- Email: [email protected]
Some Observations on the Tectonic Framework of Southeastern Indian Shield
Goiidzunria Rimirch (Gondzonnn Nrzuslrtter Section) V 2, No. 4, pp. 651-653. 01999 Intrrnntioiml Association for Gondzunnn Resrnrch, Japan.
GNL CORRESPONDENCE
Some Observations on the Tectonic Framework of Southeastern Indian Shield T.R.K. Chetty National Geoplzysical Research Institute, Hyderabad 500 007, lndia
The southeastern Indian shield represents a crucial segment in the evolutionary history of the pre-breakup east Gondwana supercontinent. This has attracted the attention of geoscientists from all over the globe leading to fairly detailed studies, but mostly in isolated areas of small extent. Here, an attempt has been made to compile a tectonic map of the southeastern Indian shield (Fig. 1) by bringing together all the available information mostly based on the author’s own interpretation and results (e.g. Chetty and Murthy, 1998). This map basically involves structural interpretation of Landsat TM data on 1:l million scale and reconnaissance field traverses. The major geological units in tlus part of the Indian slueld are Archaean granite-greenstone sequence of Dharwar and Bastar Cratons separated by NW-SE trending Godavari rift filled with coal bearing Gondwana sediments. Linear belts of high grade gneisses shoulder the flanks of the Godavari rift on either side. Mahanadi rift, comprising Gondwana sediments associated with coal seams, separates the Singhbhum Craton to the north and Bastar Craton to the south. The cratons are skirted by Precambrian mobile belts namely, the Eastern Ghats Mobile Belt (EGMB) along the east coast of India, and the Southern Granulite Terrane (SGT) in the south. It is observed that the EGMB takes a southeasterly trend near Ongole and disappears in the Bay of Bengal. It reemerges near Madras and forms a part of the SGT. The Proterozoic sedimentary basins occur adjacent to these mobile belts overlying the cratonic areas, perhaps related to collisional processes involved with the evolution of mobile belts (Chetty, 1999). One of the most remarkable features is the presence of a network of Proterozoic shear zones in these Precambrian mobile belts. While the shear zones in SGT are dominantly east-west, the shear zones in the EGMB show varied orientations despite their linear geometry (Chetty, 1995a, 1996). The shear zones in the southern part of the EGMB are NE-SW; in the northern part they are E-W and in the central part they are nearly N-S, which is nearly orthogonal
to the regional trend of the EGMB. These shear zones in general, are significant in many ways. They are: (i) the sites of mineralization; (ii) the terrane boundaries; (iii) the channels for fluid flow; (iv) the zones of intense deformation; (v) the zones of recurring tectonic activity; (vi) the hazardous zones of seismic activity; (vii) the zones of retrograde metamorphism and grain size reduction; (viii) the zones of igneous activity etc. The shear zones in the mobile belts vary in their size, geometry, and their attitude in depth. The recognition of shear zones has resulted in major reinterpretation of structural hstory, tectonic style and produced new models capable of being tested (Drury et al., 1984). It has been well established that the rocks in the shear zones are affected by several processes like partial melting, mixing with fluids transmitted through shear zones and strain variations combined with rheological properties. Distinct variations are expected between sheared and nonsheared rocks in their chemical a n d mineralogical characteristics and isotopic signatures (Beach, 1976). An integrated study of petrology, geochemistry, geochronology in combination with structural and geophysical studies along and across these shear zones is essential. The geometry of the shear zones and their implications of the collision direction are critical to all conceptual models dealing with tectono-metamorphic history of the mobile belts. Complex fold patterns are important deformational features in EGMB as well as in SGT. Traditional description of such folds and their events in a sequence may not be of much relevance to regional tectonic studies. However, identifying the sheath fold structures (closed fold forms) would be extremely useful in view of their occurrence either at the base of nappe or in other high strain areas. Sheath folds are common fea tures in mobile belts of lndia and they are observable both in mesoscopic as well as megascopic scales (Chetty and Murthy, 1994). They plunge at shallow angles and are cut by different erosional surfaces giving rise to apparent complex fold trends.
