calculated mineral equilibria for eclogites in cao na2o feo mgo al2o3 sio2 h2o application to the pouebo terrane pam peninsula new caledonia the high p medium t pouebo terrane of the pam peninsula northern new caledonia includes barroisite and glaucophane bearing eclogite and variably rehydrated equivalents the metamorphic evolution of the pouebo terrane is inferred from calculated p t and p t xh2o pseudosections for bulk compositions appropriate to these rocks in the model system cao na2o feo mgo al2o3 sio2 h2o the eclogites experienced a clockwise p t path that reached p 19 kbar and t 600 c the eclogitic mineral assemblages are preserved because reaction consequent upon decompression consumed the rocks fluid extensive reaction occurred only in rocks with fluid influx during decompression of the pouebo terrane

a recipe for microcontinent formation accreted slivers of continental margins are common in the geologic record but the processes that lead to their formation are poorly understood we observe an association of plume related microcontinent isolation and subsequent long term asymmetries in oceanic crustal accretion based on four recent examples the seychelles in the indian ocean jan mayen in the norwegian greenland sea and the east tasman plateau and the gilbert seamount complex in the tasman sea these microcontinents formed by rerifting of a young continental margin 25 m y old in the vicinity of a mantle plume stem followed by asymmetric seafloor spreading two dimensional numerical stochastic basin modeling suggests that a yield strength minimum along the landward edge of a rifted margin thermally enhanced by heating from a mantle plume may cause a spreading ridge to jump onto this zone of weakness this action isolates a passive margin segment the association of large igneous provinces and microcontinents should be useful for identifying similar events in the geological record

the relationship between rifting and magmatism in the northeastern arabian sea the rifting of the seychelles microcontinent from india involved two phases of extensional activity the initial separation of the laxmi ridge from india was accompanied by extensive magmatism but the later separation of the seychelles from the laxmi ridge was only weakly magmatic the causal mechanisms linking continental flood basalts lithospheric extension and mantle plumes as well as the relative timing of extension and volcanism are controversial1 2 3 4 the eruption of the deccan flood basalts was approximately contemporaneous with the separation of the seychelles microcontinent from india however between these continental blocks lies the enigmatic laxmi ridge and the sequence of extensional events that formed these various tectonic elements is poorly understood here we present wide angle seismic data along a profile across laxmi ridge that permit delineation of offshore igneous bodies associated with the deccan magmatism these bodies are similar to those associated with flood basalt volcanism and rifting in the atlantic region and elsewhere1 from the geometry of these bodies we infer that there were two periods of extension the first phase which involved extension between laxmi ridge and the indian subcontinent was accompanied by significant deccan related magmatism full development of a continental margin was achieved during the second phase of weakly magmatic extension between laxmi ridge and the seychelles we suggest that between these rifting events the region passed beyond the reach of lateral flow from the source region of the deccan flood basalts

lawsonite omphacite bearing metabasites of the pam peninsula ne new caledonia evidence for disrupted blueschist to eclogite facies conditions

gravity anomalies and associated tectonic features over the indian peninsular shield and adjoining ocean basins abstract a combined gravity map over the indian peninsular shield ips and adjoining oceans brings out well the inter relationships between the older tectonic features of the continent and the adjoining younger oceanic features the nw se ne sw and n s precambrian trends of the ips are reflected in the structural trends of the arabian sea and the bay of bengal suggesting their probable reactivation the simple bouguer anomaly map shows consistent increase in gravity value from the continent to the deep ocean basins which is attributed to isostatic compensation due to variations in the crustal thickness a crustal density model computed along a profile across this region suggests a thick crust of 35 40 km under the continent which reduces to 22 20 24 km under the bay of bengal with thick sediments of 8 10 km underlain by crustal layers of density 2720 and 2900 2840 kg m3 large crustal thickness and trends of the gravity anomalies may suggest a transitional crust in the bay of bengal up to 150 200 km from the east coast the crustal thickness under the laxmi ridge and east of it in the arabian sea is 20 and 14 km respectively with 5 6 km thick tertiary and mesozoic sediments separated by a thin layer of deccan trap crustal layers of densities 2750 and 2950 kg m3 underlie sediments the crustal density model in this part of the arabian sea east of laxmi ridge and the structural trends similar to the indian peninsular shield suggest a continent ocean transitional crust cotc the cotc may represent down dropped and submerged parts of the indian crust evolved at the time of break up along the west coast of india and passage of reunion hotspot over india during late cretaceous the crustal model under this part also shows an underplated lower crust and a low density upper mantle extending over the continent across the west coast of india which appears to be related to the deccan volcanism the crustal thickness under the western arabian sea west of the laxmi ridge reduces to 8 9 km with crustal layers of densities 2650 and 2870 kg m3 representing an oceanic crust

evidence for seafloor spreading in the laxmi basin northeastern arabian sea abstract marine magnetic anomaly data from the laxmi basin for the first time reveal the presence of fairly correlatable nnw trending magnetic lineations these magnetic lineations are symmetric about a central negative magnetic anomaly and the axis of symmetry coincides with a characteristic short wavelength free air gravity low the anomalies are interpreted as representing a two limbed seafloor spreading sequence which can be equated to the a28 a33 interval of the geomagnetic polarity reversal timescale these results suggest that the laxmi basin is underlain by an oceanic crust evidence of seafloor spreading in this basin possibly implies a pre a27 spreading episode in the evolutionary history of the arabian sea

tectono magmatic evolution of the west coast of india abstract the west and east coasts of india wci eci have distinct histories of their own the wci originated subsequent to eci which has the imprint of two hotspots marion and reunion evolved through several stages of rifting magmatism and isostatic movements important among them are felsic magmatism associated with doming 93 ma mafic magmatism related to rifting 88 ma origin of the western ghats of india and the east facing scarp of madagascar all the three related to separation of madagascar from india mafic deccan volcanism in the north western parts of india 67 ma rifting of seychelles micro continent and lava cover from the north western parts of india along the carlsberg ridge 62 ma a 27 isostatic subsidence relating to loading of deccan basalts subsidence of bombay offshore region due to reactivation of sonata rift separation of laccadive chagos ridge from the southern part of mascarene plateau because of shifting of the central indian ridge 40 ma buckling of south india and tilting of the peninsula northward due to collision and subduction these events make the wci unique and endowed with a great deal of dynamism

