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The tectonic framework of the South Indian craton: A reconnaissance involving LANDSAT imagery
Tectonophysics, 65 (1980) T1--T15
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© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands Letter Section The tectonic framework of the South involving L A N D S A T i m a g e r y
S.A. D R U R Y
Indian craton:
a reconnaissance
and R.W. H O L T
Department of Earth Sciences, The Open University, Milton Keynes M K 7 Britain) (Received September 3, 1979; accepted December 1, 1979)
6 A A (Great
ABSTRACT Drury, S.A. and Holt, R.W., 1980. The tectonic framework of the South Indian craton: a reconnaissance involvingL A N D S A T imagery. Tectonophysics, 65: T1--T15. Interpretation of 1:1,000,000 scale L A N D S A T imagery and reconnaissance structural analysis has revealed a pattern of tectonic evolution in the South Indian craton extending from Archaean times to the late Proterozoic. The precursors of the Archaean granitoid gneisses and supracrustal rocks were disrupted by a series of N--S late Archaean shear belts with various senses of movement, leading to a divisioninto high and low finite-straindomains and the main tectonic "grain" of much of the craton. Climatic metamorphism to as high as granulite facies was superimposed on these domains. The dehydration associated with granulite facies metamorphism may have been due to guiding of CO2-rich vapours by these shear belts. Postkinematic late Archaean to early Proterozoic granites are distributedin close relationto the shear belt fabrics.Mid to late Proterozoic deformation resulted in the overthrusting of the intercratonic Proterozoic Cuddapah Basin by Archaean rocks from the east, and the development of a major dextral shear system about which the northern part of the craton moved at least 70 k m eastwards relativeto the southern part.
INTRODUCTION T h e South Indian craton comprises the Archaean and Proterozoic rocks of Kerala, Tamfl Nadu, Karnataka and A n d h r a Pradesh. It is m a s k e d to the northwest by late Mesozoic to early Tertiary D e c c a n flood basalts, and to the west and east by Mesozoic, Tertiary and Quaternary sediments. T o the northeast it is b o u n d e d by the Godavari graben, active f r o m Palaeozoic to late Mesozoic times. T h e roughly triangular shape of the craton probably reflects the lines of tectonic separation during break-up of the G o n d w a n a supercontinent. Conceivably, the major Archaean and Proterozoic tectonic features of South India have counterparts in the originally adjacent cratonic areas of southern Africa, Madagascar and Antarctica (e.g. Katz and Premoli,
1979). L A N D S A T i m a g e r y m o s t clearly displays a l i g n m e n t s o f geological signif-
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icance, such as near-vertical faults, dykes, major joint patterns and steep to vertical compositional banding or planar tectonic fabrics. Such features appear as linear-son the images. If the last two categories can be distinguished from the others, they permit tracing of regional scale fold and shear belt patterns, and their analysis. This is aided if sharp contrasts exist on the images between important lithological units. The imagery used in this synoptic interpretation was 1:1,000,000 scale multispectral scanner positive black and white prints of Band 5 (red) acquired by LANDSAT 1 between November 1972 and February 1973. The compilation was prepared on an uncontrolled mosaic which shows little distortion when compared with similar scale maps. The interpretation was controlled by geological literature on the region, observations during about 150 man days field work between 1977 and 1979, and discussions with several Indian colleagues. In contrast to the only other detailed publication on satellite remote sensing data for the area (Srinivasan and Sreenivas, 1977) we attempt to separate information about Archaean and Proterozoic tectonic features from the wealth of later linears, and propose a generalized tectonic model as a guide to more detailed ground investigations. RAW DATA
The distribution and trends of linears that correspond to lithological boundaries, compositional banding and/or tectonic planar fabrics are shown on Fig.1. Also shown are major lithostratigraphic boundaries, such as Archaean supracrustal-granitoid interfaces, the distribution of pyroxene and the unconformities beneath Proterozoic gneisses (“charnockites”)* supracrustal sequences. This info~ation was transferred from the published, large scale Geological Survey of India maps of Kamataka, Tamil Nadu and Kerala. In Karnataka, these boundaries closely correspond with distinct contrasts on the imagery. Also outlined are areas of inselberg topography that are consistently related to homogeneous granitoids of the craton. No attempt has been made to infer the boundaries between these rocks and the gneissic granitoids of the craton. A notional line joining Mangalore, Bangalore and Madras separates the Archaean of South India into two units differing in me~orphism and lithology grounds. To the north, the craton is dominated by abundant Archaean supracrustal belts (the “Dharwar Schist Belts”) set in a “matrix” of highly variable gneissic granitoids (the “Peninsula Gneisses”). This bimodal association is punctuated by homogeneous granitoid plutons. The metamorphic grade of this area is generally from upper greenschist to upper amphibolite facies and it is referred to as the low grade terrain, To the south *Terms in parentheses are those conventionally used in literature prior to 1976. stratigraphic terminology of Swami Nath et al. (1976) is not used here.
The
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the dominant feature is the presence of large massifs of pyroxene gneisses (the “chamockites”) and metamorphic grade ranges from upper amphibolite to pyroxene granulite facies. This area is termed the high grade terrain. Though Archaean supracrustals are present in the high grade terrain, they are in narrow bands and enclaves which cannot be resolved on the imagery.
