a mechanism for magmatic accretion under spreading centres the most active volcanic zone on earth occurs along the global mid ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km myr 1 this volcanism is segmented and is concentrated along the axial valleys and highs of slow and fast spreading ridges respectively with transform fault zones separating and generally offsetting each active ridge segment volcanic activity may be nearly absent at the ridge transform intersection where a slow spreading ridge is offset and greatly reduced where a fast spreading ridge is offset implying thinner crust in these regions1 the source of magmas erupted along ocean ridges is the underlying mantle which undergoes decompression melting when rising to fill the gap between the spreading plates the resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock the melt is believed to pool in crustal magma chambers whence it periodically erupts to the surface typically the formation ascent and aggregation of magmas along the mid ocean ridges is regarded two dimensionally and modelled in a section through the crust and mantle across the strike of the ridges see ref 2 here however we consider the problem with a three dimensional model for which we provide supporting geological evidence in which the magmas rise as a result of a gravitational instability modified rayleigh taylor in the underlying partial melt zone

upper mantle anisotropy beneath the geoscope stations seismic anisotropy has been widely studied this last decade particularly by measuring splitting of vertically propagating core shear waves the main interest in this technique is to characterize upper mantle flow beneath seismic stations on the other hand the major restriction in this method is that a single station gives a single anisotropy measurement alternative methods have been developed in order to avoid this restriction an accurate determination of upper mantle seismic anisotropy beneath a seismic station may allow one by doing anisotropy correction to characterize remote or deeper anisotropy the geoscope network is ideal for this purpose because it is composed of a large set about 26 of high quality broadband seismometers globally distributed and because some of these stations have run for more than 10 years and most of them for more than 5 years we selected about 100 events at each site generally of magnitude mb 6 0 and we performed systematic measurements of the splitting parameters fast polarization direction and delay time t on sks skks and pks phases splitting on oceanic islands has been difficult to observe owing to the low quality of the signal but also perhaps owing to complex upper mantle structures beneath the stations station kip kipapa hawaii in the pacific is the only oceanic geoscope station with a clear anisotropy we determined well constrained splitting parameters for 10 of the 17 continental stations that may be explained by a single anisotropic layer the poor correlation between fast polarization directions and the absolute plate motion together with the apparent incoherence between the plate velocities and the observed delay times suggest that a simple drag induced asthenospheric flow alone fails to explain most of the observations for some stations located on or near major lithospheric structures tam tamanrasset algeria for instance we observe a good correlation between fast polarization directions and regional structures at station scz santa cruz california we found clear variations of the splitting parameters as a function of the event backazimuth compatible with two layers of anisotropy three stations can canberra hyb hyderabad india and ssb saint sauveur badole france seem to be devoid of detectable anisotropy

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

the role of bay of bengal convection in the onset of the 1998 south china sea summer monsoon abstract assimilated analysis fields from the south china sea monsoon experiment and the outgoing longwave radiation data from the national center for atmospheric research ncar have been employed to describe the large scale and synoptic features of the subtropical circulation during the bay of bengal bob 6 20 n 80 100 e and south china sea scs 7 20 n 110 120 e monsoon onsets in 1998 the results show that the asian monsoon onset during may 1998 exhibited a typical eastward development from the bob region to the scs domain the weakening and retreat of the subtropical anticyclone from the scs were preceded by the intrusion of westerlies and the development of convective activities over the northern part of the scs nscs 15 20 n 110 120 e as the vertical shear of zonal wind changes in sign the ridge surface of the subtropical anticyclone tilted northward and the summer pattern was established over the scs based on these observational results version 4 of the ncar climate model ccm3

signals of spring thermal contrast related to the interannual variations in the onset of the south china sea summer monsoon abstractland sea thermal contrast is one of the key factors modulating the onset and strength of summer monsoons this study investigates the precursory thermal contrast signal of the onset of the

surface deformation and lower crustal flow in eastern tibet field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern tibetan plateau since about 4 million years ago instead central eastern tibet has been nearly stationary relative to southeastern china southeastern tibet has rotated clockwise without major crustal shortening and the crust along portions of the eastern plateau margin has been extended modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle this model also predicts east west extension on the high plateau without convective removal of tibetan lithosphere and without eastward movement of the crust east of the plateau

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

crustal accretion at high temperature spreading centres rheological control of crustal thickness abstract new determinations of lateral crustal thickness variations at anomalous oceanic spreading centres such as iceland have shown that the crust may be thinner at the ridge axis above the plume thickening towards the sides bjarnason and schmeling 2009 to understand this behaviour crustal accretion models have been carried out solving the conservation equations of mass momentum and energy with melting melt extraction and feedback of extracted material as newly formed crust for an extending lithosphere system underlain by a hot mantle plume the dynamics of rifting are thermally and rheologically controlled by the feedback due to accreted new crust four accretional modes with characteristic crustal thickness variations are identified depending on the width of the volcanic emplacement zone the accretional heating rate which can be associated with the thickness of the surface layer in which magmatic emplacement takes place and the spreading rate mode 1 zero crustal thickness at the spreading axis develops for cool accretion and a wide emplacement zone mode 2 strongly or moderately crustal thickening away from the axis develops in case of warm deep reaching accretion and wide emplacement zones mode 3 nearly constant crustal thickness develops in case of warm deep reaching accretion but narrow emplacement zones dynamic topography of mode 3 shows only a weak or no regional minimum at all near the axis modes 2 or 3 may be identified with the situation in iceland mode 4 a stagnating central crustal block evolves for cool accretion and narrow emplacement this mode disappears for increasing spreading rates no accretional mode with maximum crustal thickness above the plume at the rift axis has been found the absence of mode 1 accretion with zero crust at ridge axis on earth may be an indication that in general crustal accretion is not cold and shallow the model is also applied to other hotspot ridge settings azores galapagos and suggests modes 2 3 accretion

