wetland expansion on the continental shelf of the northern south china sea during deglacial sea level rise abstract to identify environmental causes for past changes in vegetation in subtropical east asia we present carbon isotope compositions of plant wax n alkanes and provide estimates of the c4 plant contribution across the past four glacial terminations and interglacials based on cores recovered from the northern south china sea our results show a comparable c4 plant contribution between the last glacial maximum lgm and the holocene an increase of the c4 plant contribution by 15 20 is found for terminations iv ii and i relative to subsequent interglacial peaks coeval with an expansion of cyperaceae and poaceae in contrast termination v reveals a lower c4 plant contribution than marine isotope stage mis 11c the data exhibit a long term trend with a stepwise increase of the c4 plant contribution across interglacials mis 11c 9e 7e and 1 we suggest that no substantial changes in humidity levels over glacial interglacial cycles occurred facilitating a similar c3 c4 plant ratio for the lgm and the holocene instead deglacial sea level rises caused an extensive development of floodplains and wetlands on the exposed continental shelf providing habitats for the spread of c4 sedges and grasses the progressive subsidence of chinese coastal areas and the broadening of the continental shelf over the late quaternary explains the nearly absence of c4 plant occurrence during termination v and a gradual increase of the c4 plant contribution across interglacial peaks taken together changes in coastal environments should be considered when interpreting marine based vegetation reconstructions from subtropical asia

the heterogeneous upper mantle low velocity zone abstract the upper mantle low velocity zone lvz is a depth interval with slightly reduced seismic velocity compared to the surrounding depth intervals the zone is present below a relatively constant depth of 100 km in most continental parts of the world both in cratonic areas with high average velocity and tectonically active areas with low average velocity evidence for the low velocity zone arises from controlled and natural source seismology including studies of surface waves and of primary and multiple reflections of body waves from the bounding interfaces calculations of receiver functions and absolute velocity tomography the available data indicates a more pronounced reduction in seismic velocity and q value for s waves than p waves as well as high electrical conductivity in the lvz seismic waves are strongly scattered by the zone which demonstrates the existence of small scale heterogeneity the depth to the base of the lvz is systematically shallower in cold stable cratonic areas than in hot active regions of the world because of its global occurrence below a relative constant depth of 100 km the lvz cannot be explained by metamorphic or compositional variation and rheological changes calculated upper mantle temperatures indicate that the rocks are close to the solidus in an interval with variable thickness below 100 km depth provided that the rocks contain water and carbon dioxide the presence of even small amounts of such fluids in the mantle rocks will lower the solidus by several hundred degrees and introduce a characteristic kink on the solidus curve around 80 100 km depth the seismic velocities and q values are significantly reduced of rocks which are close to the solidus or contain small amounts of partial melt hence the lvz may be explained by upper mantle temperatures being close to the solidus in a depth interval below 100 km assuming that the rocks contain only limited amounts of fluids this mechanism may explain the low velocities q values and resistivity as well as the intrinsic scattering and the characteristic variation in thickness of the low velocity zone

deep crustal structure of the indian shield from joint inversion of p wave receiver functions and rayleigh wave group velocities implications for precambrian crustal evolution 1 the s wave velocity structure of the crust and uppermost mantle of the indian shield has been investigated by jointly inverting p wave receiver functions and rayleigh wave group velocities at 38 broadband stations in the subcontinent the indian shield is an amalgamation of several terranes of archean and proterozoic age that were partly flooded by deccan trap volcanism during cenozoic times and that make up a natural laboratory for assessing models of precambrian crustal evolution our results reveal significant variations in crustal thickness and deep crustal velocities 45 50 km thick under the archean west dharwar craton and southern granulite terrane with lower crustal velocities around 4 1 km s 32 35 km thick beneath the archean east dharwar and bundelkhand cratons with lower crustal velocities around 3 8 3 9 km s 50 65 km thick under the proterozoic bhandara craton with lower crustal velocities around 4 2 4 3 km s and 55 km thick under the proterozoic aravalli delhi belt with lower crustal velocities around 4 2 km s s velocities in the 4 1 4 3 km s range in the deep crust can be attributed to mafic lithologies suggesting there has been no secular change in the precambrian evolution of the south indian shield moreover pervasive mafic dike swarming throughout the indian shield suggests that the layer of mafic cumulates is 2 5 1 6 ga old and that it delaminated from some archean terranes our interpretation is that mafic underplating of the terranes making up the indian shield occurred in proterozoic times and that a refractory root developed under the archean terranes after the proterozoic event