Archean low grade terrain The overall tectonic grain of the northern part of the craton is picked out by the distribution and shape of Archaean supracrustal belts. From west to east, the width and surface area of supracrustals relative to gneissic granitoids decreases. Some of the supracrustal belts clearly define an arcuate pattern, trend changing from NW-SE, to roughly N-S from north to south. This is shown particularly well by the chain of narrow supracrustal belts BadamiHagari-Ramagiri-Hesargatta (16-20) and by the Chitradurga (12), supracrustal belt. Between Chickmagalur and Kudremukh is a zone, 15-20 km wide and up to 400 km long of strong NNW-SSE linears, some of which are narrow arms of supracrustal belts. This zone separates an area to the west with NW-SE trends from that immedia~ly to the east which displays E-W trends and evidence of low dips. At many places there is clear evidence of linears swinging dramatically to parallel the NNW-SSE trend of the zone. There is some evidence for profound decrease in thickness of supracrustal belts as they enter this zone. Its appearance is of a dextral, ‘oblique-slip shear belt. Several large, pre-shear belt open folds with E-W axial traces are picked out by the southern interface between the Bababudan supracrustal belt (9) and the older gneissic granitoids. The eastern boundary of the Shimoga-Goa suprac~st~ belt (13) is marked by a sudden swing in strike within the belt from NW-SE through E-W to N-S, to the east and northeast of Shimoga. A similar feature is shown by the small, trident-shaped Holenarsipur supracrustal belt (2) 80 km to the south. East of a line joining the hinges of these major changes in strike, the majority of linear-s and supracrustal-gneiss interfaces define the arcuate pattern described earlier. This line may be interpreted as the western boundary of a major N-S shear system in which the large Chitradurga supracrustal belt (12) has been deformed. To the east of Chitradurga there are several parallel, arcuate arrays of minor Archaean supracrustal belts separated by gneisses. A parallel feature is the linear array of homogeneous granitoid inselbergs (the “Closepet granites”). North of Bangalore there are several curved E-W linears within the gneisses which separate two N-S arrays of supracrustal belts. This may reflect the presence of areas of low finite strain between shear belts. If so, the N-S trends of supracrustal belts reflect trends of shear belts rather than any preexisting grain in the craton. Returning to the western low grade terrain, the roughly N-S linears of the
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two possible major shear belts Belgaum-Sargar and Chitrad~ga-Mysore appear to merge and pinch out southwards. This feature is matched by the narrowing of outcrop in the Chitradurga supracrustal belt. It may indicate a southward increasing E-W crustal shortening and corresponding N-S extension within the shear belts. Archaean high grade terrain As the domin~t N-S trend of linears is followed south, met~o~hic grade increases. The linear features of the low grade terrain pass without break into a terrain dominated by pyroxene gneisses. The pyroxene gneisses form a series of massifs (Fig.l), within which strong patterns of parallel linears can be seen. Many of these massifs, in particular the Nilgiri (B) and Biligirirangan (C) massifs, are separated from one another by wide major lineaments, such as the Moyar and Bhavani valleys. The massifs also display narrower lineaments which parallel the strike. From north to south across the Moyar valley the strike swings from roughly N-S through NE-SW to roughly E-W in the valley itself, and then to NE-SW in the Nilgiri massif (B). The Moyar lineament (M) is up to 20 km wide, 200 km long and appears to be a major complex shear zone with an overall dextral sense of movement. Linears associated with the N-S shear belts in the low grade terrain are rotated in the Moyar lineament and it therefore post-dates them. The Moyar structure meets and appears to coalesce with a similar structure along the Bhavani valley (Bh) which forms the southem boundary of the Nilgiri massif. The latter (Bh) is essentially parallel to the strike in the Nilgiris but its eastward extension with the Moyar structure (M-Bh) does show dextral rotation of linears in the Biligirirangan massif (C) from N-S to ENE-WSW. Tracing the Moyar-Bhavani shear to the east, we find that it branches into several curved lineaments which are eventually parallel the NNE-SSW strike in the Shevaroi (I)) and North Arcot (E) massifs. Some of these strikeparallel lineaments can be traced over 250 km until they disappear at the southern tip of the Proterozoic Cuddapah-Kumool Basin. The southern boundary of this complex shear system is not readily distinguishable on the LANDSAT imagery as it is concealed in the near featureless plains of the Noyil-Cauvery drainage system. A branch appears to pass through the Attur valley east of Salem, separating the pyroxene gneiss massifs of the Shevaroy and Kalrayan Hills (D and E) from that of the Kollimalal and PachamaIai Hills (3’) (Srinivasan, 1974). The latter are bounded to the south by the E-W trend of the Cauvery valley, the north bank of which is characterized by E-W linears and which may be a further branch of the shear system (see Grady, 1971). A remarkable E-W lineament forms the northern flank of the Anaimalai and Palni Hills (G and EI) but has no structural counterpart on the ground, the only swing in strike being recorded by ground measurements south of Coimbatore in the plains of the Noyil drain-
.. ... ..
0
A-K. KB M-Bh
Fig. 1. Tectonic interpretation of LANDSAT imagery for South India. Supracrustal belts are numbered I-24, for ease of reference in the text. Similarly, important upland massifs in the high grade terrain are labelled Proterozoic basins: = Kaladgi Basin, CB = Cuddapah-Kumool Basin. Major Proterozoic shear belts: Bh = Bhavani, M = Moyar, = MoyarBhavani, N-Co = Noyil-Cauvery.
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age system. On Fig.1 the southern boundary is indicated as the NoyilCauvery lineament (N-06). South of the Noyil-Cauvery lineament the structures revealed by linears in the Anaimalai (G), Palni (H), Cardomom (R) and other massifs (1 and J) are very complex and may reflect numerous small shear systems anastomosing around pyroxene gneiss massifs. The most prominent feature is a profound swing in trend of linears from SE near ~~chirap~li and Madurai, to SW in the Palni (H) and Cardomon Hills (R). This structure is displaced by a prominent SE fault passing through Madurai along the Vaigai valley. It may extend as far as the Kerala coast north of Trivandrum. The age of this structure is uncertain, but it seems to be truncated by the Noyil-Cauvery lineament to the north, and its detailed structure is complex as it refolds large, earlier E-W trending folds. There is some evidence that there is a close, possibly genetic relationship between the structure and the nearby pyroxene gneiss massifs (H, 1, J and E), which, if true, would suggest it to be of late Archaean age. Llykes
A 250 km wide belt of many very prominent E-W linears crossing the tectonic grain of the Archaean is seen immediately to the south and west of the Cuddapah-Kumool Basin. Some of the linears are distinct lithological units made up of smaller en echelon linears. They are interpreted as igneous dykes. They appear to be terminated by the western interface between the Proterozoic Cuddapah supracrustals and the underlying Archaean. They could reflect profound N-S crustal extension after the main Archaean deformations but before Proterozoic sedimentation. This being the case, their detailed study could provide much information about post-Cuddapah strain in the critical area south of the Cuddapah Basin. Proterozoic terrain Proterozoic supracrustals occupy two main basins which rest with clear angular unconfo~i~ upon the Archaean rocks; the Kaladgi basin in the northwest and the Cuddapah basin in the northeast. The structure of both is clearly brought out by linears related to ridges and escarpments of resistant strata, and by the shape of the boundary with Archaean rocks (Fig. 1). The Kaladgi basin is clearly folded into four or five major open fold pairs with axial traces parallel to the structures in the underlying Archaean. This folding must also have affected the Archaean basement, though the profound overstepping at the base of the Kaladgi basin suggests that much of the structural complexity in the Archaean is older than the Kaladgi strata. The western portion of the crescentic Cuddapah basin is essentially unfolded and shallow dips towards the basin centre are indicated by scarp shapes. The base is clearly unconformable on the Archaean and all local
