phlogopite and quartz lamellae in diamond bearing diopside from marbles of the kokchetav massif kazakhstan exsolution or replacement reaction exsolution lamellae of pyroxene in garnet grt coesite in titanite and omphacite from uhpm terranes are widely accepted as products of decompression however interpretation of oriented lamellae of phyllosilicates framework silicates and oxides as a product of decompression of pyroxene is very often under debate results are presented here of fib tem feg emp and synchrotron assisted infrared ir spectroscopy studies of phlogopite phlog and phlogopite quartz qtz lamellae in diamond bearing clinopyroxene cpx from ultra high pressure uhp marble these techniques allowed collection of three dimensional information from the grain boundaries of both the single phlogopite two phase lamellae phlogopite quartz and fluid inclusions inside of diamond included in k rich cpx and understanding their relationships and mechanisms of formation the cpx grains contain in their cores lamellae i which are represented by topotactically oriented extremely thin lamellae of phlogopite that generally are two units cell wide but locally can be seen to be somewhat broader and microdiamond the core composition is ca sub 0 94 k sub 0 04 na sub 0 02 al sub 0 06 fe sub 0 08 mg sub 0 88 si sub 1 98 al sub 0 02 o sub 6 00 fluid inclusions rich in k and si are recognized in the core of the cpx having no visible connections to the lamellae i lamellar ii inclusions consist of micron size single laths of phlogopite and lens like quartzmore or slightly elongated phlogopite quartz intergrowths all are situated in the rim zone of the cpx the composition of the rim is ca sub 0 95 fe sub 0 03 na sub 0 02 al sub 0 05 fe sub 0 05 mg sub 0 90 si sub 2 o sub 6 and the rim contains more ca mg than the core with no k there such chemical tests support our microstructural observations and conclusion that the phlogopite lamellae i are exsolved from the k rich cpx precursor during decompression it is assumed that cpx precursor was also enriched in h sub 2 o because diamond included in the core of this cpx contains fluid inclusions the synchrotron ir spectra of such diamond record the presence of oh sup stretching and h sub 2 o bending motion regions lamellar ii inclusions are interpreted as forming partly because of modification of the lamellae i in the presence of fluid enriched in k fe and si during deformation of the host diopside the latter is probably related to the shallower stage of exhumation of the uhp marble this study emphasizes that in each case to understand the mechanism of lamellar inclusion formation more detailed studies are needed combining both compositional structural and three dimensional textural features of lamellar inclusions and their host less

ca eskola component in clinopyroxene experimental studies at high pressures and high temperatures in multianvil apparatus abstract in order to investigate the pressure implications of silica exsolution from clinopyroxene in ultrahigh pressure metamorphic rocks we have measured systematically the values of the calcium eskola ca esk component of clinopyroxenes over a considerable range of pressure and temperature a series of anhydrous experiments using powdered glass of composition wt sio2 59 29 al2o3 23 12 mgo 5 44 cao 10 37 and na2o 1 78 were conducted in a walker style multianvil apparatus in the p t range of 6 12 gpa and 900 1200 c we find that all clinopyroxenes synthesized in equilibrium with garnet kyanite and sio2 coesite or stishovite are cation deficient the highest value of the ca esk component 32 38 mol is reached at 6 gpa followed by slight decrease beginning at 8 gpa as the pyroxene progressively dissolves into garnet and more rapid decrease at 10 12 gpa when stishovite becomes stable in the assemblage comparison of our data with those available in the literature which all together covers a wider range of p and t shows that the ca esk activity depends strongly on bulk chemistry especially the silica content in general integrated analyses of published experiments on similar compositions show that the ca esk component rises rapidly to its highest values from 1 to 4 6 gpa then slightly decreases to 9 gpa followed by remarkable decrease to 15 gpa culminating in replacement of pyroxene by garnet in equilibrium with stishovite we conclude that the sio2 exsolution lamellae observed in clinopyroxenes from many ultrahigh pressure terranes can be explained by destabilization of the ca esk component during decompression however because silica solubility in clinopyroxene reaches its maximum at 6 8 gpa the practical upper pressure limit that can be inferred from this system is 200 250 km

formation and evolution of the tancheng lujiang wrench fault system a major shear system to the northwest of the pacific ocean abstract in the northwestern peri pacific tectonic belt on the continental margin of east asia there is a series of major faults trending ne nne i e the tancheng lujiang wrench fault system this fault system extends more than 5000 km with a width of 800 1000 km in the continent and a general strike of n30 e on the basis of the results of special research on the major faults and a lot of new information about the fault system the authors discuss early mesozoic triassic jurassic period sinistral transcurrent faulting in the different regions it is suggested that in the eastern continent of east asia the e w trending tectonic pattern of the paleozoic and beginning of the mesozoic appear in the nuclei of the sino korean continent massif and the yangtze continent massif these ancient continent massifs accreted both southwards and northwards which finally led to their collision and suturing to the angura continent massif later this tectonic framework was changed greatly by the tancheng lujiang wrench fault system along which left lateral horizontal displacement of 1000 1500 km took place since the mesozoic the dislocation and deformation of horizontal displacement of the tancheng lujiang fault system produced a series of structures trending ne nne in the continental margin which have greatly transformed the ancient structures of the preceding mesozoic this is an important diktyogenese in the continent of east asia during the horizontal displacement motion of these faults the folds nappes and decollement structures pull apart basins and subductions within the continent occurred in the continent at the same time some large sinistral ductile shear zones of horizontal displacement appeared the depth of the deformed horizon of ductile shear which belongs to simple shear was controlled by the tectonic element and distance away from the continental margin whilst the ductile shear deformation occurred the fault dynamo thermal metamorphism and the fault remelting took place and magma activity appeared within the continent in the mesozoic the fault zones used to be high energy zones and recently became the seismic belts within the continent formation of the tan lu fault system was due to the collision between the southern and northern blocks its large sinistral horizontal displacement in the mesozoic was closely related to the kula plate moving northwards therefore the continent disintegrated by shearing which produced some new terranes the shear edge belt was formed between the continent and ocean the change of relative motion between the continent and pacific ocean plate transformed the tan lu fault system into first extension and then compression clarifying the characteristics of the tan lu fault system is an important key in understanding the evolution of the east asia continent since the mesozoic

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

systematic analysis of crustal anisotropy along the karadere duzce branch of the north anatolian fault s u m m a r y we perform a systematic analysis of crustal anisotropy along and around the karadere duzce branch of the north anatolian fault naf which ruptured during the 1999 mw 7 4 izmit and mw 7 1 duzce earthquakes a method consisting of an iterative grid search for the best shear wave splitting parameters in sliding time windows is applied to 22 000 measurements recorded in the 6 month period after the izmit main shock based on objective criteria 6600 measurements are assigned high quality and used for further detailed analysis most stations near the rupture zone have fast polarization directions that are parallel to and change with the nearby fault strike the average delay times for ray paths that propagate along the rupture zone are larger than for the other paths these results suggest the existence of an approximately 1 km broad zone around the karadere duzce branch with fault parallel cracks or shear fabric however some fault zone fz stations record bimodal or scattered polarization directions while stations near large structural complexities e g branching and offsets show average fast polarization directions that are almost perpendicular to the local fault strike the average fast polarization directions from ray paths that propagate inside the almacik block south of the karadere duzce branch are neither parallel to the local fault strike nor to the expected regional maximum compressive stress direction the large overall spatial variations of the results imply that multiple structures and mechanisms contribute to the observed crustal anisotropy in our study area most stations do not exhibit a clear dependency of shear wave splitting delay time with increasing depth and hypocentral distance indicating that the anisotropy is confined primarily to the top 3 4 km of the crust using the observed average delay time at fz stations and assumed propagation distance of 3 5 km we estimate the apparent crack density in the damaged shallow fz rock to be approximately 7 per cent

why fast polarization directions of sks seismic waves are parallel to mountain belts abstract the splitting of sks waves shows that the fast velocity is commonly parallel to the trend of mountain belts this has been related to the anisotropy induced by frozen mantle flow at lithospheric depth when the flow direction is parallel to mountain belts this flow reflects a transcurrent motion parallel to the boundaries of the plates whose collision is responsible for mountain building considering the width of major shear zones transcurrent flow should not reorient the mantle fabric on a transverse width larger than 20 40 km unless the flow is partitioned between several major transcurrent shear zones therefore such a scale seems to be too small to explain the sks results another contribution to the observed mantle anisotropy is suggested to be the fabric induced by mantle flow during a prior event of rifting at the colliding plate boundaries which are derived from passive margins

constraints on the timing and duration of orogenic events by combined lu hf and sm nd geochronology an example from the grenville orogeny abstract grenville age basement rocks of the virginia blue ridge province preserve a record of over 150 million years of tectonism and crustal reworking associated with the construction of the supercontinent rodinia although the igneous origins of these rocks have been the subject of much previous work few studies have examined the complex metamorphic history of these rocks during the grenville orogeny to better constrain the timing and duration of metamorphism we determined lu hf and sm nd garnet and u pb zircon rim dates of garnet bearing rocks exposed in the virginia blue ridge multiple periods of mesoproterozoic metamorphism and deformation have been previously recognized with a major deformational event occurring before 1065 ma and subsequent local episodes occurring at least until 1000 ma zircon overgrowths and monazite dates suggest metamorphic events between 1050 and 960 ma our new lu hf garnet dates range from 1032 3 to 1019 3 ma new sm nd dates determined on the same garnet dissolutions define a younger range from 965 6 to 949 6 ma our zircon dates are consistent with previously measured ages 1090 940 ma and often overlap with the lu hf garnet dates from the same samples there is a large and systematic difference 60 80 m y between the garnet dates from the two isotopic systems although sm nd dates slightly younger than those recorded by lu hf are not uncommon the large discrepancies between the two systems that we see here are unusual the dated garnets are unzoned and yield lu hf dates that coincide with the zircon overgrowth dates sm nd ages overlap with the younger monazite dates we interpret the discrepancy in garnet dates as reflecting the difference in closure temperature between the two isotopic systems further these data require slow cooling following large scale crustal thickening after the assembly of rodinia during grenville orogenesis these data illustrate that the use of the coupled lu hf and sm nd chronometers in garnet can provide important constraints on the conditions of metamorphism during past orogenic events