gravity field of the southeast central pacific from geosat exact repeat mission data efficient techniques for mapping the marine gravity field with geosat exact repeat mission erm data are demonstrated geosat has completed more than fifty two 17 day global erm repeat cycles and has collected roughly 50 million observations of geoid undulations or more precisely sea surface topography because the global ground track repeats every 17 days the erm observations are highly redundant by averaging the repeat cycles very precise profiles of alongtrack geoid undulations are obtained for a region of the southeast central pacific without any crossing arc orbit adjustments these profiles are shown to be repeatable to within 10 cm over their entire 3000km length i e to three parts in 108 this southeast central pacific region contains a portion of the east pacific rise as well as some cross grain gravity field lineations and is therefore of considerable geophysical interest in addition to profiles of alongtrack deflections of the vertical regularly gridded gravity anomalies are derived for this region and portrayed in color the procedure used for recovering gravity has two stages in stage 1 alongtrack deflections of vertical are derived by fitting cubic splines to alongtrack heights computing alongtrack slopes at standardized latitudes and then averaging slopes for coincident passes in stage 2 gridded band limited gravity anomalies are recovered from deflections by using fast fourier transform techniques to accomplish the necessary inverse vening meinesz transformation high precision gem t1 orbits are used and crossover adjustments are not performed the operational navy astronautics group orbits are shown to have systematic errors which may be as large as 10 m

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

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

shear wave splitting in the appalachians and the urals a case for multilayered anisotropy observations of shear wave splitting in the northeastern u s appalachians and in the foredeep of the urals vary significantly with the back azimuth and incidence angle of the incoming phase these variations suggest two or more layers within the upper mantle with different anisotropic properties synthetic seismograms for simple multilayered anisotropic structures show that shear wave splitting parameters tend to vary substantially with the direction of approach relying on a subset of back azimuth and incidence angle may strongly bias the model inferred especially if the observations are averaged on the other hand the azimuthal splitting pattern provides additional constraints on vertical or lateral variation of anisotropic properties in the earth using a new error estimation technique for splitting we find that individual measurements from broadband data have errors of the order of 3 7 for the fast direction and 0 1 0 2 s for the delay of split shear waves the azimuthal variation of splitting parameters is broadly consistent throughout the appalachian terranes in the northeast united states especially for two long running stations in the northeast united states hrv harvard massachusetts and pal palisades new york observations can be separated into two distinct populations with mean fast axis azimuths of n60 e 4 and n119 e 2 delay values within each population range from near zero to 1 s azimuthal splitting variation for aru arti russia in the foredeep of uralian mountains is characterized by sharp transitions between different groups of observations using synthetic seismograms in simple structures we develop one dimensional anisotropic models under stations hrv and aru the model for hrv includes two layers of anisotropic material under an isotropic crust with fast axis azimuths n53 e and n115 e for the bottom and the top layers respectively the model for the upper mantle under aru includes a layer with a fast axis at n50 e atop a layer with fast axis azimuth n90 e our modeling confirms the need for a layer of strong anisotropy with a slow axis of symmetry in the lower crust under aru reported by levin and park 1997a our results suggest that both urals and appalachians possess a relict anisotropy in the tectosphere associated with past continental collision and accretion underlain by anisotropy with orientation similar to the local absolute plate motion suggesting an asthenospheric component to the signal

three dimensional flow beneath a slow spreading ridge axis a dynamic contribution to the deepening of the median valley toward fracture zones the axial valley along a slow spreading mid ocean ridge may be explained by a vertical pressure gradient due to viscous flow of asthenosphere upwelling in a relatively narrow conduit beneath the ridge axis along a ridge segment the axial valley floor deepens by as much as 2 3 km over distances of several tens of kilometers approaching a ridge transform intersection this deepening may be explained in part by a pressure gradient associated with horizontal flow in the ridge axis conduit vanishing vertical velocity on a conduit end wall at a ridge transform intersection results in reduced vertical flow within about one conduit width of the intersection horizontal flow along the conduit toward the intersection must occur to form lithosphere at a uniform rate along the ridge axis a simple model with a conduit of uniform width that terminates with vertical planar endwalls at ridge transform intersections is considered the model is based on an analytical solution for flow in a narrow conduit and boundary layer approximations for the flow structure near the vertical end walls this model indicates that the induced horizontal flow and pressure gradient extend for a distance along the ridge axis of several lithosphere thicknesses from an intersection for reasonable values of the conduit width and asthenosphere viscosity the model predicts that induced horizontal flow can contribute significantly to the deepening of an axial valley approaching an intersection

the deep structure of ocean ridges in a convecting mantle abstract plate tectonic models have frequently assumed that mid ocean ridges are the surface expression of the hot rising limb of a convection cell measured half spreading rates show that in any reference frame the horizontal transverse velocity of some ridges is comparable to the individual plate velocities if the above assumption is correct this observation implies that the thermal structure beneath the ridge can adjust rapidly to the changing surface configuration of the plates numerical experiments in a two dimensional convecting layer of constant viscosity fluid show that such a rapid adjustment is probably unrealistic a symmetrically spreading ridge moving horizontally is simulated by specifying a velocity boundary condition on the upper surface of the layer the other boundary conditions are uncertain so several different combinations are used in the experiments including periodic or reflecting lateral boundaries and stress free or no slip lower boundary in all the experiments the rising sheets of the convection cells are anchored firmly in the mean horizontal flow of the layer unless the mean flow coincides approximately with the ridge velocity the ridge quickly moves away from a rising sheet which is initially beneath it if the aspect ratio width depth of the convection cells is significantly greater than one this conclusion is even more evident in the earth it seems unlikely that there is a systematic deep of the order of the convecting layer thickness structure beneath the worldwide network of ocean ridges

