gravity anomalies and the indian lithosphere review and analysis of existing gravity data abstract the upper part of the lithosphere has been actively involved in various exogenic and endogenic processes which have left their imprint on the gravity field on the indian peninsula and the himalaya analysis of the gravity field over the dharwar craton shows that the greenstone belts of this craton have been formed as a result of development of deep fractures in the earth s crust during archaean times precambrian mountain ranges such as the aravallies vindhyans satpura and eastern ghats are located peripheral to archaean cratons most of these mountain belts are characterized by gravity highs suggesting that the underlying crust is of higher than normal density these mountain ranges with the exception of the eastern ghats do not appear to be locally compensated regional compensation seems to prevail over all these areas eastern ghats ranges are also underlain by a crust of higher than normal density relative to the dharwar and bastar cratons and exist with a sharp contact with the cratons in the west isostatic compensation in the eastern ghats appears to have been achieved by thickening of the underlying crust the himalaya has attained a fairly high degree of isostatic compensation

seismic constraints on a large mafic intrusion with implications for the subsidence history of the danish basin 1 seismic refraction data from the estrid 1 profile are used for seismic velocity modeling along the strike of a large mafic intrusion in the norwegian danish basin central denmark the p wave velocity structure identifies a 8 km thick sedimentary succession with velocities between 1 8 and 5 7 km s the top basement is defined by a step to vp 6 2 km s in the middle to lower crust a high velocity body vp 6 7 km s with its top located at about 10 12 km depth is interpreted as a high velocity and high density gabbroic intrusion of permian age this high velocity body explains the large 50 mgal positive gravity anomaly known as silkeborg gravity high the intrusion has a minimum volume of 40 000 km3 which implies that the magma influx and the consequent cooling of the lithosphere from high temperature could have had profound effects on the subsidence of the danish basin in particular because the magma probably intruded during only a few events and other similar structures cover much of the basin an anomalously high velocity gradient from 7 0 km s in the middle crust to 7 7 km s at 30 32 km depth in the central part of the intrusion coincides with an interval without moho reflections indicative of a gradual transition zone between the crust and the mantle this feature may show the location of the feeder dykes of the intrusion

sveconorwegian igneous complexes beneath the norwegian danish basin abstract gravity and magnetic anomalies have previously been interpreted to indicate strongly magnetic permian or even tertiary intrusive bodies beneath the skagerrak waterway such as the skagerrak volcano and beneath silkeborg in denmark our combined modelling of the magnetic and gravity anomalies over these rock bodies indicates that a steep upward magnetisation is required to explain the magnetic anomalies at the surface reminiscent of the magnetic direction in the sveconorwegian rocks of the rogaland igneous province in southern norway the younger rocks of the permian oslo rift region have intermediate and flat magnetisation that is inadequate to explain the observed magnetic field the positive part of the skagerrak aeromagnetic anomaly is continuous with the induced anomalies associated with the eastward extension of the rogaland igneous province this relation also suggests that rocks of the rogaland igneous province and its offshore extension are responsible for the skagerrak anomalies both the negative remanence dominated aeromagnetic anomaly and the positive gravity anomaly can be modelled using constraints from seismic reflection lines and available density data and rock magnetic properties a 7 km thick complex of ultramafic mafic intrusions is located below a southward dipping 1 4 km thick section of mesozoic sediments and 1 2 km of palaeozoic sediments the enormous body of dense ultramafic mafic rocks implied by the modelling could be the residue of the parental magma that produced the voluminous rogaland anorthosites the application of similar petrophysical properties in the forward modelling of the silkeborg source body provides an improved explanation of the observed gravity and magnetic anomalies compared with earlier studies the new model is constrained by magnetic depth estimates from the located euler method ranging between 6 and 8 km forward modelling shows that a model with a reverse magnetic body anorthosite situated above a dense mafic ultramafic body may account for the silkeborg anomalies the anorthosites may have formed by differentiation of the underlying mafic intrusion similar to the intrusive relations in the rogaland igneous province we conclude that there is strong evidence for a sveconorwegian age for both the skagerrak and the silkeborg anomalous rock bodies

marine geophysical study of the comorin ridge north central indian basin marine geophysical data are used to interpret the continental margin structure south of india and west of sri lanka in the northern central indian basin free air and isostatic anomaly maps are presented they show the prominent indian ocean gravity low with values more negative than 100 mgal south west of sri lanka a steep gradient in the free air and isostatic anomaly forms a nnw sse trending line extending south and striking parallel to the southwest coast of india this line marks the eastern boundary of a topographic and basement high here named the comorin ridge which is also clearly seen on a sediment isopach map the ridge seems to act as a barrier to the passage of ganges cone sediments to the west a comparison of the eastern landward edge of the comorin ridge with similar structures of other passive continental margins such as those off southern south africa agulhas fracture zone off the falkland plateau and off western norway is made it is suggested that the eastern edge of the comorin ridge may mark a significant structural crustal boundary