magmatic underplating of crust beneath the laccadive island nw indian ocean summary we investigate the crust and uppermost mantle velocity structure beneath the laccadive island through the inversion of teleseismic receiver functions following the neighbourhood algorithm the velocity structure suggests that the 16 km thick oceanic crust is underplated by 8 km thick high velocity layer vs 4 25 4 4 km s 1 vp 7 35 7 6 km s 1 the uppermost mantle shear velocity is 4 6 km s 1 vp 8 18 km s 1 the moho at 24 km is possibly formed due to underplating of mantle derived picritic melt at the base of oceanic crust produced during the interaction of the indian plate with the reunion hotspot continuity of crustal underplated material beneath the laccadive could be traced to the laxmi ridge in the north and to the reunion island in the south suggesting this to be possible trace of the reunion hotspot

fractionation of bulk rock composition due to porphyroblast growth effects on eclogite facies mineral equilibria pam peninsula new caledonia chemically zoned porphyroblasts in metamorphic rocks indicate that diffusional processes could not maintain equilibrium conditions on a grain scale during porphyroblast growth or establish it afterwards an effect of this inability to maintain equilibrium is the progressive removal of elements forming garnet cores from any metamorphic reaction that occurs at the porphyroblast boundaries or in the matrix of the rock to examine this effect on mineral assemblages the bence albee matrix correction was applied to x ray intensity maps collected using eclogite samples from northern new caledonia in order to determine the chemical composition of all parts of the sample the manipulation of these element maps allows a quantitative analysis of the fractionation of the bulk rock composition between garnet cores and the matrix a series of calculated equilibrium volume compositions represents the change in matrix chemistry with progressive elemental fractionation as a consequence of prograde garnet growth under high p conditions pressure temperature pseudosections are calculated for these compositions in the cao na2o feo mgo al2o3 sio2 h2o system assemblages modal proportions and mineral textures observed in the new caledonian eclogites can be closely modelled by progressively removing elements forming garnet cores from the bulk rock composition the pseudosections demonstrate how chemical fractionation effects the peak metamorphic assemblage prograde textures and the development of retrograde assemblages

hydration of eclogite pam peninsula new caledonia garnet glaucophanite and greenschist facies assemblages were formed by the recrystallization of barroisite bearing eclogite facies metabasites in northern new caledonia the mineralogical evolution can be modelled by calculated p t and p xh2o diagrams for appropriate bulk rock compositions in the model system cao na2o feo mgo al2o3 sio2 h2o the eclogites having developed in a clockwise p t path that reached p 19 kbar and t 590 c underwent decompression with the consumption of free h2o as the volume of hydrous minerals increased eclogite is preserved in domains that experienced no fluid influx following the loss of this fluid garnet glaucophanite formed at p 16 kbar during semi pervasive fluid influx fluid influx after further isothermal decompression was focused in shear zones and resulted in chlorite albite bearing greenschist facies mineral assemblages that reflect p 9 kbar

nature of the crust in the laxmi basin 14 20 n western continental margin of india 1 the nature of the crust in the laxmi basin western margin of india is an uncertain issue more importantly this has implications on paleogeographic reconstructions of the western indian ocean we have analyzed three geophysical data sets and modeled gravity and magnetic anomalies for determining nature of the crust basement of the laxmi basin includes numerous highs which make the basement uneven and shallower compared to the western basin the laxmi basin is characterized by a broad gravity high and a narrower prominent gravity low within it while within the basin the broad anomaly gradually increases toward north the panikkar ridge is associated with the gravity low which is comparable at least in sign to known negative gravity anomaly of the laxmi ridge intrusive structures mapped in the laxmi basin coincide with significant magnetic anomalies which were earlier interpreted as seafloor spreading anomalies model studies reveal that the laxmi basin consists of 14 km thick stretched continental crust in which magmatic bodies have been emplaced whereas the panikkar ridge remains less altered stretched continental crust the crust of the laxmi basin is mostly thinner than crust under the laxmi ridge and continental margin in addition to the rift drift related stretching of the continental margin the laxmi basin possibly has undergone extra stretching in e w direction during the pre tertiary period at 68 ma deccan volcanism on western india may have disrupted the initial conditions that were leading to onset of spreading in the basin subsequently the reunion hot spot had emplaced the volcanic material within the stretched thinned continental crust we interpret the laxmi basin as a failed rift undergone stretching following intraplate kinematics prior to deccan volcanism

crustal structure of the indian shield new constraints from teleseismic receiver functions teleseismic earthquake waveform data from 10 broadband stations spread over the indian shield and in operation since 1997 were analyzed to infer the crustal structure using the receiver function technique the south indian shield is characterized by a 33 39 km thick and remarkably simple crust with an average poisson s ratio close to 0 25 the archaean crust is devoid of any prominent intra crustal discontinuities the velocity contrast at the well developed moho is large resulting in very clear p to s conversions as well as first order multiples in contrast the predominantly proterozoic crust in the northern part of the shield exhibits a complex character due to the presence of additional seismic discontinuities moho conversions which are considerably weaker compared to the archaean terrains indicate crustal thicknesses of more than 40 km

teleseismic tomographic evidence for contrasting crust and upper mantles in south indian archaean terrains abstract we compare the velocity structure of the crust and upper mantle of the archaen granulite terrain of southernmost india with that of the archaean granite greenstone dharwar craton terrain just to the north through p wave teleseismic travel time measurements the teleseismic rays recorded by a coarse network of 11 portable seismic stations show an anomalous pattern of late arrivals delays over the granulite terrain in contrast to fast arrivals on the dharwar craton such a pattern of time residuals amongst these archaean terrains which have remained inert in the last 2 ga may indicate the presence of compositional heterogeneity within the crust and upper mantle beneath them three dimensional p wave velocity tomography using teleseismic rays from a variety of azimuths indicates the existence of contrasting p wave crust and upper mantle velocity patterns 1 in the crust 0 40 km the western dharwar craton and the granulite terrain have lower velocity up to 3 compared with the higher velocity 1 3 observed over the eastern dharwar craton and 2 in the upper mantle 40 177 km there is 2 3 lower velocity beneath the granulite terrain compared with the western dharwar craton the existence of such lateral velocity variation in the crust and upper mantle and its preservation since late archaean times points to the presence of possibly thick and chemically distinct lithospheres that did not participate in mantle convection