alpe arami a peridotite massif from depths of more than 300 kilometers the abundance of fetio 3 and chromite precipitates in olivine of the alpe arami peridotite massif switzerland requires a much higher solubility for highly charged cations than is found in mantle xenoliths from depths of 250 kilometers three previously unknown crystal structures of fetio 3 were identified that indicate that the originally exsolved phase was the high pressure perovskite polymorph of ilmenite implying a minimum depth of origin of 300 kilometers these observations coupled with the unique lattice preferred orientation of olivine further suggest that the original composition of olivine may reflect that of a precursor phase perhaps wadsleyite that is stable only at depths greater than 400 kilometers

deep shallow coupling response of the cenozoic bohai bay basin to plate interactions around the eurasian plate abstract the bohai bay basin bbb is a meso cenozoic intracontinental petroliferous basin where four pre existing continental scale fault zones intersect in east china the bbb has experienced complex deformational and evolutionary history that records significant information about how the lithosphere there has responded to the coeval plate interactions around the eurasian plate and or their resulting deep crust mantle processes in the cenozoic now both deep and shallow deformational and evolutionary processes of the bbb have been well studied but many geodynamic mechanisms previously proposed for those processes are too general to explain them well based on various data and principles we first summarized both deep and shallow responses and processes of the bbb in the cenozoic and then matched reasonable geodynamic mechanisms for each of them the results show as follows the cenozoic differential reactivation of pre existing continental scale weak zones controlled the coeval deep shallow differential coupling evolution of the bbb the paleogene extension in the bbb was mainly caused by crust mantle processes triggered by the stagnant izanagi slab the cenozoic strike slip faulting was influenced by various plate interactions in which the oblique subduction and anticlockwise rotation of young pacific plate resulted in the complex pre oligocene strike slip history of the tan lu fault zone to the south of the zhang peng fault zone while the india eurasia collision has caused the intense and synchronous strike slip faulting of various faults there since 35 ma which was then weakened by the oblique subduction and clockwise rotation of young philippine sea plate in the neogene the compression resulting from both the india eurasia collision and the philippine sea plate s subduction contributed to the latest paleogene tectonic inversion in the bbb the arrival of mantle flow triggered by both the india eurasia collision and the stagnant slab of the pacific plate resulted in the tectonic reactivation there in the late neogene

oriented precipitates of quartz and amphibole in clinopyroxene of metabasites from the greek rhodope a product of open system precipitation during eclogite granulite amphibolite transition in metabasites from the high grade metamorphic kimi complex of the greek rhodope lamellae of quartz and amphibole were observed together with either rutile or titanite precipitates in the cores of clinopyroxene that underwent a prolonged exhumation history from eclogite via granulite and amphibolite facies conditions lamellae are crystallographically oriented along clinopyroxene cleavage planes 010 and interpreted as the result of precipitation during a two step intracrystalline compositional adjustment from an omphacite to a ca tschermak rich clinopyroxene and finally to diopside even though retrograde diffusion profiles develop in clinopyroxene the diffusion rates of highly charged ions like si4 al3 and ti4 are too slow so precipitates rich in si4 quartz al3 amphibole and ti4 rutile or titanite form as viable kinetic alternatives these precipitation reactions are not possible isochemically and additional exchange of ions with the matrix is required for precipitation to occur this exchange of mainly rapidly diffusing low charge ions is postulated to proceed by bulk diffusion self diffusion and does not require special pathways or fluid even though these may play an additional role during metamorphism this hitherto undescribed process of precipitation made possible by diffusional exchange with the matrix through an intact host crystal lattice is termed here as open system precipitation osp no prior ca eskola rich ultrahigh pressure uhp composition of omphacite is required to explain the observed precipitates it is proposed that many precipitation phenomena that were hard to explain in a closed system scenario can now be explained more readily by osp calculation of an original solid solution composition as in an uhp phase from precipitate and host mineral analyses becomes imprecise to the degree that bulk diffusion and hence osp have been important

a thinned lithospheric image of the tanlu fault zone eastern china constructed from wave equation based receiver function migration 1 we apply the newly proposed wave equation based receiver function poststack migration method to the northern china interior structure project broadband data to image the lithospheric structure of the tanlu fault zone area in eastern china our migration result reveals a 60 to 80 km thick present day lithosphere beneath the study region significantly thinned from the paleozoic lithosphere of 180 km the lithosphere asthenosphere boundary lab is coherently imaged along the 300 km east west profile displaying an arc like shape with its apex roughly coincident with the transverse location of the tanlu fault zone on the surface an obvious uplift from 36 km to 32 km of the moho is also clearly detected right below this fault zone the coincidence of the imaged moho uplift and the lab apex with the surface location of the tanlu fault zone provides seismological evidence for the steep geometry and deep penetration of the fault system and indicates that the tanlu fault zone might have acted as a major channel for anthenosphere upwelling during the mesozoic cenozoic continental extension and lithospheric thinning in eastern china frequency analysis and synthetic modeling suggest that both the moho and the lab are sharp and strong the latter in particular is constrained to have a 3 7 drop in s wave velocity over a depth range of 10 km or less such a rapid velocity change at the base of the lithosphere in the study region cannot be solely explained by thermal variation but likely reflects the presence of volatiles or melt in the asthenosphere or is partially attributed to the compositional contrast between the preserved depleted and dehydrated cratonic lithospheric veneer and the uplifted hydrated and fertile asthenospheric materials

diamond from the dabie shan metamorphic rocks and its implication for tectonic setting diamond occurs in ultrahigh pressure metamorphic rocks from dabie shan anhui province eastern china diamond bearing rocks include eclogite gamet pyroxenite and jadeitite diamond occurs in a mineral assemblage with coesite and jadeite the diamonds and diamondiferous rocks of dabie shan are interpreted to be the products of ultrahigh pressure metamorphism in the underthrust basement of the yangtze continental plate during the early mesozoic at greater than 4 0 gigapascals and 900 c this interpretation is based on the distribution of rock units the stability field of diamond and isotopic data indicating a crustal origin for the rocks most diamonds occur as euhedral inclusions in garnets and are 10 to 60 micrometers across although some are up to 700 micrometers across

shear wave splitting in the crust in north china stress faults and tectonic implications summary this paper uses data from the capital area seismograph network in north china nc currently the largest regional seismograph network in china to investigate the compressional stress orientations from the polarizations of faster shear waves pfs a parameter of shear wave velocity anisotropy data are restricted to earthquakes deeper than 5 km to reduce the influence of the heterogeneous uppermost few kilometres of the crust the results show that the regional compressional stress direction is ene wsw to nearly ew in nc which agrees with seismic mechanisms and drilling and gps data predominantly pfs are also nearly ew in nc and are influenced by faults geology and tectonics pfs orientations at stations on seismically active faults are consistent with the strike of strike slip faults and indicate large seismically active faults breaking or nearly breaking the free surface pfs orientations are influenced by both regional compressional stress and by nearby faults it is an effective way to study detailed spatial distribution of crustal compressional stress by shear wave anisotropy with dense seismograph network shear wave splitting may also be used to indicate hitherto unknown faults

supersilicic clinopyroxene and silica exsolution in uhpm eclogite and pelitic gneiss from the kokchetav massif kazakhstan abundant exsolved quartz rods occur in matrix clinopyroxene of eclogite from the kokchetav massif kazakhstan these rocks are diamond grade ultrahigh pressure uhp metamorphic rocks that recrystallized at p 6 gpa and t 1000 c zircon is an excellent container which effectively protects peak uhp metamorphic phases from retrogression therefore to ascertain the pre exsolution composition of the clinopyroxene we analyzed clinopyroxene inclusions in zircon of the eclogite and a diamond bearing biotite gneiss clinopyroxene in zircon has an excess of fe 3 vi al over iv al na k and calculated cation totals significantly less than 4 0 per six o atoms the stoichiometry of these pyroxenes can be reconciled if the ca eskola end member ca 0 5 0 5 alsi 2 o 6 is considered the zircon hosted clinopyroxene in the eclogite contains up to 9 6 mol of the ca eskola component and in the biotite gneiss up to 18 2 mol whereas the matrix clinopyroxene contains much less 1 3 mol on average recalculation of the composition of the matrix clinopyroxene prior to exsolution of quartz rods yields 6 8 mol ca eskola component which is consistent with the composition of the clinopyroxene inclusions in zircon we conclude that the ca eskola component in the peak metamorphic clinopyroxenes broke down by a retrograde reaction 2 ca 0 5 square 0 5 alsi 2 o 6 rightarrow caal 2 sio 6 3 sio 2 resulting in exsolution of the quartz rods in the matrix clinopyroxene subducted crustal and supracrustal rocks form the ca eskola clinopyroxene at high pressures and temperatures the vacancy containing clinopyroxene may have an important bearing on the physico chemical properties of the subducted slab at upper mantle depth

