u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

deep upper mantle melting beneath the tasman and coral seas detected with multiple scs reverberations abstract multiple scs reverberations are used to search for mantle reflectors beneath the tasman and coral seas with a hierarchical waveform inversion migration method in addition to the major transition zone discontinuities a low velocity layer above the 410 km discontinuity is detected the top of the low velocity layer lies at an average depth of 352 km indicating that the layer could be more than 70 km thick if it persists to the 410 km discontinuity which occurs at an average depth of 420 km along paths containing the low velocity layer we attribute the low velocities to partial melt resulting from volatile induced melting the considerable thickness of the partial melt layer may require thin films of a hydrous melt with a zero degree dihedral angle surrounding grains or the combined effect on melting of the addition of both water and carbon to the deep upper mantle via subduction although the depths of the transition zone discontinuities do not indicate that the transition zone itself is rich in water the impedance contrasts do contain a subtle signature that could be related to transition zone water namely a decrease in the impedance contrast across the 410 km discontinuity and a relatively strong 520 km discontinuity

sharpness of upper mantle discontinuities determined from high frequency reflections an understanding of the nature of seismic discontinuities in the earth s upper mantle is important for understanding mantle processes in particular the amplitude and sharpness of these discontinuities are critical for assessing models of upper mantle phase changes and chemical layering so far seismic studies aimed at determining the thickness and lateral variability of upper mantle discontinuities have yielded equivocal results particularly for the discontinuity at 410km depth1 2 here we present short period 0 8 2 0 s recordings of upper mantle precursors to the seismic phase p p pkppkp from two south american earthquakes recorded by the 700 station short period array in california our results show that the 410 and 660 km discontinuities beneath the indian ocean are locally simple and sharp corresponding to transi tion zones of 4 km or less these observations pose problems for mineral physics models3 5 which predict a transitional thickness greater than 6 km for the peridotite to spinel phase transition in contrast to the results of long period studies6 7 we observe no short period arrivals from near 520 km depth

the effect of water on the 410 km discontinuity an experimental study 1 in order to evaluate the effect of h on the pressure and transition interval of the olivine wadsleyite transformation synthesis experiments were carried out in hydrous and anhydrous peridotite compositions at 1400 c under hydrous near saturated conditions the pressure of the transition is 1 0 0 5 gpa lower than under anhydrous conditions in addition the two phase interval appears to broaden from about 0 4 gpa 12 km in the anhydrous system to as much as 1 3 gpa 40 km in the hydrous system under hydrous conditions the spatial distribution of olivine and wadsleyite appears to be controlled by h diffusion extrapolating previously measured diffusion coefficients for h in olivine and in liquids it appears that h diffusion driven by gravitational equilibration may serve to narrow the transition interval between olivine and wadsleyite

seismic scattering at the top of the mantle transition zone abstract we document strong seismic scattering from around the top of the mantle transition zone in all available high resolution explosion seismic profiles from siberia and north america this seismic reflectivity from around the 410 km discontinuity indicates the presence of pronounced heterogeneity in the depth interval between 320 and 450 km in the earth s mantle we model the seismic observations by heterogeneity in the form of random seismic scatterers with typical scale lengths of kilometre size 10 40 km by 2 10 km in a 100 140 km thick depth interval the observed heterogeneity may be explained by changes in the depths to the spinel transformations caused by an unexpectedly high iron content at the top of the mantle transition zone the phase transformation of pyroxenes into the garnet mineral majorite probably also contributes to the reflectivity mainly below a depth of 400 km whereas we find it unlikely that the presence of water or partial melt is the main cause of the observed strong seismic reflectivity subducted oceanic slabs that equilibrated at the top of the transition zone may also contribute to the observed reflectivity if this is the main cause of the reflectivity a substantial amount of young oceanic lithosphere has been subducted under siberia and north america during their geologic evolution subducted slabs may have initiated metamorphic reactions in the original mantle rocks