zs9
Fig. 1. Tectonic framework of Southeastern Indian Shield. Gondwana Research, V.2, No.4,1999
65 3 ~~
~~
Mineral stretching lineations are one of the most important tectonic fabrics. They are well developed in the mobile belts. They show regional as well as local variation in their attitude. Both steep as well as shallow plunging lineations are observable in the EGMB as well as SGT (Chetty and Bhaskar Rao, 1998). However, the following are pertinent in their interpretations (Tikoff and Greene, 1997). (i) stretching lineations do not necessarily correlate with the tectonic transport direction particularly in high strain zones, (ii) the orientation of stretching lineations may vary either along strike or at a single outcrop, due to variations in accumulated finite strain and the amount of simple shear partitioned within the shear bands, (iii) a wrench dominated transpression could result in either horizontal or vertical lineations, or vary between the two depending on the magnitude of the strain, and (iv) development of stretching lineations in three dimensional deformation, such as transpressions, can be quite complex and could potentially result in simultaneous formation of two orthogonal lineations that are kinematically related. Considering all these factors, the stretching lineations must be paid adequate attention which would unravel the deformational history and help in the recognition of terranes in orogenic belts. The mobile belts of south Indian shield are widely interpreted to be the products of collisional orogeny. Studies in various Precambrian orogenic belts led to the recognition of large scale thrust related horizontal tectonics and resultant features (Daly, 1986). Although, some workers including the present author advocate such collision related thrust tectonic regime for the mobile belts, no systematic effort has, so far, been attempted in recognizing or considering such fiat lying tectonic features. The utility of satellite or airborne data in the interpretation of megastructures (10-100 km) is remarkable for regional tectonic synthesis of mobile belts (Chetty, 1995b)because of the following: (i)limited perception of a field geologist; (ii) dense vegetation; (iii) poor exposure and (iv) poor accessibility. It is interesting to note that the potential of satellite data increases proportionately with the scale of structure under consideration. More effective interpretation of large structures such as lithologic contacts, folds, shear zones etc. is possible from satellite data. The structures in mobile belts are of crustal size and therefore geophysical input would be immensely useful for understanding the tectonics. The evolution of ancient tectonic system in forms of terranes and realistic plate tectonics has increasingly gained importance in recent years. Such an effort must be directed in recognizing terranes in this part of the Indian shield. Although, several geoscientists directed their efforts, the mobile belts have not been fully understood in their totality. Interpretations continue to vary widely, but have more or less tended to revolve around
Gondwana Research, V. 2, No. 4, 1999
collision related processes, getting away from the traditional geosynclinal concepts. The map presented here hopefully throws much light on the features described above. This part of the Indian shield provides plenty of opportunity, with its splendid exposures, to have a broader view about mechanism of collision regime, the timing of deformation, nature of shear zones and their kinematics. It is clear that no single discipline will be able to give us a conclusive picture. Hence integrated geological and geophysical approaches must be considered in unraveling the tectono-metamorphic history of the southeastern Indian shield.
Acknowledgments Dr. H.K. Gupta, Director, is acknowledged for his encouragement and kind permission to publish this note.
References Beach, A. (1976) The interrelations of fluid transport, deformation, geochemistry and heat flow in early Proterozoic shear zones in Lewisian complex. Phil. Trans. R. Soc.Lond., A. 280, pp. 569-604. Chetty, T.R.K. (1995a) A correlation of Proterozoic shear zones between Eastern Ghats,India and Enderby land, East Antarctica, based on Landsat imagery. India and Antarctica during the Precambrian. In: Yoshida, M. and Santosh, M. (Eds.), Geol. SOC.India, Memoir No. 34, pp.205-220. Chetty, T.R.K. (199513) Structural interpretation of high grade belts of south India using remotely sensed data. NNRMS Bull., V. 19, pp. 35-37. Chetty, T.R.K. (1996) Proterozoic shear zones in Southern Granulite Terrrain, India. In: Santosh, M. and Yoshida, M. (Eds.), The Archaean and Proterozoic terrains in southern India within East Gondwana. Gond. Res. Memoir No. 3, pp. 77-89. Chetty, T.R.K. (1999) Tectonics of Cudappah basin: A reinterpretation. Geol. SOC.India, Memoir (in press). Chetty, T.R.K. and Bhaskar Rao, Y. J. (1998) Behaviour of stretching lineations in the Salem-Attur shear belt, Southern Granulite Terrane, south India. J. Geol. SOC.India, v. 52, pp. 443-448. Chetty, T.R.K. and Murthy, D.S.(1994)Collision tectonics in the Eastern Ghats Mobile Belt: mesoscopic to satellite scale structural observations. Terra Nova, v. 6, pp. 72-81. Chetty, T.R.K. and Murthy, D.S.N. (1998) Regional tectonic framework of Eastern Ghats Mobile Belt: A new interpretation. Geol. Surv. India. spl. pub. No.44, pp. 39-50. Daly, M.C. (1986) Crustal shear zones and thrust belts: their geometry and continuity in central Africa. Phil. Trans. R. SOC.Lond.,A. 317, pp. 111-128. Drury, S.A., Harris, N.B.W., Reeves-Smith,G.J. and Wightmann, R.T. (1984) Precambrian tectonics and crustal evolution in south India. J. Geol., v. 92, pp. 3-20. Tikoff, B. and Greene, D. (1997) Stetching lineations in transpressional shear zones: an example from the Sierra Nevada batholith, California. J. Str. Geol., v. 19, pp. 29-38.