extension of structural and tectonic trends from the indian subcontinent into the eastern arabian sea abstract trends of basement highs faults and structural patterns on the india pakistan indus continental shelf and slope and in the deep waters extending to the lakshmi laxmi chagos laccadive ridge complex in the eastern arabian sea are parallel to the ancient precambrian structural and tectonic grain of the indian subcontinent these trends the occurrence of horst graben style features of the basement of the lakshmi ridge and of the region north and east of it both in deep waters and shelf areas and the oceanic nature of the basement west of the ridge suggest that the rifted transitional crust extends as far as the lakshmi chagos laccadive ridge complex the extension of rifted crust with horst graben style tectonic features into deep offshore areas of the arabian sea has implications for future hydrocarbon exploration in the region

estimation of effective elastic thickness from gravity and topography data under the deccan volcanic province india abstract effective elastic thickness eet under the deccan volcanic province is estimated based on admittance and coherence between the gravity field and the topography the observed admittance function shows a good fit for the regional compensation model with an eet of 10 2 km the effect of a long wavelength geoidal gravity anomaly on the computation of eet has been investigated and found not to significantly influence the value of eet although a considerable change in compensation depth was noticed the estimated eet is less than the thickness of the seismically active layer in this region these results are consistent with observations in other parts of the world that eet should be smaller than the depth of the 350 100 c isotherm

smnd ages of archaean metavolcanics of the dharwar craton south india abstract well constrained sm nd arrays of whole rocks are reported for the first time from three important stratigraphic units of the dharwar volcano sedimentary sequence of south india if interpreted as isochrons these arrays will correspond to the following ages and initial nd ratios relative to chur 2 911 0 049 ga and 0 20 0 40 for the kalasapura mafic basal metavolcanics of the bababudan greenstone belt 2 848 0 070 ga and 0 88 0 82 for the santaveri formation comprising mafic to felsic metavolcanics overlying the kalasapura rocks and 2 747 0 015 ga and 1 72 0 10 for the ingaldhal mafic felsic metavolcanic suite of the chitradurga greenstone belt the possibility that these sm nd arrays are mixing lines between unrelated components depleted and enriched respectively in light rare earth elements has been discussed leading to the conclusion that the ages closely correspond to their time of eruption the older suites from the bababudan belt from a mantle source of chur like composition the younger suite from the chitradurga belt from one with a prior history of lree depletion

eclogites and blueschists of the pam peninsula ne new caledonia a reappraisal high p rocks of the pam peninsula ne new caledonia are divided into three zones 1 an uppermost ferroglaucophane lawsonite zone of cretaceous to eocene metasediments and metavolcanics of the diahot terrane that experienced peak conditions involving p 7 9 kbar and t 400 58 c 2 albite epidote omphacite zone diahot terrane rocks that experienced blueschist facies conditions of p 12 6 1 2 kbar and t 570 36 c 3 lowermost metabasic eclogites of uncertain age that form the pouebo terrane which experienced high p conditions of p 23 9 3 0 kbar and t 600 c eclogite occurs as metre to kilometre scale pods in coarse grained hydrous mineral rich glaucophanite formed during hydration and decompression of the pouebo terrane metamorphism and deformation were consequent to 44 51 ma eocene convergence when sedimentary and ophiolitic nappes were thrust over the eclogites in a sw direction white mica ages constrain metamorphism to have ended by 37 1 ma large steps in metamorphic grade are coincident with sw dipping and ne dipping faults that separate the three zones and were formed during two stages 1 comparatively slow uplift and hydration of the pouebo terrane before it was juxtaposed with the albite epidote omphacite zone at p 14 kbar 2 comparatively rapid uplift of both the pouebo terrane and the albite epidote omphacite zone to form a domal core of eclogite flanked by significantly lower grade rocks to the sw and ne

structure and tectonics of the southwestern continental margin of india abstract a nnw sse trending graben in the inner shelf 40 60 m isobaths a mid shelf basement ridge 60 150 m isobaths a shelf margin basin 150 1000 m isobaths and the prathap ridge complex in the slope 1000 2000 m isobaths extend as linear features along the southwestern continental margin of india between 12 20 n and 15 00 n the basic structural framework of the margin seems to be influenced by the pre existing precambrian structural fabric of the continent the mid shelf basement ridge and the prathap ridge complex are interpreted as manifestation of the rift phase volcanism when india separated from madagascar around 84 ma

crustal structure from deep seismic soundings along the koyna ii kelsi loni profile in the deccan trap area india abstract the crustal depth section obtained from deep seismic soundings along the koyna ii kelsi loni profile which lies near latitude 18 n roughly in the east west direction in that part of the deccan trap maharashtra state india shows a number of reflection segments below the deccan traps down to the moho discontinuity a deep fault below the deccan traps 13 km east of mahad divides the entire cross section including the moho boundary into two crustal blocks the reflection segments show updip towards the west coast in the western block the moho discontinuity which is at a depth of 39 km near the deep fault starts rising towards the coast reaching a depth of 31 5 km at the west coast the eastern block is thrown up by 1 5 km with respect to the western block along the deep fault a structural contour map of the moho discontinuity for the koyna reservoir area has been prepared from the present results and the crustal information obtained along the koyna i profile kaila et al 1979a shows that the deep fault in the koyna area is aligned in the nnw sse direction refraction seismic data analysis by the wave front method reveals that the thickness of the deccan trap increases towards the west coast the deccan trap is 600 700 m thick in the eastern region between nira sp 130 and loni sp 200 and attains a thickness of 1500 m at 10 km east of the west coast the longitudinal wave velocity in the deccan traps along the profile varies from 4 8 to 5 0 km sec and in the crystalline basement from 6 0 to 6 15 km sec a tentative isopach contour map of the deccan traps and a tentative structural contour map of the pre deccan trap contact have been prepared for the koyna reservoir area from the results along the koyna ii and koyna i profiles a flexure aligned in a nnw sse direction in the pre deccan trap contact which is an expression of the deep fault into the basement has been clearly brought out the flexure coincides in general with the orientation of the deccan volcanic scarp in this area

the importance of rift history for volcanic margin formation rifting and magmatism are fundamental geological processes that shape the surface of our planet a relationship between the two is widely acknowledged but its precise nature has eluded geoscientists and remained controversial largely on the basis of detailed observations from the north atlantic ocean mantle temperature was identified as the primary factor controlling magmatic production with most authors seeking to explain observed variations in volcanic activity at rifted margins in terms of the mantle temperature at the time of break up however as more detailed observations have been made at other rifted margins worldwide the validity of this interpretation and the importance of other factors in controlling break up style have been much debated one such observation is from the northwest indian ocean where despite an unequivocal link between an onshore flood basalt province continental break up and a hot spot track leading to an active ocean island volcano the associated continental margins show little magmatism here we reconcile these observations by applying a numerical model that accounts explicitly for the effects of earlier episodes of extension our approach allows us to directly compare break up magmatism generated at different locations and so isolate the key controlling factors we show that the volume of rift related magmatism generated both in the northwest indian ocean and at the better known north atlantic margins depends not only on the mantle temperature but to a similar degree on the rift history the inherited extensional history can either suppress or enhance melt generation which can explain previously enigmatic observations