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tectonic and granitic activity is pre-Cuddapah. The eastern portion is a 50 km wide arcuate zone of steep dip with indications of tight-~oclin~ folding, coinciding with the Nallamalai hills. The eastern boundary of the basin with the Archaean is sharply marked and parallels the structural trend in the Nallamalai fold belt. The folding parallels the strike in the Archaean and part of the structure of the Archaean to the south must be of a similar age. FIELD DATA
Detailed structural data are at present available for only small parts of the craton. Apart from confirmation of the direct relation between LANDSAT linears and strike of lithological units, compositional banding and dominant planar tectonic fabrics, the following information is relevant to the tectonic framework of the craton: Arc~aea~ lout grade termin (1) Strain indicators such as lava pillows and one spherical amygdales show consistent increase in finite strain as volcanic units in the West Coast (10) and Holenarosipur (2) supracrustal belts are followed into N-S shear belts recognized on LANDSAT imagery. Between these shear belts, volcanogenie supracrustals are virtually undeformed. These patterns are confirmed by strain analysis of conglomerates in the low grade terrain (Chadwick et al., 1979). (2) Linear fabrics are common in the shear belts of the low grade terrain but absent in the intervening rocks. East of Kudremukh, lineations have low plunges parallel to the trend of the shear belt, confirming its oblique-slip sense of movement. In the N-S arm of the Holenarosipur supracrustal belt, identified as being within a major shear belt, lineations plunge down dip or steeply dipping foliation -planes, suggesting a component of vertical. movement within the shear belt. (3) Gr~itoid gneisses between shear belts and in large areas of low strain within them retain original igneous relationships between different phases. They are poorly foliated, except in minor shear zones and contain few, if any, enclaves of older materials. An Rb-Sr age of 3.4 Ga has been obtained from such gneisses in Hassan District (Beckinsale et al., 1980). In contrast the granitoids within shear belts are intensely deformed polyphase banded gneisses, which are often strongly lineated. They contain abundant supracrustal and other enclaves. The relative volume of enclaves in the gneisses of the western part of the low grade terrain increases southwards. This feature coincides with narrowing and disappearance of discrete supracrustal belts where the strike converges to the south of Mysore. The convergence coincides with a large area of gneisses around Mysore containing a huge number and variety of enclaves (Ramakrishnan et al., 1976). This association may represent a tectonic melange of those diverse lithologies comprising the Archaean prior to shearing, but may equally reflect the original intrusive relations between the gneisses’ precursors and older suprac~s~l suite.
(4) In sheared Archaean supracrustals, the growth of climax metamorphic minerals, such as garnet, chloritoid, staurolite and cordierite is static and post-shearing, although there is evidence of syn- and pre-shearing metamorphism at lower grades.
ATchaean high grade terrain (1) As the shear belts of the low grade terrain are traced southwards, hornblence-biotite gneisses give way to pyroxene-bearing assemblages with colinear fabrics. The onset of dehydration is clearly marked by irregular diffuse zones of pyroxene gneiss in hydrous gneiss, which follow earlier structures such as fold axial traces and basic-acid interfaces. These zones overprint banding and foliation in the host gneisses (R~ieng~ et al., 1976). Quite clearly, the granulite facies metamorphism post-dates the major Archaean deformation of the gneiss complex and is a relatively young facet of the craton’s evolution. Crawford (1969) obtained a regional Rb-Sr isotope pattern for the pyroxene gneisses that suggests an age of metamorphism around 2.6 Ga. Janardhan et al. (1979) suggest that the complex was purged of HZ0 by influx of CO,-rich vapour guided by earlier structures. The massifs indicated on Fig.1 are almost totally composed of near anhydrous pyroxenic rocks, possessing essentially the same structural features as the lower grade gneisses. (2) Retrogression of the pyroxene gneisses is mainly focussed along the late shear belts of the Moyar, Bhavani and other lineaments. There, entirely new planar and linear fabrics have been produced, giving extensive augen gneisses along the shears. Reconnaissance mapping south of Coimbatore reveals that the southern boundary of the shear system (N-Ca) is marked by a swing in strike from NNE-SSW in the Anaimalai Hills (G) through NE-SW to E-W, with increasing deformation (K. Gop~ak~hn~, personal communication, 1979). This confirms the dextral sense of the system as a whole, and this southern boundary can be termed the Noyil-Cauvery lineament. (3) Related zones of new shear fabric are ubiquitous throughout the plains between Coimbatore and Salem, and separate small areas of low finite strain. Within the Moyar-Bhavani, Noyil-Cauvery shear zone system, numerous enclaves of pyroxene gneiss and ultramafic to anorthositic high-pressure, garnet ~anulites show only locally developed shear fabrics. The mech~~m of formation and propagation of small-scale shear zones is such that they coalesce around relatively undeformed blocks of pre-shear zone material, Thus the Nilgiri massif itself and the high-pressure garnet granulite and pyroxene gneiss enclaves may be large scale analogues of this mechanism (compare Coward, 1976, fig. 2 for a small-scale example). Outside the shear system, similar shears form an integral part of local structural sequences. The variable dip of new planar and linear fabrics suggests complex patterns of movement in the E-W shear system. (4) The vaiious late shear belts have been exploited by a variety of post-
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kinematic granitic and syenitic are extensively fenitized.
plutons.
Some shear belts in North Arcot (E)
Proterozoic The Cuddapah Basin is tectonically overlain to the east by a thrust mass of Archaean rocks. Deep seismic sounding shows that this thrust and several smaller inbrications within the basin, are steeply inclined and displace the Moho (Kaila, 1979). The thrusting is related to the Nallamalai fold belt within the basin, where the Proterozoic strata are inverted. Crystal shortening associated with the eastern thrust is at least 5 km (Kaila, 1979) and taken together with that associated with the Nallamalai fold belt and other smaller thrusts, may be much more. DISCUSSION
Figure 2 is a schematic map of the main tectonic elements in the South Indian craton based on Fig.1. It shows the two generations of major shear belts, the pre-Cuddapah dyke swarm, and lines along which small Archaean suprac~st~ belts are arranged, perhaps reflecting major dislocations, and major areas of low finite strain where original Archaean relationships may be preserved. Inferred movement directions for each major shear belt are also shown, where there is evidence. In the absence of high-precision, geologically controlled ages from the craton, and of widespread detailed structural analysis, it is not yet possible to make any final conclusions about tectonic evolution. However, several general features are clearly indicated: Stage 0. Formation of a bimodal ~~itoid~uprac~s~ Archaean crust probably involving several early tectonothermal events. The controversy over subdivision of Archaean supracrustal belts into several different stratigraphic groups, perhaps including one group that predates the bulk of the granitoid component (e.g., Swami Nath et al., 1976; Geochemical Group, 1977; Chadwick et al., 1979) is complicated by the fact that most of the areas regarded as critical lie within Stage 1 shear belts. There is evidence on the imagery for early open folds along the southern margin of the Bababudan supracrustal belt (9). Field studies by the authors in the West Coast supracrustal belt (10) indicate the presence of large nappe-like structures that pre-date the Stage 1 shear belts. Similar structures are recorded from the Chitradurga belt (12) (Naqvi, 1973). Stage 1. An Archaean bimodal association of granitoids and supracrustals was disrupted by a series of major N-S shear belts with various senses of movement. The present configuration of Archaean supracrustal belts is largely a consequence of this shearing and probably not directly related to their original shapes. Stage 2. The climatic metamorphic event in the low grade and high grade
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..
