strain heating as a mechanism for partial melting and ultrahigh temperature metamorphism in convergent orogens implications of temperature dependent thermal diffusivity and rheology 1 we explore the importance of strain heating in ductile shear zones for production of leucogranites and high temperature metamorphism in collisional orogens with temperature dependent thermal diffusivity d and rock rheology new measurements on metamorphic rocks from the bohemian massif show that d is 1 3 mm2 s at 25 c and decreases exponentially to as low as 0 4 mm2 s at 600 c this temperature dependence causes model lithospheric geotherms to be straighter compared to geotherms calculated with a constant d using power law parameters for rheology we show that deformation of quartzite at strain rate of 3 10 13 s 1 produces 100 w m3 at 550 c and 7 w m3 at 800 c deformation of stronger clinopyroxenite at this strain rate produces 8 w m3 even at 1050 c when strain heating is introduced by a 3 km thick ductile shear zone with quartz rheology at the depth of 35 km in a lithosphere that has 70 km thick crust and 60 km thick lithospheric mantle the schist solidus is reached by 8 myr in the vicinity of the shear zone deforming at the strain rate of 3 10 13 s 1 for clinopyroxenite rheology ultrahigh temperature uht metamorphic conditions are reached in 40 myr after initiation of strain heating two dimensional models that replicate the exhumation of the greater himalaya crystalline rocks above the main central thrust produce extensive partial melting and an inverted metamorphic field gradient occurrences of leucogranites within crustal shear zone systems may be evidence of the coupling between deformation and heat production in the crust

dynamic topography produced by lower crustal flow against rheological strength heterogeneities bordering the tibetan plateau summary dynamic stresses developed in the deep crust as a consequence of flow of weak lower crust may explain anomalously high topography and extensional structures localized along orogenic plateau margins with lubrication equations commonly used to describe viscous flow in a thin gap geometry we model dynamic stresses associated with the obstruction of lower crustal channel flow due to rheological heterogeneity dynamic stresses depend on the mean velocity u viscosity and channel thickness h uniquely through the term u h 2 these stresses are then applied to the base of an elastic upper crust and the deflection of the elastic layer is computed to yield the predicted dynamic topography we compare model calculations with observed topography of the eastern tibetan plateau margin where we interpret channel flow of the deep crust to be inhibited by the rigid sichuan basin model results suggest that as much 1500 m of dynamic topography across a region of several tens to a hundred kilometres wide may be produced for lower crustal material with a viscosity of 2 10 18 p asfl owing in a 15 km thick channel around a rigid cylindrical block at an average rate of 80 mm yr 1

neotectonic deformation in central eurasia a geodynamic model approach funding was granted by the spanish government through the project mite cgl2014 59516 p and alpimed piecsic 201530e082

a 3d shear wave velocity model of the upper mantle beneath china and the surrounding areas we present a three dimensional model of shear wave velocity for the upper mantle of china and the surrounding region by analyzing 50 338 vertical component multi mode rayleigh wave seismograms recorded at 144 permanent and more than 300 temporary broadband stations in and around china the procedure involves combination of 1 d path average models obtained by modeling each rayleigh waveform up to the 4th higher mode in a tomographic inversion scheme the dense station network and the use of multi mode analysis help to achieve a lateral resolution of a few hundred kilometers down to 400 km depth the seismic lithosphere as it is defined by the crust and the high velocity mantle lid is to the first order thin in east china and thick in the west with a high velocity lid extending down to about 200 km depth beneath much of the tibet pamir plateau beneath india the thickness of the seismic lithosphere gradually increases from 100 km in south india to more than 150 km in north india where it underthrusts the tibetan plateau to approximately the jinsha river suture high velocity lid extending down to 100 150 km depth is also observed in the tarim basin sichuan basin and ordos block in the eastern part of the north china craton the seismic lithosphere is probably close to or thinner than 70 km adjacent to these areas the high velocity lid in the eastern yangtze craton and south china fold system extends down to 70 80 km depth a large scale subhorizontal high velocity body is observed at depths of 150 350 km beneath the entire east china cratonic areas this high velocity body might be the remnant of a delamination process which resulted in the decratonization of the north china and the yangtze cratons

shear wave model of southern africa from regional rayleigh wave tomography with 2 d sensitivity kernels summary rayleigh wave phase velocities are computed at the period range of 20 167 s in southern africa from a two plane wave tpw tomography method using 2 d born approximation sensitivity kernels these phase velocities are compared with the results of the tpw method using gaussian sensitivity functions and the correlation between the two methods is good up to 100 s the correlation coefficient becomes less than 0 5 at periods greater than 111 s indicating significant finite frequency effects at long periods the correlation of phase velocity between adjacent frequencies increases with period for inversions using the 2 d sensitivity kernels while such a systematic trend is not observed for inversions using gaussian sensitivity functions in addition low phase velocity anomalies at long periods 111 167 s are better and more consistently imaged from inversions using 2 d sensitivity kernels to account for finite frequency effects and improve resolution the 2 d sensitivity kernels are therefore recommended in regional tpw tomography a 3 d shear wave model is developed from the phase velocities obtained in southern africa the new model confirms the existence of a fast mantle lid underlain by a low velocity zone revealed in a previous model constrained from the tpw method using gaussian sensitivity functions one new feature in this model is the vertical alignment of a shallow low velocity anomaly with a deep high velocity anomaly at the western bushveld province the alignment provides new evidence to support the interpretation of the slow anomaly as the result of high iron content from the bushveld intrusion and the fast as a more depleted residual mantle a low velocity channel at the depths of 220 310 km from the namaqua natal belt to the northwest of the kaapvaal craton is also imaged it suggests that the hot asthenosphere beneath the mobile belt can migrate into the craton area through a weak channel and produce thermal disturbance at the base of the craton further more low velocity anomalies from 100 to 140 km in the 3 d model agree well with the localities of kimberlites erupted at 65 104 ma in the kaapvaal craton this observation supports that kimberlites are generated in the lithosphere by thermal disturbance and provides additional evidence for the depth extent of mantle xenoliths

multiscale finite frequency rayleigh wave tomography of the kaapvaal craton summary we have measured phase delays of fundamental mode rayleigh waves for 12 events recorded by the southern africa seismic experiment at frequencies between 0 005 and 0 035 hz a novel multiscale finite frequency tomographic method based on wavelet decomposition of 3 d sensitivity kernels for the phase of rayleigh waves is used to map the shear velocities in the upper mantle beneath southern africa the kernels are computed by summing coupled normal modes over a very fine grid surrounding the seismic array to estimate and minimize the biases in the model resulting from structures outside the tomographic grid a jackknife inversion method is implemented the contribution of heterogeneities outside the target volume is significant but produces artefacts in the tomographic model that are easily identified and discarded before interpretation with structures on length scales as short as 100 km retrieved beneath the array the deep structure of the kaapvaal craton is revealed with unprecedented detail outside the array the corresponding resolution is 200 km high velocity cratonic roots are confined to the archean craton and extend to depths of at least 250 km confirming earlier surface structural studies we recognize two distinct units in the kaapvaal craton the eastern witwatersrand block and the western kimberley block are separated by a major near vertical translithospheric boundary which coincides with the colesberg lineament lower than average velocities south and east of the kaapvaal craton reveal extensive metasomatism and heating of the lithosphere probably related to the karoo magmatic event and to the opening of the south atlantic ocean

northward channel flow in northern tibet revealed from 3d magnetotelluric modelling abstract the kunlun fault defines one of the major northern tectonic boundaries of the tibetan plateau previous geophysical studies have detected a major change in rheology across this boundary but it is not clear how this is related to models that have invoked crustal flow the lithospheric resistivity structure of the kunlun fault has been investigated by both the indepth iii and iv magnetotelluric mt transects all the mt data were processed using modern statistically robust methods and have been analysed for directionality and dimensionality in order to improve understanding of the anisotropic distribution of melt previously revealed by our remodelling of the indepth iii mt data a variant approach on 3d inversion of 2d profiles was investigated to explore and improve lateral resolution in addition to the apparent surficial deformation associated with the sinistral strike slip kunlun fault the 3d modelling of the indepth mt data reveals that complex deformation processes are occurring at mid crustal depths in northern tibet the 3d mt inversion results supported by synthetic modelling particularly confirm and highlight the presence of separate north south intrusions of conductive material crossing the kunlun fault into the more resistive kunlun qaidam block these north south intrusions are interpreted to be associated with the horizontal channel flow of partially molten songpan ganzi crust into two or more separated channels moving northwards and crossing the surficial trace of the kunlun fault at mid to lower crustal depths

