Editor's Introduction

Introduction

Organised research during the last decade has generated new insights into the deep structure of the Indian continental lithosphere, largely through a multi-institutional participation and synthesis Of Multi-disciplinary data. The selected papers in this volume provide a window on to the large benefits that are accruing through these researches and the future directions in which further work may be organized. The papers presented here address several frontier ,areas of research and some of the specific episodes in the long history of the evolution of the Indian continent. They have been grouped under three categories that will reflect the relative emphasis that is being laid on researches on ft Deep Continental Studies. Six papers present data from the recently completed transacts and related studies; nine papers relate to regional studies using both geological and geophysical methods and also a few reviews of past achievements and four papers discuss different aspects of the thermal structure. Included in this section are also two papers on numerical modeling, a combination of convenience.

Transects and Related Studies

The nineteen nineties may be called the decade of transacts, when several transacts were selected and covered by multidisciplinary approaches following the prescriptions of the International Commission on Lithosphere. Several new transacts were completed and include the Nagaur-Jhalawar (N-J) transect across the Aravalli- Delhi Fold belt of Rajasthan; Miingwani-Seoni- Katangi-Kalimati (MSKK) transect across Central Indian Shear Zone (CISZ) and the Kuppam-Palani (K-P) transect cross the southern India high-grade domain. An ongoing project translating the transect concept is the HIMPROBE project that combines several scientific traverses across the NW Himalaya, covering notably the area between the Tso Morati Crystallines and the Karakorum Mountains.

The approach to these transacts have varied from terrain to terrain especially with regard to the choice of methodology. While the K-P transect combines coincident reflection-refraction experiments with almost all available geophysical methodology, reflection profiling has dominated over refraction experiments in the other two profiles due largely to logistic reasons. In the HIMPROBE project the terrain difficulties render seismic profiling difficult and expensive. Extensive geological and magneto-telluric and other potential under processing. Despite these limitations, interesting models of evolution have emerged from each of the transect programmes that can stimulate further enquiry into the deep continental structure and continental evolution.

HIMPROBE Project

The HIMPROBE project started in 1998 is still in a preliminary stage of data collection and analysis. Interpreting the available data, Jain et al. present tectonic evolutionary model, wherein the open oceans in the region of the Karakoram Ranges had a subduction-related island arc volcanic belt during the period - 140-120 Ma. The ocean closed sequentially along the Karakoram shear zone (KSZ) and the Shyok Shear Zone (SSZ).Me KSZ is being proposed for the first time to define a distinct dextral transpressional shear zone that juxtaposes the lithologies of the SSZ against the frontal edge of the Eurasian plate. Subsequent to the closure of the Tethyan Ocean by - 55 Ma, intra- continental shortening of the mid- to upper crust imposed a distinctive coesite- bearing ultra-high-pressure (UHP) mineralogy on the eclogites and related petro-assemblages in the Tso Morari crystallines belt, suggestive of continental subduction to maximum depths of 60-100 km. The Tso-Morari complex was possibly exhumed between 45-30 Ma. The orogenic impact seems to have shifted thereafter to the Palaeo-Proterozoic Himalayan Metamorphic belt (HMB) and its Tethyan cover leading to active over thrusting along the Main Central Thrust and the Main Boundary Thrust, spanning the period of 45 to 20 Ma. The thrust- rnovements have continued into the forelands basin along the MBT and the Indogangetic Plains thereafter. While continental collision tectonics has coalesced the Himalaya and the Karakoram, underthrusting of the Indian Plate still continues along the Main Himalayan Thrust, contributing to seismogenesis.

MT studies have revealed a low-dipping 70 km wide conductive zone beneath the Trans-Himalaya and the Karakoram at a shallow depth of 15 km. This is believed to be a zone of partial melts.