Granulite
f
‘16”-
facies
Archaean high fmite strain Proterozoic high finite strain Proterozoic supracrustals Mesozoic
cover
Proterozoic dykes Marams of Archaean she; belts Margins of Proterozoic shear belts Inferred lateral Ar&nnan motions inferred vertical A’*“aG
I
* @ 2
mntions
t
Inferred Proterozoic motions
lateral
Fig. 2. Schematic tectonic map of South India showing majar Archaean shear belts and movement senses, where known, and major Proterozoic shear belts and movement senses.
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Archaean terrains is clearly static and post dates the Stage 1 shear belts. Development of pyroxenic assemblages in the high grade terrain seems to have been related in some way to the Stage I shear belts. The steep foliation in them may have guided the mantle-derived CO2 rich vapour phase thought to be responsible for purging of Hz0 from these rocks (J~~han et al., 1979). These “charnockites” are clearly not new additions to the crust but are reflections of a high grade metamorphism superimposed on earlier, structurally complex sial. Stage 3. Post-kinematic granitoid plutons are younger than the Stage 1 shear belts, yet are often arranged in curvilinear arrays parallel to the Stage 1 dominated structural grain of the low grade terrain. There may be some genetic connection between post-kinematic granitoids, Stage 2, and Stage 1 structures. Stage 4. Retrogression in the high grade terrain is related to a second series of major shear belts along which large massifs have been dislocated. The sense of movement of Stage 4 shear belts is variable, as is their orientation. They are probably connected to the thrusting and crustal shortening which has affected the supracrustals of the Proterozoic Cuddapah Basin. The disposition of pyroxene gneiss (“charnockite”) massifs about the E-W Stage 4 Moy~Bhav~i, Noyil-Cauvery shear system suggests a possible 60-80 km dextraI shift of the southern high grade terrain relative to the terrain to the north of the system. Folding of the Proterozoic Kaladgi supracrustal basin along axial traces parallel to the structural grain in the underlying Archaean terrain suggests that Stage 4 deformation here was taken up by reactivation of earlier Stage 1 structures. The broad arcuate pattern of structural grain in the northern part of the craton possibly reflects a younger tectonic event. There is no doubt that the tectonic evolution of the South Indian craton has far more complexities than those outlined in the above scheme. LANDSAT imagery is, for example, incapable of resolving small or nearhorizontal tectonic structures that may have been present prior to Stage 1. However, it is hoped that the scheme, together with Figs. 1 and 2 will provide a framework for orientation of new work within the craton. In particular, it identifies areas of low finite strain where the earliest history of the craton will probably be revealed. ACKNOWLEDGEMENTS
The bulk of this work would never have been possible without the logistic support of the Nationat Geophysical Research Institute (Hyderabad), the Karnataka State Department of Mines and Geology, and the Geological Survey of India (Tamil Nadu Circle) to whom our gratitude is extended. Our ideas have been formulated partly by discussions of South Indian geology with numerous Indian colleagues, three of whom have figured largely in our
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field work: S.M. Naqvi (NGRI), S. Jayaram (KSDMG) and K. Gopalakrishnan (GSI). Their friendship is highly valued. Visits to India have been financed by the Open University, the Royal Society and NERC (Research Grant GB3/3665) to whom thanks are due. Graphics were designed by Jenny Hill. The manuscript was typed by Anna Goulas.
REFERENCES Beckinsale, R.D., Drury, S.A. and Holt, R.W., 1980. 3360 M yr old gneisses from the South Indian craton. Nature, 283: 469-470. Chadwick, B., Ramakrishnan, M., Viswanatha, M.N. and Srinavasa Murthy, V., 1979. Structural studies in the Archaean Sargur and Dharwar supracrustal rocks of the Karnataka craton. J. Geol. Sot. India, 19: 531-549. Coward, M.P., 1976. Strain within ductile shear zones. Tectonophysics, 34: 181-197. Geochemistry Group, 1977. Archaean Geochemistry - contributions and programmes of NGRI. Geophys. Res. Bull., 15: l-54. Grady, J.C., 1971. Deep main faults in India. J. Geol. Sot. India, 12: 56-62. Janardhan, A.S., Newton, R.C. and Smith, J.V., 1979. Ancient crustal metamorphism at low PH,o: charnockite formation at Kabbaldurga, South India. Nature, 278: 511-514. Kaila, K.L., Chowdhury, K.R., Reddy, P.R., Krishna, V.G., Narain, H., Subbotin, S.I., Sollogub, V.B., Chekunov, A.V., Kharetchko, G.E., Lazarenko, M.W. and Ilchenko, T.V., 1979. Crustal structure along Kvali-Udipi profile in the Indian Peninsular Shield from deep seismic sounding. J. Geol. Sot. India, 20: 307.-333. Katz, M.B. and Premoli, C., 1979. India and Madagascar in Gondwanaland based on matching Precambrian lineaments. Nature, 279: 312-315. Naqvi, S.M., 1973. Geological structure and aeromagnetic and gravity anomalies in the central part of the Chitaldrug schist belt, Mysore, India. Geol. Sot. Am. Bull., 84: 1721-1732. Ramakrishnan, M., Viswanatha, M.N. and Swami Nath, J., 1976. Basement-cover relationships of peninsular gneiss with high grade schists and greenstone belts of southern Karnataka. J. Geol, Sot. India, 17: 97-111. Ramiengar, A.S., Ramakrishnan, M. and Viswanatha, M.N., 1976. Charnokite-gneiss complex relationship in Southern Karnataka and its bearing on crustal development. \ J. Geol. Sot. India, 17: 149-158. Srinivasan, R. and Sreenivas, B.L., 1977. Some new geological features from the Landsat imagery of Karnataka. J. Geol. Sot. India, 18: 589-597. Srinivasan, V., 1974. Geological structures in Attur valley, Tamil, Nadu, based on photo interpretation. J. Geol. Sot. India, 15: 89-92. Swami Nath, J., R~akrishnan, M. and Viswanathan, M.N., 1976. Dharwar strati~aphic model and Karnataka craton evolution. Rec. Geol. Surv. India, 107: X49-175.
