tectonothermal evolution of the mayuan assemblage in the cathaysia block implications for neoproterozoic collision related assembly of the south china craton the mayuan assemblage in the cathaysia block southeast china consists of felsic paragneiss pelitic schist greenschist amphibolite marble calcsilicate and quartzite that underwent three episodes of deformation d1 d3 and four episodes of metamorphism m1 m4 in the early neoproterozoic the m1 assemblage consists of mineral inclusions defining an early foliation s1 within porphyroblasts represented by chlorite muscovite biotite plagioclase quartz in pelitic schist and actinolite chlorite epidote albite quartz enclosed in amphibolite m2 coincides with the development of the regional schistosity s2 and represents the formation of the porphyroblasts and growth of matrix minerals resulting in development of prograde metamorphic zones chlorite biotite garnet staurolite and kyanite zones m3 is simultaneous with the third phase of deformation d3 and produced sillimanite bearing mineral assemblages in pelitic schist and hornblende bearing assemblages in amphibo lite the last metamorphic episode m4 gave rise to the retrogressive assemblage chlorite muscovite in pelitic rock and actinolite chlorite epidote in amphibolite the sequence of mineral assemblages and history of metamorphic reactions built from the petrogenetic grid of pelites suggest a near isothermal decompression clockwise p t path for the mayuan pelitic schists using the tweequ software program the garnet biotite thermometer and garnet muscovite biotite plagioclase barometer yield p t conditions for m1 of 5 5 to 6 0 kb and 450 to 500 c and conditions for the garnet staurolite and kyanite zones of m2 of 6 0 to 7 0 kb and 550 to 600 c 6 0 to 7 5 kb and 600 c and 11 0 to 11 5 kb and 600 c the p t conditions of m3 were estimated at 570 to 625 c and 4 0 to 4 5 kb using the muscovite biotite thermometer and hornblende plagioclase geothermo barometer the garnet chlorite thermometer yields temperatures of 300 to 400 c for m4 but the pressures of m4 cannot be quantitatively estimated because of the lack of a suitable geobarometer these p t estimates also define an near isothermal decompression clockwise p t path which is involved in initial crustal thickening followed by rapid exhumation and final cooling and is related to amalgamation of the cathaysia and yangtze blocks to form the south china craton

seismic evidence for sharp lithosphere asthenosphere boundaries of oceanic plates the mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics and thus the understanding of the processes at the lithosphere asthenosphere boundary lab is fundamental to understand how our planet works it is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the lab data from borehole broadband ocean bottom seismometers show that the lab beneath the pacific and philippine sea plates is sharp and age dependent the observed large shear wave velocity reduction at the lab requires a partially molten asthenosphere consisting of horizontal melt rich layers embedded in meltless mantle which accounts for the large viscosity contrast at the lab that facilitates horizontal plate motions

two dimensional three dimensional waveform modeling of subducting slab and transition zone beneath northeast asia subduction plays a fundamental role in dynamics of the mantle convection and in material circulations of the earth s interior slabs have been imaged to subduct near horizontally in the transition zone tz beneath the northeast asia in seismic tomography triplication waveform modeling is an effective tool to study the detailed seismic structure in the tz however tz triplication modeling has traditionally relied on 1 d models in this study we use the spectral element method to explore influences of 2 d 3 d slab structure on tz triplication waveforms and to model for the first time the slab and tz structures beneath the northeast asia synthetic tests suggest that for a subduction zone earthquake slab structure can have important influences on tz triplication waveforms and that even in a narrow azimuth range the effects from 2 d 3 d slab structure on the wave propagation can lead to erroneous conclusions with 1 d modeling our data are high quality triplicated sh waveforms at distances of 10 32 from a deep event below 410 km discontinuity in the pacific subducting slab our 2 d 3 d waveform modeling results suggest that a simple model of the subducting slab 5 high velocity anomaly and 100 km thick down to 560 km but normal below 560 km can match most of the observed waveforms remarkably well the bottom of the tz of the sampling region north of the yellow sea contains a patch of very slow anomaly the results indicate that subhorizontal slab above the 660 discontinuity is not everywhere beneath northeast asia and the subducting slab is not everywhere continuous to the bottom of the tz compared with the traditional 1 d modeling our new 2 d 3 d approach provides better fits to the data and allows us to constrain the slab geometry and to separate tz structure from slab structure

new insights on 3 d plates interaction near taiwan from tomography and tectonic implications abstract recent tomographic results are used to trace the south china sea and philippine sea subducting slabs south and northeast of taiwan along the manila and ryukyu trenches respectively in particular the 3 d plates interaction beneath taiwan is discussed based on a close up view of the tomographic sections together with earthquake hypocenters distribution our study indicates that 1 the east dipping south china sea slab can be followed to the north up to the latitude of hualien 2 the eurasian plate subducts beneath most part of the taiwan island down to the 670 km depth discontinuity 3 the north dipping philippine sea slab can be followed slightly west of the longitude of hualien both plates thus interact beneath northern taiwan where the arc continent collision is paroxysmal 4 slab detachment is envisaged at the northern edge of the subducted eurasian plate beneath the coastal range of taiwan which may facilitate the northwestward motion of the philippine sea plate with respect to eurasia slabs geometries obtained from tomographic sections allow us to reconstruct the late neogene plate kinematics and dynamics in this region our main conclusions are 1 the size of the original south china sea was about twice its present size 2 the subducted part of the west philippine basin i e the largest oceanic basin of the philippine sea plate extends only 400 km north of the ryukyu trench 3 slab detachment might have occurred 3 5 my ago beneath the central and northern ryukyu arc along a weak zone that is aligned with the gagua ridge an ancient plate boundary 4 the ryukyu trench has propagated westward from 126 e of longitude southeast of miyako island to the locus of the present arc continent collision along a major lithospheric tear that cut through the continent ocean boundary first and then through the continental lithosphere as a consequence the southern ryukyu margin developed progressively from east to west as a subduction zone during the last 8 my it has been elaborated onto a passive margin which corresponded to the edge of the eurasian continental shelf bordering the south china sea prior to pliocene 5 the southern okinawa trough rifted during the last 2 1 ma following the propagation of the subduction zone 6 subduction of the eurasian margin probably stopped recently beneath northern taiwan and a flip of subduction from west to east is being initiated near hualien

mid miocene propagation of the yellowstone mantle plume head beneath the columbia river basalt source region columbia river flood basalt volcanism is the product of mid miocene distortion of the yellowstone mantle plume head against the precambrian margin of north america the overall migration pattern of columbia river basalt vents reflects northward propagation of the plume head beneath the west southwest migrating north american plate thermal uplift mimics the vent migration over the 1 5 m y of grande ronde basalt extrusion the projected crest of the plume head migrated 105 km 7 cm yr from the chief joseph dike swarm east oortheast into the clearwater embayment where uplift occurred over the same period at a rate of 0 67 mm yr chemical variations reflect progressive melting of the plume and mantle lithosphere followed by the mixing of these components with an enriched source from the overriding craton rapid thickening of the plume head against the cratonic margin led to decompressional melting and magma rise beneath a thin oceanic lithosphere of accreted terranes

global adjoint tomography first generation model we present the first generation global tomographic model constructed based on adjoint tomography an iterative full waveform inversion technique synthetic seismograms were calculated using gpu accelerated spectral element simulations of global seismic wave propagation accommodating effects due to 3d anelastic crust mantle structure topography bathymetry the ocean load ellipticity rotation and self gravitation frechet derivatives were calculated in 3d anelastic models based on an adjoint state method the simulations were performed on the cray xk7 named titan a computer with 18 688 gpu accelerators housed at oak ridge national laboratory the transversely isotropic global model is the result of 15 tomographic iterations which systematically reduced differences between observed and simulated three component seismograms our starting model combined 3d mantle model s362ani kustowski et al 2008 with 3d crustal model crust2 0 bassin et al 2000 we simultaneously inverted for structure in the crust and mantle thereby eliminating the need for widely used crustal corrections we used data from 253 earthquakes in the magnitude range 5 8 mw 7 0 for the first 12 iterations we combined 30 s body wave data with 60 s surface wave data the shortest period of the surface waves was gradually decreased and in the last three iterations we combined 17 s body waves with 45 s surface waves we started using 180 min long seismograms after the 12th iteration and assimilated minor and major arc body and surface waves the 15th iteration model features enhancements of well known slabs an enhanced image of the samoa tahiti plume as well as various other plumes and hotspots such as caroline galapagos yellowstone and erebus furthermore we see clear improvements in slab resolution along the hellenic and japan arcs as well as subduction along the east of scotia plate which does not exist in the starting model point spread function tests demonstrate that we are approaching the resolution of continental scale studies in some areas for example underneath yellowstone this is a consequence of our multi scale smoothing strategy in which we define our smoothing operator as a function of the approximate hessian kernel thereby smoothing gradients less wherever we have good ray coverage such as underneath north america

