (1985) and automated by Blakely and Simpson(1986). The magnitude of the horizontal gradient of the gravityanomalies is calculated using GEOSOFT software (XcellarationGravity, 2002). Fig. 3a presents the computed horizontal gradientof the gravity field of the South Indian shield together with theestablished structural domain boundaries. Typically most gradientvalues are less than about 1.0 mGal/km, yet the maximum gradientin South India is 11.22 mGal/km. Short-wavelength noise in thedata may interfere with geologically meaningful lineaments, andis magnified when the subtle and short-wavelength anomaliesare enhanced. This noise should be suppressed before processing,albeit at a price of sacrificing some useful anomaly information.Three noise-suppression techniques were considered: band passwavelength filtering, slight upward continuation, or smoothingwith convolution filters. Band pass filtering requires an estimationof the cut-off wavelengths, can smear the separation due to non-vertical filter roll-off, and can contaminate the data by Gibbs ring-ing. By experimentation, upward continuation was found to be themost effective for the gravity data in the Carmel fault zone, north-ern Israel (Achmon and ben-Avraham, 1997). To minimise the ef-fects of very small wavelength features; the gridded data werecontinued upward to 1.0 km (Jacobsen, 1987). The horizontal gra-dient analysis was applied again, and the results are presented inthe same way in Fig. 3b. Visual inspection for the locations and sig-

Fig. 1. Generalized geology and tectonic map of the Southern Indian shield, south of 14 �N (GSI, 1998) that comprises EDC: Eastern Dharwar Craton, EGMB: Eastern GhatsMobile Belt, SGT: Southern Granulite Terrain, and WDC: Western Dharwar Craton. SGT is sub-divided into the Northern Marginal Zone (NMZ), the Central Zone (CZ) and theSouthern Zone (SZ). The SZ is further sub-divided into the Madurai Granulite Block and Kerala Khondalite Belt. Orthopyroxene isograd popularly known as the Fermor Line(shown by thick dash line) separates the SGT from the Dharwar Craton. Sequence of stars defines the major structural elements (shear zones), namely ACSZ: Achankovil ShearZone, GSZ: Gangavalli Shear Zone, KKPTSZ: Karur–Kambam–Painavu–Trichur Shear Zone, KOKSZ: Karur–Oddanchatram–Kodaikanal Shear Zone, M-B-Me-P- SZ: Moyar–Bhavani–Mettur–Palar Shear Zones, PCSZ: Palghat–Cauvery Shear Zone, SASZ: Salem–Attur Shear Zone. Major geological formations are: (1) Quaternary sediment, (2)Proterozoic sediment (CB: Cuddapah Basin), (3) Alkaline complex, (4) K-Granite (CG: Closepet Granite), (5) Khondalite, (6) Charnockite, (7) Archaean schist belt (CSB:Chitradurga schist belt; KSB: Kolar schist belt; SSB: Shimoga schist belt), (8) Gneiss (undifferentiated), and (9) Shear Zones.

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nificance of linear maxima indicates that the maximum horizontalgradients in both the maps tend to have similar trends and extentsand are located in essentially the same position. The analysis of thesurface gravity data before the application of the upward continu-ation filter (Fig. 3a) shows prominent linear belts of gradient max-ima coinciding with known contact zones. However, in the upwardcontinued map (Fig. 3b), the first order gravity gradients and ele-ment along the coastlines, while diminished in amplitude, becomemore prominent in comparison to those gradient belts associatedwith shallower source and/or contacts where the density contrastis not strong. The magnitude of the horizontal gradient of the grav-ity anomalies is also calculated using automated procedure of Blak-ely and Simpson (1986) to a pre-defined significance level of 3,which determines the directional nature of the maximum beingcomputed. The horizontal gravity gradients in the form of sinuoussequences of dots corresponding to their magnitude are shown inFig. 4. The stronger group of the maxima points are more likely

to represent the real geological features and the weaker signalsare the probable ‘‘noise”. It would, of course, be unwise to inter-pret all the stronger group of maxima in the figures as gravity do-main boundaries; each linear belt of maxima should be carefullycompared with the original structural map. Any linear belt of max-ima on the gravity gradient map not evident on the structural mapshould be regarded with caution.

4. Gravity trends and domains

These images (Figs. 3a and b and 4) may be regarded as a pseu-do structural image of the Peninsular India, with each linear seg-ment of gravity gradient representing the boundary betweenrock packages with contrasting densities. By experimentation, anarbitrary distinction between gradient features with magnitudesP and <1.5 mGal/km has been made and is used in recognisingfirst order structural features in both exposed and buried areas of

Fig. 2. Bouguer anomaly (in mGal) map of the South India, south of 14 �N latitude. Gravity data distribution in the study region is shown by solid circles. Red solid circles andblue solid circles indicate the recently collected and old available data, respectively (NGRI, 1978; Krishna Brahmam et al., 1986).

