The southern Indian shield consists of three major tectonic provinces viz., (1) Dharwar Craton, (2) Eastern Ghat Mobile Belt, and (3) Pandyan Mobile Belt. An understanding of their mutual relations is crucial for formulating crustal evolution models. The tectonic evolution of these provinces is summarized

Ramakrishnan, M.

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TECTONIC EVOLUTION OF THE ARCHAEAN HIGH-GRADE TERRAIN OF SOUTH INDIA 
M. Ramakrishnan, Geological Survey of India ,  Precambrian Geology Divis ion,  
Hyderabad. 
/ /y77,/’5 
The southern Indian sh ie ld  (Fig.  1) c o n s i s t s  o f  three major t e c t o n i c  
provinces viz. ,  (1) Dharwar Craton, (2) Eastern Ghat  Mobile Belt and (3) 
Pandyan Mobile Belt. An understanding of  their  mutual r e l a t i o n s  is  c ruc ia l  
f o r  formulating c r u s t a l  evolut ion models. 
Dharwar Craton is  d i v i s i b l e  i n to  Western and Eastern Blocks separated by 
the linear Closepet Granite (1). The supracrus ta l  b e l t s  of  the Western Block 
are comparable t o  the Early Proterozoic  ’geosynclines’ of Canada and Aus t r a l i a  
and those of the Eas te r  Block are typ ica l  la te  greenstone b e l t s .  
N-S t rending  supracrustals are co-eval (2600 Ma) and their  d i f f e rences  a r e  due 
t o  minor r e a c t i v a t i o n  of 3000-4000 Ma o ld  basement i n  the Western Block, i n  
contrast t o  the extensive juvenile plutonism and l a r g e  s c a l e  c r u s t a l  
remobil izat ion of the Eastern Block, r e s u l t i n g  from anomalous heat  flow from 
mantle. 
s i a l i c  crust w i t h  proto-ocean opening and i t s  p a r t i a l  c losu re  due t o  regional  
compression ( 2 ) .  S t i l l  o l d e r  sup rac rus t a l s  (Sargur Group) a r e  found i n  the 
g n e i s s i c  basement a s  small enclaves (3) and their  o r i g i n  i s  obscure. 
Both types of 
The favoured model for their evolut ion is sagging and r i f t i n g  o f  
Orthogonal t o  the t rend  of the supracrus ta l  b e l t s  is the E-W t rending  
charnockite b e l t  extending from Madras t o  Mangalore (4) .  As the supracrus ta l  
b e l t s  approach this b e l t  they become narrower, more highly metamorphosed and 
migmatised. Tra ins  o f  supracrus ta l  enclaves cut throught the  charnocki te  b e l t  
and af ter  passing through a series of small dex t r a l  shear zones (Kabini, 
Gundlupet, Moyar, Bhavani) are terminated by the major Palghat-Cauvery shear  
zone (5 ) .  Curving i n t o  this  main shear zone are the numerous no r the r ly  v e r t i c a l  
f a u l t  zones (Chitradurga,  Bababudan). The f a u l t s  a r e  developed 
contemporaneously w i t h  the fo ld ing  of  Dharwar sup rac rus t a l s  and a r e  fomed a s  a 
consequence of subhor.izonta1 shortening and basement up1 i f t  t o  the e a s t  (6) .  
The Palghat-Cauvery shear  zone is  marked by f iss i le  gneisses  containing r o o t s  of 
supracrustal b e l t s  and dismembered 1 ayered bas i c  complexes. The high grade 
terrain occurr ing t o  the north of this shear zone represents deeper c r u s t a l  
l e v e l s  of the Dharwar craton (7) brought up due t o  nor ther ly  t i l t  of the 
Peninsular s h i e l d  during Himalayan c o l l i s i o n .  
Pandayan Mobile Belt: This terrain which l ies  t o  the south of the Cauvery 
shear  zone is  d i s c i n c t l y  d i f f e r e n t  from the Dharwar Craton and is  d i v i s i b l e  i n t o  
two zones, the northern and southern. The northern zone consists e s s e n t i a l l y  of  