distinct slab interfaces imaged within the mantle transition zone oceanic lithosphere descends into earth s mantle at subduction zones and drives material exchange between earth s surface and its deep interior the subduction process creates chemical and thermal heterogeneities in the mantle with the strongest gradients located at the interfaces between subducted slabs and the surrounding mantle seismic imaging of slab interfaces is key to understanding slab compositional layering deep water cycling and melting yet the existence of slab interfaces below 200 km remains unconfirmed here we observe two sharp and slightly dipping seismic discontinuities within the mantle transition zone beneath the western pacific subduction zone that coincide spatially with the upper and lower bounds of the high velocity slab based on a multi frequency receiver function waveform modelling we found the upper discontinuity to be consistent with the mohorovicic discontinuity of the subducted oceanic lithosphere in the mantle transition zone the lower discontinuity could be caused by partial melting of sub slab asthenosphere under hydrous conditions in the seaward portion of the slab our observations show distinct slab mantle boundaries at depths between 410 and 660 km deeper than previously observed suggesting a compositionally layered slab and high water contents beneath the slab two seismic discontinuities in the mantle transition zone beneath the western pacific represent subducted slab interfaces that could be the slab moho and partially molten sub slab asthenosphere according to an analysis of seismic data

why cold slabs stagnate in the transition zone oceanic lithosphere sinks stagnates and is deflected sub horizontally beneath western pacific island arcs requiring buoyancy in the slab that is inconsistent with a thermal origin the transformation of pyroxene to majoritic garnet occurs by extremely slow diffusion and pyroxene is therefore unlikely to transform at equilibrium pressures and temperatures in the cold interior of slabs we present high resolution numerical simulations showing that when slow diffusion inhibits the dissolution of pyroxene into garnet the slab becomes buoyant relative to the ambient mantle and stagnates whereas when the phase transformations occur in equilibrium there is no effect on the slab we test the model by comparing slab temperature and geometry and find that sub horizontal slabs are more likely colder than average consistent with our numerical simulations

adjoint tomography of the southern california crust using an inversion strategy based on adjoint methods we developed a three dimensional seismological model of the southern california crust the resulting model involved 16 tomographic iterations which required 6800 wavefield simulations and a total of 0 8 million central processing unit hours the new crustal model reveals strong heterogeneity including local changes of 30 with respect to the initial three dimensional model provided by the southern california earthquake center the model illuminates shallow features such as sedimentary basins and compositional contrasts across faults it also reveals crustal features at depth that aid in the tectonic reconstruction of southern california such as subduction captured oceanic crustal fragments the new model enables more realistic and accurate assessments of seismic hazard

long term sea level fluctuations driven by ocean basin dynamics earth9s long term sea level history is characterized by widespread continental flooding in the cretaceous period 145 to 65 million years ago followed by gradual regression of inland seas however published estimates of the late cretaceous sea level high differ by half an order of magnitude from 40 to 250 meters above the present level the low estimate is based on the stratigraphy of the new jersey margin by assimilating marine geophysical data into reconstructions of ancient ocean basins we model a late cretaceous sea level that is 170 85 to 270 meters higher than it is today we use a mantle convection model to suggest that new jersey subsided by 105 to 180 meters in the past 70 million years because of north america9s westward passage over the subducted farallon plate this mechanism reconciles new jersey margin based sea level estimates with ocean basin reconstructions

global shear speed structure of the upper mantle and transition zone s u m m a r y the rapid expansion of broad band seismic networks over the last decade has paved the way for a new generation of global tomographic models significantly improved resolution of global upper mantle and crustal structure can now be achieved provided that structural information is extracted effectively from both surface and body waves and that the effects of errors in the data are controlled and minimized here we present a new global vertically polarized shear speed model that yields considerable improvements in resolution compared to previous ones for a variety of features in the upper mantle and crust the model sl2013sv is constrained by an unprecedentedly large set of waveform fits 3 4 of a million broad band seismograms computed in seismogram dependent frequency bands up to a maximum period range of 11 450 s automated multimode inversion of surface and s wave forms was used to extract a set of linear equations with uncorrelated uncertainties from each seismogram the equations described perturbations in elastic structure within approximate sensitivity volumes between sources and receivers going beyond ray theory we calculated the phase of every mode at every frequency and its derivative with respect to sand p velocity perturbations by integration over a sensitivity area in a 3 d reference model the normally small perturbations of the 3 d model required to fit the waveforms were then linearized using these accurate derivatives the equations yielded by the waveform inversion of all the seismograms were simultaneously inverted for a 3 d model of shear and compressional speeds and azimuthal anisotropy within the crust and upper mantle elaborate outlier analysis was used to control the propagation of errors in the data source parameters timing at the stations etc the selection of only the most mutually consistent equations exploited the data redundancy provided by our data set and strongly reduced the effect of the errors increasing the resolution of the imaging our new shear speed model is parametrized on a triangular grid with a 280 km spacing in well sampled continental domains lateral resolution approaches or exceeds that of regionalscale studies the close match of known surface expressions of deep structure with the distribution of anomalies in the model provides a useful benchmark in oceanic regions spreading ridges are very well resolved with narrow anomalies in the shallow mantle closely confined near the ridge axis and those deeper down to 100 120 km showing variability in their width and location with respect to the ridge major subduction zones worldwide are well captured extending from shallow depths down to the transition zone the large size of our waveform fit data set also provides a strong statistical foundation to re examine the validity field of the jwkb approximation and surface wave ray theory our analysis shows that the approximations are likely to be valid within certain time frequency portions of most seismograms with high signal to noise ratios and these portions can be identified using a set of consistent criteria that we apply in the course of waveform fitting

p and s wave tomography and anisotropy in northwest pacific and east asia constraints on stagnant slab and intraplate volcanism we determined three dimensional p and s wave velocities and p wave azimuthal anisotropic tomography of the northwest pacific subduction zones by inverting 1 225 086 p wave and 335 117 s wave arrival times from 13 413 earthquakes our results show some differences between p and s wave images for the stagnant pacific slab in the mantle transition zone mtz beneath northeast china the stagnant slab looks thicker in the p wave image than that in the s wave image which may reflects the effects of both hydration and lower temperature in the mtz though differences in the resolution of p and s wave tomography may also have some effects the changbai intraplate volcanism is caused by hot and wet upwelling in the big mantle wedge above the stagnant pacific slab our p wave anisotropy tomography shows that the fast velocity direction fvd in the subducting philippine sea plate beneath the ryukyu arc is ne sw trench parallel which is consistent with the spreading direction of the west philippine basin during its initial opening stage suggesting that it may reflect the fossil anisotropy a striking variation of the fvd with depth is revealed in the subducting pacific slab beneath the northeast japan arc which may be caused by slab dehydration that changed elastic properties of the slab with depth the fvd in the mantle wedge beneath the northeast japan and ryukyu arcs is trench normal which reflects subduction induced convection beneath the kuril and izu bonin arcs where oblique subduction occurs the fvd in the mantle wedge is nearly normal to the moving direction of the downgoing pacific plate suggesting that the oblique subduction together with the complex slab morphology have disturbed the mantle flow

the s velocity structure of the east asian mantle from inversion of shear and surface waveforms summary the structure of the mantle beneath east asia down to 800 km depth is investigated using full waveforms of seismic shear and surface waves epicentral distances are limited to less than 40 in contrast with previous waveform inversions we avoid ray theoretical or path integral approaches instead we use 1 exact 3 d waveform sensitivity kernels that correctly reflect off path sensitivity and the existence of fresnel zones we apply 2 an accurate 3 d forward modelling technique based on a coupled mode multiple forward scattering approach allowing us 3 to iterate the inversion procedure through several 3 d models and 4 to evaluate the true misfit between the data and the synthetics for the 3 d model average lateral resolution of the model in regions with good path coverage is 400 km throughout the upper mantle in the depth range from 100 to 250 km the lateral resolution even approaches 200 km since wave front healing is taken into account the amplitudes of velocity perturbations are larger than in other tomographic models moreover the waveform sensitivity kernels provide an intrinsic physical smoothing of the model stabilizing the inversion finally owing to the use of exact sensitivities better resolution can be achieved with a given set of seismograms major features of the model are high velocity subducting slabs along the west pacific subduction zones stagnating at the 660 km discontinuity a strong low velocity zone between 80 and 250 km depth in the west pacific backarc regions a plume like low velocity feature beneath the southern tip of the baikal rift zone extending into the transition zone a low velocity region under the tien shan with connection into the transition zone and thick crust under tibet reaching its maximum thickness of approximately 80 km close to the 35th parallel the lithospheric mantle underneath southern tibet is very fast indicating underthrusting of the indian lithosphere in contrast the upper mantle beneath northern tibet exhibits average to slow values from the moho down to the 660 km discontinuity very high s velocities are again observed beneath the tarim basin the high velocity mantle lithospheres under southern tibet and the tarim join beneath the karakorum range at the western tip of the tibet plateau

seismic velocity variations beneath central mongolia evidence for upper mantle plumes abstract central mongolia is marked by wide spread recent volcanism as well as significant topographic relief even though it is far from any plate tectonic boundaries the cause of the recent magmatism and topography remains uncertain partially because little is known of the underlying mantle seismic structure due to the lack of seismic instrumentation in the region from august 2011 through august 2013 69 broadband seismic stations were deployed in central mongolia teleseismic traveltime residuals were measured using waveform correlation and were inverted to image upper mantle p and s velocity variations significant lateral variations in seismic velocity are imaged in the deep upper mantle 100 to 800 km depth most significant are two continuous slow anomalies from the deep upper mantle to near the surface one slow feature has been imaged previously and may be a zone of deep upwelling bringing warm mantle to beneath the hangay dome resulting in uplift and magmatism including the active khanuy gol and middle gobi volcanoes the second deep low velocity anomaly is seen in the east from 800 to 150 km depth the anomaly ends beneath the gobi desert that is found to have fast shallow mantle indicating a relatively thick lithosphere we interpret the second deep slow anomaly as a mantle upwelling that is deflected by the thick gobi desert lithosphere to surrounding regions such as the hentay mountains to the north the upwellings are a means of feeding warmer than normal asthenospheric mantle over a widely distributed region beneath mongolia resulting in distributed volcanic activity and uplift there is no indication that the upwellings are rooted in the deep lower mantle i e classic plumes we speculate the upwellings may be related to deep subduction of the pacific and indian plates and are thus plumes anchored in the upper mantle