nukundamite cu3 38fe0 62s4 bearing copper ore in the bingham porphyry deposit utah result of upflow through quartzite nukundamite is an important ore mineral in a geologic resource of approximately 90 million metric tons mt of 0 8 wt percent cu in quartzite on the northeast contact of the bingham stock in contrast with other deposits where nukundamite occurs as a minor phase in relatively low temperature sericitic assemblages nukundamite at bingham is widespread in higher temperature potassic assemblages the coexistence of this high sulfidation sulfide with potassic assemblages suggests an unusual evolutionary path for magmatic hydrothermal fluids in quartzite wall rocks nukundamite bearing copper ore restricted to a 240 m thick quartzite unit of the pennsylvanian bingham mine formation extends 170 m from the northeast margin of the mineralized stock along the strike of beds and plunges 1 km down the dip of beds 30 nnw the quartzite unit is cut off at depth by a thrust fault below which beds of the bingham mine formation are vertical and cut by abundant northeast striking steeply dipping faults dikes and isoclinal folds the fortuna zone ore forming hydrothermal assemblages in quartzite can be classified as main and late stage main stage minerals are predominantly disseminated quartz phlogopite k feldspar with nukundamite replaced by chalcopyrite and bornite and no pyrite replacement of nukundamite by chalcopyrite and bornite reduced its abundance from possibly as high as 90 to 10 vol percent of total sulfides at the end of the main stage late stage assemblages consist of quartz sericite kaolinite with abundant pyrite lesser amounts of bornite digenite enargite and traces of nukundamite based on experimental data and retrieved thermodynamic properties nukundamite is stable within a highly limited range of a fe 2 a h 2 f s2 and f o2 at temperatures between 501 and 223 c at very high sulfidation states values of a fe 2 a h 2 in the nukundamite field are also appropriate for the stable assemblages chalcopyrite bornite and chalcopyrite pyrite but nukundamite requires higher values of f s2 and f o2 than the latter more common assemblages phase equilibria indicate that nukundamite bearing sulfide assemblages of the main stage are metastable supersaturated with respect to pyrite we hypothesize that early brine and vapor known to be of magmatic origin on the basis of isotopic and fluid inclusion evidence traveled upward through several kilometers of quartzite along the stock contact and the fortuna zone and then up to the south southeast along bedding at its source region the brine had so2 g h2s g 1 typical of degassing magmas of intermediate composition on cooling and due to lack of redox buffers in quartzite the brine became increasingly oxidized and evolved to so2 g h2s g 0 1 at 450 c at constant f s2 precipitation of nukundamite with k feldspar phlogopite at temperatures below 400 c so2 g h2s g 0 01 was possible due to relatively high concentrations of s and k in the brine s aq 0 2 m k 0 5 m also resulting from lack of exchange reactions with quartzite subsequent replacement of nukundamite by bornite chalcopyrite during the late main stage required a different fluid at lower sulfidation and oxidation states and slightly higher a fe 2 a h 2 we suggest that this late main stage fluid rising within the faulted northeast margin of the stock had been enriched in iron and depleted in sulfur during main stage potassic alteration and ore deposition in the stock

structure and early evolution of the arabian sea and east somali basin summary the laxmi ridge is a large scale basement high buried beneath the sediments of the indus fan the location of the ocean continent transition oct on this margin has previously been proposed at either the southern edge of the laxmi ridge or beyond it towards the india pakistan shelf the former explains the margin parallel laxmi basin as thinned continental crust the latter as a failed rift of earlier seafloor spreading to examine the structure of this margin a reassessment of marine magnetic data has detailed seafloor spreading magnetic anomalies prior to anomaly 24 in both the arabian and east somali basins the previously identified anomaly 28 is not interpreted as a seafloor spreading anomaly but as a magnetized basement feature adjacent to and merging with the ridge the laxmi spur new gravity models across the laxmi ridge and adjacent margin using ship and satellite data corroborate the existence of underplated crust beneath the laxmi ridge and basin and the location of the oct at the southern edge of the ridge the results are not compatible with the existence of a pre anomaly 28 phase of seafloor spreading although large scale intrusions may be the origin of some of the basement features in the laxmi basin the models also identify the laxmi spur as a low density feature with a natural remanent magnetization nrm compatible with serpentinization the laxmi ridge is mapped to the southeast where it appears to terminate at a point coinciding with the appearance of e w magnetic lineations and gravity anomalies at 15 5 n thereafter it becomes indistinct this is interpreted as necessary in the reconstruction to the mascarene plateau to avoid continental overlap

three dimensional lithospheric structure of the western continental margin of india constrained from gravity modelling implication for tectonic evolution this paper describes a 3 d lithospheric density model of the western continental margin of india wcmi based on forward modelling of gravity data derived from satellite altimetry over the ocean and surface measurements on the indian peninsula the model covers the north eastern arabian sea and the western part of the indian peninsula and incorporates constraints from a wide variety of geophysical and geological information salient features of the density model include 1 the moho depth varying from 13 km below the oceanic crust to 46 km below the continental interior 2 the lithosphere asthenosphere boundary lab located at depths between 70 km in the southwestern corner under oceanic crust and about 165 km below the continental region 3 thickening of the crust under the chagos laccadive and laxmi ridges and 4 a revised definition of the continent ocean boundary the 3 d density structure of the region enables us to propose an evolutionary model of the wcmi that revisits earlier views of passive rifting the first stage of continental scale rifting of madagascar from india at about 90 ma is marked by relatively small amounts of magmatism a second episode of rifting and large scale magmatism was possibly initiated around 70 ma with the opening of the gop rift subsequently at around 68 ma the drifting away of the seychelles and formation of the laxmi ridge was a consequence of the down faulting of the northern margin during this second episode of rifting the northern part of the wcmi witnessed massive volcanism attributed to interaction with the reunion hotspot at around 65 ma subsequent stretching of the transitional crust between about 65 and 62 ma formed the laxmi basin the southward extension of the failed gop rift as the interaction between plume and lithosphere continued the chagos laccadive ridge was emplaced on the edge of the nascent oceanic crust rifted continental margin in the south as the indian plate was moving northwards