elastic wave velocities of forsterite and its β spinel form and chemical boundary hypothesis for the 410 km discontinuity a method for measuring elastic wave velocities at pressures up to 15 gpa has been developed velocities of forsterite and spinel were measured using a piston cylinder apparatus with a pulse echo overlap method for pressures up to 4 gpa and using a split sphere apparatus with a pulse transmission technique for higher pressures to validate the results volume compression incompressibility and rigidity were estimated from measured travel times and comparison was made between the present estimates and data from x ray brillouin and acoustic measurements the v p and k values for spinet are 3 and 8 higher than previous measurements at 10 gpa respectively velocity jumps in v p and v s due to olivine to spinel transition were about 1 34 and 0 80 km s 1 respectively very different from seismological observations of velocity jumps at the 410 km discontinuity which were concentrated around 0 45 km s 1 for the p wave and 0 22 km s 1 for the s wave velocity jumps in mantle peridotite were calculated using the experimental results and comparison was made between these estimates and seismological observations at the 410 km discontinuity the evidence does not support the phase change hypothesis and a chemical boundary was proposed as the origin of the 410 km discontinuity an eclogitic mineral assemblage might be an alternative for the composition of the transition layer

upper mantle seismic shear discontinuities of the pacific 1 shear wave impedance discontinuities are inventoried for eight paths connecting circum pacific earthquakes with seismic stations on hawaii and oahu in addition to the transition zone discontinuities we observe a consistent impedance decrease at a depth of 80 km that marks the transition from the fast seismic lid to the low velocity zone the interval over which this transition occurs is less than 30 km the requisite impedance decrease ascribed almost entirely to diminished velocities in the low velocity zone exceeds predictions for peridotite chemistries of appropriate lithospheric age in all cases the transition is better matched by lithosphere of roughly half of the true age four paths show clear evidence of the x discontinuity an impedance increase of 3 8 near 300 km depth the only viable explanations would require extensive eclogite or basalt depleted mantle in the upper mantle of the western pacific the impedance contrast of the 410 km discontinuity which depends on the modal fraction of mantle olivine does not signal the presence of either although a mixture of the two remains a possibility we find no evidence of a low velocity layer atop of the 410 km discontinuity if present it is either consistently thin 15 km highly variable in thickness topography in excess of 60 km or has little impedance contrast 2 3 with overlying mantle the apparent absence of melt atop the 410 km discontinuity an intermittent and weak 520 km discontinuity and a thinned transition zone are consistent with relatively dry conditions in the deep upper mantle of the central pacific

tectonics of the western margin of the shan plateau central myanmar implication for the india indochina oblique convergence since the oligocene abstract we present new data and interpretations on the neogene tectonics of the shan scarp area central myanmar and its relationship with the india indochina oblique convergence we describe ductile and brittle fabrics associated with the major features in this area the mogok metamorphic belt mmb the shan scarp and the sagaing fault from these data we identify a succession of two tectonic regimes first a dominant nnw sse trending extension marked by ductile stretching that characterizes the mmb and associated n70e brittle normal faults later from middle or upper miocene to the present these structures were cross cut by brittle right lateral faults among which the most important are the n20w transpressive shan scarp fault zone and the n s sagaing fault to explain this transition from a dominant transtensive to a transpressive stress regime that occurred during miocene we place our data within a larger geodynamic context we suggest that like the intraplate deformation in the indian ocean the end of spreading in the south china sea the opening of the andaman basin or the end of subduction within the indo burma range the change in the tectonic regime in central myanmar could be in response to a major miocene regional plate kinematic reorganization

understanding mesozoic accretion in southeast asia significance of triassic thermotectonism indosinian orogeny in vietnam results from a zircon u pb study of the metamorphic basement of vietnam reveal that a large part of the continental crust was affected by a short lived episode of ductile deformation and high temperature metamorphism between 258 6 ma and 243 5 ma although coincident with final stages of north south china collision qinling orogenesis the thermotectonism in vietnam was caused by accretion of sibumasu to indochina south china this accretion event indosinian orogeny has regional significance because it contributed to the final stages of north south china collision an aspect not explicitly recognized in qinling orogenic models