Nagaur - Jhalawar (N-J) Transect

Tewari and Vijay Rao, present the results of seismic reflection prof-lbng along the Nagaur-Jhalawar (N-J) transect across the Aravalli- Delhi Fold belt Rajasthan. The profile offers Only limited velocity data but a gross velocity model has been still possible by dynamic forward modeling using ray method and by synthesis with gravity data. The crustal structure below the transect is complex. A thickened (48 km) crust is dissected by deep penetrating faults and ffimsts, some extending down to Moho depths. The reflective pattern shows diversity correlatable with the different tectonics settings. Changes in the character of reflectivity demarcate tectonic boundaries. A prominent feature is a dome structure above the Moho under the relatively younger Delhi fold belt and the Sandmata granulite complex fringing it. The dome structure defines a zone of 7.3 knvsec P-wave velocity. Synthesising the seismic data with gravity and electrical conductivity data, the authors present an evolutionary sequence centering around a plate tectonic regime in the Palaeo- and Meso-proterozoic involving rifting opening and closing of oceans between the Bundelkhand block on the east and the Mewar block on the west.

Mungwani-Seoni-Katangi-Kalimati (MSKK) Transect

Reddy and Rao present a review of the coincident reflection-refraction experiments carried out along the NW-SE trending Mungwani-Seoni-Katangi- Kalimati (MS KK) profile across Central Indian Shear Zone (CISZ) and integrate the results with 5 DSS profiles prepared across the Narmada-Son Lineament earlier. 7he MSKK profile reveals a distinct change in the reflectivity of the crust across the CISZ and a convergence of dips of the reflectors on either side. The line of convergence nearly coincides with the CISZ along which the Amgaon Gneisses in the south are thrust up against the Sausar-Tirodi gneisses with some granulite bands (charnockites). A 5-layer velocity structure is proposed along the transect within a thickened crust, 44-46 km thick, which includes two low velocity layers, Reddy and Rao propose a collision model to explain the convergence. Synthesizing the MSKK profile data with the data from earlier DSS profiles across the Narmada-Son Lineament, it is suggested that several of the features may be viewed as resulting from the impact of the Reunion hot spot on the western margins of the Indian shield.

South Indian Transect

The more significant results accruing from this project have already been published. (Ramakri shnan, M. (Ed.), Memoir 50, Geol. Soc. India., 2003). A review of these and related results and their bearing on the geological evolution of the Southern Indian shield in the Archaean through the Proterozoic is presented by Mahadevan. The high grade- domain of the Peninsular Shield has many major unique features that include a four-layered velocity-density structure above the Moho with a rnid- to lower crustal low velocity layer with distinctive electrical conductivity and magnetic attributes; a distinctive pattern of both dipping and in places domical high reflectivity and a distinctive thermal structure. It is suggested that the characteristic reflective crust and the low velocity layer may be features developed during the differential exhumation of the lower crust through a dominantly isothermal decompression path and the several punctuated episodes of ultrabasic, basic, alkaline and granitic magmatism in the span of-2500 to 550 Ma. The deep crustal structure deciphered is consistent with a geological history of extensive fluid-mobilisation and numerous mantle-related magmatic episodes of the Proterozoic. Mahadevan links the diachronous evolution of the Archaean Dharwar craton and the Archaean-Proterozoic HGD to the coffesponding diachronous change in both the thermal structure and chemistry of the mantle from the Archaean to the Proterozoic.

Teleseismic Receiver Function

Studies on teleseisniie receiver funcfion carried out in India since recently constitute a new dimension to deep continental Studies. Prakasam and Rai outline results of such studies in modeling the crustal structure below the Eastern Dharwar Craton.(EDC) based on recordings in a network of broadband stations in the EDC and the fringing Deccan Volcanic Province.Mey model a relatively homogeneous crust across the EDC and the DVP with a sharp Moho at variable depths. The crust is 31-36 km thick and the Poisson's ratio ranges 0.24-0.27. The upper crust is up to 12 km thick and the Lower Crust isfelsic to intermediate in composition.