heterogeneous modification and reactivation of a craton margin beneath the korean peninsula from teleseismic travel time tomography abstract margins of old continental lithosphere are likely prone to ongoing modification processes therefore constraining detailed structures beneath the margin can be essential in understanding the evolution of the continental lithosphere the eastern margin of the eurasian plate is a natural laboratory that allows us to study the strong effects from multiple episodes of continental collision and subduction of different oceanic plates since their formation to reveal the detailed evolution of cratons at their margins we describe for the first time the upper mantle structures beneath the southern korean peninsula skp based strictly on teleseismic relative arrival time data from densely deployed local seismic arrays which allows us to constrain the details of the lithospheric structures beneath the archean proterozoic basement we imaged a thick 150 km high velocity anomaly mainly beneath the proterozoic yeongnam massif with large velocity contrasts dlnvp 4 0 and dlnvs 6 0 at its boundaries suggesting the presence of a long lasting cratonic root in the southwestern skp on the other hand low velocity anomalies were found beneath the proterozoic gyeonggi massif gyeongsang arc back arc basin and along the eastern margin of the skp indicating significantly modified regions the possible existence of a remnant cratonic root beneath the skp and contrasting lithospheric structures across the different precambrian massifs suggests the highly heterogeneous modification of cratonic lithosphere at the eastern eurasian plate margin strong velocity reductions which indicate a thermally elevated upper mantle potentially with partial melts correspond to areas of cenozoic basalts high surface heat flow and high topography along the eastern kp margin we interpret this coincidence as a result of recent reactivation of a craton margin which is controlled by intense interaction between the convective upper mantle and heterogeneous continental lithosphere

anisotropic shear wave velocity structure of the earth s mantle a global model 1 we combine a new large data set of surface wave phase anomalies long period waveforms and body wave travel times to construct a three dimensional model of the anisotropic shear wave velocity in the earth s mantle our modeling approach is improved and more comprehensive compared to our earlier studies and involves the development and implementation of a new spherically symmetric reference model simultaneous inversion for velocity and anisotropy as well as discontinuity topographies and implementation of nonlinear crustal corrections for waveforms a comparison of our new three dimensional model s362ani with two other models derived from comparable data sets but using different techniques reveals persistent features 1 strong 200 km thick high velocity anomalies beneath cratons likely representing the continental lithosphere underlain by weaker fast anomalies extending below 250 km which may represent continental roots 2 weak velocity heterogeneity between 250 and 400 km depths 3 fast anomalies extending horizontally up to 2000 3000 km in the mantle transition zone beneath subduction zones 4 lack of strong long wavelength heterogeneity below 650 km suggesting inhibiting character of the upper mantle lower mantle boundary and 5 slow velocity superplumes beneath the pacific and africa the shear wave radial anisotropy is strongest at 120 km depth in particular beneath the central pacific lateral anisotropic variations appreciably improve the fit to data that are predominantly sensitive to the uppermost and lowermost mantle but not to the waveforms that control the transition zone and midmantle depths tradeoffs between lateral variations in velocity and anisotropy are negligible in the uppermost mantle but noticeable at the bottom of the mantle

controls of the structure of subducted slabs numerical simulations of subducting slabs are formulated in which the shape and dip of the slab are determined by the dynamics of the flow rather than imposed a priori the dip of slabs is a function of the time since the initiation of subduction slabs fold develop a kink in dip and thicken on entry into a high viscosity lower mantle comparison of the simulations with seismic observations suggest that the lower mantle is at least 10 30 times more viscous than the upper mantle

slab temperature and thickness from seismic tomography 2 izu bonin japan and kuril subduction zones delay times from teleseismic and local p wave arrivals are used to invert for a high resolution three dimensional velocity model beneath the northwest pacific the model shows high velocity slabs with average velocity anomalies of the order of 3 4 assuming the positive velocity deviations in the subducting lithosphere are to first order due to a temperature anomaly the results of a theoretical slab temperature profile based on the diffusion equation are converted to a synthetic slab velocity model temperature variations between the ambient mantle and the interior of the slab are converted to p wave velocity perturbations using dvp dt 4 8 10 4 km s 1 c 1 a nonlinear optimization scheme compares the tomograms obtained via tomography to the theoretically predicted models in order to determine the optimal values for slab thickness and mantle potential temperature using 1180 100 c as the potential temperature thickness estimates of 88 8 km 85 8 km and 84 8 km are obtained for the izu bonin japan and kuril slabs respectively a correlation exists between slab thickness and age which is strong if mantle temperature variations along the slab strike can be ruled out in the process of estimating slab thickness the predicted slab velocity model is used as a filter to enhance the initial minimum norm tomographic result the initial tomogram is modified to closely resemble the synthetic slab tomogram by using only null space components the use of the null space components guarantees that the enhanced solution will satisfy the original seismic delay times the enhanced slab images show very continuous and narrow slabs compared to the initial tomographic results

receiver function images of the mantle transition zone beneath ne china new constraints on intraplate volcanism deep subduction and their potential link abstract in order to better understand the deep subduction geometry of the pacific plate and genesis of intraplate volcanism in northeast china ne china we computed a total of 45 505 receiver functions from 788 teleseismic events recorded by 255 stations necessarray temporal and permanent stations in ne china we used a common conversion point stacking ccp method to generate a 3d reflectivity volume beneath the study area to position the p to s conversions to the correct depths we employed 3d crustal and mantle models as references to make time to depth conversion the 3d reflectivity volume was generated in an area between 115 135 e and 40 49 n in the depth range of 300 to 800 km we found significant topographic relief on the 660 km discontinuity across the study area in particular in a westward pacific plate subduction section between 40 n and 45 5 n the 660 km discontinuity is depressed by as much as 30 40 km along the western extension of the deep seismicity the depression is elongated along the strike of the deep seismicity and is confined to a 200 300 km region in the e w direction of subduction to the west of this depression the 660 km discontinuity is uplifted by 5 10 km in a rectangular area of 100 km by 200 km centered at about 125 e and 43 n in the north the 660 km discontinuity is moderately depressed 20 km in a broad area that extends further west the high and low regions in the 660 km topographic map correlate respectively with low and high velocity anomalies in the p and s wave tomographic velocity images at the same depth our results suggest that slab stagnation might not be occurring in the southern part of the ne china where the changbaishan volcanic complex is located thus the magmatism is unlikely caused by dehydration of the flat lying pacific slab in the transition zone the low velocity mantle upwelling arising from a gap of stagnant slabs is a likely source that feeds the volcanic complex in ne china

bifurcation of the yellowstone plume driven by subduction induced mantle flow in the northwestern united states two neighbouring volcanic chains the snake river plain and high lava plains decrease in age in opposing directions laboratory experiments show that diverging volcanic chains can form where vigorous mantle circulation beneath a subduction zone causes an upwelling mantle plume to split into two

a three dimensional radially anisotropic model of shear velocity in the whole mantle summary we present a 3 d radially anisotropic s velocity model of the whole mantle saw642an obtained using a large three component surface and body waveform data set and an iterative inversion for structure and source parameters based on non linear asymptotic coupling theory nact the model is parametrized in level 4 spherical splines which have a spacing of 8 the model shows a link between mantle flow and anisotropy in a variety of depth ranges in the uppermost mantle we confirm observations of regions with vsh vsv starting at 80 km under oceanic regions and 200 km under stable continental lithosphere suggesting horizontal flow beneath the lithosphere we also observe a vsv vsh signature at 150 300 km depth beneath major ridge systems with amplitude correlated with spreading rate for fast spreading segments in the transition zone 400 700 km depth regions of subducted slab material are associated with vsv vsh while the ridge signal decreases whilethemid mantlehasloweramplitudeanisotropy vsv in the lowermost 300 km which appears to be a robust conclusion despite an error in our previous paper which has been corrected here the 3 d deviations from this signature are associated with the large scale low velocity superplumes underthecentralpacificandafrica suggestingthatvsh vsv isgeneratedinthepredominant horizontal flow of a mechanical boundary layer with a change in signature related to transition to upwelling at the superplumes