thermo mechanical structure of the southern part of the indian shield and its relevance to precambrian basin evolution abstract the thermal and mechanical structures of the southern part of the precambrian indian shield have been estimated using available heat flow data and shear stress profiles from olivine rheology these and other geological geochronological and geophysical data including deep seismic studies dss profiles of proterozoic cuddapah basin on south indian shield are utilized to examine thermal models for the evolution of precambrian intracratonic platform basins on the archean lithosphere of indian shield evidence of mantle perturbations and cycles of thermal events are documented to be important in the cuddapah basin s evolution haxby et al s 1976 thermal model has been shown to explain the cuddapah basin s flexuring and magnitude of subsidence

observations of winter polynyas and fractures using noaa avhrr tir images and nimbus 7 smmr sea ice concentration charts abstract remotely sensed data routinely acquired by the nimbus 7 scanning multichannel microwave radiometer smmr are processed to provide daily sea ice concentration charts currently these charts are the exclusive source of information on cloud covered ice drifting in the polar oceans hence it is vital to test the validity of these charts to this end winter polynyas and fractures recorded on cloud free three thermal infrared tir noaa avhrr 4 images with a 1 1 km 1 1 km resolution were analyzed and compared to the same polynyas and fractures recorded on ice concentration charts processed for the same time and over the same area these charts have a 50 0 km 50 0 km resolution it was concluded that although there is a loss of detail due to the coarser relolution of smmr data in all three cases the ice concentration charts provided valuable information on openings in the ice the smmr information is consistent with the location orientation and size of polynyas and fractures as recorded on the avhrr images these fractures are shown to have a similar orientation on the isobaric contours finally it was concluded that the location and orientation of polynyas and fractures were related to the direction of surface winds

induction models of southern india and the effect of off shore geology abstract simple numerical tests based on thin sheet modelling are used to define the conductors most probably responsible for current channelling in the region including southern india a combination of crustal conductors located in the indo ceylon graben and underneath the comorin ridge best reproduces the observations at periods of 50 100 min in so far as channelling is concerned daytime variations do not help to define the location of conductors any better than night time data and may thus be ignored

the bangui magnetic anomaly its geological origin satellite magnetometer data have revealed a long wave length magnetic anomaly in the vicinity of bangui central african republic substantiation of the anomaly has been made through analysis of air and ground magnetic surveys a correlative gravity anomaly has also been observed in ground gravity data the anomaly in an area of highly metamorphosed precambrian rocks does not correlate with any surficial features but is definitely crustal in origin an interpretation consistent with all available geophysical and geological data is that the anomaly could be the result of an early intrusion into the crustal rocks of this region followed by subsidence and deformation

correlation of deep seismic reflectivity patterns with tectonic units in western and central europe abstract the dense network of deep seismic reflection lines in western and central europe makes a systematic comparative study possible the reflectivity of the continental crust is not the same but can change significantly from one seismic line to the next different seismic reflectivity patterns can be observed and correlations to specific tectonic units are found

palaeozoic terranes and their lithospheric boundaries within the trans european suture zone tesz a review the trans european suture zone tesz is a broad and complex zone of terrane accretion separating ancient lithosphere of the baltic shield and east european craton eec from the younger lithosphere of western and southern europe there is debate about the number of terranes involved and their origins the most significant terrane boundaries originally oceanic sutures are poorly exposed and are defined using faunal provinciality prominent geophysical lineaments may represent sub terrane boundaries within composite terranes the avalonia composite terrane was amalgamated to baltica in latest ician time the provenance of the malopolska and moravo silesian terranes whose neoproterozoic basement is suspect with respect to baltica but with which they nonetheless share faunal affinities is still debated the extent of the variscide rhenohercynian zone defines the southern limit of the early palaeozoic accreted terranes terranes within the bohemian massif exhibit hp metamorphism recording subduction driven collision prior to incorporation in the tesz collage itself associated with a hp event in romania the saxothuringian zone and rheic suture are directly juxtaposed with the eec and the crustal structure of the tesz has been much modified by post palaeozoic tectonism deep seismic reflection data where available show that the oceanic sutures are frequently associated with inclined reflectivity zones irz in the lithospheric mantle here inferred to mark relict subduction zones interpreting the age of subduction is complicated by offset of originally coplanar mantle and crustal segments of the irz resulting from early orogenic lithospheric delamination at the moho interface and or post orogenic reordering processes the mantle irz appear to indicate a n directed polarity for early palaeozoic subduction apparently incompatible with the geological evidence which indicates subduction of the tornquist sea beneath avalonia and a large amount of underthrusting of avalonia by baltica possible explanations for these apparently incompatible observations are evaluated

