the philippine sea new survey results reveal the structure and the history of the marginal basins the hydrographic department of japan initiated a long term geological and geophysical survey of the philippine sea in 1983 we here summarize the bathymetry and magnetic anomalies of this area focusing on the structure and the evolutionary process of the marginal basins the shikoku and parece vela basins developed as one backarc system in the later phase of their formation the significant difference between the two basins however is clearest near the extinct spreading centers the steep oki daito escarpment located in the northern west philippine basin was mapped for the first time during this survey the n s trending spreading fabric north of the escarpment is in striking contrast to the nw se to e w lineament in the south our data reveal a more complicated history of the western philippine sea than has been proposed in previous studies

effects of seismic anisotropy on p velocity tomography of the baltic shield the upper mantle structure of swedish part of baltic shield with its isotropic and anisotropic seismic velocity characteristics is investigated using telesesismic body waves i e p waves and shear waves recorded by the swedish national seismological network snsn nonlinear high resolution p and sv and sh wave isotropic tomographic inversions reveal velocity perturbations of 3 down to at least 470 km below the network separate sv and sv models indicate several consistent major features many of which are also consistent with p wave results a direct cell by cell comparison of sh and sv models reveals velocity differences of up to 4 numerical tests show that differences in the two s wave models can only be partially caused by noise and limited resolution and some features are attributed to the effect of large scale anisotropy shear wave splitting and p travel time residual analyses also detect anisotropic mantle structure distinct back azimuth dependence of sks splitting excludes single layer anisotropy models with horizontal symmetry axes for the whole region joint inversion using both the p and s data reveals 3d self consistent anisotropic models with well defined mantle lithospheric domains these domains of differently oriented anisotropy most probably retain fossil fabric since the domains origin supporting the idea of the existence of an early form of plate tectonics during formation of continental cratons already in the archean the possible disturbing effects of anisotropy on seismic tomography studies are investigated and found to be potentially significant p wave arrival times were adjusted based on the estimates of mantle anisotropy and re inverted the general pattern of the velocity perturbation images was similar but changed significantly in some places including the disappearance of a slab like structure identified in the inversion with the original data thus the analysis demonstrates that anisotropy of quite plausible magnitude can have a significant effect on the tomographic images and should not be ignored if as we believe our estimates of anisotropy are reasonably correct then the model based on the adjusted data should give a more robust and correct image of the mantle structure

including anisotropy in 3 d velocity inversion and application to marlborough new zealand summary two types of seismic velocity heterogeneity are often observed in a given region pronounced patterns of anisotropy shown by local earthquake shear wave splitting and seismic velocity structure imaged by local earthquake traveltimes we seek to combine these types of observations by including anisotropy in 3 d velocity inversion the fast polarization directions from local earthquake shear wave splitting are used to define an initial anisotropy model the ray paths are estimated for each of the shear wave splitting observations and the 3 d initial model is determined via an averaging process and a coherence analysis the initial magnitude of anisotropy at a node is dependent on the number of adjacent ray paths and the consistency of the shear wave splitting anisotropy measurements the traveltime inversion is parametrized with an isotropic component and two azimuthal anisotropy parameters for each node the ray paths are defined with approximate 3 d ray tracing and for each point along a ray path the velocity is defined as a function of the ray path azimuth at that point a blended approach is used to damp the perturbations to the magnitude of anisotropy and or perturbations to the fast direction tests with synthetic traveltime data show that the anisotropic velocity inversion method is reliable application to the marlborough region shows variations in anisotropy in the brittle crust ductile lower crust mantle and subducted slab the patterns are consistent with the shear wave splitting observations but show some additional features the maximum anisotropy is approximately 12 per cent oriented northeast between the awatere and wairau faults this high anisotropy may reflect deformation and extensive fracturing in a region of high total shear strain in the region of interaction of the ductile lower crust with the shallow subducting slab moderate anisotropy with east west orientation is imaged this is consistent with ductile deformation roughly parallel to the plate velocity

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

p wave anisotropic tomography of the nankai subduction zone in southwest japan 1 the active subduction of the young philippine sea phs plate and the old pacific plate has resulted in significant seismic heterogeneity and anisotropy in southwest sw japan in this work we determined a detailed 3 d p wave anisotropic tomography of the crust and upper mantle beneath sw japan using 540 000 p wave arrival times from 5 249 local earthquakes recorded by 1095 stations the phs slab is imaged clearly as a high velocity high v anomaly which exhibits considerable lateral variations significant low velocity low v anomalies are revealed above and below the phs slab the low v anomalies above the phs slab may reflect the upwelling flow in the mantle wedge and the phs slab dehydration and they form the source zone of the arc volcanoes in sw japan the low v zones under the phs slab may reflect the upwelling flow in the big mantle wedge above the pacific slab the anisotropy in the crust and upper mantle is complex in kyushu the p wave fast velocity direction fvd is generally trench normal in the mantle wedge under the back arc which is consistent with the corner flow driven by the phs slab subduction the fvd is trench parallel in the subducting phs slab under kyushu we think that the intraslab seismicity is a potential indicator to the slab anisotropy that is the phs slab with seismicity has kept its original fossil anisotropy formed at the mid ocean ridge while the aseismic phs slab has reproduced the anisotropy according to its current deformation

b type olivine fabric in the mantle wedge insights from high resolution non newtonian subduction zone models several hypotheses have been proposed to explain trench parallel shear wave splitting in the mantle wedge of subduction zones these include 3 d flow effects parallel melt filled cracks and b type olivine fabric we predict the distribution of btype and other fabrics with high resolution thermal and stress models of subduction zones a composite viscous rheology is used that incorporates wet diffusion creep wet dislocation creep and a stress limiting approximated peierls creep law rheological parameters for dislocation and diffusion creep are based on deformation experiments the effects of variable viscous coupling between the slab and mantle are explored using kinematic boundary conditions that change along the top of the slab two end member models are observed which depend on the depth of partial viscous coupling between the slab and mantle 1 deep partial coupling which gives rise to cold high stress conditions in the forearc mantle and high temperature low stress conditions in the arc and back arc mantle 2 full viscous coupling at the base of a 40 km conducting lid which is characterized by high temperature and low stress the case with deep partial coupling which produces a better match with attenuation tomography and heat flow shows a large region with suitable conditions for b type fabric in the forearc mantle and a rapid transition toward the back arc to conditions that are more suitable for a c or e type fabrics full viscous coupling gives rise to high temperature low stress conditions unsuitable for b type fabric our modeling predicts that the best candidates for regions with b type fabric are the forearc mantle and a cold 10 15 km layer above the slab d 2005 elsevier b v all rights reserved

three dimensional structure of p wave anisotropy in the presence of small scale convection in the mantle wedge 1 as a possible explanation of hot fingers in the mantle wedge below the tohoku region japan the existence of small scale convection has been proposed in this study we performed numerical calculations around the subduction zone in 3d with composite rheology i e a combination of linear and non linear rheology and found that small scale convection could arise when the rheology determined from laboratory experiments is considered we also calculated 3d structure of expected p wave anisotropy for the case with and without small scale convection based on a theory of lpo development and an assumption that anisotropy is approximately represented as hexagonal symmetry and found that the fast axis of the p wave propagation projected on the horizontal cross sections is nearly in the same direction as that of plate motion in many places it implies that effects of large scale mantle flow associated with subducting slab is still dominant even in the presence of small scale convection in mantle wedge however in vertical cross sections the projected fast axis of the p wave propagation could tilt vertically while that without small scale convection is almost horizontal therefore future seismological studies that determine the fast direction of p wave propagation in 3d would give us critical information on the possible existence of small scale convection in the mantle wedge

petrological constraints on seismic anisotropy the purpose of this paper is to investigate the correlations between anisotropic parameters for different orientations and mineralogies for realistic mineralogical and petrological models of the upper mantle such correlations make it possible to reduce the number of independent parameters required in seismic modelling of the upper mantle the variation with depth and with degree of random orientation is also investigated it is shown that the anisotropic combinations involved in an equivalent transversely isotropic medium azumithally averaged velocities with a vertical symmetry axis are strongly correlated but the combinations involved in azimuthal anisotropy are more weakly correlated comparison with available seismic data does not yet allow discrimination based on anisotropy between different petrological models it is shown that even if a proportion of random orientation is introduced it should be possible to achieve petrological discrimination from seismic data for some orientation configurations if the inverse problem is well posed i e takes account of the correlations between anisotropic parameters these correlations are incorporated in an inversion process to show that it is possible to explain the dataset used to derive the prem by alternative anisotropic models for the uppermost 220 km