east asia mantle tomography new insight into plate subduction and intraplate volcanism abstract we present a new p wave tomographic model beneath east asia by inverting 1 401 797 arrival time data recorded by the regional seismic networks in east asia and temporary seismic arrays deployed on the tibetan plateau our high resolution tomography shows that the subducted pacific slab is revealed clearly as a high velocity high v zone and it becomes stagnant in the mantle transition zone beneath eastern china prominent high v anomalies atop the 410 km discontinuity are detected beneath the eastern north china craton and northeast china suggesting that lithospheric delamination may have occurred and greatly affected the thermal state intraplate magmatism and surface topography of this region the indian lithosphere is characterized by a high v anomaly and it is subducting nearly horizontally beneath the entire or most parts of western tibet and with a small dipping angle to the southernmost part of eastern tibet the intraplate magmatism in different parts of east asia has different origins the active tengchong volcano is underlain by a prominent low velocity low v anomaly in the shallow mantle which may be caused by the subduction and dehydration of the burma microplate plate or indian plate the hainan volcano is underlain by a plume like low v anomaly that extends down to at least 1000 km depth and seems to be related to the deep subductions of the burma microplate or indian plate in the west and the philippine sea plate in the east prominent low v anomalies are detected beneath the changbai longgang and xianjindao volcanoes in northeast asia which may be related to the rollback of the subducting pacific slab as well as the convective circulation process in the big mantle wedge above the stagnant slab the origin of the wudalianchi volcano in northeast china seems associated with the upwelling of asthenospheric materials caused by the subduction induced lithospheric delamination

shear wave tomography of china using joint inversion of body and surface wave constraints 1 the india eurasia collision and the decratonization of the north china craton have drawn much attention from the scientific community here we provide the first large scale s wave velocity model for china ch11 s based on constraints from both teleseismic surface and body waves we take advantage of the recent deployment of the 140 permanent stations of the chinese digital seismic network and temporary network deployments to resolve both the lithospheric and deeper mantle structure slow velocities are widespread in the crust and upper mantle deeply rooted fast anomalies are located beneath the stable yangtze craton and the western ordos block of the north china craton an upper mantle fast anomaly is observed beneath the eastern block of the north china craton and could represent thermally eroded or delaminated precambrian lithosphere another flat and fast feature appears beneath the tibetan plateau from 50 to 250 km depth this may represent the indian slab stalled in the mantle due to its buoyancy or a lithospheric instability triggered by the india eurasia collision a large fast anomaly apparently stagnant in the transition zone is observed beneath the yangtze craton and may play a role in the stability of this block in contrast on both sides of the south china block active and reactivated areas coincide with oceanic slab material that has already sunk into the lower mantle and that may have enhanced tectonic activity by forcing convection finally the upper mantle beneath tibet seems almost completely surrounded by adjacent high velocity and presumably strong blocks

timing of collision of the north and south china blocks controversy and reconciliation the qinling orogen was formed by the joining of the north and south china blocks but the timing of their integration has been debated for more than a decade the controversies obviously stem from different approaches to reconstruction of the integration history two contrasting lines of evidence yield two different ages for collision of the north and south china blocks middle paleozoic and late triassic the shangdan suture within the qinling was regarded in previous studies as the trace along which the north and south china blocks collided our studies however demonstrate that there are two sutures within the qinling the well documented shangdan suture and the newly discovered mianlue suture we show in this paper that the late proterozoic to early mesozoic evolution of the qinling involved interactions between the north china block the north and south qinling orogens and the south china block the middle paleozoic collision along the shangdan suture as constrained by some evidence accreted only the south qinling orogen to the southern part i e the north qinling of the north china block contemporaneous rifting of the south china block and subsequent drifting separated the south qinling from the south china block during the middle to late paleozoic the separation of the south from the north china blocks is supported by other evidence in particular geomagnetic data evidently it was the late triassic collision of the south china block with the south qinling orogen along the mianlue suture that led to final integration of the north and south china blocks

waveform modeling of the slab beneath japan 1 the tomographic p wave model for the japan subduction zone derived by zhao et al 1994 has two very striking features a slab about 90 km thick with p wave velocities 3 6 higher than the surrounding mantle and a mantle wedge with 6 low velocity anomalies we study three component seismograms from more than 600 hi net stations produced by two earthquakes which occurred in the downgoing pacific plate at depths greater than 400 km we simulate body wave propagation in the three dimensional 3 d p wave model using 2 d finite difference fdm and 3 d spectral element sem methods as measured by cross correlation between synthetics and data the p wave model typically explains about half of the traveltime anomaly and some of the waveform complexity but fails to predict the extended sh wave train in this study we take advantage of the densely distributed hi net stations and use 2 d fdm modeling to simulate the p sv and sh waveforms our 2 d model suggests that a thin elongated low velocity zone exists atop the slab extending down to a depth of 300 km with an s wave velocity reduction of 14 if a thickness of 20 km is assumed further 3 d sem simulations confirm that this model explains a strong secondary arrival which cannot easily be imaged with standard tomographic techniques the low velocity layer could explain the relatively weak coupling associated with most subduction zones at shallow depths 50 km generally involving abundant volcanic activity and silent earthquakes and it may also help to further our understanding of the water related phase transition of ultramafic rocks and the nature of seismicity at intermediate depths 70 300 km

fractal heterogeneities in sonic logs and low frequency scattering attenuation cycles in sedimentary strata exist at different scales and can be described by fractal statistics we use von karman s autocorrelation function to model heterogeneities in sonic logs from a clastic reservoir and propose a nonlinear parameter estimation our method is validated using synthetic signals when applied to real sonic logs it extracts the fractal properties of high spatial frequencies and one dominant cycle between 2 5 and 7 m results demonstrate non gaussian and antipersistent statistics of sedimentary layers we have derived an analytical formula for the scattering attenuation of scalar waves by 3d isotropic fractal heterogeneities using the mean field theory penetration of waves exhibits a high frequency cutoff sensitive to heterogeneity size therefore shear waves can be attenuated more than compressional waves because of their shorter wavelength

an upper mantle s wave velocity model for east asia from rayleigh wave tomography abstract we present a new shear velocity model of the upper mantle beneath the east asia region derived by inverting rayleigh wave group velocity measurements between 10 and 145 s combined with previously published rayleigh wave phase velocity measurements between 150 and 250 s rayleigh wave group velocity dispersion curves along more than 9500 paths were measured and combined to produce 2d dispersion maps for 10 145 s periods the group velocity maps benefit from the inclusion of new data recorded by the china national seismic network and surrounding global stations the increase in available data has resulted in enhanced resolution compared with previously published group velocity maps the horizontal resolution across the region is about 3 for the periods used in this study the new shear wave velocity models indicate varying velocity structure beneath eastern china which yields estimates of a lithosphere asthenosphere boundary depth from around 160 km beneath the yangtze block to approximately 140 km beneath the western part of the north china craton ncc up to depths of 70 100 km beneath the eastern ncc northeast china and the cathaysia block the models reveal the subduction of two opposite facing continental plates under the southern and northern margin of tibet an obvious low velocity anomaly appears in the top 200 km of the upper mantle beneath northern tibet which is inconsistent with the presence of subducted asian or indian mantle lithosphere beneath northern tibet the cenozoic volcanism fields in the mongolian plateau are characterized by an obvious upper mantle negative anomaly but no signature of deep seated plume was observed

seismic structure of the deep kurile subduction zone three dimensional modeling of p wave travel time residuals was performed to investigate the deep slab related velocity structure within the kurile subduction zone travel times of p waves from earthquakes at all depths within the kurile subduction zone to world wide standard seismograph network and canadian stations were measured instead of analyzing the travel time residuals of a single event relative travel time residuals of p waves to common stations have been analyzed and used in this study to isolate near source contributions from the total travel time residuals the effect of uncertainties in long wavelength heterogeneity far from the source in the lower mantle and the effect of uncertainties in the reference earth model have been significantly reduced by analyzing relative travel time residuals the effect of uncertainties in station corrections are eliminated by relative travel time residuals our measured p wave travel time data clearly indicate that high velocity anomalies exist in the lower mantle beneath the kurile subduction zone but they do not appear to be caused by simple slab continuation into the lower mantle the travel time modeling was accomplished using a fully three dimensional finite difference technique for wave propagation within the source region the modeling results show that high velocity anomalies extend several hundred kilometers into the lower mantle but they appear to be very complicated a large number of models consisting of simple slab continuation into the lower mantle were tested and the results show that none ofthese models was able to produce synthetics which match the observed p wave travel time data the slab related velocity anomalies in the upper mantle of the kurile subduction zone are generally consistent with the seismicity the high velocity anomalies in the lower mantle below the kurile subduction zone are broad and geometrically change from the southern to the northern kuriles below the southern kuriles the high velocity anomalies appear to be subhorizontal however beneath the central kuriles there are broad about 600 km wide high velocity anomalies on the continental side of the slab which extend several hundred kilometers into the lower mantle the high velocity anomalies found below the northern kuriles are broad and on both the continental and the ocean side of the slab and extend several hundred kilometers into the lower mantle

high resolution mantle tomography of china and surrounding regions 1 a high resolution p wave tomographic model of the crust and mantle down to 1100 km depth under china and surrounding regions is determined by using about one million arrival times of p pp pp and pcp waves from 19 361 earthquakes recorded by 1012 seismic stations the subducting pacific slab is imaged clearly as a high velocity zone from the oceanic trenches down to about 600 km depth and intermediate depth and deep earthquakes are located within the slab the pacific slab becomes stagnant in the mantle transition zone under east china the western edge of the stagnant slab is roughly coincident with a surface topographic boundary in east china the active changbai and wudalianchi intraplate volcanoes in northeast china are underlain by significant slow anomalies in the upper mantle above the stagnant pacific slab these results suggest that the active intraplate volcanoes in ne china are not hot spots but a kind of back arc volcano associated with the deep subduction of the pacific slab and its stagnancy in the transition zone under the mariana arc however the pacific slab penetrates directly down to the lower mantle the active tengchong volcano in southwest china is related to the eastward subduction of the burma microplate the subducting indian and philippine sea plates are also imaged clearly the indian plate has subducted down to 200 300 km depth under the tibetan plateau with a horizontal moving distance of about 500 km high velocity anomalies are revealed in the upper mantle under the tarim basin ordos and sichuan basin which are three stable blocks in china