fine structure of the 410 km discontinuity the april 14 1995 earthquake in western texas m w 5 7 produced a strong topside reflection off the 410 km discontinuity which was recorded on a multitude of seismic arrays throughout the southwestern united states data from 394 vertical short period and 24 broadband instruments provide dense coverage of this event from distances of 11 to 19 and provide a detailed look at the subcontinental 410 km structure the salient features of this data set are 1 the strong dependence on wavelength of the 410 km triplication range 2 the uniform amplitude ratio of the direct p and reflected p 410 phases on both short period and broadband recordings throughout the triplication and 3 the abrupt termination of the short period p 410 phase at 13 3 these features are best modeled by a composite discontinuity in which a sharp velocity jump of 3 is overlain by a linear velocity jump of 3 5 spread over 14 km the interference of energy turning in the diffuse and sharp portions of this discontinuity structure reproduces both the long and short period triplication range and the step like behavior of the p 410 short period amplitude which cannot be reproduced with either a simple linearly diffuse or a purely sharp discontinuity this composite structure produces a triplication range which depends on source frequency and has an apparent depth which depends on observation frequency additionally this is the structure expected from mineralogical arguments for the to olivine phase transition

olivine spinel transitions experimental and thermodynamic constraints and implications for the nature of the 400 km seismic discontinuity the sequence of high pressure phase transitions in olivine is traditionally used as a model for seismic velocity variations in the 200 to 650 km depth interval in a mantle of peridotitic bulk composition it has been proposed that the observed seismic velocity increase at 400 km depth is too sharp and of too small a magnitude to be attributable to the phase change and that the upper mantle must thus be chemically stratified with the 400 km discontinuity being due either to a combination of phase changes in a layer of pyroxene garnet rich piclogite composition or to a chemical boundary between such a piclogite layer and an overlying peridotitic layer using available calorimetric thermoelastic and synthesis data and their associated experimental uncertainties we have derived internally consistent high pressure phase relations for the mg2sio4 fe2sio4 join we find that the divariant transition which is generally regarded as occurring over a broad depth interval for mantle olivine compositions is in fact extremely sharp the seismic discontinuity corresponding to the transition in mg0 9fe0 1 2sio4 should occur over a depth interval isothermal of about 6 km at a depth of approximately 400 km the sharpness of this transition is quite insensitive to uncertainties in the constraining calorimetric thermoelastic and synthesis data in addition we have computed seismic velocity profiles for a model mantle consisting of pure olivine of mg0 9fe0 1 2sio4 composition comparison of these computed profiles to those derived from recent seismic studies indicates that the magnitude of the observed velocity increase at 400 km depth is consistent with a mantle transition zone composed of about 60 70 olivine we conclude that there is no need to infer the existence of pyroxene garnet rich compositions such as eclogite or piclogite in the transition zone since an upper mantle of homogeneous olivine rich peridotitic composition is consistent with the available seismic velocity data