short wavelength infra red swir characteristics of hydrothermal alteration minerals in skarn deposits example from the jiguanzui cu au deposit eastern china abstract the jiguanzui cu au deposit is an important skarn deposit in the edong ore district of the middle lower yangtze river valley metallogenic belt mlyrb eastern china the mineralization occurred dominantly in the lower middle triassic jialingjiang formation marble either along interlayer fracture zones or along the intrusive contact with the early cretaceous quartz diorite based on the mineral assemblages and textural relationships the jiguanzui alteration mineralization can be divided into five stages namely i prograde skarn alteration garnet pyroxene ii retrograde alteration hornblende epidote actinolite iii hematite mineralization iv quartz sericite pyrite chalcopyrite mineralization main cu au mineralization stage and v calcite sulfide mineralization pyrite chalcopyrite molybdenite galena sphalerite systematic short wavelength infrared swir spectral analyses show that alteration minerals at jiguanzui include mainly sericite group minerals montmorillonite illite and muscovite kaolinite and carbonate minerals calcite ankerite and dolomite with minor chlorite halloysite and dickite spectral features of stage iv sericite group minerals indicate that the al oh absorption feature wavelength positions pos2200 have shifted toward longer wavelengths 2209 nm in the ore proximal area which may be caused by the substitution of fe and mg in the octahedral al site the vertical distance of mineralization indication can reach up to ca 450 m but can be affected by the wall rock lithology and the jiguanshan fault zone the fe oh absorption feature wavelength positions pos2250 of chlorite in the middle triassic puqi formation hornfels have also shifted toward longer wavelengths in the ore proximal area which may be caused by the relative mg rich nature of the chlorite we conclude that the high pos2200 2209 nm of sericite group minerals and the high pos2250 of chlorite can be used as prospecting tools for the jiguanzui cu au skarn mineralization

crustal shear velocity structure of the south indian shield 1 the south indian shield is a collage of precambrian terrains gathered around and in part derived from the archean age dharwar craton we operated seven broadband seismographs on the shield along a n s corridor from nanded nnd to bangalore bgl and used data from these to determine the seismic characteristics of this part of the shield surface wave dispersion and receiver function data from these sites and the geoscope station at hyderabad hyb give the shear wave velocity structure of the crust along this 600 km long transect inversion of rayleigh wave phase velocity measured along the profile shows that the crust has an average thickness of 35 km and consists of a 3 66 km s 1 12 km thick layer overlying a 3 81 km s 1 23 km thick lower crust at all sites the receiver functions are extremely simple indicating that the crust beneath each site is also simple with no significant intracrustal discontinuities joint inversion of the receiver function and surface wave phase velocity data shows the seismic characteristics of this part of the dharwar crust to be remarkably uniform throughout and that it varies within fairly narrow bounds crustal thickness 35 2 km average shear wave speed 3 79 0 09 km s 1 and vp vs ratio 1 746 0 014 there is no evidence for a high velocity basal layer in the receiver function crustal images of the central dharwar craton suggesting that there is no seismically distinct layer of mafic cumulates overlying the moho and implying that the base of the dharwar crust has remained fairly refractory since its cratonization index terms 7203 seismology body wave propagation 7205 seismology continental crust 1242 7255 seismology surface waves and free oscillations keywords continental crust archean crust receiver function indian shield

evidence for a compositional boundary within the lithospheric mantle beneath the kalahari craton from s receiver functions abstract s and p receiver functions from the southern african seismic experiment are analyzed for lithospheric discontinuities beneath the kalahari craton besides the moho the most prominent feature is a discontinuous reduction in seismic velocity of about 4 5 at approximately 150 km depth the discontinuity appears to have a width of about 10 km termed the k discontinuity this feature is restricted to the northern half of the array extending from the zimbabwe craton south to the tml thabazimbi murchison lineament and in the west from botswana to the edge of the kalahari craton in the east it spans several archean sutures and is thus unlikely related to archean tectonics it does however appear to be related to subsequent magmatic episodes the strongest anomaly is coincident with the most intense karoo volcanism and it extends to the northern edge of the bushveld intrusion from mantle xenoliths and xenocrysts the entire lithosphere in this region appears to have experienced a long term infiltration of basaltic melt and metasomatic fluids we propose that the k discontinuity reflects the influence of this melt metasomatic infiltration which has over time intruded and refertilized the lithosphere based on kimberlites pipes that show obvious signs of melt metasomatism and likely karoo influence the observed reduction in seismic velocity is plausibly consistent with the observed major element and volatile enrichment at 150 km depth in such kimberlites if this interpretation is correct then the high temperature kimberlite nodules that most clearly reflect this perturbation likely represent the general state of the lower lithosphere rather than only reflecting local mantle properties in the immediate vicinity of the kimberlite eruption