high resolution body wave tomography models of the upper mantle beneath eastern china and the adjacent areas 1 we present new 3 d tomographic models of vp vs and vp vsratio anomalies in the upper mantle beneath ec and adjacent areas this data was collected and interpreted with the goal of clarifying geodynamic processes that caused spatially variable event histories throughout eastern china ec during mesozoic to cenozoic time the tomographic images were constructed by inverting body wave travel times recorded at 1300 stations within the upgraded china national seismic network and 9 temporary arrays resolution tests for different depths and the featured velocity anomalies verify that the tomographic images capture the velocity heterogeneities in the upper mantle to depths of 700 km the salient features of vp vs and vp vsratio anomalies can be clearly identified these include strong multiscale heterogeneities occupying the upper mantle beneath ec and differences in the spatial scale of anomalies found beneath northern and southern areas of ec these features demonstrate a degree of spatial variability in the geodynamic evolution of ec we propose two mechanisms to explain these patterns first the western front of the subducted slab may have imparted greater horizontal compressional stress in areas where it impinged further eastward into ec these areas would experience stronger convection and an altered stress regime in the upper mantle creating significant thermal anomalies beneath the south china block scb relative to the eastern north china craton ncc second differing thermal states and viscosities for the eastern ncc and the cathaysia block cab resulted in differing responses to regional deformation the archean hinterland of the eastern ncc specifically has a colder thermal state and higher viscosity and therefore exhibits only small scale heterogeneities due to the effect of shear localization the neoproterozoic cab has a relatively warm thermal state with lower viscosity and thus deformed more continuously

philippine sea and east asian plate tectonics since 52 ma constrained by new subducted slab reconstruction methods we reconstructed philippine sea and east asian plate tectonics since 52 ma from 28 slabs mapped in 3 d from global tomography with a subducted area of 25 of present day global oceanic lithosphere slab constraints include subducted parts of existing pacific indian and philippine sea oceans plus wholly subducted proto south china sea and newly discovered east asian sea mapped slabs were unfolded and restored to the earth surface using three methodologies and input to globally consistent plate reconstructions important constraints include the following 1 the ryukyu slab is 1000 km n s too short to account for 20 philippine sea northward motion from paleolatitudes 2 the marianas pacific subduction zone was at its present location 200 km since 48 10 ma based on a 1000 km deep slab wall 3 the 8000 2500 km east asian sea existed between the pacific and indian oceans at 52 ma based on lower mantle flat slabs 4 the caroline back arc basin moved with the pacific based on the overlapping coeval caroline hot spot track these new constraints allow two classes of philippine sea plate models which we compared to paleomagnetic and geologic data our preferred model involves philippine sea nucleation above the manus plume 0 150 e near the pacific east asian sea plate boundary large philippine sea westward motion and post 40 ma maximum 80 clockwise rotation accompanied late eocene oligocene collision with the caroline pacific plate the philippine sea moved northward post 25 ma over the northern east asian sea forming a northern philippine sea arc that collided with the sw japan ryukyu margin in the miocene 20 14 ma

the 2018 fiji mw 8 2 and 7 9 deep earthquakes one doublet in two slabs abstract the cold fiji tonga subduction zone accounts for 75 of cataloged deep earthquakes but none of the largest ten in the last century on 19 august 2018 and 06 september 2018 a deep earthquake doublet with moment magnitude m w 8 2 and 7 9 struck the fiji area providing a rare opportunity to interrogate the behaviors of great deep earthquakes in cold slabs while the aftershock productivity of the 2018 m w 8 2 event is similar to the 2013 okhotsk m w 8 3 event in a cold slab the inferred compact rupture dimensions of both the m w 8 2 and 7 9 events appear to be similar to the 1994 bolivia earthquake in a warm slab this seems to contradict the traditional view that slab temperature controls deep earthquakes however we find that neither event was confined within the cold tonga slab core the m w 8 2 ruptured mostly in the warmer rim of the tonga slab and the m w 7 9 occurred in a warm relic slab leaning on top of the tonga slab the fiji doublet demonstrates local slab temperature as the critical factor for deep earthquakes and reveals complex interaction of subducted slabs in tonga

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

subduction hinge migration abstract it has been argued that back arc tectonics is controlled principally by the absolute motion of the overriding plate and the migration of the hinge of the downgoing plate thus hinge migration rollback is fundamentally important to understanding the geodynamics of back arc regions but even rates of hinge migration are unknown the rate of hinge migration can be estimated if the rate of back arc extension and the motion of the overriding plate are known rates of rollback in a hotspot reference frame range from near zero in the case of the new hebrides arc to 50 70 mm a at the south sandwich arc and may be as high as 100 mm a at the tonga trench but the hinge is actually advancing along the izu bonin trench as evidenced by the fact that there is no significant active extension behind the izu bonin arc the rate of hinge advance increases from north to south along the izu bonin trench and correlates well with the dip of the subducted slab active spreading in the mariana trough may be attributed to a rate of hinge advance which is lower than the rate of retreat of the philippine plate all of the possible motions of overriding plates and subduction hinges which might give rise to back arc opening are represented among the examples considered thus there is no unique circumstance in which back arc extension occurs finally there appears to be no clear correlation between hinge motion and the age of the subducted lithosphere

variation of transition zone high velocity anomalies and depression of 660 km discontinuity associated with subduction zones from the southern kuriles to izu bonin and ryukyu the seismic structure of the transition zone beneath the northwestern pacific ocean is studied by comparing regional distance seismic waveforms with synthetic seismograms by modeling waveforms we incorporate the travel times of secondary waves that are not included in traveltime data used in most p wave tomography experiments these secondary waves are highly sensitive to the velocity above the 660 km discontinuity and thus add information on the seismic structure of the transition zone independent of that used in many tomography experiments we used just two one dimensional models in the waveform comparison the iasp91 slightly modified in the shallower depths as aiasp is used as a global average structure and model m3 11 is used as representative of transition zone structure near a deep subducting slab model m3 11 is faster than iasp91 from 525 to 660 km depth but also has a 30 km deeper 660 km discontinuity our results show strong variations in the transition zone structure beneath the northwestern pacific data that sample beneath the northwestern japanese subduction zone and the southern kuriles are well fit by model m3 11 the deep structure beneath the bohai sea to the west of korea also appears to show slab related velocity anomalies in the transition zone in contrast the seismic data sampling beneath the yellow sea to the east china sea are better modeled using the global average model iasp91 data that sampled beneath the philippine sea showed evidence for anomalously high velocity in the transition zone but there is little apparent change in the depth of the 660 km discontinuity there the overall results are in agreement with past work indicating fairly broad regions where subducting plate is lying flat or piling up within the transition zone although we find that the region where this occurs and thus the total volume of slab within the upper mantle is considerably less than that seen in past studies the unusual structure beneath the philippine plate may be due to the relatively recent descent of slab beneath this region the slab itself is cold and thus anomalously fast but the deeper mantle may not have cooled enough for the 660 km discontinuity to have deepened

the dynamics of plate tectonics and mantle flow from local to global scales plate tectonics is regulated by driving and resisting forces concentrated at plate boundaries but observationally constrained high resolution models of global mantle flow remain a computational challenge we capitalized on advances in adaptive mesh refinement algorithms on parallel computers to simulate global mantle flow by incorporating plate motions with individual plate margins resolved down to a scale of 1 kilometer back arc extension and slab rollback are emergent consequences of slab descent in the upper mantle cold thermal anomalies within the lower mantle couple into oceanic plates through narrow high viscosity slabs altering the velocity of oceanic plates viscous dissipation within the bending lithosphere at trenches amounts to 5 to 20 of the total dissipation through the entire lithosphere and mantle

modeling the dynamics of subducting slabs cold dense subducting lithosphere provides the primary force driving tectonic plates at earth s surface the force available to drive the plates depends on a balance between the buoyancy forces driving subduction and the mechanical and buoyancy forces resisting subduction because both the buoyancy and rheology of the slab and mantle depend on temperature composition grain size water content and melt fraction unraveling which of these processes exert a first order control on slab dynamics and under what circumstances other processes become first order effects can be challenging laboratory and numerical models of slab dynamics provide a powerful method for testing the combined effects of buoyancy and strength changes that accompany the slab evolution in the upper mantle transition zone and lower mantle recent studies have focused on understanding how rheologic variations newtonian versus non newtonian viscosity or water content geometry 2d versus 3d and plate motions trench roll back or advance influence the evolution of slabs in the upper mantle and how they sink into the lower mantle these models suggest that spatial and temporal variations in slab strength and the history of subduction determine whether slabs sink directly into the lower mantle or are trapped in the transition zone