seismic signature of the collision between the east tibetan escape flow and the sichuan basin abstract gps displacement vectors show that the crust in east tibet is being squeezed in an easterly direction by the northward motion of the indian plate and the sichuan basin is resisting this stream and redirecting it mainly towards indochina the longmen shan containing the steepest rise to the high plateau anywhere in tibet results from the strong interaction between the east tibetan escape flow and the rigid yangtze block sichuan basin but the kinematics and dynamics of this interaction are still the subject of some debates we herein present results from a dense passive source seismic profile from the sichuan basin into eastern tibet in order to study the deep structure of this collision zone using p and s receiver function images we observe a sudden rise of the lithosphere asthenosphere boundary lab from 120 to 150 km beneath the sichuan basin and from 70 to 80 km beneath eastern tibet in contrast the depth of the crust mantle boundary moho increases from 36 to 40 km beneath the sichuan basin and from 55 to 60 km beneath eastern tibet the 410 km discontinuity is depressed below eastern tibet by about 30 km although the 660 remains at nearly the same depth throughout the lms from these observations we conclude that the mode of collision that occurs between tibet and the sichuan basin is very different to that found between india and tibet in southern tibet we observe in essence the subduction of the indian plate which penetrates northwards for several hundred kilometers under central tibet the very thin mantle part of the lithosphere beneath eastern tibet may indicate delamination or removal of the bottom of the lithosphere by hot asthenospheric escape flow this process leads to the exceptionally steep topography at the eastern tibetan margin as a result of gravitational buoyancy this view is supported by the very unusual depression of the 410 km discontinuity beneath eastern tibet which could be caused by the dynamics of the sub vertical downward asthenospheric flow

sm nd and rb sr dating of pyroxene garnetite from north dabie in east central china problem of isotope disequilibrium due to retrograde metamorphism abstract oxygen isotopes may serve as a guide to evaluate whether equilibria are preserved in the sm nd and or rb sr systems in eclogites that experience retrograde metamorphism this study demonstrates that sm nd rb sr and o isotopes for minerals from two portions of a clinopyroxene garnetite from north dabie in east central china record both equilibrium peak metamorphic conditions and disequilibrium conditions resulting from anhydrous retrograde metamorphism the minerals are homogeneous in terms of major elements and show no variation across the sample but clinopyroxene shows variable degrees of alteration pure primary mineral separates that are in o isotope equilibrium yield triassic sm nd isochron ages of 219 11 and 229 13 ma that match previously published zircon u pb ages for peak metamorphism thus the sm nd system in these minerals equilibrated during peak metamorphism and remained unaffected by retrograde metamorphism in contrast altered and impure clinopyroxenes that display oxygen isotope disequilibria fall off the sm nd isochron defined by the pure minerals implying that like oxygen the sm nd systems in these clinopyroxene were disturbed during retrogression no meaningful isochrons are found from the rb sr dating even in the pure primary minerals implying that these systems were disturbed during retrograde metamorphism and the diffusion rate of sr is faster than those of sm nd and o in mafic minerals our results suggest that the oxygen isotopes in metamorphic minerals can be used to evaluate the validity of sm nd mineral isochrons in dating peak metamorphic events in addition variations in the sm nd concentration and isotopic compositions of altered and impure clinopyroxenes from the two portions suggest heterogeneous effects on the minerals from external and internal fluids

quartz exsolution topotaxy in clinopyroxene from the uhp eclogite of weihai china abundant oriented silica precipitates of quartz 4 0 1 0 vol in part coexisting with calcic amphiboles 0 2 vol have been identified in clinopyroxenes of eclogites from the weihai area sulu ultrahigh pressure terrane eastern china electron backscatter diffraction ebsd analyses demonstrate that the majority 97 of quartz precipitates have topotactic relationships with their host clinopyroxenes three types of crystallographic topotactic relationship have been identified between quartz and host clinopyroxene 1 52 quartz precipitates share the same orientation for the c axes with 0001 qz 001 cpx 2 34 quartz precipitates share the same orientation for the a axes with 112 0 qz 001 cpx and 3 11 quartz precipitates share the same orientation for the s planes with 112 1 qz 100 cpx other quartz axes and planes disperse in large or small girdles around the shared axes or planes many quartz rods needles are elongated parallel to the 001 cpx with the long axes of quartz being either 0001 qz or 112 0 qz amphibole precipitates have also a strong crystallographic relationship with host clinopyroxene i e 100 amp 100 cpx 010 amp 010 cpx and 001 amp 001 cpx these results provide quantitative microstructural evidence supporting an exsolution origin for oriented quartz needles rods in clinopyroxene and demonstrate that the exsolution of quartz from clinopyroxene occurred within the stability field of quartz rather than coesite the oriented precipitates of quartz in part coexisting with calcic amphiboles in host clinopyroxene are probably promoted by supercritical fluid or partial melting during the early exhumation of eclogites our results suggest that oriented quartz precipitates in clinopyroxene cannot be used as an indisputable uhp indicator

silica precipitates in omphacite from eclogite at alpe arami switzerland evidence of deep subduction observations of oriented sio2 precipitates in omphacite from eclogite with tholeiitic basalt protolith bordering the alpe arami garnet peridotite massif ticino switzerland and petrological studies of the eclogitic mineral assemblages suggest that this rock was subjected to higher pressure metamorphism than previously realized we employed various calibrations of the fe2 mg exchange thermometer and calculations of equilibria with thermodynamic data considering the calcium tschermak s component caal2sio6 of garnet pyroxene pairs from these calculations it is concluded that the eclogitic lenses have recorded at least four stages of mineral growth corresponding to the following stage i prograde c 2 4 gpa 700 c stage iia maximum recorded grade c 7 0 gpa 1100 c stage iib retrograde c 3 7 gpa 900 c stage iii retrograde c 2 1 gpa 750 c because of the preservation of stage i a relatively rapid subduction and exhumation of alpe arami eclogite is suggested the exhumation path of the eclogitic rock is in good agreement with most exhumation paths inferred for the alpe arami garnet lherzolite proposed previously by several authors based upon a variety of different observations although the eclogite and peridotite exhumation paths may diverge at depths greater than 120 km

clinoenstatite in alpe arami peridotite additional evidence of very high pressure observations by transmission electron microscopy show that lamellae of clinoenstatite are present in diopside grains of the alpe arami garnet lherzolite of the swiss alps the simplest interpretation of the orientation crystallography and microstructures of the lamellae and the phase relationships in this system is that the lamellae originally exsolved as the high pressure c centered form of clinoenstatite these results imply that the rocks were exhumed from a minimum depth of 250 kilometers before or during continental collision

precipitates of α cristobalite and silicate glass in uhp clinopyroxene from a bohemian massif eclogite

seismic anisotropy and its relation with crust structure and stress field in the reggio emilia region northern italy summary shear wave splitting is measured at 14 seismic stations in the reggio emilia region above local background seismicity and two sequences of seismic events the good quality of the waveforms together with the favourable distribution of earthquake foci allows us to place strong constraints on the geometry and the depth of the anisotropic volume it is about 60 km 2 wide and located between 6 and 11 km depth inside mesozoic age carbonate rocks the splitting results suggest also the presence of a shallower anisotropic layer about 1 km thick and few km wide in the pliocene quaternary alluvium above the mesozoic layer the fast polarization directions n30 e are approximately parallel to the maximum horizontal stress 1 is ssw nne in the region and also parallel to the strike of the main structural features in the reggio emilia area the size of the delay times suggests about 4 5 per cent shear wave velocity anisotropy these parameters agree with an interpretation of seismic anisotropy in terms of the extensivedilatancy anisotropy model which considers the rock volume to be pervaded by fluid saturated microcracks aligned by the active stress field we cannot completely rule out the contribution of aligned macroscopic fractures as the cause of the shear wave anisotropy even if the parallel shear wave polarizations we found are diagnostic of transverse isotropy with a horizontal axis of symmetry this symmetry is commonly explained by parallel stress aligned microcracks

near fault anisotropy following the hector mine earthquake 1 we present anisotropy measurements from shear wave splitting along the hector mine rupture zone six major arrays were deployed in four locations in the year following the m7 1 hector mine earthquake the dense station coverage wide distribution of the arrays and repeated deployments show a clear predominant fast direction and spatial variation of splitting along the fault but no resolvable temporal variations we determined splitting parameters using an automated cross correlation method discarding fast directions with initial source polarizations near crack parallel or perpendicular directions only two of the four array locations give reliable measurements of anisotropy at depth fast directions and delay times are constant across the 1 km wide array length however some spatial variation of splitting is observed along fault strike delay times decrease from north to south with greater splitting in areas of higher slip a change in splitting parameters along fault strike likely reveals the orientation of cracking in the near fault region during a major quake average fast directions are between fault parallel and the regional maximum compressive stress direction we do not see temporal evolution in anisotropy however measured splitting suggests a heterogeneous stress field partially created during rupture that persists over at least a 1 year timescale