configuration of subducting philippine sea plate beneath southwest japan revealed from receiver function analysis based on the multivariate autoregressive model 1 we construct a detailed image of velocity discontinuity beneath southwest japan by using receiver functions we first propose an improved receiver function estimation method based on the statistical multivariate autoregressive model then we apply the new method to the teleseismic waveform data recorded by the high density seismograph network in southwest japan the results show a clear velocity discontinuity at 30 km depth beneath the southern shikoku region this discontinuity corresponding to the boundary between the oceanic crust and the high velocity layer the oceanic moho of the philippine sea plate phs continues down to the north indicating that the aseismic phs extends to the central chugoku region the continental moho is also clearly imaged beneath the chugoku region the depth contour of the phs shows a rather complicated feature with ridges and valleys the most significant ridge is located around longitude 133 e west of which the contours are basically directed north south changing to west east to the east beneath the western part of this ridge hypocenters of microearthquakes are located above this velocity discontinuity while earthquakes mainly occur below the discontinuity within the oceanic mantle beneath the eastern part of the ridge

p wave anisotropic tomography of the crust and upper mantle under hokkaido japan abstract we determined a 3 d anisotropic velocity structure beneath hokkaido in northern japan using a large number of first p wave arrival times the travel time inversion is parameterized with an isotropic component and two anisotropic parameters for each grid node by assuming p wave azimuthal anisotropy with hexagonal symmetry axis distributed horizontally the geometries of the conrad and moho discontinuities and the upper boundary of the subducting pacific plate are taken into account in the model our results show that low velocity zones exist beneath active arc volcanoes in the crust and in the central portion of mantle wedge above the high velocity pacific slab which are similar to the previous tomographic images p wave anisotropy is revealed in the crust mantle wedge and the subducting pacific slab in the upper crust the anisotropy is possibly caused by microcracks and cracks in the lower crust the anisotropic structure is different from that in the upper crust and it may be affected considerably by the plastic flow deformation in the mantle wedge the fast velocity direction fvd is generally trench normal in most of the study area while it becomes trench parallel in the pacific coast areas close to the trench which agrees with the shear wave splitting results beneath the volcanic front the fvd is also trench parallel which may suggest the existence of complex 3 d mantle flow in the mantle wedge the fvd in the subducting pacific slab is nearly north south which likely keeps the original fossil anisotropy of the pacific plate formed at the mid ocean ridge or is affected by the olivine fabric transition from a type to b type fabric due to the changes in water content stress and temperature

subduction of the philippine sea slab in view of p wave anisotropy abstract three dimensional 3d structure of isotropic and anisotropic velocities in the chugoku and shikoku districts southwest japan is retrieved from the first p arrival times from local earthquakes by assuming p wave azimuthal anisotropy with hexagonal symmetry axis the main result of isotropic velocity structure is that the philippine sea phs slab is represented by a high velocity body with the thickness of 20 30 km lying at the around depth of 40 km beneath the shikoku and southern chugoku districts the phs slab subduction appears to be interrupted by a small scale low velocity body under middle chugoku and is not clear under north chugoku these features of 3d isotropic velocity structure are basically similar to those obtained in previous tomographic studies using recent seismic data this result suggests that seismic anisotropy has no significant effect on estimation of isotropic velocity structure from the p arrival time data on the other hand fast propagation directions of p wave azimuthal anisotropy in the crust are basically in close agreement with fast polarization directions estimated from polarization anomaly of shear waves azimuthal anisotropy in the crust of shikoku district is characterized by the fast propagation direction with the e w orientation trend which is nearly parallel to trajectory of maximum principal stress acting on the southwest japan arc and strike directions of zonal geological structure crustal anisotropy in the chugoku district is consistent with the orientation of the geological structure and not with the maximum principal stress direction these results indicate that the crustal anisotropy is attributable not only to alignment of the stress induced cracks but also to lineament of geological structures we could also point out the difference in the upper mantle anisotropy between the shikoku and chugoku districts i e p wave velocity tends to be fast in the n s directions beneath shikoku and southern chugoku and in e w directions under northern chugoku since the n s axis trend of p wave anisotropy is highly correlated with high velocity anomaly of phs slab it is regarded as slab anisotropy the reason why the p wave anisotropy shows the high propagation velocity in the e w direction beneath northern chugoku is discussed on the example of two cases in which the phs slab is absent and present in the upper mantle beneath the region although we could not locate exactly the north leading edge of the phs slab in view of anisotropic velocity structure if the phs slab is defined as a high velocity body with fast propagation axes in the directions of n s system it turns out to exist at least beneath shikoku to south chugoku

seismic anisotropy near source region in subduction zones around japan abstract broad band seismographs have been distributed widely over the japanese islands and have provided us with high quality digital waveform data in recent years we have investigated splitting of scs waves from deep earthquakes in the kuril and izu bonin subduction zones recorded on the sts seismograms we used scs waves from 13 events observed in japan and recognized shear wave splitting which is mainly caused by anisotropy in the lithospheric slab the anisotropy was confirmed around the source region by comparing scs wave splitting from deep earthquakes in different regions and at different depths with s wave splitting from deep events just beneath central japan we have found that the anisotropic regions exist within the subducting slab although they are distributed locally in and around the source possibly as patches with a diameter of 100 km or less discrepancy in the direction of the fast polarized shear waves between two nearby events is observed in the izu bonin subduction zone the splitting of the shallower event 289 km shows its fast polarization direction to be parallel to the fossil motion of the pacific plate but that of the deeper event 361 km immediately beneath is parallel to the current motion these observations suggest that the change in shear wave splitting with depth comes from the modification of preferred orientation of minerals depending on depth we propose in this study that the reorientation of minerals olivine occurs owing to the change in physical conditions associated with the phase transformation of olivine phase to modified spinel phase

garnet and pyroxenes in the mantle a test of the majorite fractionation hypothesis experimental data are reported for the compositions of garnets and pyroxenes that coexist with liquid in the system cao mgo al2o3 sio2 cmas at 80 160 kbar all experiments were run using the uniaxial split sphere apparatus at stony brook our data have permitted a refinement of gasparik s 1990a thermodynamic analysis of garnets and pyroxenes that occur at arid below the solidus our results should provide precise estimates of the temperature pressure conditions at which peridotite and garnet xenoliths equilibrated in the mantle an examination is also made of the composition of garnet that crystallizes as a liquidus phase it is demonstrated to be a function of both composition and pressure pressure has the effect of changing the composition of garnet from pyropic at around 80 kilobars to pyroxene like majorite at pressures in excess of 140 kbar changes that mirror those seen for garnets in the subsolidus failure to consider the effects of pressure on the composition of liquidus garnet can result in an erroneous test of the hypothesis that garnet fractionation occurred in early earth history it is shown that primitive mantle peridotite could have formed as a liquid by 35 majorite garnet fractionation from a cv chondrite like allende at 200 260 kbar these results support the hypothesis that extensive or possibly total melting occurred during the growth of the earth by accretion of planetesimals e g davies 1985 abe and matsui 1985 stevenson 1987 wetherill 1990 and that majorite fractionation could have occurred during this time ohtani and sawamoto 1987 removal of majorite to the lower mantle during the crystallization of a magma ocean is a successful way of explaining differences in mgo sio2 and cao al2o3 between mantle peridotite and chondrite

meso neoproterozoic coated grains and palaeoecology of associated microfossils the deoban limestone lesser himalaya india abstract organically preserved microfossil assemblages occurring in association with the coated grains ooids and pisoids are being reported from the petrographic thin sections of the black chert samples have been collected from the deoban limestone garhwal lesser himalaya like any other proterozoic microfossil assemblage present assemblage also exhibits domination of cyanobacterial community and rare occurrence of bacterial and acritarchean affinity the microfossils occur in three principal palaeoecological modes a clast bound microfossils these are found in detrital grains which also served as a nuclei for ooid growth b epilithic or interstitial microfossils these occur on the outer surface of the coated grains as well as within the cement c euendolithic microfossils these microorganisms actively bored into the coated grains coated grains comprising the microbial structures can be considered as the indicators of shallow water environments their size is at least partly dependant on current strength at larger sizes increasing rates of abrasion may come to equal rates of carbonate precipitation or accretion since filamentous population of specific dimensions dominate the present assemblage it is inferred that tubular filaments ranging in diameter from 4 to 5 m played the most significant role in the genesis of coated grains irrespective of their mode of occurrence as an epilith endolith or as the clast bound microfossils the assemblage adds one more record of proterozoic endoliths which are rarely recorded world wide

