crustal structure of the western part of the southern granulite terrain of indian peninsular shield derived from gravity data abstract the southern granulite terrain sgt is composed of high grade granulite domain occurring to the south of dharwar craton dc the structural units of sgt show a marked change in the structural trend from the dominant north south in dc to east west trend in sgt and primarily consist of different crustal blocks divided by major shear zones the bouguer anomaly map prepared based on nearly 3900 gravity observations shows that the anomalies are predominantly negative and vary between 125 mgal and 22 mgal the trends of the anomalies follow structural grain of the terrain and exhibit considerable variations within the charnockite bodies two dimensional wavelength filtering as well as zero free air based zfb analysis of the geoid corrected bouguer anomaly map of the region is found to be very useful in preparing regional gravity anomaly map and inversion of this map gave rise to crustal thicknesses of 37 44 km in the sgt crustal density structure along four regional gravity profiles cutting across major shear zones lineaments plateaus and other important geological structures bring out the following structural information the bavali shear zone extending at least up to 10 km depth is manifested as a plane separating two contrasting upper crustal blocks on both sides and the gravity high north of it reveals the presence of a high density mass at the base of the crust below coorg the steepness of the moyar and bhavani shears on either side of nilgiri plateau indicates uplift of the plateau due to block faulting with a high density mass at the crustal base the bhavani shear zone is manifested as a steep southerly dipping plane extending to deeper levels along which alkaline and granite rocks intruded into the top crustal layer the gravity high over palghat gap is due to the upwarping of moho by 1 2 km with the presence of a high density mass at intermediate crustal levels the gravity low in periyar plateau is due to the granite emplacement mid crustal interface and the thicker crust the feeble gravity signature across the achankovil shear characterized by sharp velocity contrast indicates that the shear is not a superficial structure but a crustal scale zone of deformation reaching up to mid crustal level

evolution of the passive continental margins of india a geophysical appraisal abstract the passive continental margins of india have evolved as india broke and drifted away from east antarctica madagascar and seychelles at various geological times in this study we have attempted to collate and re examine gravity and topographic bathymetry data over india and the adjoining oceans to understand the structure and tectonic evolution of these margins including processes such as crustal lithosphere extension subsidence due to sedimentation magmatic underplating and so on the eastern continental margin of india ecmi seems to have evolved in a complex rift and shear tectonic settings in its northern and southern segments respectively and bears similarities with its conjugate in east antarctica crustal extension rates are uniform along the stretch of the ecmi in spite of the presence or absence of crustal underplated material variability in lithospheric strength and tectonic style of evolution ranging from rifting to shearing the krishna godavari basin is underlain by a strong 30 km elastic lithosphere while the cauvery basin is underlain by a thin elastic lithosphere 3 km the coupling between the ocean and continent lithosphere along the rifted segment of the ecmi is across a stretched continental crust while it is direct beneath the cauvery basin the western continental margin of india wcmi seems to have developed in an oblique rift setting with a strike slip component unlike the ecmi the wcmi is in striking contrast with its conjugate in the eastern margin of madagascar in respect of sedimentation processes and alignment of magnetic lineations and fracture zones the break up between eastern india and east antarctica seems to have been accommodated along a proterozoic mobile belt while that between western india and madagascar is along a combination of both mobile belt and cratonic blocks

crustal structure and rift architecture across the krishna godavari basin in the central eastern continental margin of india based on analysis of gravity and seismic data abstract the krishna godavari basin is a rifted passive margin basin that developed orthogonally to the nw se trending pranhita godavari graben along the central eastern continental margin of india in response to the continental rifting process and the subsequent seafloor spreading between india and eastern antarctica during the early cretaceous period the 3 d gravity interpretation of both the onshore and offshore sections of the basin integrated with detailed seismic reflection and refraction data provided significant new information about the crustal architecture and the early breakup history of the basin the gravity derived crustal models indicate that the crust at the eastern indian shield margin is 39 41 km thick and thins to as much as 20 23 km at the ocean continent transition oct in the offshore there is a significant variation in the nature of crust and the configuration along the margin between the three crustal domains encompassing the basin the cuddapah basin pranhita godavari graben and the eastern ghat mobile belt that indicates the lateral segmentation of the margin a zone of high density 3 0 g cm3 crustal material at the oct separates the pure continental and oceanic crusts on either side the moho is shallowest along this zone and brought the upper mantle notably close to the surface which indicates that it could be comprised of proto oceanic crustal rocks furthermore the present study highlights a major basement structural high in the deep offshore area of the margin the geophysical signatures over this structure suggest that it is a crustal scale feature that is characterized by intrusive volcanic rocks comprised of low density crust and lies close to the oct towards offshore based on the inferred tectonic reconstruction we believe that this structural high could be a continental fragment that was left behind during the process of breakup between india and the elan bank

structural and tectonic interpretation of geophysical data along the eastern continental margin of india with special reference to the deep water petroliferous basins abstract the study area encompasses the eastern continental margin of india ecmi and the adjoining deep water areas of bay of bengal the region has evolved through multiple phases of tectonic activity and fed by abundant supply of sediments brought by prominent river systems of the indian shield detailed analysis of total field magnetic and satellite derived gravity data along with multi channel seismic reflection sections is carried out to decipher major tectonic features basement structure and the results have been interpreted in terms of basin configuration and play types for different deep water basins along the ecmi interpretation of various image enhanced gravity and magnetic anomaly maps suggest that in general the ene wsw trending faults dominate the structural configuration at the margin these maps also exhibit a clear density transition from the region of attenuated continental crust proto oceanic crust to oceanic crust based on which the continent ocean boundary cob has been demarcated along the margin basement depths estimated from magnetic data indicate that the values range from 1 to 12 km below sea level and deepen towards the bengal fan in the north and reveal horst graben features related to rifting a comparison of basement depths derived from seismic data indicates that in general the basement trends and depths are comparable in cauvery and krishna godavari basins whereas in the mahanadi basin basement structure over the 85 e ridge is clearly revealed in seismic data further eight multichannel seismic sections across different basins of the margin presented here reveal fault pattern rift geometries and depositional trends related to canyon fills and channel levee systems and provide a basic framework for future petroleum in this under explored frontier