wind and buoyancy induced transport of the norwegian coastal current in the barents sea 1 the focus of this study is on the fluxes and forcing of the norwegian coastal current ncc at the entrance to the barents sea the structure and dynamics of the ncc which consists of a slope branch and an offshore branch are investigated using 1 a recent 1 year full depth current meter record in the core of the slope region 2 repeated hydrographic data at an inshore monitoring station 3 broader regional hydrographic surveys and 4 atmospheric reanalysis data the total fluxes are estimated to 1 8 sv for volume and 26 msv for freshwater relative to a reference salinity of 34 8 with the largest contributions from baroclinic offshore branch the heat flux calculated for the slope branch only is 34 tw these estimates are higher compared to earlier estimates but are based on more comprehensive data the major mode of variability in the slope branch is consistent with a continental shelf wave on time scales in the range of 3 16 days forced by the along coast component of the wind stress maximum along slope currents during fall winter corresponding to the stronger wind forcing during these seasons suggest that the observed seasonality in the ncc can be attributed to a similar mechanism

water mass transformation in the barents sea inferred from radiogenic neodymium isotopes rare earth elements and stable oxygen isotopes abstract nearly half the inflow of warm and saline atlantic water aw to the arctic ocean is substantially cooled and freshened in the barents sea which is therefore considered a key region for water mass transformation in the arctic mediterranean am numerous studies have focused on this transformation and the increasing influence of aw on arctic climate and biodiversity yet geochemical investigations of these processes have been scarce using the first comprehensive data set of the distributions of dissolved radiogenic neodymium nd isotopes expressed as nd rare earth elements ree and stable oxygen isotope 18o compositions from this region we are able to constrain the transport and transformation of aw in the barents sea and to investigate which processes change the chemical composition of the water masses beyond what is expected from circulation and mixing inflowing aw and norwegian coastal water ncw both exhibit distinctly unradiogenic nd signatures of 12 4 and 14 5 respectively whereas cold and dense polar water pw has considerably more radiogenic nd signatures reaching up to 8 1 locally formed barents sea atlantic water bsaw and barents sea arctic atlantic water bsaaw are encountered in the northeastern barents sea and have intermediate nd values resulting from admixture of pw containing small amounts of riverine freshwater from the ob this article is part of a special issue entitled cycles of trace elements and isotopes in the ocean geotraces and beyond edited by tim m conway tristan horner yves plancherel and aridane g gonzalez

indepth iii seismic data from surface observations to deep crustal processes in tibet 1 during the summer of 1998 active source seismic data were collected along a transect running 400 km nnw sse across the central tibetan plateau as the third phase of project indepth international deep profiling of tibet and the himalaya the transect extends northward from the central lhasa block across the jurassic bangong nujiang suture bns at 89 5 e to the central qiangtang block a seismic velocity model for the transect to 25 km depth produced by inversion of p wave first arrivals on 3000 traces shows 1 a 50 km wide region of low velocity at least 5 less than surrounding velocities extending to the base of the model at the bns 2 sedimentary cover for the southern qiangtang block that is 3 5 km thick 3 a distinct interface between sedimentary cover and qiangtang basement or underplated jurassic melange in the central qiangtang block and 4 evidence that the bangoin granite extends to a depth of at least 15 km the bns has little geophysical signature and appears unrelated to the 5 km northward shallowing of the moho which is associated with the bns in central tibet geophysical data along the main indepth iii transect show little evidence for widespread crustal fluids in contrast to the seismic bright spots found in southern tibet and to magnetotelluric evidence of fluid accumulations in eastern tibet a comparison between the global average and tibetan velocity depth functions offers constraints for models of plateau uplift and crustal thickening taken together with the weak geophysical signature of the bns these velocity depth functions suggest that convergence has been accommodated largely through pure shear thickening accompanied by removal of lower crustal material by lateral escape likely via ductile flow although we cannot resolve the details we believe lateral lower crustal flow has overprinted or destroyed evidence in the deep crust for the earlier assembly of tibet as a series of accreted terranes

tomographic evidence for wholesale underthrusting of india beneath the entire tibetan plateau we analyzed a global tomographic model for the tibet himalayan collision zone which indicates that the indian lithospheric slab has been subducted subhorizontally beneath nearly the entire tibetan plateau to depths of 165 260 km tibetan velocity structure is low in its crust and high in its lithospheric mantle at depths between 75 and 120 km we interpret an asthenospheric layer positioned above the subducted indian slab at depths between 120 and 165 km beneath the tibetan plateau beneath the central portion of the plateau a lowvelocity anomaly exists from the crust down to 310 km depth indicating mantle upwelling through a weakened part of the subducted slab we present a model which explains that the uplift history and low relief of the tibetan plateau is a result of subhorizontal subduction and heating of indian lithosphere that is separated from tibetan lithosphere by a thin channel of asthenosphere two predictions made by our model are 1 the amount of shortening in the himalayas is equivalent to the amount of underthrusted indian mantle lithosphere and 2 a young mantle geochemical signature should be present along the entire southern portion of the tibetan plateau q 2004 elsevier ltd all rights reserved

circulation of water masses in the baltic proper revealed through iodine isotopes abstract tracer technology has been used to understand water circulation in marine systems where the tracer dose is commonly injected into the marine waters through controlled experiments accidental releases or waste discharges anthropogenic discharges of 129i have been used to trace water circulation in the arctic and north atlantic ocean here 129i together with 127i is utilized as a tracer of water pathways and circulation in the baltic sea through collection of seawater depth profiles the results indicate the presence of 129i signatures which are distinct for each water mass and provide evidence for 1 inflow water masses through the drogden sill that may reach as far as the sw of the arkona sea 2 a portion of north atlantic water in the bottom of arkona basin 3 cyclonic upwelling which breaks through the halocline in a pattern similar to the baltic haline conveyor belt and 4 more influx of fresher water from the gulf of finland and bothnian sea in august relative to april these findings provide advances in labeling and understanding water pathways in the baltic sea

two crustal low velocity channels beneath se tibet revealed by joint inversion of rayleigh wave dispersion and receiver functions abstract competing geodynamic models such as rigid block extrusion continuous deformation and the mid lower crustal flow have been proposed to describe the growth and expansion of eastern tibet however the dynamic processes responsible for plateau evolution and deformation remain poorly understood partly due to resolution limitations of previous models of lithospheric structure on the basis of joint inversion of rayleigh wave dispersion and receiver functions using data from a newly deployed seismic array we have obtained a high resolution 3d image that reveals the distribution of low velocity zones lvzs with unprecedented clarity the prominent feature of our model is two low velocity channels that bound major strike slip faults in se tibet and wrap around the eastern himalaya syntaxis consistent with the clockwise movement of crustal material in this region most large earthquakes in this region occurred in the boundaries of the lvzs we propose that ductile flow within these channels in addition to shear motion along strike slip faults played a significant role in accommodating intensive lithospheric deformation during the eastward expansion of tibet in the cenozoic

seismic polarization anisotropy beneath the central tibetan plateau sks and skks shear waves recorded on the indepth iii seismic array deployed in central tibet during 1998 1999 have been analyzed for the direction and extent of seismic polarization anisotropy the 400 km long nnw trending array extended south to north from the central lhasa terrane across the karakoram jiali fault system and banggong nujiang suture to the central qiangtang terrane substantial splitting with delay times from 1 to 2 s and fast directions varying from e w to ne sw was observed for stations in the qiangtang terrane and northernmost lhasa terrane no detectable splitting was observed for stations located farther south in the central lhasa terrane the change in shear wave splitting characteristics occurs at 32 n approximately coincident with the transcurrent karakoram jiali fault system but 40 km south of the surface trace of the banggong nujiang suture this location is also near the southernmost edge of a region of high sn attenuation and low upper mantle velocities found in previous studies the transition between no measured splitting and strong anisotropy 2 2 s delay time is exceptionally sharp 15 km suggesting a large crustal contribution to the measured splitting the e w to ne sw fast directions are broadly similar to the fast directions observed farther east along the yadong golmud highway suggesting that no large scale change in anisotropic properties occurs in the east west direction however in detail fast directions and delay times vary over lateral distances of 100 km in both the n s and e w direction by as much as 40 and 0 5 1 s respectively the onset of measurable splitting at 32 n most likely marks the northern limit of the underthrusting indian lithosphere which is characterized by negligible polarization anisotropy taken in conjunction with decades of geophysical and geological observations in tibet the new anisotropy measurements are consistent with a model where hot and weak upper mantle beneath northern tibet is being squeezed and sheared between the advancing indian lithosphere to the south and the tsaidam and tarim lithospheres to the north and west resulting in eastward flow and possibly thickening and subsequent detachment due to gravitational instability in northern tibet crustal deformation clearly follows this large scale deformation pattern

