change from late tertiary compression to quaternary extension in southern tibet during the india asia collision field studies in southern tibet show that subsequent to folding resulting from the india asia collision 50 40 ma reverse faults and backthrusts formed as a result of nne ssw and nnw sse trending compressions they affect conglomerates of oligocene miocene age and are probably contemporaneous with shearing on the himalayan main central thrust and folding of paleogene basins along the northern border of southern tibet 25 16 ma rift systems postdate compressional structures they were active during the pleistocene and probably during the pliocene 5 2 ma change from a compressional to an extensional regime in tibet probably occurred during the late miocene 10 5 ma this change is tentatively correlated with uplift inferred by chinese workers from paleogene to present day variations of the tibetan vegetation recent and active faulting in the high plateau results from a wnw ese striking extension this is in agreement with tibetan focal mechanisms the himalayan lowlands are subjected to a nne ssw trending compression as demonstrated by superficial deformations and seismicity therefore the horizontal maximum compressive stress trajectory is roughly parallel to india asia convergence hmax being 1 in the lowlands and 2 in the high plateau this change results from the increase of the vertical principal stress value due to the weight of the elevated mass zz is 3 in the lowlands and 1 in the high plateau thus the third principal stress axis is 2 in the lowlands and 3 in the high plateau allowing extension to occur in the wnw ese direction extensional tectonics are significantly different in southern and northern tibet southern tibet being constrained between the himalayan syntaxes the e w extensional principal deviatoric stress value 3 is of rather small magnitude and the n s trending 2 value is extensional as demonstrated by analysis of recent faults normal components prevail along all the azimuths in the field this favors formation of rift systems of large axial extension on the contrary north of the himalayan syntaxes northern tibet flows easily eastward the e w extensional principal deviatoric stress value 3 is of higher magnitude and the n s stress 2 is compressional as shown by inversion of focal mechanism data this favors strike slip components on faults which deviate from the n s direction as expressed in the field by shorter n s trending graben linked with normal strike slip faults these faults permit some n s shortening in northern tibet without thickening in response to india asia collision in high plateaus which are subjected to lateral compression such as the andean altiplano and tibet the extensional direction is basically controlled by the convergence direction being roughly orthogonal to the latter

crustal structure and earthquake focal depths beneath northeastern india and southern tibet summary we use broad band teleseismic data recorded at eight sites along a north south profile from karimganj 24 84 n 92 34 e south of the eastern shillong plateau to bomdilla 27 27 n 92 41 e in the eastern lesser himalaya to determine the seismic characteristics of the crust in northeastern india we also analyse data from the chinese digital seismic network station at lhasa and indepthii stations located on the southern tibetan plateau north of our profile to extend the seismic images of the crust further northwards although the northeastern indian stations and the tibetan stations do not lie along a linear profile across the himalaya the wellrecognized uniformity of the himalaya along strike make this comparison of the two profiles meaningful receiver functions calculated from these data show that the crust is thinnest 35 38 km beneath the shillong plateau receiver functions at cherrapunji on the southern edge of the shillong plateau have a strong azimuthal dependence those from northern backazimuth events show that the moho beneath the southernmost shillong plateau is at a depth of 38 km while receiver functions from southern backazimuth events indicate that the moho beneath the northernmost bengal basin is at a depth of 44 km receiver functions from sites on the brahmaputra valley demonstrate that the moho is deeper by 5 7 km than below the shillong plateau a result which agrees with the hypothesis that the shillong plateau is supported by shearing stress on two steep faults that cut through the crust further north of the eastern himalayan foredeep the moho dips gently northwards reaching a depth of 48 km beneath bomdilla in the lesser himalaya and 88 km below lhasa in tibet using the crustal velocity models obtained from receiver function inversions we redetermined focal depths of wellrecorded earthquakes across this part of the indo tibetan collision zone and find all of these to occur within the crust hence we find no evidence for bimodal depth distribution of earthquakes beneath this region of northeastern india

lithospheric and upper mantle structure of southern tibet from a seismological passive source experiment fifteen seismic stations were operated with about 20 km spacing in southern tibet across the zangbo suture the collision zone between india and asia between may and october 1994 as part of the international deep profiling of tibet and the himalaya project indepth ii for wide angle recording of the controlled source experiment and for passive earthquake recording in addition a dense deployment 4 km spacing of stations within the german depth profiling of tibet and the himalayas gedepth project also recorded a number of teleseismic earthquakes the third data source used in this study is the records of the permanent broadband station at lhasa teleseismic records have been obtained in sufficient quantity and quality to derive an image of the structure of the lithosphere and upper mantle from p to s converted phases important results are as follows the moho at 70 80 km and a second discontinuity at 50 60 km depth are observed over the entire profile south and north of the zangbo suture the data from the gedepth dense array enable the detection of inclined structures penetrating the crust at the zangbo suture a pronounced low velocity zone exists north of the zangbo suture at about 10 20 km depth the locations of the upper mantle discontinuities at 410 and 660 km depth are in agreement with the global reference model iasp91 kennett 1991 over a large region of the himalaya and southern tibet

tibetan tectonic evolution inferred from spatial and temporal variations in post collisional magmatism cenozoic magmatism on the tibetan plateau shows systematic variations in space and time that must be considered in models concerning tibetan tectonic evolution after the india asia collision which started in the early tertiary and terminated the gangdese arc magmatism in the lhasa terrane made of the southern tibetan plateau widespread potassium rich lavas and subordinate sodium rich basalts were generated from f50 to 30 ma in the qiangtang terrane of northern tibet subsequent post collisional magmatism migrated southwards producing ultrapotassic and adakitic lavas coevally between f26 and 10 ma in the lhasa terrane then potassic volcanism was renewed to the north and has become extensive and semicontinuous since f13 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations enable us to elaborate a geodynamic evolution model which depicts when and how the indian continental lithospheric mantle started thrusting under asia by involving rollback and breakoff of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root we propose that only after the lithospheric removal which occurred at f26 ma could the indian mantle lithosphere have commenced its northward underthrusting and henceforth served as a pivotal control to the himalayan tibetan orogenesis consequently the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder owing to the push of the underthrust indian mantle lithosphere d 2004 elsevier b v all rights reserved