madagascar a continental fragment of the paleo super dharwar craton of india the morphological kinship of madagascar to its immediate neighbors on the west african continent and east indian subcontinent during the early and middle cretaceous has been debated for the past two decades on the basis of available geologic tectonic and paleomagnetic information most of the paleoreconstructions of madagascar have shown its attachment to the east african continent we present magnetic satelite and gravity data and morphological geophysical and geotectonic similarities to hypothesize that in the period before the breakup of gondwana madagascar was a continental fragment of the paleo super dharwar craton of india

crustal evolution in south india constraints from nd isotopes nd isotope analyses from a range of lithologies including charnockites metasediments and granitoids that comprise the high grade terrain of south india define contrasting age provinces north of the palghat cauvery shear zone the karnataka craton and the granulite blocks of nilgiri and madras are characterized by model nd ages from 3 4 to 2 4 ga with significant crustal growth during the late archean south of the palghat cauvery shear zone a pan african granulite facies event resulted in sm nd fractionation as observed in several incipient charnockite localities model nd age calculations that allow for sm nd fractionation at 550 ma yield ages in the range of 2 9 to 1 3 ga this range compares closely with published model nd ages from the high grade terrain of sri lanka mid proterozoic crustal growth 1 1 ga observed in the vijayan complex of sri lanka may have provided source material for metasediments characterized by young model nd ages in southern peninsular india the palghat cauvery shear z

u pb single zircon pb evaporation and sm nd isotopic study of a granulite domain in se madagascar madagascar is a poorly known precambrian domain at the eastern border of the mozambique belt strongly affected by the pan african orogeny this large island represents a key part of gondwanaland and the accurate dating of the major metamorphic event is critical for the reconstruction of that supercontinent in the southeastern zone of the island metamorphism has reached high temperature and low pressure conditions 850 c and 5 kbar with moderate retrogression a u pb conventional multigrain single zircon stepwise pb evaporation and sm nd geochronological study on meta felsic rocks has shown that basement of at least lower proterozoic age was strongly metamorphosed around 570 580 ma this tectometamorphic event nearly obliterated any pre pan african zircon isotopic memory the granulite facies metamorphism is associated with the syn metamorphic emplacement of the anosyan charnock ites and granites a strong channelized fluid contribution at 545 ma has produced the re crystallization of biotite apatite

new zircon ages for precambrian granites gneisses and granulites from central and southern madagascar significance for correlations in east gondwana institiif fiir geozuissenschnffen universitiit mainz 55099 mainz gevniany 2depnrtnienf of geology university of leicesfer leiccsfev le 1 7r h uk 3mnx plnnck lnsfitzit fiiv clzeniie postjnclz 3060 55020 mcriiiz geuniany 4depf of applzed geology ciirfin university of t c c z j i o o pevflz wa 6001 airstvaliii 5dc ppnrfeniei f des sciences nnfirreiles uiiiversitt de toliavn tolinrn mndngascav

gold occurrences in madagascar south india and sri lanka significance of a possible pan african event abstract gold occurs in various quantities mainly in the central and southern parts of sri lanka although gold is recovered mainly from river sediments at present it has been mined in the past also from a concordant quartz reef in central sri lanka and from other types of sediments such as marshy beds surrounding dykes of ultramafic composition in southern sri lanka the sedimentary gold has been interpreted as being derived from the mineralizations of the latest deformational features such as ductile and brittle shears and fractures consisting of quartz veins minor pegmatites and sulphide mineralizations mainly of pyrite pyrrhotite and chalcopyrite the mineralization of gold is exclusively in the highland complex rocks of sri lanka which represent archaean late proterozoic metamorphic complexes in madagascar gold is present in the precambrian basement rocks consisting of some greenstone belts and metasediments as a diffused mineral in the rocks concordant quartz reefs and more recent discordant quartz veins the concordant quartz veins and diffused gold are related to both archaean and proterozoic rock associations and the discordant veins which have been identified as permo triassic are found mainly in the north within the contact zone of proterozoic rocks and the phanerozoic cover gold is also recovered form ancient and recent alluvial accumulations and lateritic earth the mineralization of vein deposits is mainly gold quartz and gold sulphide associations the sulphides being galena chalcopyrite and pyrite in south india gold occurs in archaean metamorphic units comprising of granite greenstone complexes in the kolar classic gold field and in granulite terranes in more southerly occurrences such as the historic wynad fields in both cases gold occurs in gold quartz veins which occupy fissures and shears within the archaean metavolcanics gold also occurs in the attapadi area of kerala both in sediments and in primary epigenetic gold deposits which are hosted by precambrian amphibolites and granitic gneisses the attapadi lode veins are predominantly structurally controlled and are closely associated with the proterozoic bhavani shear zone gold is recovered both from the primary and placer deposits and the primary deposits contain gold quartz veins gold quartz carbonate veins and gold quartz sulphide veins the sulphide minerals are mainly galena pyrite chalcopyrite and arsenopyrite the rocks in the southern madagascar southern india and sri lanka belong to granulite terranes that have peak metamorphic imprints and tectonic development during the pan african the emplacement of gold mineralization mainly in madagascar and south india can be correlated in the archaean the gold deposition in the later deformational structures with concordant and discordant gold bearing veins can be correlated also with those in sri lanka as a pan african mineralizing event it is also evident that co2 rich fluids from a deep seated source has continued from archaean and were still active during the proterozoic being responsible for the pan african event of gold deposition particularly in the younger structures in these countries