geomagnetic variation anomalies in peninsular india summary geomagnetic substorms recorded by an array of magnetometers over peninsular india have been analysed to investigate the subsurface electric configuration of the area the study considered only night time events when the external inducing field is uniform over the indian peninsula and the conventional interpretational techniques of geomagnetic deep sounding can be readily applied contour maps of fourier transform amplitudes and phases along with real and quadrature parkinson arrows are presented their features reveal a complex anomaly pattern the observed anomalies can be classified into i southern peninsular anomalies ii palk sirait anomalies iii pondicherry rift anomalies and iv anomalies in the central region a large scale induction process seems to be disturbed by localized perturbations the prominent anomaly observed over the southern tip seems to have a direct relevance to the tectonic setting of the indian ocean and adjoining areas

seismic velocity structure of a large mafic intrusion in the crust of central denmark from project estrid abstract the origin of regional sedimentary basins is being investigated by the estrid project explosion seismic transects around a rift in denmark this project investigates the mechanisms of the formation of wide regional basins and their interrelation to previous rifting processes in the danish norwegian basin in the north sea region in may 2004 a 143 km long refraction seismic profile was acquired along the strike direction of a suspected major mafic intrusion in the crust in central denmark the data confirms the presence of a body with high seismic velocity 6 5 km s extending from a depth of 10 12 km depth into the lower crust there is a remarkable moho relief between 27 and 34 km depth along this new along strike profile as based on ray tracing modelling of pmp reflections the lack of pmp reflections at a zone of very high velocity in the lowest crust 7 3 7 5 km s suggests a possible location of a feeder channel to the batholith the presence of volcanic rocks of carboniferous permian age above the intrusion mafic batholith suggests a similar age of the intrusion an older obliquely crossing profile and two new fan profiles deployed perpendicular to the main estrid profile show that the batholith is about 30 40 km wide the existence of this large mafic batholith supports the hypothesis that the origin of the danish norwegian basin is related to cooling and contraction after intrusion of large amounts of mafic melts into the crust during the late carboniferous and early permian the data and interpretations from project estrid will form the basis for subsidence modelling tentatively we interpret the formation of the danish norwegian basin as a thermal subsidence basin which developed after widespread rifting of the region

super mobility of hot indian lithosphere negi j g pandey o p and agrawal p k 1986 super mobility of hot indian lithosphere tectonoph vsics 131 147 156 the great mobility of the indian subcontinent during the last 180 million years 9000 km of south north motion with an anticlockwise rotation of about 60 o has long been a major puzzle for theorists of plate tectonics a relative analysis of the available heat flow data the estimated temperature depth regime of india and the gravity anomalies reveals that the indian lithosphere does not have the characteristics of a typical shield zone and that it has higher radioactivity smaller viscous drag decidedly low viscosity of 10 p lower density and a well defined low velocity zone as compared to other shield areas of the world these factors appear to contribute significantly to the faster northward movement of the thin indian lithosphere as compared to the almost immobile african plate

deep seismic investigation of crustal extensional structures in the danish basin along the estrid 2 profile summary the crust and uppermost mantle in the danish basin are investigated by modelling the p wave velocity distribution along the north south trending seismic profile estrid 2 seismic tomography and ray inversion modelling demonstrate a variable depth to the top of the crystalline crust from 10 km in the northern part of the profile to 2 km depth in the southernmost part the crystalline crust shows very high p wave velocity in the central part of the profile with 6 7 km s 1 at depths as shallow as 12 km and 7 3 7 5 km s 1 in the lowermost crust these values confirm previous results obtained along the orthogonal estrid 1 profile and the eugeno s profile 2 this high velocity zone in the middle to lower crust is interpreted as a mafic intrusion which explains a positive gravity anomaly of 50 mgal silkeborg gravity high the total length of the intrusion is at least 80 km in the east west direction and 25 35 km in the north south direction the estimated thickness of the intrusion from its top to the moho level is 18 20 km which gives a total minimum volume of 40 50 000 km3 the reflectivity properties of the moho discontinuity are variable along the profile below the intrusion the pmp signal is very weak due to the small velocity contrast between the lowermost crust 7 4 km s 1 and uppermost mantle 7 6 7 7 km s 1 the main moho reflection has a reverberative character to the south of the intrusion this feature is interpreted by layering at the moho level possibly due to magmatic underplating the occurrence of a large crustal mafic intrusion associated with magmatic underplating may be related to extensional transtensional tectonism in the tornquist fan area in the late palaeozoic the extensional event probably caused the opening of a plumbing system for intrusion of mantle derived magma into the crust the ascending magma may have been injected at upper middle crustal levels and during the late phases of the development squeezed laterally along the moho