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Precambrian basement (Fig. 3a and b; Thomas et al., 1992). Thisfeature has been used to subdivide the map into a number of grav-ity domains on the basis of orientation, continuity, magnitude andpattern of the gradient features (Figs. 3 and 4; Thomas et al., 1992).The domains have deliberately been made as large as possible,while still being distinct from adjacent domains. Most domainsare characterized by a relatively simple pattern of sub-paralleltrends (Thomas et al., 1992). For example NW–SE trending gravitytrends in Domain-I coincide more or less with the structural pat-tern of the EGMB (Chetty and Murthy, 1994). On the other hand,Domain II: the extended EGMB(?), in the south-eastern coastal re-gion with Phanerozoic sediments, is characterized essentially bythe NE–SW trending gravity gradients. Western part of Domain-III, corresponding to the EDC, is characterized by dominant N–Strends at various intervals of longitude. The part to the east ofthe Kolar schist belt is, however, dotted by weaker NW–SE trendsand presents itself as a separate domain. Domain-IV, which coin-cides with the WDC, is characterised essentially by the markeddiversity of its trends, although visual inspection gives an impres-sion of overall NW–SE trend somewhat similar to its structuralgrain (Naqvi and Rogers, 1987). Domain-V, corresponding withthe SGT, is identified on the basis of curvilinear trends convex to-

wards south and mostly dominant in the Northern Marginal Zone.The gravity trends of WDC and EDC are rectilinear to this trend inplaces. The central part of the Central Zone of the SGT is mostlydominated by concentric trends whereas the Southern Zone ofthe SGT does not show a definite pattern of gravity trends.

The largest gradient magnitudes within areas of Precambrianshield generally occur along boundaries between structural prov-inces (Thomas et al., 1987, 1988, 1992). As expected, one of themost prominent feature in the present horizontal gradient mapcoincides with the well-defined suture between the EGMB andthe EDC (Subrahmanyam, 1978; Chetty and Murthy, 1994; Singhand Mishra, 2002; Singh et al., 2004). This linear belt of gradientmaxima extends discontinuously as far south as Kanyakumari withan offset left-laterally c. 100 km by the PCSZ, as if the main EasternGhats orogen extends southwards along the east coast (Subrah-manyam, 1978; Bhaskar Rao et al., 2003; Leelanandam et al.,2006). The northern boundary of this extended feature runs sub-parallel to the Gangavalli Shear Zone; the structural boundary ofthe southern part is not yet ascertained. The aeromagnetic mapof the SGT shows that the contours are oriented E–W trend (Reddiet al., 1988). These E–W magnetic trends probably represent thebasement grain, while the NE–SW trending gravity gradients may

Fig. 3. (a) Horizontal gradient map of the Bouguer gravity field. (b) Horizontal gradient map of the Bouguer gravity field upward continued to 1 km. Note the close correlationbetween first order maxima (P1.5 mGal/km) lines that defines the gravity domains (GD I–V) and the known major structural elements (shear zones) are shown in the figure.Name of the shear zones are as in Fig. 1.

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suggest the influence of dense material such as the oceanic rocks ofthe Bay of Bengal formed during the break up of Gondwanaland.The western margin of the gravity gradient coincides with coastalgrowth faults, and therefore does not indicate the southern exten-sion of the main EGMB to Kanyakumari (Ramakrishnan, 1993).However, the gradient may still be attributed to a boundary be-tween two crustal domains (GD II and GD V) having basic differ-ences in the nature of the crust and mantle.

Another prominent feature of the map is a discontinuous but longlinear belt of gravity gradient between the WDC and the EDC. The lo-cus of the different shorter linear belts of gravity gradient defines thecryptic suture between the WDC and the EDC. Possibly due to theoverprints of later tectonic events Rao and Prasad (2000) mistookthe subdued paired gravity anomaly over their contact zone as notbeing typical of a gravity signature of ancient suture. Removal ofthe gravity effect of the high-density Chitradurga schist belt andlow-density Closepet granites enhances the relative amplitudes ofthe paired gravity anomaly, producing sharper gravity gradients,similar to the characteristic gravity signature observed over ancientsuture zones. Modelling of the paired anomaly provided two distinctcrustal domains beneath the WDC and the EDC with Chitradurgaschist belt as the divide line between the two (Singh et al., 2004).