the terrace like feature in the mid continental slope region off trivandrum and a plausible model for india madagascar juxtaposition in immediate pre drift scenario abstract bathymetry of the southwestern continental margin of india reveals the presence of an anomalous terrace like feature in the mid continental slope region off trivandrum the genesis of this terrace of large areal extent 9000 sq km is yet to be established based on exercises with several existing paleogeographic reconstruction models and updated compilation of identified offshore tectonic elements this study attempts to identify a plausible model of india madagascar juxtaposition in immediate pre drift scenario which provides idea about genesis of this terrace it is inferred that the terrace off trivandrum and an anomalous bathymetric notch located in the northern madagascar ridge are conjugate features related to india madagascar separation and the rifted and sheared segments of the pre drift plate boundary have shaped their outlines the drifting of india from madagascar is suggested to have commenced at about 86 5 ma

pink sapphire from southern kerala s india implications on india madagascar correlation within gondwana assembly abstract we report here the occurrence of pink sapphires in association with a variety of gemstones from the trivandrum granulite block south of the achankovil shear zone in southern india the mineralization is associated with pegmatites or veins emplaced within granulite facies aluminous supracrustals the sapphires show near pure a1 0 composition 98 43 99 48 wt with traces of cr o 0 02 0 12 wt and feo 0 01 0 12 wt the available radiometric age of 513 2 ma for gem quality zircon associated with pink sapphire in the melankode locality confirms that the mineralization is of late pan african age pink sapphires have been widely reported from a number of localities in southern madagascar including betroka illakaka antranondambo and ambossary sapphires of various hues also occur in the ratnapura gem district in the southwestern part of sri lanka the pink sapphire occurrences in southern madagascar and southern kerala provide strong evidence for india madagascar juxtaposition in the gondwana assembly with the ranotsara shear zone in southern madagascar extending into the achankovil shear zone in southern india

correlations between silicic volcanic rocks of the st mary s islands southwestern india and eastern madagascar implications for late cretaceous india madagascar reconstructions abstract the st mary9s islands southwestern india expose silicic volcanic and sub volcanic rocks rhyolites and granophyric dacites emplaced contemporaneously with the cretaceous igneous province of madagascar roughly 88 90 ma ago the st mary9s islands rocks have phenocrysts of plagioclase clinopyroxene orthopyroxene and opaque oxide moderate enrichment in the incompatible elements e g zr 580 720 ppm nb 43 53 ppm la yb n 6 9 7 2 relatively low initial 87 sr 86 sr 0 7052 0 7055 and near chondritic initial 143 nd 144 nd 0 51248 0 51249 they have mineral chemical whole rock chemical and isotopic compositions very close to those of rhyolites exposed between vatomandry ilaka and mananjary in eastern madagascar and are distinctly different from rhyolites from other sectors of the madagascan province we therefore postulate that the st mary9s and the vatomandry ilaka mananjary silicic rock outcrops were adjacent before the late cretaceous rifting that split madagascar from india if so they provide a valuable tool to check and aid traditional cretaceous india madagascar reconstructions based on palaeomagnetism matching precambrian geological features and geometric fitting of continental shelves supplementary material mineral analyses mass balance calculations and locality information are available at http www geolsoc org uk sup18332

the timescale and mechanism of granulite formation at kurunegala sri lanka incipient charnockite formation at kurunegala in sri lanka is characterized by the growth of orthopyroxene at the expense of amphibole and biotite in an originally homogeneous gneiss mineral equilibria in the charnockite assemblage record pressure temperature p t conditions of 738 60 c and 6 9 1 2 kbar at 17 0 1 2 log fo2 and ah2o 0 18 0 16 wholerock trace element and isotopic measurements show that charnockite formation was accompanied by a systematic depletion of sm rb pb u sr nd with a fractionation of rb sr sm nd and th u ratios and crystallization of the charnockite assemblage at 535 5 ma major element fe mg ca and sm nd equilibration between minerals occurred at 524 9 ma whereas pb and rb sr underwent continued exchange to 501 5 ma and 486 1 ma respectively trace element data for both amphibolite and charnockite minerals show that depletion on a whole rock scale can be accounted for either by changes in mineral modes or trace element abundances within the immediate area of dehydration the fractionation of sm nd on a whole rock scale is controlled by the breakdown of amphibole without the growth of a major new host phase for sm in the charnockite rubidium and sr are dependent on the relative behaviour of biotite plagioclase and alkali feldspar modelling of dehydration melting involving the breakdown of amphibole biotite and alkali feldspar reproduces the observed sm nd and rb sr fractionation and indicates the loss of small melt fractions on a cm scale from the charnockite these observations suggest that partial melting is the most plausible means of effecting both the dehydration and depletion that accompanies charnockite formation

the rayner complex of east antarctica complex isotopic systematics within a proterozoic mobile belt new isotopic rb sr u pb zircon and sm nd and petrological data are presented for part of an extensive proterozoic mobile belt locally known as the rayner complex in east antarctica much of the belt is the product of mid proterozoic 1800 2000 ma juvenile crustal formation melting of this crust at about 1500 ma ago produced the felsic magmas from which the dominant orthogneisses of this terrain were subsequently derived deformation and transitional granulite amphibolite facies conditions which peaked at 750 50 c and 7 8 kbar 0 7 0 8 gpa produced open to tight folding about e w axes and syn tectonic granitoids about 960 ma ago subsequent felsic magmatism occurred at about 770 ma and not as has been widely advocated at 500 550 ma which appears to have been a time of widespread upper greenschist facies 400 500 c metamorphism localized shearing and faulting sm nd model ages of 1 65 2 18 ga disprove a previously favoured hypothesis that the rayner complex mostly represents reworked archaean rocks from the neighbouring craton napier complex models that involve rehydration of the napier complex are no longer required since the rayner complex was its own source of water two episodes of proterozoic crustal growth are identified the later of which occurred between about 1200 ma and 1000 ma and was relatively minor sedimentation took place only shortly before late proterozoic orogenesis the multiphase history of the rayner complex has resulted in complex isotopic behaviour three temporally discrete episodes of pb loss from zircon have been identified the earliest two of which are responses to the c 960 ma and 540 ma tectonothermal events fluid leaching was operative during the later event for there is a good correlation between degree of isotopic discordance and secondary mineral growth pb loss during the high grade event was probably governed by the same process or by lattice annealing some zircon suites also document recent pb loss most lower concordia intercepts have no direct geological meaning and are explicable as mixed ages produced by incomplete pb loss during two or more secondary events whereas all zircon separates from the orthogneisses produce u pb isotopic alignments zircons from the only analysed paragneiss produce scattered data in part reflecting a range of provenance the 960 ma event was also associated with the growth of a characteristically low u zircon 300 g g in rocks of inferred high zr content there is ubiquitous evidence for the resetting of rb sr total rock isochrons even samples separated by up to 10 km fail to produce igneous crystallization ages minor mineralogical changes produced by the 540 ma upper greenschist facies metamorphism were sufficient to almost completely reset some rb sr isochrons and to produce open system conditions on outcrop scale at least in one location

