South Indian Shield Geotransect -An Introduction

The South Indian Shield represents one of the shield areas of the world that has preserved an early-formed crust (prior to 2500 Ma). Major events of plutonisrn, volcanism and sedimentation, and several periods of deformation and metamorphism' have been identified in this region. Many significant

problems related to the unearthing of deeper structure of the Indian Shield, such as outlining of the stages in crustal growth, the nature of the sub-crustal lithospheric mantle and its role in crustal evolution etc. still need to be understood. It was felt during the early nineteen nineties that there is a need for concretizing and then launching an integrated multidisciplinary programme on South Indian Shield to resolve some of the critical issues regarding its evolution.

The Department of Science and Technology in collaboration with Geological Society of India organized a Group Discussion on 26-27th July, 1991 under Deep Continental Studies Programme with a view to identify a N-S Transect in the Southern Peninsular Region. It was felt advisable to collect all the data that was available - geological, geophysical and structural - so that this dataset becomes readily accessible to all those en,-,aged in this study. Accordingly, the Geological Society of India brought out a special volume on "Continental Crust of South India" (Mem. 25, Geol. Soc, India, 1993, Ed. B.P. Radhakrishna). Radhakrishna (1993) summarized in the following words, the various scientific problems/issues that need to be resolved with new evidences and data sets, and by launching multi-disciplinary and multi- parametric studies.

"The nature of seismic reflections in the lower crust, the demarcation of crustal blocks through identification of shears, mytonite zones and regions of high fluid pressure; the outlining of the stages in crustal growth; the tracing of .geochemical trends in crustal evolution; the identification of source of precious and base metal concentration in the lower crustal the dating of major crustal events through geochronotogical studies - these are some of the items on which much new evidence has still to be gathered. This is possible only through multi-disciplinary and multi-institutional interactions, involving geologists, geophysicists and geochemists. Seismic profiling, palaeomagnetism and rock magnetism, acromagnetic surveys, gravity mapping, electromagnetic studies, geochemistry, geochronology and fluid inclusion studies, have a major role to play in this quest for new knowledge."

Based on the debate in the Group Discussion, a N-S Transect in the Southern Peninsular India from Raichur in the north to Kanyaktimarl in the south was identified for detailed geological, geophysical, geochemical and geochronological studies.

Considering the expertise and infrastructure available with NGRI, it was desired that its expert scientists prepare a detailed proposal in this regard. NGRI scientists prepared and submitted a proposal, and the Programme Advisory and Monitoring Committee (PAMC-D-CS), after due deliberation and discussions, recommended that a Sub-Committee under the chairmanship of Prof. D. Mukhopadhyay may interact with the NGRI scientists and work out the focussed objectives, details of profiles, operational sequences, experimental parameters etc. Accordingly, a Sub-Comn-@ttee comprising Prof. D. Mukhopadhyay, Prof. B. Mahabaleshwar, Prof. I.B. Ramaprasada Rao, Dr. U. Raval, Prof. T.M. Mahadevan and Dr. K.R. Gupta met at NGRI, Hyderabad on July 5-6, 1996. NGRI scientists made presentations on different geological and geophysical aspects of the southern Indian geotransect. This was followed by observations by the members of the Sub-,Committee and in-depth discussions. As a consequence of these discussions, the following components of the transect programme were identified with the objective of addressing some of the fundamental aspects of the Southern Granulite Terrain (SGT):

• Nature and kinematics of the Moyar-Bhavani, Paighat-Cauvery and Achankovil shear zones.

• Deep crustal structure below the Southern Granulite Terrain using geophysical tools.

• Tectonic scenario of the assembly of the Southern Granulite Terrain. • Age and tectonic setting of granulite
metamorphism.

• Geochronology and dating of tectono-thermal events.

• Synthesis of geophysical and geochemical data to erect an evolutionary model for this terrain.