Electrical Conductivity Traverses

The last decade witnessed new developments in the methodology of electromagnetic investigations and its extensive application. A significant finding has been the presence of n-iid-crustal conductors in the dominantly Precambrian Peninsular shield. While the causative factors of the conductivity may be debated, correlation with low velocity zones bestows on these zones both a tectonic (Theological) and petrological significance. Possibilities of fluid enriched zones within an otherwise brittle crust lend to these discov -eries of seismogenic significance. In his contribuion Gokarn of TIG compares the electrical structure below a transect across the south Indian Dharwar craion; across the central Indian region and the NW Himalaya. Interestingly, in all the three regions of distinctive but younging temporal evolution, a refractory crust is underlain by a more conductive lower crust. While no electrical Moho is evident in any of the profiles, presumably because of a conductive lower crust, these findings point to a fluid-enriched lower crust in all the three regions transacted.

Exhumation of Deep Domains

A major characteristic of the Indian shield are the extensive dominantly Proterozoic domains of high-grade rocks that seem to encircle a central core of Archaean-early Proterozoic greenstone-granite belts. Researches during the last two decades have addressed the question of the styles of exhumation of these bigh-grade domains and a prominent approach has been to trace their P-T-t paths of evolution through mainly petro-mineralogy. In the first paper Anand Mohan traces the evolutionary history in terms of the P-T-t paths of the high- grade rocks of south India using refined calibrations of mineral equilibria and thermodynamic bases. It is inferred that near thermal peak of granulite facies metamorphism at >900"C at 9 kb pressure were followed by isothermal decompressional path These results support several earlier findings and provide a sharper definition to the style and manner of exhumation of the high grade domain of southern India

Phanerozoic Rifting and Voicanism

The Rajamahal and Deccan Volcanic episodes have been the two great events during the Phanerozoic that have had a major impact on the deep continental structure of the Indian shield. Ghose and Ray Kent present a review of the field, petrochemical and geochronological aspects of the Rajmahal flows in the type area of Rajmahal Volcanism. These flows are largely tholeiites grading rather locally into basaltic andesite and more acid fields. 'Mey are chemically distinct from those of Deccan Basalts, implying a distinct mantle source. They are divided into two chemical groups, the early phases of eruption being enriched in transition elements (Fe, Ni, Cr, V and Sc), TiO. and CaO and Ti/Zr and e Nd(t) and relatively depleted in incompatible elements. The second types have strong differentiation patterns with negative Nb and Ta anomalies, and positive Rb, Ba, K, Th, Zr and Hf. REE chemistry reveals lesser fractionation in the former. The authors infer that the two groups have evolved through variable but low degrees of cnistal contamination of mantle -derived magmas followed by fractional crystallisation ol'the contaminated iiiagilia ill the upper crust. Hf and Ar/Ar isotope systematics date the Rajmahal events - 1 15 Ma. The authors infer that the Kerguelen hotspot was close to the Eastern Indian margin around this time. The data that have accrued through these researches have the potential to model the chemical evolution of the mantle source areas through time.

The Deccan voicanism is One of the most intensively investigated volcanic suites. Chandrasekharam presents the status of our present knowledge on the Deccan continental basaits and identifies the gaps in our understanding. The paper focuses on the diversity of views with regard to the extent, age and duration of the Deccan volcanism, the chemical stratigraphy, crustal contamination, the tectonic and plume settings associated with the vocalism and eruptive models. Deccan Volcanism being one of the largest of continental basalt eruptions in the world, a resolution of several divergent views on almost every aspect of this great phenomenon is fundamental to get a clear insight into the evolution the earths's crust in the latter part of the Phanerozoic.