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

structure of the upper mantle and transition zone beneath southeast asia from traveltime tomography 1 tomographic images of the mantle beneath east asia were obtained from the inversion of traveltime data from global and regional seismograph networks and from temporary arrays on and around the tibetan plateau our results are consistent with previous studies but the unprecedented resolution of mantle heterogeneity provides new insight into the large scale tectonic framework of the continental india asia collision in the western part of the study region and subduction of the oceanic lithosphere in the east in the realm of continental collision west of 100 e a relatively slow p wave speed characterizes the upper mantle beneath much of the tibetan plateau but the wave speed is high beneath cratonic india the southern and western part of the tibetan plateau hindu kush and the tian shan in the subduction realm east of 110 e the main structures are i pronounced low wave speed anomalies at a depth of between 100 and 400 km beneath asia s southeastern seaboard and the back arc regions of ongoing subduction ii narrow fast anomalies in the upper mantle beneath major subduction zones and iii widespread fast anomalies at a depth of 500 700 km beneath the sea of japan the northern part of the philippine sea plate and southeastern china if the latter anomalies represent stagnant slabs their fragmented nature and large lateral extent suggest that they are produced by different episodes of subduction beneath western pacific island arcs along the old se margin of asia or during the mesozoic collision of cratonic units in southeast asia attribution to ancient subduction systems implies that slab fragments can reside in the transition zone for at least several tens of millions of years shallow slow anomalies beneath the red river fault region connect to deep anomalies beneath the south china fold belt and south china sea suggesting a causal relationship between the evolution of the continental lithosphere of sw china and deeper mantle processes between the collision and the subduction realms tomography reveals high wave speed continental roots beneath the western part of the north china craton ordos block and the south china or yangtze craton sichuan basin to a depth of 300 km

simultaneous melting and compaction in deformable two phase media summary melt generation and extraction are typically modelled using the two phase equations developed by mckenzie or scott and stevenson various approximations are often made to simplify the problem which may lead to some unphysical results e g thermodynamically inconsistent conditions of melting and unrealistic porosity profiles we discuss a generalized version of the set of equations introduced by bercovici et al that allows for mass transfer between the two phases in a single component system and consider a self consistent set of equations in our description the two phases solid and melt are submitted to individual pressure fields whose difference is related to the surface tension at the interfaces changes in porosity and the melting rate a kinetic relation for the melting rate arises from the second law of thermodynamics the condition of chemical equilibrium corresponds to the usual univariant equality of the chemical potentials of each phase when the matrix and melt are motionless in the most general form phase equilibrium is influenced by both the gibbs thomson effect that arises naturally from thermodynamic considerations on surface tension and by the viscous deformation of the phases we apply these new equations to a steady state problem of pressure release melting in a univariant system we treat melting and compaction simultaneously and observe several new effects including multiple domains near the onset of melting that correspond to various force balances a consequence of matrix compaction and melt expulsion or matrix dilation and melt accumulation is a pressure difference between melt and solid that facilitates inhibits melting for parameters corresponding to mid oceanic ridge magmatism compaction permits melting to start as much as 2k m below the standard solidus numerical solutions are necessary to determine the magnitude of the melt zone shift numerical results support the boundary layer solutions obtained analytically and suggest that in most of the melting zone the movement of melt and matrix should be close to the darcy equilibrium where the buoyancy of melt is balanced by the viscous drag between the phases the darcy equilibrium follows an initial stage where the matrix viscous stresses balance darcy drag in all situations the steady state porosity profile remains a monotonic function of depth the existence of a compaction layer following a melting zone where the porosity is maximum as described in various earlier publications has never been found

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

an observational study of the south china sea monsoon during the 1979 summer onset and life cycle abstract the onset and life cycle of the 1979 south china sea summer monsoon were examined in the context of the 30 60 and 12 24 day monsoon modes the former intraseasonal mode formed the northward migrating monsoon trough ridge while the latter intraseasonal mode propagated westward in the south china sea the monsoon in this region exhibited three cycles over the summer may august with the onset taking place about one cycle ahead of the onset of the indian and japanese monsoons climatologically a summer trough line radiated out from the indian monsoon trough region across indochina to the northern south china sea the monsoon onset occurred when the 30 60 day monsoon trough and the 12 24 day low center arrived simultaneously at the northern south china sea close to the climatological summer trough line in the middle of may the breaks occurred when the 30 60 day monsoon ridge lines and the 12 24 day high center met near 15 20 n in the northern south china sea the south china sea monsoon wa

two types of interannual variability of south china sea summer monsoon onset related to the sst anomalies before and after 1993 94 abstractan advance in the timing of the onset of the south china sea scs summer monsoon scssm during the period 1980 2014 can be detected after 1993 94 in the present study the interannual variability of the scssm onset is classified into two types for the periods before and after 1993 94 based on their different characteristics of vertical coupling between the upper and lower tropospheric circulation and the differences in their related sea surface temperature anomalies sstas on the interannual time scale type i scssm onset is characterized by anomalous low level circulation over the northern scs during 1980 93 whereas type ii scssm onset is associated with anomalies of upper level circulation in the tropics during 1994 2014 the upper tropospheric thermodynamic field and circulation structures over the scs are distinct between the two types of scssm onset and this investigation shows the importance of the role played by the spring sstas in the southern indian ocean sio and that of enso e

seismic anisotropy beneath the southern himalayas tibet collision zone shear wave phases scs sks and skks collected from the project international deep seismic sounding of tibet and himalayas ii indepth ii array in the eastern himalayas and tibet are analyzed for the orientation and extent of polarization seismic anisotropy beneath the southern himalayas tibet collision zone no evidence is seen for any shear wave splitting for stations in this part of the collision zone except for two northern stations located about 100 km north of the indus tsangpo suture the lack of shear wave splitting beneath the stations in the vicinity of suture and the tethyan himalayas indicates that there is no appreciable large scale mantle polarization seismic anisotropy beneath this part of the collision zone one explanation for the lack of polarization seismic anisotropy would be a subvertical mantle shear strain field resulting from a downwelling mantle flow of a thickened indian lower continental lithosphere north of the gandese belt the fast directions of the split shear waves are oriented in a northeast southwest direction approximately parallel to the trend of a major left lateral strike slip fault zone that bounds the gulu rift the coincidence of fast polarization directions parallel to the surficial geologic shear zones and the direction of shear planes within the upper crustal strain rate field suggests lithospheric wide coherent deformation