structure of the lower crust revealed by one and two dimensional modeling of wide angle reflections west bengal basin india the crustal structure in the west bengal basin india has been investigated by means of wide angle reflection data recorded along i bishnupur palashi kandi 227 km long profile in the north south direction and ii taki arambagh 120 km long profile in the east west direction the data were acquired using multichannel digital seismic instruments with close station spacing the crustal model initially derived by 1 d forward modeling of the wide angle reflection data has been iteratively refined by 2 d ray tracing and modeling of travel time observations and the corresponding synthetic seismograms computation the structural contour map of the moho prepared from the present data set indicates the crustal thickness of about 37 km in the western margin of the basin thinning to about 28 km in the east with an upwarp in the moho boundary the upwarp in the moho and the inferred structural features may be indicative of crustal rifting the well known gravity anomaly in the west bengal basin calcutta gravity high appears to have resulted from the moho upwarp in combination with the huge thickness of sediments deposited east of the steep flexure of the crystalline basement representing the hinze zone

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

observations of regional phase propagation across the tibetan plateau we present observations of regional phase velocity and propagation characteristics using data recorded during a 1 year deployment of broadband digital seismic stations across the central tibetan plateau along the qinghai tibet highway from golmud to lhasa previous seismological studies within this region have had to rely on earthquakes recorded almost exclusively at stations outside of the plateau we have the opportunity to study numerous source receiver paths confined entirely within the tibetan plateau our analysis concentrates on travel time amplitude and frequency content measurements of the pg pn and sn phases pn can be clearly picked for all observed paths and propagates at an average velocity of 8 16 0 07 km s within the tibetan plateau sn however shows dramatic variations in propagation efficiency across the tibetan plateau that is strongly dependent on frequency we observe that sn rapidly decreases in frequency and amplitude as it passes through the northern portion of the plateau we show that in general sn propagation efficiency decreases with increasing frequency content we use 122 events from outside of the plateau and 61 from within to refine the boundaries of a region of inefficient high frequency sn propagation specifically we show that a larger portion of the northern tibetan plateau attenuates sn energy than was previously suggested in the southern plateau where high frequency sn is observed we computed an average velocity of 4 59 0 18 km s we also observed that the pn velocity within this region of inefficient high frequency sn propagation is nearly 4 slower than the pn velocity computed for paths restricted to the southern plateau and that the crust is about 10 km thinner than in the south the coincident locations of inefficient sn propagation and slow pn velocity is commonly observed in regions of active tectonics our results add constraints to the velocity structure of the lithosphere beneath the tibetan plateau and require first order lateral variations in the uppermost mantle structure despite the relatively uniform topography of the plateau

crustal structure of central tibet as derived from project indepth wide angle seismic data summary in the summer of 1998 project indepth recorded a 400 km long nnw sse wide angle seismic profile in central tibet from the lhasa terrane across the banggong nujiang suture bns at about 89 5 e and into the qiangtang terrane analysis of the p wave data reveals that 1 the crustal thickness is 65 5 km beneath the line 2 there is no 20 km step in the moho in the vicinity of the bns as has been suggested to exist along strike to the east based on prior fan profiling 3 a thick high velocity lower crustal layer is evident along the length of the profile 20 35 km thick 6 5 7 3 km s 1 and 4 in contrast to the southern lhasa terrane there is no obvious evidence of a mid crustal low velocity layer in the p wave data although the data do not negate the possibility of such a layer of modest proportions combining the results from the indepth iii wide angle profile with other seismic results allows a cross section of moho depths to be constructed across tibet this cross section shows that crustal thickness tends to decrease from south to north with values of 70 80 km south of the middle of the lhasa terrane 60 70 km in the northern part of the lhasa terrane and the qiangtang terrane and less than 60 km in the qaidam basin the overall northward thinning of the crust evident in the combined seismic observations coupled with the essentially uniform surface elevation of the plateau south of the qaidam basin is supportive of the inference that northern tibet until the qaidam basin is underlain by somewhat thinner crust which is isostatically supported by relatively low density hot upper mantle with respect to southern tibet

configuration of the indian moho beneath the nw himalaya and ladakh 1 teleseismic receiver function analysis of seismograms recorded on a 700 km long profile of 17 broadband seismographs traversing the nw himalaya shows a progressive northward deepening of the indian moho from 40 km beneath delhi south of the himalayan foredeep to 75 km beneath taksha at the karakoram fault similar studies by wittlinger et al 2004 to the north of the karakoram fault show that the moho continues to deepen to 90 km beneath western tibet before shallowing substantially to 50 60 km at the altyn tagh fault the continuity of the indian moho imaged in the receiver functions reported here along with those of wittlinger et al 2004 suggest that in this part of the himalayan orogen the indian plate may penetrate as far as the bangong suture and possibly as far north as the altyn tagh

high resolution global phase velocity distributions we present global phase velocity distributions for fundamental mode love and rayleigh waves in the period range 40 to 150 seconds the models expressed in terms of spherical harmonic expansions up to degree and order 40 have beed derived from 28 479 love wave and 33 662 rayleigh wave measurements the measurements were made using an automatic procedure based on non linear waveform inversion we show that the results are characterized by lateral resolving radii between 500 and 850 km a significant reduction from previous work the improved global resolution is largely due to the inclusion of many more major arc measurements the power spectra of the models are much whiter than has previously been found the results show great similarity to the phase velocity distributions derived from the recent a priori model 3 smac in which the primary relevant features are the crustal thickness and crustal velocity distributions the cooling oceanic lithosphere and the thick 300km lithosphere of the continental cratons the results thus confirm these features differences for example in the strengths of the crust ridge and craton signatures should lead to refinement in the thermal and constitutive parameters upon which the a priori model depends