crustal structure and domain tectonics of the dharwar craton india insight from new gravity data abstract the precambrian dharwar craton of peninsular india comprises three distinct crustal domains namely the western dharwar craton wdc the eastern dharwar craton edc and the eastern ghats mobile belt egmb the domain boundaries are demarcated by bipolar positive and negative gravity anomalies measured along a 600 km long udipi kavali seismic line the two and half dimensional modelling of the gravity anomalies well constrained by available seismic depth sections identified the two contact zones one occurs between the wdc relatively low density and the edc intermediate density to the east of the chitradurga schist belt and the second occurs between the edc and the egmb high density to the east of cuddapah basin as west verging thrust faults the three layer crustal structure is up to 41 km thick beneath the wdc thinning to 37 km beneath the west coast under the edc the depth of the moho varies from 38 km towards the west to 40 km beneath the cuddapah basin and decreases to a depth of 35 km beneath the egmb a high density steeply inclined ridge like body is modelled between 5 and 20 km depth beneath the closepet granite and a similar body is modelled under the egmb it is suggested that the three domains of the dharwar craton were brought and welded together at the suture zones in the period from late archaean to late proterozoic through collisional tectonics the apparent offsets in the moho depth may be the imprint of the relative domain movement that probably was initiated by the convergent plate motion the steeply inclined ridge like high density bodies are interpreted as stacked lower crustal blocks from the convergent tectonic setting the cuddapah basin with a 10 km thick shelf marine sedimentary sequence is interpreted as a peripheral foreland basin abutting the egmb a lopolith of likely mafic composition is modelled at 10 km depth and is interpreted as a product of juvenile magmatism beneath the edc

crustal and upper mantle seismic structure of the australian plate south island new zealand abstract seismic reflection and refraction data were collected west of new zealand s south island parallel to the pacific australian plate boundary the obliquely convergent plate boundary is marked at the surface by the alpine fault which juxtaposes continental crust of each plate the data are used to study the crustal and uppermost mantle structure and provide a link between other seismic transects which cross the plate boundary arrival times of wide angle reflected and refracted events from 13 recording stations are used to construct a 380 km long crustal velocity model the model shows that beneath a 2 4 km thick sedimentary veneer the crust consists of two layers the upper layer velocities increase from 5 4 5 9 km s at the top of the layer to 6 3 km s at the base of the layer the base of the layer is mainly about 20 km deep but deepens to 25 km at its southern end the lower layer velocities range from 6 3 to 7 1 km s and are commonly around 6 5 km s at the top of the layer and 6 7 km s at the base beneath the lower layer the model has velocities of 8 2 8 5 km s typical of mantle material the mohorovicic discontinuity moho therefore lies at the base of the second layer it is at a depth of around 30 km but shallows over the south central third of the profile to about 26 km possibly associated with a southwest dipping detachment fault the high variable sub moho velocities of 8 2 km s to 8 5 km s are inferred to result from strong upper mantle anisotropy multichannel seismic reflection data cover about 220 km of the southern part of the modelled section beneath the well layered oligocene to recent sedimentary section the crustal section is broadly divided into two zones which correspond to the two layers of the velocity model the upper layer down to about 7 9 s two way travel time has few reflections the lower layer down to about 11 s two way time contains many strong subparallel reflections the base of this reflective zone is the moho bi vergent dipping reflective zones within this lower crustal layer are interpreted as interwedging structures common in areas of crustal shortening these structures and the strong northeast dipping reflections beneath the moho towards the north end of the mcs line are interpreted to be caused by paleozoic north dipping subduction and terrane collision at the margin of gondwana deeper mantle reflections with variable dip are observed on the wide angle gathers travel time modelling of these events by ray tracing through the established velocity model indicates depths of 50 110 km for these events they show little coherence in dip and may be caused side swipe from the adjacent crustal root under the southern alps or from the upper mantle density anomalies inferred from teleseismic data under the crustal root

mean crustal velocity a critical parameter for interpreting crustal structure and crustal growth abstract mean crustal velocity is a critical parameter for genesis of continental crystalline crust because it is a function of mean crustal composition and therefore may be used to resolve continental crustal growth in space and time although the best values of mean crustal velocity are determined from wide angle reflection measurements most studied here necessarily come from vertical averages in crustal refraction determinations the mode of 158 values of mean crustal velocity is 6 3 km s a velocity which corresponds to a mean crustal composition of granodiorite to felsic quartz diorite archean crust may be slightly more mafic mean crustal velocities range from 5 8 to 7 0 km s the lowest values invariably are found in thermally disturbed rift zones and the highest values correspond to velocities in gabbro velocities in island arcs may be as low as 6 0 km s but are typically 6 5 6 9 km s which corresponds to andesitic composition estimates of island arc composition are andesitic if values of mean crustal velocity are not biased this observation suggests that continental crust did not grow simply by addition of island arc material possibilities are that crust formed from fusion of island arcs and was later changed to more felsic composition by addition of material from the mantle or that the late archean episode of major crustal growth did not involve processes similar to younger island arcs some crustal blocks might be changed in composition and thickness by such processes as underplating interthrusting necking and sub crustal erosion specially designed experiments are suggested to determine this parameter so critical for understanding genesis of continental crust