Mantle Xenoliths

Xenoliths of both 10wez'crustal and mantle rocks entrained in magmas generated in the mantle by partial melting are, the only real messengers of the source areas Of these magmas and have the potential to throw light on the composition of their source depths. Integration of their petrology and chemistry with several geophysical signals, such as gravity., seismic velocity structure, electrical conductivity and so on can greatly constrain the models of the deep continental structure. A ]inciting factor is that the incidence of mantle xenoliths in mantle-derived magmatic intrusives in India seems to be low and confined to a few areas. Nevertheless a beginning has been made in integrated studies on the known xenoliths. From among these are a suite of xenoliths of pyroxenites, websterites, rare wehrlite and pyroxene granulites occurring in alkaline dyke swarms of Janjira- Munid areas south of Mumbai and the spinel Iherzolite xenoliths occurring in the early alkaline basaits of Kutch. These are related respectively to the late and early phases of Deccan voicailism of late Cretaceous-Palaeocene ages. Dessai in Ns contribution discusses the inferred P-'T' conditions of the source mantle/lower crust sites of the xenoliths. A synthesis with the known seismic profiles across these regions confirms a 3-layer velocity model of the crust beneath the craton. He further concludes that the exhumed lower crust beneath the attenuated margin of the continent is largely dominated by mafic granulites interstratified with pyroxenites and websterites. A transitional Moho is suspected in these regions of peri-continental late Mesozoic rifting.

Interpretation of Magnetic Fields

The largevolulyle of the magnetic data thlt flow from satellite and aerial platforms and ground surveys provides unprecedented opportunities for applying magnetic methods to earth science research in general and deep continental studies in particular. The data from these three sources are characterized by a range of wavelengths and have distinctiveness in application to modeling source depths contributing to the respective fields measured. Addressing these sources Ntta Rajaram reviews some of the outstanding achievements in interpreting magnetic data in relation to the Indian subcontinent. A notable development is the use of the analytical signal map, in wtuch the maxima mark the edges of the magnetic bodies, thereby bringing out the presence of larger magnetic sources and filtered maps in which one can remove, minimize or enhance relevant source components and source depths. Comparison of aeromagnetic data with digitized gravity data, using high-pass filtering techniques helps to interpret distinctive source depths. Estimating the ,average spectral depths from power spectra plots of the filtered aeromagnetic anomaly for wavelengths of <80km, I 00 Ian, 180km and 2OOkm, it is established that the Dharwar cratonic region of largely Archaean age has distinctive magnetic fields compared to the high-grade domain. Significantly the high- grade domain has a thin magnetic crust, the crust below - 22 km not contributing to the magnetic anomalies. This may be due to any of two reasons (i) compositional changes below 22 kms or alternatively due to the magnetization having been lost due to Curie isotherm. The findings are consistent with-a low velocity layer below - 24 km in the velocity structure of the region, generated through a coincident reflection-refraction experiment recently completed along the Kuppam- Palrii transect.

Palaeomagnetic Insights

From its early application to palaeocontinental reconstruction, palaeomagnatic research has expanded into several applied areas such as providing temporal constraints on stratigraphy, sedimentation and evolution of basins, dating magmatism and dating palaeo-environmental changes. Radhakrishna presents a review of the status of the pal aeomagnetic researches in India since its inception. The available palaeomagnetic data are evaluated by basic reliability criteria. Several new directions for future research are suggested.

Constraints on Hinialayan Seisniicity

One of the major objectives of the Deep Continental Studies programme is to constrain seismicity through well-defined deep structure and reliable velocity models. This is a challenging task but some insights haN7e been gained on crustal constraints on stress accumulations through teleseismic studies, micro-earhquake (MF-Q) studies, monitoring after-shocks; and electrical conductivity studies. In his contribution, Kayal outlines some of the insights gained regarding the seismotectonics in the Himalaya on the basis of micro- earthquake studies. The seismotectonic models generated differ from west to east Himalaya and the NE Himalaya. In the NE Himalaya earthquakes are generated much below the plane of detachment of the currently accepted steady state model and are caused by the influence of the Himalayan collision tectonics in the north and the Bunnese arc subduction in the east. The earthquakes in the Shillong Plateau are generated by locally active reverse faults. In the east Himalaya, in Sikkim and the Darjeeling, seismic activity is generated at depths of 0 to 50 km below the trace of the MBT. In the North Himalaya (Fhmachal Pradesh) hypocenters are confined above the plane of detachment, as exemplified by the recent 1991 Uttarkashi earthquake of 6.6 Mb and the 1999 Chamoli earthquake of K6.3). 'Me MCT is not active in any of the arm. Shocks are generated by thrust faulting in the Basement Thrust Front. 'Me tectonic models, therefore, are distinctive in each of the tectonic provinces.