north atlantic nordic seas exchanges abstract the northeastern part of the north atlantic is unique in the sense that it is much warmer in the surface than other ocean areas at similar latitudes the main reason for this is the large northward transport of heat that extends to high latitudes and crosses the greenland scotland ridge to enter the nordic seas and the arctic there the warm atlantic water is converted to colder water masses that return southwards over the ridge partly as surface outflows and partly as overflows through the deep passages across the ridge in this paper the state of knowledge on the exchanges especially across the eastern part of the greenland scotland ridge is reviewed based on results from the ices nansen north atlantic norwegian sea exchanges project from the nordic woce project and from other sources the accumulated evidence allows us to describe the exchanges in fair detail the origins of the waters the patterns of their flow towards and over the ridge and their ultimate fate there is also increasing information on temporal variations of the exchanges although dynamical changes are still not well understood quantitative estimates for the volume transport of most of the overflow branches seem reasonably well established and transport measurements of the atlantic inflows to the nordic seas are approaching acceptable levels of confidence which allows preliminary budgets to be presented the deep overflows are driven by pressure gradients set up by the formation of deep and intermediate water the dominance of deep overflows over surface outflows in the water budget argues that this thermohaline forcing also dominates over direct wind stress and estuarine forcing in driving the atlantic water inflow across the greenland scotland ridge while wind stress seems to influence the characteristics and distribution of the atlantic water north of the ridge

petrogenesis and tectonic implications of granitoids from western north altun northwest china abstract the altun orogenic belt in northwest china is part of the northern margin of the tibetan plateau the north altun ophiolitic melange belt is an important tectonic unit within the altun orogenic belt that contains voluminous early paleozoic granitoids in this study we report the petrological features geochemical compositions and zircon u pb and hf isotope data of three granitic plutons from the western segment of the north altun ophiolitic melange belt zircon u pb dating yields magmatic crystallization ages of 499 493 and 496 ma for samples of granodiorite quartz diorite and syenogranite respectively the granitoids have metaluminous to weakly peraluminous and medium k to high k calc alkaline characteristics and display relative enrichments in large ion lithophile elements rb th u k and light rare earth elements lree and relative depletions in nb ta sr p and ti suggesting an arc related origin the granodiorites 499 ma have positive zircon ehf t values ranging from 1 87 to 6 59 with two stage hf model ages tdmc of 1 05 to 1 35 ga implying that the granodiorites were derived from juvenile crust the quartz diorites 493 ma have similar hf isotopic characteristics to the granodiorites ehf t 2 59 to 6 04 tdmc 1 08 to 1 30 ga indicating derivation from juvenile crust the syenogranites 496 ma have high total ree and k2o contents and low zircon ehf t values 1 69 to 1 54 suggesting that they were derived mainly from juvenile crust mixed with ancient crustal materials combined with data from previous studies we conclude that magmatism in the north altun ophiolitic melange belt can be subdivided into three episodes episode 1 520 470 ma granitoids are related to subduction episode 2 460 425 ma granitoids formed in a continent continent collisional setting and episode 3

seismic imaging of the downwelling indian lithosphere beneath central tibet a tomographic image of the upper mantle beneath central tibet from indepth data has revealed a subvertical high velocity zone from 100 to 400 kilometers depth located approximately south of the bangong nujiang suture we interpret this zone to be downwelling indian mantle lithosphere this additional lithosphere would account for the total amount of shortening in the himalayas and tibet a consequence of this downwelling would be a deficit of asthenosphere which should be balanced by an upwelling counterflow and thus could explain the presence of warm mantle beneath north central tibet

3d imaging of subducting and fragmenting indian continental lithosphere beneath southern and central tibet using body wave finite frequency tomography abstract we perform a finite frequency tomographic inversion to image 3d velocity structures beneath southern and central tibet using teleseismic body wave data recorded by the tibet 31n passive seismic array as well as waveforms from previous temporary seismic arrays high velocity bodies dip 40 northward beneath the himalaya and the lhasa terrane we interpret these high velocity anomalies as subducting indian continental lithosphere icl the icl appears to extend further north in central tibet than in eastern tibet reaching 350 km depth at 31 n along 85 e but at 30 n along 91 e low p and s wave velocity anomalies extend from the lower crust to 180 km depth beneath the tangra yum co rift yadong gulu rift and the cona rift suggesting that rifting in southern tibet may involve the entire lithosphere the anomaly beneath tangra yum co rift extends down to about 180 km whereas the anomalies west of the yadong gulu rift and east of the cona rift extend to more than 300 km depth the low velocity upper mantle west of the yadong gulu rift extends furthest north and appears to connect with the extensive upper mantle low velocity region beneath central tibet thus the northward subducting indian plate is fragmented along north south breaks that permit or induce asthenospheric upwellings indistinguishable from the upper mantle of northern tibet

present day crustal motion within the tibetan plateau inferred from gps measurements 1 using the measurements of 726 gps stations around the tibetan plateau we determine the rigid rotation of the entire plateau in a eurasia fixed reference frame which can be best described by an euler vector of 24 38 0 42 n 102 37 0 42 e 0 7096 0 0206 ma the rigid rotational component accommodates at least 50 of the northeastward thrust from india and dominates the eastward extrusion of the northern plateau after removing the rigid rotation to highlight the interior deformation within the plateau we find that the most remarkable interior deformation of the plateau is a glacier like flow zone which starts at somewhere between the middle and western plateau goes clockwise around the eastern himalayan syntaxis ehs and ends at the southeast corner of the plateau with a fan like front the deformation feature of the southern plateau especially the emergence of the flow zone could be attributed to an eastward escape of highly plastic upper crustal material driven by a lower crust viscous channel flow generated by lateral compression and gravitational buoyancy at the later developmental stage of the plateau the first order feature of crustal deformation of the northeastern plateau can be well explained by a three dimensional elastic half space dislocation model with rates of dislocation segments comparable to the ones from geological observations in the eastern plateau although gps data show no significant convergence between the eastern margin of the plateau and the sichuan basin a small but significant compressional strain rate component of 10 5 2 8 nstrain yr exists in a relatively narrow region around the eastern margin in addition a large part of the eastern plateau northeast of the ehs is not undergoing shortening along the northeastward convergence direction of the ehs but is stretching

the boundary between the indian and asian tectonic plates below tibet the fate of the colliding indian and asian tectonic plates below the tibetan high plateau may be visualized by in addition to seismic tomography mapping the deep seismic discontinuities like the crust mantle boundary moho the lithosphere asthenosphere boundary lab or the discontinuities at 410 and 660 km depth we herein present observations of seismic discontinuities with the p and s receiver function techniques beneath central and western tibet along two new profiles and discuss the results in connection with results from earlier profiles which did observe the lab the lab of the indian and asian plates is well imaged by several profiles and suggests a changing mode of india asia collision in the east west direction from eastern himalayan syntaxis to the western edge of the tarim basin the indian lithosphere is underthrusting tibet at an increasingly shallower angle and reaching progressively further to the north a particular lithospheric region was formed in northern and eastern tibet as a crush zone between the two colliding plates the existence of which is marked by high temperature low mantle seismic wavespeed correlating with late arriving signals from the 410 discontinuity poor sn propagation east and southeast oriented global positioning system displacements and strikingly larger seismic sks anisotropy

seismotectonics of the himalayan collision zone geometry of the underthrusting indian plate beneath the himalaya fault plane solutions and well determined focal depths of medium sized earthquakes topography and landsat imagery in conjunction with seismicity maps cross sections and available geological information are used to investigate the present tectonics of the himalayan continental collision zone most of the accurately located epicenters of events along the himalayan arc 78 e 95 e that occurred between 1961 and 1981 are concentrated in a narrow zone about 50 km wide lying between the northerly dipping main boundary mbt and main central mct thrusts most of these events are located just south of the mct though the epicenters of the events are in general well located their depths as determined by teleseismic travel time data are very unreliable events with accurately determined depths obtained from identification of surface reflected phases define a simple planar zone from about 10 km and 20 km depth with an apparent dip of about 15 this result is all the more remarkable considering that the events used were located along about an 1800 km length of the himalyan arc except for one all available focal mechanisms of events within this zone indicate shallow 30 north dipping thrusts this shallow north dipping zone apparently defines a part of the detachment that separates the underthrusting indian plate from the lesser himalayan crustal block the spatial extent and the geometry of this interplate thrust zone strongly indicate that the mbt and nearby subsidiary surface and blind thrusts rather than the mct are currently the most active structures of the himalayan arc we suggest that the great himalayan earthquakes m 8 occur along the same detachment surface as defined by the thrust type medium sized events events located to the south of the mbt and beneath the ganges foredeep show normal faulting with t axes perpendicular to the himalayan trend the above results suggest that the indian continental plate is underthrusting the himalayan crustal blocks in a relatively coherent and simple geometry and that this geometry is not much different from that observed along oceanic subduction zones the november 19 1980 earthquake that occurred near the mct near 88 5 e shows a predominantly strike slip focal mechanism one of the nodal planes of this mechanism is transverse to the himalayan structural grain and moreover this plane has a trend similar to that of the recently mapped yadong gulu rift in the tethyan himalaya and in southern tibet just northeast of the earthquake we interpret this predominantly left lateral strike slip mechanism to indicate a possible genetic relationship between transverse structural features in the underthrusting indian plate the kishangang basement fault and the upper himalayan blocks and tibet