the karnataka late cretaceous dykes as products of the marion hot spot at the madagascar india breakup event evidence from 40ar 39ar geochronology and geochemistry two late cretaceous mafic dykes with an ene strike that is orthogonal to the west coast of india and located nearly 200 km inland around huliyardurga karnataka state yield 40ar 39ar plateau ages of 90 0 1 0 and 87 5 0 9 ma these fe ti enriched tholeiites are essentially co eval with at least four other igneous suites widely scattered in southern india namely the south and north kerala dykes the agali anaikatti dykes of central kerala tamil nadu and lavas of st mary islands off the west coast of india the karnataka cretaceous dykes are also co eval and compositionally very similar to the fe ti enriched tholeiitic lavas and dykes around mananjary a major phase of late cretaceous magmatism along the eastern rifted margin of madagascar which are believed to be products of the marion hot spot that extruded at 88 ma synchronous with the india madagascar break up event the age and compositional similarities between these late cretaceous magmatic rocks from india and madagascar constitute a clear evidence for extension of the thermal manifestations deep into the indian peninsula

the rapid drift of the indian tectonic plate the breakup of the supercontinent gondwanaland into africa antarctica australia and india about 140 million years ago and consequently the opening of the indian ocean is thought to have been caused by heating of the lithosphere from below by a large plume whose relicts are now the marion kerguelen and reunion plumes plate reconstructions based on palaeomagnetic data suggest that the indian plate attained a very high speed 18 20 cm yr 1 during the late cretaceous period subsequent to its breakup from gondwanaland and then slowed to approximately 5 cm yr 1 after the continental collision with asia approximately 50 myr ago the australian and african plates moved comparatively less distance and at much lower speeds of 2 4 cm yr 1 refs 3 5 antarctica remained almost stationary this mobility makes india unique among the fragments of gondwanaland here we propose that when the fragments of gondwanaland were separated by the plume the penetration of their lithospheric roots into the asthenosphere were important in determining their speed we estimated the thickness of the lithospheric plates of the different fragments of gondwanaland around the indian ocean by using the shear wave receiver function technique we found that the fragment of gondwanaland with clearly the thinnest lithosphere is india the lithospheric roots in south africa australia and antarctica are between 180 and 300 km deep whereas the indian lithosphere extends only about 100 km deep we infer that the plume that partitioned gondwanaland may have also melted the lower half of the indian lithosphere thus permitting faster motion due to ridge push or slab pull

pn velocity and cooling of the continental lithosphere average upper mantle p sub n velocities and heat flow were computed within continental physiographic provinces in north america from published data p sub n velocity and heat flow data display an inverse relationship and were found to correlate with a statistically significant correlation coefficient temperatures at the crust mantle boundary were estimated from the heat flow values and these were used to demonstrate a correlation between p sub n velocity and temperature of upper mantle material the value of partialp sub n partialt p thus obtained 4 4 x 10 sup 4 to 8 1 x 10 sup 4 km s sup 1 sup 0 c sup 1 is within the range of temperature derivatives determined from laboratory studies of ultramafic rocks this dependence of p sub n velocity on temperature implies that one possible explanation for the observed geographical distribution in upper mantle seismic velocity is that the variation in p sub n velocity is primarily a temperature effect combined with the relationship between heat flow and crustal age for continents the p sub n versus heat flow correlation suggests a relationship between p sub n velocity and crustal age probably due to progressive cooling of the continental lithosphere after a thermotectonic event

