GEOLOGICAL STRUCTURES IN VALLEY. . BASED ON PHOTO·INTERPRETATION·

V. SRINIVASAN Tamil Nadu Geology Branch. Madras

Introduction: Attur valley forms part of the area of study of the V.N.D.P. in Tamil Nadu the aerial photographs of which were interpreted by Grady (1970) wherein he has made a reference (p. 10) to the 'E-W trends in the southern part of the area. The present work of the writer included the detailed photo interpretation of the area (Fig. I) to the north of Sweta Nadi river, and limited field checks in the area

LEGEND GEOLOGICAL SYMBOLS t§J aalic Oyk.. , .... ,.Geological Conlacl ...... -- Faull m Ullra Dallc. 4 Allilude wll~ 40"-70' m Meta ..dim.nl. ~ 70.....o'Vertlco' A' AtlltuGo m.olured on ground ~ C~a,nackil.. X Synclinal Aall- normal o Gn'/Sllc Complex ..Jt:1"" II • oVlrturn.d to louth X Anticlinal A.il-normal ~ .. .. (o..rlurn.d lOloulo) o ~lYer•• Fault (Higo AnQlI Thralll-Arrow • X Indicot .. Ill. direction of Incllnoflon (70 -75"1 \l 01 I~' Faull Plonl (U' uP. 0= down) " ...... S.clion Lin. (The _0' baUd on l>I'lO'Olr'lt.rpruatloft~,nfo' ...atjon fro'" U.N .... in.ral DUllopmut ProJlet ond 010100,. 8roncl'l In Tamil Hod") Figure I. Photo-geotectonic map of Attur valley and the adjoining areas. south of it i.e. in the northern flanks of Kollimalai- (for which aerial photographs were not available) in order to throw more light on the geological struc­ tures in general and on the effects of Vellar and Sweta Nadi faults in particular. The aerial photographs used for interpretation are on scale I : 36000 (approx). The area forms part of the region represented in the Survey of toposheet • Published with the permission of the Government of Tamil Nadu. 90 SHORTER COMMUNICATIONS No. 58 I and lies between the east longitudes 78°02' and 78°51' and north latitudes 11°18' and llo49'. Geology: With the usual limitations of photo interpretation of the Archaean complex, the litholigical units exhibiting different photographic characters (recogni­ tion elements) are classified into the following rock types. Basic dykes: Dark tone, fine texture, linear form and landscape and discordant relationship. Ultrabasics : Subdued landscape, less resistance to erosion, presence of white toned veins and quarry dumps in the dark toned rocks. Charnockites: Light grey tone, resistance to erosion, less foliated and folded appearance.

...••c CD .. ~ ::I ~ e, o Kolllmolol .. a:LL Hilla c: .. .. " ~~ 1<01r010n Hilla

Figure 2. Geological cross section along A·B in fig. 2. Gneisses: Light grey tone, coarse texture, well foliated with broad folded struc-­ tures. The charnockites and gneisses form the basement complex which occupies the area fully. The metasediments, magnetite-quartzite and amphibolite, are found in the synclinal outliers in Attur Valley. The ultrabasics occur in the central Valley portion. Structures: As the structural features are different in Attur valley and in the areas north and south of it, separate azimuthal frequency diagrams (Figs. 3 and 4) have been prepared and the following major trends (Table-I) have been recognised.

TABLE I

INFERENCES FROM THE AZIMUTHAL FREQUENCY DIAGRAMS

Frequency

Structural Areas north and Attur valley trends south of Attur valley Faults and Fold axes Faults and dykes Fold axes ---- dykes

NW-SE Less not present More not present

NE-SW More More More Less

E-W Less not present More More (minor) (major) . SHORTER COMMUNICATIONS 91

The E-W fold axes and foliations in Attur valley and those of NE-SW in Shevaroy­ ·Chitteri-Kalrayan and Kollimalai-Pachaimalai hills make the respective regions to form different tectonic domains. Synchronizing with this interpretation of different tectonic domains, the magnetic contours produced by the aeromagnetic geophysical survey (U.N.D.P.) of these regions reflect similar structural patterns. NW-SE faults: A number of dykes in the area and a fault in the south-western part of belong to this category and they are off-set by the NW-SW faults.

~L-. -L --' o 25% Scalet=:::t

FOLD A XES Figure 3. Azimuthal frequency diagrams for the areas north and south of Attur valley.

NE-SW faults: These are predominant and step faults with their eastern blocks -successively downthrown as suggested by the fault line scarps and successive lowering -of elevations towards east. The ultrabasics align themselves along this group of faults near Salem and west of Attur. Shearing (vertical) is common along these faults. The dykes and faults with NNE - ssw trends in the area have intersecting (conjugate) relationship with these NE-SW faults. The NE-SW faults are found to have been reactivated and dissected during later period of Vellar and Sweta Nadi faults. E-W Faults: Vellar Faults: This fault, which is partly followed by the Vellar river, forms the northern boundary for Attur valley. A zone of mylonite and tec­ tonite about 100 metre wide has been observed along this fault. The fault plane 0 (zone) dips at a high angle towards north, the hade being between 10° and 20 , The northerly overturned synclinal structure of Godumalai that rests (fault-scarp) on the fault plane also indicates the inclination of the fault plane towards north. The groove lineations present on the fault planes indicate a high angle oblique slip and the associated steps (Billings, 1960, p. 149) suggest an upward or thrust movement for the northern block in relation to the southern block (Attur valley). The small strike shift produced by the oblique slip is exhibited by the NE-SW faults which otherwise would have been continuous. 92 SHORTER COMMUNICATIONS Sweta Nadi fault: This fault forming the southern boundary for Attur Valley­ has controlled the course of Sweta Nadi river along which the mylonites are exposed. The mylonites outcrop as E-W linear ridges particularly west of Tammampatty N N (78°29'43 : 11°25'43 ) where they attain a width of about 1.5 km. This fault dips towards south at high angle. Here also the movement is high angle oblique slip and upward in the case of southern block in relation to the northern block (Attur valley), The strike shift is about 100 metres as exhibited by a dyke east of Tammampatty.