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© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands Letter Section The tectonic framework of the South involving L A N D S A T i m a g e r y
S.A. D R U R Y
Indian craton:
a reconnaissance
and R.W. H O L T
Department of Earth Sciences, The Open University, Milton Keynes M K 7 Britain) (Received September 3, 1979; accepted December 1, 1979)
6 A A (Great
ABSTRACT Drury, S.A. and Holt, R.W., 1980. The tectonic framework of the South Indian craton: a reconnaissance involvingL A N D S A T imagery. Tectonophysics, 65: T1--T15. Interpretation of 1:1,000,000 scale L A N D S A T imagery and reconnaissance structural analysis has revealed a pattern of tectonic evolution in the South Indian craton extending from Archaean times to the late Proterozoic. The precursors of the Archaean granitoid gneisses and supracrustal rocks were disrupted by a series of N--S late Archaean shear belts with various senses of movement, leading to a divisioninto high and low finite-straindomains and the main tectonic "grain" of much of the craton. Climatic metamorphism to as high as granulite facies was superimposed on these domains. The dehydration associated with granulite facies metamorphism may have been due to guiding of CO2-rich vapours by these shear belts. Postkinematic late Archaean to early Proterozoic granites are distributedin close relationto the shear belt fabrics.Mid to late Proterozoic deformation resulted in the overthrusting of the intercratonic Proterozoic Cuddapah Basin by Archaean rocks from the east, and the development of a major dextral shear system about which the northern part of the craton moved at least 70 k m eastwards relativeto the southern part.
INTRODUCTION T h e South Indian craton comprises the Archaean and Proterozoic rocks of Kerala, Tamfl Nadu, Karnataka and A n d h r a Pradesh. It is m a s k e d to the northwest by late Mesozoic to early Tertiary D e c c a n flood basalts, and to the west and east by Mesozoic, Tertiary and Quaternary sediments. T o the northeast it is b o u n d e d by the Godavari graben, active f r o m Palaeozoic to late Mesozoic times. T h e roughly triangular shape of the craton probably reflects the lines of tectonic separation during break-up of the G o n d w a n a supercontinent. Conceivably, the major Archaean and Proterozoic tectonic features of South India have counterparts in the originally adjacent cratonic areas of southern Africa, Madagascar and Antarctica (e.g. Katz and Premoli,
1979). L A N D S A T i m a g e r y m o s t clearly displays a l i g n m e n t s o f geological signif-
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icance, such as near-vertical faults, dykes, major joint patterns and steep to vertical compositional banding or planar tectonic fabrics. Such features appear as linear-son the images. If the last two categories can be distinguished from the others, they permit tracing of regional scale fold and shear belt patterns, and their analysis. This is aided if sharp contrasts exist on the images between important lithological units. The imagery used in this synoptic interpretation was 1:1,000,000 scale multispectral scanner positive black and white prints of Band 5 (red) acquired by LANDSAT 1 between November 1972 and February 1973. The compilation was prepared on an uncontrolled mosaic which shows little distortion when compared with similar scale maps. The interpretation was controlled by geological literature on the region, observations during about 150 man days field work between 1977 and 1979, and discussions with several Indian colleagues. In contrast to the only other detailed publication on satellite remote sensing data for the area (Srinivasan and Sreenivas, 1977) we attempt to separate information about Archaean and Proterozoic tectonic features from the wealth of later linears, and propose a generalized tectonic model as a guide to more detailed ground investigations. RAW DATA
The distribution and trends of linears that correspond to lithological boundaries, compositional banding and/or tectonic planar fabrics are shown on Fig.1. Also shown are major lithostratigraphic boundaries, such as Archaean supracrustal-granitoid interfaces, the distribution of pyroxene and the unconformities beneath Proterozoic gneisses (“charnockites”)* supracrustal sequences. This info~ation was transferred from the published, large scale Geological Survey of India maps of Kamataka, Tamil Nadu and Kerala. In Karnataka, these boundaries closely correspond with distinct contrasts on the imagery. Also outlined are areas of inselberg topography that are consistently related to homogeneous granitoids of the craton. No attempt has been made to infer the boundaries between these rocks and the gneissic granitoids of the craton. A notional line joining Mangalore, Bangalore and Madras separates the Archaean of South India into two units differing in me~orphism and lithology grounds. To the north, the craton is dominated by abundant Archaean supracrustal belts (the “Dharwar Schist Belts”) set in a “matrix” of highly variable gneissic granitoids (the “Peninsula Gneisses”). This bimodal association is punctuated by homogeneous granitoid plutons. The metamorphic grade of this area is generally from upper greenschist to upper amphibolite facies and it is referred to as the low grade terrain, To the south *Terms in parentheses are those conventionally used in literature prior to 1976. stratigraphic terminology of Swami Nath et al. (1976) is not used here.
The
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the dominant feature is the presence of large massifs of pyroxene gneisses (the “chamockites”) and metamorphic grade ranges from upper amphibolite to pyroxene granulite facies. This area is termed the high grade terrain. Though Archaean supracrustals are present in the high grade terrain, they are in narrow bands and enclaves which cannot be resolved on the imagery.