oriented quartz calcic amphibole inclusions in omphacite from the saualpe and pohorje mountain eclogites eastern alps an assessment of possible formation mechanisms based on ir and mineral chemical data and water storage in eastern alpine eclogites abstract the composition of mineral phases and their modal proportions have been determined for three representative eoalpine eclogites from the saualpe type locality eastern austria sample skp31 and the pohorje massif slovenia cm31 03 and cm15 01 using electron microprobe laser icp ms ir spectroscopy and modal analysis to evaluate possible mechanisms for the formation of composite oriented calcic amphibole quartz inclusions cois in omphacite and to assess the relative importance of hydrous and nominally anhydrous phases as h 2 o carriers in these eclogites for omphacites in cm31 03 with a zonal distribution of cois a comparison of water and trace element concentrations of areas containing cois and those free of cois and a comparison with the trace element concentration of calcic amphibole indicate that cois have formed through an open system alteration of clinopyroxene and not through a closed system exsolution process in sample skp31 both textural and mineral chemical evidence suggests that cois did not form by exsolution involving a ca eskola component in clinopyroxene but formed by progressive growth under eclogite facies p t conditions and prior to the onset of retrogressive symplectite formation analogous to the formation of poikiloblastic quartz calcic amphibole grains in the matrix bulk h 2 o contents of the eclogites are between ca 750 and 2150 ppm with 6 25 of the total water contributed by nominally anhydrous minerals nams because of high modal amounts of 37 65 omphacite is the major nominally anhydrous water carrier containing 145 580 ppm h 2 o with significant concentration variations on a thin section scale due to their very low h 2 o concentrations of 2 o content calcic amphibole forming part of cois may be a major carrier of h 2 o as evidenced by cm31 03 containing both cois and texturally primary calcic amphibole in this sample calcic amphibole of the cois contributes 63 whereas primary calcic amphibole only accounts for 13 of the bulk water the relative order of h 2 o concentrations in nams is h 2 o rutile h 2 o omphacite h 2 o garnet for cm31 03 and cm15 01 and h 2 o rutile h 2 o omphacite h 2 o garnet in skp31

an experimental investigation of hydroxyl solubility in jadeite and na rich clinopyroxenes the solubility and incorporation mechanisms of water in synthetic water saturated jadeite and na rich clinopyroxenes have been experimentally investigated infrared spectra for water saturated jadeite synthesised from 2 0 to 10 gpa show two prominent sharp peaks at 3 373 and 3 613 cm 1 together with several weaker features in the oh stretching region indicating that there are at least 5 distinct modes of hydrogen incorporation in the structure water solubility in pure jadeite reaches a maximum of about 450 ppm by weight at 2 gpa and slowly decreases with increasing pressure to about 100 ppm at 10 gpa solubility can be described by the function coh a fh2o0 5 exp p vsolid rt where coh is water solubility in ppm h2o by weight a is 7 144 ppm bar0 5 fh2o is water fugacity and vsolid 8 019 cm3 mol is the volume change of the clinopyroxene upon incorporation of oh jadeite provides a good model for understanding hydrogen incorporation mechanisms in more complex omphacite compositions assignment of absorption bands in ir spectra verifies the importance of cation vacancies on the m2 site in providing mechanisms for hydrogen incorporation however results also suggest that substitution of lower valency cations onto the m1 site may also be important solid solution of jadeite with diopside and in particular with ca eskola component leads to a drastic increase of water solubility and the bulk composition has a more important effect on the capacity of omphacite to store water than pressure and temperature omphacite is expected to be the major carrier of water in a subducted eclogite after the breakdown of hydrous minerals

no direct correlation of mantle flow beneath the north china craton to the india eurasia collision constraints from new sks wave splitting measurements summary the long range effects of the ongoing india eurasia collision and their role in the cenozoic tectonic evolution of eastern china are not well understood here we investigate deformation beneath the north china craton and the qilian mountain region between the haiyuan and kunlun faults both of which are located immediately to the northeast of the tibetan plateau we present teleseismic shear wave splitting parameters from 299 broad band stations these stations provided good spatial coverage in mapping upper mantle deformation beneath the north china craton and the qilian mountain the backazimuth dependence of fast directions fromstationslocatedwithintheqilianmountainregionindicatesacomplexanisotropypattern a multilayer anisotropy and or anisotropy with a dipping symmetry axis may exist beneath the qilian mountain the overlapping or underthrusting of material evident in anisotropy from this region may be a long range effect of the india eurasia collision the central north china craton showed spatially coherent fast directions and the shear wave velocity anomalies within the upper mantle these patterns suggest horizontal deflection of a regional mantle upwelling possibly originating from the mantle transition zone anisotropy with large delay times was observed in the vicinity of shanxi rift system high lnvs lnvp values for the upper mantle indicate that anisotropy in this area is enhanced by vertically aligned melt filled fissures our results suggest that the dynamic processes beneath the central north china craton are not directly related to the india eurasia collision

collision between the north and south china blocks a crustal detachment model for suturing in the region east of the tanlu fault a crustal detachment model based on the interpretation of linear aeromagnetic anomalies surface geological observations and deep seismic profiles is proposed for the continent continent collision between the north and south china blocks east of the tanlu fault the model suggests that during the mid mesozoic collision between the two continental blocks the upper crust of the south china block in the subei yellow sea region was detached from the lower crust and thrust over the north china block for 400 km whereas the lower part of the lithosphere was subducted under the north china block along a subsurface suture running east from nanjing the sinistral offset of the qinling suture by the tanlu fault is only 110 120 km in the deep crust much less than previously suggested

modelling the compliance of crustal rock ii response to temporal changes before earthquakes summary there have been several claims that seismic shear waves respond to changes in stress before earthquakes the companion paper develops a stress sensitive model ape for the behaviour of low porosity low permeability crystalline rocks containing pervasive distributions of fluid filled intergranular microcracks and this paper uses ape to model the behaviour before earthquakes modelling with ape shows that the microgeometry and statistics of distributions of such fluid filled microcracks respond almost immediately to changes in stress and that the behaviour can be monitored by analysing seismic shear wave splitting the physical reasons for the coupling between shear wave splitting and differential stress are discussed in this paper we extend the model by using percolation theory to show that large crack densities are limited at the grain scale level by the percolation threshold at which interacting crack clusters lead to pronounced increases in rock matrix permeability in the simplest formulation the modelling is dimensionless and almost entirely constrained without free parameters nevertheless ape modelling of the evolution of fluid saturated rocks under stress reproduces the observed fracture criticality and the narrow range of shear wave azimuthal anisotropy in crustal rocks it also reproduces the behaviour of temporal variations in shear wave splitting observed before and after the 1986 m 6 north palm springs earthquake southern california and several other smaller earthquakes the agreement of ape modelling with a wide range of observations confirms that fluid saturated crystalline rocks are stress sensitive and respond to changes in stress by critical fluid rock interactions at the microscale level this means that the effects of changes in stress and other parameters can be numerically modelled and monitored by appropriate observations of seismic shear waves

multiple breakdown and chemical equilibrium of silicic clinopyroxene under extreme metamorphic conditions in the kontum massif central vietnam clinopyroxene in ultrahigh temperature mafic granulites from the kontum massif in central vietnam records multiple metamorphic stages manifested as exsolution textures quartz rods and orthopyroxene hornblende plagioclase needles and as symplectitic intergrowths involving clinopyroxene plagioclase these textures suggest a metamorphic evolution characterized by decompression and subsequent cooling from eclogite facies to amphibolite facies conditions through ultrahigh temperature conditions quartz rods in clinopyroxene and clinopyroxene plagioclase symplectites were formed under eclogite conditions prior to ultrahigh temperature metamorphism the orthopyroxene hornblende plagioclase needles in clinopyroxene are regarded as cooling products after ultrahightemperature metamorphism recalculated compositions of precursor clinopyroxene show supersilicic composition during the metamorphic evolution the chemical composition varies from silicic ca eskola rich via sodic jadeite rich to aluminous ca tschermak rich compositions presence of supersilicic clinopyroxene suggests that the granulite decompressed from possible ultrahigh pressure conditions ca 800 900 c at 2 3 gpa preceding the ultrahigh temperature stage 1050 c at 1 3 gpa which provide strong constraints on the tectonic evolution of the indochina region and it also provides insights on crustal exhumation at a continental collision zone another significant aspect of this study is that the breakdown textures of clinopyroxene and its chemical variations may provide important information in establishing pre and post peak evolution especially for extremely high temperature or high pressure metamorphic rocks

mantle flow pattern and geodynamic cause of the north china craton reactivation evidence from seismic anisotropy 1 we investigate the mantle flow pattern and geodynamic cause of north china craton ncc reactivation using shear wave splitting measurements from 140 broadband stations using a newly developed method for simulating wave propagation in a two dimensional anisotropic medium we first examined the influence of sedimentary structures on sks splitting measurements the simulations show that a sedimentary layer whether isotropic or anisotropic significantly influences the waveform however the shear wave splitting parameters can be retrieved with negligible errors as a result this study included new splitting measurements at stations which were deployed within basins and not used previously among 121 broadband stations that contribute valid splitting results 55 stations are newly added which were deployed within basins this significantly improved the sampling coverage on the ncc the complicated spatial patterns of the splitting parameters indicate that complex upper mantle deformation has occurred in the ncc to obtain both deep kinematic and geometric information we interpret our splitting measurements in the context of new high resolution tomographic results for the ncc by comparing our observations with three end member conceptual models upwelling wedge flow and lithospheric delamination we found that the observed anisotropy pattern beneath the ncc is not completely consistent with any of them thus we prefer a hybrid mantle flow model where the subduction of the pacific plate causes a mantle wedge flow beneath the eastern archean block and a regional upwelling beneath the central block which has been imaged as a low velocity anomaly in seismic tomography thus we speculate that the subduction of the pacific plate compared to the ncc yangtze craton amalgamation and the india eurasian collision is most likely the geodynamic cause of the reactivation of the ncc eastern block during the late mesozoic to cenozoic