shear wave anisotropy of the upper and lower crusts estimated by stripping analysis of ps converted waves abstract the anisotropic structure of the crust is investigated by measuring the shear wave polarization anisotropy of the ps phases converted at the conrad and moho discontinuities the ps converted phases are identified on the p wave receiver functions which are constructed from teleseismic records at the f net and hi net stations in the kinki chugoku shikoku and kyushu districts in southwest japan the ps phase traveling through an anisotropic layer undergoes shear wave splitting which is described by splitting parameters fast polarization direction fpd and split time the upper and lower crustal anisotropies are estimated from the splitting parameters of the conrad and moho ps phases respectively which are determined by a waveform cross correlation method to reliably estimate the lower crustal anisotropy the moho ps phase is corrected for the shear wave splitting effect in the upper crust by a stripping method because the upper crustal anisotropy masks the anisotropic behavior of the ps phase in the lower crust the split times estimated for the upper and lower crusts are less than 0 2 s but their fpd characteristics differ from each other the fpds in the upper crust are predominantly in the e w direction the fpds in the lower crust range from nw se to ne sw in the region with the philippine sea phs slab subduction but from ne sw to se nw in the region without slab subduction thus seismic anisotropy exists in both the lower and upper crusts and slab subduction significantly influences the lower crustal anisotropy the most likely cause for the upper crustal anisotropy is the alignment of vertical cracks induced in the upper crust by the tectonic stress while the lower crustal anisotropy is attributed to the lattice preferred orientation of rock forming minerals along with ductile deformation of the lower crust

the amurian plate motion and current plate kinematics in eastern asia we use global positioning system gps velocity data to model eastern asian plate kinematics out of 15 stations in korea russia china and japan studied here three sites considered to be on the stable interior of the hypothetical amurian plate showed eastward velocities as fast as 9 10 mm yr with respect to the eurasian plate they were stationary relative to each other to within 1 mm yr and these velocity vectors together with those of a few additional sites were used to accurately determine the instantaneous angular velocity euler vector of the amurian plate the predicted movement between the amurian and the north american plates is consistent with slip vectors along the eastern margin of the japan sea and sakhalin which reduces the necessity to postulate the existence of the okhotsk plate the euler vector of the amurian plate predicts left lateral movement along its boundary with the south china block consistent with neotectonic estimates of the displacement at the qinling fault possibly the southern boundary of the amurian plate the amurian plate offers a platform for models of interseismic strain buildup in southwest japan by the philippine sea plate subduction at the nankai trough slip vectors along the baikal rift the boundary between the amurian and the eurasian plates are largely inconsistent with the gps based euler vector suggesting an intrinsic difficulty in using earthquake slip vectors in continental rift zones for such studies

melting of enstatite mgsio3 from 10 to 16 5 gpa and the forsterite mg2sio4 majorite mgsio3 eutectic at 16 5 gpa implications for the origin of the mantle a split sphere multianvil apparatus clissa 2000 has been used to determine the melting curve of enstatite mgsio3 from 10 to 16 5 gpa and the forsterile mgio majoritc mgsio3 euteetic at 16 5 gpa use of a new capsule material rhenium allowed long runs of up to 2 hours at temperatures up to 2380oc with no observed contamination of the sample and with stable temperature control melting temperatures of enstatite are up to 160oc higher than those of kato and kumazawa 1986a two invariant points have been located one for orthoenstatite linoenstatite liquid at 2230oc 11 9 gpa and one for clinoenstatite majoritc liquid at 2370oc 16 4 gpa forsteritc melts incongruently at 2350oc 16 5 gpa to liquid and anhb 0vlglnsiso24 in the system mg2sio4 mgsio3 at 16 5 gpa the forsterite majorite eutectic has been located at 34 wt mg2sio4 a composition similar to that determined by karo and kumazawa 1986a at 20 gpa but much lower in mg2sio4 than several previous estimates based on melting experiments on mantle peridotitc compositions this forstefite poor location for the cuteerie is in apparent conflict with the suggestion that the peridotitic upper mantle is a crystallized cuteeric like liquid formed at about 16 gpa

receiver functions of seismic waves in layered anisotropic media application to the estimate of seismic anisotropy abstract we investigate the effect of seismic anisotropy on p wave receiver functions calculating synthetic seismograms for p wave incidence on multilayered anisotropic structure with hexagonal symmetry the main characteristics of the receiver functions affected by the anisotropy are summarized as 1 appearance of seismic energy on radial and transverse receiver functions 2 systematic change of p to s ps converted waveforms on receiver functions as ray back azimuth increases and 3 reversal of the ps phase polarity on the radial receiver function in a range of the back azimuth another important influence is shear wave splitting of the ps converted waves and other later phases reverberated as s wave by numerical experiments using synthetic receiver functions we demonstrate that the waveform cross correlation analysis is applicable to splitting ps phases on receiver functions to estimate the seismic anisotropy of layer structure advantages to utilizing the ps phases are 1 they appear more clearly on receiver functions than on seismograms and 2 they inform us about what place along the seismic ray path is anisotropic real analysis of shear wave splitting is executed to the moho generated ps phases that are identified on receiver functions at six seismic stations in the chugoku district southwest japan the time lags between the two arrivals of the split ps phases are estimated at 0 2 0 7 sec and the polarization directions of the fast arrival components are from north south to northeast southwest this result is consistent with recent results of shear wave splitting measurements and the trend of linear epicenter distributions of crustal earthquakes and active fault strikes in the chugoku district

microfossils from silicified stromatolitic carbonates of the upper proterozoic limestone dolomite series central east greenland silicified flake conglomerates and in situ stratiform stromatolites of the upper proterozoic c 700 800 ma limestone dolomite series central east greenland contain well preserved microfossils five stratigraphic horizons within the 1200 m succession contain microbial mat assemblages providing a broad palaeontological representation of late proterozoic peritidal mat communities comparison of assemblages demonstrates that the taxonomy and diversity of mat builder dweller and allochthonous populations all vary considerably within and among horizons the primary mat builder in most assemblages is siphonophycus inornatum a sheath forming prokaryote of probable but not unequivocally established cyanobacterial affinities an unusual low diversity unit in bed 17 is dominated by a different builder tenuofilum septatum while a thin cryptalgal horizon in bed 18 is built almost exclusively by siphonophycus kestron although variable taphonomic histories contribute to observed assemblage variation most differences within and among horizons appear to reflect the differential success or failure of individual microbial populations in colonizing different tidal flat microenvironments twenty two taxa are recognized of which two are described as new myxococcoides stragulescens n sp and scissilisphaera gradata n sp

seismic consequences of warm versus cool subduction metamorphism examples from southwest and northeast japan warm and cool subduction zones exhibit differences in seismicity seismic structure and arc magmatism which reflect differences in metamorphic reactions occurring in subducting oceanic crust in southwest japan arc volcanism is sparse and intraslab earthquakes extend to 65 kilometers depth in northeast japan arc volcanism is more common and intraslab earthquakes reach 200 kilometers depth thermal petrologic models predict that oceanic crust subducting beneath southwest japan is 300 to 500 c warmer than beneath northeast japan resulting in shallower eclogite transformation and slab dehydration reactions and possible slab melting

three dimensional structure of p wave anisotropy beneath the tohoku district northeast japan 1 we retrieve three dimensional structures of isotropic and anisotropic velocities of p waves of the tohoku district from first p arrival time data assuming azimuthal anisotropy to be caused by hexagonal symmetry axes distributed horizontally in the earth the results show that the high velocity pacific slab is clearly imaged in the isotropic velocity structure even though the azimuthal anisotropy is taken into account in addition small scale low velocity regions and prominent low velocity anomalies are found just below the active volcanoes and in the mantle wedge above the high velocity pacific slab respectively the fast propagation axis of p waves is in mostly e w direction in the upper crust nearly n s and e w directions in the lower crust e w direction in the mantle wedge and n s direction in the descending pacific slab these features of the p wave anisotropy structure are consistent with those of lateral variations of the fast polarization directions measured previously by shear wave splitting observations the plausible factor that causes the crust anisotropy is interpreted as being alignment or preferred orientation of microcracks and crust minerals the mantle wedge anisotropy is attributed to lattice preferred orientation of the mantle minerals arising from present day mantle process such as the mantle wedge convection and the plate motion however the fast propagation axis of p waves in the slab is almost perpendicular to the magnetic lineation of the oceanic plate under the northwest pacific and thus the slab preserves the original anisotropic property that the pacific plate gained when it formed

subducted seamount imaged in the rupture zone of the 1946 nankaido earthquake the nankai trough is a vigorous subduction zone where large earthquakes have been recorded since the seventh century with a recurrence time of 100 to 200 years the 1946 nankaido earthquake was unusual with a rupture zone estimated from long period geodetic data that was more than twice as large as that derived from shorter period seismic data in the center of this earthquake rupture zone we used densely deployed ocean bottom seismographs to detect a subducted seamount 13 kilometers thick by 50 kilometers wide at a depth of 10 kilometers we propose that this seamount might work as a barrier inhibiting brittle seismogenic rupture