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

gps measurements from the ladakh himalaya india preliminary tests of plate like or continuous deformation in tibet observations of relative motion in a geodetic network in ladakh india and across southern tibet indicate slow shear on the karakorum fault rapid east west extension across the whole of southern tibet and constant arc normal convergence between india and southern tibet along the himalayan arc measurements of ten campaign style and six permanent sites with global positioning system gps precise geodesy provide these bounds on the style and rates of the large scale deformation in the tibet himalaya region divergence between sites at leh ladakh india and shiquanhe western tibet as well as slow relative motion among sites within the ladakh network limit right lateral slip parallel to the karakorum fault to only 3 4 5 mm yr this low rate concurs with a recent estimate of 3 4 mm yr for late holocene time but disagrees with the much higher rate of 30 35 mm yr that has been used to argue for plate like behavior of the tibetan plateau convergence between ladakh and the indian subcontinent at 18 8 3 mm yr at 224 17 1 differs little from estimates of convergence across the central segment of the himalaya finally lengthening of the baseline between leh ladakh and lhasa in southeastern tibet at 17 8 1 mm yr or between leh and bayi farther to the southeast at 18 3 mm yr is consistent with an extrapolation of rates of east west extension of the tibetan plateau based both on shorter gps baselines e g lhasa simikot and on diverging slip vectors of earthquakes in the himalaya we interpret these results to indicate that tibet behaves more like a fl uid than like a plate

crust and upper mantle shear velocity structure beneath the tibetan plateau and surrounding regions from interevent surface wave phase velocity inversion average a priori shear velocity models are constructed for the tien shan tarim basin pamir hindu kush himalaya and ne india these models are shown to account for most of the lateral velocity heterogeneity at wavelengths 1000 km interevent fundamental mode rayleigh and love wave phase velocities were measured in the period range 32 200 s and were inverted for path averaged shear velocity structures these in turn were regionalized to estimate average isotropic structures beneath the six regions listed above low upper mantle velocities are observed beneath the western and eastern tien shan the tarim basin structure is poorly constrained but exhibits very low upper crustal velocities probably due to deep sedimentary accumulation and either very high upper mantle velocities exist 4 8 km s in a layer 70 km thick or the lithosphere is very thick 180 km low upper mantle velocities are observed beneath the chang thang region the central and the northeast plateau but not beneath the southern or southeastern plateau the crustal velocity gradient with depth is zero in the east central plateau consistent with lower crustal basaltic intrusions there but is positive to the west and southeast of this zone upper mantle structures show that the indian lithosphere could have subducted beneath the entire plateau only if either 1 mantle in the depth range 115 185 km has been altered or removed to reduce its average velocity or 2 the indian lithosphere is only 85 km thick

surface wave array tomography in se tibet from ambient seismic noise and two station analysis i phase velocity maps summary empirical green s functions egfs between pairs of seismographs can be estimated from the time derivative of the long time cross correlation of ambient seismic noise these egfs reveal velocity dispersion at relatively short periods which can be used to resolve structures in the crust and uppermost mantle better than with traditional surface wave tomography we combine rayleigh wave dispersion estimates from egfs and from traditional two station ts analysis into a new approach to surface wave array tomography with data from dense receiver arrays we illustrate the methodology with continuous broad band recordings from a temporary seismographic network on the southeastern part of the tibetan plateau in sichuan and yunnan provinces sw china the egfs are robust under temporal changes in regional seismicity and the use of either ambient noise approximated by records without signal from events with magnitude mb 5 or 4 or surface wave coda produces similar results the egfs do not strongly depend on the presence of large earthquakes but they are not reciprocal for stations aligned in the n s direction this directionality reflects the paucity of seismicity to the north of the array using a far field representation of the surface wave green s function and an image transformation technique we infer from the egfs the rayleigh wave phase velocity dispersion in the period band from 10 30 s a classical ts approach is used to determine rayleigh wave phase velocity dispersion between 20 120 s together they constrain phase velocity variations for t 10 120 s which can be used to study the structure from the crust to the upper mantle beneath se tibet short and intermediate period 10 80 s phase velocities are prominently low suggesting that the crust and upper mantle beneath se tibet is characterized by slow shear wave propagation

coal fly ash interaction with environmental fluids geochemical and strontium isotope results from combined column and batch leaching experiments abstract the major element and sr isotope systematics and geochemistry of coal fly ash and its interactions with environmental waters were investigated using laboratory flow through column leaching experiments sodium carbonate acetic acid nitric acid and sequential batch leaching experiments water acetic acid hydrochloric acid column leaching of class f fly ash samples shows rapid release of most major elements early in the leaching procedure suggesting an association of these elements with soluble and surface bound phases delayed release of certain elements e g al fe si signals gradual dissolution of more resistant silicate or glass phases as leaching continues strontium isotope results from both column and batch leaching experiments show a marked increase in 87 sr 86 sr ratio with continued leaching yielding a total range of values from 0 7107 to 0 7138 for comparison the isotopic composition of fluid output from a fly ash impoundment in west virginia falls in a narrow range around 0 7124 the experimental data suggest the presence of a more resistant highly radiogenic silicate phase that survives the combustion process and is leached after the more soluble minerals are removed strontium isotopic homogenization of minerals in coal does not always occur during the combustion process despite the high temperatures encountered in the boiler early released sr tends to be isotopically uniform thus the sr isotopic composition of fly ash could be distinguishable from other sources and is a useful tool for quantifying the possible contribution of fly ash leaching to the total dissolved load in natural surface and ground waters