recently formed elastic anisotropy and petrological models for the continental subcrustal lithosphere in southern germany this paper advances new evidence for elastic anisotropy in the continental subcrustal lithosphere in southern germany the range of petrological models compatible with the observed azimuthal variation of seismic p wave velocity is explored the azimuthal distribution of amplitudes of mantle phases and the observed increase of p velocity with depth both indicate a continuation of anisotropy with depth together with an increase of preferred orientation even depletion of the upper mantle in basaltic components as suggested by mantle xenoliths from various parts of germany cannot explain the velocity depth and azimuthal amplitude observations without an increase of anisotropy with depth preferred orientation of olivine is the most likely mechanism for the observed phenomena its fast a axis at the moho level is directed towards n22 5 e the b axis is also required to be horizontal i e the b plane one of the preferred glide planes of olivine is vertical with a strike of n22 5 e therefore this preferred glide plane of olivine practically coincides with the plane of maximum horizontal shear stress deduced from fault plane solutions of earthquakes in western germany this is a strong indication that the preferred orientation of olivine is formed in the recent west european crustal stress field leaking into the upper mantle the distribution of velocities to a depth of at least 50 km requires slight horizontal rotation of the a axis with depth by 10 towards n32 e and a change in the modal composition towards a depletion increasing with depth compatible with the composition of mantle xenoliths from western germany further experiments are needed to substantiate this suggestion which could lead to a better understanding of the interaction of crustal and upper mantle stress strain fields

late archaean 2550 2520 ma juvenile magmatism in the eastern dharwar craton southern india constraints from geochronology nd sr isotopes and whole rock geochemistry the results of field geochronologic geochemical and isotopic studies are presented for the granitoids that occur east of the closepet batholith up to the kolar schist belt ksb field data such as common foliation strong shear deformation occasionally leading to mylonitization together with petrographic data including reduction in grain size with corroded borders show characteristics of the syn kinematic emplacement of the granitoids single zircon evaporation ages define a minimum age of 3127 ma for the tonalitic trondhjemitic granodioritic ttg basement and 2552 2534 ma plateau ages for the emplacement of the granitoids which slightly predate 20 30 ma the emplacement of the 2518 ma closepet batholith major and trace element data together with isotopic data suggest at least four magmatic suites from closepet batholith to the east which have independent magmatic evolution histories the observed data are compatible with magma mixing for the closepet batholith melting of ttg and assimilation fractional crystallization processes for bangalore granites either melting of heterogeneous source or different degree of melting of the same source for the granitoids of hoskote kolar and fractional crystallization for the western margin of the ksb isotopic nd sr and geochemical data lree and lil elements suggest highly enriched mantle and ancient ttg crust for the closepet batholith enriched mantle and ttg crust for the bangalore granites c a chondritic mantle source for the granitoids of hoskote kolar and the quartz monzonites of the western margin of the ksb and slightly depleted mantle for granodiorites of the eastern margin of the ksb we interpret all these geochronologic geochemical and isotopic characteristics of granitoids from the closepet batholith to the east up to the ksb in terms of a plume model the centre of the plume would be an enriched hot spot in the mantle that lies below the present exposure level of the closepet batholith melting of such an enriched mantle hot spot produces high temperature magmas closepet that penetrate overlying ancient crust where they strongly interact and induce partial melting of the surrounding crust these magmas cool very slowly as the hot spot maintains high temperatures for a long time thus they appear younger 2518 ma on the contrary to the east the plume induces melting of c a chondritic or slightly depleted mantle that produces relatively colder and less enriched magmas which show less or no interactions with the surrounding crust and cool rapidly and appear slightly older 2552 2534 ma this plume model can also account for late archaean geodynamic evolution including juvenile magmatism heat source for reworking inverse diapirism and granulite metamorphism in the dharwar craton

archean and proterozoic crustal evolution evidence from crustal seismology seismic velocity models for archean and proterozoic provinces throughout the world are analyzed the thickness of the crust in archean provinces is generally found to be about 35 km except at collisional boundaries whereas proterozoic crust has a significantly greater thickness of about 45 km and has a substantially thicker high velocity 7 0 km s layer at the base we consider two models that may explain these differences the first model attributes the difference to a change in the composition of the upper mantle the higher temperatures in the archean mantle led to the eruption of komatiitic lavas resulting in an ultradepleted lithosphere unable to produce significant volumes of basaltic melt proterozoic crust developed above fertile mantle and subsequent partial melting resulted in basaltic underplating and crustal inflation in the second model convection in the hot archean mantle is considered to have been too turbulent to sustain stable long lived subduction zones by the proterozoic the mantle had cooled sufficiently for substantial island and continental arcs to be constructed and the high velocity basal layer was formed by basaltic underplating

tectonic delamination and subcrustal imbrication of the precambrian lithosphere in northwestern canada mapped by lithoprobe seismic reflection profiling of the precambrian lithosphere in northwestern canada has produced images of delamination structures tectonic wedges at crustal scale to about 30 km depth and for the first time at lithospheric scale to about 100 km depth and has further delineated subcrustal structures that were likely caused by imbrication of subducted lithosphere coupled with observations of similar structures in thrust and fold belts delamination and lower plate imbrication are persistent features at many scales