Archean low grade terrain The overall tectonic grain of the northern part of the craton is picked out by the distribution and shape of Archaean supracrustal belts. From west to east, the width and surface area of supracrustals relative to gneissic granitoids decreases. Some of the supracrustal belts clearly define an arcuate pattern, trend changing from NW-SE, to roughly N-S from north to south. This is shown particularly well by the chain of narrow supracrustal belts BadamiHagari-Ramagiri-Hesargatta (16-20) and by the Chitradurga (12), supracrustal belt. Between Chickmagalur and Kudremukh is a zone, 15-20 km wide and up to 400 km long of strong NNW-SSE linears, some of which are narrow arms of supracrustal belts. This zone separates an area to the west with NW-SE trends from that immedia~ly to the east which displays E-W trends and evidence of low dips. At many places there is clear evidence of linears swinging dramatically to parallel the NNW-SSE trend of the zone. There is some evidence for profound decrease in thickness of supracrustal belts as they enter this zone. Its appearance is of a dextral, ‘oblique-slip shear belt. Several large, pre-shear belt open folds with E-W axial traces are picked out by the southern interface between the Bababudan supracrustal belt (9) and the older gneissic granitoids. The eastern boundary of the Shimoga-Goa suprac~st~ belt (13) is marked by a sudden swing in strike within the belt from NW-SE through E-W to N-S, to the east and northeast of Shimoga. A similar feature is shown by the small, trident-shaped Holenarsipur supracrustal belt (2) 80 km to the south. East of a line joining the hinges of these major changes in strike, the majority of linear-s and supracrustal-gneiss interfaces define the arcuate pattern described earlier. This line may be interpreted as the western boundary of a major N-S shear system in which the large Chitradurga supracrustal belt (12) has been deformed. To the east of Chitradurga there are several parallel, arcuate arrays of minor Archaean supracrustal belts separated by gneisses. A parallel feature is the linear array of homogeneous granitoid inselbergs (the “Closepet granites”). North of Bangalore there are several curved E-W linears within the gneisses which separate two N-S arrays of supracrustal belts. This may reflect the presence of areas of low finite strain between shear belts. If so, the N-S trends of supracrustal belts reflect trends of shear belts rather than any preexisting grain in the craton. Returning to the western low grade terrain, the roughly N-S linears of the
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two possible major shear belts Belgaum-Sargar and Chitrad~ga-Mysore appear to merge and pinch out southwards. This feature is matched by the narrowing of outcrop in the Chitradurga supracrustal belt. It may indicate a southward increasing E-W crustal shortening and corresponding N-S extension within the shear belts. Archaean high grade terrain As the domin~t N-S trend of linears is followed south, met~o~hic grade increases. The linear features of the low grade terrain pass without break into a terrain dominated by pyroxene gneisses. The pyroxene gneisses form a series of massifs (Fig.l), within which strong patterns of parallel linears can be seen. Many of these massifs, in particular the Nilgiri (B) and Biligirirangan (C) massifs, are separated from one another by wide major lineaments, such as the Moyar and Bhavani valleys. The massifs also display narrower lineaments which parallel the strike. From north to south across the Moyar valley the strike swings from roughly N-S through NE-SW to roughly E-W in the valley itself, and then to NE-SW in the Nilgiri massif (B). The Moyar lineament (M) is up to 20 km wide, 200 km long and appears to be a major complex shear zone with an overall dextral sense of movement. Linears associated with the N-S shear belts in the low grade terrain are rotated in the Moyar lineament and it therefore post-dates them. The Moyar structure meets and appears to coalesce with a similar structure along the Bhavani valley (Bh) which forms the southem boundary of the Nilgiri massif. The latter (Bh) is essentially parallel to the strike in the Nilgiris but its eastward extension with the Moyar structure (M-Bh) does show dextral rotation of linears in the Biligirirangan massif (C) from N-S to ENE-WSW. Tracing the Moyar-Bhavani shear to the east, we find that it branches into several curved lineaments which are eventually parallel the NNE-SSW strike in the Shevaroi (I)) and North Arcot (E) massifs. Some of these strikeparallel lineaments can be traced over 250 km until they disappear at the southern tip of the Proterozoic Cuddapah-Kumool Basin. The southern boundary of this complex shear system is not readily distinguishable on the LANDSAT imagery as it is concealed in the near featureless plains of the Noyil-Cauvery drainage system. A branch appears to pass through the Attur valley east of Salem, separating the pyroxene gneiss massifs of the Shevaroy and Kalrayan Hills (D and E) from that of the Kollimalal and PachamaIai Hills (3’) (Srinivasan, 1974). The latter are bounded to the south by the E-W trend of the Cauvery valley, the north bank of which is characterized by E-W linears and which may be a further branch of the shear system (see Grady, 1971). A remarkable E-W lineament forms the northern flank of the Anaimalai and Palni Hills (G and EI) but has no structural counterpart on the ground, the only swing in strike being recorded by ground measurements south of Coimbatore in the plains of the Noyil drain-
.. ... ..
0
A-K. KB M-Bh
Fig. 1. Tectonic interpretation of LANDSAT imagery for South India. Supracrustal belts are numbered I-24, for ease of reference in the text. Similarly, important upland massifs in the high grade terrain are labelled Proterozoic basins: = Kaladgi Basin, CB = Cuddapah-Kumool Basin. Major Proterozoic shear belts: Bh = Bhavani, M = Moyar, = MoyarBhavani, N-Co = Noyil-Cauvery.
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age system. On Fig.1 the southern boundary is indicated as the NoyilCauvery lineament (N-06). South of the Noyil-Cauvery lineament the structures revealed by linears in the Anaimalai (G), Palni (H), Cardomom (R) and other massifs (1 and J) are very complex and may reflect numerous small shear systems anastomosing around pyroxene gneiss massifs. The most prominent feature is a profound swing in trend of linears from SE near ~~chirap~li and Madurai, to SW in the Palni (H) and Cardomon Hills (R). This structure is displaced by a prominent SE fault passing through Madurai along the Vaigai valley. It may extend as far as the Kerala coast north of Trivandrum. The age of this structure is uncertain, but it seems to be truncated by the Noyil-Cauvery lineament to the north, and its detailed structure is complex as it refolds large, earlier E-W trending folds. There is some evidence that there is a close, possibly genetic relationship between the structure and the nearby pyroxene gneiss massifs (H, 1, J and E), which, if true, would suggest it to be of late Archaean age. Llykes
A 250 km wide belt of many very prominent E-W linears crossing the tectonic grain of the Archaean is seen immediately to the south and west of the Cuddapah-Kumool Basin. Some of the linears are distinct lithological units made up of smaller en echelon linears. They are interpreted as igneous dykes. They appear to be terminated by the western interface between the Proterozoic Cuddapah supracrustals and the underlying Archaean. They could reflect profound N-S crustal extension after the main Archaean deformations but before Proterozoic sedimentation. This being the case, their detailed study could provide much information about post-Cuddapah strain in the critical area south of the Cuddapah Basin. Proterozoic terrain Proterozoic supracrustals occupy two main basins which rest with clear angular unconfo~i~ upon the Archaean rocks; the Kaladgi basin in the northwest and the Cuddapah basin in the northeast. The structure of both is clearly brought out by linears related to ridges and escarpments of resistant strata, and by the shape of the boundary with Archaean rocks (Fig. 1). The Kaladgi basin is clearly folded into four or five major open fold pairs with axial traces parallel to the structures in the underlying Archaean. This folding must also have affected the Archaean basement, though the profound overstepping at the base of the Kaladgi basin suggests that much of the structural complexity in the Archaean is older than the Kaladgi strata. The western portion of the crescentic Cuddapah basin is essentially unfolded and shallow dips towards the basin centre are indicated by scarp shapes. The base is clearly unconformable on the Archaean and all local