indian mantle corner flow at southern tibet revealed by shear wave splitting measurements 1 we constrain anistropic seismic structure in the southernmost tibet and the tethyan himalays using sks and skks phases recorded from a temporary seismic network operated from july 2004 to august 2005 in conjunction with the hi climb project shear wave splitting is not detected at 16 stations most of which are located near and to the north of the indus yalong suture iys for the first time anisotropy with an n s fast direction and 0 4 1 s delay times is observed to the region about 50 km south of the iys this weak anisotropy correlates with the flat part of the subducting indian lithosphere and could be caused by northward asthenospheric flow and shear at the base of the indian lithosphere the null measurements in the vicinity of the iys are most likely the result of a vertical asthenospheric flow the corner flow induced by the subvertical subduction of the indian lithosphere just south of the bangong nujiang suture bns observations of the systematic changes in mantle anisotropy from southern tibet to the central tibet provide evidence for different mantle flow fields between the indian and the eurasian asthenospheric mantle which are separated by a subvertical indian lithosphere at the bns

dynamical consequences of compositional and thermal density stratification beneath spreading centers extraction of partial melt to form oceanic crust reduces the density of upwelling mantle beneath spreading centers this compositional buoyancy can significantly affect both the rate of upwelling and the pattern of mantle flow the relative importance of buoyant upwelling and upwelling due to plate spreading depends on the mantle viscosity for a low enough mantle viscosity buoyant upwelling can significantly enhance the thickness of the crust relative to that which would be produced by plate spreading alone for a mantle viscosity of 1022 pa s upwelling due to plate spreading is dominant and crustal thickness is predicted to be a strong function of spreading rate however for a mantle viscosity of 1019 pa s buoyancy dominates mantle upwelling and the calculated crustal thickness like the oceanic crust is nearly independent of spreading rate thermal and compositional density variations result in opposing buoyancy forces that can cause time dependent upwelling for a mantle viscosity of 1020 pa s and a low 1 cm yr spreading rate this behavior takes the form of undamped crustal thickness oscillations with a single well defined period for even slightly lower mantle viscosity the frequency increases and crustal thickness oscillations become more complex with two or more frequencies present while numerical experiments have thus far been carried out only for viscosities significantly greater than that likely to characterize the mantle beneath spreading centers we speculate that these crustal thickness oscillations may explain important aspects of seafloor structure

multi stage onset of the summer monsoon over the western north pacific the climatological summer monsoon onset displays a distinct step wise northeastward movement over the south china sea and the western north pacific wnp 110 160 e 10 20 n monsoon rain commences over the south china sea philippines region in mid may extends abruptly to the southwestern philippine sea in early to mid june and finally penetrates to the northeastern part of the domain around mid july in association three abrupt changes are identified in the atmospheric circulation specifically the wnp subtropical high displays a sudden eastward retreat or quick northward displacement and the monsoon trough pushes abruptly eastward or northeastward at the onset of the three stages the step wise movement of the onset results from the slow northeastward seasonal evolution of large scale circulation and the phase locked intraseasonal oscillation iso the seasonal evolution establishes a large scale background for the development of convection and the iso triggers deep convection the iso over the wnp has a dominant period of about 20 30 days this determines up the time interval between the consecutive stages of the monsoon onset from the atmospheric perspective the seasonal sea surface temperature sst change in the wnp plays a critical role in the northeastward advance of the onset the seasonal northeastward march of the warmest sst tongue sst exceeding 29 5 c favors the northeastward movement of the monsoon trough and the high convective instability region the seasonal sst change in turn is affected by the monsoon through cloud radiation and wind evaporation feedbacks

finite frequency tomography in southeastern tibet evidence for the causal relationship between mantle lithosphere delamination and the north south trending rifts 1 while several mechanisms have been suggested to explain the evolution of the tibetan plateau observational constraints on the deep lithospheric processes have been sparse and previous seismic studies were mostly along profiles perpendicular to the collision front of the indian and eurasian plates in this study we show tomographic evidence for the delamination of the mantle lithosphere beneath southeastern tibet a process in which the entire mantle lithosphere peels away from the crust along the moho and thus is a mechanism for rapid thinning of the lithosphere our p and s wave velocity models show the presence of a low velocity anomaly in the crust and upper mantle down to 300 km depth beneath a north south trending rift zone in southeastern tibet this low velocity anomaly is situated above a tabular high dipping angle high velocity anomaly that extends into the upper mantle transition zone the vp vs ratio of this high velocity anomaly suggests that temperature variations are not the only cause of the anomaly and a highly melt depleted mantle is required these observations suggest a causal relationship between the delamination of mantle lithosphere and the formation of the north south trending rift in southeastern tibet

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

convergence of the indian and eurasian plates under eastern tibet revealed by seismic tomography 1 a three dimensional p wave velocity model of the crust and upper mantle down to 400 km depth beneath eastern tibet is obtained using many temporary seismic stations of the ascent project and the namche barwa broadband seismic network we collected 16 508 arrival times of p pn and pg phases from 573 local and regional earthquakes and 7 450 p wave arrivals from seismograms of 435 teleseismic events our high quality data set enables us to reconstruct the 3 d velocity structure under eastern tibet in more detail than the previous studies in the shallow depth our results show that the low velocity zones are not interconnected well no wide spread low velocity zones which may reflect the complex pattern of material flowing in the study region below the moho we find that the indian lithospheric mantle underthrusts sub horizontally under eastern tibet and the extent of the northward advancing indian lithosphere decreases from west to east in the north the asian lithospheric mantle is detected under the vicinity of the qaidam basin between the indian and asian lithospheric mantles there is an obvious low velocity anomaly which may reflect an upwelling mantle diapir

energy dissipation rates at ridge and transform fault segments a model of oblique spreading we present an energy dissipation model that can easily be applied to oblique transform faults as well as to oblique spreading ridges by using a simple model of a rising incompressible newtonian fluid in a vertical conduit of the ridge transform fault system the energy dissipation rates of the ridge and transform fault segments are calculated as a function of the angles between the strikes of segments and the spreading directions and the spreading rates the stability is estimated for about 400 present day segments from the energy dissipation rates of ridge and transform fault segments it is concluded that most of the present day configurations of the ridge transform fault system are nearly stable even though the strikes of ridge and transform fault segments are oblique to the spreading directions several exceptions seem to be associated with extraordinary phenomena such as the occurrence of an extremely large earthquake and overlapping spreading centers