recent seismicity in valles marineris mars insights from young faults landslides boulder falls and possible mud volcanoes abstract current seismicity of mars is poorly known geodynamic models and analysis of surface faults indicate that mars could be seismically active globally particularly within the tharsis region however published geologic constraints on such activity are sparse in this study we use geomorphologic observations and crater size frequency age determinations gleaned from high resolution orbiter data to demonstrate that the valles marineris region is recently seismo tectonically active the chasmata shows evidence of reactivated dip slip faults that cross cut the chasmata walls and floors and indicates up to 1 2 km total vertical displacement along the trough bounding faults more than 16000 boulder fall occurrences with pristine trails are observed throughout the chasmata wall with an average slope of 25 degrees the boulder falls are interpreted to have been triggered by recent seismic shaking from the shallow marsquakes occurring along the chasmata faults possibly in the last thousands of years synthetic ground motion models indicate several m w 4 6 marsquakes at shallow depths 1 6 km are required to produce these canyon wide boulder falls in addition the presence of many young landslides 22 790 ma proximal to the reactivated trough bounding faults and formation of thousands of possible mud volcanic cones throughout the chasmata floor all suggest marsquake triggered shaking in the past tens to hundreds of million years corresponding to late to middle amazonian epochs we estimate formation of 20 m to 1 3 km diameter fresh impact craters around the chasmata had negligible contribution to the recent seismicity therefore valles marineris tectonism is an important source of marsquakes that may be readily detectable by the upcoming insight seismometers

non stoichiometry deformation and high pressure properties of the humite group of minerals in the marbles of the kerala khondalite belt india

anorogenic acid volcanic rocks in the kundal area of the malani igneous suite northwestern india geochemical and petrogenetic studies abstract acid volcanic rocks from the kundal area of the neoproterozoic 725 7 ma malani igneous suite mis are predominantly represented by rhyolite porphyritic and non porphyritic with minor amounts of trachyte welded tuff and volcanic ash beds the non porphyritic rhyolite is peralkaline acmite normative 1 ai and a cnk ratio

electron microprobe dating of monazites from high grade gneisses and pegmatites of the kerala khondalite belt southern india monazites of five samples one leptynitic garnet biotite gneiss one khondalite one augen gneiss and two pegmatites from the central and northern part ponmudi unit of the kerala khondalite belt kkb in southern india were analyzed with the electron microprobe dating technique monazites in the augen gneiss and the pegmatites yield grain sizes between 200 800 m th abundances are rather low 10 wt and the distribution of th u and pb within single grains is fairly homogeneous contrasting to this monazites in the leptynitic gneiss and the khondalite are small 150 m they often display very complex th u pb patterns and contain high th concentrations up to 20 wt the statistical treatment of individual ages from the investigated samples revealed three populations of lower proterozoic 1 9 ga upper proterozoic 580 ma and ordovician age 470 ma as well as mid proterozoic ages between 0 8 1 7 ga which are not regarded to be of geological significance lower proterozoic ages are preserved in the cores of monazites from leptynitic gneisses and khondalites they fairly agree with sm nd model ages for similar rocks of the kkb and give a minimum age for first monazite growth or complete homogenization the prominent pan african population with mean values between 540 and 580 ma is present in the leptynitic gneiss the khondalite and the augen gneiss and in line with published isotope ages for the kkb the ordovician population finally marks the emplacement of granitic pegmatites subsequent to the pan african high grade metamorphic event there is an obvious discrepancy between khondalites and leptynitic gneisses on the one hand and augen gneisses on the other concerning the presence of lower proterozoic ages while these are abundant in the former often rimmed by upper proterozoic ages they are completely absent in the latter it appears unlikely that lower proterozoic ages were completely reset during a pan african event exclusively in the augen gneisses while they were preserved in leptynitic gneisses and khondalites it is further concluded that the augen gneisses are of magmatic origin and were derived from porphyritic granites thus the upper proterozoic age of 605 37 ma calculated for the investigated sample approximates the time of magma emplacement which slightly precedes the peak stage of pan african high grade metamorphism in the kkb and of monazite crystallization from the granitic melt a characteristic feature of the investigated monazites is the resetting of lower proterozoic and pan african ages to significantly younger values due to partial pb loss monazites not shielded by other minerals e g garnet suffered selective mobilization of pb along fractures or at their rims while th and u concentrations remained almost unchanged the results presented in this study indicate that this was mainly due to fluid rock interaction it is concluded that thermal diffusion of pb even at the suggested temperatures of 900 c only had minor influence on the th u pb composition in monazite and that the closure temperature for this system must be significantly higher than previously assumed 750 c

alkali granites with pan african affinities from kerala s india in the kerala region of the south indian shield alkali granite plutons ranging in age from 550 to 740 ma intrude precambrian charnockites and migmatites all of them are spatially related to regional fault lineaments their geochemistry is similar to pan african post kinematic plutons from the afro arabian shield they are of granitic to adamellitic composition with high sio 2 k 2 o up to 12 and na 2 o and low cao feo and mgo their major element composition together with a low 87 sr 86 sr i 0 703 from one pluton denote a type affinities however local charnockites have such low rb and radiogenic 87 sr that the granites could have formed by crustal melting their overall trace element characteristics are typical of alkaline granites in general but the details show that most resemble more recent volcanic arc granites and one pluton is typical of a within plate setting the presence of co 2 rich fluid inclusions supports the notion that the enrichment of rare earth an