p v vp vs t measurements on wadsleyite to 7 gpa and 873 k implications for the 410 km seismic discontinuity the compressional p and shear wave s velocities for mg2sio4 wadsleyite have been measured to 7 gpa and 873 k using simultaneous ultrasonic interferometry and in situ x ray diffraction techniques from the velocity measurements we obtained the pressure and temperature derivatives for the elastic shear g and adiabatic bulk ks moduli g p t 1 5 1 g t p 0 017 1 gpa k ks 173 2 gpa ks p t 4 2 1 and ks t p 0 012 1 gpa k for the p and s waves we obtained vs p t 0 021 1 km s gpa vs t p 0 035 2 km s k vp p t 0 065 2 km s gpa and vp t p 0 038 2 km s k values in parentheses are standard deviations e g 1 5 1 1 5 1 independent equation of state analysis of p v t data provided an estimation of the temperature dependence for the isothermal bulk modulus of kt t p 0 022 12 gpa k and thermal expansion a bt coefficients of a 2 10 5 k 1 and b 2 5 10 8 k 2 using these data along with elastic properties for other mantle phases a velocity depth profile for a pyrolite model to 670 km depth is constructed using a finite strain method along a 1673 k adiabat in the transition zone the pyrolite model has a smaller gradient between 410 and 660 km than the body wave models from synthetic waveform analyses but converges with the seismic profiles at the bottom of the transition zone just above the 660 km discontinuity the pyrolite model has velocity jumps of 6 9 and 7 9 for p and s waves respectively over a thickness of 10 km for the phase transformation from olivine to wadsleyite which is in good agreement with short period p wave reflection data and a recent fine structure model c4 for both the velocity jumps and the thickness of the 410 km seismic discontinuity

jurassic to miocene magmatism and metamorphism in the mogok metamorphic belt and the india eurasia collision in myanmar situated south of the eastern himalayan syntaxis at the western margin of the shan thai terrane the highgrade mogok metamorphic belt mmb in myanmar occupies a key position in the tectonic evolution of southeast asia the first sensitive high resolution ion microprobe u pb in zircon geochronology for the mmb shows that strongly deformed granitic orthogneisses near mandalay contain jurassic 170 ma zircons that have partly recrystallized during 43 ma high grade metamorphism a hornblende syenite from mandalay hill also contains jurassic zircons with evidence of eocene metamorphic recrystallization rimmed by thin zones of 30 9 plus or minus 0 7 ma magmatic zircon the relative abundance of jurassic zircons in these rocks is consistent with suggestions that southern eurasia had an andean type margin at that time mid cretaceous to earliest eocene 120 to 50 ma i type granitoids in the mmb myeik archipelago and western myanmar confirm that prior to the collision of india an up to 200 km wide magmatic belt extended along the eurasian margin from pakistan to sumatra metamorphic overgrowths to zircons in the orthogneiss near mandalay date a period of eocene 43 ma high grade metamorphism possibly during crustal thickening related to the initial collision between india and eurasia at 65 to 55 ma this was followed by emplacement of syntectonic hornblende syenites and leucogranites between 35 and 23 ma similar syntectonic syenites and leucogranites intruded the ailao shan red river shear belt in southern china and vietnam during the eocene oligocene to miocene and the wang chao and three pagodas faults in northern thailand that most likely link with the mmb were also active at this time the complex history of eocene to early miocene metamorphism deformation and magmatism in the mmb provides evidence that it may have played a key role in the network of deformation zones that accommodated strain during the northwards movement of india and resulting extrusion or rotation of indochina

mantle discontinuities under southern africa from precursors to p p df we investigate the reflection properties of upper mantle discontinuities beneath southern africa using precursors to the df branch of pkppkp p p the p p df branch is weaker than the ab and bc branches but it does not have the complication of a caustic and appears across a wider distance range stacks from hundreds of short period seismograms recorded in california from the march 9 1994 tonga earthquake mw 7 6 show an 5 reflection at 3 5 s dominate period from 660 km depth indicating a sharp 660 under southern africa a 3 5 s period reflection from 410 km depth is also visible in these stacks but only 2 the strength of p p df this result contrasts with the observation of the 410 and the 660 reflecting comparable amounts of high frequency energy under the indian ocean benz and vidale 1993a indicating either a diffuse 410 boundary under southern africa or global variations in the impedance change across the 410 a 1 5 s period reflection may indicate the existence of fine scale heterogeneity near 320 km depth reflectivity synthetic seismograms also show that a previously claimed reflection from 785 km depth has the more likely explanation as pcppkp