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

intraseasonal variability of the south china sea summer monsoon the objective of this study is to explore based on the national centers for environmental prediction national center for atmospheric research ncep ncar reanalysis data the intraseasonal variability of the south china sea scs summer monsoon sm in terms of its structure and propagation as well as interannual variations a possible mechanism that is responsible for the origin of the 10 20 day oscillation of the scs sm is also proposed the 30 60 day hereafter the 3 6 mode and 10 20 day hereafter the 1 2 mode oscillations are found to be the two intraseasonal modes that control the behavior of the scssm activities for most of the years both the 3 6 and 1 2 modes are distinct but may not always exist simultaneously in a particular year and their contributions to the overall variations differ among different years thus the interannual variability in the intraseasonal oscillation activity of the scs sm may be categorized as follows the 3 6 category in which the 3 6 mode is more significant in terms of the percentage of variance explained than the 1 2 mode the 1 2 category in which the 1 2 mode is dominant and the dual category in which both the 3 6 and 1 2 modes are pronounced composite analyses of the 3 6 category cases indicate that the 30 60 day oscillation of the scs sm exhibits a trough ridge seesaw in which the monsoon trough and subtropical ridge exist alternatively over the scs with anomalous cyclones anticyclones along with enhanced suppressed convection migrating northward from the equator to the midlatitudes the northward migrating 3 6 mode monsoon trough ridge in the lower troposphere is coupled with the eastward propagating 3 6 mode divergence convergence in the upper troposphere it is also found that for the years in the dual category the scs sm activities are basically controlled by the 3 6 mode but modified by the 1 2 mode composite results of the 1 2 mode category cases show that the 10 20 day oscillation is manifest as an anticyclone cyclone system over the western tropical pacific propagating northwestward into the scs a close coupling also exists between the upper level convergence divergence and the low level anticyclone cyclone it is found that the 1 2 mode of the scs sm mainly originates from the equatorial central pacific although a disturbance from the northeast of the scs also contributes to this mode the flow patterns from an inactive to an active period resemble those associated with a mixed rossby gravity wave observed in previous studies

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

crustal and upper mantle velocity structure beneath central tibet by p wave teleseismic tomography summary a detailed 3 d p wave velocity model beneath central tibet was obtained using 26 741 arrival times from 1025 teleseismic events recorded by the portable stations of the hi climb project in the crustal correction we consider both vertical and lateral velocity variations our teleseismic p wave tomography result reveals that the indian lithospheric mantle underthrusts no further than 32 5 n in addition the presence of low velocity anomalies under the indus tsangpo suture suggests that subduction is not a simple and continuous process we suggest that the delamination of indian mantle lithosphere induces mantle upwelling beneath the rifts which in turn created cracks or a break in the subducted plate moreover the formation of active rifts near the profile is related to the mantle upwelling

upper crustal anisotropy of the conjugate strike slip fault zone in central tibet analyzed using local earthquakes and shear wave splittingupper crustal anisotropy of the conjugate strike slip fault zone

finite strain calculations of continental deformation 1 method and general results for convergent zones a finite element method is used to calculate stresses and strain rates in a thin viscous sheet representing the continental lithosphere that is bordered in one part by an indenting boundary we calculate solutions for velocity fields crustal thickness distributions strain rates rotation time integrated vorticity and finite strain ellipsoids to show the dependence of these quantities on the controlling parameters the stress strain exponent n and the strength of the sheet relative to the gravitational forces which may be expressed as a dimensionless number the argand number ar for n 3 and ar between 1 and 10 a plateau of thickened crust is formed in front of the indenter the strain histories of individual elements within the viscous sheet may be quite complex and the finite strain at the end of deformation may bear no obvious relation to the principal stress orientations during the deformation the strain rate fields are interpreted in terms of the style of faulting that would be expected in the brittle upper crust if it were coupled to the ductile but stronger upper mantle in general the length scale of deformation increases with ar and time since the start of the collision and decreases with n for ar 3 the increase in length scale is accompanied by a change in the style of deformation from predominantly crustal thickening with thrust faulting in the brittle layer to predominantly transcurrent deformation strike slip faulting in the brittle layer

constraints on shallow mantle viscosity from morphology and deformation of fast spreading ridges 1 we show that the morphology and the deformation of a fastspreadingridgeconstrainshallowuppermantleviscosity a fast spreading center is simulated in a numerical model that couples tectonic deformation due to plate spreading and periodic dike emplacements the amount of magma intruded into dikes or extruded is enough to make a 7 km thick crust and the axial lithosphere is fixed at 2 km thickness for low viscosities 10 19 pa s the model spreadingcenter is marked by an axial high with relief governed by the axial density structure for high viscosities 10 20 pa s the axial morphology is a valley and extensional brittle deformation is distributed away from the near axis region the observed morphology and deformation of fast spreading ridges are not consistent with the high mantle viscosity estimated for very dry olivine as some suggest may result from partial melting related dehydration citation choi e and w r buck 2010 constraints on shallow mantle viscosity from morphology and deformation of fast spreading ridges geophys res lett 37 l16302 doi 10 1029 2010gl043681