interpretation of sks waves using samples from the subcontinental lithosphere abstract the seismic properties of five south african kimberlite nodules have been calculated from petrofabric measurements these garnet lherzolite and harzburgite composition nodules 56 80 olivine are known from previous studies to have originated at depths of 120 170 km in the subcontinental lithosphere their in situ seismic properties have been calculated by extrapolating the elastic constants to the appropriate temperature 900 1050 c and pressure 3 0 3 5 gpa conditions the average of the five samples has a maximum s wave anisotropy v s of 3 7 which is sufficiently high to explain previously reported teleseismic sks delay times t in continental shield areas of up to 1 7 s the biggest delay time is observed when the foliation is vertical the lineation horizontal and the fastest s wave polarized parallel to the lineation i e an orientation for transcurrent motion delay times of over 1 s can only be generated by this orientation a strong mantle crust mechanical coupling is suggested for such situations the potential use of seismic anisotropy as an indicator of strain in the lithosphere has been investigated using experimental and simulated fabric data for olivine the most abundant phase 70 in the upper mantle the v p and v s seismic anisotropy increases with fabric strength and finite strain recrystallization tends to reduce fabric strength and seismic anisotropy resulting in saturation values for experimental and simulated fabrics which correspond to approximately 50 60 strain or 8 9 v s for pure olivine aggregates a survey of 25 naturally deformed peridotites of oceanic origin suggests an average maximum s wave anisotropy for the olivine component of 9 or about 8 for a lherzolite or harzburgite rock composition when the orthopyroxene component is taken into account the ophiolite samples are twice as anisotropic as the kimberlite nodules if an average anisotropy value is representative of the lithosphere then the sks delay times represent variations in anisotropic layer thickness for delay times over 1 s as the orientation is constrained to be foliation vertical and lineation horizontal the magnitude of s wave anisotropy is less sensitive than v p to variations in olivine volume fraction in range 50 100 and to deformation or fabric intensity further suggesting that s wave anisotropy is particularly apt for the determination of the anisotropic lithosphere thickness for smaller delay times there is some trade off between structural orientation and thickness

regional variation in shear wave polarization anisotropy of the crust in southwest japan as estimated by splitting analysis of ps converted waves on receiver functions abstract regional variation in shear wave polarization anisotropy of the crust is investigated in the kyushu chugoku shikoku and kinki districts southwest japan using ps converted phases generated at the moho discontinuity the ps phases on radial and transverse receiver functions are analyzed to determine shear wave splitting parameters of fast polarization direction and time lag by waveform cross correction analysis as a result the fast polarization directions are shown to be obviously different between the chugoku district and the kyushu and shikoku districts which are separated from each other by the seto inland sea in the kyushu shikoku and south kinki districts the ps phases exhibit fast polarization directions ranging from e w to nw se the polarization directions are not only consistent with those of s waves from shallow crustal earthquakes but also parallel to the direction of maximum horizontal compression acting on southwest japan and the strike directions of metamorphic belts accretionary zones and rift zones thus the polarization anisotropy is understood to reflect mainly the upper crustal anisotropy that is attributable to stress induced vertical cracks and geological lineament structure on the other hand in the chugoku and north kinki districts fast polarization directions are in a range from n s to ne sw they are not consistent with the tectonic stress field and the s wave splitting observed for shallow crustal earthquakes the disagreement between the fast polarization directions estimated from the s waves and the ps converted phases is interpreted as arising because the lower crustal anisotropy attributed to mineral preferred orientation has a significant influence on the ps phase splitting the regional variation in ps phase polarization anisotropy of the crust in southwest japan is dependent on which anisotropy of the upper and lower crust is prominently reflected in the moho ps phase splitting

effect of temperature on the b to c type olivine fabric transition and implication for flow pattern in subduction zones abstract the effect of temperature on the b to c type fabric transition in olivine was investigated by large strain shear deformation experiments a new series of deformation experiments was conducted at 1273 1373 k 2 0 gpa and strain rates ranging from 3 8 10 4 to 3 2 10 5 s 1 under water saturated conditions and combined with previous results at 1473 k the influence of temperature on deformation fabrics was investigated two types of olivine fabrics were identified one in which the olivine 0 0 1 axis is subparallel to the shear direction and the 0 1 0 plane is parallel to the shear plane b type and another one in which the 0 0 1 axis is parallel to the shear direction and the 1 0 0 plane is parallel to the shear plane c type the b type olivine fabric occurs at higher stress than the c type but the stress magnitude at which the b to c type fabric transition occurs decreases significantly with decreasing temperature the temperature dependence of the transition stress is parameterized using a range of exponential flow law that is constrained by the experimental study when the experimental results are extrapolated to lower temperatures the fabric transition between b and c type is predicted to occur at 1 mpa and 1000 k this prediction is consistent with the olivine fabrics in naturally deformed peridotites found in plate convergent regions the present study indicates that when some amounts of water 200 ppm h si are present the b type fabric will dominate in low temperature regions of the upper mantle whereas the c type or e type fabric will dominate at high temperatures the complex seismic anisotropy in subduction zones can be explained by changes in the dominant type of olivine fabrics caused by spatial variations in physical and chemical variables such as temperature and water content rather than by complexities in mantle wedge flow pattern

origin of crustal anisotropy shear wave splitting studies in japan shear wave splitting manifested as leading shear wave polarization that is parallel alignment of leading shear wave particle motions from a variety of sources has been observed at a number of seismograph stations in japan detected on shear wave seismograms from crustal earthquakes over a wide range of source zones and source receiver azimuths the shear wave splitting can be attributed to crustal anisotropy this paper discusses the relation between leading shear wave polarization directions and tectonic features of japan to explain the observed shear wave splitting the author proposes that at least three phenomena should be taken into account stress induced microcracks primarily aligned in vertical or subvertical planes cracks or fractures in the vicinity of active faults having their orientation parallel to the fault planes and intrinsic rock anisotropy resulting from preferred orientation of minerals travel time differences between leading and slower split shear waves from crustal and upper mantle earthquakes analyzed for about one third of the stations suggest that the crustal anisotropy which causes the observed shear wave splitting may be limited to the upper 15 25 km this implies that the density of nonhorizontally aligned cracks or fractures below 15 25 km and into the upper mantle is muchmore smaller than that in the crust above 15 25 km less

melting of pyrope mg3al2si3o12 up to 10 gpa possibility of a pressure induced structural change in pyrope melt the melting of pyrope mg3al2si3o12 has been determined experimentally at pressures up to 10 gpa pyrope melts congruently in the pressure range 3 5 10 gpa at pressures below 7 gpa aluminous enstatite of pyrope composition crystallizes during quenching whereas quench pyrope appears at higher pressures moreover the slope of the melting curve of pyrope decreases significantly at pressures above 7 gpa these changes in both the nature of the quench phase and the slope of the melting curve suggest a pressure induced coordination change for al3 in pyrope melt at around 7 gpa this pressure induced coordination change of al3 probably occurs even in magmatic liquids generated in the deep upper mantle and should affect some of their physical properties

tomographic evidence for the mantle upwelling beneath southwestern japan and its implications for arc magmatism abstract the seismotectonics and volcanotectonics of southwestern sw japan have been affected by the subduction of the old pacific and the young philippine sea slabs we have attempted to determine the three dimensional p and s wave velocity structures of the upper mantle in sw japan by using a large amount of high quality arrival time data recorded by the dense seismograph network on the japanese islands to understand the tectonic framework of sw japan our results show the existence of a large low velocity anomaly below the philippine sea slab in the chugoku and shikoku districts at least down to depths of 300 km we infer that the low velocity zone is an upwelling flow from depth two shallower low velocity zones derived from this low velocity zone are detected one is toward the quaternary volcanoes in the chugoku district and the other is toward the uppermost mantle beneath the kii peninsula the former suggests that the magma for the quaternary volcanoes in the chugoku district is attributable to the upwelling flow in the upper mantle this idea supports the geochemical and petrological observations that the erupted alkalic basalt in the chugoku district should have originated from a more primitive or enriched mantle the latter is responsible for the absence of a high velocity anomaly corresponding to the subducted philippine sea slab beneath the kii peninsula the philippine sea slab could have been more hydrated locally beneath the kii peninsula than that in the surrounding regions by the interaction with the mantle upwelling consequently dehydration reactions could be involved during the progressive metamorphism accompanied by intensive intraslab seismicity lowering the seismic velocity in the slab aqueous fluids thus released to the overriding plate might be a possible source of mantle type helium and trigger a long term swarm activity in the crust of the overriding plate our results indicate the importance of the heterogeneous structure below the philippine sea slab with regard to the tectonics of sw japan