initial scalar magnetic anomaly map from magsat a scalar magnetic anomaly map between 50 geographic latitude was derived from magsat data acquired during the november 1979 to june 1980 mission lifetime separation of anomaly fields from core and external fields was accomplished utilizing techniques developed for pogo data mayhew 1979 langel et al 1980 comparison with a similar map from pogo data shows that the basic anomaly patterns are essentially reproducible except in a few regions such as the atlantic and pacific at latitudes south of 15 increased resolution due to a lower measurement altitude is evident in the magsat map

spatial resolution and repeatability of magsat crustal anomaly data over the indian ocean preliminary results from analysis of individual magsat tracks in the eastern indian ocean demonstrate the resolution capability and repeatability of these data spectrum analysis indicates that 250 km is the absolute lower limit for resolving crustal magnetic anomalies in magsat along track data spectral coherence analysis of closely spaced tracks shows that magnetic anomaly features with wavelengths greater than 700 km are highly repeatable from track to track

geophysical characteristics of the tornquist fan area northwest trans european suture zone indication of late carboniferous to early permian dextral transtension

precambrian continental crust of india and its evolution the indian precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes the bulk of this crust was formed prior to 2600 m y ago and remobilized at least twice between 2600 2000 m y ago early proterozoic mobile belt epmb and 2000 1500 m y ago middle proterozoic mobile belt mpmb three early precambrian nucleii karnataka kn jeypore bastar jbn and singhbhum sn appear to have survived in the craton and are characterized by low grade supracrustals and tonalitic trondhjemite gneisses formed 3800 2600 m y ago the epmb event involved sedimentation amphibolite granulite facies metamorphism and co 2 k metasomatism and produced amphibolite facies rocks and k granites in the north and charnockite and other granulite facies rocks in the south gold sporadically distributed in the supracrustal rocks of the craton was remobilized during the epmb event k granites form a garland around the central dharwar craton sugg

the role of rifting in the tectonic development of the midcontinent u s a abstract recent studies have proposed the existence of several major ancient rift zones in the midcontinent region of north america although the dating of some of these rifts and even the rift interpretations are subject to question an analysis of these paleo rifts reveals three major episodes of rifting keweenawan 1 1 b y b p eocambrian 600 m y b p and early mesozoic 200 m y b p the extent of these events documents that rifting has played a major role in the tectonic development of the midcontinent region this role goes well beyond the initial rifting event because these features display a strong correlation with paleozoic basins and a strong propensity for reactivation for example the eocambrian reelfoot rift was reactivated in the mesozoic to form the mississippi embayment and is the site of modern seismicity which suggests reactivation in a contemporary stress field of ene compression even though the importance of rifting can be established recognition of rifts and delineation of their complexities remain a major problem which requires more study

gravity study along the kavali udipi deep seismic sounding profile in the indian peninsular shield some inferences about the origin of anorthosites and the eastern ghats orogeny abstract the crustal section along a 600 km long profile across the indian peninsular shield between kavali on the east coast and udipi on the west coast running approximately along 14 n latitude can be represented by a generalised three layer crustal model in accordance with the interpretation of gravity data along this profile based on the structural picture obtained from deep seismic soundings dss kaila et al 1979 these three crustal layers are an upper layer of density 2 70 g cm 3 corresponding to an average p wave velocity v p of 6 2 km sec an intermediate layer of density 2 75 g cm 3 v p 6 5 km sec and a deeper layer of density 2 85 g cm 3 v p 6 8 km sec crust of varied thicknesses and comprising the above three layers sits in general on upper mantle rocks of 3 3 g cm 3 corresponding to v p 8 2 km sec the regional gravity anomaly pattern along the profile can convincingly be explained as being due to the combined effect of intracrustal structural features and the configuration of the moho boundary as developed through tectonic activities along a number of faults detected by dss in the eastern portion of the profile corresponding to the cuddapah basin there exists an additional deep crustal layer of density 3 05 g cm 3 v p 7 2 km sec and an emplacement of highdensity material at shallow depths along weak zones on the eastern margin of the cuddapah basin this magmatic high density mass at shallow depth may extend as a linear subsurface feature along the entire eastern ghats belt as evidenced by the continuity of the similar gravity anomaly pattern and may be responsible for the eastern ghats orogeny and uplift this feature may be responsible for the plutonic activity to which the origin of anorthosite rocks found along the eastern ghats can be associated in the western part of the cuddapah basin the basaltic layer has thickened considerably with its top reaching a shallow depth of about 8 km this thickening of the basaltic layer is considered responsible for the generation of numerous dykes and sills in this region