an experimental replication of upper mantle metasomatism interaction of melts ascending from the asthenosphere with the lithospheric mantle through which they pass is widely accepted to be a significant process by which the lithosphere can acquire distinctive trace element and isotopic signatures1 3 despite the importance of this process in the genesis of the lithosphere the nature of these metasomatic reactions is still debated2 5 here i report the results of an experiment designed to investigate the mechanism by which such metasomatism could occur a natural sample of group i kimberlite a low volume partial melt of asthenospheric origin is reacted with a refractory peridotite harz burgite in a thermal gradient at high pressure to simulate the interaction between a dyke and its host rock in the lower lithosphere the harzburgite develops several contrasting mineralogical zones the coolest of which contains olivine orthopyroxene clino pyroxene phlogopite amphibole nb cr rutile and minor zircon the formation of mica and amphibole is accompanied by increases in trace element concentrations and rb sr rb ba and pb u ratios features characteristic of metasomatized xenoliths recovered from southern african kimberlites5 these results support the suggestion based on temporal association and isotopic data that at least some mantle metasomatism is related to the interaction of group i kimberlite with refractory peridotite in the lower lithosphere6

progressive metamorphism of mafic rocks from greenschist to granulite facies in the dharwar craton of south india in the archean dharwar craton of southern india n s trending belts of metabasic rocks are exposed which underwent regional metamorphism at about 2 5 ga ago the progressive changes in the assemblages and mineral chemistry of metabasites was studied in a n s traverse covering the chitradurga and nagaman gala belts the sargur area and the nilgiri hills towards the south with increasing metamorphic grade greenschists chl act ab ep pm carb qtz give way to amphibolites hbl plag pm qtz gar cumm and mafic granulites pyx plag gar pm hbl qtz the amphibole composition changes from actinolite in the greenschist zone to tschermakitic hornblende in the amphibolite zone and pargasitic hastingsitic hornblende in the granulite zone the ti content of the amphiboles systematically increases in this direction the plagioclase composition changes from albite in the greenschist zone to oligoclase andesine in the amphibolite zone and andesine labradorite in the granulite zone almandin

lithospheric and upper mantle structure of the indian shield from teleseismic receiver functions receiver functions derived from teleseismic p waveforms of 297 earthquakes recorded at broadband station hyderabad hyb are analysed to investigate the lithospheric and upper mantle structure in the vicinity of the station located in the northeastern part of the archaean dharwar craton the internal structure of the 33 2 km thick crust is simple it can be approximated by a single layer with s velocity linearly increasing with depth from 3 4 to 4 0 km s the average being 3 7 0 2 km s the crustal poisson s ratio is 0 25 0 01 a seismic discontinuity at 90 km depth is inferred from a signal particularly prominent on the sh component and possibly indicative of a layer of depth localized anisotropy in the lithospheric mantle the two principal mantle discontinuities are at depths of 406 and 659 km which are close to the global averages

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

seismic reflection coefficients from mantle fault zones summary several bright reflections from structures within the mantle can be seen on birps deep seismic reflection profiles we have calculated apparent reflection coefficients for the brightest of these events and obtain values around 0 1 it is not possible to produce such large reflections by either compositional layering or seismic anisotropy if olivine and pyroxene are the only significant minerals in the mantle these large reflections can be produced by a mafic layer or a partially hydrated layer within normal peridotite the brightest reflections seem to be best explained as major faults or shear zones within the mantle

anomalous lithosphere beneath the proterozoic of western and central australia a record of continental collision and intraplate deformation a new surface wave seismic tomography model of australia is presented which provides a means of investigating the lithospheric structure beneath the proterozoic regions in the west north and centre of the continent with improved resolution and reliability the dominant feature of the model is a region of low seismic wavespeeds in the uppermost mantle at approximately 75 km depth beneath central australia the zone of slow wavespeeds is underlain by a region of fast wavespeeds more typical of continental lithosphere this layered velocity structure and strong positive wavespeed gradient makes the shallow anomaly hard to explain in terms of high mantle temperatures and typical steady state continental geotherms a possible thermal explanation requires the impact of the redistribution of high heat producing elements within the crust alternatively a mineral or minerals with low seismic velocities such as amphibole in the shallowest part of the lithosphere with a more conventional lithology in the deep continental root below could explain the seismic wavespeeds the anomaly is located directly beneath the zone where the australian cratons amalgamated in the proterozoic and where subsequently there have been periods of intraplate tectonic activity suggesting a correlation between the prolonged history of deformation and a highly unusual lithospheric structure in western australia the capricorn orogen and pilbara craton have a similar lithospheric thickness whereas a thicker lithosphere is observed beneath the yilgarn to the south in northern australia large regions appear to be underlain by fast wavespeeds similar to those observed beneath the yilgarn craton variations in the shear wavespeeds beneath the arunta also indicate that there is not always an obvious correlation between the overlying surface geology and seismic structures observed in the upper mantle