mantle layering from scs reverberations 2 the transition zone this is the second paper in a four part sequence that examines the nature of mantle layering using the scsn phases and internal reflections observed within the reverberative interval of sh polarized seismograms mantle reflectivity profiles for 18 seismic corridors sampling portions of indonesia australia and the western and central pacific are constructed from low frequency 25 mhz scs reverberations i e scsn type phases reflected once from internal mantle discontinuities modeling the reflectivity profiles using synthetic seismograms yields precise path specific estimates of the travel times to the 410 km and 660 km discontinuities and their reflection coefficients the data are consistent with an olivine dominated model of the transition zone marked by two major phase transitions having average apparent depths of 414 2 km and 660 2 km and impedance contrasts of 0 046 0 010 and 0 072 0 010 respectively the interpath variations observed in the reverberation travel times imply that the topography on the two discontinuities is of the order of 20 km peak to peak and is negatively correlated consistent with an endothermic transition at 660 km whose clapeyron slope magnitude is comparable to that of the 410 km exothermic transition in the case of the 660 km discontinuity we have been able to supplement these long wavelength 1500 5000 km observations with an estimate of intrapath topography inferred from the frequency dependence of the 660 km reflection coefficient which gives peak to peak variations less than 40 km on scale lengths of 500 1500 km for the 660 km discontinuity the observed topography is significantly less than the dynamic topography expected for a simple compositional interface in regions of subduction however we do observed an intriguing negative correlation between the apparent depth and the reflection coefficient of the 660 km discontinuity which may involve small compositional variations plausible explanations for this correlation include heating of an initially cool chemical boundary layer gravitationally trapped above an endothermic phase transition loss of reflected energy due to local curvature or roughness of the reflector and or extreme topography occurring on a small compositional component to the discontinuity 40 of the total impedance contrast by stripping the 410 km and 660 km peaks from the composite path reflectivity profile we identify three minor reflectors at mean depths of 520 710 and 900 km the shallowest may mark the phase spinel phase transition the 710 km discontinuity can be attributed to the ilmenite gamet perovskite phase transition although changes in perovskite symmetry can potentially provide explanations for both the 710 km and 900 km features

global variations of earth s 520 and 560 km discontinuities abstract we investigate seismic discontinuities in the mantle transition zone mtz by analyzing ss precursors recorded at global seismic stations our observations confirm the global existence of the 520 km discontinuity although substantial regional depth variations in the 520 km discontinuity are generally correlated with temperature in the mid mtz they cannot be fully explained by the clapeyron slope of the wadsleyite ringwoodite phase transition suggesting both thermal and compositional heterogeneities in the mtz a second discontinuity at 560 km depth previously interpreted as splitting of the 520 km discontinuity is most commonly detected in cold subduction zones and hot mantle regions the depth separation between the 520 and 560 km discontinuities varies from 80 km in cold regions to 40 km in hot areas the exsolution of calcium perovskite ca pv from majorite garnet has been proposed to explain the velocity and density changes across the 560 km discontinuity however the gradual exsolution of perovskite and partitioning of ca and al between perovskite and garnet appear inconsistent with the relatively sharp discontinuity in seismic observations and thus need to be revisited in the future nevertheless because the only known transition in major minerals at this depth in the mtz is the formation of ca pv the existence of the 560 km discontinuity may imply localized high calcium concentrations in the mid mtz possibly related to the recycling of oceanic crust

ar ar and fission track ages in the song chay massif early triassic and cenozoic tectonics in northern vietnam the song chay massif is the northeasternmost metamorphic complex in vietnam to the east of the red river shear zone it shows a large antiformal structure involving orthogneisses and migmatites overlain on its northern flank by muscovite bearing marbles an e w striking fault bounds the dome to the south kinematic indicators along a s n section reveal top to the n shear sense along the interface between the orthogneissic core and the overlying metasediments radiometric ages were obtained by the 40ar 39ar method using purified mica separates across the dome ages range from 236 ma at the southern edge to 160 ma in the core attesting to a strong imprint in the early triassic time a clear difference is seen between these mesozoic ages and the eocene to miocene ages from 40 to 24 ma that obtained in the nearby red river shear zone using the same method these data show that the song chay massif was already high in the crust when the high temperature deformation of the red river shear zone took place the final exhumation of the song chay orthogneiss constrained by fission track analysis on samples along the same transect occurred during the early miocene and could be interpreted as the consequence of a first normal sense of motion along the fault which bounds the massif to the south timing is similar to that of exhumation in the red river shear zone