sensitivity studies with a coupled ice ocean model of the marginal ice zone an analytical coupled ice ocean model is considered which is forced by a specified wind stress acting on the open ocean as well as the ice the analysis supports the conjecture that the upwelling dynamics at ice edges can be understood by means of a simple analytical model in similarity with coastal problems it is shown that the ice edge upwelling is determined by the net mass flux at the boundaries of the considered region the model is used to study the sensitivity of the upwelling dynamics in the marginal ice zone to variation in the controlling parameters these parameters consist of combinations of the drag coefficients used in the parameterization of the stresses on the three interfaces atmosphere ice atmosphere ocean and ice ocean the response is shown to be sensitive to variations in these parameters in that one set of parameters may give upwelling while a slightly different set of parameters may give downwelling

evaluation of indian coastal response an integrated approach abstract since its separation 130 ma from the mainland the indian subcontinent has experienced major evolutionary changes a comparison of the structure evolution and tectonic elements of the east and west coast reveals characteristic differences thus while the eastern coast shows significant anomalous behaviour in all geophysical observations in contrast the corresponding observations on the western coast are relatively devoid of such anomalies in this investigation available magsat gravity electromagnetic gds and heatflow data have been utilized to search for a possible coherent cause for these different anomalies and thereby some inferences are sought to relate these causes to the geodynamics of this part of the indian plate

estimating surface wind direction over drifting open pack ice surface wind direction data vital for the study of drifting pack ice are not routinely available for polar oceans these data may be estimated from the difference between the direction of motion of detached ice floes as determined from sequential satellite images and the angle of sea ice deflection which varies with the geostrophic wind speed

multi genetic origin of the continental moho insights from lithoprobe this paper reviews four hypotheses for the origin of the continental crust mantle boundary and discusses seismic parameters with which these hypotheses might be tested the relict moho hypothesis posits that the oceanic moho is preserved during continental assembly the magmatic underplating hypothesis posits formation of a new moho by episodic emplacement of sill like intrusive bodies the metamorphic or metasomatic front hypothesis posits that the moho is overprinted by a phase transformation and the regional decollement hypothesis posits that the moho behaves as a structural detachment these hypotheses are not mutually exclusive and examples from the canadian lithoprobe program suggest that all four may be applicable in different regions of north america comparison of seismic images from a fossil subduction zone with modern subduction at cascadia suggests that serpentinization of the forearc mantle a previously unrecognized mechanism for overprinting and erasing the reflection moho may have occurred in the proterozoic

geophysical evidence for early permian igneous activity in a transtensional environment denmark abstract integrated interpretation of gravity magnetic and seismic data in the area of the strong silkeborg gravity high in central denmark has led to a structural geological model involving a transtensional rift feature which probably formed in connection with permo carboniferous wrench faulting and the development of the oslo horn graben system reinterpretation of the deep seismic data from profile 2 of the eugeno s project in the area of the silkeborg gravity high detected a new thin high velocity layer 6 25 km s at about 7 km deep within the palaeozoic sedimentary sequence in the crystalline basement a separate body of high velocity 6 9 7 0 km s was found at 11 km deep beneath this body the moho reaches its shallowest position in the area the seismic results compare well with an interpretation based on magnetism of a shallow volcanic or subvolcanic layer at a depth of 6 8 km this layer shows strong remanent and induced magnetization with a southward inclination of about 10 gravity reinterpretation of the area allows the separation of an upper body of density contrast 0 1 g cm3 from a lower body of density contrast 0 2 g cm3 the upper body roughly coincides with the magnetic body the lower body the major one coincides with the deep seismic body and it is responsible for the main gravity anomaly in the area

effects of asthenospheric heat flow in basin modelling exemplified with the danish basin a mathematical model is developed to simulate the process of basin formation by rifting subsidence is driven by crustal thinning and influenced by the transient temperature variation within the lithosphere the model offers possibilities for multiple phases of rifting superimposing non steady state conditions important features in the model are two layered heat production and a definition of the lower boundary in terms of heat flow instead of one of constant temperature as opposed to other models furthermore phase transformations caused by rift induced pressure and temperature changes are suggested as an additional syn rift subsidence mechanism the relevant phase transformations are assumed to be those occurring during an amphibolite to granulite facies transition perturbation of the steady state temperature profile is performed by one or several phases of rifting with finite stretching rates by sill intrusion or by changes in asthenospheric heat flow the greatest deviation from the steady state geotherm occurs at the lower boundary because of the flow definition of the lower boundary the consequence of this is a much longer duration of anomalous heat flow the model is applied to the danish basin north jylland denmark this basin was formed in permian times and since then it has been thermally subsiding apart from a minor event of stretching in the late cretaceous the higher present day heat flow in the basin compared to the adjacent fennoscandian shield area is successfully explained by the model presented in this paper it is concluded that transient temperature conditions still today contribute significantly to present surface heat flow although main rifting took place for more than 240 m y ago this cannot be explained by models using a fixed temperature definition of the lower boundary the modelled contribution by phase transformations to subsidence explains why the syn rift early and middle triassic subsidence was accompanied by insignificant faulting activity as evidenced by the seismic data available