experimental investigation of eclogite rheology and its fabrics at high temperature and pressure eclogite plays an important role in mantle convection and geodynamics in subduction zones an improved understanding of processes in the deeper levels of subduction zones and collision belts requires information on eclogite rheology however the deformation processes and associated fabrics in eclogite are not well understood incompatible views of deformation mechanism have been proposed for both garnet and omphacite we present here deformation behaviour of eclogite at temperatures of 1027 1427 c confining pressures of 2 5 3 5 gpa and strain rates of 1 10 5 s 1 to 5 10 4 s 1 we obtained a power law creep for the high temperature and pressure deformation of a dry eclogite 50 vol garnet 40 omphacite and 10 quartz with a 103 3 1 0 n 3 5 0 4 e 403 30 kj mol 1 and v 27 2 cm3 mol 1 the two principal minerals of eclogite have greatly different strengths progressive increase of garnet results in a smooth increase in strength analysis by electron back scattered diffraction shows that 1 garnet displays pole figures with near random distributions of misorientation angle under both dry and wet conditions 2 omphacite shows pronounced lattice preferred orientations lpos suggesting a dominant dislocation creep mechanism further investigation into the water effects on eclogite show 3 water content does not influence the style of omphacite fabric but increases slightly the fabric strength 4 grain boundary processes dominate the deformation of garnet under high water fugacity or high shear strain conditions yielding a random lpo similar to that of non deforming garnet despite the strong shape preferred orientation spo observed 110 001 slip may dominate the deformation of rutile quartz displays complicated and inconsistent lpos in eclogite these results are remarkably similar to observations from deformed eclogites in nature

quartz exsolution in clinopyroxene is not proof of ultrahigh pressures evidence from eclogites from the eastern blue ridge southern appalachians u s a oriented quartz needles in clinopyroxene have become one of the diagnostic indicators of ultrahigh pressure uhp metamorphism the presence of apparently exsolved quartz is taken as evidence of decompression of a non stochiometric ca eskola component ca0 5 0 5alsi2o6 caes that is presumed to be stable only at uhp conditions eclogite from the eastern blue ridge north carolina contains clinopyroxene jd20cats5ac5caesdi65hd5 with oriented needles of quartz and calcic amphibole that appear to have exsolved together the quartz amphibole intergrowths are surrounded by 1 5 m haloes of neoformed pyroxene jd10cats10ac5caesdi70hd5 the modes of quartz amphibole and clinopyroxene haloes were determined using bse images and reintegrated with the host clinopyroxene viewing the quartz and amphibole needles down the c axis of the pyroxene host provides a better estimate of their proportions than in prismatic sections reintegrated pyroxene compositions were nearly identical to the analyzed host pyroxene with no caes component clinopyroxene with caes solid solution has been repeatedly synthesized at uhp conditions however examination of the phase equilibria usually cited as evidence for caes stability at conditions of 25 kbar shows that clinopyroxene with 10 mol caes is stable well within the quartz field and provides a pressure minimum similar to the albite jadeite quartz barometer exsolution of quartz and associated amphibole is commonplace in clinopyroxene from the blue ridge eclogite that lacks coesite or other evidence for uhp metamorphism the presence of a diluted 5 10 caes component in clinopyroxene does not require uhp conditions

complex subduction and small scale convection revealed by body wave tomography of the western united states upper mantle abstract new high resolution p and s wave tomography of the united states upper mantle from the pacific coast to the great plains reveals strong multi scale heterogeneity closely correlated with tectonic and magmatic activity we invert teleseismic travel time residuals from the earthscope transportable array and more than 1700 additional temporary and permanent stations for 3 d velocity perturbations to a depth of 1000 km the inversion uses recent advances in western u s crust models to better isolate the mantle component of travel time residuals and frequency dependent 3 d sensitivity kernels to map travel time residuals measured in multiple frequency bands into velocity structure in addition to separate v p and v s models we jointly invert the two datasets for v p v s perturbations by imposing a smoothness constraint on the ln v s ln v p field the joint inversion helps us identify regions where partial melt is probable the amplitude of v p v s and v p v s variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and the occurrence of widespread small scale convection partially molten mantle is inferred beneath yellowstone and the eastern snake river plain srp the salton trough and the clear lake volcanic field the inferred depth extent of partial melt is consistent with a generally hydrated upper mantle and elevated temperatures beneath the eastern srp and yellowstone despite continuous subduction since the cretaceous the distribution of sub lithospheric high velocity anomalies is dissected similar to other recent studies based on our new tomography models western u s geologic history and plate tectonic reconstructions we infer patchy and incomplete removal of the flat subducting laramide slab and slab tearing associated with eocene accretion in the northwestern u s

a possible uhp eclogite in the leaota mts south carpathians and its history from high pressure melting to retrograde inclusion in a subduction melange abstract an unusual eclogite pod containing impure jadeite to omphacite pyroxene with anti phase domain apd visible under the optical microscope is hosted in a medium grade matrix in the leaota massif textural relationships support the hypothesis of garnet formation by peritectic reactions involving high jadeite clinopyroxene while inclusions entrapped in garnet mainly a wide range of amphibole compositions seemingly crystallised from a fluid phase in disequilibrium with the garnet host thermobarometric estimates indicate uhp ht peak conditions 2 8 3 2 gpa 780 840 c inconsistent with those of the associated rocks followed by a complicated exhumation path the p t array of the retrograde path suggests syn subduction exhumation and tectonic inclusion in a melange with a matrix derived from the accretionary wedge partial melting of the subducting slab which induces tectonic weakening may be an effective mechanism in detaching and enclosing small eclogite pods in metamorphic terrains

ultrahigh pressure metamorphism and multistage exhumation of eclogite of the luotian dome north dabie complex zone central china evidence from mineral inclusions and decompression textures abstract the studied eclogite is located in the luotian dome in the southwestern part of the north dabie complex zone ndz central china and is a portion of deeply subducted mafic lower continental crust of the south china block petrologic analysis suggests that the eclogite underwent ultrahigh pressure uhp and high pressure eclogite facies metamorphism and subsequent hp granulite facies overprinting and amphibolite facies retrogression during continental subduction and exhumation as a result multiple decompression textures were produced suggesting a multistage exhumation history from eclogite to granulite to amphibolite facies conditions a striking feature of this eclogite is the widespread exsolution of different phases oriented needles of rutile clinopyroxene amphibole apatite in garnet quartz amphibole hypersthene sodic plagioclase in clinopyroxene and pyrrhotite in apatite most importantly we provide for the first time conclusive evidence of quartz pseudomorphs after coesite in garnet and relic coesite confirmed by in situ raman spectroscopy in zircon in addition there are two groups of apatite one is fluor f apatite that occurs as inclusions in garnet or in matrix which is rich in f 3 wt lakes exsolution features and formed at uhp metamorphic conditions the second is associated with the oriented pyrrhotite exsolution and formed during decompression microtextural and petrologic analysis suggests that the eclogites from the southwestern segment of the ndz are similar to eclogites from the northern segment of the ndz e g huangweihe and baizhangyan and underwent uhp metamorphism with a peak pressure 5 7 gpa these results combined with published geochronological data imply that the ndz experienced triassic uhp metamorphism as a coherent unit

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

natural occurrence of keatite precipitates in uhp clinopyroxene from the kokchetav massif a tem investigation we report the first natural occurrence of keatite also known as silica k discovered as a precipitate in the core of ultrahigh pressure uhp clinopyroxene cpx within garnet pyroxenite from the kokchetav massif kazakhstan high resolution transmission electron microscopy and electron diffraction demonstrate that sub micrometer and nano scale keatite precipitates have a definite crystallographic relationship with the host pyroxene diopside di 90 cpx 100 keatite 100 and cpx 010 keatite 001 clinopyroxene provides a template for keatite nucleation due to the close structural relationship and excellent lattice match between the diopside and keatite we propose that keatite micro precipitates are formed in localized low pressure micro environments produced as a result of exsolution of extra silica and vacancies held within uhp host diopside and stabilized by the pyroxene lattice low density metastable keatite and its relationship to the host pyroxene likely reflects the important influence of pyroxene precipitate interfacial energy on the micro and nano scales in controlling the nature of exsolved phases in exhumed uhp minerals

crustal structure beneath china from receiver function analysis 1 we collected and processed a large amount of high quality broadband teleseismic waveform data recorded by the 48 chinese national digital seismic network stations to estimate large scale lateral variations of crustal thickness and vp vs ratio hence poisson s ratio beneath china a statistical method was used to select mutually coherent receiver functions at each station which yielded over 200 traces for most of the stations with the conventional h the crustal thickness and vp vs ratio approach there is a large trade off between h and consequently multiple maxima are frequently observed in the h domain we introduced a weight function that measures the coherence between the p to s conversion and the reverberation phases at each h grid to reduce the trade off a 4th root stacking method was further applied to reduce uncorrelated noise relative to the linear stack these modifications turned out to be very effective in reducing the h trade off and yielded reliable estimates of crustal thickness and vp vs ratio the crust beneath eastern china is as thin as 31 33 km and the underlying moho is relatively flat and sharp in the western part of china the crust is considerably thicker and shows large variations the moho is observed at about 51 km depth along the tian shan fold system and about 84 km deep beneath the central part of the tibetan plateau the transition occurs at the so called n s belt between about 100 and 110 e which is featured by unusually high seismicity and large gravity anomalies the average vp vs ratio over the mainland china crust is about 1 730 0 249 significantly lower than the global average 1 78 0 27 of the continental crust this lower vp vs ratio may suggest a general absence of mafic lowermost crustal layer beneath china