shear wave anisotropy beneath the tibetan plateau eleven broadband digital seismic stations were deployed across the central tibetan plateau in the first extensive passive source experiment attempted within the tibetan plateau one year of recording resulted in 186 event station pairs which we analyze to determine the characteristics of shear wave splitting in the upper mantle beneath the array measurements of the fast polarization direction and delay time t for sks and direct s arrivals reveal systematic variations along the north south oriented array in the north central region of the plateau very large delay times are observed at three stations the largest of which is budo with of t 2 4 s however at tunl which is off the northern edge of the plateau and 110 km from budo and at sites in the south central plateau t decreases by nearly a factor of 3 we also observe systematic rotation of from about 45 ne to 90 e w from south to north along the array a previously identified zone of inefficient sn propagation correlates well with our region of large t observations the large delay times suggest that a relateively high number of anisotropic crystals are preferentially alligned within the mantle lid beneath the north central portion of the tibetan plateau in most cases fast polarization directions appear to be parallel to surface geologic features suggesting as much as 200 km of the upper mantle has been involved in the collisional deformation that has produced the tibetan plateau

effects of partial melting on the uranium decay series distinction between possible partial melting processes using stable incompatible trace elements depends largely on the mass fraction of liquid produced at low degrees of melting less than 5 both incompatible element concentrations and interelement fractionations are greater for batch fractional and accumulated fractional melting than for dynamic or continuous melting at higher degrees of melting uncertainty in initial source composition and small fractionations between incompatible elements make it unlikely that the most realistic melting models batch and dynamic could be discriminated however it is possible for these melting processes to be distinguished by the magnitude of radioactive disequilibria that is produced within the 238u decay series even for durations of partial melting much longer than the half lives of 230th and 226ra both230th 238u and 226ra 230th disequilibria can occur because of differences in the bulk solid liquid partition coefficients for these elements the magnitude of these disequilibria depends on melting process the 230th 232th of a dynamic melt can differ significantly from that of a batch melt and will not necessarily indicate the original 238u 232th of the source if morb is formed by a dynamic melting process then the th u of the upper mantle near ridge axes may be higher than that calculated from the 230th 232th of the basalts

geochemistry of chromitites and host rocks from the pindos ophiolite complex northwestern greece abstract representative chromite samples of schlieren and massive ore types from various localities of the pindos ophiolite complex were analyzed for platinum group elements pge s ni co and cu and mineral chemistry host dunites and harzburgite were also analyzed for the chalcophile elements the chromite ores show a wide compositional variation the cr cr al ratio ranges from 0 47 to 0 83 while mg mg fe 2 ranges between 0 42 and 0 73 but two populations can be distinguished the high cr and high al types which show a more restricted variation ranges of pge contents in ppb are os 8 150 ir 4 320 ru 15 550 rh 2 82 pt 3 150 and pd 1 5 20 gold ranges between 1 and 13 ppb the pge and less chalcophile elements ni co and cu the incompatible compatible element ratios the pge s ratio and pge patterns provide valuable information for the discrimination of chromite ores with a similar major element composition a fractionation trend in both chromitites and associated dunites can be used for stratigraphic orientation in the mantle sequences of ophiolite complexes and evaluation of their chromite potential

evidence from core reflected shear waves for anisotropy in the earth s mantle seismologists have begun to recognize that the upper mantle at depth is anisotropic and that seismological data are not fully explicable if anisotropy is ignored1 5 on entering an anisotropic zone a plane shear wave splits into two orthogonally polarized phases6 an effect observed by several researchers7 notably by ando et al 5 8 9 the near vertical scs wave an s wave reflected once at the core mantle boundary is one of the most suitable phases to study the phenomenon of shear wave birefringence because it is isolated from other phases and because its particle motion is nearly horizontal and thus free from s to p conversion the split scs shear waves travel with different velocities in the anisotropic zone and consequently there is a time delay between their arrival at stations my aim was to determine this time delay and the azimuth of polarization using the scs records of two horizontal components from the japanese islands for a deep shock beneath the sea of okhotsk from the total of 41 stations 34 show a polarization of the maximum velocity phase in the nnw sse direction with an average time advance of 0 8s from the minimum velocity phase the relevant anisotropic zone seems to be deep seated

correlation between seismic anisotropy and bouguer gravity anomalies in tibet and its implications for lithospheric structures summary as a baseline measurement for understanding the himalayan tibetan orogen a product of continent continent collision between india and eurasia we analyse digital seismic data in order to constrain the seismic anisotropy of the indian shield based on spatially sparse data that are currently available in the public domain there is little shear wave birefringence for sks phases under the indian shield even though it is part of a fast moving plate in the hotspot frame of reference if most of the northern indian mantle has little transverse anisotropy the onset of significant anisotropy under tibet marks the northern terminus of intact indian lithosphere that is thrusting under the himalayan tibetan orogen beyond this terminus tectonic fabric such as that associated with the deforming lithospheric mantle of eurasia must be present in the upper mantle along the profile from yadong to golmud the only profile in tibet where a number of shear wave birefringence data are available the amount of birefringence shows two marked increases near 30 and 33 n between which a local high in bouguer gravity anomaly is observed such a correlation between patterns of shear wave birefringence and gravity anomalies is explained by the juxtaposition of indian lithosphere against the overlying eurasian lithosphere while the eurasian lithospheric mantle appears only to the north of 30 n the indian lithospheric mantle extends northwards to near 33 n