crustal velocity inhomogeneities along the hirapur mandla profile central india and its tectonic implications abstract wide angle seismic and gravity data across the narmada son lineament nsl in central india are analyzed to determine crustal structure velocity inhomogeneities and hence constrain the tectonics of the lineament we present the 2 d crustal velocity structure from deep wide angle reflection data by using a ray trace inverse approach the main result of the study is the delineation of fault bounded horst raised to a subsurface depth 1 5 km and the moho upwarp beneath the nsl the crust below the basement consists of three layers with velocities of 6 45 6 7 6 2 6 5 and 6 7 6 95 km s and interface depths of about 5 5 8 7 14 17 and 18 23 km along the profile the low velocity 6 2 6 5 km s layer goes up to a depth of 5 km and becomes the thickest part 13 km while the overlying high velocity 6 45 6 7 km s layer becomes the thinnest 3 km and upper boundary lies at a depth of 1 5 km beneath the nsl the overall uncertainties of various velocity and boundary nodes are of the order of 0 12 km s and 1 40 km respectively the up lifted crustal block and the up warping moho beneath the nsl indicate that the north and south faults bounding the nsl are deeply penetrated through which mafic materials from upper mantle have been intruded into the upper crust gravity modeling was also undertaken to assess the seismically derived crustal features and to fill the seismic data gap the lateral and vertical heterogeneous nature of the structure and velocity inhomogeneities in the crust cause instability to the crustal blocks and played an important role in reactivation of the narmada south fault during the 1997 jabalpur earthquake

seismic evidence for stepwise thickening of the crust across the ne tibetan plateau we performed a receiver function analysis on teleseismic data recorded along two 550 km long profiles crossing the northeastern tibetan plateau results from time to depth migration grid search vp vs determination and simulated annealing inversion of waveforms reveal that the crust thickens from 50 km near the northern edge of the plateau to 80 km south of the jinsha suture in the qiang tang block crustal thickening occurs in staircase fashion with steps located beneath the main reactivated sutures the vp vs ratio close to the global continental average does not suggest widespread partial melting but rather a more usual separation between an upper felsic and a lower mafic part within the northeastern tibetan crust

strontium isotopes as tracers of airborne fly ash from coal fired power plants fly ash generated by coal fired power plants is in part collected by filters in the emission stacks while a small portion is vented into the atmosphere since many of the coalfired power plants in the western united states are located in the desnrt the ability to monitor fly ash emissions requires a chemical tracer that utilizes desert soil and plant interactions with the fly ash deposited in the desert environment this investigation presents the results of a controlled greenhouse experiment in which a native desert plant the brittlebush encelia farinosa was grown on admixtures of desert soils and fly ash the fly ash is strongly enriched in sr and the brittlebush is a sr accumulator the data demonstrate that 1 the brittlebush isotopically equilibrates with desert soils whose fly ash components are as low as 0 25 by weight 2 the fly ash sr is apparently more available to the plant than sr derived from the soils and 3 the difference between the87sr 86sr ratio of the fly ash 0 70807 and soils 0 71097 to 0 71117 warrants further investigations in the natural environment to determine the practicality of this method as a natural tracer of fly ash in the environment

thickness composition and evolution of the indian precambrian crust inferred from broadband seismological measurements abstract we present seismological constraints to model the evolution of indian precambrian crust through receiver function analysis of teleseismic waveforms recorded on 22 broadband seismographs along a n s corridor from southern granulite terrain to delhi on the aravalli craton archean terrains eastern dharwar bastar bundelkhand and aravalli cratons are underlain by crust with felsic intermediate composition vp vs 1 65 1 75 in contrast to mafic composition vp vs 1 78 of crust beneath vindhyan basin godavari basin and the narmada son lineament nsl of proterozoic age the moho depth varies between 33 and 43 km with no definite relationship with age of the crust duplex moho with highly mafic underplated lower crust is characteristic feature of the nsl signifying its paleo rift character present day felsic character of the indian archean crust could be a consequence of post formation modification of the initial mafic lower crust through the process of lithospheric delamination this major tectonic event also created layered lithosphere beneath parts of india imprinted as positive velocity contrast at a depth of 80 km usually referred to as hales discontinuity the upper part of the lithosphere has shear velocity vs 4 52 km s and the lower one 4 77 km s we speculate that such a layered lithosphere could be due to a low density depleted spinel peridotite with archean affinity lying over higher density fertile garnet peridotite of proterozoic age

argon 39 argon 40 dating of metamorphic and plutonic events in the north and high himalaya belts southern tibet china new 39ar 40ar radiometric data are presented they concern the metasedimentary series and the orthogneiss pluton of kangmar ages as young as 13 ma are found which may be related either to the metamorphism or to the resetting of the minerals during the emplacement of the numerous intrusive bodies into the north himalaya belt inherited excess argon is found in micas from the orthogneiss previously dated at 562 ma these bodies are constituted by leucogranitic magmas the biotites and the muscovites of which the granites intrude lhagoi kangri massif same range of ages is found in the high himalaya belt for micas from leucogranites ages as young as 4 ma are found on micas from the southern part of the cross section between nyalam and zham the recent uplifts and or the movement of recent normal faults may be related to these young ages

west east variation in crustal thickness in northern lhasa block central tibet from deep seismic sounding data 5 30 km depth 5 0 vp 6 4 km s a middle crust 33 45 km depth 6 5 vp 6 8 km s and a lower crust depths below 48 km 7 0 vp 7 4 km s the west to east increase in crustal thickness is accomplished by a 50 thickening of the middle and lower crust larger vertical velocity gradients separate these three layers and bound them above surficial and sedimentary rocks and below moho transition zone the most notable low velocity zone in the crust lies at the base of the upper crustal layer s wave velocity structure is less well constrained but parallels the p wave structure except that vp vs ratios may decrease from west to east in the lower crust our data suggest considerable variation in structure along strike of the tibetan plateau and show that interpretations of tibet as a purely two dimensional orogen are overly simplistic the west east increase in crustal thickness may occur across the karakorum jiali fault system and be an indicator of lateral tectonic escape of the qiangtang terrane