precambrian tectonics of the south indian shield inferred from geophysical data geophysical parameters such as magsat landsat gravity deep seismic sounding seismicity and heat flow data have identified cratonic and mobile units in the precambrian continental crust of the south indian shield landsat imagery and gravity field data suggest that the shield is divided into four geophysical domains from north to south namely 1 the deccan trap covered gneissic basement and the gneiss terrain around hyderabad 2 the dharwar craton subdivided into its eastern and western blocks 3 the granulite terrains of kerala karnataka and tamil nadu and 4 the eastern ghats granulite belt the smooth transition in the gravity field from the dharwar greenstone terrain to the deccan trap covered area to the north indicates the northward continuity of the granite gneiss basement however the trap covered region lacks any significant short wavelength anomalies of the type associated with the dharwar greenstone belts indicating the probable absence of any such large scale high density bel

a coupled ice ocean model of upwelling in the marginal ice zone a dynamical coupled ice ocean numerical model for the marginal ice zone miz is suggested and used to study upwelling dynamics in the miz the nonlinear sea ice model has a variable ice concentration and includes internal ice stress the model is forced by stresses on the air ocean and air ice surfaces the main coupling between the ice and the ocean is in the form of an interfacial stress on the ice ocean interface the ocean model is a linear reduced gravity model the wind stress exerted by the atmosphere on the ocean is proportional to the fraction of open water while the interfacial stress ice ocean is proportional to the concentration of ice a new mechanism for ice edge upwelling is suggested based on a geostrophic equilibrium solution for the sea ice medium the upwelling reported in previous models invoking a stationary ice cover is shown to be replaced by a weak downwelling due to the ice motion most of the upwelling dynamics can be understood by analysis of the divergence of the across ice edge upper ocean transport on the basis of numerical model an analytical model is suggested that reproduces most of the upwelling dynamics of the more complex numerical model

heat flow in the indian peninsula its geological and geophysical implications abstract the distribution of surface heat flow in the various geological units of the indian peninsula is presented and discussed in the light of other related geodata the heat flow data show a large variation 26 to 107 mw m 2 but indicate in general a thermal regime characteristic of stable continental areas the dharwar schist belts greenstone belts and greenstone like geosynclinal piles arc characterised by low heat flow it appears very probable a that favourable conditions for the generation of the earth s crust and the melting near the surface developed at the very early stages of the earth s evolution and b that true greenstone belts which represent the relics of the primitive basic ultrabasic crust may have been more or less continuous once it is our contention that their association with low surface heat flow is primarily due to their initial evolution and growth the low heat flow values in green stone belts show a general increase in archaean proterozoic granitic and gneisic terrains the average heat flow in archaean areas is significantly lower than in proterozoic parts of the indian shield in general the average heat flow shows an inverse relation with the age of precambrian orogeny further the proterozoic geosynclinal belts in which sedimentation and mobility continued up to upper proterozoic have been found to mark the highest heat flow areas of precambrian india surprisingly the deccan traps the cretaceous paleocene basic effusives are associated with low heat flow values 40 mw m 2 the observed high heat flow in the northern part of a basin in which a full sequence of tertiary to recent sediments overlie the deccan traps has been ascribed to a shallow basic crustal miocene pliocene intrusive body the gondwanas which occur in intracratonic small isolated elongate trough like and semi elliptical depressions show a contrasting geothermal character with many zones of high surface heat flow for which there is no clear explanation at present most of the indian peninsula is comprised of three cratons the dharwar the aravallis and the singhbhum the dharwar craton appears to be associated with the lowest surface heat flow and the heat flow from the mantle and the lower crust is q r 23 mw m 2 this is followed by q r 30 mw m 2 for the aravalli craton and about 38 mw m 2 for the singhbhum craton thus for the same surface radioactivity and heat flow crustal temperatures are lowest in the dharwar craton for this craton the temperatures at the base of the crust are similar to those in the western australian shield and in the superior province of the canadian shield we further suggest that u th and k in the top radioactive layer probably do not follow the same concentration depth pattern i e the characteristic depth d for these heat producing radioactive elements is most likely different and is the main cause of a wide scatter in the values of q r for various heat flow heat production pairs within a heat flow province we demonstrate that it is not worthwhile to study the correlation of surface heat flow to the gravity anomaly in the indian peninsula without removing the effects of the pronounced indian ocean gravity low from the gravity anomalies surface heat flow q s and crustal thickness zm as obtained from dss and earthquake data show a positive slightly exponential correlation which is compatible with crustal properties and dynamics it is demonstrated that the correlation between q s and zm in precambrian shields can be positive or negative or even negligible depending upon their past geological history