The line joining the Moyar–Bhavani–Metture–Palar shear zonesmarks the location of a Palaeoproterozoic thrust boundary be-tween the Dharwar Craton and the Northern Marginal Zone ofthe SGT (Raith et al., 1999; Meissner et al., 2002; Chetty et al.,2003; Gopalakrishnan, 2003; Chetty and Bhaskar Rao, 2006). Largemagnitude (steep) gravity gradient along this boundary suggestsan important domain boundary between the Dharwar Craton andthe SGT (Singh et al., 2003, 2006). The absence of the E–W trendingpaired gravity anomaly across this structure was otherwise inter-preted to indicate that the Dharwar Craton and the SGT forms asingle crustal domain (Subrahmanyam, 1978; Narain and Subrah-manyam, 1986).

In some cases, however, structural provinces defined on geolog-ical criteria do not manifest themselves as corresponding gravity do-mains at this broad scale of investigation. For example, the narrowbelt of Closepet granite was believed to mark the geo-suture joiningthe two Dharwar cratons. Because the linear Closepet granite mightbe the consequence of intracrustal melting of the stacked crustal do-mains that collided east of the Chitradurga schist belt (Subba Raoet al., 1998) the subdued gravity gradient along the Closepet granitedoes not support the granite belt as divide line between the WDCand the EDC. Similarly, the Kolar schist belt is proposed as a site of

Fig. 3 (continued)

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Late Archaean or Early Proterozoic suture along which the easternpart of the EDC accreted to the Archaean nucleus (Krogstad et al.,1989). An intra arc setting with mixed mode basins is proposed forthe linear schist belt (Chadwick et al., 1996, 1997). The subduedand/or limited extent of the gravity gradient along the Kolar schistbelts does not support the belts as divide line of two geologically dis-tinct crustal terranes. Rather Raval (2005) supports the lateral zona-tion within the EDC since western EDC has remained dormant sinceLate Proterozoic while eastern EDC has been mobilised many timesover the Proterozoic era. Opening and subsequent block closurealong the Kolar schist belt is a distinct possibility.

The PCSZ has also been suggested as the suture zone betweenthe northern and the southern blocks of the SGT (Drury et al.,1984; Harris et al., 1994; Chetty and Bhaskar Rao, 2006; Collinset al., 2007). The subdued nature of the gravity gradient along thebelt is consistent with the intraterrane thrust belt interpretationof Mishra and Vijaya Kumar (2005). The quasi-circular pattern ofthe gravity gradient vis-à-vis Bouguer anomaly centred in the Cen-tral Zone is a reflection of a large scale intrusion at shallow depths(Singh et al., 2003, 2006). A belt of strong gravity gradient coincideswith the N–S course of Cauvery River, and divides the Central Zoneinto western and eastern sectors (Gopalakrishnan, 2003). Accordingto Bhaskar Rao et al. (2003) and Ghosh et al. (2004) KOKSZ andKKPTSZ represent decollément zones and thus possibly demarcate

the Archaean-Neoproterozoic terrane boundaries in the SGT. Nei-ther any field evidence for a major tectonic boundary at the appro-priate sites (Cenki et al., 2004; Cenki and Kriegsman, 2005) nor thisshear zone as a whole is found reflected in the airborne magneticdata (Mishra et al., 2006). According to Ramakrishnan (1993) theseare not major terrane boundaries. A first order gravity gradientalong these equivocal lineaments and the sub-dividing crustalblocks is conspicuously missing. This may be accounted for by theloss of the detail resulting from the sparse nature of the data setin a rugged terrain, and the present data as such can’t confirm themas potential geo-sutures. ACSZ is another litho-tectonic divide linein the SGT with Kerala Khondalite Block to the south and MaduraiGranulite Block to the north (Drury et al., 1984; Ram Babu and Pra-santi Lakshmi, 2003; Guru Rajesh and Chetty, 2006) whereas to oth-ers the two terrains across the ACSZ form a single crustal domain(Radhakrishna et al., 1990; Cenki et al., 2004; Cenki and Kriegsman,2005). A significant gravity gradient along the ACSZ too is not de-fined at all, and the two provinces appear to be related by a contin-ual progression in single metamorphic terrain.

Some prominent gravity gradients, however, do not correspondto well known structural boundaries in this region. For example,the gravity gradient occurring in northern half of the GD IV doesnot negate its rank in tectonic importance with respect to the knownstructural boundaries of the WDC. In fact it is a possible contact zone,

Fig. 4. Graphic representation of the maxima in the horizontal gradient field as sinuous sequences of dots corresponding to their magnitude. It was calculated usingautomated procedure of Blakely and Simpson (1986). Locus of the maximum horizontal gravity gradient features defines the gravity domains (GD I and GD III–V) map akin tostructural domains (EGMB, EDC, WDC and SGT, respectively) of the Peninsular India. The E–W trending thick dash line (Fermor Line) separates the SGT from the DharwarCraton. Another NE–SW trending thick dash line with displacement along the PCSZ marks the western boundary of GD II, which is considered to be: an equivocal extension ofthe EGMB towards south.