tibetan plate overriding the asian plate in central and northern tibet at the southern boundary between india and tibet the indian tectonic plate subducts northwards beneath the tibetan plateau seismic imaging shows that at the northern boundary of tibet the asian plate is also subducting southwards beneath the plateau and the tibetan lithosphere is separate

reconciling lithospheric deformation and lower crustal flow beneath central tibet a viscous region with deformable boundaries is used to model simultaneous crustal flow and lithospheric coupling in northern tibet this model suggests that 1 the deformation of northern tibet is different from that of southern tibet because of structural differences between the regions 2 the viscosity contrast between the crust and the mantle lithosphere is relatively small beneath northern tibet and 3 crustal flow is compatible with crust mantle coupling under these conditions

seismic images of crust and upper mantle beneath tibet evidence for eurasian plate subduction seismic data from central tibet have been combined to image the subsurface structure and understand the evolution of the collision of india and eurasia the 410 and 660 kilometer mantle discontinuities are sharply defined implying a lack of a subducting slab beneath the plateau the discontinuities appear slightly deeper beneath northern tibet implying that the average temperature of the mantle above the transition zone is about 300 c hotter in the north than in the south there is a prominent south dipping converter in the uppermost mantle beneath northern tibet that might represent the top of the eurasian mantle lithosphere underthrusting the northern margin of the plateau

tsunami magnitudes determined from ocean bottom pressure gauge data around japan 1 we develop a method to calculate tsunami magnitude from ocean bottom pressure gauge data around japan direct application of abe s equations to ocean bottom pressure gauge data underestimates tsunami magnitudes by about an order of magnitude this is because abe s equations were derived only from tsunami amplitudes measured at coastal tide gauges where tsunamis are amplified by the shoaling of waves and reflection at the coastline for offshore bottom pressure records we introduce correction factors to abe s original equations to account for tsunami amplitudes measured in open oceans the modified equations produce pressure gauge determined tsunami magnitudes close to those determined using coastal tide gauges

the norwegian atlantic current in the lofoten basin inferred from hydrological and tracer data 129i and its interaction with the norwegian coastal current 1 from three hydrological sections taken across the lofoten basin in may 2000 we estimated geostrophic transports of 7 2 sv sverdrup 106 m3 s 1 for the norwegian atlantic current nac and its division between northern 4 8 sv and eastern 2 4 sv branches from 129i 127i concentration ratio measurements and transport estimations we calculated 129i mass flux across the three sections it appears that in the lofoten basin a 129i tracer laden norwegian coastal current ncc is transporting northward 55 kg y of 129i assuming a volume flux of 0 7 sv b the estimated mass fluxes of 129i by the nac and the ncc are comparable c the total mass flux of 129i by the nac and ncc accounts only for about 1 3 of the 129i annual discharge 350 kg y from two reprocessing plants based in france and uk if these measurements are representative of annual mean it suggests an important transfer of 129i outside the nac ncc system

129i anthropogenic budget major sources and sinks data are presented here on the anthropogenic 129i inventory in regions that have been strongly affected by releases from european reprocessing facilities which to the authors knowledge presently account for 90 of the global isotope source in the earth s surface environment the results show that 90 of the isotope inventory occurs in marine waters with the nordic seas and eurasian basin of the arctic ocean containing most of the 129i within the terrestrial environment of europe soils contain the largest part of the isotope inventory however the inventory of the terrestrial system did not provide clues on the most plausible atmospheric source of 129i to europe thus supply from both gaseous and marine releases is proposed the sum of the total inventory in both the marine and terrestrial environments did not match the estimated releases this imbalance is likely to relate to unconstrained inventory estimates for marine basins irish sea english channel and north sea close to the facilities but also to the occurrence of 129i in the biosphere and possible overestimated releases from the nuclear reprocessing facilities there is no doubt that the available data on 129i distribution in the environment are far from representative and further research is urgently needed to construct a comprehensive picture

shear wave birefringence and current configuration of converging lithosphere under tibet article i nfo new data from west central tibet show that birefringence of s waves has two pronounced increases in magnitude toward the hinterland null birefringence persists to about 75 km north of the indus yarlung suture iys between the indian shield and the lhasa terrane of southern tibet a second rapid increase occurs about 100 km farther north of the bangong nujiang sutures between the lhasa terrane and the qiangtang terrane in central tibet the latter feature is consistently observed along three long transects that collectively span a lateral orogen parallel distance of about 600 km and is likely to mark the northern leading edge of sub horizontally advancing mantle lithosphere of the indian shield the greater india an interpretation consistent with the latest results of finite frequency tomography using both p and s wave travel times previous results of modeling gravity anomalies and a host of other seismic observations similarly complementary constraints indicate that the sudden onset of significant birefringence north of the iys is likely to be the southern termination of eurasian mantle lithosphere curiously the shortest of three transects showed null birefringence through much of the lhasa terrane a pattern inconsistent with those of he isotopes and gravity

the meridional distribution of dissolved iodine in near surface waters of the atlantic ocean abstract dissolved total iodine and iodate have been measured along an approximately north east south west transect between the british isles and falkland isles total iodine showed very little variation between the surface and 200 m for the surface samples the coefficient of variation was 3 5 iodate concentrations varied between 0 30 and 0 45 m with the lowest occuring at the equatorial surface these results together with an earlier similar set for the pacific ocean are used to argue that in general iodate reduction can be modelled separately from iodine uptake regeneration correlation between iodate and nitrate concentrations was high r 2 0 93 in the permanently stratified waters of the sub tropics and tropics however upwelling off cape verde shows that these correlations are the result of hydrographic coupling rather than direct chemical coupling of the two systems

moho offset across the northern margin of the tibetan plateau anomalous double pulse teleseismic p wave arrivals were observed at one station near the northern margin of the tibetan plateau the azimuthal dependence of the waveform distortion and its absence at nearby stations indicated that the distortion was produced by receiver side crustal heterogeneity modeling of the three component data revealed a 15 to 20 kilometer moho offset that occurs over a narrow lateral range of less than 5 kilometers this east west striking offset separates the thick tibetan plateau crust from the qaidam basin crust such a sharp crustal thickness change implies a weak tibetan plateau crust that thickens vertically in response to penetration by india from the south and to blockage caused by a strong qaidam basin crust to the north

rayleigh wave phase velocity maps of tibet and the surrounding regions from ambient seismic noise tomography ambient noise tomography is applied to the significant data resources now available across tibet and surrounding regions to produce rayleigh wave phase speed maps at periods between 6 and 50 s data resources include the permanent federation of digital seismographic networks five temporary u s program for array seismic studies of the continental lithosphere passcal experiments in and around tibet and chinese provincial networks surrounding tibet from 2003 to 2009 totaling 600 stations and 150 000 interstation paths with such a heterogeneous data set data quality control is of utmost importance we apply conservative data quality control criteria to accept between 5000 and 45 000 measurements as a function of period which produce a lateral resolution between 100 and 200 km across most of the tibetan plateau and adjacent regions to the east misfits to the accepted measurements among passcal stations and among chinese stations are similar with a standard deviation of 1 7 s which indicates that the final dispersion measurements from chinese and passcal stations are of similar quality phase velocities across the tibetan plateau are lower on average than those in the surrounding nonbasin regions phase velocities in northern tibet are lower than those in southern tibet perhaps implying different spatial and temporal variations in the way the high elevations of the plateau are created and maintained at short periods 20 s very high velocities are imaged in the tarim basin the ordos block and the sichuan basin these phase velocity dispersion maps provide information needed to construct a 3 d shear velocity model of the crust across the tibetan plateau and surrounding regions

distribution of the crustal low velocity zones beneath the central and northeastern tibetan plateau insights from joint analysis of receiver functions and surface wave dispersion observations abstract a crustal and uppermost mantle shear wave velocity model beneath the central and northeastern tibetan plateau is obtained from the joint analysis of rayleigh wave phase dispersions and teleseismic p wave receiver functions at 188 broadband seismic stations deployed in this region this newly presented model reveals wide spread low velocity zones in the middle crust and these zones are mainly confined to the region south of the kunlun fault the velocities in the low velocity zones are extremely low