estimation of the regional bouguer gravity field over the indian peninsula using two dimensional filtering abstract the regional bouguer gravity field for india has been obtained objectively by digitising the gravity data at 0 25 interval and by subjecting it to two dimensional symmetrical filtering with cutoff of 0 004 cycles km 200 km the data at continental edges were appended with co sinusiods of appropriate wave number to produce a uniform rectangular grid this process also avoids gibb s phenomenon the data set thus obtained was divided into four overlapping independent rectangles for the purpopse of filtering this process also provided a check on the filtered values the low pass filtered deeper and the residual shallower maps so obtained are correlated with geology and tectonic regimes of the india subcontinent the high gradients in the contours of the residual map and the gentle gradients in the regional map reflect the effects of shallower and mainly deeper structures respectively the regional field indicates 1 1 a characteristic difference between the eastern and the western coasts 2 2 a central high encompassing the godavari graben and satpura fold belt which bends syntaxially towards aravalli trend and 3 3 a nw se trending low in southern peninsular india the salient observations from the residual map are 1 1 a clear rift like structural signature with negative gravity values in the centre flanked by positive gravity values over the godavari graben 2 2 the ne sw aravalli trend merges with the e w satpura trend in the south 3 3 the gravity signature of the narmada son lineament is not very conspicuous and 4 4 an isolated patch of negative gravity values south of bangalore is prominent

a deep structural ridge beneath central india a joint inversion of magnetic satellite magsat and free air gravity data has been conducted to quantitatively investigate the cause for bouguer gravity anomaly over central indian plateaus and possible fold consequences beside himalayan zone in the indian sub continent due to collision between indian and eurasian plates the appropriate inversion with 40 km crustal depth model has delineated after discriminating high density and magnetisation models for the first time about 1500 km long hidden ridge structure trending nw se the structure is parallel to himalayan fold axis and the indian ocean ridge in the arabian sea a quantitative relief model across a representative anomaly profile confirms the ridge structure with its highest point nearly 6 km higher than the surrounding crustal level in peninsular india the ridge structure finds visible support from the astro geoidal contours

ocean feedback to wind driven coastal polynyas for lack of observations a coupled numerical model of an ice layer and a coastal ocean is used to conduct thought experiments on the evolution of wind driven coastal polynyas attention is focused on the possible ocean feedback to polynya developments before and after wind relaxation the coupled system is initially motionless having an ice layer of uniform concentration and thickness in the ocean the salinity is initially uniform and the temperature is at the freezing point subsequent cold air outbreaks of several days in duration push the ice layer offshore frazil is produced and collected in the polynya owing to continuous heat loss to the atmosphere ocean feedback is particularly effective after wind relaxation under large scale seaward winds the displaced ice edge does not return landward after wind abatement if the wind event is strong and long over a sizable coastal bottom obstacle however anticyclonic circulation develops and its landward arm may force a landward return of the ice edge after wind relaxation further dipole vortices in the ocean may develop under a mesoscale seaward wind event landward ocean currents around the rim of a dipole may also force a postwind return of the ice edge the ocean feedback is generally stronger and faster in shallower basins this can be demonstrated by a one dimensional stress driven coastal ocean model

geotectonic remobilisation of the lower crustal segment of southern peninsular india abstract deep crustal exposures on various continental platforms have recently attracted much attention with regard to their evolution and relevance to present day tectonics the granulite terrain of the southern part of peninsular india exhibits unique characteristics the small dimensions bounded by two rifted margins high relief as a result of recent uplift and anomalous gravity magnetic and electromagnetic signatures the geophysical anomalies over this region have been cohered to produce a tentative geodynamic model in which the break up of gondwana 150 my bp subsequent rifting of the western margin of india 70 my bp and the collision with asia and post collision stresses 50 my bp play a role in the remobilisation of the crust mantle regime of south india the geophysical signatures over this deep crustal cross section differ markedly from those of its counterparts in other parts of the world and thus may reflect the consequences of recent remobilisation rather than features frozen in since proterozoic time in this sense deep crustal imaging of southern peninsular india may differ from that in other continental platforms