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marking a lithological subdivision between Shimoga schist belt andpeninsular gneisses within the WDC. Western Ghats is also charac-terised by high gravity gradient signifying its dimensional extentall along the west coast. It may also be noted here that the south-western part of the Cuddapah Basin is characterised by a concentricpattern in gradient trends. In the absence of any known faultedboundary the concentric pattern in gradient trend, also apparent inthe Bouguer anomaly maps, supports the argument for a shallowintrusion beneath the region (Singh and Mishra, 2002; Singh et al.,2004).

5. Tectonic significance

The presented images and discourse on gravity gradients do notprovide an unequivocal explanation of the origin of the Southern In-dian shield. However, recognition of major geophysical boundariesprovides new constraints on the crustal architecture, which can beused to infer tectonic models for its evolution. Orientation, continu-ity, magnitude and pattern of the gravity gradient bring out at leastfive gravity domains related to the EDC, the EGMB, the extendedEGMB(?), the SGT, and the WDC. The Archaean-Proterozoic sutures

of Southern Indian shield that separate the five proposed domainsare well characterized by first order gravity gradient. Through pas-sage of time the areal extent of the Southern Indian shield has in-creased in collisional processes (Naqvi and Rogers, 1987). Thisprocess of crustal accretion started during Late Archaean whenthe EDC accreted to the WDC along the eastern margin of the pres-ent day Chitradurga schist belt (Naqvi, 1985; Naqvi and Rogers,1987; Chadwick et al., 1997, 2000; Singh et al., 2004). The coherentblocks amalgamated in the SGT docked with the combined domainof the EDC and the WDC at Moyar–Bhavani–Mettur–Palar shearzones: the leading margin of the Northern Marginal Zone of theSGT (Harris et al., 1994; Gopalakrishnan, 2003; Chetty and BhaskarRao, 2006; Rao et al., 2006; Singh et al., 2006). Finally, the EGMB ac-creted to the SGT at the east of the Cuddapah Basin (Leelanandam,1990; Chetty and Murthy, 1994; Singh and Mishra, 2002; Singhet al., 2004). However, its southern extension the towards Kanyaku-mari and association with the SGT is equivocal (Subrahmanyam,1978; Ramakrishnan, 1993; Ghosh et al., 2004; Leelanandamet al., 2006).

The collisional collage arising from the amalgamation in theSouthern Indian shield was periodically reworked, involving com-

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ponents of transpressional movement (Chadwick et al., 1989;Chetty, 1996; Chetty et al., 2003; Guru Rajesh and Chetty, 2006).The SGT accreted to the unified Dharwar craton around Archae-an-Proterozoic boundary (Bhaskar Rao et al., 1996) but not laterthan 1.6 Ga (Radhakrishna et al., 1999). The transpressional move-ment along the eastern margin of the Chitradurga schist belt isconsistent with this N–S crustal shortening (Chadwick et al.,1989). Middle to Late Proterozoic accretion of the EGMB resultedin the development of marginal foreland Cuddapah Basin alongthe eastern margin of the EDC (Leelanandam, 1990; Chetty andMurthy, 1994; Singh et al., 2004). The dextral sense of movementalong the PCSZ is consistent with this E–W crustal shortening (Dru-ry et al., 1984). The two transpressional deformations of the ACSZwere intimately associated with the Neoproterozoic ‘East AfricanOrogeny’ and ‘Kuunga Orogeny’, respectively (Guru Rajesh andChetty, 2006).

The absence of significant gravity gradients along the Closepetgranite, Kolar schist belt, PCSZ, KOKSZ, KKPTSZ and ACSZ supportsthe idea of them being intracratonic litho-tectonic features ratherelements of ancient geo-sutures. Uncertainty in regard to intracru-stal melting at the end of the Archaean (Swamy Nath et al., 1976;Subba Rao et al., 1998) and the presence of second order gravitygradient bordering the Closepet granite calls into question the pos-sibility that a palaeo-suture is represented by the Closepet granite(Krishna Brahmam, 1993). Similarly, intra arc setting with mixedmode basin (Chadwick et al., 1997) together with the limited ex-tent of the gravity gradient does not support the linear Kolar schistbelt as a divide line of two geologically distinct terranes. Absenceof first order gravity gradients along PCSZ, KOKSZ, KKPTSZ andACSZ also make it very likely that the respective flanking domainsshared a common Pan-African tectono-metamorphic evolution,and were in a neighbouring position during the Gondwana assem-bly at the end of the Proterozoic when Pan-African thermal pertur-bation reactivated the structure and reworked partly or totallyobliterating earlier domain fabric (Cenki and Kriegsman, 2005;Leelanandam et al., 2006). Further study of these domains isneeded before definitive conclusions can be reached about thedrivers of past tectonism in the Southern Indian shield.