anisotropic 3d full waveform inversion abstractwe have developed and implemented a robust and practical scheme for anisotropic 3d acoustic full waveform inversion fwi we demonstrate this scheme on a field data set applying it to a 4c ocean bottom survey over the tommeliten alpha field in the north sea this shallow water data set provides good azimuthal coverage to offsets of 7 km with reduced coverage to a maximum offset of about 11 km the reservoir lies at the crest of a high velocity antiformal chalk section overlain by about 3000 m of clastics within which a low velocity gas cloud produces a seismic obscured area we inverted only the hydrophone data and we retained free surface multiples and ghosts within the field data we invert in six narrow frequency bands in the range 3 to 6 5 hz at each iteration we selected only a subset of sources using a different subset at each iteration this strategy is more efficient than inverting all the data every iteration our starting velocity model was obtained using standard psdm model bui

imaging subducted slab structure beneath the sea of okhotsk with teleseismic waveforms the structure of subducted slabs is not well imaged in most global tomographic models with anomalies typically less than 1 synthetic waveforms for such models are not noticeably different from their 1d reference models in contrast we find that observed teleseismic waveforms sampling slabs in the down dip direction display multipathing features indicative of substantial structure such waveform complexity patterns appear distinctly different for various source locations with outer rise events generally displaying relatively simple waveforms here we model these waveform patterns for a portion of the kuril subduction zone the best fitting slab models have relatively fast cores 5 with smooth edges compatible with thermal modeling in shape complete seismograms for these 2d models indicate that multibounce p and s waves are slab sensitive and can be used in refining models

asthenospheric upwelling beneath northeastern margin of ordos block constraints from rayleigh surface wave tomography abstract phase velocities at periods of 20 143 s are inverted from rayleigh surface waves recorded at 201 stations of permanent and temporary seismic network deployed in the northern ordos block we obtained the shear wave velocities in the upper mantle from phase velocities our velocity models clearly reveal a striking low velocity anomaly zone in the upper mantle beneath the northeastern margin of the ordos block the low velocity zone at periods of 50 100 s extends from the north end of the luliangshan mountain to the north border of the western north china craton the minimum shear wave velocity is approximately 4 2 km s and the minimum lithospheric thickness is approximately 80 km in the low velocity anomaly zone our shear wave velocity results indicate that lithospheric thinning asthenospheric upwelling and partial melting may occur in the region during the cenozoic high temperature and thinned lithosphere may be significant factors that cause partial melting in the asthenospheric mantle during the cenozoic asthenospheric upwelling may be associated with the lithospheric stretching caused by the extension stress in the northeastern margin of the ordos block the extension stress may be contemporaneous with the regional extension of various stages which is related to the pacific subduction and the far field effect of the indian eurasian collision in the cenozoic

seismic evidence for significant lateral variations in lithospheric thickness beneath the central and western north china craton abstract we have migrated teleseismic s receiver functions to construct detailed lithospheric structure images that cover the three constituent parts of the north china craton ncc our images show that in contrast to the eastern ncc where significantly thinned lithosphere 60 100 km is widespread the central and western ncc are characterized by the coexistence of both preserved thick and dramatically thinned lithosphere the thick lithosphere 200 km is present beneath the stable ordos plateau and the thinned lithosphere up to 80 km is found in the late cenozoic yinchuan hetao and shaanxi shanxi rift areas with sharp changes occurring over a lateral distance of

destruction mechanism of the north china craton insight from p and s wave mantle tomography abstract the eastern part of the north china craton ncc was reactivated and its lithosphere was destructed during the mesozoic to cenozoic several destruction models have been proposed by geochemical and geologic studies in this work we determined detailed p and s wave velocity and poisson s ratio structures under the ncc which show different structural patterns in different blocks of the ncc the present results provide important new constrains on the ncc evolution mechanism integrating geochemical and geologic observations and our tomographic results we consider that deep subduction of the pacific slab under east asia caused local or regional scale lithospheric delamination that first took place along some special locations such as the trans north china orogen dabie sulu and yanshan orogenic belts in early mesozoic or paleoproterozoic as a result asthenospheric upwelling in the big mantle wedge above the stagnant pacific slab led to widespread magmatism in and around the eastern part of the ncc hence the thermal erosion and or chemical metasomatism might be a main dynamic mechanism for the lithospheric evolution of eastern ncc during the mesozoic to cenozoic we suggest that the tanlu fault zone and trans north china orogen may have acted as two main conduits for the asthenospheric upwelling which have played an important role in the mesozoic cenozoic destruction of the pre existing archean lithospheric mantle

subduction triggered magmatic pulses a new class of plumes article i nfo a variety of atypical plume like structures and focused upwellings that are not rooted in the lower mantle have recently been discussed and seismological imaging has shown ubiquitous small scale convection in the uppermost mantle in regions such as the mediterranean region the western us and around the western pacific we argue that the three dimensional return flow and slab fragmentation associated with complex oceanic subduction trajectories within the upper mantle can generate focused upwellings and that these may play a significant role in regional tectonics the testable surface expressions of this process are the outside arc alkaline volcanism topographic swell and low velocity seismic anomalies associated with partial melt using three dimensional simplified numerical subduction models we show that focused upwellings can be generated both ahead of the slab in the back arc region though five times further inward from the trench than arc volcanism and around the lateral edges of the slab in the order of 100 km away from slab edges vertical mass transport and by inference the associated decompression melting in these regions appears strongly correlated with the interplay between relative trench motion and subduction velocities the upward flux of material from the depths is expected to be most pronounced during the first phase of slab descent into the upper mantle or during slab fragmentation we discuss representative case histories from the pacific and the mediterranean where we find possible evidence for such slab related volcanism

mantle transition zone stagnant slab and intraplate volcanism in northeast asia 3 d p and s wave velocity structures of the mantle down to a depth of 800 km beneath ne asia are investigated using similar to 981 000 high quality arrival time data of local earthquakes and teleseismic events recorded at 2388 stations of permanent and portable seismic networks deployed in ne china japan and south korea our results do not support the existence of a gap or a hole in the stagnant slab under the changbai volcano which was proposed by a previous study of teleseismic tomography in this work we conducted joint inversions of both local earthquake arrival times and teleseismic relative traveltime residuals leading to a robust tomography of the upper mantle and the mantle transition zone mtz beneath ne asia our joint inversion results reveal clearly the subducting pacific slab beneath the japan islands and the japan sea as well as the stagnant slab in the mtz beneath the korean peninsula and ne china a big mantle wedge bmw has formed in the upper mantle and the upper part of the mtz above the stagnant slab localized low velocity anomalies are revealed clearly in the crust and the bmw directly beneath the active changbai and ulleung volcanoes indicating that the intraplate volcanism is caused by hot and wet upwelling in the bmw associated with corner flows in the bmw and deep slab dehydration as well

full 3d tomography for the crustal structure of the los angeles region we apply full 3d tomography f3dt to a regional dataset in the los angeles area in f3dt the starting model as well as the model perturbation is three dimensional and the sensitivity frechet kernels are computed using numerical schemes that account for the full physics of 3d wave propagation f3dt can account for the nonlinearity of structural inverse problem through iteration thus providing the most efficient means for assimilating seismic observations into dynamic ground motion models we have successfully applied a scattering integral si formulation of f3dt to improve a 3d elastic structure model the southern california earthquake center scec community velocity model version 3 0 cvm3 0 with lowest shear velocity clamped at 1 km sec in the los angeles region our data are time and frequency localized measurements of phase delay anomalies relative to synthetics computed from the 3d elastic starting model the frechet kernels for our measurements were computed by convolving the earthquake wave fields generated by point earthquake sources with the receiver green tensors rgts which are the spatial temporal fields produced by three orthogonal unit impulsive point forces acting at the receiver locations we inverted 7364 phase delay measurements of p and s body waves by using the lsqr method the revised 3d model laf3d provides substantially better fit to the observed waveform data than the 3d starting model to our knowledge this study is the first successful application of f3dt using real data in structural seismology

stagnant slab a review a stagnant slab is a subducted slab of oceanic lithosphere subhorizontally deflected above across or below the 660 km discontinuity this phenomenon has now been widely recognized beneath subduction zones around the circum pacific and in the mediterranean collaboration of seismic and electromagnetic observations mineral physics measurements and geodynamic modeling has begun to provide a consistent picture of stagnant slab

non linear 3 d born shear waveform tomography in southeast asia s u m m a r ysoutheast se asia is a tectonically complex region surrounded by many active source re gions thus an ideal test bed for developments in seismic tomography much recent develop ment in tomography has been based on 3d sensitivity kernels based on the rst order bornapproximation but there are potential problems with this approach when applied to waveformdata in this study we develop a radially anisotropic model of se asia using long period multi mode waveforms we use a theoretical cascade approach starting with a large scale eurasianmodel developed using 2d nact sensitivity kernels and then using a modi ed born approxi mation nborn shown to be more accurate at modeling waveforms in order to invert a subsetof the data for structure in a subregion longitude 75

changbaishan volcanism in northeast china linked to subduction induced mantle upwelling the changbaishan volcanic complex in china cannot be easily explained as the consequence of a mantle plume seismic images from the region identify buoyant mantle material that may have been entrained and dragged downwards by the subducting pacific plate but is now escaping upwards through a gap in the plate and producing the intraplate volcanism