imaging the indian subcontinent beneath the himalaya new seismic images reveal the geometry of the indian plate as it passes beneath the himalayan plateau several important subsurface features of vanished india are revealed including a shear zone above the base of the indian crust beneath the himalaya and conversion of the deep indian crust into denser rock the rocks of the indian subcontinent are last seen south of the ganges before they plunge beneath the himalaya and the tibetan plateau they are next glimpsed in seismic reflection profiles deep beneath southern tibet1 yet the surface seen there has been modified by processes within the himalaya that have consumed parts of the upper indian crust and converted them into himalayan rocks2 3 the geometry of the partly dismantled indian plate as it passes through the himalayan process zone has hitherto eluded imaging here we report seismic images both of the decollement at the base of the himalaya and of the moho the boundary between crust and mantle at the base of the indian crust a significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths north of the himalaya the lower indian crust is characterized by a high velocity region consistent with the formation of eclogite a high density material whose presence affects the dynamics of the tibetan plateau

observations of frequency dependent sn propagation in northern tibet summary we present new observations of the frequency dependent propagation efficiency of the seismic phase sn over the tibetan plateau our measurements are the ratio of the sn amplitude to the pg coda amplitude on tibetan regional seismograms and we map the lateral variation in the maximum and mean values of this ratio across the plateau good path density and azimuthal coverage allow the area of sn blockage identified by previous studies to be better constrained an important result is that at low frequencies 0 2 hz sn propagates efficiently over the entire plateau whereas at higher frequencies 1h z sn is blocked for a region of the northern plateau as we take measurements at higher frequencies we find that the southern boundary of the region of inefficient propagation migrates to the south the observation that low frequency sn propagates efficiently across the whole plateau suggests that at the length scales sampled by these waves the upper mantle has an overall positive shear velocity gradient which could indicate that the high velocity seismic lid or seismic lithosphere beneath tibet is still intact and has not delaminated as some researchers have previously proposed the observation that high frequency sn does not propagate beneath northern tibet suggests that at the shorter length scales sampled by the higher frequency waves the upper mantle has a negative shear velocity gradient this could indicate that at shallow depths the sub moho upper mantle is hot and may contain some melt

sediment volume and mass beneath the bay of bengal rates of sediment accumulation and the amount of sedimentary fill in depocenters lying downstream of erosion in the himalayas and tibet can provide some insight into tectonics and geological history the objective of this paper is to put on record the best estimates which are possible with existing data of the volume and mass of sediments sedimentary rock and metasedimentary rock beneath the sea floor of the bay of bengal the sedimentary section in the bay of bengal is divided into two parts 1 eocene through holocene sediments and sedimentary rocks which post date the initial india asia collision volume 12 5 106 km3 mass 2 88 1016 t this is most of the bengal fan including its eastern lobe the nicobar fan plus some of the outer bengal delta 2 early cretaceous through paleocene pre collision sedimentary and metasedimentary rocks volume 4 36 106 km3 mass 1 13 to 1 18 1016 t these are interpreted as continental rise and pelagic deposits

teleseismic imaging of subducting lithosphere and moho offsets beneath western tibet teleseismic images suggest that the tarim plate plunges 45 s down to 300 km depth beneath nw tibet the 410 km discontinuity shallows by 10 km under the plateau implying 100 c cooler upper mantle the deepest moho on record 90 km lies under w qiangtang it rises abruptly by 20 and 10 km beneath the altyn tagh fault and bangong suture respectively vp vs ratios are normal except in the yecheng flexural basin and deep under the south karakax volcanics 1 92 w kunlun s neogene tectonics are simply accounted for by oblique subduction of lithospheric mantle beneath an upward extruding thrust wedge of the tarim crust

high resolution rayleigh wave slowness tomography of central asia 1 we compute short period high resolution surface wave slowness maps for central asia using bayesian tomography we focus on the region between 69 and 108 e and 29 and 54 n and used seismograms from more than 1100 events using multiple filter and phase matched filter techniques we measured the dispersion characteristics of the signals between 6 and 30 s period these rayleigh wave group velocity dispersion curves were used to compute high resolution half degree cell size slowness tomographic maps because short periods are primarily sensitive to upper crustal structures the images display low velocities associated with the tarim junggar and qaidam basins relatively high velocities are associated with mountainous tectonic features such as the tian shan we validated our maps using dispersion curves from 640 events that were not used to construct the tomographic model the model predictions show a significant variance reduction at short and intermediate periods 6 15 s with respect to the prior model our model also shows 15 improvement in surface wave detection capability with respect to previous one dimensional models these high resolution short period tomography maps can help improve regional magnitude estimations for construction of mb ms discriminants moreover the short period surface wave tomographic results show unprecedented resolution that reveals greater geologic detail than has previously been achieved using surface waves and that give us insight into the shear velocity structure of the crust underlying this part of asia