new insights into the basement structure of the west siberian basin from forward and inverse modeling of grace satellite gravity data 1 the oil and gas rich west siberian basin is underlain by a layer of flood basalts of late permian triassic age that are coeval with the siberian traps the extent and thickness of the basalts are unknown but knowing their thickness is important for discussions on the end permian mass extinction because basalt volume constrains estimates of emitted volatiles we have used grace satellite and terrestrial gravity data to study the structure of the crust and basalt distribution published seismic sections are used to constrain the sediment isopachs and to estimate a depth density function we use published models of crustal thickness and basement depth to reduce the observed gravity field to the basement level the resulting three dimensional density model gives information on density anomalies in the lower crust and upper mantle and on the basalt thickness we identify several rift graben structures that are presumably filled with basalt the lower crust below the west siberian basin shows considerable density variations and these variations allow the region to be divided into four major blocks the eastern part of the basin toward the siberian platform shows an arch shaped density increase in the lower crust that is accompanied by a linear high density anomaly at shallower depths our work demonstrates the way in which the grace gravity field can be applied to map geological structures like buried rifts and large basins the same techniques can be used for other large remote basins such as those in cratonic south america

continental geotherm and the evolution of rifted margins models of melting accompanying mantle upwelling predict far more melt than is observed at nonvolcanic margins the discrepancy may be explained if the paradigm of a uniform asthenosphere is incorrect work on the velocity structure of the continents has shown that the convecting sublithospheric mantle may have a potential temperature as low as 1200 c 100 c cooler than that beneath the oceans the continental geotherm derived from studies on xenoliths brought up by kimberlites is also compatible with a cool sublithospheric mantle except where perturbed by mantle plumes upwelling of such cool mantle during rifting leads to little melt production even for rapid extension rates explaining the formation of amagmatic margins away from mantle plumes however the transition to seafloor spreading and the development of normal thickness oceanic crust requires the invasion of the amagmatic rift by hotter oceanic asthenosphere and or plume material this influx may cause a transient thermal uplift recorded as a breakup unconformity conversely at volcanic margins the onset of seafloor spreading is accompanied by the escape of the hot plume puddle along the mid ocean ridge system away from the volcanic margin leading to a pulse of rapid subsidence

sulfide oxidation as a process for the formation of copper rich magmatic sulfides typical magmatic sulfides are dominated by pyrrhotite and pentlandite with minor chalcopyrite and the bulk atomic cu fe ratio of these sulfides is typically less than unity however there are rar

a receiver function perspective of the dharwar craton india crustal structure summary teleseismic data from six broad band stations on the south indian shield have been analysed primarily to examine the differences in the crustal structure between the eastern and western units of the archaean dharwar craton sv receiver functions for these stations have been computed and modelled down to the moho level in terms of poisson s ratio and shear wave velocities results show that the crust for the entire dharwar craton is mainly simple and has a low about 0 25 poisson ratio it is usually thinner and less complex than the adjacent proterozoic crust however the western dharwar craton crust thickness 41 km with a gradational moho boundary is substantially 7 km thicker than its eastern counterpart thickness 34 km no substantial differences in average crustal s velocities 3 6 3 8 km s 1 were found the eastern dharwar crust below the proterozoic cuddapah basin was also found to have remained relatively simple and undisturbed the continental margin to the west of the dharwar craton appears to have shifted further west off the coast where a possible west coast fault has down thrown a continental crustal block under the seas the crust constituting the deccan volcanic province in the western indian shield is found to be similar to that of the eastern dharwar craton the crust underneath the neighbouring godavari graben is significantly different from the dharwar crust and resembles that of a typical rift valley

prograde sulfide metamorphism in blueschist and eclogite new caledonia in new caledonia blueschist and eclogite preserve as inclusions in porphyroblastic minerals a record of sulfide present during prograde subduction processes sulfide inclusions in prograde garnet and lawsonite became chemically isolated from the matrix whereas sulfide minerals in the matrix continued to equilibrate with matrix fluids or grew later during retrogression cu fe sulfide mineral inclusions have been found across metamorphic grade within silicate defined metamorphic mineral zones spanning a crustal profile of 30 km bulk area scans of sulfide inclusions provide compositions that represent mixtures of the solid sulfide that were included as the host silicate minerals grew in general single sulfide inclusion compositions and aggregate sulfide assemblages are distinct from those of matrix phases high cu contents in sulfide inclusions are interpreted to be a consequence of fe lost from sulfide to growing garnet rather than the result of intrinsically high cu in the bulk rock the distribution of sulfide inclusion compositions across metamorphic grade considered together with the available thermodynamic data suggests that covellite nukundamite bearing inclusions in lawsonite high in both cu and s disappear at higher grades as these sulfide minerals are no longer stable similarly clustering near the ratio fe cu 1 1 may cease with increasing grade owing to the replacement of chalcopyrite intermediate solid solution iss by the denser assemblage pyrite pyrrhotite bornite digenite

age of seychelles india break up many continental flood basalt provinces are spatially and temporally linked with continental break up establishing the relative timing of the two events is a key step in determining their causal relationship here we investigate the example of the deccan traps and the separation of india and the seychelles whilst there has been a growing consensus as to the age of the main phase of the deccan emplacement 65 5 1 ma chron 29r the age of the rifting has remained unclear we resolve this issue through detailed seafloor magnetic anomaly modeling supported by wide angle and reflection seismic results of the north seychelles and conjugate laxmi ridge gop rift margins and geochemistry and 40ar 39ar geochronology of rocks from the north seychelles margin we show that syn rift volcanics offshore the seychelles islands in the form of seaward dipping reflectors were most likely erupted during chron 28n and the first organized seafloor spreading at the carlsberg ridge also initiated during this chron at 63 4 ma the severing of the seychelles occurred by a south eastward ridge propagation that was completed by the start of chron 27n 62 ma a brief pre 28r phase of seafloor spreading occurred in the gop rift possibly as early as 31r 32n 71 ma initial extension at the margin therefore preceded or was contemporaneous with the deccan emplacement and separation of the seychelles was achieved less than 3 5 ma afterwards this is the shortest time interval between flood basalt emplacement and break up yet reported for any continental flood basalt rifted margin pair a contributing factor to the apparently short interval in the deccan case may be that rifting occurred by a ridge jump into already thinned continental lithosphere however we conclude that external plate boundary forces rather than the impact of a mantle plume were largely responsible for the rifting of the seychelles from india