slab segmentation revealed by anisotropic p wave tomography 1 seismic anisotropy is a useful indicator for identifying the physical and chemical condition of the earth s interior such as stress and flow fields and in situ constituent minerals using traveltime tomography we examined three dimensional anisotropic p wave velocity structure of the kii peninsula southwest japan where source regions of megathrust earthquakes along the nankai trough are presumed to exist the tomography revealed that the philippine sea slab beneath the peninsula is segmented i e a high velocity slab with e w anisotropy obtained during seafloor spreading is broken by an anomalous low velocity region with n s anisotropy the anomaly lies along the segmentation boundary of two previous rupture zones the 1944 tonankai and 1946 nankai earthquakes and can be explained by the occurrence of a fracture zone with n s oriented fractures and fault planes revealed by wide angle seismic data it is likely that this anomalous region will form a clear segmentation boundary during future earthquakes

mantle deformation during slow seafloor spreading constrained by observations of seismic anisotropy in the western atlantic during mantle flow at mid ocean ridges viscous shear deformation imparts a structural fabric that remains in the lithosphere as it cools preserving a record of ridge processes over time this fabric can be detected by seismic imaging as it produces an azimuthal anisotropy in the velocity of seismic waves that propagate in the shallow mantle using data from a novel seismic refraction survey in the western atlantic we found that the maximum p wave velocity in the upper 10 km of mantle lithosphere formed at the slow spreading mid atlantic ridge is parallel to the paleo spreading direction consistent with viscous shear deformation dominated by corner flow the magnitude of the p wave anisotropy is 3 4f0 3 approximately one half that found in lithosphere formed at faster spreading rates in the pacific weaker anisotropy in the atlantic suggests that more pervasive conductive cooling at slow spreading ridges increases the proportion of localized brittle deformation in the mantle lithosphere thereby limiting the degree of viscous deformation by scaling our field observations to laboratory experiments we estimate that total viscous strain during slow seafloor spreading is of order 0 5 as opposed to 1 2 for the fast spreading case finally our travel time observations display an azimuthal asymmetry that can be interpreted in one of two ways either the average elastic tensor in this region is not orthorhombic as is commonly assumed or the underlying shear fabric is rotated 158 relative to fossil spreading d 2004 elsevier b v all rights reserved

melting experiment on a model chondritic mantle composition at 25 gpa we have conducted melting experiments on a chondritic mantle composition in the five component system cao feo mgo al2o3 sio2 at 25 gpa the liquidus phase at this pressure is majorite garnet and the second crystallizing phase is mg silicate perovskite or magnesiowustite these phases seem to be consumed into the melt at almost the same temperature the present experiments indicate that majorite garnet fractionation might have occurred in the chondritic primitive mantle if the partial molten zone extended to a depth equivalent to the upper part of the lower mantle in the early stages of terrestrial evolution

paleobiology of the mesoproterozoic neoproterozoic transition the sukhaya tunguska formation turukhansk uplift siberia silicified carbonates of the latest mesoproterozoic sukhaya tunguska formation northwestern siberia contain abundant and diverse permineralized microfossils peritidal environments are dominated by microbial mats built by filamentous cyanobacteria comparable to modern species of lyngbya and phormidium in subtidal to lower intertidal settings mat dwelling microbenthos and possible coastal microplankton are abundant in contrast densely woven mat populations with few associated taxa characterize more restricted parts of tidal flats the preservation of vertically oriented sheath bundles and primary fenestrae indicates that in these mats carbonate cementation was commonly penecontemporaneous with mat growth eoentophysalis mats are limited to restricted environments where microlaminated carbonate precipitates formed on or just beneath the sediment surface most microbenthic populations are cyanobacterial although eukaryotic microfossils may occur among the simple spheroidal cells interpreted as coastal plankton protists are more securely represented by large up to 320 micrometers in diameter but poorly preserved acritarchs in basinal facies the sukhaya tunguska assemblage contains 27 species in 18 genera by virtue of their stratigraphic longevity and their close and predictable association with specific paleoenvironmental conditions including substrates proterozoic cyanobacteria support a model of bacterial evolution in which populations adapt rapidly to novel environments and thereafter resist competitive replacement the resulting evolutionary pattern is one of accumulation and stasis rather than the turnover and replacement characteristic of phanerozoic plants and animals

hypocenter velocity structure coupling in local earthquake tomography abstract local earthquake tomography involves the use of seismic wave arrival times from earthquake sources with uncertain locations for the inference of three dimensional variations in seismic velocity formally this inverse problem contains explicit coupling between the earthquake hypocenters and the velocity structure however many studies fail to treat the hypocenter velocity structure coupling explicitly an examination of previous studies provides evidence that this failure can lead to significant bias in the resulting velocity model a set of simple hypothetical test cases clearly demonstrates the seriousness of the potential bias when coupling is neglected producing models that underestimate the actual velocity perturbations by about a factor of four under the same conditions strict simultaneous inversion and simultaneous inversion with parameter separation both produce accurate inverted models the use of appropriate station corrections and iteration of the solution can improve the results when the coupling is ignored but velocity anomalies will still be underestimated by about a factor of two at best

a review of shear wave splitting in the crack critical crust summary over the last 15 years it has become established that crack induced stress aligned shear wave splitting with azimuthal anisotropy is an inherent characteristic of almost all rocks in the crust this means that most in situ rocks are pervaded by fluid saturated microcracks and consequently are highly compliant the evolution of such stress aligned fluid saturated grain boundary cracks and pore throats in response to changing conditions can be calculated in some cases with great accuracy using anisotropic poro elasticity ape ape is tightly constrained with no free parameters yet dynamic modelling with ape currently matches a wide range of phenomena concerning anisotropy stress shear waves and cracks in particular ape has allowed the anisotropic response of a reservoir to injection to be calculated predicted with hindsight and the time and magnitude of an earthquake to be correctly stress forecast the reason for this calculability and predictability is that the microcracks in the crust are so closely spaced that they form critical systems this crack critical crust leads to a new style of geophysics that has profound implications for almost all aspects of pre fracturing deformation of the crust and for solid earth geophysics and geology we review past present and speculate about the future of shear wave splitting in the crack critical crust shear wave splitting is seen to be a dynamic measure of the deformation of the rock mass there is some good news and some bad news for conventional geophysics many accepted phenomena are no longer valid at high spatial and temporal resolution a major effect is that the detailed crack geometry changes with time and varies from place to place in response to very small previously negligible changes however at least in some circumstances the behaviour of the rock in the highly complex inhomogeneous earth may be calculated and the response predicted opening the way to possible control by feedback the need is to devise ways to exploit these new opportunities in the crack critical crust recent observations from the smsites project at husavik in northern iceland gathered while this review was being written display the extraordinarily sensitivity of in situ rock to small changes at great distances the effects are far too large to occur in a conventional elastic brittle crust and their presence confirms the highly compliant nature of the crack critical crust

phase transformations in a harzburgite composition to 26 gpa implications for dynamical behaviour of the subducting slab abstract the mineralogy adopted by a depleted harzburgite composition has been studied over the pressure interval 5 26 gpa at temperatures of 1300 1400 c the pyroxene garnet component of the harzburgite composition harzburgite minus 82 wt olivine transforms to majorite garnet by 18 19 gpa and further disproportionates to the assemblage of garnet stishovite mg 2 sio 4 spinel above 20 gpa at still higher pressures first ilmenite 22 24 gpa and then perovskite mgsio 3 24 26 gpa are found to coexist with garnet garnet disappears at 26 gpa and almost complete transition to perovskite is achieved at this pressure the mineral proportions and density profiles in the subducting oceanic lithosphere modelled by a combination of 80 harzburgite 20 primitive morb compositions are calculated as a function of depth under conditions isothermal with surrounding pyrolite mantle and also for a temperature distribution in which the slab is substantially cooler than surrounding mantle to below 700 km under isothermal conditions the slab has a density similar to surrounding mantle to a depth of 600 km however between 600 and 700 km the slab is up to 0 08 g cm 3 denser than surrounding mantle this is caused primarily by the higher alumina content in pyrolite as compared to harzburgite which causes the transition to perovskite in pyrolite to occur at substantially higher pressures than in harzburgite the presence of alumina also smears out the garnet perovskite transition in pyrolite over a depth interval of 50 km whereas this transformation is much sharper in the harzburgite composition calculations based on the observed phase equilibria also show that a subducted cool slab remains much denser by 0 1 0 3 g cm 3 than surrounding mantle to a depth of 700 km but possesses a density similar to surrounding mantle below this depth these results have important implications for the dynamical behaviour of slabs possessing different thermal regimes when they encounter the 670 km discontinuity and also for the nature of this discontinuity

