One of the important aspects of the evolution of the Shield areas is the understanding of the development and nature of the continental crust which has accreted through time, and which has undergone various tectonic and geochemical changes, having spatial and temporal variability. These aspects have a great relevance to geological, geophysical and petrochemical characteristics of each terrain. In this context, the Northwest and the Central Indian Shield segments are important for these terrains contain evidences of resurgent tectonics and geological evolution through principally the Precambrian time that led to the development of superposed fold belt systems like the Aravallis, the Delhis, the Sausars and the Sakolis. There are extensive late Precambrian paratectonic cover sequences, whose tectonic -setting and development history are not fully understood. Major' dislocation zones (e.g. Great Boundary Fault), suspected palaeo-sutures, important ductile shear zones, and significant lineaments (e.g. Narmada-Son Lineament) are indi- cations of significant crustal interactions in these terrains.

In order to provide a focussed attention and to tackle some of the problems associated with the Indian Shield, the Department of Science and Technology, Government. of India, launched in 1987 a multidisciplinary research programme entitled "Deep Continental Studies in India" starting with the studies in Jaipur - Raipur Corridor. Different leading institutions and universities have and are participating in this programme. In order to discuss the results of multidisciplinary projects and studies under the programme, and to attempt an interaction between the different disciplines a Group Discussion was held at Jaipur during March 23-24,1993 which was organised by the Geological Survey of India, Western Region, and sponsored by the Department of Science and Technology, Govt. of India.

This volume contains some selected papers presented and discussed in the above Group Discussion. These relate to geology, tectonics, geochemistry and geophysics of the selected segments of the Northwestern and Central Indian Shield. These papers have brought out the characteristics and trends of evolution of the continental crust of these segments, and have also highlighted the present status of knowledge and future course of studies required to under- stand the polycyclic development of the Precambrian continental crustal segments and their interactions.

The importance of integrated studies to understand the crustal dynamics in Precambrian terrain has been emphasized by T.M. Mahadevan who has outlined the geological and geophysical at- tributes of five crustal provinces, namely, the Aravalli-Delhi tec- tonic belt, the Bundelkhand Granite-Vindhyan Basin, the Son- Narmada-Tapi belt, the Saurashtra-Kutch area, and the Cambay- Broach-Bombay area. Although a fair amount of geological and geophysical information for each of these provinces are available and based on these data, some inferences on the crustal structures of these provinces are possible, the relationship between these provinces ire not i7et clear. Future work on establishing these relationships will go a long way in working out the evolutionary history of the Northwestern and Central Indian Shield.

U. Raval discusses the geophysical characteristics, including gravity, seismicity, electrical conductivity and heat-flow of the NW and Central Indian Shield segments in the regional context of conti- nental break-up, but the significance, as emphasised, of the possible .1/thermochemical forces" and the tectonics related to Phanerozoic distensional tectonics in the Indian sub-continent on the crustal structure of the Precambrian terrains needs critical evaluation.

A number of papers have brought out in fair detail the geo- logic, structural and tectonic aspects of the transect corridor in particular, and the NW and Central Indian Shield segments in general. Some papers have dealt with specific aspects and problems related to structure and tectonics of each segment.