dextral pan african shear along the southwestern edge of the achankovil shear belt south india constraints on gondwana reconstructions the achankovil shear belt of the southern indian peninsular shield is prominent on landsat images it coincides with the boundary between charnockites of the madurai block to the north and khondalites of the trivandrum block to the south and with a major change in the aeromagnetic pattern that can be traced across southern india field investigations reveal a major shear zone along the southwestern edge of the achankovil shear belt the tenmala shear zone rocks in the tenmala shear zone include charnockite garnet biotite gneiss garnet sillimanite gneiss cordierite gneiss and granite kinematic indicators include stretched and asymmetric garnet feldspar and quartz porphyroclasts shear bands asymmetric folds extensional and contractional composite structures hook folds on rotated and deformed gash veins and lineations kinematic analysis of these features along 60 km of the shear zone indicates primarily dextral shear with a minor component of reverse shear textures as well as mineral assembl

archean proterozoic evolution of indian charnockites isotopic and geochemical evidence from granulites of the eastern ghats belt pyroxene granulites and charnockites are associated with metasedimentary khondalite garnet sillimanite gneiss and migmatites in the eastern ghats mobile belt various stages of in situ granulitization similar to those observed in southern india and sri lanka are present geochemically the granulite suite around visakhapatnam is bimodal with acid and mafic variants there is an increase in the concentration and fractionation of the rare earth elements ree from pyroxene granulites to charnockites and six sm nd models ages t dm range from 2 86 to 2 35 ga with 2 1 ga for a younger granite the charnockites a pyroxene granulite and granite define a pb pb isochron age of 1176 pm 201 ma u pb ages of zircons and monazites from a charnockite near phulbani orissa yield near concordant data with ages of 985 pm 5 ma and 965 pm 7 ma respectively the new isotopic data and the available ages from the northeastern part of the eastern ghats in orissa suggest a major charnockite formati

proterozoic events in the eastern ghats granulite belt india evidence from rb sr sm nd systematics and shrimp dating metamorphic and protolith ages of five rock types mafic granulite orthopyroxene granulite leptynite sillimanite granite and metapelite from rayagada in the north central part of the eastern ghats granulite belt eggb india were determined from rb sr and sm nd whole rock and mineral isochrons in combination with shrimp u pb zircon data most of the whole rock isochron ages in both sm nd and rb sr systems point to either 1450 or 1000 ma and the mineral isochron ages are 1000 800 and 550 ma shrimp u pb zircon ages of 940 ma were obtained from metapelite which are in close agreement with the sm nd and rb sr isochron ages from all these data four age clusters 1450 1000 800 and 550 ma have been noted the 1450 ma ages are interpreted to represent igneous protolith formation of mafic granulite and leptynite the 1000 ma age cluster is regarded as the intrusion ages of sillimanite granite and charnockite and associated granulite facies metamorphism two other age clusters 800 a

ductile shearing in attur shear zone and its relation with moyar shear zone south india abstract in the eastern part of southern peninsular india the charnockitic hills of the madras block are cut across by the e w trending attur shear zone asz which is characterised by a thick 1 to 1 5 km phyllonite zone showing intense mylonitisation due to ductile shearing steeply plunging 70 80 stretching lineation on steeply dipping mylonitic foliation within this zone indicates a relative vertical upliftment of the adjacent blocks a dextral shearing event from west to east is envisaged from the kinematic analysis of shear sense indicators such as s c fabric asymmetric folds asymmetric augens and asymmetric porphyroclasts simultaneous development of these features related to vertical and horizontal movements may be explained by the mechanism of transpressional deformation the attur shear zone may be correlated with the moyar shear zone based on distinct lithological and structural similarities

evidence for continent continent collision zone in the south indian shield region abstract with a view towards understanding the evolutionary history of the complex south indian shield several geological and geophysical studies have been carried out recent geophysical studies include magnetotelluric mt deep seismic sounding dss gravity magnetic and deep resistivity soundings drs in the present study mt results along 140 km andiyur turaiyur east west profile is presented the data are subjected to groom bailey decomposition and static shift correction before deriving a 2 d model the 2 d modeling results have shown that the upper crust up to about 15 km towards western part of the profile have exhibited high resistive character of about 40 000 ohm m as compared to the eastern part less than 5 000 ohm m the mid lower crust has shown a decrease in resistivity in western part of the profile the order of resistivity being 2 000 ohm m an anomalous steep conductive feature less than 100 ohm m is observed near sankari at mid lower crustal depths 20 km towards middle part of the profile this feature is spatially correlatable with the well known moyar bhavani shear zone mbsz the features obtained in the present study are consistent with earlier mt studies in this region and correlatable with other geophysical studies dss studies near the study region gave an evidence for differing crustal structure on either side of mbsz variation in geoelectric character along the profile both in the upper crust and mid lower crust indicate a block structure in the sgt with shear zones acting as boundaries the new evidence in the form of distinct geoelectric structure and also variation in seismic structure indicate a continent continent collision zone in this region and plays an important role for the gondwana reconstruction models of south indian shield