p wave velocity and anisotropy of lawsonite and epidote blueschists constraints on water transportation along subducting oceanic crust p wave velocity vp and the anisotropy of lawsonite and epidote blueschists were measured up to 1 0 gpa and 400 c using the ultrasonic pulse transmission technique the slowest vp in the direction normal to foliation is similar between lawsonite and epidote blueschists 7 0 7 2 km s at 1 0 gpa and room temperature while the fastest vp in the direction parallel to lineation markedly differs between lawsonite blueschists 7 4 7 6 km s at 1 0 gpa and room temperature and epidote blueschist 7 9 km s at 1 0 gpa and room temperature crystallographic orientation measurements for main constituent min erals revealed that both epidote 0 1 0 axes fastest vp direction in epidote single crystal and amphibole 0 0 1 axes fastest vp direction in amphibole single crystal are preferentially oriented parallel to lin eation to enhance vp anisotropy of the epidote blueschist in contrast lawsonite 0 0 1 axes fastest vp direction in lawsonite single crystal are oriented subnormal to foliation whereas amphibole 0 0 1 axes are oriented subparallel to lineation so that relatively weak vp anisotropy was observed in the lawsonite blueschist our experimental results in conjunction with recent seismological observations suggest that the vp of the subducting oceanic crust at 50 km has been observed in several subducting slabs however the high vp values at 50 km depth are difficult to be explained by blueschists this indicates that the blueschist would be at least partially transformed to hydrous mineral bearing eclogite at 50 km depth in subducting oceanic crusts

zonal structure of cenozoic basalts related to mantle upwelling in southwest japan cenozoic olivine bearing basaltic rocks in the central chugoku district form eight provinces each 20 to 30 km in diameter these provinces can be grouped into three zones almost parallel to the elongation of the sw japan arc based on geological and petrological characteristics the san yo zone the closest to the nankai trough is characterized by a small amount of alkaline basalt in the sekiryo zone intermediate zone transitional basalt is dominant the san in zone along the japan sea coast is characterized by subalkaline basalt and a large amount of andesitic rocks the eruption volume of these volcanics exponentially increases toward the back arc side less than 1 km3 to more than 100 km3 chemical compositions of relatively undifferentiated basalts show systematic across arc variations sio2 and al2o3increase and feo mgo and cao decrease toward the back arc side 44 to 54 13 to 18 11 to 6 13 to 7 and 12 to 8 wt at feo mgo 0 85 respectively melting experiments on the basalt h2o co2 systems show that the pressure and temperature of magma segregation decrease toward the back arc side 17 19 kbar and 1340 1320 c to 8 kbar and 1250 c and the coexisting phases change from a spinel lherzolite assemblage to a harzburgite assemblage these pt conditions define a trend which can be interpreted as a pt path for adiabatic upwelling of the mantle material consequently the observed volcanism and zonation may have been caused by upwelling of a plume from a deep part of the mantle

evidence for the location and cause of large crustal earthquakes in japan large crustal earthquakes magnitude 5 7 8 0 depth 0 20 km during a period of 115 years from 1885 to 1999 in japan occurred in or around zones of low seismic velocity revealed by seismic tomography the low velocity zones may represent weak sections of the seismogenic crust the crustal weakening is considered to be closely related to the subduction of the oceanic pacific and philippine sea plates in this region along the volcanic front and in back arc areas the crustal weakening may be caused by active volcanoes and magma chambers in the forearc areas fluids are detected in the earthquake source zones which may have contributed to the crustal weakening and the rupture nucleation the fluids may be related to the dehydration of the subducting philippine sea slab these results indicate that large crustal earthquakes do not strike anywhere but only anomalous areas that may be detected with geophysical methods

high resolution subducting slab structure beneath northern honshu japan revealed by double difference tomography the high resolution seismic velocity structure of the subducting slab beneath northern honshu japan has been obtained by double difference tomography capitalizing on the existence of two planes of seismicity the upper plane lies in the region with average to high vp vs ratios 1 72 1 85 which may correspond to the transformations of metabasalt and metagabbro to blueschist the lower plane is associated with very low vp vs ratios 1 6 1 7 in sharp contrast with high vp vs ratios 1 8 1 85 in the region between the two planes these features may be explained by forsterite enstatite h2o formation from serpentine dehydration in the lower plane and partial hydration of the region between the two planes our results provide direct constraints for petrologic modeling of the subducting slab

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

high pressure transformation of pyrope mg3al2si3o12 in a sintered diamond cubic anvil assembly transformation of pyrope garnet was investigated at 1600 c and at pressures higher than 35 gpa using a sintered diamond cubic anvil assembly in a double staged split sphere apparatus pyrope transformed into orthorhombic perovskite at ca 37 gpa whereas a small amount of corundum coexisted with perovskite at lower pressures the unit cell dimensions of perovskite were determined to be a 4 771 2 a b 4 947 2 a c 6 950 3 a and v 164 0 2 a a trace of stishovite observed in the run products may be evidence of nonstoichiometry in the perovskite these results support the view that perovskite is the major host phase of al2o3 in the lower mantle

crustal and uppermost mantle structure and seismotectonics of north china craton abstract we determined a 3 d p wave anisotropic tomography of the crust and uppermost mantle beneath north china craton ncc using 107 976 p wave arrival times from 16 073 local earthquakes recorded by 380 seismic stations our results show significant lateral heterogeneities beneath ncc the lower crust and uppermost mantle beneath the north china basin show widespread low velocity anomalies which may reflect high temperature materials caused by the late mesozoic basaltic magmatism in the ncc low velocity anomalies also exist beneath the trans north china orogen which may reflect asthenospheric upwelling since late mesozoic large crustal earthquakes generally occurred in high velocity zones in the upper to middle crust while low velocity and high conductivity anomalies that may represent fluid filled fractured rock matrices exist in the lower crust to the uppermost mantle under the source zones of the large earthquakes the crustal fluids may lead to the weakening of the seismogenic layer in the upper and middle crust and hence cause the large crustal earthquakes the nw se p wave fast velocity directions seem to be dominant in the uppermost mantle under the central parts of eastern ncc suggesting that these mantle minerals were possibly regenerated but keep the original fossil anisotropy formed before the new lithospheric mantle was produced during the mesozoic to cenozoic

topography of the seismic velocity discontinuities beneath the chugoku and shikoku districts southwest japan the first p arrival time data from 513 local earthquakes were analyzed to study lateral variation of the depth to the conrad and moho discontinuities beneath the chugoku and shikoku districts southwest japan as well as to determine earthquake hypocenters and p wave station corrections the depth to the discontinuity was estimated by minimizing the travel time residuals of more than 8700 first p arrivals observed at 55 seismic stations the conrad and moho discontinuities are located within depth ranges of 15 25 km and 30 40 km respectively the moho is deeper under the mountain area than under the seto inland sea area and especially deep under the pacific coast of the shikoku district and the mountain area in the chugoku district the depth variation of the moho is quite similar to the bouguer gravity anomaly distribution and the lateral variations of the p wave velocity the deep moho under the southern shikoku is located at the portion in which the continental moho under the island arc meets the oceanic moho that is the boundary interface between the oceanic crust and the philippine sea phs plate dipping toward the back arc although there are high mountains in the northern and middle shikoku the moho is not so deep because subduction of the phs plate prevents the moho from getting deep while the moho is deep due to isostatic balance under the mountain area in the chugoku district in addition we indicated the possibility that the upper boundary of the oceanic crust just above the high velocity phs plate is in contact with the deep moho under the western chugoku the contact of the moho with the oceanic crust can explain the markedly negative gravity anomaly observed in the western chugoku and the later phase that appears just after the first p arrival from local earthquakes

anisotropic pn tomography in the western united states pn travel times are affected not only by lateral variations in crust and mantle velocity but also by significant amounts of laterally varying anisotropy to investigate uppermost mantle anisotropy a tomography algorithm was reformulated to include lateral variations in both velocity and horizontal anisotropy and it was applied to pn travel time data from the western united states results show that anisotropy is as important in explaining the travel time residuals as are the velocity variations a detailed resolution study examined the trade off between the velocity variations and the anisotropy variations and showed that both could be resolved for regions with good ray path coverage pn anisotropy beneath the western united states has maximum amplitudes of 0 3 km s 7 5 when resolved on a length scales of around 3 the orientations of pn anisotropy often correlate well with those inferred from shear wave splitting results this correlation suggests that orientation of mantle anisotropy does not change significantly with depth for many regions the orientation of the pn anisotropy can be correlated with some of the tectonic processes which occur within the western united states for example the fast direction of pn anisotropy parallels the northeast direction of subduction of the juan de fuca plate beneath the northwest pacific coast suggesting that there the anisotropy is related to subduction driven deformation the fast direction of pn anisotropy also parallels the strike of the san andreas fault system in central california indicating that shear strains along the plate boundary may be responsible for the anisotropy there within the great basin the pn anisotropy varies substantially and both partial melting and small scale convection within the uppermost mantle could be responsible for these anisotropy variations