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

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

genesis of ilmenite rods and palisades along humite type defects in olivine from alpe arami rod shaped ilmenite inclusions in olivine have been observed in numerous peridotite lenses of the adula cima lunga unit central alps the rods are elongated parallel to 010 ol and identical in size shape and concentration to those reported from garnet peridotite at alpe arami aa tem investigation of the rods at aa cima di gagnone cdg and monte duria md demonstrates that the titanate inclusions correspond to a mg bearing ilmenite with exclusively a trigonal crystal structure no other fetio3 polymorphs have been identified the shape of the ilmenite inclusions concurs with minimum topotactic misfits between the lattices of olivine and mg bearing ilmenite optical microscopy reveals that the ilmenite rods are not randomly distributed within olivine but have at all localities a preferred alignment parallel to 001 ol forming palisades tem images of the ilmenite palisades at aa and cdg show that they are controlled by the presence of 001 planar defects in olivine along which the rods nucleated the defects have been characterised by high resolution tem and identified as 4 4 a wide humite type layers i e mg2sio4 mg oh f 2 intergrown within olivine in such layers substitution of up to 50 mol mgh2 by ti4 is possible assuming ti saturated humite slabs the maximum density of 001 faults observed at aa approximately 1900 mm 1 can accommodate about 170 ppm tio2 in olivine this represents about half the values obtained by uv laser ablation icp ms and empa analysis evidence for annealing of the defects however suggests that only parts of the defects are still preserved the formation of ilmenite rods is considered to be a consequence of the breakdown of isolated humite layers in olivine the apparent tio2 content in olivine at aa cdg and md is regarded as a function of the ti humite defect density

relict coesite exsolution in omphacite from western tianshan eclogites china exsolution rods of relict coesite together with quartz were identified in omphacite in eclogites from western tianshan china they are oriented along the c axis of the host clinopyroxenes and have grain size up to 30 m long and 2 3 m wide raman spectra of exsolved lamellae yield consistent but weak bands at 521 270 181 151 and 118 cm 1 typical for coesite in addition to those of quartz and the host omphacite such occurrences together with textured observations suggest a two stage evolution of sio 2 exsolution rods in omphacite lamellae of coesite were apparently exsolved from supersilicic omphacite at p 5 0 gpa and the transformation from coesite to quartz occurred during retrograde metamorphism

the solubility of tio2 in olivine implications for the mantle wedge environment abstract one characteristic of many subduction zone garnet peridotites is that they contain titanium bearing phases not otherwise found in mantle rocks in particular titanoclinohumite tich and or its breakdown assemblage consisting of symplectic intergrowths of olivine and ilmenite is common in many of these bodies the alpe arami garnet lherzolite of the lepontine alps while lacking tich displays instead large numbers of fetio 3 rod shaped precipitates in the oldest generation of olivine amounting to approximately 1 by volume indicating that at some time in its past the peridotite experienced conditions under which the solubility of tio 2 in olivine was greater than 0 6 wt in order to test the hypothesis that the environment of very high solubility of tio 2 in olivine is to be found at very high pressures we have conducted experiments on lherzolite compositions with added ilmenite at pressures between 5 and 12 gpa and temperatures of 1350 1700 k our results on anhydrous compositions show that whereas solubility of tio 2 was not detected in olivine at 5 gpa 1400 k where it coexists with rutile when rutile disappeared from the paragenesis the solubility climbed to 0 5 wt at 8 and 10 gpa and to 1 wt at 12 gpa all at 1600 k these results support our previous interpretations from titanate morphology and abundance that the alpe arami massif has surfaced from depths 300 km corresponding to pressures 10 gpa but remove the need to suggest a deeper origin and possible precursor phase such as wadsleyite they also support the hypothesis that garnet peridotites with unusual ti bearing phases reflect a unique mantle environment occurring in the mantle wedge overlying subduction zones 2000 elsevier science b v all rights reserved

structure of crust and upper mantle beneath the ordos block and the yinshan mountains revealed by receiver function analysis a temporary seismological network of broadband three component stations has been deployed n s to investigate the crust and upper mantle structure across the ordos block and the yinshan mountains p wave receiver functions reveal the moho depth to be about 41 km beneath the central ordos block and down to 45 km beneath the northern ordos block a slight uplifting to 42 43 km beneath the hetao graben increasing to 47 48 km beneath the yinshan mountains and then decreasing to 44 km beneath the northern yinshan mountains along the profile in the ordos block the crustal vp vs ratio about 1 80 south to the hetao graben differs from that about 1 75 beneath the center ordos block the crustal vp vs ratio is significantly lower about 1 65 1 70 beneath the yinshan mountains the p wave receiver function migration imaging suggests relatively flat discontinuities at 410 and 660 km indicating the lack of a strong thermal anomaly beneath this profile at these depths and a low s wave velocity anomaly in the upper mantle beneath the hetao graben we suggest that the low s wave velocity anomaly may be attributable to heat and that the thermal softening advances the evolution of the hetao graben while the lower crustal ductile flows transfer from the hetao graben to the northern ordos block resulting in crustal thickening

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

global mantle flow and the development of seismic anisotropy differences between the oceanic and continental upper mantle 1 viscous shear in the asthenosphere accommodates relative motion between earth s surface plates and underlying mantle generating lattice preferred orientation lpo in olivine aggregates and a seismically anisotropic fabric because this fabric develops with the evolving mantle flow field observations of seismic anisotropy can constrain asthenospheric flow patterns if the contribution of fossil lithospheric anisotropy is small we use global viscous mantle flow models to characterize the relationship between asthenospheric deformation and lpo and compare the predicted pattern of anisotropy to a global compilation of observed shear wave splitting measurements for asthenosphere 500 km from plate boundaries simple shear rotates the lpo toward the infinite strain axis isa the lpo after infinite deformation faster than the isa changes along flow lines thus we expect the isa to approximate lpo throughout most of the asthenosphere greatly simplifying lpo predictions because strain integration along flow lines is unnecessary approximating lpo with the isa and assuming a type fabric olivine a axis parallel to isa we find that mantle flow driven by both plate motions and mantle density heterogeneity successfully predicts oceanic anisotropy average misfit 13 continental anisotropy is less well fit average misfit 41 but lateral variations in lithospheric thickness improve the fit in some continental areas this suggests that asthenospheric anisotropy contributes to shear wave splitting for both continents and oceans but is overlain by a stronger layer of lithospheric anisotropy for continents the contribution of the oceanic lithosphere is likely smaller because it is thinner younger and less deformed than its continental counterpart

global patterns of azimuthal anisotropy and deformations in the continental mantle summary we present a summary of measurements of azimuthal anisotropy in the continental mantle based on the sks technique and performed mostly with the active participation of the authors the directions of polarization of the fast quasi shear wave and the time delays between the quasi shear waves are obtained at nearly 70 locations in all continents except antarctica these data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the mantle the depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km the fast directions in sks do not show clear correlation with the fast directions of the teleseismic p at the same seismograph stations in the regions of present day convergence the fast direction of anisotropy usually aligns with the plate boundary this correlation implies that the direction of shortening is the same in the crust and the upper mantle in the regions of rifting the inferred direction of mantle flow usually aligns with the direction of extension in the crust outside the regions of recent tectonic activity we most likely observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere on a global scale in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy the correlation is especially strong in central and eastern parts of north america a clear absence of any evidence of large scale azimuthal anisotropy in the data of long range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in sks is generated mainly at greater depths in the asthenosphere orientation of olivine at these depths reflects recent and present day flow in the mantle rather than processes of a distant geologic past

quantitative 3d measurement of ilmenite abundance in alpe arami olivine by confocal microscopy confirmation of high pressure origin a critical aspect of the debate about the origin and conditions of metamorphism of the alpe arami aa peridotite is the disagreement over how much ilmenite is contained in the older generation of olivine and therefore how much tio 2 might have been dissolved at high pressure and temperature we have now determined quantitatively the 3 dimensional distribution of ilmenite in aa olivine by confocal laser scanning microscopy clsm the clsm measurements show an average concentration of 0 31 vol of ilmenite in olivine with individual grains containing up to 1 2 vol this translates into average and maximum concentrations of 0 23 and 0 9 wt tio 2 in olivine respectively and confirms the original estimation of maximum concentration of 1 vol tio 2 the vast majority of ilmenite in aa olivine is distributed randomly although topotactically oriented and in all cases is accompanied by chromite in a ratio of 4 1 these observations are consistent with an origin of the ilmenite and chromite by exsolution from an olivine solid solution at p 9 12 gpa and temperatures above the stability field of titanian clinohumite but are not consistent with suggested breakdown of titanian clinohumite combining these results with other recent findings suggests that exsolution followed deformation under relatively high fugacity of h 2 o and that the high solubility of tio 2 is probably explained by pressure induced accommodation of ti in the tetrahedral site of silicates

the upper mantle discontinuity structure beneath eastern china 1 we apply a common conversion point stacking technique of receiver functions from the north china interior structure project ncisp to image the upper mantle discontinuity structure beneath eastern china the stacking results of dense receiver functions reveal a prominent discontinuity structure the appearance of the 410 km discontinuity is sharp and consistent with little influence by western pacific subduction slabs the 520 km discontinuity is relatively weak and consists of splitting phases at depths ranging from 500 km to 550 km as for the 660 km discontinuity double discontinuities and a narrow depression zone are detected near a depth of 660 km indicating a more complex structure there phase transformations in non olivine components such as from garnet to perovskite seem to be a more plausible interpretation for a complex 660 km discontinuity in this region