a model of the crust and mantle structure down to 700 km depth beneath the lhasa to golmud transect across the tibetan plateau as derived from seismological data abstract a 700 km deep seismic velocity cross section beneath the lhasa to golmud transect across the tibetan plateau is presented in contrast to the first version of this cross section which comprised an 800 km wide swath centred on the lhasa to golmud transect due to the recent proliferation of publications concerning the mantle structure beneath tibet this study is based only on seismic profiles which either run along or cross the transect and arrays or studies which at least partly cover the transect the results from the recent indepth iv project indicate that the crustal thickness change from 70 km beneath the songpan ganzi terrane and kunlun mountains to 54 km beneath the qaidam basin is located about 100 km north of the kunlun fault and almost 45 km north of the north kunlun thrust the qaidam basin moho is underlain by crustal velocity material for almost 45 km and the apparently overlapping crustal material may represent songpan ganzi lower crust underthrusting or flowing northward beneath the qaidam basin moho thus the high tibetan plateau may be thickening northward into south qaidam as its weak thickened lower crust is injected beneath the stronger qaidam crust beneath the crust high velocity dense cold indian lithospheric mantle extends northwards until about the banggong nujiang suture northwards asian lithospheric mantle is overlain by a low velocity less dense warm tibetan plate consisting of an upper lithospheric and a lower asthenospheric part the apparent northwards deepening of the 410 and 660 km discontinuities by about 20 km implies that the upper mantle beneath north tibet is slower less dense and warmer than under south tibet in agreement with the observed uppermost mantle velocities this in turn could provide some of the isostatic support for the high elevations in the north where the crust is somewhat thinner than in the southern plateau

structure and resilience of overwintering habitats of calanus finmarchicus in the eastern norwegian sea abstract very high concentrations of overwintering calanus finmarchicus were found in the eastern lofoten basin of the norwegian sea close to the shelf break in january 2001 2002 a coupled 3d hydrodynamic and ecological model was used to study the formation of this deep overwintering aggregation and its stability the ecological model includes nutrients phytoplankton and microzooplankton in addition to a stage structured model of c finmarchicus using a eulerian approach the model was initiated with an overwintering stock evenly distributed in the oceanic regions of the norwegian sea i e where depths 600 m spawning and development of the new generation take place in response to vertical mixing and phytoplankton development animals are assumed to begin their descent to overwintering depths of 700 1000 m as late stage vs model results show that in late summer high concentrations of animals were found at overwintering depths near the shelf break north of the north sea off the northeastern voring plateau and in the eastern lofoten basin along the slope of the barents sea shelf they remained there for months due to deep eddies and southward deep currents along the norwegian shelf the simulation experiments indicate that the combined effect of deep anticyclonic circulation and vertical migration behavior of the animals may explain the high concentrations of overwintering c finmarchicus found in field surveys in the eastern lofoten basin close to the shelf break

imaging the colliding indian and asian lithospheric plates beneath tibet 1 a high resolution image of the base of the lithosphere from s to p converted seismic waves revealed the collision architecture of the indian and asian continental plates beneath the tibetan plateau the base of the indian lithosphere dips northward from a depth of 160 km beneath the himalayas to a depth of 220 km just south of the bangong suture the base of the asian lithosphere is nearly horizontal at the depth of 160 180 km from central to northern tibet there is a vertical gap of about 50 km between indian and asian lithospheres our observation of a well defined thick lithosphere throughout the entire plateau is not consistent with models of wholesale convective instability of a thickened mantle lithosphere which would predict a very thin asian lithosphere the hypothesized sequential southward subduction of tibetan and asian continental lithospheres leading to the growth of the tibetan plateau if correct cannot be occurring below 180 km depth our results along with available geological and geophysical data strongly support that the plateau is predominantly formed by a relatively coherent north dipping subducted indian continental lithosphere in the south which presently can be traced to the middle of the plateau and a south dipping subducted asian lithosphere in the north at a shallower depth

upper mantle velocity structure beneath the tibetan plateau from pn travel time tomography we inverted 1510 p arrival times from regional distances 333 1600 km in and around the tibetan plateau to map the lateral velocity variation within the uppermost mantle previous studies have placed first order constraints on upper mantle velocities but relied on data recorded almost exclusively at stations outside of the plateau we improve resolution by using 40 events recorded at stations within the tibetan plateau we combine these data with observations obtained from the international seismological centre isc to extend our coverage by including pn arrivals from 85 additional plateau events relocated in previous studies and recorded at stations in and around the tibetan plateau we use synthetic travel time data to evaluate the resolution of our data set the observations provide good resolution to about 1 over most of the plateau and surrounding regions our results show average pn velocities that are about 3 lower in the northern plateau relative to the southern plateau these variations correlate well with major tectonic features and previous geophysical observations in the qiangtang terrane of the northern plateau an area known to be inefficient for sn propagation pn is slow relative to both the plateau south of the banggong nujiang suture and the tectonically stable tarim basin north of the plateau this is strong evidence for the existence of partial melt within the uppermost mantle beneath the northern tibetan plateau however when laboratory estimates of relationships between temperature velocity and attenuation are applied a relatively small temperature variation 240 to 370 c is required to explain our pn velocity observations when combined with geochemical constraints from volcanics in the northern plateau our results strongly suggest that the mantle lid is intact beneath the northern plateau this result would preclude tectonic models involving wholesale delamination of the mantle lithosphere in the northern tibetan plateau

subduction of the indian lithosphere beneath the tibetan plateau and burma abstract subduction of the indian lithosphere under eurasia plays an important role in the tectonic evolution of the tibetan plateau and surrounding regions to improve our knowledge of pertinent mantle structures through tomographic imaging we combine p wave arrival time data from temporary arrays in tibet and stations of the chinese seismograph network with reprocessed data from the international seismological centre the new images reveal considerable spatial variations in mantle structure along the collision zone and the horizontal distance over which presumed continental indian lithosphere slides northward beneath the plateau decreases from west where it underlies the himalayas and the entire plateau to east where no indication is found for present day underthrusting beyond the himalayan block and indus tsangpo suture p wavespeed appears low in the shallow mantle beneath much of central and eastern tibet these observations suggest that indian lithosphere underlies only the southwestern part of the plateau and that the central and northeastern part is underlain by lithosphere of asian origin the continental parts of the indian plate that currently underthrust or subduct below the plateau appear generally detached from the oceanic slab fragments that subducted longer ago and that have been detected deeper in the mantle except perhaps in the central part of the collision zone 80 90 e this suggests that ongoing india eurasia collision must increasingly be driven by other forces such as subduction of the indian plate further west and east e g beneath indochina our images reveal structures associated with eastward subduction along the burmese arc southeast of the eastern himalayan syntaxis the tengchong volcanic complex in southwest china is marked by slow wave propagation to 150 km depth suggesting a causal link to subduction along the burmese arc the low velocities beneath the red river fault region extend to greater depth and may be related to upper mantle processes further southeast

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

seismic tomography of northern tibet and kunlun evidence for crustal blocks and mantle velocity contrasts although the crust and mantle of the tibet plateau reveal vital information for understanding the interplay of dynamic processes that has governed its recent uplift and growth their deep physical and thermal structure remains poorly understood in order to throw light on the structure and hence to constrain the processes and models tied to them we performed a teleseismic experiment on a 600 km long profile across the northern part of the plateau the kunlun range and the qaidam basin the 400 km deep tomographic image we obtain has a resolution 50 km over one order of magnitude better than achieved in previous broader scale studies at relatively shallow depth 100 km the tomographic cross section clearly reveals that the mid lower crust and upper lithosphere of northern tibet is an assemblage of blocks with different velocities and thicknesses hence different natures histories and ages the crust of the qiantang block appears to be thickest 70 km with the lowest velocity along the north edge of that block the jinsha suture although not remarkable in the local geology and topography stands as a particularly sharp and prominent crustal boundary the crust and lithosphere of the region between that suture and the qaidam bayan har songpan south kunlun appear to be thinnest 50 km and with the highest velocity at greater depth 150 km our experiment confirms the existence of a bulky low velocity zone in the mantle beneath the northernmost reaches of the plateau the tomogram helps assess with unprecedented accuracy the location and shape of this low velocity anomaly which forms a 250 300 km wide dome rising to about 150 200 km south of the kunlun range roughly coincident with the region where the strongest shear wave splitting hence horizontal anisotropy has recently been found that dome lies between two relatively high velocity zones with opposite dips one towards the north under the qaidam basin the other towards the south beneath the quangtang platform the implications that this new detailed image of the deep structure of northern tibet has upon the uplift and thermo mechanical evolution of the plateau are discussed