lithospheric mantle deformation beneath the indian cratons abstract the nature of deformation of the deep continental roots beneath the archean early proterozoic terrains opens the question whether these ancient terrains have had stable roots since the precambrian or whether recent plate motions have deformed them in view of this we make an attempt to study the thermal structure beneath the cratonic regions of the indian shield which vary in lithospheric thickness from 65 km in the singhbhum craton to 148 km in the archean dharwars the average depth of 104 km to the top of the underlying asthenosphere is consistent with other termination methods and is in fact less than half the 200 400 km depth found in other stable areas of the earth similarly the average reduced heat flow of about 35 mw m2 and moho temperature of about 550 c range 400 730 c for the indian cratons are also much higher than their counterparts elsewhere our study indicates a large scale deformation of the cratonic mantle lithosphere beneath the indian shield since the mesoproterozoic c

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

sub crustal lvz below dharwar craton india an evidence for mantle metasomatism and tectonothermal activity in the archean crust abstract improved knowledge of seismic structure of the continental crust provides deep insights into the mechanism of the crustal growth since the archean times reprocessing of deep seismic sounding data acquired over western dharwar craton india reveals presence of i a high velocity magmatic layer 7 30 km s at the base of the crust ii a higher pn velocity 8 35 km s at a deeper depth of 45 km than in an usual archean crust and iii an unusually low velocity layer 7 40 km s at a depth range of 48 60 km in the upper mantle such shallow subcrustal lithospheric lvzs are rarely found in a stable archean craton which is associated with low surface heat flow and devoid of recent tectonic activities considering various geological tectonic and geothermal scenarios occurrence of such a low velocity zone has been ascribed to chemical alteration caused by mantle metasomatism rather than mafic partial melts or high temperature sub solidus the derived velocity structure and distribution of volcanic rocks of different ages in the studied area indicate multiple crust mantle thermal interaction in the archean crust

qp melting temperature relation in peridotite at high pressure and temperature attenuation mechanism and implications for the mechanical properties of the upper mantle the anelastic properties of compressional waves in a peridotite have been determined in the laboratory at sufficiently high temperatures to 1280 c and pressures to 0 73 gpa to warrant comparison with seismic measurements of the earth a substantial decrease of qp is observed at temperatures well below the onset of partial melting qp systematically increases with increasing pressure over the entire temperature range of major significance is the finding that qp is dependent on the ratio of the temperatures to the melting solidus temperature i e qp depends on the homologous temperature the pressure dependence of qp appears through the pressure dependence of the solidus of the peridotite within the uncertainties of measurement of both qp and the phase diagram it appears that melting and high temperature anelastic properties have a common origin in peridotite the homologous temperature dependence of qp suggests that we may estimate the temperature and pressure dependence of qp for peridotites of different compositions and possibly even for hydrous peridotites if solidus temperatures are known as a function of pressure a far easier measurement than elastic and anelastic properties the activation volume of qp is greatly reduced at high pressure since the slope of solidus versus pressure rapidly decreases with increasing pressure pressure dependence of seismic velocity and melt fraction in peridotite also appears to be related to the homologous temperature the qp homologous temperature relation suggests a connection between qp and the properties of the grain boundaries that is the major loss of seismic energy occurs at the grain boundaries grain boundary relaxation or high temperature background attenuation is a possible mechanism for the grain boundary damping no frequency dependence of qp is resolved 0 0 2 in qp f over the pressure temperature and frequency ranges of the measurement the present results and the model of grain boundary relaxation suggest that an appropriate choice of grain size may give an ultrasonic q that is applicable to the earth experimentally determined anelastic properties of a peridotite are critical for modeling mechanical properties of the upper mantle implications of the results are as follows 1 seismic data commonly interpreted as indicating a partially molten asthenosphere may instead reflect a hot solid asthenosphere at 90 100 of the solidus temperature 2 partial melting may not produce any abrupt change of seismic velocity and q rather elastic and anelastic properties of the upper mantle will change gradually at the boundary where the geotherm crosses the solidus 3 there may be no sharp mechanical boundary between the lithosphere and the asthenosphere