TlO
tectonic and granitic activity is pre-Cuddapah. The eastern portion is a 50 km wide arcuate zone of steep dip with indications of tight-~oclin~ folding, coinciding with the Nallamalai hills. The eastern boundary of the basin with the Archaean is sharply marked and parallels the structural trend in the Nallamalai fold belt. The folding parallels the strike in the Archaean and part of the structure of the Archaean to the south must be of a similar age. FIELD DATA
Detailed structural data are at present available for only small parts of the craton. Apart from confirmation of the direct relation between LANDSAT linears and strike of lithological units, compositional banding and dominant planar tectonic fabrics, the following information is relevant to the tectonic framework of the craton: Arc~aea~ lout grade termin (1) Strain indicators such as lava pillows and one spherical amygdales show consistent increase in finite strain as volcanic units in the West Coast (10) and Holenarosipur (2) supracrustal belts are followed into N-S shear belts recognized on LANDSAT imagery. Between these shear belts, volcanogenie supracrustals are virtually undeformed. These patterns are confirmed by strain analysis of conglomerates in the low grade terrain (Chadwick et al., 1979). (2) Linear fabrics are common in the shear belts of the low grade terrain but absent in the intervening rocks. East of Kudremukh, lineations have low plunges parallel to the trend of the shear belt, confirming its oblique-slip sense of movement. In the N-S arm of the Holenarosipur supracrustal belt, identified as being within a major shear belt, lineations plunge down dip or steeply dipping foliation -planes, suggesting a component of vertical. movement within the shear belt. (3) Gr~itoid gneisses between shear belts and in large areas of low strain within them retain original igneous relationships between different phases. They are poorly foliated, except in minor shear zones and contain few, if any, enclaves of older materials. An Rb-Sr age of 3.4 Ga has been obtained from such gneisses in Hassan District (Beckinsale et al., 1980). In contrast the granitoids within shear belts are intensely deformed polyphase banded gneisses, which are often strongly lineated. They contain abundant supracrustal and other enclaves. The relative volume of enclaves in the gneisses of the western part of the low grade terrain increases southwards. This feature coincides with narrowing and disappearance of discrete supracrustal belts where the strike converges to the south of Mysore. The convergence coincides with a large area of gneisses around Mysore containing a huge number and variety of enclaves (Ramakrishnan et al., 1976). This association may represent a tectonic melange of those diverse lithologies comprising the Archaean prior to shearing, but may equally reflect the original intrusive relations between the gneisses’ precursors and older suprac~s~l suite.
(4) In sheared Archaean supracrustals, the growth of climax metamorphic minerals, such as garnet, chloritoid, staurolite and cordierite is static and post-shearing, although there is evidence of syn- and pre-shearing metamorphism at lower grades.
ATchaean high grade terrain (1) As the shear belts of the low grade terrain are traced southwards, hornblence-biotite gneisses give way to pyroxene-bearing assemblages with colinear fabrics. The onset of dehydration is clearly marked by irregular diffuse zones of pyroxene gneiss in hydrous gneiss, which follow earlier structures such as fold axial traces and basic-acid interfaces. These zones overprint banding and foliation in the host gneisses (R~ieng~ et al., 1976). Quite clearly, the granulite facies metamorphism post-dates the major Archaean deformation of the gneiss complex and is a relatively young facet of the craton’s evolution. Crawford (1969) obtained a regional Rb-Sr isotope pattern for the pyroxene gneisses that suggests an age of metamorphism around 2.6 Ga. Janardhan et al. (1979) suggest that the complex was purged of HZ0 by influx of CO,-rich vapour guided by earlier structures. The massifs indicated on Fig.1 are almost totally composed of near anhydrous pyroxenic rocks, possessing essentially the same structural features as the lower grade gneisses. (2) Retrogression of the pyroxene gneisses is mainly focussed along the late shear belts of the Moyar, Bhavani and other lineaments. There, entirely new planar and linear fabrics have been produced, giving extensive augen gneisses along the shears. Reconnaissance mapping south of Coimbatore reveals that the southern boundary of the shear system (N-Ca) is marked by a swing in strike from NNE-SSW in the Anaimalai Hills (G) through NE-SW to E-W, with increasing deformation (K. Gop~ak~hn~, personal communication, 1979). This confirms the dextral sense of the system as a whole, and this southern boundary can be termed the Noyil-Cauvery lineament. (3) Related zones of new shear fabric are ubiquitous throughout the plains between Coimbatore and Salem, and separate small areas of low finite strain. Within the Moyar-Bhavani, Noyil-Cauvery shear zone system, numerous enclaves of pyroxene gneiss and ultramafic to anorthositic high-pressure, garnet ~anulites show only locally developed shear fabrics. The mech~~m of formation and propagation of small-scale shear zones is such that they coalesce around relatively undeformed blocks of pre-shear zone material, Thus the Nilgiri massif itself and the high-pressure garnet granulite and pyroxene gneiss enclaves may be large scale analogues of this mechanism (compare Coward, 1976, fig. 2 for a small-scale example). Outside the shear system, similar shears form an integral part of local structural sequences. The variable dip of new planar and linear fabrics suggests complex patterns of movement in the E-W shear system. (4) The vaiious late shear belts have been exploited by a variety of post-
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kinematic granitic and syenitic are extensively fenitized.
plutons.
Some shear belts in North Arcot (E)
Proterozoic The Cuddapah Basin is tectonically overlain to the east by a thrust mass of Archaean rocks. Deep seismic sounding shows that this thrust and several smaller inbrications within the basin, are steeply inclined and displace the Moho (Kaila, 1979). The thrusting is related to the Nallamalai fold belt within the basin, where the Proterozoic strata are inverted. Crystal shortening associated with the eastern thrust is at least 5 km (Kaila, 1979) and taken together with that associated with the Nallamalai fold belt and other smaller thrusts, may be much more. DISCUSSION
Figure 2 is a schematic map of the main tectonic elements in the South Indian craton based on Fig.1. It shows the two generations of major shear belts, the pre-Cuddapah dyke swarm, and lines along which small Archaean suprac~st~ belts are arranged, perhaps reflecting major dislocations, and major areas of low finite strain where original Archaean relationships may be preserved. Inferred movement directions for each major shear belt are also shown, where there is evidence. In the absence of high-precision, geologically controlled ages from the craton, and of widespread detailed structural analysis, it is not yet possible to make any final conclusions about tectonic evolution. However, several general features are clearly indicated: Stage 0. Formation of a bimodal ~~itoid~uprac~s~ Archaean crust probably involving several early tectonothermal events. The controversy over subdivision of Archaean supracrustal belts into several different stratigraphic groups, perhaps including one group that predates the bulk of the granitoid component (e.g., Swami Nath et al., 1976; Geochemical Group, 1977; Chadwick et al., 1979) is complicated by the fact that most of the areas regarded as critical lie within Stage 1 shear belts. There is evidence on the imagery for early open folds along the southern margin of the Bababudan supracrustal belt (9). Field studies by the authors in the West Coast supracrustal belt (10) indicate the presence of large nappe-like structures that pre-date the Stage 1 shear belts. Similar structures are recorded from the Chitradurga belt (12) (Naqvi, 1973). Stage 1. An Archaean bimodal association of granitoids and supracrustals was disrupted by a series of major N-S shear belts with various senses of movement. The present configuration of Archaean supracrustal belts is largely a consequence of this shearing and probably not directly related to their original shapes. Stage 2. The climatic metamorphic event in the low grade and high grade
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..