global marine gravity field from the ers 1 and geosat geodetic mission altimetry satellite altimetry from the geosat and the ers i geodetic missions provide altimeter data with very dense spatial coverage therefore the gravity field may be recovered in great detail as neighboring ground tracks are very closely distributed cross track variations in the sea surface heights are extremely sensitive to sea surface variability to avoid errors in the gravity field caused by such effects sea surface variability needs to be carefully eliminated from the observations initially a careful removal of gross errors and outliers was performed and the tracks were fitted individually to a geoid model and crossover adjusted using bias and tilt subsequently sea surface heights were gridded using local collocation in which residual ocean variability was considered the conversion of the heights into gravity anomalies was carried out using the fast fourier transform fft in this process filtering was done in the spectral domain to avoid the so called orange skin characteristics comparison with marine gravity was finally carried out in three different regions of the earth to evaluate the accuracy of the global marine gravity field from ers 1 and geosat

reunion hotspot magma chemistry over the past 65 m y results from leg 115 of the ocean drilling program leg 115 of the ocean drilling program recovered basalts from four locations along the hotspot track that leads from the deccan flood basalts in india to reunion island in the western indian ocean sites 706 707 713 and 715 the drilled basalts range in age from 35 ma site 706 to 64 ma site 707 and including the deccan basalts 66 to 68 ma mauritius island 0 2 to 8 ma and reunion island 0 to 2 ma seven sites are provided for sampling the volcanic record of the 5 000 km long hotspot track chemical and age comparisons indicate that site 707 lavas correlate with basalt units near the top of the deccan flood basalt sequence the lavas of site 715 55 to 60 ma are most similar to the islands of mauritius and reunion site 713 basalts 48 ma are similar to the earliest lavas of the deccan province and site 706 basalts are intermediate in chemistry between those of central indian spreading ridge basalts and reunion differences in lava compositions along the hotspot track can be related to variable mixing of plume and asthenospheric mantle depending on the changing position of spreading ridge segments and the hotspot during the opening of the indian more ocean alternatively time dependent changes in the composition of hotspot melts may be due to a decrease in partial melting of a heterogeneous plume or to intrinsic changes in the composition of material supplied by the plume less

flexural and stratigraphic development of the west indian continental margin subsidence curves for 27 wells from the western continental margin of india show a characteristic late oligocene to early miocene 24 5 ma rapid increase in subsidence rate superposed on the long lived slow subsidence typical of the thermal subsidence phase of passive continental margins by subtracting a best fit negative exponential subsidence from the observed subsidence curves we obtain an estimate of the distribution and magnitude of the excess subsidence affecting the neogene development of the west indian margin the magnitude of this excess subsidence increases seaward from the coast ranging from a few meters to 2000 m near the shelf edge we examine the following hypotheses to explain the distribution and timing of this excess subsidence 1 modification of basin stratigraphy due to the effects of lithospheric in plane compression 2 creation of accommodation space on the margin by flexural effects associated with indus fan loading and 3 rapid growth of the continental margin and associated flexural effects of the three hypotheses tested the least important mechanism to account for the observed excess subsidence is that of variations of lithospheric in plane force principally because maximum in plane compression within the indo australian plate was only achieved in the late miocene because indus fan sediment deposition began in the late oligocene to early miocene we investigated three dimensional flexural effects associated with fan loading as a cause of the excess subsidence beginning at 24 ma the distribution and magnitude of modeled flexural deflection however are not consistent with the observed excess subsidence interpretation of seismic reflection data indicates that the margin has aggraded and prograded by 100 km basin ward since the oligocene therefore we evaluate the flexural effects of this margin growth by estimating the amount of space infilled by margin progradation and aggradation since 24 ma and computing the resulting deflection this deflection matches the distribution and magnitude of observed excess subsidence along the margin in addition the distribution of the flexural bulge predicted from the combined deflections due to indus fan and margin loading is spatially coincident with the distribution of exposed marine terraces and drainage divides in the saurastra peninsula and the regions surrounding the gulfs of cambay and kutch respectively available gravity seismic reflection refraction and well data are consistent with our prediction of a 4000 to 5000 m thick sediment load developed during the neogene along the outer margin we propose that flexural deformation due to sedimentary loading provides a potential tectonic feedback mechanism that affects coastal and fluvial depositional processes as regions in close proximity to the load are depressed regions farther from the load experience uplift i e the peripheral bulge which is sufficient to cause subaerial exposure of large portions of the shelf and to modify existing drainage networks this feedback represents a mechanism for inducing relative sea level changes without invoking glacial eustasy

regional tectonic framework structure and evolution of the western marginal basins of india abstract the kutch saurashtra cambay and narmada basins are pericontinental rift basins in the western margin of the indian craton these basins were formed by rifting along precambrian tectonic trends interplay of three major precambrian tectonic trends of western india dharwar nnw sse aravalli delhi ne sw and satpura ene wsw controlled the tectonic style of the basins the geological history of the basins indicates that these basins were formed by sequential reactivation of primordial faults the kutch basin opened up first in the early jurassic rifting was initiated in late triassic along the delhi trend followed by the cambay basin in the early cretaceous along the dharwar trend and the narmada basin in late cretaceous time along the satpura trend the evolution of the basins took place in four stages these stages are synchronous with the important events in the evolution of the indian sub continent its breakup from gondwanaland in the late triassic early jurassic its northward drifting during the jurassic cretaceous and collision with the asian continent in the early tertiary the most important tectonic events occurred in late cretaceous time the present style of the continental margins of india evolved during early tertiary time the saurashtra arch the extension of the aravalli range across the western continental shelf subsided along the eastern margin fault of the cambay basin during the early cretaceous it formed an extensive depositional platform continuous with the kutch shelf for the accumulation of thick deltaic sediments a part of the saurashtra arch was uplifted as a horst during the main tectonic phase in the late cretaceous the present high thermal regime of the cambay bombay high region is suggestive of a renewed rifting phase