melting relations in the system mg4si4o12 en mg3al2si3o12 py at high pressures melting experiments in the system mg4si4o12 en mg3al2si3o12 py have been carried out at 15 and 25 gpa using a uniaxial split sphere high pressure apparatus quenched melt was identified by textural and compositional observation by scanning electron microscopy and energy dispersive electron probe microanalysis melting on the join clinoenstatite pyrope at 15 gpa is eutectic and the eutectic temperature at which clinoenstatite en99 7py0 3 liquid en98py2 and garnet en91py9 coexist is 2250 c the melting temperatures of clinoenstatite and pyrope at 15 gpa are 2350 c and 2275 c respectively consequently garnet and liquid coexist over a very narrow temperature interval in the range of composition from the eutectic to pyrope the melting temperature of pyrope at 25 gpa exceeds 3000 c which is substantially higher than that of mgsio3 perovskite the available data suggest that the melting temperature of perovskite increases with incorporation of al2o3 this is consistent with peritectic melting on the en py join at 25 gpa the wide stability of garnet both in temperature and composition suggests that garnet could be the predominant liquidus phase for ultrabasic bulk compositions up to the pressure at which the perovskite rich in al2o3 is stabilized

solidus and liquidus temperatures and mineralogies for anhydrous garnet lherzolite to 15 gpa abstract a review of experimental data for systems pertaining to anhydrous fertile garnet lherzolite shows strong convergence in the liquidus and solidus temperatures for the range 6 5 15 gpa these can converge either to a common temperature or to temperatures which differ by only 100 c the major element composition of magmas generated by even minor degrees of partial melting may be similar to the primordial bulk silicate earth composition in an upper mantle stratigraphic column extending over 160 km in depth the convergence of the solidus and liquidus temperatures is a consequence of the highly variable d t d p of the fusion curves for minerals which crystallize in peridotite systems in particular d t d p for the forsterite fusion curve is much less than that for diopside and garnet whether or not the solidus and liquidus intersect the liquidus mineralogy for undepleted garnet lherzolite compositions changes from olivine at low pressures to pyroxene garnet or a complex pyroxene garnet solid solution at pressures in excess of 10 15 gpa geochemical data for the earliest archean komatiites are consistent with an upper mantle phase diagram having garnet as a liquidus phase for garnet lherzolite compositions at high pressures all estimates of the anhydrous solidus and liquidus for the range 10 15 gpa are consistent with silicate liquid compressibility data which indicate that olivine may be neutrally buoyant in ultramafic magmas at these pressures

links between palaeoproterozoic palaeogeography and rise and decline of stromatolites fennoscandian shield through a review of literature and new data we have documented two major events in the palaeoproterozoic history of stromatolites as indicated by palaeontological and palaeoenvironmental studies with a time resolution of between 40 and 200 ma we confirm semikhatov and raaben s and awramik s albeit approximately maximum in diversity and abundance of stromatolites between 2330 and 2060 ma ago jatulian diversification we suggest that this taxonomic diversity was driven by a major phase of cratonisation formation of the karelian carbonate platform and numerous rift related shallow water carbonate basins supersaturated with ca 2 mg 2 and co2 the jatulian stromatolite explosion is synchronised with a positive 13ccarb shift of jatulian age carbonates we also document stromatolite decline which occurred on the fennoscandian shield somewhere between 2060 and 1900 ma ago this decline both in abundance and in taxonomic diversity is interpreted as having been caused by the first phase of oceanisation the oceanisation led to the considerable reduction in ecological niches that could be utilised by cyanobacteria the post jatulian decline of stromatolites coincides with an abrupt downward 13corg shift from 19 to 38 and is roughly coeval with the appearance of the first eukaryotic algae documented elsewhere the systematics of the fennoscandian diversity of palaeoproterozoic stromatolites is identical to that reported from india and china and reveals a dissimilarity with abundance and diversity patterns in australia and northern america

the meaning of stromatolites stromatolites document microbial interactions with sediments and flowing water throughout recorded earth history and have the potential to illuminate the long term history of life and environments modern stromatolites however provide analogs to only a small subset of the structures preserved in archean and proterozoic carbonates thus interpretations of secular trends in the shapes and textures of ancient columnar stromatolites require nonuniformitarian scale dependent models of microbial responses to nutrient availability seawater chemistry influx of sediment grains shear and burial models that integrate stromatolite scales macroscopic organization and shapes could also help test the biogenicity of the oldest stromatolites and other structures whose petrographic fabrics do not preserve direct evidence of microbial activity an improved understanding of stromatolite morphogenesis in the presence of oxygenic and anoxygenic microbial mats may illuminate the diversity of microbial metabolisms tha

scs wave splitting of deep earthquakes around japan summary scs wave splitting of five deep earthquakes in subduction zones near japan is investigated using horizontal seismograms recorded al jma stations for each earthquake we clearly observe uniform scs wave splitting in all stations over japan especially for the events located south of honshu in 1982 1984 and 1993 however the directions of fast polarized waves of these events differed by a maximum of about 50 from one another the orientation of fast polarized waves in the 1982 event was nnw sse those in the two later events wnw ese we also recognize this discrepancy in the results of the analysis of the 1971 sea of okhotsk event reported by fukao 1984 the sakhalin islands event in 1990 reveals a linear particle motion without such a change in direction of the second arrivals implying no anisotropy these observations are interpreted as indicating an anisotropic region within the slab near the earthquake sources but not beneath the receivers since the orientations of fast polarized waves recorded at each station are not common to all the earthquakes furthermore we consider that anisotropy exists non uniformly within the slab the event in 1982 which occurred in almost the same area as those in 1984 and 1993 showed a fast direction different from the events in 1984 and 1993 the 1982 event was 179 km deep but the two later events were at 398 km and 360 km respectively the fast direction observed from the 1982 event is parallel to the fossil plate motion whereas those from the events in 1984 and 1993 are parallel to the compression axis within the subducting slab the depth of 400 km is a phase boundary where olivine changes to spinel we consider that the most likely cause of the change in anisotropy direction is the re orientation of crystals associated with the phase change of olivine to spinel due to subduction of the slab

moho depth variation beneath southwestern japan revealed from the velocity structure based on receiver function inversion abstract the philippine sea plate is subducting under the eurasian plate beneath the chugoku shikoku region southwestern japan we have constructed depth contours for the continental and oceanic mohos derived from the velocity structure based on receiver function inversion receiver functions were calculated using teleseismic waveforms recorded by the high density seismograph network in southwestern japan in order to determine crustal velocity structure we first improved the linearized time domain receiver function inversion method the continental moho is relatively shallow 30 km at the coastline of the sea of japan and at the seto inland sea and becomes deeper greater than 40 km around 35 n and 133 8 e near the seto inland sea a low velocity layer of thickness 10 km lies under the continental moho this low velocity layer corresponds to the subducting oceanic crust of the philippine sea plate the oceanic moho continues to descend from south to northwest and exhibits complicated ridge and valley features the oceanic moho runs around 25 km beneath the pacific coast and 45 km beneath the seto inland sea and it extends to at least to 34 5 n the depth variation of the moho discontinuities is in good qualitative agreement with the concept of isostasy from the configurations of both the continental and oceanic mohos we demonstrate that the continental lower crust and the subducting oceanic crust overlap beneath the southern and central part of shikoku and that a mantle wedge may exist beneath the western and eastern part of shikoku the southern edge of the overlapping region coincides with the downdip limit of the slip area of a megathrust earthquake

novel anisotropic teleseismic body wave tomography code anitomo to illuminate heterogeneous anisotropic upper mantle part i theory and inversion tuning with realistic synthetic data