6. Conclusions

The application of the horizontal gradient analysis to the newgravity data suggests a complex gravity domain architecture andtectonic development of the region. The study reveals curvilinearsegments of maxima in the horizontal gradients of the gravity field.Orientation, continuity, magnitude and pattern of the gravity gra-dient features define five gravity domains akin to the EDC, theEGMB, the extended EGMB(?), the SGT and the WDC in the South-ern Indian shield, which has grown outward from the WDC. Thelinear elements of first order maxima in the horizontal gradientof the gravity field are found associated with the structural domainboundaries and interpreted as being caused by the tectonic suturezones. The absence of significant gravity gradient across the Close-pet granite and the Kolar schist belt supports the idea of thembeing intracratonic litho-tectonic features rather than elementsof ancient geo-sutures. Some of the structural domains of theSGT also do not manifest themselves as corresponding gravity do-mains at this broad scale of investigation. That absence may be aconsequence of the data resolution and/or overprinting related toLate Proterozoic (c. 550 Ma) tectonic events in the area.

Acknowledgements

The authors thank Director, NGRI, Hyderabad for his encourage-ment and permission to publish this work. Thanks are also due to

Prof. R.J. Blakely for sharing his codes and to Dr. M.D. Thomas andDr. P. Betts for critical comments and suggestions that have helpedgreatly to improve the quality of the manuscript. A part of the datawas collected under Grant-in-aid Project, supported by Depart-ment of Science and Technology, New Delhi. N.K. thanks CSIR,New Delhi for the senior research fellowship.

References

Achmon, M., ben-Avraham, Z., 1997. The deep structure of the Carmel fault zone,Northern Israel, from gravity field analysis. Tectonics 16 (3), 563–569.

Bhaskar Rao, Y.J., Chetty, T.R.K., Janardhan, A.S., Gopalan, K., 1996. Sm–Nd and Rb–Srages and P–T history of the Archean Sittampundi and Bhavani layered meta-anorthosite complex in Cauvery shear zone, South India: evidence forNeoproterozoic reworking of Archean crust. Contributions to Mineralogy andPetrology 125, 237–250.

Bhaskar Rao, Y.J., Janardan, A.S., Vijaya Kumar, T., Narayana, B.L., Dayal, A.M., Taylor,P.N., Chetty, T.R.K., 2003. Sm–Nd model ages and Rb–Sr isotopic systematics ofcharnockites and gneisses across the Cauvery shear zone, southern India:implications for the Archaean-Neoproterozoic Terrane boundary in theSouthern Granulite Terrain. Geological Society of India Memoir 50, 297–317.

Blakely, R.J., Simpson, R.W., 1986. Approximating edges of source bodies frommagnetic or gravity anomalies. Geophysics 51 (7), 1494–1498.

Cenki, B., Broun, I., Brocker, M., 2004. Evolution of the continental crust in the KeralaKhondalite Belt, Southernmost India: evidence from Nd isotope mapping, U–Pband Rb–Sr geochronology. Precambrian Research 134, 275–292.

Cenki, B., Kriegsman, L.M., 2005. Tectonics of the Neoproterozoic SouthernGranulite Terrain, South India. Precambrian Research 138, 37–56.

Chadwick, B., Ramakrishnan, M., Vasudev, V.N., Viswanatha, M.N., 1989. Faciesdistributions and structure of a Dharwar volcanosedimentary basin: evidencefor late Archaean transpression in Southern India? Journal of Geological Societyof London 146, 825–834.

Chadwick, B., Vasudev, V.N., Ahmed, N., 1996. The Sandur schist belt and itsadjacent plutonic rocks implications for Late Archaean crustal evolution inKarnataka. Journal of the Geological Society of India 47, 37–57.

Chadwick, B., Vasudev, V.N., Hegde, G.V., 1997. The Dharwar craton, Southern India,and its Late Archaean plate tectonic setting: current interpretations andcontroversies. Proceedings of the Indian Academy of Sciences (Earth andPlanetary Sciences) 106, 249–258.

Chadwick, B., Vasudev, V.N., Hegde, G.V., 2000. The Dharwar craton, Southern India,interpreted as the result of Late Archaean oblique convergence. PrecambrianResearch 99, 91–111.

Chapman, M.E., Bordine, J.H., 1979. Considerations of the indirect effect in marinegravity modelling. Journal of Geophysical Research 84, 3889–3892.

Chetty, T.R.K., 1996. Proterozoic shear zones in Southern Granulite Terrain, India. In:Santosh, M., Yoshida, M. (Eds.), The Archaean and Proterozoic Terrains inSouthern India within East Gondwana, vol. 3. Gondwana Research GroupMemoir, pp. 77–89.