phase relations in the cu fe zn s system between 500 degrees and 300 degrees c under hydrothermal conditions phase relations in the cu fe zn s system were studied between 500 degrees and 300 degrees c using both the thermal gradient transport method and the isothermal in situ recrystallization method under hydrothermal conditions in addition the compositional fields of chalcopyrite intermediate solid solution iss and sphalerite were determined at temperatures between 500 degrees and 300 degrees c the stable univariant phase assemblages in the central portion of the system at both 500 degrees and 400 degrees c are covellite nukundamite pyrite sphalerite covellite bornite nukundamite sphalerite bornite nukundamite pyrite sphalerite bornite chalcopyrite pyrite sphalerite bornite chalcopyrite iss sphalerite chalcopyrite iss pyrite sphalerite and iss pyrite pyrrhotite sphalerite at 300 degrees c stable assemblages of chalcopyrite pyrite pyrrhotite sphalerite and chalcopyrite iss pyrrhotite sphalerite appear in place of the last two assemblages at 500 degrees and 400 degrees c because of a tie line between chalcopyrite and pyrrhotite also a stable tie line between bornite and pyrite commonly found in natural ores occurs in the present system instead of the tie line between nukundamite and chalcopyrite observed in the cu fe s system chalcopyrite has a very small solid solution field close to the stoichiometric cufes 2 and dissolves small amounts of zinc 0 9 at at all three temperatures intermediate solid solution has an extensive solid solution field which dissolves up to 3 3 at percent at 500 degrees c and 1 2 at percent at 300 degrees c with a slight reduction in solubility due to pressure the solid solution area of intermediate solid solution becomes narrower as the temperature decreases and becomes richer in copper with increasing pressure as shown in the system cu fe s by sugaki et al 1982 sphalerite solid solution dissolves a maximum of about 2 4 mole percent cus at the three temperatures and has no relation to fes content and sulfur fugacity

analysis of airborne magnetic and gravity anomalies of peninsular shield india integrated with seismic and magnetotelluric results and gravity anomalies of madagascar sri lanka and east antarctica abstract modelling of gravity and airborne magnetic data integrated with seismic studies suggest that the linear gravity and magnetic anomalies associated with moyar bhavani shear zone mbsz and palghat cauvery shear zone pcsz are caused by high density and high susceptibility rocks in upper crust which may represent mafic lower crustal rocks this along with thick crust 44 45 km under the southern granulite terrain sgt indicates collision of dharwar craton towards north and sgt towards south with n s directed compression during 2 6 2 5 ga this collision may be related to contemporary collision northwards between eastern madagascar western dharwar craton wdc and eastern dharwar craton edc arcuate shaped n and s verging thrusts mbsz mettur shear and pcsz gangavalli shear respectively across cauvery shear zone system csz in sgt also suggest that the wdc edc and sgt might have collided almost simultaneously during 2 6 2 5 ga due to nw se directed compressional forces with csz as central core complex in plate tectonics paradigm preserving rocks of oceanic affinity gravity anomalies of schist belts of wdc suggest marginal and intra arc basin setting the gravity highs of egfb along east coast of india and regional gravity low over east antarctica are attributed to thrusted high density lower crustal upper mantle rocks at a depth of 5 6 km along w verging thrust which is supported by high seismic velocity and crustal thickening respectively it may represent a collision zone at about 1 0 ga between india and east antarctica paired gravity anomalies in the central part of sri lanka related to high density intrusives under western margin of highland complex and crustal thickening 40 km along eastern margin of highland complex with several arc type magmatic rocks of about 1 0 ga in vijayan complex towards the east may represent collision between them with w verging thrust as in case of egfb the gravity high of sri lanka in the central part falls in line with that of egfb in case it is fitted in gulf of mannar and may represent the extension of this orogeny in sri lanka

spatial distribution of hotspot material added to the lithosphere under la reunion from wide angle seismic data wide angle seismic lines recorded by ocean bottom and land seismometers provide a pseudo three dimensional investigation of the crust and upper mantle structure around the volcanically active hotspot island of la reunion the submarine part of the edifice has fairly low seismic velocities without evidence for intrusives an upper unit with a velocity depth gradient is interpreted as made of material erupted subaerially then transported and compacted downslope between this unit and the top of the oceanic plate imaged by normal incidence seismic reflection a more homogeneous unit indicated by shadow zones on several wide angle sections may correspond to lavas of a different nature extruded underwater in the earlier phase of volcanism coincident wide angle and normal incidence reflections document that the oceanic plate is not generally downwarping toward the island but doming instead toward its southeastern part with limited evidence for some intracrustal intrusion deeper in the lithosphere the presence of a layer of intermediate velocity between the crust and mantle is firmly established it is interpreted as resulting from the advection of hotspot magmatic products possibly partially molten and of a composition for which the crust is a density barrier the extensive wide angle coverage constrains the extent of this body it does not show the elongated shape expected from plate drift above a steady hotspot supply alternative propositions can hence be considered for example that la reunion is caused by a solitary wave of hotspot material or by a young hotspot the size of the underplate 140 km wide and up to 3 km thick corresponds to less than half the volume of the edifice on top of the plate

the nature of the crust in southern india implications for precambrian crustal evolution 1 we present crustal thickness and poisson s ratio determinations from receiver function analyzes at 32 sites on the archaean and proterozoic terrains of south india the crustal thickness in the late archaean 2 5 ga eastern dharwar craton varies from 34 39 km similar crustal thickness is observed beneath the deccan volcanic province and the cuddapah basin the most unexpected result is the anomalous present day crustal thickness of 42 51 km beneath the mid archaean 3 4 3 0 ga segment of the western dharwar craton since the amphibolite grade metamorphic mineral assemblages 5 7 kbar paleopressures in this part of western dharwar craton equilibrated at the depths of 15 20 km our observations suggest the existence of an exceptionally thick 57 70 km crust 3 0 ga ago beneath the exhumed granulite terrain in southernmost india the crustal thickness varies between 42 60 km the poisson s ratio ranges between 0 24 0 28 beneath the precambrian terrains indicating the presence of intermediate rock type in the lower crust these observations of thickened crust suggest significant crustal shortening in south india during the archaean