high resolution lithospheric structure beneath mainland china from ambient noise and earthquake surface wave tomography abstract we present a new high resolution shear velocity model of the lithosphere down to about 160 km beneath china using rayleigh wave tomography we combined ambient noise and earthquake data recorded at 1316 seismic stations the largest number used for the region to date more than 700 000 dispersion curves were measured to generate group and phase velocity maps at periods of 10 140 s the resolution of our model is significantly improved over previous models with about 1 2 in eastern china and 2 3 in western china we also derived models of the study region for crustal thickness and averaged s velocities for upper and mid lower crust and uppermost mantle these models reveal important lithospheric features beneath china and provide a fundamental data set for understanding continental dynamics and evolution different geological units show distinct features in the moho depth lithospheric thickness and shear velocity in particular the north china craton ncc lithosphere shows strong east west structural variations with thin and low velocity lithosphere in eastern ncc and thick and high velocity lithosphere beneath western ncc and the lithosphere of the ordos block seems to have undergone strong erosion the results support the progressive destruction of the ncc lithosphere from east to west at least partly caused by the thermal chemical erosion of the cratonic lithosphere from the asthenosphere another pronounced feature of our model is the strong lateral variations of the mantle lithosphere beneath the tibetan plateau tp the indian lithosphere beneath the tp shows variable northward advancement with nearly flat subduction in western and eastern tp and steep subduction in central tp with evidence for the tearing of indian lithosphere beneath central tp which may be important for the riftings at the surface in himalayas and southern tp the low velocity zone in northern tp shows strong correlation with the region of the mid miocene to quaternary potassic magmatism suggesting that delamination of lithosphere may have played an important role in the rise of the tp

lithospheric and upper mantle structure of the northeastern tibetan plateau 1 we use receiver functions calculated for data collected by the indepth iv seismic array to image the three dimensional geometry of the crustal and upper mantle velocity discontinuities beneath northeastern tibet our results indicate an average crustal thickness of 65 to 70 km in northern tibet in addition we observe a 20 km moho offset beneath the northern margin of the kunlun mountains a 10 km moho offset across the jinsha river suture and gently northward dipping moho beneath the qaidam basin a region in the central qiangtang terrane with higher than normal crustal vp vs ratio of 1 83 can be the result of the eocene magmatic event in the qiangtang terrane we observe a significant lithospheric mantle discontinuity beneath the bangong nujiang suture at 80 km depth which dips 10 to the north reaching 120 km depth we interpret this feature as either a piece of lhasa terrane or remnant oceanic slab underthrust below northern tibet we detect a 20 km depression of the 660 km discontinuity in the mantle transition zone beneath the northern lhasa terrane in central tibet which suggests this phase transition has been influenced by a dense and or cold oceanic slab a modest 10 km depression of the 410 km discontinuity located beneath the northern qiangtang terrane may be the result of localized warm upwelling associated with small scale convection induced by the penetration of the sinking indian continental lithosphere into the transition zone beneath the central tibetan plateau

was the atlantic water temperature in the west spitsbergen current predictable in the 1990s 1 a major source of heat and salt for the arctic ocean is the atlantic water aw imported from the norwegian sea by the west spitsbergen current wsc analysis of temperature records from the wsc has helped to link the warming of the arctic ocean to changes in the north atlantic oscillation nao index here we analyze using summer hydrographic data from a meridional section at 15 e in the northern norwegian sea the interannual variability in the aw hydrography and its relation to the nao index in the 1990s we show that while the aw temperature exhibited a tendency to lag the nao index the aw salinity exhibited a tendency to lead the index a surprising conclusion is that the aw temperature in the wsc was predictable one year in advance from the aw salinity

significant and vertically coherent seismic anisotropy beneath eastern tibet 1 shear wave splitting measurements using teleseismic sks and skks phases recorded by the indepth iv arrays has revealed a strong upper mantle anisotropic fabric in northeastern tibet with large delay times of up to 2 2 s suggesting that anisotropy exists in both the lithospheric and asthenospheric mantle the coherence among fast polarization orientations of split core phases and the left lateral slip on eastern striking southern striking faults in eastern tibet and the surface deformation fields calculated from both gps observations and quaternary fault slip rates support the idea that left lateral shear strain is the predominant cause of the orientation of the upper mantle petrofabrics we suggest the bending of the eastern himalayan syntaxis around the foundering burma andaman sumatra slab also contributes to the observed seismic anisotropy in the eastern himalayan syntaxis region two plausible competing processes are proposed for the flow of asthenosphere in eastern tibet in the first the deforming lithosphere glides over the passive asthenosphere inducing flow in the asthenospheric mantle in the second the asthenosphere beneath northeastern tibet is squeezed between the advancing indian continental lithosphere and the thick tarim and qaidam lithospheric blocks to the north a westward retreat of the burma slab from eurasia may induce flow that is toroidal and located exclusively around the northern edge of the slab the rotation of fast orientations for stations in the eastern himalayan syntaxis region are consistent with the toroidal flow pattern as well as the rotational deformation of the overlying lithosphere

receiver function imaging of the lithosphere at the kunlun qaidam boundary northeast tibet abstract we studied the crustal structure and tectonics in the north tibetan plateau from the songpan ganzi terrane to the qaidam basin using teleseismic receiver function imaging across a major lithospheric boundary the kunlun qaidam boundary where previous studies suggest a 15 20 km change in crustal thickness from thicker crust in the kunlun mountains to thinner crust in the qaidam basin we report p receiver functions for 70 stations largely the international deep profiling of tibet and the himalaya indepth phase iv experiment our most dense station coverage is located along the roughly north south indepth iv active source seismic profile at approximately 95 e longitude azimuthal and geographical changes in the receiver functions reveal significant changes in crustal structure and vp vs from across the study area receiver functions show strong converters that we interpret as the moho at 70 km depth beneath the qiangtang songpan ganzi terranes and kunlun mountains and at 50 km depth beneath the central qaidam basin this large change in crustal thickness occurs 50 km north of the north kunlun strike slip fault on which the 2001 m8 1 kunlun earthquake occurred receiver functions for some of the stations north of the thickness change at the kunlun qaidam boundary also show a deeper 70 km bright converter in addition to the 50 km converter the two converters appear to overlap by up to 30 km in some locations along the south qaidam basin we combine previous results with these new results to discuss implications for mechanisms for crustal thickening in the north tibetan plateau including crustal flow and crustal injection at depths imaged here shallower than 100 km we see no evidence of southward subduction of eurasian lithosphere

iodine 129 137cs and cfc 11 tracer transit time distributions in the arctic ocean 1 time series measurements of the nuclear fuel reprocessing tracers 129i and 137cs and ventilation tracer cfc 11 were used to determine circulation time scales for atlantic water aw in the arctic ocean measurements in surface water are consistent with an advection model and transit times from the north sea of 1 4 years to the barents sea 3 6 years to the kara sea and 9 12 years to the north pole tracer ages for intermediate waters do not support an advection model because they vary with time the tracer relationships were employed to determine transit time distributions ttd using inverse gaussian functions that implicitly incorporate the effects of mixing in simulating fluid flow tracer concentrations were used to constrain the ratio of the width of the ttd to the mean age a measure of the relative importance of mixing giving values for most intermediate water regimes of 1 these results are in contrast to values 1 typical of the north atlantic indicating the greater relative importance of advection compared to mixing in the arctic ocean mean ages for aw increase from 12 20 years in the southern makarov basin to 30 40 years near the arctic outflow north of greenland a decline in with distance along the direction of aw boundary flow from the makarov into the canada basin may indicate relatively stronger mixing in regimes upstream of the makarov basin but also implies that ttds may differ from inverse gaussian functions for flow in the arctic domain

extreme cross shelf transport induced by eddy interactions southwest of iberia in winter 2001 1 unusually high for winter time satellite derived chlorophyll a concentrations southwest of iberia were detected in february 2001 analysis of satellite data seawifs avhrr sea level anomaly from topex poseidon and ers revealed that the phytoplankton rich waters were associated with an extremely long filament up to 400 km induced by eddy eddy interactions occurring in the vicinity of the continental margin the eddy interaction occurred simultaneously with an accumulation of coastal fresh water over the shelf the event promoted an export of biogenic material offshore along with an estimated shelf flushing in a 5 to 6 day period that resulted in a dramatic offshore growth of phytoplankton a description of the event is given and preliminary estimates of the cross shelf transport are presented and discussed