dynamic phase boundary topography by latent heat effects abstract when radial flow passes through a mantle phase boundary the interplay of advection and diffusion of latent heat creates dynamic topography of this boundary which means a deflection that is related to the sense and magnitude of radial velocity even though the flow does not advect temperature differences towards the phase boundary a one dimensional stationary model is used to quantify the effect which does not dependent on the sign of the clapeyron slope significant deflection requires slow radial flow and a narrow phase loop these conditions are most probably met for the phase transition from spinel to perovskite and magnesiowustite at 660 km depth up to 4 km of topography peak to peak are created with a mass anomaly that is equivalent to 440 m of dynamic topography at the earth s surface other phase transitions of the olivine component also contribute to a degree that is less certain the time scale of adjustment of phase boundary topography is studied in a two dimensional model where a deep mass anomaly in an isentropic mantle drives flow through the 660 km phase boundary the adjustment time depends on the inverse square of the vertical velocity and is on the order of 10 ma in the earth which is shorter than the convective time scale for a high viscosity lower mantle dynamic phase boundary topography created by this mechanism can replace a significant part of the surface topography required in dynamic geoid models and can help to reduce the misfit between predicted dynamic surface topography and the upper limit that observations pose on its amplitude

410 km discontinuity sharpness and the form of the olivine α β phase diagram resolution of apparent seismic contradictions summary estimates of the thickness of the 410 km seismic discontinuity believed to be due to the olivine modified spinel transformation in olivine are as low as 4 km based on discontinuity reflectivity the seismically estimated thickness is however biased to values narrower than the true transformation interval if linear interpolation of properties is used for modelling a 5 km linear velocity gradient yields an average reflection coefficient identical to that of a 10 km transition interval based on olivine phase diagram features moreover alternative forms of the phase diagram equally consistent with experimentally determined iron magnesium partitioning can yield true transition intervals as narrow as 4 km this reconciles a discrepancy between phase equilibrium and seismic measures of discontinuity thickness in two ways 1 seismic thickness estimates are too narrow and 2 narrow transition intervals are permissible given existing phase equilibrium constraints incorporating recent results on the influence of h2o on discontinuity properties it appears that 410 km discontinuity reflectivity is much more sensitive to varying h2o concentration than to temperature suggesting that discontinuity reflectivity variations reflect changes in mantle chemistry

detailed structure and sharpness of upper mantle discontinuities in the tonga subduction zone from regional broadband arrays 1 recordings of deep tonga earthquakes from two arrays of 12 broadband seismographs each in the fiji and tonga islands are stacked and searched for reflections and conversions from upper mantle discontinuities in the tonga subduction zone the arrays operated as part of the seismic arrays in fiji and tonga saft experiment from july 2001 to august 2002 in comparison with the commonly used teleseismic approaches the short path lengths for the local data provide smaller fresnel zones and high frequency content for precise mapping of discontinuity topography and sharpness to enhance the low amplitude discontinuity phases s410p p660p and s660p deconvolved seismograms from each event array pair are aligned on the maximum amplitude of the direct p wave and subsequently slant stacked for the 410 km discontinuity the results show no systematic variations in depth with distance to the cold slab the 660 km discontinuity varies between 656 and 714 km in depth for the southern and central parts of the subduction zone the largest depths occur in the core of the tonga slab for the northern part two separate depressions of the 660 km discontinuity are observed these anomalies are interpreted as being induced by the active steeply subducting tonga deep zone and a subhorizontally lying remnant of subducted lithosphere from the fossil vityaz trench respectively interpreting the deflections of the 660 km discontinuity in terms of local temperatures implies a thermal anomaly of 800 k to 1200 k at 660 km depth except for the southern region where it may thicken the width of the depressed 660 km discontinuity region implies that the tonga slab seems to penetrate the 660 km discontinuity with little deformation waveform modeling suggests that both the 410 and 660 km discontinuities are sharp the 660 km discontinuity is at most 2 km thick in many parts of the region and a first order discontinuity cannot be precluded the 410 km discontinuity thickness shows somewhat more variability and ranges from 2 to 10 km outside the slab and is at most 10 km thick within the slab this suggests that the subduction process does not produce dramatic effects on the sharpness of the discontinuities