episodic kinematics in continental rifts modulated by changes in mantle melt fraction gps measurements are used to investigate the continental rift system of north island new zealand and to develop a model for short term deformation involving flexing of an elastic upper crust caused by episodic changes in mantle viscosity and melt fraction direct observation of mantle flow dynamics beneath spreading oceanic crusts is difficult but similar processes occur below rifts in the continental crust and so can be inferred from satellite observations of the moving crust simon lamb et al use ten years of continuous gps measurements from a back arc continental rift system in north island new zealand to investigate the dynamics of mantle flow beneath seafloor spreading centres they observe deformation which they conclude to be the result of flexure of an elastic upper crust pulled downwards or pushed upwards along the rift axis by a driving force at depth they propose that this flexure is caused by melt induced episodic changes in mantle viscosity in their model a drop in melt fraction due to melt extraction raises the mantle viscosity and drives subsidence whereas melt accumulation reduces viscosity and allows uplift oceanic crust is created by the extraction of molten rock from underlying mantle at the seafloor spreading centres found between diverging tectonic plates modelling studies have suggested that mantle melting can occur through decompression as the mantle flows upwards beneath spreading centres1 but direct observation of this process is difficult beneath the oceans continental rifts however which are also associated with mantle melt production are amenable to detailed measurements of their short term kinematics using geodetic techniques here we show that such data can provide evidence for an upwelling mantle flow as well as information on the dimensions and timescale of mantle melting for north island new zealand around ten years of campaign and continuous gps measurements in the continental rift system known as the taupo volcanic zone reveal that it is extending at a rate of 6 15 millimetres per year however a roughly 70 kilometre long segment of the rift axis is associated with strong horizontal contraction and rapid subsidence and is flanked by regions of extension and uplift these features fit a simple model that involves flexure of an elastic upper crust which is pulled downwards or pushed upwards along the rift axis by a driving force located at a depth greater than 15 kilometres we propose that flexure is caused by melt induced episodic changes in the vertical flow forces that are generated by upwelling mantle beneath the rift axis triggering a transient lower crustal flow a drop in the melt fraction owing to melt extraction raises the mantle flow viscosity and drives subsidence whereas melt accumulation reduces viscosity and allows uplift processes that are also likely to occur in oceanic spreading centres

chemical and isotopic variations along the superfast spreading east pacific rise from 6 to 30 s chemical data of 39 fresh basaltic glasses from the east pacific rise epr between 6 and 30 s and pb sr and nd isotopic compositions of 12 basalt glasses are presented major and trace element data indicate a wide compositional range including primitive basalts mg 0 67 and highly evolved feti basalts mg 0 34 molmg mg fe2 the compositional range can be attributed to low pressure fractional crystallization fractionation corrected major element concentrations provide evidence for varying mantle melting conditions calculations of the melting conditions suggest melt generation in a rising upper mantle column between 20 and 10 kbar at temperatures between 1430 and 1280 c and total degrees of partial melting between 17 and 20 by weight leached and hand picked basalt glasses display large variations in 87sr 86sr 0 70235 0 70270 143nd 144nd 0 51312 0 51323 and 206pb 204pb 18 064 18 665 but are similar to other n type morb from the epr the isotopic ratios of basalts from 13 to 23 s show strong correlations and delineate two systematic trends from 23 to 17 s 87sr 86sr and pb isotope ratios increase and 143nd 144nd decrease in agreement with previous results mahoney et al 1989 a reverse trend is indicated by basalts from 17 to 13 s however k ti and la sm n continuously increase from 23 to 13 s this opposite behavior indicates a recent decoupling of isotopic and minor element ratios in the mantle between 13 and 17 s north of 13 5 s garrett fracture zone isotopic data show no systematic variation with ridge location and display an overall weaker covariation the results suggest that the isotopic variations and ridge segmentation appear to be unrelated and that major ridge offsets apparently coincide with changes in mantle melting conditions p t f f degrees of melting there is no evidence for a systematic relationship between calculated melting conditions and second order ridge segmentation our isotopic data provide further evidence for regionally confined chemical variations in the mantle at 5 to 30 s we interpret the isotopic trends as reflecting melting of distinct smallvolume and old enriched mantle components in contrast variations in trace elements are attributed to young mantle differentiation processes

extremely early summer monsoon onset in the south china sea in 2019 following an el nino event abstractthe el nino southern oscillation enso is regarded as one of the most important factors for onset of the south china sea summer monsoon scssm previous studies generally indicated that a

on the dynamics of mid ocean ridges most extant flow models for ocean ridges suffer either from unrealistic geometry vertical conduit models or a stress singularity at the ridge axis corner flow models i present a model which avoids these difficulties by incorporating a weak plate boundary zone pbz between diverging surface plates an analytical solution for the flow of a constant viscosity asthenosphere and a coexisting melt phase is obtained in elliptic coordinates and used to predict the dynamically supported topography axial valley on the pbz the force required to pull the plates apart and the rate of melt extraction at the ridge stress in the asthenosphere increases linearly with spreading rate but decreases as the plates become thinner and the dynamics of ridge crests is controlled by the balance between these opposing effects the depth of the axial valley is controlled by the plate thinning effect and is therefore a decreasing function of spreading rate the depth of the axial valley on the mid atlantic ridge can be explained if the viscosity of the asthenosphere is 6 1019 1021 pa s and the lower surface of the plate is the 600 c isotherm the plate separation force is controlled by the spreading rate effect and therefore increases with spreading rate about five times as much force is required to maintain a fast spreading ridge 9 cm yr 1 half rate as a slow spreading ridge 1 cm yr 1 melt extraction can produce 6 km of basaltic crust if the volume fraction of melt present in the asthenosphere total degree of melting ranges from about 2 5 4 0 for a slow spreading ridge to 4 0 6 5 for a fast spreading ridge the dynamics of melt extraction does not depend significantly on the finite width of the pbz

platinum group element distribution in chromite ores from ophiolite complexes implications for their exploration abstract compilation of some new data on ophiolites for greece and yugoslavia and published data from previous studies indicate that platinum group element pge and gold concentrations in chromite ores are generally low ranging from less than 100 ppb to a few hundred ppb however samples from several ophiolite complexes exhibit an enrichment of a few ppm a only in os ir and ru b only in pt and or pd or c in all pge this enrichment up to 10s ppm is mainly related with chromitites hosted in supra moho dunites and dunites of the uppermost stratigraphic levels of the mantle sequence and it seems to be local independent of the chromitite major element composition and the chromite potential of the ophiolite complexes the contents of pge combined with less chalcophile elements ni co cu the ratios of incompatible compatible elements and pge patterns provide evidence for discrimination between chromitites derived from primitive magmas and those derived from partially fractionated magmas although they have a similar major element composition thus they can be used for a stratigraphic orientation in the mantle sequence and therefore for exploration targets moreover pge data offer valuable information for the evaluation of the chromite potential in ophiolite complexes the most promising ophiolites seem to be those which apart from the petrological and geochemical characteristics indicating extensive degree of partial melting in the mantle source contain only one chromite ore type the other type being only in small proportion of limited compositional variation in both major elements and pge low ratios of pd ir while pge enriched chromitites in the mantle sequence are only occasionally present in contrast ophiolites which contain both high cr and al chromitites and where their chalcophile element data implies relatively extensive fractionation trend are not good exploration targets for chromite ores although they are related with a ssz environment