Chetty, T.R.K., Murthy, D.S.N., 1994. Collision tectonics in the late PrecambrianEastern Ghats Mobile Belt: mesoscopic to satellite-scale structural observations.Terra Nova 6, 72–81.

Chetty, T.R.K., Bhaskar Rao, Y.J., 2004. Tectonics and evolution of the PrecambrianSouthern Granulite Terrain, India and Gondwanian correlations. Abstract andGeological Excursion Guide, IFW-SGT-2004. National Geophysical ResearchInstitute, Hyderabad, India.

Chetty, T.R.K., Bhaskar Rao, Y.J., 2006. The Cauvery shear zone, Southern GranuliteTerrain: a crustal scale flower structure. Gondwana Research 10, 77–85.

Chetty, T.R.K., Bhaskar Rao, Y.J., Narayana, B.L., 2003. A structural cross-sectionalong Krishnagiri–Palani corridor, Southern Granulite Terrain, India. GeologicalSociety of India Memoir 50, 255–277.

Cordell, L., Grauch, V.J.S., 1985. Mapping basement magnetization map zone fromaeromagnetic data in the San Juan basin, New Mexico. In: Hinze, W.E. (Ed.), TheUtility of Regional Gravity and Magnetic Anomaly Maps. Society of ExplorationGeophysicist Publication, pp. 181–197.

Collins, A.S., Clark, C., Sajeev, K., Santosh, M., Kelsey, D.E., Hand, M., 2007. Passagethrough India: the Mozambique Ocean suture, high-pressure granulites and thePalghat–Cauvery shear zone system. Terra Nova 19, 141–147.

Drury, S.A., Harris, N.B.W., Holt, R.W., Reeves-Smith, G.J., Wightman, R.T., 1984.Precambrian tectonics and crustal evolution in South India. Journal of Geology92, 3–20.

Fountain, D.M., Salisbury, M.H., 1981. Exposed cross-sections through thecontinental crust: implications for crustal structure, petrology, and evolution.Earth and Planetary Science Letters 56, 263–277.

Ghosh, J.G., de Wit, M.J., Zartman, R.E., 2004. Age and tectonic evolution ofNeoproterozoic ductile shear zone in the Southern Granulite Terrain of India,with implications for Gondwana studies. Tectonics 23 (TC3600), 1–38.

Gibb, R.A., Thomas, M.D., 1976. Gravity signature of fossil plate boundaries in theCanadian shield. Nature 262, 199–200.

Gopalakrishnan, K., 2003. An overview of Southern Granulite Terrain, India-Constraints and reconstruction of Precambrian assembly of Gondwanaland.Geological Society of India Memoir 50, 47–78.

GSI, 1998. Geological Map of India on 1:2,000,000 scale. Geological Survey of IndiaPublication, Kolkata, India.

N. Kumar et al. / Journal of Asian Earth Sciences 34 (2009) 577–585 585

Guru Rajesh, K., Chetty, T.R.K., 2006. Structure and tectonics of the Achankovil ShearZone, Southern India. Gondwana research 10, 86–98.

Narain, Hari, Subrahmanyam, C., 1986. Precambrian tectonics of the southernIndian shield inferred from geophysical data. Journal of Geology 94, 187–198.

Harinarayana, T., Naganjaneyulu, K., Patro, B.K.P., 2006. Detection of collision zonein South Indian shield region from magnetotelluric studies. Gondwana Research10, 48–56.

Harris, N.B.W., Santosh, M., Taylor, P.N., 1994. Crustal evolution in South India:constraints from Nd isotopes. Journal of Geology 102, 139–150.

Jacobsen, B.H., 1987. A case for upward continuation as a standard separation filterfor potential-field maps. Geophysics 52, 1138–1148.

Kaila, K.L., Roy Chowdhury, K., Reddy, P.R., Krishna, V.G., Narain, Hari, Subbotin, S.I.,Sollogub, V.B., Chekunov, A.V., Kharetchko, G.E., Lazarenko, M.A., Ilchenko, T.V.,1979. Crustal structure along Kavali–Udipi profile in the Indian peninsularshield from deep seismic sounding. Journal of the Geological Society of India 20,307–333.

Kaila, K.L., Bhatia, S.C., 1981. Gravity study along the Kavali–Udipi deep seismicsounding profile in the Indian peninsular shield: some inferences about the originof anorthosites and the Eastern Ghats Orogeny. Tectonophysics 79, 129–143.

Krishna Brahmam, K., 1993. Gravity in relation to crustal structure, Palaeo-suturesand seismicity of Southern India (South of the 16th parallel). Geological Societyof India Memoir 25, 165–201.