360

o 10% Scale l::==I FAULTS and DYKES

360

~L••"'~-'~.,.••

FOLD AXES

Figure 4. Azimuthal frequency diagrams for Attur valley.

Conclusion: The effects of the two major E-W faults of Vellar and Sweta Nadi are: (l) The northern and southern hill masses of Shevaroy-Chitteri-Kalrayan and Kollimalai-Pachaimalai hills (respectively) have suffered upward (thrust) movements. in relation to Attur valley which is hence structurally a graben, the relative throw being about 700 metres; The E-W fault line scarps of the hill masses, the altitude difference (700-1100 metres) between the hill masses and Attur valley, the similar difference in the disposition of metasediments in the above two areas, the preservation of metasediments in basin structures (e.g. Kanjamalai, Palaniyapuri basin, etc.) in Attur valley and the minor structure of steps found on the fault planes suggest these movements; the concomitant N·S compression has partly modified the earlier NE-SW structural styles of Attur valley into E-W ones; (2) The Attur valley has suffered a minor eastward shift as suggested by the offsets of dykes and faults along these two­ faults; (3) The general eastward plunges of the E-W folds in Attur valley indicate a tilt of this block in the west. . Further, it is interesting to know in this context, that a major E-W fault along Cauvery river south ofKollimalai-Pachairnalai hills has been observed and picked up. in the geophysical traverses of the Oil and Natural Gas Commission. Probable existence of an E-W fault (with a downthrow on the north) north of the Shevaroy­ Chitteri-Kalrayan hills mass cannot be ruled out as the geomorphology completely SHORTER COMMUNICATIONS 93 changes, and hence the 'metasediments occur in the basin structures (e.g. iron ores near Thiruvannamalai) as in Attur valley. Another well known E-W fault still to the north is along the southern border of the Cuddapah basin near Tirupathi in . Hence the origin of E-W faults throughout Peninsular India in the post-Cuddapah period is a possible tectonic phenomenon. The E-W and NE-SW faults appear to be mainly responsible for the developing into isolated hill masses (unlike the continuous hill mass of Western ghats); and they appear to have dissected the crust into rhohlbic blocks to whose sides the major trends (NE-SW and E-W) of folia­ tions and fold axes are parallel suggesting contemporaneity. The NW-SE trends observed in the Western ghats are parallel to the oldest faults and dykes (NW-SE) of this area indicating their concomitancy. Therefore these three (NW-SE, NE-SW and E-W in the chronological order) systems of faults have bearings on the respective trends of foliations and fold axes. Acknowledgements: The writer gratefully thanks Mr. A. Hatton, Project Manager and Mr. V. Gopal, Project Co-ordinator of V.N.D.P. Mineral Develop­ ment Project in Tamil Nadu for their kind permission to interpret the aerial photo­ graphs and for their valuable discussions during interpretation, and preparation of the manuscript. He is thankful to Mr. T. S. Paramasivan, Additional State Geologist for his kind permission to work on this problem. The assistance of MIs P. Dorai­ samy and K. Shanmugam during field checks are acknowledged.

REFERENCES BILLINGS, M. P., (1960) Structural Geology, Asia Publishing House, Bombay, etc., p. 149. GRADY,- J. C., (1970) Photogeology of the Project Area; Project Report, U. N. Mineral Develop­ ment Project, Tamil Nadu, p. 10.

GRAVITY ANOMALIES OVER GONDWANAS OF INDlA

S. C. BHATIA AND D. V. SUBBA RAO National Geophysical Research Institute, Hyderabad Introduction: The study of Gondwana formations of India has been of great interest to earth scientists not merely because of their economic importance in being the main coal bearing formations but also because of certain tectonic features asso­ dated with them. -The study of tectonics of Gondwana formations of India by Fox (1931) brought out the idea that these formations were formed in the block-faulted basins, i.e., rift valley-like basins. This idea ofFox though doubted by some authors like Ahmad (1961) has not yet been written off. The conclusions arrived at by Fox as well as Ahmad are based on almost the same data which are mainly geological. Girdler (1963) pointed out the importance of geophysical studies such as gravity, magnetic and seismic surveys in studying the problems associated with rift valley-like structures with particular emphasis on the steep negative gravity anomalies associated with well established East African rifts (Bullard, 1936). A good deal of gravity data from the Survey of India, Oil & Natural Gas Commission and National Geophysical Research Institute are available over the Gondwana deposits of India, such as that of Damodar valley, basin, Satpura basin, Godavary valley -and the small 6