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

seismic anisotropy and global geodynamics for many years seismic anisotropy was often neglected mostly because of the inherent heavy mathematical and computational tools needed to describe and model its effects on seismic waves the usual basic knowledge about propagation in isotropic media cannot easily apply to anisotropic media where new phenomena come up such as birefringence or shear wave splitting or difference between directions of propagation of phase velocity and of group velocity consequently geophysicists often claimed that it was a second order effect and considered the earth as isotropic this hypothesis was assumed to be a good approximation because of the random orientation of crystals in most parts of the earth and of the random sampling of anisotropic regions by seismic rays this assumption furthermore made easier the description of wave propagation as well as the parameterization of media in inverse problems an isotropic elastic medium can be described by two independent elastic parameters and lame parameters but the simplest anisotropic medium transverse isotropy with a vertical symmetry axis requires 5 independent parameters love 1927 anderson 1961 to date seismic observations have been explained in terms of isotropic and often thermal lateral heterogeneities ignoring manifestations of anisotropy however since the 1960s it was recognized that most parts of the earth are not only laterally heterogeneous but also anisotropic and that seismic anisotropy provides a simple explanation of different observational data seismic anisotropy contrary to isotropy is reflecting some inherent

joint inversion of surface waves and teleseismic body waves across the tibetan collision zone the fate of subducted indian lithosphere our study has included data from gsn including ic iu and ii china digital seismograph network geoscope iris ida pacific 21 kyrgyz digital network kyrgyz seismic telemetry network and iris usgs permanent seismic networks and the manas tien shan continental dynamics tibetan plateau broadband experiment indepth ii indepth iii indepth iv ascent himnt bhutan nanga parbat pakistan and ghengis passcal temporary seismic deployments we thank iiees and lgit for seismic data from iran and also seisuk for provision and assistance with instruments operated in northeast india cn was supported by a natural environment research council studentship grant ne h52449x 1 with case funding from awe blacknest we thank nick rawlinson and an anonymous reviewer for their constructive and helpful reviews figures were prepared with generic mapping tools gmt software wessel smith 1998

ocean circulation and freshwater pathways in the arctic mediterranean based on a combined nd isotope ree and oxygen isotope section across fram strait the water masses passing the fram strait are mainly responsible for the exchange of heat and freshwater between the nordic seas and the arctic ocean the arctic mediterranean am disentangling their exact sources distribution and mixing however is complex this work provides new insights based on a detailed geochemical tracer inventory including dissolved nd isotope end rare earth element ree and stable oxygen isotope 18o data along a full water depth section across fram strait we find that nd isotope and ree distributions in the open am primarily reflect lateral advection of water masses and their mixing seawater particle interactions exert important control only above the shelf regions as observed above the ne greenland shelf advection of northward flowing warm atlantic water aw is clearly reflected by an end signature of 11 7 and a nd concentration nd of 16 pmol kg in the upper 500 m of the eastern and central fram strait freshening and cooling of the aw on its way trough the am are accompanied by a continuous change towards more radiogenic end signatures e g 10 4 of dense arctic atlantic water this mainly reflects mixing with intermediate waters but also admixture of dense kara sea waters and pacific derived waters the more radiogenic end signatures of the intermediate and deep waters reaching 9 5 are mainly acquired in the sw nordic seas through exchange with basaltic formations of iceland and se greenland inputs of nd from svalbard are not observed and surface waters and nd on the svalbard shelf originate from the barents sea shallow southward flowing arctic derived waters 200 m form the core of the east greenland current above the greenland slope and can be traced by their relatively radiogenic end reaching 8 8 and elevated nd 21 to 29 pmol kg these properties are used together with 18o and standard hydrographic tracers to define the proportions of pacific derived 30 based on nd isotopes and atlantic derived waters as well as of river waters 8 shallow waters 150 m on the ne greenland shelf share some characteristics of arctic derived waters but exhibit less radiogenic end values reaching 12 4 and higher nd up to 38 pmol kg in the upper 100 m this suggests local addition of greenland freshwater of up to 6 in addition to these observations this study shows that the pronounced gradients in end signatures and ree characteristics in the upper water column provide a reliable basis for assessments of shallow hydrological changes within the am

cirque glacier activity in arctic norway during the last deglaciation abstract numerous cirques of the lofoten vesteralen archipelago in northern norway have distinct moraine sequences that previously have been assigned to the allerod younger dryas 13 400 to 11 700 yr bp interval constraining the regional distribution of the equilibrium line altitude ela of cirque and valley glaciers here we present evidence from a once glacier fed lake on southern andoya that contests this view analyses of radiocarbon dated lacustrine sediments including rock magnetic parameters grain size organic matter dry bulk density and visual interpretation suggest that no glacier was present in the low lying cirque during the younger dryas allerod the initiation of the glacial retreat commenced with the onset of the bolling warming 14 700 yr bp and was completed by the onset of allerod interstade 13 400 yr bp the reconstructed glacier stages of the investigated cirque coincide with a cool and dry period from 17 500 to 14 700 yr bp and a somewhat larger last glacial maximum lgm advance possibly occurring between 21 050 and 19 100 yr bp

geodynamics of the southeastern tibetan plateau from seismic anisotropy and geodesy ongoing plate convergence between india and eurasia provides a natural laboratory for studying the dynamics of continental collision a fi rst order process in the evolution of continents regional climate and natural hazards in southeastern tibet the fast directions of seismic anisotropy determined using shear wave splitting analysis correlate with the surfi cial geology including major sutures and shear zones and with the surface strain derived from the global positioning system velocity fi eld these observations are consistent with a clockwise rotation of material around the eastern himalayan syntaxis and suggest coherent distributed lithospheric deformation beneath much of southeastern tibet at the southeastern edge of the tibetan plateau we observe a sharp transition in mantle anisotropy with a change in fast directions to a consistent e w direction and a clockwise rotation of the surface velocity surface strain fi eld and fault network toward burma around the eastern himalayan syntaxis the coincidence between structural crustal features surface strain and mantle anisotropy suggests that the deformation in the lithosphere is mechanically coupled across the crust mantle interface and that the lower crust is suffi ciently strong to transmit stress at the southeastern margin of the plateau in yunnan province a change in orientation between mantle anisotropy and surface strain suggests a change in the relationship between crustal and mantle deformation lateral variations in boundary conditions and rheological properties of the lithosphere play an important role in the geodynamic evolution of the himalayan orogen and tibetan plateau and require the development of three dimensional models that incorporate lateral heterogeneity

seismic evidence for the north china plate underthrusting beneath northeastern tibet and its implications for plateau growth abstract lithospheric deformation of the tibetan plateau is caused by subduction of the indian northward and asian southward plates the effects of this interaction on inner reaches of the asian continent between the tibetan plateau and the north china craton ncc for example remain uncertain due to poor geophysical data coverage in northeastern tibet ne tibet we provide here detailed knowledge of the lithospheric structure beneath ne tibet as determined from a dense broadband seismic profile traversing ne tibet to the westernmost ncc receiver function imaging reveals several significant features including a north dipping intracrustal converter nc moho offset overlap beneath major fault zones and a low velocity layer lvl in the middle lower crust the lithosphere asthenosphere boundary lab is clearly defined and appears as a south dipping interface that runs continuously from the alxa interior to the qilian orogen interpretation of these observations combined with other seismic evidence implies that the ncc lithospheric mantle has been persistently underthrust beneath the qilian orogen this process forms the thick skinned crustal accretionary wedges which develop above a middle lower intracrustal decollement our results provide further deep geophysical constraints on the cenozoic post collisional evolution of the convergent boundary between ne tibet and the ncc and help clarify the mechanism of plateau growth in this boundary area

underplating in the himalaya tibet collision zone revealed by the hi climb experiment we studied the formation of the himalayan mountain range and the tibetan plateau by investigating their lithospheric structure using an 800 kilometer long densely spaced seismic array we have constructed an image of the crust and upper mantle beneath the himalayas and the southern tibetan plateau the image reveals in a continuous fashion the main himalayan thrust fault as it extends from a shallow depth under nepal to the mid crust under southern tibet indian crust can be traced to 31 n the crust mantle interface beneath tibet is anisotropic indicating shearing during its formation the dipping mantle fabric suggests that the indian mantle is subducting in a diffuse fashion along several evolving subparallel structures

3d seismic evidence of buried iceberg ploughmarks from the mid norwegian continental margin reveals largely persistent north atlantic current through the quaternary over 7500 buried linear and curvilinear depressions interpreted as iceberg ploughmarks were identified within the quaternary naust formation from an extensive three dimensional seismic dataset that covers 40 000 km2 of the mid norwegian continental margin the morphology and net orientation of ploughmarks were mapped and analysed these features are up to 28 km long 700 m wide and are incised up to 31 m deep on average ploughmarks are incised 5 m deep with median width of 185 m and median lengths ranging from 1 2 to 2 7 km for individual palaeo surfaces width to depth ratio ranges from 8 1 to 400 1 and is on average 36 1 the presence of ploughmarks buried deeply within some palaeo slope surfaces implies the occasional presence of very large icebergs since the middle quaternary suggesting that thick ice sheet margins with fast flowing ice streams were present in order to calve icebergs of such dimensions into the norwegian sea the wide geographical distribution of ploughmarks suggests unrestricted iceberg drift and an open norwegian sea during the periods of iceberg calving since the early quaternary ploughmark trajectory analysis demonstrates that the ocean current circulation now dominated by the northeasterly flowing norwegian atlantic current nwac has largely persisted throughout the quaternary despite the overall strikingly consistent pattern of iceberg drift ploughmark mapping also shows evidence for short lived nwac reductions possibly related to major phases of iceberg discharge and or meltwater pulses from the fennoscandian ice sheet during the middle and late quaternary