GNL CORRESPONDENCE
Some Observations on the Tectonic Framework of Southeastern Indian Shield T.R.K. Chetty National Geoplzysical Research Institute, Hyderabad 500 007, lndia
The southeastern Indian shield represents a crucial segment in the evolutionary history of the pre-breakup east Gondwana supercontinent. This has attracted the attention of geoscientists from all over the globe leading to fairly detailed studies, but mostly in isolated areas of small extent. Here, an attempt has been made to compile a tectonic map of the southeastern Indian shield (Fig. 1) by bringing together all the available information mostly based on the author’s own interpretation and results (e.g. Chetty and Murthy, 1998). This map basically involves structural interpretation of Landsat TM data on 1:l million scale and reconnaissance field traverses. The major geological units in tlus part of the Indian slueld are Archaean granite-greenstone sequence of Dharwar and Bastar Cratons separated by NW-SE trending Godavari rift filled with coal bearing Gondwana sediments. Linear belts of high grade gneisses shoulder the flanks of the Godavari rift on either side. Mahanadi rift, comprising Gondwana sediments associated with coal seams, separates the Singhbhum Craton to the north and Bastar Craton to the south. The cratons are skirted by Precambrian mobile belts namely, the Eastern Ghats Mobile Belt (EGMB) along the east coast of India, and the Southern Granulite Terrane (SGT) in the south. It is observed that the EGMB takes a southeasterly trend near Ongole and disappears in the Bay of Bengal. It reemerges near Madras and forms a part of the SGT. The Proterozoic sedimentary basins occur adjacent to these mobile belts overlying the cratonic areas, perhaps related to collisional processes involved with the evolution of mobile belts (Chetty, 1999). One of the most remarkable features is the presence of a network of Proterozoic shear zones in these Precambrian mobile belts. While the shear zones in SGT are dominantly east-west, the shear zones in the EGMB show varied orientations despite their linear geometry (Chetty, 1995a, 1996). The shear zones in the southern part of the EGMB are NE-SW; in the northern part they are E-W and in the central part they are nearly N-S, which is nearly orthogonal
to the regional trend of the EGMB. These shear zones in general, are significant in many ways. They are: (i) the sites of mineralization; (ii) the terrane boundaries; (iii) the channels for fluid flow; (iv) the zones of intense deformation; (v) the zones of recurring tectonic activity; (vi) the hazardous zones of seismic activity; (vii) the zones of retrograde metamorphism and grain size reduction; (viii) the zones of igneous activity etc. The shear zones in the mobile belts vary in their size, geometry, and their attitude in depth. The recognition of shear zones has resulted in major reinterpretation of structural hstory, tectonic style and produced new models capable of being tested (Drury et al., 1984). It has been well established that the rocks in the shear zones are affected by several processes like partial melting, mixing with fluids transmitted through shear zones and strain variations combined with rheological properties. Distinct variations are expected between sheared and nonsheared rocks in their chemical a n d mineralogical characteristics and isotopic signatures (Beach, 1976). An integrated study of petrology, geochemistry, geochronology in combination with structural and geophysical studies along and across these shear zones is essential. The geometry of the shear zones and their implications of the collision direction are critical to all conceptual models dealing with tectono-metamorphic history of the mobile belts. Complex fold patterns are important deformational features in EGMB as well as in SGT. Traditional description of such folds and their events in a sequence may not be of much relevance to regional tectonic studies. However, identifying the sheath fold structures (closed fold forms) would be extremely useful in view of their occurrence either at the base of nappe or in other high strain areas. Sheath folds are common fea tures in mobile belts of lndia and they are observable both in mesoscopic as well as megascopic scales (Chetty and Murthy, 1994). They plunge at shallow angles and are cut by different erosional surfaces giving rise to apparent complex fold trends.