Indian lithosphere : Some Issues related to Evolution

Avasthi discusses several problems related to the evolution of the Indian lithosphere, with special reference to the litho-tectonic boundaries within the Indian plate,-the Himalayan front in the north and the oceanic front in the eastern, western and southern boundaries. Within the continental segment of specific importance is the deciphering of the characteristic relationship between the seismic velocity profile, density profile and the thermal conductivity profile. A major issue to be clarified in the off-shore and oceanic parts is the constitution of several of the oceanic ridges, whether they are continental or oceanic in character. these emphasize the need to extend deep continental studies into the Greater Indian plate as a whole and not merely to the continental land mass.

THERMAL STRUCTURE AND NUMERICAL MODELING

Thermal Structure

Systematic heat now studies in India date back to the past five decades, but have been based on logging of boreholes drilled for mineral exploration and groundwater. For the first time heat flow estimates were refined by using the down-the-hole-hammer (DT14) non-coring holes to depths of 300-600ni in some 12 select spots in the Southern Indian granulite terrain. The sites were selected on the basis of homogeneity of formations, low depths of weathering and low groundwater potential so that a more or less stabilised heat flow can be measured. Heat generation measurements, hither-to-fore, were based on radio-nucleiide estimates in the laboratory on single specimens. But these were carried out in situ on carefully selected flat outcrops in the field using a 4 channel gamma-ray spectrometer. The method permits enlarging sample size to some 40 kg in contrast to less < 1 kg in laboratory samples.

Rao et al. review the overall data that have accrued on the geothermal gradients in different crustal segments of the Peninsular Shield through measurements in some 183 boreholes and two deep mines. The heat flow characteristics in the dominantly Archaean Dharwar and Bastar cratonic regions as,,also the Proterozoic Aravalli-Delhi fold belt in NW part of the Shield and the Singhbhum Shear Zone in Eastern India show a wide but overlapping range of 25 to 62 mWm-1; the coldest is the western Dharwar craton. The Mesozoic Gondwana basins and the late Mesozoic-Cenozoic Cambay rift zones have overlapping heat flow ranges of 46-107 m W m-2.

Estimates of crustal heat generation viewed against the total heat flow lead to a robust estimate of 12 to 19 mWm-2 in the Dharwar province but 25-30 mWm-2 in the northern part of the Southern Granulite Province. This is consistent with an enriched mantle that may exist below the Southern Granulite terrain, inferred through researches in geochemistry of some of the plutonites in the region.

Heat flow measurements of 43 mWm-2 in a bore hole drilled at the site of the 6.2 M Killari- 1993 earthquake point to a brittle crust up to some 30 km depth. The centroid depth of the Killari event estimated at 2.6 km tilts the causative factor to increase of fluid pore pressure over hydrostatic pressure.

Electrical Conductivity Studies across Northern Indian Ocean

A new. significant trend in recent researches is the extension of electrical conductivity studies to constrain models of the plume lithosphere interactions in the Indian Ocean. Arora et al. present results of recent researches in this direction. Transient geomagnetic field variations recorded at a number of sites distributed across the Peninsular Shield and in the Arabian Sea, interpreted through a thin sheet conductance model, lends definition to the source region of high conductivity in the offshore region SW of the southern tip of the Indian Peninsula. The anomaly is named the South India Offshore Conductive anomaly" (SIOCA). This anomaly coincides with the low velocity zone and low magnetisation anomaly centered near the southern tip of the Indian shield. Arora et al. interpret these anomalies as the manifestation of the impact of the Marion Plume on the Indian Shield. The poorly constrained weak induction features below the 850 E Ridge may point to localized magmatic intrusions and the relatively pronounced induction anomalies at stations flanking the Ridge may be due to sources of sediment filled troughs, on either side of the Ridge.