reflectivity of the 410 km discontinuity from pp and ss precursors a number of studies have confirmed the global existence of a transition zone discontinuity at 410 km depth by aligning large numbers of long period seismograms on a surface reflection phase before stacking in particular ss and pp precursors from the 410 km discontinuity termed p410p and s410s have revealed long wavelength topography of this discontinuity here we extend these techniques to examine the reflection coefficient of the 410 km discontinuity using our measurements of p410p and s410s amplitudes we constrain the impedance contrasts across the 410 km discontinuity we also show lateral variations in the p wave impedance contrast at 410 km which is typically low under north america and china and higher beneath the north pacific the s wave impedance contrast shows less variability on the regional scale however analysis of p410p and s410s amplitudes over smaller areas by binning traces into spherical caps shows that the s wave reflection coefficient varies over much shorter scale lengths than that for p waves the different patterns of variation for p410p and s410s reflection amplitudes could be due to the presence of melt water or other chemical heterogeneities in the transition zone other factors such as temperature or mantle olivine content variations could also influence precursor amplitudes but they would be expected to lead to correlated variations and so they cannot explain all the variation that we observe

p t t constraints on the development of the doi inthanon metamorphic core complex domain and implications for the evolution of the western gneiss belt northern thailand abstract the western gneiss belt in northern thailand is exposed within two overlapping cenozoic structural domains the extensional doi inthanon metamorphic core complex domain located west of the chiang mai basin and the mae ping strike slip fault domain located west of the tak batholith new p t estimates and u pb and 40ar 39ar age determinations from the doi inthanon domain show that the gneiss there records a complex multi stage history that can be represented by a clockwise p t t path u pb zircon and titanite dating of mylonitic calc silicate gneiss from the mae wang area of the complex indicates that the paragneissic sequence experienced high grade medium pressure metamorphism m1 in the late triassic early jurassic ca 210 ma in good agreement with previously determined zircon ages from the underlying core orthogneiss exposed on doi inthanon late cretaceous monazite ages of 84 and 72 ma reported previously from the core orthogneiss are attributed to a thermal overprint m2 to upper amphibolite facies in the sillimanite field u pb zircon and monazite dating of granitic mylonite from the doi suthep area of the complex provides an upper age limit of 40 ma late eocene for the early stage s of development of the actual core complex by initially ductile low angle extensional shearing under lower amphibolite facies conditions m3 accompanied by near isothermal diapiric rise and decompression melting 40ar 39ar laserprobe dating of muscovite from both doi suthep and doi inthanon provided miocene ages of ca 26 15 ma representing cooling through the ca 350 c isotherm and marking late stage development of the core complex by detachment faulting of the cover rocks and isostatic uplift of the sheared core zone and mantling gneisses in the footwall similarities in the thermochronology of high grade gneisses exposed in the core complex and shear zone domains in the western gneiss belt of northern thailand and also in northern vietnam laos yunnan and central myanmar suggest a complex regional response to indentation of southeast asia by india