zs9
Fig. 1. Tectonic framework of Southeastern Indian Shield. Gondwana Research, V.2, No.4,1999
65 3 ~~
~~
Mineral stretching lineations are one of the most important tectonic fabrics. They are well developed in the mobile belts. They show regional as well as local variation in their attitude. Both steep as well as shallow plunging lineations are observable in the EGMB as well as SGT (Chetty and Bhaskar Rao, 1998). However, the following are pertinent in their interpretations (Tikoff and Greene, 1997). (i) stretching lineations do not necessarily correlate with the tectonic transport direction particularly in high strain zones, (ii) the orientation of stretching lineations may vary either along strike or at a single outcrop, due to variations in accumulated finite strain and the amount of simple shear partitioned within the shear bands, (iii) a wrench dominated transpression could result in either horizontal or vertical lineations, or vary between the two depending on the magnitude of the strain, and (iv) development of stretching lineations in three dimensional deformation, such as transpressions, can be quite complex and could potentially result in simultaneous formation of two orthogonal lineations that are kinematically related. Considering all these factors, the stretching lineations must be paid adequate attention which would unravel the deformational history and help in the recognition of terranes in orogenic belts. The mobile belts of south Indian shield are widely interpreted to be the products of collisional orogeny. Studies in various Precambrian orogenic belts led to the recognition of large scale thrust related horizontal tectonics and resultant features (Daly, 1986). Although, some workers including the present author advocate such collision related thrust tectonic regime for the mobile belts, no systematic effort has, so far, been attempted in recognizing or considering such fiat lying tectonic features. The utility of satellite or airborne data in the interpretation of megastructures (10-100 km) is remarkable for regional tectonic synthesis of mobile belts (Chetty, 1995b)because of the following: (i)limited perception of a field geologist; (ii) dense vegetation; (iii) poor exposure and (iv) poor accessibility. It is interesting to note that the potential of satellite data increases proportionately with the scale of structure under consideration. More effective interpretation of large structures such as lithologic contacts, folds, shear zones etc. is possible from satellite data. The structures in mobile belts are of crustal size and therefore geophysical input would be immensely useful for understanding the tectonics. The evolution of ancient tectonic system in forms of terranes and realistic plate tectonics has increasingly gained importance in recent years. Such an effort must be directed in recognizing terranes in this part of the Indian shield. Although, several geoscientists directed their efforts, the mobile belts have not been fully understood in their totality. Interpretations continue to vary widely, but have more or less tended to revolve around
Gondwana Research, V. 2, No. 4, 1999
collision related processes, getting away from the traditional geosynclinal concepts. The map presented here hopefully throws much light on the features described above. This part of the Indian shield provides plenty of opportunity, with its splendid exposures, to have a broader view about mechanism of collision regime, the timing of deformation, nature of shear zones and their kinematics. It is clear that no single discipline will be able to give us a conclusive picture. Hence integrated geological and geophysical approaches must be considered in unraveling the tectono-metamorphic history of the southeastern Indian shield.
Acknowledgments Dr. H.K. Gupta, Director, is acknowledged for his encouragement and kind permission to publish this note.
References Beach, A. (1976) The interrelations of fluid transport, deformation, geochemistry and heat flow in early Proterozoic shear zones in Lewisian complex. Phil. Trans. R. Soc.Lond., A. 280, pp. 569-604. Chetty, T.R.K. (1995a) A correlation of Proterozoic shear zones between Eastern Ghats,India and Enderby land, East Antarctica, based on Landsat imagery. India and Antarctica during the Precambrian. In: Yoshida, M. and Santosh, M. (Eds.), Geol. SOC.India, Memoir No. 34, pp.205-220. Chetty, T.R.K. (199513) Structural interpretation of high grade belts of south India using remotely sensed data. NNRMS Bull., V. 19, pp. 35-37. Chetty, T.R.K. (1996) Proterozoic shear zones in Southern Granulite Terrrain, India. In: Santosh, M. and Yoshida, M. (Eds.), The Archaean and Proterozoic terrains in southern India within East Gondwana. Gond. Res. Memoir No. 3, pp. 77-89. Chetty, T.R.K. (1999) Tectonics of Cudappah basin: A reinterpretation. Geol. SOC.India, Memoir (in press). Chetty, T.R.K. and Bhaskar Rao, Y. J. (1998) Behaviour of stretching lineations in the Salem-Attur shear belt, Southern Granulite Terrane, south India. J. Geol. SOC.India, v. 52, pp. 443-448. Chetty, T.R.K. and Murthy, D.S.(1994)Collision tectonics in the Eastern Ghats Mobile Belt: mesoscopic to satellite scale structural observations. Terra Nova, v. 6, pp. 72-81. Chetty, T.R.K. and Murthy, D.S.N. (1998) Regional tectonic framework of Eastern Ghats Mobile Belt: A new interpretation. Geol. Surv. India. spl. pub. No.44, pp. 39-50. Daly, M.C. (1986) Crustal shear zones and thrust belts: their geometry and continuity in central Africa. Phil. Trans. R. SOC.Lond.,A. 317, pp. 111-128. Drury, S.A., Harris, N.B.W., Reeves-Smith,G.J. and Wightmann, R.T. (1984) Precambrian tectonics and crustal evolution in south India. J. Geol., v. 92, pp. 3-20. Tikoff, B. and Greene, D. (1997) Stetching lineations in transpressional shear zones: an example from the Sierra Nevada batholith, California. J. Str. Geol., v. 19, pp. 29-38.