a plausible model for evolution of schist belts and granite plutons of dharwar craton india and madagascar during 3 0 2 5 ga insight from gravity modeling constrained in part from seismic studies abstract gravity modeling of an e w profile across dharwar craton india and madagascar integrated with the results of deep seismic sounding dss across the dharwar craton suggest a thick crust of 40 42 km under the eastern part of eastern dharwar craton edc the western dharwar craton wdc and the central part of the madagascar towards east of these blocks the crustal thickness is reduced to 36 38 km along the eastern ghat fold belt egfb shear zone between the edc and the wdc and the east coast of madagascar respectively these zones of thin crust are also characterized by high density lower crustal rocks associated with thrusts the seismic section across dharwar craton shows domal shaped reflectors in the lower crust and upper mantle under the wdc which may be related to asthenopheric upwelling during an extension phase the occurrences of large schist belts with volcano sedimentary sequences of marine origin of late archean period 3 0 2 7 ga as rift basins in the wdc and madagascar also suggest an extensional phase in this region during that period it is followed by a convergence between the wdc and the edc giving rise to collision related shear and thrust zones between the wdc and the edc associated with high density lower crustal rocks the seismic section shows upwarped reflectors in the upper crust which may be related to this convergence eastward dipping reflectors under wdc and edc and west verging thrusts suggest convergence from the west to the east which resulted in easterly subduction giving rise to subduction related k granite plutons of the edc of 2 6 2 5 ga in this regard the closepet granite in the edc which extends almost parallel to the shear zone between the wdc and edc and shows an i type calk alkaline composition may represent relict of an island arc and the linear schist belts with bimodal volcanics of the edc east of it might have developed as back arc rift basins subsequent collision between india and antarctica along the egfb during middle proterozoic indicated by eastward dipping reflectors in the crust and the upper mantle and west verging thrust gave rise to contemporary high grade rocks of the egfb 1 6 1 0 ga and associated mafic and felsic intrusives of this belt the part of adjoining cuddapah basin contemporary to the egfb towards the west consisting of marine shelf type of sediments which are highly disturbed and thickest at its contact with the egfb may represent a peripheral foreland basin gravity modeling provides thickest crust of 42 km in the southern part of the wdc and does not support sharp increase in crustal thickness of 50 60 km with high velocity upper mantle as suggested from receiver function analysis it may represent some foreign material of high density trapped in this section such as part of oceanic crust during convergence and subduction that is referred to above it is supported from eastward dipping reflectors in lower crust and upper mantle in adjoining region

precambrian tectonics and crustal evolution in south india about 3 4 ga ago voluminous calc alkaline magmas represented by the granitoid gneisses of southern india were emplaced into a now poorly preserved non continental crust unstable ensialic basins initiated at about 3 0 ga were filled with volcanic and sedimentary rocks up to about 2 6 ga this basement cover sequence was deformed at the close of the archaean first by northward accretion and thickening of several crustal slabs formerly separated by prisms of stable shelf sediments structures produced by this episode were refolded and dislocated by large n s strike slip shear belts to impart intense steep planar fabrics to large volumes of the crust fluids rich in co possibly derived from sedimentary material driven beneath the crustal slabs by the thickening mechanism purged the deep crust of h o to form granulites upward migration of hot h o rich fluids encouraged partial melting at intermediate crustal levels these late archaean tectonic and thermal events began soon after the close of basin f

smnd isotopic systematics of enderby land granulites and evidence for the redistribution of sm and nd during metamorphism measurements of143nd 144nd and147sm 144nd are reported for whole rocks and mineral separates from granulites of the napier complex at fyfe hills charnockites leuconorites and gabbros yield a whole rock smnd isochron age of3060 160m y and an initial143nd 144nd ratio of0 50776 10 nd 3060m y 2 0 1 8 the negative nd value and the presence of geologically induced dispersion in the data suggest that the isochron age does not represent the time of primary crystallization of the complex but instead indicates a time of later redistribution of sm and nd and partial re equilibration of143nd 144nd ratios this probably occurred during the upper granulite facies metamorphism which has also been dated at 3100m y by rbsr and upb zircon studies 1 coexisting clinopyroxene apatite and total rock fractions in two adjacent samples define an approximately linear array corresponding to an age of 2300 300 m y this array indicates that redistribution of sm and nd and re equilibration of143nd 144nd ratios occurred on an intermineral scale during the upper amphibolite to lower granulite facies metamorphism at 2450m y due to the resetting of the smnd system on both whole rock and mineral scales the primary crystallization age of the igneous protolith is not well constrained by the present data although it is clearly3100m y if it is assumed that the complex was derived initially from a depleted mantle reservoir nd t 2 evolution of the negative nd value of 2 0 with the observed sm nd ratios requires a prehistory of 380m y this implies a primary age of 3480m y however substantially older primary ages can be inferred if the source reservoirs had nd t 2 and or substantial reductions in the sm nd ratio occurred in whole rocks during the granulite facies metamorphism at 3100 m y such an inferred reduction in the sm nd ratio may have been the result of preferential loss of sm relative to nd or introduction of a low sm nd fluid with nd 0 during granulite facies metamorphism

geological geophysical and inherited tectonic imprints on the climate and contrasting coastal geomorphology of the indian peninsula abstract the indian subcontinent comprises accreted cratonic fragments that underwent subsequent rifting subduction collision processes along major mobile belts it is also a storehouse of a prolonged history of crustal evolution involving repeated episodes of magmatism volcanism sedimentation and metamorphism and the formation of intracratonic and foreland basins the geologic geomorphic evolution of the peninsula is recorded in relic faults suture zones that confine paleozoic mesozoic and cenozoic sedimentary basins and landforms in addition escarpments plateaus waterfalls deltas planation surfaces and strandlines are either aligned parallel and or formed over them asymmetric relief across the western eastern parts of the peninsula orographic effect on the precipitation pattern and climatic zones provide important constrains on the geologic and geomorphic evolution of the region though extensive continental and coastal deposition occurred during permo triassic and early cretaceous only the downwarped grabens of gondwana basins preserved the sedimentary records occurrences of extensive cenozoic deposits offshore and neogene holocene deltaic deposits along the entire east coast as opposed to the absence of comparable deltaic sequences in the west coast are linked to intense and recurrent episodes of denudation indicating first order control of basement structures over landscape evolution inheritance and continuum of basement structures over climate and landscape evolution of peninsular india can also be witnessed by the unique and diverse drainage patterns of major rivers while most of the major rivers follow a general easterly direction the landscape morphology of their catchments indicates youthful character and tectonically active nature the east flowing rivers show an asymmetry between the areal extents of drainage basin size and delta whereas the west flowing rivers are short and straight and their catchments are miniscule in comparison with the rivers flowing toward east the western ghats escarpment forms an orographic barrier and acts as the main divide between these two drainage systems there is a disparity between the quanta of sediment discharge as well in addition the eastern and western parts of india across the western ghats escarpment show significant differences in terms of climatic zones humid to per humid on the west while it is arid to subhumid on the east the west coast is characterized by 3000 mm year annual rainfall that often reaches 5000 mm year dominance of sw monsoon winds occurrence of narrow here we propose a conceptual tectono geomorphic model demonstrating the inheritance and continuum of gondwanan structural trends on the transient landscape of peninsular india