the or thoquartzi te-carbonate-pel  i t e  suite (with minor bas ics )  w i t h i n  a 
migmatit ic and charnocki t ic  terrain. 
p a t t e r n s  1 i ke the c e n t r a l  Limpopo o r  Greenland. These swir l ing  structures a r e  
probably r e l a t e d  t o  movements on the Cauvery dex t r a l  shear  i n  the north and 
Achankovil sinistral shear i n  the south. The southern zone is a l i nea r  b e l t  o f  
khondal ite-leptynite-charnockite, which  is  an extension of  the South-West Group 
of S r i  Lanka and Androyan Group of Malagasy. Contrary t o  the p i c t u r e  painted by 
Drury e t  a1 . (5 ) ,  the Achankovil shear does n o t  t runca te  d iscordant  s t r u c t u r a l  
t r ends  from the north. The comparison of this b e l t  w i t h  the Eastern Ghat  b e l t  
i s  not v a l i d  due t o  the absence of manganese-marble a s soc ia t ion ,  abundance o f  
quar tz  arenites and unfavourable structural t rends .  The l ack  of  worthwhile 
geochronological information Jeaves us i n  doubt whether this forms p a r t  of an 
I t  has curving and swir l ing  s t r u c t u r a l  
https://ntrs.nasa.gov/search.jsp?R=19890012871 2020-03-20T02:30:09+00:00Z
Tectonic Evolution of the Archaean High-Grade Terrain 
Ramakrishnan, M. 
145 
o lde r  gneiss ic  t e r r a i n  o r  a gounger (Proterozoic?) mobile b e l t .  
d is locat ion,  bu t  i n  p o i n t  o f  d e t a i l  t h i s  i s  a zone of highly d u c t i l e  s t ructures 
w i t h  both the t e r r a i n s  i n t e r a c t i n g  i n  a d i f f u s e  mobile zone. There i s  no 
evidence i n  t h i s  zone f o r  the c o l l i s i o n a l  suture v i sua l i zed  by Drury e t  a l .  (5). 
The contact  o f  the Dharwar craton and t h i s  b e l t  i s  a zone o f  t ranscurrent  
Eastern Ghat Mobile &: This i s  a long mobile b e l t  f r i n g i n g  the  Singhbhum 
and Central Ind ian cratons and extending t o  the north-east o f  Dharwar craton. 
It i s  predominantly composed o f  khondalites, charnockites, l e p t y n i t e s  and minor 
amounts o f  manganiferous marbles and quar tz i tes.  This b e l t  i s  cu t  o f f  a t  the 
cont inenta l  margin near Ongole, where i t  extends i n t o  Napier Complex o f  
Antarc t ica and Highland Group of S r i  Lanka. The t h r u s t  a t  the eastern margin o f  
the Middle t o  Late Proterozoic Cuddapah basin and s i m i l a r  basins t o  the  no r th  i s  
a l a t e  event i n  the polymetamorphic evolut ion o f  t h i s  b e l t  and i s  not  l i n k e d  t o  
the main movement o f  Palghat-Cauvery shear zone as suggested by Drury e t  a l .  
(5). The eastern Ghat b e l t  appears t o  be a product of cont inent-cont inent 
c o l l i s i o n .  
References 
1. Swami Nath J. and Ramakrishnan M. (1981) Geol. Surv. India,  Mem 112, p. 350. 
2. Ramakrishnan M. (1987) Indian Mineralogist  27, p. 1-9. 
3. Ramakrishnan M., Viswanatha M.N. and Swami Nath J. (1976) Jour. Geol. SOC. 
I n d i a  17, p. 97-111. 
4 .  Fermor L.L. (1936) Geol. Surv. I n d i a  Mem. 70, p. 218. 
5. Drury S.A., H a r r i s  N.B.W., H o l t  R.W., Reeves-Smith 6.3. and Wightman R.T. 
(1984) Jour. Geology 92, p. 3-20. 
6. Chadwick B. ,  Ramakrishnan M. and Viswanatha M.N. (1985) Jour. Geol. SOC. 
I n d i a  26, p. 769-821. 
7. Shackleton R.M. (1976) In: B.F. Windley (Ed.) Ear ly  H i s t o r y  o f  the Earth, 
Wiley, London, p. 317-321. 
Tectonic Evolution of the Archaean High-Grade Terrain 
Ramakrishnan, M. 
146 
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Tectonic evolution of the Archaean high-grade terrain of South India

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