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

upper mantle deformation beneath central southern tibet revealed by shear wave splitting measurements abstract seismic anisotropy in the upper mantle has been inferred from shear wave splitting measurements at the temporary seismic array antilope 2 in the central southern tibet from september 2005 to october 2006 teleseismic shear waves mainly sks and skks are used to determine the splitting parameters fast polarization direction and delay time weak splitting with average delay time of 0 3 s is observed at stations in the southern lhasa terrane and northernmost tethyan himalaya significant seismic polarization anisotropy with delay times from 0 8 to 1 5 s and ne oriented fast direction is obtained for stations located farther north in the northern lhasa terrane the transition in shear wave splitting characteristics occurs at 30 5 n and suggests distinct upper mantle deformation from south to north the negligible anisotropy south of this transition indicates that there is no appreciable large scale mantle seismic polarization anisotropy the substantial anisotropy in the north may be caused by the eastward flow in a lithospheric crush zone which is squeezed and sheared between the advancing indian plate to the south and eurasian plate to the north

three dimensional temperature structure of a ridge transform ridge system abstract we have solved the problem of three dimensional heat transfer by conduction and material transport within a ridge transform ridge system the boundary conditions are those of a finite plate with temperatures fixed at the top and bottom we do not solve for the pattern of flow so must arbitrarily describe the material transport within triangular regions beneath the spreading centers flow is assumed to be vertical and at constant velocity with the volume of upwelling matching the volume of horizontal transport by the plates the steady state solution is approached by repeated finite steps in time each involving two stages first to represent material transport temperatures in the grid of points are shifted horizontally and or vertically to the point at which the associated particle would arrive given the time period and assumed velocity field second a three dimensional fourier transform technique is used to calculate the cooling which would occur in the static case during the same period we find that the geometry of flow in the upwelling region here parameterized by velocity of upwelling or width of the upwelling zone is more important than spreading rate in determining the temperature structure the results indicate that conductive cooling cannot alone account for deepening of the median valley towards the fracture zone for a 10 m y offset on the transform fault conductive cooling across the fracture zone to the older lithosphere affects the upwelling material only to a distance of about 10 km from the fault in contrast to the deepening of the median valley over several tens of kilometers observed on the mid atlantic ridge

crustal thickening prior to 38 ma in southern tibet evidence from lower crust derived adakitic magmatism in the gangdese batholith article thepetrogenesisandgeodynamicimplicationsofthecenozoicadakitesinsoutherntibetremaintopicsofdebate here we report geochronological and geochemical data for host granites and mafi ce nclaves from wolong in the easterngangdesebatholith southerntibet zirconla icp ms datingindicatesthatthe wolonghostgranitesand enclaves were synchronously emplaced at ca 38 ma the host granites are medium to high k calc alkaline metaluminous a cnk 0 93 0 96 with high al2o3 15 47 17 68 low mgo 0 67 1 18 very low abundances of compatible elements e g cr 3 87 8 36 ppm ni 3 04 5 71 ppm and high sr y ratios 127 217 similar to those typical of adakite the mafi ce nclaves sio2 51 08 56 29 have 3 83 5 02 mgo

crustal seismic anisotropy in central tibet implications for deformational style and flow in the crust 1 receiver functions obtained at indepth iii stations located near the bangong nujiang suture in central tibet display a weak moho signal and strong p to s conversions within the first 5 s that vary systematically with back azimuth a single station with representative azimuthal variations located at the sharp onset of strong sks splitting is modeled for both dipping layers and seismic anisotropy by using a global minimization technique inversion results indicate strong anisotropy 10 near the surface and in the middle crust separated by a south dipping 25 layer possibly related to the earlier phase of crustal shortening near surface anisotropy has a fabric dipping steeply southward and trending wnw ese that correlates with the suture and younger strike slip faults in contrast mid crustal anisotropy occurs in a low velocity zone and has a fabric dipping gently 18 northward that might be related to a well developed near horizontal rock fabric induced by crustal flow

geochemistry of miocene trachytes in bugasi lhasa block tibetan plateau mixing products between mantle and crust derived melts abstract it is generally believed that cenozoic potassic and ultrapotassic volcanic rocks of the tibetan plateau were generated by partial melting of an enriched mantle region or lower crustal materials the miocene bugasi volcanic rocks bvr in the western part of the lhasa block are composed mainly of trachyandesites and trachytes both of which belong to the shoshonite series the trachytes show somewhat transitional compositions between the mantle derived trachyandesites of the bvr and the crust derived potassic rocks of konglongxiang most evident in their sr ba and ni concentrations nb ta rb sr th nb zr nb and ba rb trace element ratios and sr and nd isotopic compositions these features coupled with the relatively high cr and ni concentrations and mg suggest that the trachytes are the product of mixing between mantle derived and lower crust derived melts

the mid ocean ridge axial valley as a steady state neck abstract in this paper the mid ocean ridge axial valley is modelled as a steady state lithospheric neck in which lithospheric stretching balances lithospheric accretion conversely the axial high is a steady state lithospheric bulge the lithosphere is modelled as a thin plate with a newtonian rheology it is shown that an axial valley will occur if the rate of viscosity increase away from the ridge axis is faster than the rate at which accretion decreases an axial high will occur if the opposite condition holds this is consistent with the observation that axial valleys occur at low spreading rates and axial highs at high spreading rates by fitting our model to profiles across the mid atlantic ridge and the east pacific rise and assuming the lithospheric thickness at the ridge axis to be 5 km we find accretion widths of 6 8 km we find the width over which there is a significant increase in lithospheric viscosity to be also 6 8 km