what drove late mesozoic extension of the northern china mongolia tract the northern china mongolia tract exhibited a tectonic transition from contractional to extensional deformation in late mesozoic time late middle to early late jurassic crustal shortening is widely thought to have resulted from collision of an amalgamated north china mongolia block and the siberian plate but widespread late late jurassic early cretaceous extension has not been satisfactorily explained by existing models some prominent features of the extensional tectonics of the northern china mongolia tract are 1 late jurassic voluminous volcanism prior to early cretaceous large magnitude rapid extension 2 overlapping in time of contractional deformation in the yinshan yanshan belt with development of extensionrelated basins in the interior of the northern china mongolia tract and 3 widespread occurrence of alkali granitic plutonism extensional basins and metamorphic core complexes in the early cretaceous a new explanation is advanced in this study for this sequence of events the collision of amalgamated north china mongolia with siberia led to crustal overthickening of the northern china mongolia tract and formation of a high standing plateau subsequent breakoff at depth of the north dipping mongol okhotsk oceanic slab is suggested as the main trigger for late mesozoic lithospheric extension of that tract slab breakoff resulted in mantle lithospheric stretching of the adjacent northern china mongolia tract with subsequent ascent of hot asthenosphere and magmatic underplating at the base of the crust collectively these phenomena triggered gravitational collapse of the previously thickened crust leading to late late jurassic early cretaceous crustal extension and importantly coeval contraction along the southern margin of the plateau in the yinshan yanshan belt the proposed model provides a framework for interpreting the spatial and temporal relationships of distinct processes and reconciling some seemingly contradictory phenomena such as the synchronous extension of northerly terranes during major contraction in the neighboring yanshan yinshan belt d 2003 elsevier b v all rights reserved

crust and uppermost mantle beneath the north china craton northeastern china and the sea of japan from ambient noise tomography 1 a 3 d shear velocity model of the crust and uppermost mantle to a depth of 100 km is presented beneath the north china craton ncc northeastern china the korean peninsula and the sea of japan ambient noise rayleigh wave tomography is applied to data from more than 300 broadband seismic stations from chinese provincial networks cearray the japanese f net and the iris global seismic network continuous data from 2007 to 2009 are used to produce group and phase velocity maps from 8 s to 45 s periods the model is motivated to constrain the distributed intraplate volcanism crustal extension cratonic rejuvenation and lithospheric thinning that are hypothesized for the study region numerous robust features are observed that impose new constraints on the geometry of these processes but discussion concentrates only on four 1 the north south gravity lineament follows the 40 km contour in crustal thickness and crustal thickness is anticorrelated with water depth beneath the sea of japan consistent with crustal isostasy for a crust with laterally variable composition 2 the lithosphere is thin 70 km beneath the songliao bohai graben but seismically fast 3 even thinner more attenuated lithosphere bounds three sides of the eastern ncc in a horseshoe shape identifying a region of particularly intense tectonothermal modification where lithospheric rejuvenation may have reached nearly to the base of the crust 4 low velocity anomalies reach upward in a y shape in the mantle beneath the eastern and western borders of the sea of japan extending well into continental east asia in the west and are separated by a 60 km thick lithosphere beneath the central sea of japan this anomaly may reflect relatively shallow slab dehydration in the east and in the west may reflect deeper dehydration and convective circulation in the mantle wedge overlying the stagnant slab

seismic image and origin of the changbai intraplate volcano in east asia role of big mantle wedge above the stagnant pacific slab abstract we determined p wave tomography of the crust and upper mantle under the changbai volcanic area in northeast asia by using 289 318 arrival time data from 4802 local earthquakes and 9599 teleseismic events a prominent low velocity anomaly is clearly imaged in the crust and upper mantle down to about 410 km depth beneath the changbai volcano a broad high velocity anomaly is revealed in the mantle transition zone and deep earthquakes occurred actively there suggesting that the subducting pacific slab is stagnating in the mantle transition zone under east asia our results suggest that the formation of the changbai and other intraplate volcanoes in northeast asia is related to the upwelling of hot and wet asthenospheric materials in the big mantle wedge above the stagnant pacific slab hence the active intraplate volcanoes in northeast asia are not hotspots related to deep mantle plumes but are caused by plate tectonic processes in the upper mantle

upper plate controls on deep subduction trench migrations and deformations at convergent margins abstract thus far relatively simplistic models of free subduction in which the trench and plate motions are emergent features completely driven by the negative buoyancy of the slab have investigated the dynamics of a single isolated subducting plate here we extend such models to incorporate an overriding plate and present the results of how such an overriding plate feedbacks into the dynamics of free subduction in this study we address three fundamental aspects of these dynamics 1 how does the presence of an overriding plate change the force balance at the convergent margins 2 how are the forces from deep subduction propagated to the surface and 3 what controls the stress regime in a system of coupled upper and subducting plates and how is it expressed in the deformations and plate motions in general we find that the evolution of subduction zones is strongly controlled by both the interactions between the slab and the upper lower mantle discontinuity as well as the strength of the upper plate when either the subducting or upper plates are unable to move subduction motions are steady state and partitioned entirely into either slab rollback or plate advance respectively when conditions favour a quasi stationary trench the subducted lithosphere can form into a pile with multiple recumbent folds of slab material atop the lower mantle alternating between forwards and backwards draping slab the corresponding horizontal trench motions at the surface are frontward and rearward respectively resulting in either a compressive or extensional regime in the back arc time dependent forcing arising from the slab piling behaviour can have a feedback with upper plate and produce strongly non steady state intermittent phases of upper plate deformation as those commonly observed on earth two types of discontinuous back arc strain evolution are identified 1 periodic when recurrent phases of strain over finite durations are accommodated by viscous stretching thickening of the plate and 2 episodic when upper plate deformation localizes plastic strain and allows for punctuated episodes these phases can include extension quiescence and compression giving rise to a large variety of possible tectonic evolutions the models presented here provide insight into the dynamics behind the non steady state evolution of subduction which can help unravel seemingly erratic motions of major convergent margins and back arc deformations around the pacific and indian oceans during the cenozoic

distinct lateral variation of lithospheric thickness in the northeastern north china craton abstract a detailed knowledge of the thickness of the lithosphere in the northeastern north china craton ncc is important for understanding the significant tectonic reactivation of the craton in the mesozoic and cenozoic time we achieve this goal by applying the newly proposed wave equation based migration technique to the s receiver functions recently collected in the region distinct negative signals are identified below the moho in all the s receiver function migrated images and stacks which we interpret as representing the s to p conversions from the lithosphere asthenosphere boundary lab the imaged lab is as shallow as 60 70 km in the southeast basin and coastal areas and deepens to no more than 140 km in the northwest mountain ranges and continental interior these observations indicate widespread lithospheric thinning in the study region in comparison with the 180 km lithospheric thicknesses typical of most cratonic regions the revealed topography of the lab generally agrees with the lateral variation in upper mantle seismic anisotropy previously measured through sks splitting analysis in particular a sharp lab step of 40 km is detected at the triple junction of the basin and mountains at almost the same place where an abrupt change from nw se to ne sw in fast polarization direction of shear waves was found these findings suggest a close correlation between the seismic anisotropy and hence deformation of the upper mantle the lithospheric thickness and the surface tectonics of the northeastern ncc while the thinned lithosphere and the nw se fast shear wave polarizations in the east areas probably are related to the dominant nw se tectonic extension in the late mesozoic cenozoic time the thicker lithosphere and the ne sw fast polarization direction in the west mountain ranges may reflect earlier contractional deformations of the region synthetic tests indicate that the lab beneath the northeastern ncc is a well defined zone 10 20 km thick combined with seismic tomography results and geochemical and petrological data this suggests that complex modification of the lithosphere probably accompanied significant lithospheric thinning during the tectonic reactivation of the old craton

dynamics of the ryukyu izu bonin marianas double subduction system trench motions represent the surface expression of the interaction between subducting plates and the underlying mantle but the inherent dynamics are not fully understood one interesting case is the migration of the izu bonin marianas trench ibm that accommodates the subduction of the pacific beneath the philippine sea plate psp which is in turn subducting beneath the eurasian plate along the ryukyu trench the history of the ibm trench is dominated by fast episodic retreat from 40 to 15 ma however around 10 5 ma the ibm trench reversed its motion from retreat to advance the switch in trench motion occurred soon after the breakoff of the psp slab along the ryukyu trench and the onset of new subduction and represents a fundamental change in the dynamics of the western pacific subduction zones here following the modelling study of ci kova and bina 2015 which suggested a link between ibm trench advance and ryukyu subduction we run 2 d numerical experiments to test the influence of a newly formed ryukyu slab on the established ibm subduction zone we run two dimensional numerical experiments to test the influence of this newly formed ryukyu slab on the ibm subduction zone the results from our geodynamic model compare favourably with the reconstructed trend indicating that the switch in trench motion along the ibm trench may indeed be related to the onset of a new subduction zone along the ryukyu trench our analysis substantiates the idea that advancing trench motions in the western pacific are due to the establishment of a double subduction system further analysis of such dynamics provides insights for the mechanisms controlling subducting plate and trench motions and mantle force transmission