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

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

3 35ga komatiite volcanism in the western dharwar craton southern india constraints from nd isotopes and whole rock geochemistry abstract we present field petrographic sm nd whole rock isochron and whole rock geochemical data for komatiites from sargur group greenstone belts of the western dharwar craton field evidence such as pillow structure indicates their eruption in a marine environment petrographic data reveal that the igneous mineralogy has been altered during post magmatic hydrothermal alteration processes corresponding to greenschist to lower amphibolite facies conditions with rarely preserved primary olivine and orthopyroxene a 16 point sm nd whole rock isochron gives an age of 3352 110 ma for the timing of eruption of komatiite lavas about 60 of the studied komatiite samples show al depletion whilst the remaining are al undepleted the al depleted komatiites are characterised by high cao al2o3 ratios 1 01 1 34 and low al2o3 tio2 5 16 whereas al undepleted komatiites show lower cao al2o3 ratios 0 59 0 99 and higher al2o3 tio2 17 26 trace element distribution patterns of komatiites suggest that most of the primary geochemical and nd isotopic compositions are preserved with only minor influence of post magmatic alteration processes and negligible crustal contamination the chemical characteristics of al depleted komatiites such as high gd yb n together with lower hree y zr and hf imply their derivation from deeper upper mantle with garnet majorite involvement whereas lower gd yb n slightly higher hree y zr and hf suggest derivation from shallower upper mantle without garnet involvement the observed chemical characteristics cao al2o3 al2o3 tio2 mgo ni cr nb zr y hf ree indicate derivation of the komatiite magmas from different depths in a plume setting whereas sub contemporaneous felsic volcanism and ttg accretion can be attributed to an arc setting in order to explain the spatial association of komatiite volcanism with contemporaneous mafic felsic volcanism and ttg accretion we propose a combined plume arc setting nd isotope data of the studied komatiites indicate depleted mantle reservoirs which may have evolved by early 4 53 ga global differentiation of the silicate earth as suggested by boyet and carlson boyet m carlson r w 2005 142nd evidence for early 4 53 ga global differentiation of silicate earth science 309 577 581 or extraction of continental crust during the early archaean

southern african crustal evolution and composition constraints from receiver function studies 1 stacking of approximately 1500 radial receiver functions recorded at about 80 broadband seismic stations deployed in southern africa reveals systematic spatial variations in the ratio of crustal p and s wave velocities crustal thickness h and the amplitude of the converted moho phases r the eastern zimbabwe and the southern kaapvaal cratons are characterized by small h 38 km small 1 73 and large r 0 15 values suggesting that the relatively undisturbed archean crust beneath southern africa is separated from the mantle by a sharp moho and is felsic in composition the limpopo belt which was created by a collisional event at 2 7 ga displays large h 43 km but similar and r values relative to the cratonic areas the bushveld mafic intrusion complex and its surrounding areas show large 1 78 large h 43 km and small r 0 11 values reflecting the intrusion of mafic material into the original crust as a result of the bushveld event at 2 05 ga excluding the bushveld the spatially consistent and age independent low accentuate the difference between felsic crustal composition and more mafic island arcs that are thought to be the likely source of continental material within such an island arc model our data combined with xenolith data excluding mantle delamination in cratonic environments suggest that the modification to a felsic composition e g by the partial melting of basalt and removal of residue by delamination is restricted to have occurred during the collision between the arcs and the continent

modeling wide angle seismic data for crustal structure southeastern grenville province a modeling methodology for obtaining two dimensional 2 d crustal structure from wide angle seismic data is applied to data from the southeastern grenville province pre modeling steps include 1 assignment of arrival pick uncertainties for appropriate data fitting and weighting using an empirical relationship based on signal to noise ratio 2 using a modified form of travel time reciprocity to avoid unreasonable levels of model heterogeneity and 3 identifying data unsuitable for 2 d modeling the goal of the travel time inversion amplitude modeling approach is to obtain a minimum structure and minimum parameter model that takes into account both horizontal and vertical variations in the resolution of typical wide angle data each step of a layer stripping procedure involves a series of inversions in which a one dimensional or simple starting model is improved with additional velocity and or interface nodes until a satisfactory trade off between travel time fit parameter resolution and complete ray coverage of all source receiver pairs is achieved using zero vertical velocity gradient layers and head waves during preliminary first arrival inversion can 1 decrease the number of intermediate models 2 allow greater lateral heterogeneity to be imaged and 3 simplify incorporation of amplitude modeling constraints into the final model using amplitude distance curves allows quantitative modeling of the relative amplitude and offset variations of phases discrepancies between observed and calculated reflection amplitudes are used to infer fundamental non step like velocity changes at layer boundaries later arrivals due to unresolved velocity anomalies are modeled using reflecting segments that float within the model without an associated velocity structure these reflectors provide a spatial image like that obtained from vertical incidence reflection data as opposed to a velocity image the model of grenville crustal structure is more detailed than a model obtained from a previous interpretation of the data and includes elements analogous to those imaged in nearby deep reflection data a crustal scale zone of wide angle reflectors with an average easterly apparent dip of 13 defines a major grenvillian terrane boundary