Krishna Brahmam, N., Sarma, J.R.K., Aravamadhu, P.S., Subba Rao, D.V., 1986.Bouguer gravity anomaly map (NGRI/GPH-6) of Cuddapah Basin (India), first ed.National Geophysical Research Institute Publication, Hyderabad, India.

Krogstad, E.J., Balakrishnan, S., Mukhopadhyay, D.K., Rajamani, V., Hanson, G.N.,1989. Plate tectonics 2.5 billion years ago: evidence at Kolar, South India.Science 243, 1337–1340.

Krogstad, E.J., Hanson, G.N., Rajamani, V., 1995. Sources of continental magmatismadjacent to the late Archean Kolar suture zone, South India: distinct isotopicand elemental signatures of two late Archaean magmatic series. Contributionsto Mineralogy and Petrology 122, 159–173.

Leelanandam, C., 1990. The Kandra volcanics in Andhra Pradesh: possible ophiolite?Current Science 59, 785–788.

Leelanandam, C., Burke, K., Ashwal, L.D., Webb, S.J., 2006. Proterozoic mountainbuilding in Peninsualr India: an analysis based primarily on alkaline rockdistribution. Geological Magazine 143 (2), 195–212.

Meissner, B., Deters, P., Srikantappa, C., Khöler, H., 2002. Geochronological evolutionof the Moyar, Bhavani and Palghat shear zones of Southern India: implicationsfor East Gondwana correlations. Precambrian Research 114, 149–175.

Mishra, D.C., Singh, B., Tiwari, V.M., Gupta, S.B., Rao, M.B.S.V., 2000. Two cases ofcontinental collisions and related tectonics during the Proterozoic period inIndia – insights from gravity modelling constrained by seismic andmagnetotelluric studies. Precambrian Research 99, 149–169.

Mishra, D.C., Vijaya Kumar, V., 2005. Evidence for Proterozoic collision from airbornemagnetic and gravity studies in Southern Granulite Terrain, India and signaturesof recent tectonic activity in the Palghat Gap. Gondwana Research 8, 43–54.

Mishra, D.C., Vijaya Kumar, V., Rajasekhar, F., 2006. Analysis of airborne magneticand gravity anomalies of peninsular shield, India integrated with seismic andmagnetotelluric results and gravity anomalies of Madagaskar, Sri Lanka andEast Antarctica. Gondwana Research 10, 6–17.

Morelli, C.G., Gantar, G., Honkasalo, T., McConnell, R.K., Tanner, J.G., Szabo, B., Uotila,U., Whalen, C.T., 1974. The International Gravity Standardisation Net, 1971.International Association of Geodesy, Special Publication No. 4, Paris, p. 194.

Moritz, H., 1980. Geodetic reference system 1980. Journal of Geodesy 54, 395–405.Naqvi, S.M., 1985. Chitradurga schist belt – an Archaean suture? Journal of the

Geological Society of India 26, 511–525.Naqvi, S.M., Rogers, J.J.W., 1987. Precambrian Geology of India. Oxford University

Press, New York. p. 223.NGRI, 1978. Gravity Map Series of India (NGRI/GPH 1–5), first ed. National

Geophysical Research Institute Publication, Hyderabad, India.Radhakrishna, B.P., 1989. Suspect tectono-stratigraphic terrane elements in the

Indian subcontinent. Journal of the Geological Society of India 34, 1–24.Radhakrishna, T., Mathai, J., Yoshida, M., 1990. Geology and structure of the high-

grade rocks from Punalur–Achankovil sector, South India. Journal of theGeological Society of India 35, 263–272.

Radhakrishna, T., Maluski, H., Mitchell, J.G., Joseph, M., 1999. 40Ar–39Ar and K/Argeochronology of the dykes from the South Indian granulite terrain.Tectonophysics 304, 109–129.

Raith, M.M., Srikantappa, C., Buhl, D., Koehler, H., 1999. The Nilgiri enderbites, SouthIndia: nature and age constraints on protolith formation, high-grademetamorphism and cooling history. Precambrian Research 98, 129–150.

Ramakrishnan, M., 1993. Tectonic evolution of the granulite terrains of SouthernIndia. Geological Society of India Memoir 25, 35–44.

Ram Babu, H.V., Prasanti Lakshmi, M., 2003. Aeromagnetic characteristics of theAchankovil Shear Belt, South India. CSIR Diamond Jubilee Seminar on Frontiersof Geophysical Research.. National Geophysical Research Institute, Hyderabad.pp. 86–89.

Rao, C.V., Prasad, E.R., 2000. Gravity studies along Raichur–Panaji subtransect: apreliminary investigation of Precambrian plate tectonic process. VisakhaScience Journal 4, 37–44.

Rao, V.V., Sain, K., Reddy, P.R., Mooney, W.D., 2006. Crustal structure and tectonicsof the northern part of the Southern Granulite Terrane, India. Earth andPlanetary Science Letters 251, 90–103.