flood basalts and large igneous provinces from deep mantle plumes fact fiction and fallacy abstract flood basalts large igneous provinces and all intraplate magmatism are almost unanimously ascribed to features called mantle plumes in current geodynamics literature in the starting plume model flood basalts and any associated hotspot tracks have been ascribed to the melting of a large plume head and a narrow tail respectively and the deccan flood basalt province of india and the laccadives maldives chagos reunion island volcanic chain in the indian ocean are widely thought to be an excellent example supporting this model however the existing geodynamic thermal petrological geochemical and geochronological constraints on deccan volcanism do not require or support a mantle plume origin a new interpretation made here rules out any role for the reunion plume in deccan volcanism and even questions the notion that the reunion hotspot is the manifestation of an underlying reunion plume the laccadives reunion hotspot track is viewed here as caused by a southward propagating fracture based on the published radiometric age data and the ultimate cause of deccan volcanism is argued to be mega scale lithospheric rifting the appreciation of the great original areal extent of the deccan nearly as far as the southern tip of peninsular india solves the hitherto unsolved problem of the cause of the uplift of the 1500 km long western ghats escarpment this cause being combined surface erosion and magmatic underplating hypothesized mantle plumes do not appear responsible for most large igneous provinces instead their very existence is questionable no geological evidence of any kind geochemical petrological thermal topographic requires mantle plumes the self contradictions and special pleading that proliferate in plume explanations are unjustified and unnecessary if one accepts based on various grounds that a globe encircling enriched mantle layer the perisphere may overlie a deeper depleted mantle which is the source of morb the systematic geochemical variations along the laccadive reunion chain can be explained by this model and are not consistent with a plume tail experiencing entrainment from surrounding morb mantle as in the current plume model hotspot tracks may not reflect plate motion above stationary deep mantle upwellings but the stress state of the lithosphere as originally proposed decades ago the locations of large igneous provinces hotspots and hotspot tracks are strongly controlled by lithospheric architecture and history they are not placed randomly on the globe indifferent to the surface geology neither are large igneous provinces sudden chance events within the vastness of geological time which plume theories typically allege

analyses of multichannel seismic reflection gravity and magnetic data along a regional profile across the central western continental margin of india abstract analyses of multichannel seismic reflection gravity magnetic and bathymetry data along a regional profile across the central western continental margin of india have revealed the depositional pattern of sediments crustal structure and tectonics the four most distinct and varied crustal regions of the margin are palaeo shelf edges shelf margin basin prathap and laccadive ridges and the arabian basin the shelf margin basin is carpeted by 4 5 km maximum thick aggraded and prograded paleocene to holocene sediments six major seismic sequences of the sediments of the margin are identified and their ages are assigned on correlation with drill well results development of the sequence boundaries is attributed to the events of rifting of western india eustatic sea level changes indian and eurasian plate collision and himalayan orogeny tilted fault blocks half grabens located almost equi distance from the igneous construct of the prathap ridge in the shelf margin basin suggest a failed rift associated with stretched continental crust of the basin 2 d model studies of gravity and magnetic anomalies constrained by the seismic results have revealed 6 to 27 km thick crust across the margin the laccadive ridge crust limited by two volcanic intrusives and a steep scarp at its western end is 16 km thick it gradually thins towards offshore and juxtaposed with early tertiary normal oceanic crust 6 km thick of the arabian basin the crustal thickness and velocity and density structure of the ridge are comparable to that of the laxmi ridge a continental sliver the inferences and abrupt change in magnetic and gravity anomaly signatures across the western end of the laccadive ridge mark the zone of transition from continental to oceanic crust

modelling the rate sensitive characteristics of the gloucester foundation soil stages 1 1967 and 2 1982 of the gloucester test embankment are studied using a fully coupled finite element model the rate sensitive characteristics of the foundation soil are modelled using a

variation of effective elastic thickness and melt production along the deccan reunion hotspot track we estimate the effective elastic thickness te along the deccan reunion hotspot track using admittance analysis of seafloor topography and the free air gravity field both corrected for the thermal effects of a cooling lithosphere our results reveal that the volcanic edifices saya de malha bank chagos maldives laccadives ridge formed in the first 30 myr after the deccan volcanism not vert similar 65 myr on lithosphere with te values of 4 2 km while the younger volcanic edifices on the african plate reunion mauritius nazareth bank were emplaced on lithosphere with te values of 17 9 km these estimates suggest that the hotspot volcanism occurred on juvenile lithosphere in the first 30 myr implying that the mid ocean ridge remained near the hotspot for not vert similar 30 myr in contrast in the last 30 myr volcanism occurred on aged lithosphere in an intraplate setting which might indicate that the mid ocean ridge migrated rapidly to the north after the african plate moved over the hotspot this conclusion of a rapid shift from plume influenced mid ocean ridge mor volcanism to intraplate plume volcanism is supported by geochemical major and trace element interpretations of data from ocean drilling program odp leg 115 an estimate of the melt production rate shows a striking increase in the small te region relative to the large te region of the hotspot track which suggest a strong interrelation between te and melt production however there is also variation of melt emplacement rates within the region of low te that may be due to unknown changes in the rates of plate motions or somewhat episodic melt production

perturbation to the lithosphere along the hotspot track of la reunion from an offshore onshore seismic transect a 250 km long ne sw lithospheric transect spanning the 40 km wide island of la reunion and its submarine edifice is derived from lines of air gun shots at sea on either side along the assumed hotspot trace seismic records were obtained from an array spanning the whole transect and including sea bottom and land receivers providing a system of reversed and overlapping observations low seismic velocity and hence density is found on average for the whole edifice above the oceanic plate we attribute high velocity anomalies within the edifice to an intrusive core confined under the central northern quarter of the island crossing segment unexpectedly the main seismic interfaces top and bottom of the prevolcanic crust do not show significant flexural downwarping under the island in addition clear multipathing in the recorded wave field requires the presence of a body with a seismic velocity intermediate between the prevolcanic crustal material and the normal mantle this lithospheric structure provides the first example where underplating occur s beneath an active volcanic island suggesting a genetic relationship the underplated body could represent residues of the evolution of primary picritic melts that yield erupted basalts evidence for reflectors deeper in the lithosphere may indicate further related heterogeneity in the plate hotspot model commonly assumed the structural variation along the transect could be interpreted as a variation with time of the amount and physical state of underplated material

extinct spreading center in the labrador sea crustal structure from a two dimensional seismic refraction velocity model the labrador sea contains a rare example of an abandoned mid ocean ridge where active accretion of oceanic crust ceased due to a change in the spreading geometry of lithospheric plates seismic refraction data were collected along a line which transversely crosses the extinct spreading center two dimensional analyses of the refraction data using ray tracing and synthetic seismogram techniques reveal major variations in crustal thickness and velocity in relation to the axis of the extinct spreading center in the extinct spreading center a crustal thickness of approximately 4 km is determined compared with 5 5 km for the flanks substantial lateral variations in p wave velocities of the upper and lower crust are observed with a marked decrease within the extinct spreading center low velocities are also observed in the uppermost mantle underlying the extinct spreading center and are interpreted as being the result of hydrothermal alteration the anomalously low crustal velocities and crustal thinning are attributed to a decreasing supply of partial melt and increasing degree of tectonism at the slow spreading rates preceding extinction