pb and sr isotopic evidence for sources of atmospheric heavy metals and their deposition budgets in northeastern north america concentrations of al ba cd cu mg mn pb rb sr and zn as well as pb and sr isotopic compositions were determined in samples of snowpack obtained during the 1998 winter season from northeastern north america median enrichment factors relative to upper crustal abundances for cd cu mn pb and zn for all samples range from ca 50 to 36 000 and are indicative of an anthropogenic origin the pb isotope ratios correlate with geographic location because snowpack from eastern ontario and the northeastern usa are characterized by the most radiogenic 206pb 207pb ratios ca 1 18 1 19 and these decrease systematically in an easterly direction towards the atlantic coast ca 1 16 the pb isotope data for the 1998 samples of snowpack indicate that atmospheric pollution in this region of north america is dominated solely by a mixture of anthropogenic emissions from us 206pb 207pb ca 1 20 and canadian 206pb 207pb ca 1 15 sources this result contrasts with that obtained for 1997 snowpack from similar geographic regions because the pb isotope data for the latter suggest the involvement of an additional anthropogenic component possibly that of eurasian pollution transported over the high arctic this difference in the distribution pattern of atmospheric pollution over northeastern north america between the 1997 and 1998 winter seasons may be related to the el nino phenomenon sr isotope data for all the 1998 snowpack samples define a large range in 87sr 86sr values but most are between 0 709 to 0 710 similar to the value of present day seawater ca 0 7092 however the majority of snowpack samples from eastern ontario are characterized by lower 87sr 86sr ratios 0 709 and these correlate negatively with their enrichment factors of pb such trends suggest the presence of a relatively unradiogenic source of anthropogenic sr possibly related to emissions from coal fired power plants located in the western and midwestern regions of the usa annual depositional budgets estimated from 1998 snowpack indicate that those for pb cd and cu are highest in eastern ontario and significantly lower in the remaining regions the former reflect a meteorological corridor in northeastern north america which coincides with heavily industrialized and densely populated areas depositional budgets of zn are extremely high in all areas investigated and their exact source s remains enigmatic these may reflect either an increase in atmospheric industrial emissions since the 1980s a lower residence time in the atmosphere or particular chemical speciation that affects the deposition of zn from the lower troposphere depositional budgets for mn are also high and may be related to the combustion of mn bearing fossil fuels by automotive vehicles

seismic heterogeneities in the indian lithosphere abstract rayleigh wave group velocity distributions over the indian region were obtained by the backus gilbert method for time periods 15 25 40 50 and 60 s the observed velocity variations reveal sharp lithospheric heterogeneities in this region southern peninsular india is marked by high velocities in contrast to the northern indo gangetic plain and the himalaya highest surface wave velocities are observed over the southern part of the deccan traps and the dharwar craton whereas the metamorphic terrains and rift zones exhibit anomalous low velocities the nepal himalaya and indo burmese collision zones are characterised by contrasting velocities which reflect their diverse orogenesis the bay of bengal is marked by an anomalous 6 12 higher velocity as compared with the indian continental region for the periods 25 50 s

two cases of continental collisions and related tectonics during the proterozoic period in india insights from gravity modelling constrained by seismic and magnetotelluric studies abstract the aravalli delhi mobile belt admb in nw india includes the proterozoic aravalli and delhi rifts and fold belts a seismic profile and gravity modelling across the admb shows several dipping reflections and suggests a thick crust 45 46 km and a high density 3 04 g cm 3 dome shaped body in the lower crust this high density body in the lower crust may represent underplated lower crust caused by extension during evolution of the proterozoic rifts part of the gravity high observed over the admb is attributed to the upthrust blocks of high density lower crustal granulite rocks the gravity low along the eastern margin of the admb is modelled as being due to a westerly dipping 45 low density body in the crust in accordance with seismic reflectors which may represent remanents of the subducted crustal rocks the hindoli group of rocks above this body is characterized by high conductivity indicating a high fluid content and a fractured nature which may be related to the collisional tectonism of this region a sharp gravity high along the western margin of the admb is attributed to the basic rocks of an ophiolite melange the gravity low and high west of the admb are attributed to plutonic activity and bimodal calc alkaline volcanics in linear basins and are related to arc and back arc rift magmatism respectively the admb therefore represents a proterozoic collision zone between the bundelkhand craton towards the east and the rajasthan craton towards the west with a plausible subduction towards the west along the eastern margin of the admb the thrusting is mainly confined to the obducted block towards the west although some thrusting is inferred even in the subducted block the central indian suture cis is located at the southern margin of the satpura mobile belt smb in central india and separates high grade metasediments and granulite towards the north from low grade proterozoic volcanogenic successions towards the south a seismic profile and gravity modelling across the cis show a thick crust with a thicker crust north of the cis compared to the south and a contact in the lower crust whose projection on the surface coincides with the cis the cis at the surface coincides with a low density 2 65 g cm 3 and conductive body that is interpreted to be fluid filled and fractured in nature the high density 2 81 g cm 3 body modelled in the upper crust north of the cis coinciding approximately with a highly conductive body and dome shaped seismic reflectors may represent an upthrust block from the lower crust as high density granulite rocks are exposed in this region short wavelength gravity highs and lows south of the cis represent exposed metavolcanics and granite intrusions which represent tholeiitic series rocks to calc alkaline series rocks associated with island arcs and back arc magmatism therefore the large wavelength gravity low south of the cis is modelled as a low density body below the moho which conforms with seismic reflectors and may represent crustal rocks subducted towards the south thrusting in this case is indicated in the subducted block towards the north of the cis the bipolar gravity anomaly across the cis therefore indicates a collision zone between the bundelkhand craton towards the north and bhandara deccan cratons towards the south

strontium isotopes as tracers of coal combustion residue in the environment we evaluated the application of sr isotopes as tracers of coal combustion residues in the environment strontium isotopes unlike light stable isotopes do not fractionate and can be used to monitor interactions between coal combustion residue and hydrogeologic systems the primary objective of this investigation was to determine if sr isotopes could detect the presence of flyash and other effluents produced by coal combustion in the environs surrounding a coal fired power plant in the southwestern united states analyses of coal flyash and bottom ash indicated a 87sr 86sr range of 0 70883 to 0 70972 for these residues this range in87sr 86sr exceeds the error of an individual analysis 0 00002 and represented our criteria to establish evidence for impact i e any sample whose 87sr 86sr ratio lies within this range received sr from fossil fuel ff residue natural background materials groundwater soil rocks have87sr 86sr ratios which exceed 0 710 and are therefore markedly distinct from the ff residue our analytical techniques were developed to selectively leach the flyash derived sr from soils this makes the detection of trace quantities 0 1 of ff residue possible soil sr leachate analyses suggest that ff residues have impacted on site soils to depths of 51 5 15 5 m the 87sr 86sr ratios of soil leachates below a ff waste evaporation pond lie within the range of the ff residue this supports our selection of the sr isotopic range defined by the ff residue as our criteria for impact off site soil analyses show that surface 0 1 cm soils collected from as far as 12 5 km in all geographic directions have interacted with flyash detailed analyses of deeper soils 1 5 cm 5 10 cm indicate that flyash impact varies with depth plants such as the brittlebush growing on such soils uptake sr from both the flyash and soil their 87sr 86sr ratios represent mixtures of the two components our data suggest that groundwater has not been severely impacted by ff residue in most cases waste evaporation pond waters are more commonly impaired temporal variations in the 87sr 86sr ratios of groundwater and waste evaporation pond water were observed the data may be used to assess the quality of these waters over time