isotopic mapping of age provinces in precambrian high grade terrains sri lanka nd model ages of amphibolite and granulite grade rocks in sri lanka form a simple regional pattern that broadly correlates with mappable geological units and is in effect an isotopic map of the island s basement the granulite grade units of the highland group and southwest group have model ages of 2 2 3 0 ga indicating derivation mainly from late archean sources they are bounded to the east and west by late proterozoic gneisses of the vijayan complex with model ages of 1 1 2 0 ga the isotopic data identify three distinct crustal provinces and are not consistent with earlier suggestions that the vijayan gneisses are retrograde equivalents of the highland granulites sri lanka is not a direct continuation of the archean dharwar craton of southern india identification of vijayan type juvenile crustal terrains in other gondwana fragments may play a key role in determining the precise attachment of southern india sri lanka in eastern gondwana

juxtaposition of india and madagascar a perspective abstract proterozoic terrains in south india and madagascar provide important clues in understanding the gondwanaland tectonics especially the assembly of this mega continent during the pan african period the archaean terrains in both madagascar and india are characterized by n s trending greenstone belts occurring within gneissose granitic rocks in the northern part extensive development of k rich granitic rocks of ca 2 5 ga is also characteristic in both areas such a broad age zonation of younger dharwar ca 2 6 3 0 ga in the north and the older sargur ca 3 0 3 4 ga in the south as in south india remains to be identified in future studies from madagascar the occurrence of greenschist facies rocks in the northeastern part and higher grade rocks in most of other parts in the north central terrain of madagascar is comparable with the general tendency of increasing metamorphic grade from northwestern to southern areas ranging from greenschist to granulite facies in south india the proterozoic crystalline rocks in both continents show pronounced lithological similarity with the wide occurrence of graphite bearing khondalite in association with charnockitic rocks while the archaean proterozoic boundary is well defined in southern india by the palghat cauvery or the kkpt shear zones as recently identified this boundary is ill defined in madagascar due to extensive pan african overprinting as well as the development of the proterozoic cover sequence the itremo group there is also a possible general correlation between the mesoproterozoic cover sequences in madagascar and india such as between the itremo group of west central madagascar and the kaladgi and cuddapah sequences of south india the pan african granulite facies metamorphism of ca 0 5 ga extensively developed in both india and madagascar is generally comparable in intensity and extent p t conditions and p t t paths also appear comparable with the general range of ca 700 1000 c and 6 9 kb and near isothermal decompressional paths a type granite plutons and alkaline rocks including anorthosites and mafic plutonic rocks of ca 500 800 ma develop in both terrains provide strong basis for the correlation of both terrains and define a pan african igneous province within east gondwanaland major shear zones in both continents are expected to play a critical role in the correlation albeit are still poorly constrained detailed elucidation of the tectonic history of the shear zones and the timing of various events along the shear zones would provide important constraints on the correlation of the two continental fragments

natural analogues for radwaste disposal elemental migration in igneous contact zones abstract chemical studies of various igneous rocks intrusive into many different rocks have been examined to see if elemental transfer has taken place systems examined include granite intrusive into shale and siltstone basalt intrusive into granodiorite diorite rhyolite and dolomite diabase intrusive into rhyolite and extrusive over arkose rhyolite extrusive over basalt lamprophyre intrusive into evaporites monzonite intrusive into tuff and andesite and quartz monzonite intrusive into metasedimentary rocks the mechanism for elemental transport in these systems is due in part to diffusion and in part to chemical reaction between intrusive and host rocks evidence for transport of elements from intrusive into intruded rocks is noted for several elements in some cases the distribution of some elements is mineralogically controlled and variations in co cr ree s ni cu zr mn ti rb sr u and th can be attributed to more or less specific reactions in a few contact zones there has been limited migration of cs but usually only within 0 5 1 0 m of the igneous rock contact thermal effects of the igneous intrusions are to reset some mineral isotopic systematics loss of 40ar redistribution of 87sr 18o but otherwise the whole rock chemistry usually remains unchanged collectively the data for elemental behavior in the vicinity of the igneous contacts studied show that elemental migration of redistribution is often restricted to within 0 5 1 0 m of the contact thus suggesting that for hypothetically stored radioactive waste canisters emplaced at much lower temperatures little if any elemental transport would take place in the event of breaching of the waste package

african linkage of precambrian sri lanka new age and isotopic data show that the high grade basement rocks of sri lanka were not linked to the archaean granulite domain of southern india but experienced their main structural and metamorphic development during the pan african event some 950 to 550 ma ago this occurred when west gondwana and east gondwana collided to form one of the longest collisional structures in the supercontinent the mozambique belt that extends from mozambique to ethiopia and sudan a major tectonic boundary interpreted as a thrust zone divides the highland southwestern complex in the central part of sri lanka from the vijayan complex in the e and se the former is interpreted to represent the remnant of a once extensive passive margin extending west in a gondwana reconstruction via madagasgar to tanzania and mozambique the vijayan complex may have been part of a separate continental margin plutonic assemblage and its collision with the highland southwestern complex marks the final amalgamation of east and west gondwana into a supercontinent some 550 ma ago the sri lankan granulites cannot be correlated with the distinctly older granulites of the eastern ghats belt of india and this suggests that sri lanka was situated close to the se coast of madagascar in a gondwana reconstruction