tibetan and indian lithospheres in the upper mantle beneath tibet evidence from broadband surface wave dispersion broadband seismic experiments over the last two decades have produced dense data coverage across tibet yet the mechanism of the india asia lithospheric convergence beneath it remains a puzzle with even its basic features debated and with very different end member models advocated today we measured highly accurate rayleigh and love wave phase velocity curves in broad period ranges up to 5 200 s for a few tens of pairs and groups of stations across tibet combining in each case hundreds to thousands of interstation measurements made with cross correlation and waveform inversion methods robust shear velocity profiles were then determined by extensive series of nonlinear inversions of the data designed to constrain the depth dependent ranges of isotropic average shear speeds and radial anisotropy temperature anomalies in the upper mantle were estimated from shear velocities using accurate petrophysical relationships our results reveal strong heterogeneity in the upper mantle beneath tibet very large high velocity anomalies in the upper mantle are consistent with the presence of underthrust beneath southwestern tibet and subducted beneath central and eastern tibet indian lithosphere the corresponding thermal anomalies match those estimated for subducted indian lithosphere in contrast to the indian lithosphere tibetan lithosphere and asthenosphere display low to normal shear speeds tibetan lithosphere is thus warm and thin radial anisotropy in the upper mantle is weak in central and strong in northeastern tibet possibly reflecting asthenospheric flow above the subducting indian lithospheric slab

fragmented indian plate and vertically coherent deformation beneath eastern tibet 1 using fundamental mode rayleigh waves from the indepth iv and namche barwa seismic experiments for periods between 20 and 143 s we have investigated the lithospheric structure beneath eastern tibet we have found a 200 km wide high velocity body starting at 60 km depth and roughly centered beneath the bangong nijuang suture which is most likely a piece of the underthrusting indian continental lithosphere the sub horizontal underthrusting of the indian lithosphere beneath eastern tibet appears to be accompanied by its lateral tearing into at least two fragments and subsequent break off of the westernmost portion at 91 e 33 n the uppermost mantle low velocity zone we observe beneath the n qiangtang and songpan ganzi terranes is most probably due to warmer and thinner lithosphere relative to southern tibet we attribute the low velocity zones concentrated along the northern and southern branches of the eastern kunlun fault at lithospheric depths to strain heating caused by shearing the azimuthal fast directions at all periods up to 143 s 200 km peak sensitivity depth beneath the n qiangtang and songpan ganzi terranes are consistent suggesting vertically coherent deformation between crust and uppermost mantle furthermore the low velocity zone below the kunlun shan reaching down to 200 km argues against a present southward continental subduction along the southern margin of qaidam basin

velocities and propagation characteristics of pn and sn beneath the himalayan arc and tibetan plateau possible evidence for underthrusting of indian continental lithosphere beneath tibet new seismological observations on velocities and propagation characteristics of pn and sn waves beneath tibet can be interpreted although not uniquely to indicate the shallow angle underthrusting of the greater indian continental lithosphere beneath the tibetan plateau this is inferred from available geological and geophysical data and is now supported by new seismological observations the most significant observation is that high frequency sn waves shear waves that travel in the mantle lithosphere propagate efficiently in the uppermost mantle beneath the tibetan plateau except beneath the north central part of tibet the chang thang terrane furthermore sn propagates efficiently across the stable block of the tarim basin along the tien shan and himalayan mountains and across the indian shield apparent velocities of pn and sn waves that traverse the uppermost mantle beneath the tibetan plateau are 8 42 and 4 73 km s respectively these velocities are very similar to those beneath the himalayan mountains and the indian shield the efficient propagation and the velocities of pn and sn waves beneath tibet are also similar to those commonly observed beneath shield and stable continental regions these results are consistent with the underthrusting model and do not favor the class of alternative models in which the tibetan plateau is formed by shortening and thickening of hot crustal and uppermost mantle material in response to the convergence of india and asia

tracing water masses with 129i in the western nordic seas in early spring 2002 two long lived halogen radionuclides 36cl t1 2 301 kyr and 129i t1 2 15 7 myr have been studied by means of accelerator mass spectrometry ams at the uppsala tandem laboratory the 36cl measurements in natural samples using a medium sized tandem accelerator 1 mev amu have been considered a gas filled magnetic spectrometer gfm was proposed for the separation of 36cl from its isobar 36s semi empirical monte carlo ion optical calculations were conducted to define optimal conditions for separating 36cl and 36s a 180 gfm was constructed and installed at the dedicated ams beam line 129i has been measured in waters from the arctic and north atlantic oceans most of the 129i currently present in the earth s surface environment can be traced back to liquid and gaseous releases from the nuclear reprocessing facilities at sellafield uk and la hague france the anthropogenic 129i inventory in the central arctic ocean was found to increase proportionally to the integrated 129i releases from these reprocessing facilities the interaction and origin of water masses in the region have been clearly distinguished with the help of 129i labeling predictions based on a compartment model calculation showed that the atlantic ocean and deep arctic ocean are the major sinks for the reprocessed 129i the variability in 129i concentration measured in seawater along a transect from the baltic sea to the north atlantic suggests strong enrichment in the skagerrak kattegat basin the 129i inventory in the baltic and bothnian seas is equal to 0 3 of the total liquid releases from the reprocessing facilities a lake sediment core sampled in northeastern ireland was analyzed for 129i to study the history of the sellafield releases in particular the nuclear accident of 1957 high 129i concentration was observed corresponding to 1990 and later while no indication of the accident was found the results of this thesis research clearly demonstrate the uniqueness and future potential of 129i as a tracer of processes in both marine and continental archives

seismic anisotropy from sks splitting beneath northeastern tibet the northeastern boundary of the tibetan high plateau is marked by a 2 km topographic drop and a coincident rapid change in crustal thickness surface tectonics are dominated by the kunlun strike slip fault system and adjacent kunlun concealed thrust the main objective of the current study is to map lateral variations of seismic anisotropy parameters in this region along the linear indepth iv array in order to investigate the link between surface and internal deformation in the context of crust and mantle structure to achieve this aim we performed minimum transverse energy based sks splitting measurements using 23 stations of the indepth iv array deployed across the northeastern margin of tibet average fast polarization directions and splitting time delays are obtained by averaging stacked misfit surfaces of all analyzed events at each station the agreement of fast directions with the strikes of major active strike slip faults and strike slip focal mechanisms but not with fossil structures such as the jinsha suture implies that the anisotropy records lithospheric petrofabric formed by recent deformation within the lithosphere rather than representing frozen in anisotropy or shear within the asthenosphere due to absolute plate motion the distribution of large splitting delays throughout the northern plateau suggests that deformation is distributed rather than focused onto narrow shear zones associated with the kunlun strike slip faults the drop in splitting delays toward the qaidam is then a natural consequence of the much lower degree of deformation there

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

a complex tibetan upper mantle a fragmented indian slab and no south verging subduction of eurasian lithosphere due to the non uniform seismic station coverage in tibet critically important questions remain about the existence of southward continental subduction of eurasia beneath northern tibet from north and the nature of the underthrusting indian lithosphere underneath southern tibet from south using differential p and s wave travel times measured from 301 stations of all the temporary experiments deployed throughout tibet we constructed a comprehensive tomographic model the upper mantle of northern tibet consists of a rather homogeneous low velocity zone with no evidence of southward asian continental subduction in contrast the upper mantle from the himalayas to central tibet exhibits laterally variable p and s wave velocities significant low velocity zones are observed that are elongated in a north south direction and extending to at least 150 km depth which we interpret as evidences for fragmentation of the underthrusting indian continental lithosphere

a low velocity belt beneath northern and eastern tibetan plateau from pn tomography 1 new pn tomography on tibetan plateau tp and neighboring region is conducted with data assembled from various sources the improved pn tomographic image delineates several distinct geophysical regimes that are approximately separated by geological boundaries at surface dense distribution of the stations in the eastern half of the plateau allows us to infer a 3d view of crustal thickness variation of the region the most striking feature is the bended low velocity belt lvb which extends from west to east beneath qiangtang block and songpan ganzi fold belt sgfb in northern tp and then turns consistently southward with sgfb beneath eastern tp and further south to indo china the consistency between geological boundaries on surface and sharp velocity contrast boundaries in uppermost mantle may suggest strong deformation coupling between crust and at least uppermost mantle if not whole lithosphere a north south striking lvb is also observed cutting through the southern tp coinciding with the yadong gulu rift at surface our tomographic results are generally consistent with dynamic models that involve with underthrusting in the southern tp shortening in the northern tp and extrusion to the east and the southeast furthermore our results suggest significant east west variations in the structure and dynamics

surface wave tomography of eastern and central tibet from two plane wave inversion rayleigh wave and love wave phase velocity mapssurface wave tomography of eastern and central tibet from two plane wave inversion rayleigh wave and love wave phase velocity maps