The Sub-Committee took into account the N-S transect identified during the Group Discussion held in 1991 and the background given in the Geological Society of India Memoir 25 (1993). However, takin- various factors into account, e.g. feasibility, scientific component and limitations of logistics and funds, it was recommended that an integrated geophysical and geological programme be launched by NGRI across the boundary between the Dharwar Craton and the Southern Granulite Terrain. This will include the transition zone, and the Moyar-Bhavani and Palghat-Cauvery shear zones. Accordingly, it was agreed that about 300 kilometre long transect from Kupparn to Palani may be covered for detailed ecological and geophysical studies by NGRI. It was further resolved that the study would have the following components:

* Controlled source coincident reflection/refraction seismic studies

* Shear wave tomography

* MT Studies

* Deep electrical soundings and CSAMT

* Gravity and magnetic studies

* Interpretation/re-look into the aeromagnetic data

* Geochemical and geochronological studies

* Complementary projects: It was felt necessary to supplement these efforts through complernentary
projects based on studies of structure, petrology role of fluids in metamorphism, mantle xenoliths, fission
track dating and palaeomagnetism, as well as geothermal and other geophysical studies.
In view of the foregoing recommendations, an integrated project "Modelling the Tectonic Evolution of Southern Granulite Belt of the Indian Shield using coincident seismic reflection/refraction, geological/ geochemical, geochronotogical, gravity/ magnetic, rnagneto-telluric and deep resistively studies along the southern geotransect" was sanctioned in March 1998. In addition to the main project, many complementary projects on various facets of geological studies were also sanctioned.

A multidisciplinary and multi-institutional research programme across the greenstone-granite and granulite-gneiss domains of the South Indian Shield was concretized during 1997. Two major streams of investigation have been planned that are mutually complementary and even overlapping. One stream was largely geological that includes projects from several thematic experts drawn from the universities of Mysore, Bangalore and Banaras, and the Centre for Earth Science Studies, Trivandrum. The other stream includes projects sponsored to the scientists of the NGRI, Hyderabad addressing the specialised geophysical and ccochernical aspects of the deep continental structure below the transect corridor. This transect passes through most of the important geological units and tectonic elements of the region. Starting from the granitic gneisses of the Archaean greenstone granite terrain of the Eastern Dharwar Craton near Kupparn, it traverses through the transition zone into the high- grade charnockite terrain, the Moyar-Bhavani (M-B) shear zone, Sathya- man-alarn supracrustals, charnockites, carbonatite and alkaline complexes, the Pal-hat-Cauvery (P-C) shear zone and ending in the migrnatite gneisses at Palani.

A Data Integration Meet was, organized at Mysore University on 3rd November 2001 and project completion document submitted by NGRI tech. Rept. NGRI-2001 -EXP-317) to Department of Science and Technology. The large volume of geo scientific data that has been acquired as a combined effort of' noted geoscientists of universities and reputed institutions, with liberal support of all kinds from the Department of Science and Technology, is being, presented in a quintessential form of a set of 17 papers including three overviews.

Among the inferences that can be drawn from a reading of these papers is that SGT is an ensemble of fragmented and imbricated crustal blocks. Its suture with the Dharwar craton is along a high angle thrust represented by the Moyar- Bhavani-Mettur shear. The other shears south of it show back thrusts. The effect of these thrusts in the zone of shears, combined with a transpressional tectonics associated with Moho up-warp, has caused crustal uplift developing into a flower structure.

South of the zone of shears, SGT appears to represent a fragment of very low angle suture-zone between two crustal blocks, one riding over the other. In this manner, the over-riding block got gradually elevated due to isostasy, and its top position was getting exhumed synchronously with the under-thrusting of the siibducting block. In this manner, over a period of over 2500 million years, the over-riding block has progressively got stripped down to the level of its- middle or lower crustal rocks. The contact between the over-riding and under-thrusting crustal blocks can easily be seen in the deep magneto-telluric and seismic velocity structures along the SGT geotraverse. The two blocks are separated by a zone of electrically conductive and seisrnically low P-wave velocity. A simple calculation from the seismic velocity and density profiles derived from the presented data set gives the density of the low velocity zone as 1.328 Kg/L, which is indicative of water-saturated rock. It appears that water squeezed out under pressure from the shear between the two blocks got lodged in the less stressed shear zone, saturating the mylonized or imbricated rock matrix. One may visualize a similar state of affairs in the southern rim of Tibet in perhaps 2000 million years hence, which will also show up as a granulite terrain!

The efforts put in by the learned scientists of various universities and institutions, the financial support and encouragement given by the Department of Science and Technology, Government of India are deeply acknowledged. Thanks and appreciations are also due to the Geolocical Society of India and to the Editor of the present Memoir in bringing out the same In its present form.

K.R.GUPTA and D.N. AVASTHI