azimuthal anisotropy in the earth from observations of sks at geoscope and nars broadband stations abstract we have analyzed observations of low frequency sks waves recorded on horizontal components at several broadband stations of the geoscope and nars arrays splitting of sks is consistently observed and in good agreement with a hypothesis of azimuthal anisotropy due to transverse anisotropy with a horizontal axis in the upper mantle beneath the receivers we have designed an inversion method which takes advantage of observations made simultaneously at different azimuths and which permits us to obtain estimates of the direction of fast axis of anisotropy as well as the time delay between the orthogonal quasi shear waves in general the directions of fast velocity obtained both for stations around the atlantic and around the pacific appear to be related to directions of past and present plate motions

zircon u pb geochronology and nd pb isotope geochemistry of blue ridge basement in the eastern great smoky mountains u s a implications for the proterozoic tectonic evolution of the southeastern laurentian margin the mesoproterozoic to paleozoic history of the southeastern laurentian margin involved repeated collisional and accretionary tectonomagmatic events that reworked and recycled older continental crust of preceding events the great smoky mountains basement complex gsmbc within the southern appalachian blue ridge exposes complexly deformed orthogneiss and paragneiss that preserve a record of laurentian margin evolution from ca 1 9 ga to 450 ma the gsmbc consists primarily of 1 1 34 to 1 31 ga pre elzevirian granodioritic orthogneiss and entrained mafic xenoliths that represent some of the oldest crust in appalachian grenville massifs correlated with pre elzevirian crustal components in the adirondack green mountains new jersey highlands and french broad massifs 2 ca l 15 to 1 05 ga augen and granitic orthogneiss produced during shawinigan and ottawan phases of grenville age magmatism and metamorphism respectively and 3 paragneiss derived from protoliths with either grenville age 1 1 1 0 ga or post grenville neoproterozoic depositional ages based on presence absence of ca 1 0 ga metamorphic zircon and 1 9 to 1 1 ga detrital zircon all lithologies experienced taconian metamorphism and variable migmatization pre ottawan paragneiss exhibits major detrital zircon ages modes at 2 0 to 1 6 and 1 4 to 1 3 ga that require a component of older proterozoic crust in the sediment source region detrital zircon grains in post ottawan paragneiss exhibits the full spectrum of grenville age modes that correlate with the five phases of grenville magmatic metamorphic events in eastern laurentia these paragneiss samples also contain scattered 750 to 600 ma detrital zircon grains that constrain their maximum depositional age to late neoproterozoic the sedimentary protoliths of the latter paragneiss consist largely of detritus from exhumation of all grenville crustal age components during post orogenic exhumation and crustal extension leading up to late neoproterozoic breakup of rodinia most zircon u pb age systematics exhibit variable discordance that can be attributed to disturbance of the u pb system and or new zircon growth during either high grade ottawan ca 1 04 ga or taconian ca 0 46 ga regional metamorphism and migmatization neodymium tdm model ages for granodioritic orthogneiss and paragneiss range from 1 8 to 1 6 ga indicating that the rocks were derived from recycling of proterozoic crust that is they are not juvenile consistent with the 1 9 to 1 6 ga detrital zircon grains in pre ottawan paragneiss and with 1 8 to 1 7 ga inherited zircon in some 1 33 ga orthogneisses and a 1 35 ga xenolith whole rock pb isotope compositions of gsmbc rocks overlap the field of compositions characteristic of amazonian crust and of other basement rocks from the south central appalachians the pb isotopes and geochronology in orthogneiss mafic xenoliths and pre ottawan paragneiss are consistent with a correlation of the gsmbc with the mars hill terrane within the french broad massif and with the greater grenvillian south central appalachian basement scab that is considered exotic to laurentia and transferred during rodinian collision prior to ca 1 2 ga similarities in protolith ages and pb isotopes point to the paragua terrane of amazonia southwestern brazil and eastern bolivia as a likely match for scab initial amazonia laurentia collision occurred at ca 1 35 to 1 32 ga with final transfer of scab to laurentia occurring after 1 20 ga within the sinistral oblique collision zone between laurentia and amazonia defined by previous workers

shear wave anisotropy in the crust around the san andreas fault near parkfield spatial and temporal analysis summary we systematically analysed shear wave splitting sws for seismic data observed at a temporary array and two permanent networks around the san andreas fault saf observatory at depth the purpose was to investigate the spatial distribution of crustal shear wave anisotropy around the saf in this segment and its temporal behaviour in relation to the occurrence of the 2004 parkfield m 6 0 earthquake the dense coverage of the networks the accurate locations of earthquakes and the high resolution velocity model provide a unique opportunity to investigate anisotropy in detail around the saf zone the results show that the primary fast polarization directions pds in the region including the saf zone and the northeast side of the fault are nw se nearly parallel or subparallel to the saf strike some measurements on the southwest side of the fault are oriented to the nne ssw direction approximately parallel to the direction of local maximum horizontal compressive stress there are also a few areas in which the observed fast pds do not fit into this general pattern the strong spatial variations in both the measured fast pds and time delays reveal the extreme complexity of shear wave anisotropy in the area the top 2 3 km of the crust appears to contribute the most to the observed time delays however substantial anisotropy could extend to as deep as 7 8 km in the region the average time delay in the region is about 0 06 s we also analysed temporal patterns of sws parameters in a nearly 4 yr period around the 2004 parkfield main shock based on similar events the results show that there are no appreciable precursory coseismic or post seismic temporal changes of sws in a region near the rupture of an m 6 0 earthquake about 15 km away from its epicentre

temporal changes in shear wave splitting at an isolated swarm of small earthquakes in 1992 near dongfang hainan island southern china summary an isolated swarm of small earthquakes occurred in 1992 near dongfang on hainan island southern china the institute of geophysics state seismological bureau of china monitored the swarm with five dcs 302 digital accelerometers for three months from 1992 june 1 18 earthquakes with magnitudes m l ranging from 1 8 to 3 6 were well located by five stations and shear wave splitting varying azimuthally was analysed on 27 seismic records from these events the mean polarization azimuth of the faster shear wave was wnw time delays between the split shear waves at two stations varied with time and space the time delays at one station fell abruptly after earthquakes of magnitudes 3 1 and 3 6 but did not change significantly at the second station this behaviour is consistent with the delay time changes being caused by changes in the aspect ratio of vertical liquid filled eda cracks thus the variation in shear wavesplitting time delay could be due to changes in crustal stress related to nearby smallmagnitude earthquake activity the connection between earthquake activity and crustal stress variation measured by shear wave splitting leaves the door open for possible observations of crustal stress transients related to the onset of an earthquake however our data cannot be considered as definite evidence for such precursors

using shear wave splitting measurements to investigate the upper mantle anisotropy beneath the north china craton distinct variation from east to west 1 a compilation of shear wave splitting measurements in the north china craton ncc is presented to investigate the upper mantle anisotropy beneath the region by applying the method of silver and chan 1991 the splitting parameters of fast polarization direction and delay time are determined from sks waveforms for 38 broadband stations the results reveal the presence of small to large seismic anisotropy in the mantle beneath the ncc the most striking result is the distinct difference of fast polarization directions between the eastern block eb and the central zone cz of the ncc the fast polarization directions trend se beneath the eb which implies that northwestward mantle flow has played a significant role in the reactivating of the eb during late mesozoic to early cenozoic and the boundary between the cz and the eb was possibly a west limit where the mantle flow was deflected however a collision event occurred in early proterozoic was also a possible cause for the anisotropy of the lithosphere beneath the cz depending on the craton lithosphere beneath the region had not been completely reformed by later tectonic activities

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

shear wave splitting and mantle flow associated with the deflected pacific slab beneath northeast asia 1 a total of 361 sks and five local s wave splitting measurements obtained at global and regional seismic network stations in ne china and mongolia are used to infer the characteristics of mantle fabrics beneath northeast asia fast polarization directions at most of the stations in the western part of the study area are found to be consistent with the strike of local geological features the dominant fast directions at the eastern part beneath which seismic tomography and receiver function studies revealed a deflected slab in the mantle transition zone mtz are about 100 from north which are almost exactly the same as the motion direction of the eurasian plate relative to the pacific plate and are independent of the direction of local geological features the splitting times at those stations are about 1 s which correspond to a layer of about 150 km thickness with a 3 anisotropy the shear wave splitting observations complemented by the well established observation that most of the eastern part of the study area is underlain by a lithosphere thinned by delamination in the paleozoic era can be best explained by the preferred alignment of metastable olivine associated with the subduction of the deflected pacific slab in the mtz or by back arc asthenospheric flow in the mantle wedge above the slab

tectonic evolution of the grenville orogen in the central appalachians abstract remnants of the grenville orogeny a globally pervasive tectonothermal event resulting from the amalgamation of the supercontinent rodinia between ca 1 3 and 1 0 ga are exposed throughout eastern north america for ca 3000 km from labrador to alabama although tectonic models for the development of the grenville have historically been based on observations from the large and well exposed canadian grenville province recent investigations suggest that initial continental collision occurred proximal to the appalachian blue ridge mesoproterozoic rocks within this portion of the orogen are exposed within a series of allochthonous terranes that were extensively reworked during paleozoic orogenesis to better understand the tectonic history of this critical region we utilized garnet and monazite geochronology to reconstruct the metamorphic history of eleven samples of ortho and paragneiss from the appalachian blue ridge province the resultant dates together with new results from trace element thermobarometry define a prolonged record of high temperature metamorphism coincident with the timing of continent continent collision as recognized in the northern portion of the orogen initial garnet and monazite growth commenced between 1100 and 1065 ma with peak granulite facies conditions 750 900 c 0 55 0 85 gpa occurring from 1050 to 1010 ma burial and heating of the paragneisses progressed rapidly with peak metamorphism occurring 20 30 myr after deposition of the sedimentary protoliths local hydrothermal alteration followed peak metamorphism and occurred at 1000 990 ma with later cooling through 660 c between 980 and 935 ma 3 2 c myr the metamorphic history outlined here together with insights from previous studies is consistent with formation of the blue ridge rocks within a continental suture zone that had an orogenic architecture comparable to that of the himalayan tibetan orogen