mapping p wave anisotropy of the honshu arc from japan trench to the back arc abstract we determined a 3 d p wave anisotropic tomography beneath the entire honshu arc using about 448 000 high quality p wave arrival times from 18 335 local earthquakes that occurred beneath the northeast ne japan land area and the fore arc area under the pacific ocean our results show that low velocity low v zones exist beneath the active arc volcanoes in the crust and in the central portion of the mantle wedge above the subducting pacific slab the low v anomalies are related to the arc magmatism low v zones are also revealed in the fore arc area which are probably caused by large volumes of water releasing upwards from dehydration of the subducting oceanic crust and sediments the anisotropic amplitude in the upper crust is weaker than that in other portions under ne japan in the mantle wedge the fast velocity direction fvd is generally trench normal in back arc area which may reflect that the olivine a axis aligns with the transport direction induced by the slab driven corner flow the fvd becomes trench parallel in the central portion of the fore arc mantle wedge which is possibly induced by the olivine of b type fabric in the slab driven corner flow the fvd shows trench parallel in the low v zones in the fore arc mantle wedge close to the upper boundary of the pacific slab which may reflect the b type olivine fabric dominating in those areas or it may be induced by the dextral shearing of the overlying crust the trench parallel fvd is also revealed beneath the volcanic front indicating complex 3 d mantle flows in the mantle wedge the fvd in the subducting pacific slab is mostly trench parallel which may reflect one or more of the following possibilities 1 the original fossil anisotropy of the pacific plate formed at the mid ocean ridge 2 cracks within the slab and 3 the olivine fabric transition due to the changes in water content stress and temperature

melting experiments on the enstatite pyrope join at 80 152 kbar melting temperatures of the assemblage pyroxene garnet on the enstatite pyrope join were experimentally determined at 11 different pressures between 80 and 152 kbar with a split sphere anvil apparatus ussa 2000 the compositions of pyroxene garnet and melt coexisting along the solidus were also determined comparison with previous data at lower temperatures revealed a large temperature dependence of the compositions of garnet coexisting with pyroxene it is proposed that the observed variation in the garnet composition is caused by disorder in garnet the obtained alumina contents of orthopyroxene coexisting with garnet on the solidus at 80 110 kbar contributed to an improvement of the garnet peridotite thermobarometry an internally consistent set of thermodynamic parameters was derived which allows calculation of the temperature pressure phase diagram for the enstatite pyrope join at 20 260 kbar

instantaneous global plate motion model from 12 years of continuous gps observations 1 we estimate a global plate motion model for 17 major and minor tectonic plates solely on the basis of analysis of data from 106 globally distributed continuous gps stations spanning the period from january 1991 to july 2003 site positions estimated from 24 hour segments of data are aligned day by day to the international gps service igs realization of the itrf2000 reference frame using a similarity transformation thereby ensuring that the itrf2000 no net rotation condition is preserved linear velocities of a carefully selected set of stations are estimated from the position time series along with annual and semiannual fluctuations and position offsets due to gps instrument changes a white noise plus flicker noise model is applied to estimate realistic uncertainties for the site velocities which are then propagated into the derived plate motion model parameters we also examine the vertical velocities in the site selection process at the scripps orbit and permanent array center we have implemented a procedure whereby the plate motion model is updated on a regular monthly basis to improve its precision and reliability as new data become available and as a baseline against which anomalous motions can be detected

regional variations of amplitudes of scsp phases observed in the japanese islands summary scsp phases are examined with special reference to their amplitude variations over seismograph networks in the japanese islands scsp of rela tively large amplitude are observed along the pacific side of the japanese islands amplitude ratios of the scsp with respect to scs are explained in terms of scs to p conversion at the upper boundary of a descending pacific plate which dips at about 30 and whose seismic wave velocity is between 5 and 10 per cent higher than that of the surrounding mantle large amplitude scsp are observed at stations nearest the kurile or japan trench the ampli tude ratios of scsp scs are found to decrease rapidly with distance inland from the trench an estimate is made of the effects of sharpness of the con version interface the incident angle of scs upon it and lateral variations of anelasticity within the asthenosphere overlying the descending pacific plate upon the scsp scs ratios we show that the sharpness of the conversion inter face is the dominant factor this result implies that the nature of the scs to p conversion interface varies with distance from the trench or equivalently with depth the presence of a low velocity zone in the vicinity of the conversion interface is inferred from the relative polarity between scsp and scs observed at the stations on the outermost side of north eastern japan scsp arrivals from shallower conversions at depths less than about 80 km observed in central and south western japan are also examined to elucidate the structure of the philippine sea plate descending beneath this region a discussion is made on the correlations between dipping subcrustal seismicity and the scs to p conversion interfaces except for the chugoku region the scs to p conversion interfaces correlate well with the dipping subcrustal seismicity our scsp observations indicate the presence of a dipping dis continuity in the uppermost mantle beneath the chugoku region where no subcrustal earthquakes have been detected

a new look at differentiation of the earth from melting experiments on the allende meteorite carbonaceous chondrites contain approximately solar abundances of the non volatile elements1 but are much more feo rich2 than upper mantle peridotites3 or hypothetical chondritic mantle compositions4 5 if the earth accreted from unfractionated primitive meteoritic debris similar to the carbonaceous chondrites how then did it become a fractionated layered body with a crust mantle and core here i report the results of high pressure melting experiments on the allende cv3 carbonaceous chondrite which address this question and provide a new look at the earth s earliest stage of differentiation multi anvil experiments at 24 26 and 26 5 gpa show that feo rich magnesiowustite is an abundant crystallizing phase at temperatures near the allende silicate liquidus if a chondritic earth experienced a high temperature molten stage then during cooling and crystallization feo rich magnesiowustite could be segregated to the deepest levels of the earth s interior magnesiowustite fractionation may thus have depleted the initial feo content of the primitive chondritic mantle and contributed to the formation and growth of the earth s core

dual subduction tectonics and plate dynamics of central japan shown by three dimensional p wave anisotropic structure abstract the central japanese subduction zone is characterized by a complex tectonic setting affected by the dual subduction of oceanic plates and collisions between the island arcs to better understand of the subduction system we performed an anisotropic tomography analysis using p wave arrival times from local earthquakes to determine the three dimensional structure of p wave azimuthal anisotropy in the overriding plate and the pacific and philippine sea phs slabs the principal characteristics of anisotropy in the subducted and subducting plates are 1 in the overriding plate the distribution pattern of fast direction of crustal anisotropy coincides with that of the strike of geological structure 2 in the two oceanic plates fast propagation directions of p wave were sub parallel to the directions of seafloor spreading additionally our tomographic images demonstrate that 1 the bottom of the median tectonic line the longest fault zone in japan reaches to the lower crust and seems to link to the source region of an inter plate earthquake along the phs slab 2 the segmentation of the phs slab the izu islands arc the nishi shichito ridge and the shikoku basin due to the formation history is reflected in the regional variation of anisotropy the tomographic study further implies that there might be a fragment of the pacific slab suggested by a previous study beneath the tokyo metropolitan area the overall findings strongly indicate that seismic anisotropy analysis provide potentially useful information to understand a subduction zone

paleogeographic reconstruction and origin of the philippine sea abstract our reconstruction of the philippine sea suggests that it formed by two distinct episodes of back arc spreading each of which resulted from seaward retreat of the trench in the first episode the protoizu bonin trench retreated northward and the west philippine basin formed behind the northern half of the palau kyushu ridge in the second episode the izu mariana trench retreated eastward and the shikoku and parece vela basins formed behind it during the last 17 ma the philippine sea basin has been moving northwestward with respect to eurasia shifting the ttt triple junction off central japan westward by about 50 km the motion of the philippine sea with respect to eurasia at the triple junction changed from north northwestward to west northwestward 10 5 ma ago for the period before 17 ma ago we construct two models retreating trench model and anchored slab model the izu bonin trench migrated from south to northeast rotating in a clock wise sense since 48 ma ago in the retreating trench model in the anchored slab model the trench has been fixed with respect to eurasian margin since 43 ma ago we prefer the retreating trench model because the deformation of the plate boundary along the eastern margin of eurasia during 30 17 ma ago is much simpler for this model than for the anchored slab model furthermore rotations of the bonin mariana islands are consistent with those predicted from the retreating trench model the 48 ma ages of the northern part of the palau kyushu ridge and of chichi jima of the bonin islands indicate that there was subduction beneath the northern half of the ridge beginning at least 48 ma ago from this and the subparallelism in trend between the northern part of the palau kyushu ridge and the central basin ridge we propose that the major part of the west philippine basin formed by back arc spreading in a n s direction behind the northern part of the palau kyushu ridge the pacific plate was moving northward with respect to hot spots from 48 to 43 ma ago which implies that the pacific plate is not likely to have been subducting beneath the west philippine basin during this time we speculate that another plate existed south of the pacific plate and thai it was subducting beneath both tile west philippine basin and australia the annihilation of this plate might he a cause for the sudden change of the pacific plate motion at 43 ma ago