crustal magnetisation model of the indian subcontinent through inversion of satellite data abstract an apparent magnetisation model of the indian subcontinent and the adjoining regions has been obtained through the inversion of magsat total field scalar and vertical field data a staggered 4 4 grid pattern with a constant continental crustal thickness of 40 km yielded magnetisation values over a 2 2 grid over the major part of the indian subcontinent the magnetisation picture brings out a prominent high sandwiched between the eastern ghats and the main boundary fault of the himalayas in contrast the identity of the western ghats comprising mainly deccan traps is missing while a distinct magnetisation low is observed over the known hot spot region of the saurashtra peninsula two magnetisation highs one over the shillong massif and the other over the central part of india covering aravallis and the bundelkhand massif are obtained interestingly even at a height of 400 km the well known narmada son lineament is clearly reflected in the total intensity map as well as in the corresponding magnetisation map the bay of bengal and the himalayas ara characterised by magnetisation lows

structure and evolution of the western continental margin of india deduced from gravity seismic geomagnetic and geochronological studies abstract available geochronological gravity seismic and geomagnetic data on the western continental margin of india between 8 20 n are presented based on the data three models describing the evolution of the crustal structure are developed model 1 describes the evolution of the crustal structure along 19 n the main basic igneous activity that accompanied the tectonic events since the late cretaceous resulted in the formation of rifts along the coast thus giving rise to the present day structure the acid volcanic event that preceded the above activity at about 93 ma ago is responsible for the observed crustal structure along 13 30 n shown in model 2 this event resulted in the formation of the st mary group of islands along this latitude counterclockwise rotation of the indian plate during the tertiary period caused rifting of the continental blocks giving rise to the laccadive kerala graben bounded by the chagos laccadive ridge on the west and the west coast fault on the east vertical movements of the rifted blocks promoted the deposition of both continental and marine sediments on the coast west of the west coast fault this tectonic event was accompanied by upwarping of the mantle crust thus giving rise to the present day structure shown in model 3

geothermal studies in the hyderabad granitic region and the crustal thermal structure of the southern indian shield based on temperature measurements in three boreholes one specially drilled for the purpose and thermal conductivity determinations heat flow density values were determined for three sites in the archaean hyderabad granitic batholith a mean heat flow density value of 40 1 s d mw m 2 has been obtained the heat generation in its rocks 5 57 w m 3 is significantly higher than in average crustal rocks it has been proposed that the hyderabad batholith has a layered structure with a thin 1 km surface layer of high radioactivity these results together with the already reported data have been used to estimate the conductive steady state temperature within and at the base of the crust of the southern indian shield yielding values of the same order as found in the western australian shield

magma traps and driving pressure consequences for pluton shape and emplacement in an extensional regime abstract the level of emplacement and final form of felsic and mafic igneous rocks of the wichita mountains igneous province southwestern oklahoma u s a are discussed in light of magma driving pressure lithostatic load and crustal magma traps deposition of voluminous a type rhyolites upon an eroded gabbroic substrate formed a subhorizontal strength anisotropy that acted as a crustal magma trap for subsequent rising felsic and mafic magma intruded along this crustal magma trap are the a type sheet granites length thickness 100 1 of the wichita granite group of which the mount scott granite sheet is typical and smaller plutons of biotite bearing roosevelt gabbro in marked contrast to the subhorizontal granite sheets the gabbro plutons form more equant stocks with flat roofs and steep side walls late diabase dikes cross cut all other units but accompanying basaltic flows are extremely rare in the volcanic pile based on magmastatic calculations we draw the following conclusions concerning the level of emplacement and the shape of these intrusions 1 magma can rise to a depth at which the magma driving pressure becomes negligible magma that maintains a positive driving pressure at the surface has the potential to erupt 2 magma ascent may be arrested at a deeper level in the crust by a subhorizontal strength anisotropy i e crustal magma trap if the magma driving pressure is greater than or equal to the lithostatic load at the depth of the subhorizontal strength anisotropy 3 subhorizontal sheet intrusions form along crustal magma traps when the magma driving pressure greatly exceeds the lithostatic load under such conditions the magma driving pressure is sufficent to lift the overburden to create the necessary space for the intrusion 4 thicker steep sided stocks or batholiths with flat roofs form at crustal magma traps when the magma driving pressure approximates that of the lithostatic load under these conditions the necessary space for the intrusion must be created by other mechanisms e g stoping 5 subvertical sheets i e dikes form when the magma driving pressure is less than the lithostatic load at the level of emplacement