Fluid Dynamics of the Mantle

An important dimension of deep continental study is mathematical modeling taking into consideration several boundary conditions. Manglik outlines the basic principles of mathematical modeling of fluid dynamics of the mantle, taking into consideration current concepts of the modes of mantle convection influenced by the presence of subducting slabs, lithosphere-mantle interaction with particular reference to decompression melting of the mantle, the effects of continents on the mantle flow pattern and plume- lithosphere interaction in a rifted lithosphere and below lithospheric roots He then proceeds to address questions regarding core-mantle interaction.

Scaling Spectral Analysis

Several physical properties of rocks such as density, magnetic susceptibility and reflectivity follow a scaling distribution. Dimri et al. demonstrate the effective application of the scaling spectral method through the interpretation of the gravity profiles along the Nagaur- Jhalawar and Jaipur- Raipur seismic transacts The results of scaling are in good agreement with the observed features in the seismic profiles. The efficacy of the method is proposed to be verified using gravity and magnetic measurements in the vicinity of a bore well of the ONGC in the Jabera-Damoh area in the Vindhyan Basin.

Mantle Plumes and Continental Evolution

Interaction between mantle plumes and continental lithosphere is recognized as a major paradigm that can explain many lithospheric features imbibed from Archaean times through the Phanerozoic. i'lava't discusses the impact of mantle plumes on the Indian plate since the Cretaceous and proposes that plume and plate tectonics have played complimentary roles, but of one predominating over the other through time and space. He emphasizes the larger influence of plumes along the earlier mobile belts generated by plate tectonics as against regimes that have been already cratonised.

Future Thrust

Each of the papers in the volume provides suggestions for future researches in the specific areas addressed that could be incorporated in the frame work of a national programme of research on the deep structure of the Indian continental lithosphere. The broad approach in this regard should be to integrate the large data-base that has been created with the even larger data- base already available, especially with regard to the potential fields, including the satellite and aeromagnetic data and the data from the numerous controlled seismology experiments, teleseisniic and electrical conductivity studies. The new data presented in and exemplified by the papers in this volume have a great potential for further processing and refinement and even re-interpretations in the light of new models that may be proposed for continental evolution in the future.

A significant new source of data is the network of seismic stations that have been now established with state-of-art equipments. The large amount of seismic data flowing from these stations can revolutionise our efforts to gain insights into the deep continental structure of the Indian continent. A direction of seismological research that remains neglected is the shear wave velocity structure. Seismic Tomography needs to be extended on a larger scale over the continent and the adjoining off-shore extensions. The Himalaya needs special attention with regard to generating a reliable velocity structure along and across so that current models of seismicity could be verified and a more realistic data base generated that can constrain earthquake studies. Electrical conductivity studies need to be enhanced on a larger scale and preparation of an electrical conductance map of India should be a long term objective. A related problem that needs to be addressed is the thermal structure both below, the Shield as well as the Himalaya. The dynamic nature of Himalaya calls for addressing this problem through innovative approaches.

While geochemical researches have been addressing some of the problems i-elating to lithosphere-mantle interactions and are integrated with geochronotogy, most modeling is based on assumptions of uniformitarianism. An alternative emerging approach is to relate geocheiwstrv of the crust with the chemical evolution of the mantle. This calls for carefully planned programmes on the one hand and larger facilities for especially isotope studies, as for example using Re-Os systematics. DST has already initiated steps in iffis direction and it is hoped that geochemical researches would register greater heights of progress.

This note may be concluded by recalling the three-pronged conceptual approach to deep continental studies that has been the driving force behind the whole programme of researches initiated by the DST in the past.

1 . Understanding continental evolution through the fourth dimension of "TIME".

2. Gaining insights into the geodynamics and seismic instability of regions thereby enabling better management of seismic environment and preparing for a more distant objective of seismic prediction and

3. Generation of new and more realistic concepts of the formation of mineral deposits and metallogenic provinces by addressing crust- mantle domains of geochernical interaction through time.

It is hoped that the next few decades would achieve substantial progress in these directions.

T.M. MAHADEVAN, B.R. ARORA and K.R. GUPTA