the global attenuation structure of the upper mantle 1 a large data set of fundamental mode rayleigh wave amplitudes is analyzed to derive a new global three dimensional model of shear wave attenuation in the upper mantle the amplitude observations span a range of periods between 50 and 250 s and are derived from earthquakes with mw 6 0 that occurred between 1993 and 2005 four separate factors may influence an amplitude anomaly intrinsic attenuation along the raypath elastic focusing effects along the raypath uncertainties in the strength of excitation and uncertainties in the response at the station in an earlier paper dalton and ekstrom 2006a dependence of the retrieved attenuation structure on these terms was shown to be significant and an approach was developed to invert the amplitudes simultaneously for each term the new three dimensional attenuation model qrfsi12 which is the subject of this paper is derived using this method the model contains large lateral variations in upper mantle attenuation 60 to 100 and exhibits strong agreement with surface tectonic features at depths shallower than 200 km at greater depth qrfsi12 is dominated by high attenuation in the southeastern pacific and eastern africa and low attenuation along many subduction zones in the western pacific resolution tests confirm that the change in pattern of attenuation above and below 200 km depth can be determined with confidence using the fundamental mode data set the new model is highly correlated with global models of shear wave velocity particularly in the uppermost mantle suggesting that the same factors may control both seismic attenuation and velocity in this depth range however forcing the lateral perturbations in attenuation to match those found in global velocity models decreases the data variance reduction which suggests that subtle differences between patterns of attenuation and velocity are robust

shear velocity structure and mineralogy of the transition zone beneath the east pacific rise abstract models of seismic velocity as a function of depth through the upper mantle provide some of the strongest constraints on the mineralogy and composition of the mantle although receiver function studies have provided new information on the depths of upper mantle discontinuities they do not provide as much information on seismic gradients between discontinuities and absolute velocities within the upper mantle the waveforms and travel times of upper mantle turning waves provide the strongest constraints on vertical variations in upper mantle velocity although in the past they suffered from the lack of dense profiles of data sampling a single part of the upper mantle that would minimize effects of 3d variations in velocity here we model three dense profiles of triplicated upper mantle broadband s and ss waves recorded by usarray canadian and nars baja stations located in western north america earthquakes along the east pacific rise were recorded along profiles within 5 back azimuth windows and with stations at a maximum of 0 5 separation the distance range covered is from 35 to 60 and thus the waves sample the mantle from the shallow upper mantle to depths near 1000 km the data were inverted using a conjugate gradient algorithm that utilizes the reflectivity synthetic technique the results show a much smaller gradient within the transition zone than the prem model with larger jumps in velocity across the 410 km and 660 km depth discontinuities these results are consistent with velocities predicted for a pyrolite composition mantle transition zone compositional models with lower olivine content such as piclogite are not consistent with our seismic model

lithospheric structure variations between the eastern and central north china craton from s and p receiver function migration abstract i imaged the lithospheric structure beneath the bohai bay basin bbb in the eastern north china craton ncc and the taihang mountains tm and areas further to the west in the central ncc by applying the wave equation migration technique to both the s and p receiver functions rfs collected in this region the data images and synthetic modeling results indicate that s rf imaging is more appropriate than p rf imaging for mapping lithospheric discontinuities especially the lithosphere asthenosphere boundary lab in various types of regions even directly applicable to basin areas that are covered by thick sediments the constructed s rf images show that the lithospheric thickness of the study region is highly variable the imaged lab is on average 80 km beneath the bbb and displays obvious undulations from 120 km under the tm to 40 km under the mountain areas in the central ncc consistent with previous p rf imaging results the observed structural differences between the eastern and central ncc are likely associated with different lithospheric tectonics in these two regions while widespread lithosphere reactivation and extension in the eastern ncc during the late mesozoic time was plausibly responsible for both the thinned crust and lithosphere in the bbb lithospheric thinning in the central ncc may be equally heterogeneous but less significant and probably occurred mainly in cenozoic such a process may have given rise to the highly variable present day lithospheric thickness in this region however it probably did not noticeably affect the overlying crust that was reported to have thickened in response to the early magmatic underplating in the mesozoic time the different tectonic processes in the eastern and central ncc may have induced the formation of the nsgl which likely serves as a major tectonic boundary separating the two regions of contrasting lithospheric structures at present

the shear wave velocity structure in the upper mantle beneath eurasia summary we develop an approach that allows us to invert for the mantle velocity structure within a finely parametrized region as a perturbation with respect to a low resolution global tomographic model we implement this technique to investigate the upper mantle structure beneath eurasia and present a new model of shear wave velocity parametrized laterally using spherical splines with 2 9 spacing in eurasia and 11 5 spacing elsewhere the model is obtained from a combined data set of surface wave phase velocities long period waveforms and body wave traveltimes we identify many features as narrow as few hundred kilometres in diameter such as subducting slabs in eastern eurasia and slow velocity anomalies beneath tectonically active regions in contrast to regional studies in which these features have been identified our model encompasses the structure of the entire eurasian continent furthermore including mantleand body wave waveforms helped us constrain structures at depths larger than 250 km which are poorly resolved in earlier models we find that up to 9 per cent faster than average anomalies within the uppermost 200 km of the mantle beneath cratons and some orogenic regions are separated by a sharp gradient zone from deeper 1t o 2 per cent anomalies we speculate that this gradient zone may represent a boundary separating the lithosphere from the continental root which might be compositionally distinct from the overlying lithosphere and remain stable either due to its compositional buoyancy or due to higher viscosity compared with the suboceanic mantle our regional model of anisotropy is not significantly different from the global one

insight into the origin of the tengchong intraplate volcano and seismotectonics in southwest china from local and teleseismic data received 18 june 2008 revised 22 february 2009 accepted 10 march 2009 published 13 may 2009 1 a high resolution tomographic image of the crust and upper mantle under yunnan province in southwest china was determined by using a large number of teleseismic data measured precisely from digital seismograms as well as local earthquake arrival times recorded by the dense yunnan seismic network the grid spacing of the tomographic model is 70 km 78 km in the horizontal directions and 4 70 km in depth our resulting model shows a clear low velocity low v 0 3 with respect to the average one dimensional velocity model column extending from the surface down to about 400 km depth under the active tengchong volcano and some high velocity high v anomalies existing in the mantle transition zone furthermore the low vanomaly extends horizontally toward the northeast at 250 to 400 km depth combining our tomographic results with geochemical and geological investigations we infer that the tengchong volcano is a rift related volcano caused by the subduction and dehydration of the indian plate as well as corner flow in the mantle wedge though the subducted slab is a continental plate our results show that the upwelling flow under tengchong originates at 400 km depth a prominent low v anomaly along the red river fault zone extends down to the upper mantle 100 km depth reflecting that the red river fault zone may have cut through the crust to the upper mantle the two dayao earthquakes m 6 1 and m 6 2 in 2003 occurred on the margin of high v anomalies and are underlain by a prominent low v anomaly in the lower crust and upper mantle suggesting that the dayao earthquakes may be associated with fluids released from the dehydration of the subducted indian slab these results are significantly improved compared with previous studies and provide new seismic constraints on the dynamic processes of the india asia collision

intensive hydration of the mantle transition zone beneath china caused by ancient slab stagnation the mantle transition zone beneath china is anomalously hydrated geochemical analyses of basalts erupted above the mantle transition zone in northeast china indicate that water may have been added to this zone during the dehydration of subducted slabs on two separate occasions over the past one billion years

finite frequency whole mantle p wave tomography improvement of subducted slab images received 23 july 2013 revised 14 october 2013 accepted 17 october 2013 1 we present a new whole mantle p wave tomographic model gap p4 we used two data groups short period data of more than 10 million picked up onset times and long period data of more than 20 thousand differential travel times measured by waveform cross correlation finite frequency kernels were calculated at the corresponding frequency bands for both long and short period data with respect to an earlier model gap p2 we find important improvements especially in the transition zone and uppermost lower mantle beneath the south china sea and the southern philippine sea owing to broadband ocean bottom seismometers bbobss deployed in the western pacific ocean where station coverage is poor this new model is different from a model in which the full data set is interpreted with classical ray theory bbobs observations should be more useful to sharpen images of subducted slabs than expected from simple raypath coverage arguments citation obayashi m j yoshimitsu g nolet y fukao h shiobara h sugioka h miyamachi and y gao 2013 finite frequency whole mantle p wave tomography improvement of subducted slab images geophys res lett 40 doi 10 1002 2013gl057401

the role of ultramafic rocks in regulating the concentrations of volatile and non volatile components during deep crustal metamorphism the common preservation of h2o and co2 bearing phases in high grade ultramafic rocks indicates that they represent reservoirs of volatile components in the deep crust the upper thermal stabilities of volatile bearing mg silicates and magnesite exceed those of volatile bearing phases in most other crustal lithologies the solubility of many rock forming components e g co2 al2o3 cao sio2 is considerably lower in fluids coexisting with ultramafic rocks than in adjacent quartzo feldspathic units metamorphism of crustal sections that include ultramafic rocks is accompanied by metasomatic reactions that regulate the chemistry of both volatile and non volatile components in deep crustal metamorphic fluids during amphibolite facies metamorphism ultramafic rocks can absorb and store volatile and non volatile components introduced from the country rock under conditions of low activities or fugacities an example is the early fixation of co2 h2o and aluminium by reaction of dunite to form chlorite magnesite and progressively more siliceous mg silicates along an early low xco2 reaction path one dimensional continuum models of fluid transport and reaction indicate that infiltration of fluids derived from quarzo feldspathic country rocks can cause enrichment of aluminium in ultramafic rocks up to the level of wt internal reaction between the metasomatic minerals within ultramafic units release fluids during prograde metamorphism fluids will be released in two pulses one with intermediate xco2 and aluminium activity at the level of aluminosilicate or corundum saturation and a later pulse of pure co2