Raval, U., 2005. Some issues for future deep continental studies. Himalayan Geology26 (1), 253–269.

Ray, L., Senthil Kumar, P., Reddy, G.K., Roy, S., Rao, G.V., Srinivasan, R., Rao, R.U.M.,2003. High mantle heat flow in a Precambrian Granulite Province: evidencefrom southern India. Journal of Geophysical Research 108 (NO B2), 2084.doi:10.1029/2001 JB000688 (ETG 6 – 1 to ETG 6 – 13).

Reddi, A.G.B., Mathew, M.P., Singh, B., Naidu, P.S., 1988. Aeromagnetic evidence ofcrustal structure in the granulite terrain of Tamil Nadu–Kerala. Journal of theGeological Society of India 32, 368–381.

Reddy, P.R., Rajendra Prasad, B., Vijay Rao, V., Sain, Kalachand, Rao, Prasad, PrakashKhare, P., Reddy, M.S., 2003. Deep seismic reflection and refraction/wide anglereflection studies along Kuppam–Palani transect in the southern Granuliteterrain of India. Geological Society of India Memoir 50, 79–106.

Rogers, J.J.W., 1986. The Dharwar craton and the assembly of Peninsular India.Journal of Geology 94, 129–143.

Roy, S., Ray, L., Senthil Kumar, P., Reddy, G.K., Srinivasan, R., 2003. Heat flow andheat production in the Precambrian gneiss-granulite province of southern India.Geological Society of India Memoir 50, 177–191.

Sharpton, V.L., Grieve, R.A.F., Thomas, M.D., Halpenny, J.F., 1987. Horizontal gravitygradient – An aid to the definition of crustal structure in North America.Geophysical Research Letters 14, 808–811.

Singh, A.P., Mishra, D.C., 2002. Tectonosedimentary evolution of Cuddapah basinand Eastern Ghats mobile belt (India) as Proterozoic collision: gravity, seismicand geodynamic constraints. Journal of Geodynamics 33, 249–267.

Singh, A.P., Mishra, D.C., Vijaya Kumar, V., Rao, M.B.S.V., 2003. Gravity-magneticsignature and crustal architecture along Kuppam–Palani geotransect, SouthIndia. Geological Society of India Memoir 50, 139–163.

Singh, A.P., Mishra, D.C., Gupta, S.B., Rao, M.R.K.P., 2004. Crustal structure anddomain tectonics of the Dharwar Craton (India): insight from new gravity data.Journal of Asian Earth Sciences 23, 141–152.

Singh, A.P., Kumar, Niraj, Singh, B., 2006. Nature of the crust along Kuppam–Palanigeotransect (South India) from gravity studies: implications for Precambriancontinental collision and delamination. Gondwana Research 10, 41–47.

Srinagesh, D., Rai, S.S., 1996. Teleseismic tomographic evidence for contrasting crustand upper mantle in south Indian Archean terrains. Physics of the Earth andPlanetary Interiors 97, 27–41.

Subba Rao, M.V., Rama Rao, P., Divakara Rao, V., 1998. The advent of the Proterozoic– a trigger for extensive intracrustal processes in the south Indian shield?Gondwana Research 1, 275–283.

Subrahmanyam, C., 1978. On the relation of gravity anomalies to geotectonics of thePrecambrian terrains of the South Indian Shield. Journal of the GeologicalSociety of India 19, 251–263.

Subrahmanyam, C., Verma, R.K., 1986. Gravity field, structure and tectonics of theEastern Ghats. Tectonophysics 126, 195–212.

Swamy Nath, J., Ramakrishnan, M., Viswanatha, M.N., 1976. Dharwar stratigraphicmodel and Karnataka craton evolution. Records Geological Survey of India 107,149–175.

Thomas, M.D., Sharpton, V.L., Grieve, R.A.F., 1987. Gravity patterns and Precambrianstructure in the North American Central plains. Geology 15, 489–492.

Thomas, M.D., Grieve, R.A.F., Sharpton, V.L., 1988. Gravity domains and assemblyof the North American continent by collisional tectonics. Nature 331, 333–334.

Thomas, M.D., Grieve, R.A.F., Sharpton, V.L., 1992. Structural fabric of the NorthAmerican continent, as defined by gravity trends. In: Mason, R. (Ed.), BasementTectonics, vol. 7. Kluwer Academic Press, pp. 257–276.

Williams, H.A., Cassidy, J., Locke, C.A., Spörli, K.B., 2006. Delineation of a largeultramafic embedded within a major SW Pacific suture using gravity methods.Tectonophysics 424, 119–133.

Xcellaration Gravity, 2002. Gravity Data Processing System for Oasis MontajTM.Geosoft, Canada.