excess silica in omphacite and the formation of free silica in eclogite silica lamellae in eclogitic clinopyroxene are widely interpreted as evidence of exsolution during decompression of eclogite however mechanisms other than exsolution might produce free silica and the possible mechanisms depend in part on the nature and definition of excess silica excess silica may occur in both stoichiometric and non stoichiometric pyroxene although the issue has been debated we show that all common definitions of excess silica in non stoichiometric clinopyroxene are internally consistent interchangeable and therefore equivalent the excess silica content of pyroxene is easily illustrated in a three component condensed composition space and may be plotted directly from a structural formula unit or recalculated end members in order to evaluate possible mechanisms for the formation of free silica in eclogite we examined the net transfer reactions in model eclogites using a thompson reaction space we show that there are at least three broad classes of reactions that release free silica in eclogite i vacancy consumption in non stoichiometric pyroxene ii dissolution of ti phases in pyroxene or garnet iii reactions between accessory phases and either pyroxene or garnet we suggest that reliable interpretation of the significance of silica lamellae in natural clinopyroxene will require the evaluation not only of silica solubility but also of titanium solubility and the possible roles of accessory phases and inclusions on the balance of free silica

crystal preferred orientations of garnet comparison between numerical simulations and electron back scattered diffraction ebsd measurements in naturally deformed eclogites observations of dislocations sub grains and elongated crystal shapes support plastic deformation of garnet in laboratory experiments and naturally deformed eclogites to evaluate the crystal preferred orientations cpo of garnet formed in axial shortening pure shear and simple shear we performed numerical simulations of cpo development during plastic flow using the visco plastic self consistent model as input for the models we use the slip systems determined by transmission electron microscopy using experimentally deformed specimens although in garnet 66 slip systems are available slip on the k111l 110 system provides over 86 of the total strain in the simulations characteristic cpo distributions are produced for the three deformation paths with the cpo being strongest for axial shortening and weakest for simple shear compared with low symmetry minerals the pole figure densities of garnet which has cubic symmetry are weak k100l axes tend to align with the shortening direction in all three deformation modes the simulations are compared with cpo of naturally deformed garnet from nine eclogite samples from the alps norway and mali which contain 20 40 garnet all samples show weak garnet cpo only two samples have a cpo pattern similar to the simulations for simple shear no samples are similar to simulations for axial strain or pure shear the presence of other weaker minerals such as omphacite and quartz with volume fractions higher than garnet probably prevented garnet from becoming highly strained and developing characteristic cpos in these eclogites higher volume fractions of garnet and higher temperature conditions may however allow the development of garnet cpo in the mantle transition zone particularly within subducted oceanic material morb q 2004 published by elsevier ltd

the shear wave splitting in the crust and the upper mantle around the bohai sea north china abstract in order to infer the distribution of local stress and the deep geodynamic process in north china this study detects seismic anisotropy in the crust and upper mantle beneath the bohai sea area a total of 535 local shear wave and 721 xks including sks pks and skks phases splitting measurements were obtained from stations in permanent regional seismograph networks and a temporary seismic network called zbnet e the dominant fast polarization orientation of local shear waves in the crust is nearly east west suggesting an east west direction of local maximum compressive stress in the area nearly north south fast orientation was obtained at some stations in the tan lu fault belt and the zhang bo seismic belt the average fast orientation from xks splitting analysis is 87 4 measured clockwise from the north the average time delays of xks splitting are range from 0 54 s to 1 92 s corresponding to a 60 210 km thick layer of anisotropy the measured results indicate that upper mantle anisotropy beneath bohai sea area even the eastern part of north china is mainly from asthenospheric mantle flow from the subduction of the pacific plate from the complicated anisotropic characteristics in this study we infer that there might be multiple mechanisms in the crust and upper mantle around the bohai sea area that led to the observed anisotropy

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

anisotropy in the shallow crust observed around the san andreas fault before and after the 2004 m 6 0 parkfield earthquake local seismic arrays were deployed at two locations along the san an dreas fault saf near parkfield california before and after the 2004 m 6 0 parkfield earthquake using local earthquakes we determine the anisotropic field within 1 2 km of the main trace of the saf at the two array locations separated by 12 km the initial array near the safod site was deployed for six weeks in october and november 2003 and the second array located near the town of parkfield was de ployed for 3 months following the 28 september 2004 m 6 0 parkfield earthquake we find the fast shear wave polarization direction nearly fault parallel n40w for stations on the main fault trace and within 100 m to the southwest of the saf at both array locations these fault parallel measurements span the 100 to 150 m wide zone of pervasive cracking and damage interpreted from fault zone trapped waves associated with the main fault core li et al 2004 2006 outside of this zone the fast orientations are scattered with some preference for orientations near n10e roughly parallel to the regional maximum horizontal compressive stress direction rh in addition fast directions are preferentially oriented parallel to a northern branch of the saf recorded on stations in the 2004 parkfield deployment the measured anisotropy is likely due to a combination of stress aligned micro cracks away from the fault and shear fabric within the highly evolved fault core the majority of our measurements are taken outside of the main fault core and we es timate the density of microcracks from the measured delay times apparent crack densities are approximately 3 with large scatter the data suggest weak depth dependence to the measured delay times for source depths between 2 and 7 km below 7 km source depth the delay times do not correlate with depth suggesting higher confining pressure is forcing the microcracks to close no coseismic variation in the anisotropic parameters is observed suggesting little to no influence on measured splitting due to the 2004 m 6 0 parkfield earthquake however the premainshock and postmainshock data presented here are from arrays separated by 12 km limiting our sensitivity to small temporal changes in anisotropy

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others

mantle dynamics of western pacific and east asia insight from seismic tomography and mineral physics abstract recent results of high resolution seismic tomography and mineral physics experiments are used to study mantle dynamics of western pacific and east asia the most important processes in subduction zones are the shallow and deep slab dehydration and the convective circulation corner flow processes in the mantle wedge the combination of the two processes may have caused the back arc spreading in the lau basin affected the morphology of the subducting philippine sea slab and its seismicity under southwest japan and contributed to the formation of the continental rift system and intraplate volcanism in northeast asia which are clearly visible in our tomographic images slow anomalies are also found in the mantle under the subducting pacific slab which may represent a small mantle plumes b upwellings associated with the slab collapsing down to the lower mantle or c sub slab dehydration associated with deep earthquakes caused by the reactivation of large faults preserved in the slab combining tomographic images and earthquake hypocenters with phase diagrams in the systems of peridotite water we proposed a petrologic model for arc volcanism arc magmas are caused by the dehydration reactions of hydrated slab peridotite that supply water rich fluids to the mantle wedge and cause partial melting of the convecting mantle wedge a large amount of fluids can be released from hydrated morb at depths shallower than 55 km which move upwards to hydrate the wedge corner under the fore arc and never drag down to the deeper mantle along the slab surface slab dehydration reactions at 120 km depth are the antigorite related 5 reactions which supply water rich fluids for forming the volcanic front phase a and mg surssasite breakdown reactions at 200 and 300 km depths below 700 c cause the second and third arcs respectively moreover the dehydration reactions of super hydrous phase b phases d and e at 500 660 km depths cause the fluid transportation to the mantle boundary layer mbl 410 660 km depth the stagnant slabs extend from japan to beijing china for over 1000 km long indicating that the arc trench system covers the entire region from the japan trench to east asia we propose a big mantle wedge bmw model herein where hydrous plumes originating from 410 km depth cause a series of intra continental hot regions fluids derived from mbl accumulated by the double sided subduction zones rather than the india asia collision and the subsequent indentation into asia are the major cause for the active tectonics and mantle dynamics in this broad region

insight into modification of north china craton from seismological study in the shandong province 1 the study region is located in the southeast edge of the north china craton ncc which is an archean craton that underwent removal and modification of the lithospheric mantle during the late mesozoic cenozoic teleseismic data from the ncisp 1 experiment were used to image the velocity structures in lithosphere and to investigate the upper mantle anisotropy seismological results revealed that the present lithosphere consists of two parts with different structural features throughout including crustal fabric lithospheric velocity stratification and anisotropic patterns the formation of the contrasting structure is attributable to one of two possibilities the trans lithospheric tanlu fault zone tlfz operated as an asthenospheric upwelling channel and facilitated the ncc reactivation or the tlfz separates the study region into two lithospheric blocks with the ancient lithospheric materials remaining within the rejuvenated lithospheric mantle these two possibilities suggest that thermal chemical erosion may have played critical roles in the ncc reactivation

petrology and geochronology of mesoproterozoic basement of the mount rogers area of southwestern virginia and northwestern north carolina implications for the precambrian tectonic evolution of the southern blue ridge province this study was supported by awards from the educational geologic mapping program administered by the u s geological survey usgs to george washington university gwu the research also benefited from additional support provided by the usgs appalachian landscape project