the application of single zircon evaporation and model nd ages to the interpretation of polymetamorphic terrains an example from the proterozoic mobile belt of south india the technique of single zircon dating from the thermal evaporation of 207pb 206pb kober 1986 1987 provides a means of dating successive periods of growth and nucleation of zircons in polymetamorphic assemblages in contrast nd model ages may provide a measure of the period of crustal residency for the sample or its protolith these two techniques have been combined to elucidate the tectonic history of the proterozoic mobile belt of southern india exposed south of the palghat cauvery shear zone that marks the southern boundary of the archaean craton of karnataka the two main tectonic units of this mobile belt comprise the madurai and trivandrum blocks both of which are characterised by massive charnockite uplands and low lying polymetamorphic metasedimentary belts that have undergone a complex tectonic history throughout the proterozoic evidence for early palaeoproterozoic magmatism is restricted to the madurai block where single zircon evaporation ages from a metagranite 2436 4 ma are similar to model nd ages from a range of lithologies suggesting crustal growth at that time the trivandrum block to the south of the achankovil shear zone is comprised of the kerala khondalite belt the nagercoil charnockites and the achankovil metasediments single zircon evaporation ages together with conventional zircon and garnet chronometry suggest that all three units underwent upper amphibolite facies metamorphism at 1800 ma an event unrecorded in the metagranite from the madurai block this implies that the madurai and trivandrum blocks represent distinct terrains throughout the palaeoproterozoic model nd ages from the achankovil metasediments are much younger 1500 1200 ma than those from the adjacent kerala khondalite belt and madurai blocks 3000 2100 ma but there is no evidence for zircon growth in these metasediments during the mesoproterozoic hence the comparatively young model nd ages of the metasediments are indicative of a mixed provenance rather than a discrete period of crustal growth zircon overgrowths from the madurai block 547 17 ma and achankovil metasediments 530 21 ma suggest that all tectonic units of the proterozoic mobile belt of south india shared the same metamorphic history from the early palaeozoic this event has been recognised in the basement lithologies of sri lanka and east antarctica confirming that the constituent terrains of east gondwana had assembled by this time

dextral pan african shear along the southwestern edge of the achankovil shear belt south india constraints on gondwana reconstructions a discussion achankovil shear zone acsz figure 1a intering charnockites garnet biotite gneiss biotite gneiss calc gneiss and granite as revealed in the preted as a major nw trending strike slip shear zone of probable proterozoic age drury et al 1984 lithological mapping carried out as part of this study figure 2 a preliminary structural assesshas prominently figured in gondwana correlative studies e g windley et al 1994 although it is ment reveals the presence of a distinctive assemblage of asymmetric structures in the gneissic exclear that there is a major lithologic change across acsz figure 1b detailed studies indicating the posures indicating a consistent sinistral sense of shear gneissosity forming the secondary compoprobable sense of shear are lacking across this 10 20 km wide and 100 km long shear zone resitional banding strikes wnw ese to nw se dipping moderately toward sw with relatively rare cently sacks et al 1997 reported field observations bearing on the probable sense of shear of the ne dip and is conformable in all the rock types of the area in conformity with the general nw se acsz after combining field criteria and kinematic analysis of asymmetric features along the sw trend of the acsz structurally the terrain has undergone four phases of deformation summarized in edge of the acsz known as the tenmala shear zone they concluded that the acsz indicated prirajesh and santosh 1997 open to isoclinal intrafolial folds encountered in marily dextral shear with a minor component of reverse shear the authors illustrate their discusthe study area are invariably asymmetric fold limbs are zones of transposed gneissosity that consion with examples of kinematic indicators including shear bands asymmetric folds stretched and tain indicators of a probable sinistral sense of shear and well developed mineral elongation lineation asymmetric garnet feldspar and quartz porphyroclasts hook folds and lineations thus they appardefined by quartz ribbons trails of xenoblastic garnet grains and the alignment of the long axes of kently question the correlation of the dextral achankovil shear zone rather tenmala shear zone with feldspar augen pinch and swell structures and boudins commonly show rotation with respect to the sinistral bongolava ranotsara shear zone brsz in the india madagascar gondwana enfoliation tight s asymmetric f2 folds and synkinematic pegmatite veins are seen in the western semble we focus the present discussion on the probable part of the study area the development of a refold structure by superposed buckling is illustrated in sense of shear of acsz in the light of new data our study area falls around punalur and its adjoinfigure 3a there are smaller asymmetric sand zfolds congruent with the larger f1 with one limb ing areas within the quilon district southern kerala south india and is located between longitudes striking e w while the other strikes n50 e showing negative extension during the development of 76 55 and 76 59 e and latitudes 9 1 and 9 3 n figure 2 represented in the survey of india topo f2 on the traces of preserved axial planar cleavage the simple shear is dextral on the left limb and sisheet no 58c 16 outcrops within the study area expose a strongly deformed suite of rocks comprisnistral on the right limb asymmetrically folded veins figure 3b the bulk asymmetry of which represents a sinistral sense of shear have been found 1 manuscript received may 14 1997 in a few locations elongate quartz lenticles and 2 centre for earth science studies pb 7250 trivandrum 695 031 india garnet and biotite aggregates define penetrative lin

deep crustal electrical signatures of eastern dharwar craton india abstract wide band magnetotelluric mt investigations were carried out along a profile from kavali in the east to anantapur towards west across the eastern ghat granulite terrain eggt eastern dhanvar craton edc and a proterozoic cuddapah basin this 300 km long profile was covered with 20 stations at an interval of 12 18 km the mt data is subjected to robust processing decomposition and static shift correction before deriving a 2 d model the model shows a resistive crust 10 000 30 000 ohm m to a depth of 8 10 km towards west of the cuddapah basin the mid crust is less resistive about 500 ohm m and the lower crust with a slight increase in resistivity about 1 500 ohm m in the depth range of 20 22 km the resistivity picture to the east of the cuddapah basin also showed a different deep crustal structure the resistivity of upper crust is about 5 000 ohm m and about 200 ohm m for mid and lower crust the sediment resistivity of cuddapah basin is of the order of 15 20 ohm m mt model has shown good correlation with results from other geophysical studies like deep seismic sounding dss gravity and magnetics the results indicate that the lower crustal layers are of intermediate type showing hydrous composition in eastern dhanvar craton