recent evolution of a mediterranean deltaic coastal zone human impacts on the inner thermaikos gulf nw aegean sea the inner thermaikos gulf is located in the northwestern aegean sea receiving water and sediment fluxes from the axios aliakmon gallikos and loudias rivers the geomorphological and sedimentological evolution of the system is reconstructed for the last 150 years 1850 2000 on the basis of detailed analysis of historical bathymetric charts late holocene history is considered within the context of changing riverine sediment supply and human activities three evolutionary stages are identified stage i 1850 1916 corresponds to a natural phase of rapid deltaic progradation and sea floor deposition with an average sediment accumulation rate of 6 5 10 6 m 3 a 1 during stage ii 1916 1956 human interference e g artificial changes in river delta plains realignment of channels and land reclamation schemes to the deltaic system increased sediment delivery to the coastal waters by a factor of three this in turn enhanced the progradation of the active river mouth areas in contrast stage iii 1956 2000 is characterized by significant coastline deltaic retreat and erosion of the adjacent sea floor net loss of 2 5 10 6 m 3 a 1 this was as a result of extensive river damming which trapped a significant part of the sediment moving seaward furthermore these human impacts have affected the character of the surficial sea bed sediments of the gulf by reducing the proportion of mud the response of the deltaic margin of the inner thermaikos gulf to various anthropogenic interventions seems to be analogous to that of other deltas in the mediterranean region where large drainage projects the development of irrigation networks and dam construction have taken place within their river basins

constraints on the structure of the tibet plateau from pure path phase velocities of love and rayleigh waves pure path phase velocities of love and rayleigh waves across the tibet plateau are determined for the period range 30 90 s and for two opposite directions of propagation along the length of the plateau the method used is a two event method similar in principle to the classical two station method where the roles of stations and events are interchanged the success of this method is due to a favorable geometry whereby events located at each end of the plateau align to within 1 of a great circle path with one wwssn station at teleseismic distance to the east and one to the west body wave modeling is used to constrain depth and source mechanism of the events in order to calculate the corresponding source phases for surface waves phase velocities are determined to within 0 02 km s or better for rayleigh waves and 0 007 km s for love waves the constraints added by these data to the crust and upper mantle structure beneath tibet are discussed in the light of previous studies the best fitting model has a 65 km thick crust with low average crustal velocities and an sn velocity of 4 65 km s the low velocity zone is limited to the depth range 100 150 km the interesting result of this study is that both love and rayleigh wave data can be matched to within 1 in phase velocity by a unique isotropic model this points to a similarity with a stable region upper mantle structure such as that of the canadian shield finally additional rayleigh wave phase velocity data are used to compare propagation on different paths in the vicinity of tibet the tarim basin to the north of tibet stands out as having a distinct crustal structure more similar to an average tectonic region structure moreover evidence is presented for the existence of a very low velocity region at the western end of tibet which correlates well with a pronounced gravity low observed in that area

ambient noise rayleigh wave tomography in western sichuan and eastern tibet abstract we perform rayleigh wave tomography in western sichuan and eastern tibet by applying the ambient noise method to broadband data recorded at 49 stations from the sichuan digital seismic network cross correlations of vertical component ambient noise data are computed in one hour segments and stacked over 7 months from june to december 2007 then rayleigh wave group dispersion curves from 6 to 48 s were measured using the multiple filter analysis method group velocity maps at periods from 8 to 44 s were inverted the resulting group velocity maps generally show good correlations with tectonic features the sichuan basin is clearly resolved with lower group velocities at the short periods 8 to 20 s due to thick sedimentary layers and a prominent low velocity zone exhibits in the middle crust beneath the eastern margin of the tibetan plateau the longmen shan thrust belt marks not only the rapid lateral velocity variation between the topographically high tibetan plateau and low elevation sichuan basin but also the transition from the thick crust beneath the tibetan plateau to the normal continental crust beneath the sichuan basin

crustal structure of northern and southern tibet from surface wave dispersion analysis 1 group and phase velocities of fundamental mode rayleigh waves in the period range of 10 to 70 s are obtained for southern and northern tibet significant variations in crustal velocity structure are found the group velocity minimum for tibet occurs at 33 s and the minimum is 0 12 km s lower for southern tibet than for northern tibet at periods greater than 50 s however group velocities are up to 0 2 km s faster in southern tibet the group and phase velocities are inverted for layered s wave models the dispersion observations in southern tibet can only be fit with a low velocity layer in the middle crust in contrast the velocity models for northern tibet do not require any lowvelocity zone in the crust the s wave velocity of the lower crust of southern tibet is 0 2 km s faster than the lower crust of northern tibet in southern tibet the sub moho velocity increases with a positive gradient that is similar to a shield while there is no velocity gradient beneath northern tibet the high velocity lower crust of southern tibet is consistent with the underthrusting of indian continental lithosphere the most plausible explanation of the mid crustal low velocity zone is the presence of crustal melt resulting from h2o saturated melting of the interplate shear zone between the underthrusting indian crust and overflowing asian crust the lack of a pronounced crustal low velocity zone in northern tibet is an indication of a relatively dry crust the low s wave velocity in the lower crust of northern tibet is interpreted to be due to a combination of compositional differences high temperatures presumably caused by a high mantle heat flux and possibly small amounts of partial melt combined with all available observations in tibet the new surface wave results are consistent with a hot and weak upper mantle beneath northern tibet index terms 7205 seismology continental crust 1242 7218 seismology lithosphere and upper mantle 7255 seismology surface waves and free oscillations 8102 tectonophysics continental contractional orogenic belts keywords tibet crustal velocity structure surface wave rayleigh waves continental collision