Granulite
f
‘16”-
facies
Archaean high fmite strain Proterozoic high finite strain Proterozoic supracrustals Mesozoic
cover
Proterozoic dykes Marams of Archaean she; belts Margins of Proterozoic shear belts Inferred lateral Ar&nnan motions inferred vertical A’*“aG
I
* @ 2
mntions
t
Inferred Proterozoic motions
lateral
Fig. 2. Schematic tectonic map of South India showing majar Archaean shear belts and movement senses, where known, and major Proterozoic shear belts and movement senses.
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Archaean terrains is clearly static and post dates the Stage 1 shear belts. Development of pyroxenic assemblages in the high grade terrain seems to have been related in some way to the Stage I shear belts. The steep foliation in them may have guided the mantle-derived CO2 rich vapour phase thought to be responsible for purging of Hz0 from these rocks (J~~han et al., 1979). These “charnockites” are clearly not new additions to the crust but are reflections of a high grade metamorphism superimposed on earlier, structurally complex sial. Stage 3. Post-kinematic granitoid plutons are younger than the Stage 1 shear belts, yet are often arranged in curvilinear arrays parallel to the Stage 1 dominated structural grain of the low grade terrain. There may be some genetic connection between post-kinematic granitoids, Stage 2, and Stage 1 structures. Stage 4. Retrogression in the high grade terrain is related to a second series of major shear belts along which large massifs have been dislocated. The sense of movement of Stage 4 shear belts is variable, as is their orientation. They are probably connected to the thrusting and crustal shortening which has affected the supracrustals of the Proterozoic Cuddapah Basin. The disposition of pyroxene gneiss (“charnockite”) massifs about the E-W Stage 4 Moy~Bhav~i, Noyil-Cauvery shear system suggests a possible 60-80 km dextraI shift of the southern high grade terrain relative to the terrain to the north of the system. Folding of the Proterozoic Kaladgi supracrustal basin along axial traces parallel to the structural grain in the underlying Archaean terrain suggests that Stage 4 deformation here was taken up by reactivation of earlier Stage 1 structures. The broad arcuate pattern of structural grain in the northern part of the craton possibly reflects a younger tectonic event. There is no doubt that the tectonic evolution of the South Indian craton has far more complexities than those outlined in the above scheme. LANDSAT imagery is, for example, incapable of resolving small or nearhorizontal tectonic structures that may have been present prior to Stage 1. However, it is hoped that the scheme, together with Figs. 1 and 2 will provide a framework for orientation of new work within the craton. In particular, it identifies areas of low finite strain where the earliest history of the craton will probably be revealed. ACKNOWLEDGEMENTS
The bulk of this work would never have been possible without the logistic support of the Nationat Geophysical Research Institute (Hyderabad), the Karnataka State Department of Mines and Geology, and the Geological Survey of India (Tamil Nadu Circle) to whom our gratitude is extended. Our ideas have been formulated partly by discussions of South Indian geology with numerous Indian colleagues, three of whom have figured largely in our
1‘15
field work: S.M. Naqvi (NGRI), S. Jayaram (KSDMG) and K. Gopalakrishnan (GSI). Their friendship is highly valued. Visits to India have been financed by the Open University, the Royal Society and NERC (Research Grant GB3/3665) to whom thanks are due. Graphics were designed by Jenny Hill. The manuscript was typed by Anna Goulas.
REFERENCES Beckinsale, R.D., Drury, S.A. and Holt, R.W., 1980. 3360 M yr old gneisses from the South Indian craton. Nature, 283: 469-470. Chadwick, B., Ramakrishnan, M., Viswanatha, M.N. and Srinavasa Murthy, V., 1979. Structural studies in the Archaean Sargur and Dharwar supracrustal rocks of the Karnataka craton. J. Geol. Sot. India, 19: 531-549. Coward, M.P., 1976. Strain within ductile shear zones. Tectonophysics, 34: 181-197. Geochemistry Group, 1977. Archaean Geochemistry - contributions and programmes of NGRI. Geophys. Res. Bull., 15: l-54. Grady, J.C., 1971. Deep main faults in India. J. Geol. Sot. India, 12: 56-62. Janardhan, A.S., Newton, R.C. and Smith, J.V., 1979. Ancient crustal metamorphism at low PH,o: charnockite formation at Kabbaldurga, South India. Nature, 278: 511-514. Kaila, K.L., Chowdhury, K.R., Reddy, P.R., Krishna, V.G., Narain, H., Subbotin, S.I., Sollogub, V.B., Chekunov, A.V., Kharetchko, G.E., Lazarenko, M.W. and Ilchenko, T.V., 1979. Crustal structure along Kvali-Udipi profile in the Indian Peninsular Shield from deep seismic sounding. J. Geol. Sot. India, 20: 307.-333. Katz, M.B. and Premoli, C., 1979. India and Madagascar in Gondwanaland based on matching Precambrian lineaments. Nature, 279: 312-315. Naqvi, S.M., 1973. Geological structure and aeromagnetic and gravity anomalies in the central part of the Chitaldrug schist belt, Mysore, India. Geol. Sot. Am. Bull., 84: 1721-1732. Ramakrishnan, M., Viswanatha, M.N. and Swami Nath, J., 1976. Basement-cover relationships of peninsular gneiss with high grade schists and greenstone belts of southern Karnataka. J. Geol, Sot. India, 17: 97-111. Ramiengar, A.S., Ramakrishnan, M. and Viswanatha, M.N., 1976. Charnokite-gneiss complex relationship in Southern Karnataka and its bearing on crustal development. \ J. Geol. Sot. India, 17: 149-158. Srinivasan, R. and Sreenivas, B.L., 1977. Some new geological features from the Landsat imagery of Karnataka. J. Geol. Sot. India, 18: 589-597. Srinivasan, V., 1974. Geological structures in Attur valley, Tamil, Nadu, based on photo interpretation. J. Geol. Sot. India, 15: 89-92. Swami Nath, J., R~akrishnan, M. and Viswanathan, M.N., 1976. Dharwar strati~aphic model and Karnataka craton evolution. Rec. Geol. Surv. India, 107: X49-175.