group velocity tomography of the indo eurasian collision zone we present results of a rayleigh and love wave group velocity dispersion study of the indo eurasian collision zone group velocity dispersion curves are measured and combined to produce dispersion maps for 10 70 s period rayleigh waves from 4054 paths and for 15 70 s love waves from 1946 paths group velocity maps benefit from the inclusion of data recorded at a large number of stations within india an advantage over previous global studies this has the largest impact at short periods as a result of the improved path length distribution synthetic tests are used to estimate resolution which ranges from 3 to 5 on the continents for rayleigh wave maps and from 5 to 7 5 for love wave maps group velocities correspond well with known geological and tectonic features and show good correlation with sediment thickness at short periods the cratons of the indian shield can be distinguished in the short period and midperiod group velocities group velocities are slow across tibet until 70 s whereas the cratonic cores of the indian shield appear as a high velocity anomaly at 70 s dispersion curves extracted from the rayleigh wave group velocity maps are inverted for shear wave velocity as a function of depth for profiles across india and tibet the relationship between shear velocity contours and the moho indicated by receiver function studies has been used to obtain a first order estimate of crustal thickness across the collision zone results suggest a slow tibetan midcrust and low sub moho velocities beneath the central and northeastern tibetan plateau

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

midcrustal low velocity layer beneath the central himalaya and southern tibet revealed by ambient noise array tomography 1 ambient noise tomography has been becoming an important tool to image the shallow lithospheric structure of the earth using 2 months of ambient noise data from 20 stations of the himalayan nepal tibet seismic experiment we investigate the upper and middle crustal structure in the central himalaya and southern tibet about 120 interstation rayleigh wave empirical green s functions with sufficient signal to noise ratio are obtained and used for group velocity dispersion analysis in the period range 6 25 s using frequency time analysis technique the obtained dispersion data are then used to construct 2 d group velocity maps at the short periods from 9 to 15 s the distribution of rayleigh wave velocities delineates several distinct low and high velocity zones separated mainly by geological boundaries the high group velocity zone is located mainly around regions with plutonic rocks and the low group velocity zone is located around regions with sedimentary or metasedimentary rocks finally we invert for the shear velocity structure in the upper and middle crust along a n s trending cross section at the longitude 86 5 e we observe a clear low velocity layer in the middle crust about 10 25 km depth distributed on both sides of the indus yarlung suture zone the existence of this midcrustal low velocity zone suggests a mechanically weaker middle crust beneath the central himalaya and southern tibet which might decouple the upper crustal deformation from that of the lower crust in the tibetan himalayan orogenic processes

structure and tectonics of the proterozoic aravalli delhi fold belt in northwestern india from deep seismic reflection studies abstract seismic imaging of the crust along a 400 km long deep seismic reflection profile across the palaeo mesoproterozoic aravalli delhi fold belt in the northwestern indian shield brings into focus its complex structure and provides clues to understand the geological processes involved in the evolution of this belt the reflectivity pattern varies considerably for different crustal units along the profile the deep crustal reflection data image two sets of oppositely dipping strong reflection bands from upper to lower crustal levels these are identified as the signatures of the collision corresponding to aravalli and delhi orogeny the data also exhibit a clear moho and strong lower crustal reflections near the collision boundaries a stack of dipping reflections from the top of the moho to the surface is identified as a major thrust fault indicating that the proterozoic collision and deformation were primarily thick skinned in nature

phase velocity structure from rayleigh and love waves in tibet and its neighboring regions deep geodynamic processes involved in the continental collision between india and asia are still controversial to address this issue phase velocities of 619 rayleigh waves and 254 love waves have been inverted by using an anisotropic tomographic technique such a technique enables us not only to retrieve phase velocity distributions but also to map azimuthal anisotropy on a large scale only phase velocity anomalies in the period range 50 200 s are dealt with here but the azimuthal anisotropy has been taken into account in the inversion process moreover because of the important influence of shallow layers on the inversion results an inversion of phase velocities corrected for shallow layers has been performed to correct raw phase velocities the 3smac crust heterogeneous reference earth model has been used the comparison of phase velocity distributions for love and rayleigh waves with and without corrections of surficial layers implies that much of the low velocity zone visible at short periods in central tibet can be explained by unusually thick crust at periods longer than 100 s tibet is overall characterized by high velocity anomalies such anomalies seem to converge toward the center of the plateau as period increases we conclude that the phase velocity structure beneath tibet results at short periods from thick crust and possible partial melting in the uppermost lithosphere and at long periods from the inward plunge of the cold lithospheric mantle surrounding the high plateau

crustal structure in tibet high seismic velocity in the lower crust a least squares procedure is used to extract pure path tibetan group velocities in the period range 8 100 s from fundamental mode rayleigh and love waves recorded along 50 mixed paths the crustal model derived from these dispersion data reveals that the lower half of the very thick 74 10 km tibetan crust has a shear velocity similar to those found in the lower crust beneath stable continental and shield regions a second interesting feature of the model is the presence of a prominent low velocity channel at midcrustal depth the uniqueness of the tibetan crustal structure places the plateau region in a class by itself among tectonic provinces characterized by high heat flow and or wide spread recent volcanism setting stringent constraints on tectonic models of the tibetan plateau