transition zone structure in a tectonically inactive area 410 and 660 km discontinuity properties under the northern north sea summary we report results from a year long deployment of broad band seismometers in scotland to study the mantle structure in the northern united kingdom the region is tectonically inactive and its underlying mantle transition zone should not show unusual structure due to thermal perturbations caused by cenozoic subduction or plume related volcanism the estimated 410 and 660 km discontinuity depths estimated by receiver function stacking are 414 5 and 655 14 km typical of average worldwide values there is no evidence for a discontinuity at 520 km the 660 km discontinuity conversion is weaker than global earth models predict by factors of 1 65 2 97 which is significant given amplitude measurement uncertainties either significant attenuation below 410 km or regional non thermal variations in 660 km discontinuity properties that decrease its velocity and density jumps to 1 9 per cent vp 4 8 per cent vs and 4 8 per cent or that broaden it to 13 35 km width could account for the lower than expected amplitudes lateral variations in transition zone water content around the 660 km discontinuity may be responsible for the broadening

global seismic data reveal little water in the mantle transition zone abstract knowledge of the earth s present water content is necessary to constrain the amount of water and other volatiles the earth acquired during its formation and the amount that is cycled back into the interior from the surface this study compares 410 and 660 km discontinuity depth with shear wave tomography within the mantle transition zone to identify regions with seismic signals consistent with water the depth of the 410 and 660 km discontinuities is determined from a large updated dataset of ss s410s and ss s660s differential travel times known as ss precursors the discontinuity depths measured from binning and stacking the ss precursor data are then compared to the shear velocity model hmsl s06 in the transition zone mapping all the possible combinations very few locations match the predictions from mineral physics for the effects of water on discontinuity depth and shear velocity the predictions although not yet measured at actual transition zone temperatures and pressures are a shallow 410 km discontinuity a deep 660 km discontinuity and a slow shear velocity only 8 of the bins with high quality data are consistent with these predictions and the calculated average water content within these bins is around 0 6 wt a few isolated locations have patterns of velocity topography that are consistent with water while there are large regional scale patterns consistent with cold hot temperature anomalies combining this global analysis of long period seismic data and the current mineral physics predictions for water in transition zone minerals i find that the mantle transition zone is generally dry containing less than one earth ocean of water although subduction zones could be locally hydrated the combined discontinuity and velocity data show no evidence that wadsleyite or ringwoodite have been globally hydrated by subduction or initial earth conditions

seismic discontinuities and subduction zones abstract the seismic discontinuities at 410 and 660 km depth in the mantle are generally believed to be due to phase transformations in its major component olivine an increasing observational base of short period regional seismic network data supports this view here we analyse cross sections of discontinuity topography in the northwest pacific subduction zones to infer the thermal and chemical state of the mantle in a subduction environment most of the data is from the southern part of the izu bonin subduction zone penetration of the lower mantle by the steeply dipping subducting slab is clearly indicated by depression of the 660 km discontinuity elevation of the 410 km discontinuity in this region by up to 60 km implies a thermal anomaly of approximately 900 1000 c relative to the ambient mantle observation of the elevated 410 in the seismically active part of the slab indicates that the olivine wadsleyite phase transformation occurs under essentially equilibrium conditions there which argues against the transformational faulting hypothesis as the mechanism for deep earthquakes upgoing p wave reflections from the 410 km discontinuity indicate that it is sharp in the mantle far from subducting slabs reflection coefficient variability in and near slabs indicates either a chemical effect or the consequences of topographic focusing

tectonics of strike slip restraining and releasing bends restraining and releasing bends are common but enigmatic features of strike slip fault systems occurring in all crustal environments and at regional to microscopic scales of observation regional scale restraining bends are sites of mountain building transpressional deformation and basement exhumation whereas releasing bends are sites of topographic subsidence transtensional deformation basin sedimentation and possible volcanism and economic mineralization because restraining and releasing bends often occur as singular self contained domains of complex deformation they are appealing natural laboratories for earth scientists to study fault processes earthquake seismology active faulting and sedimentation fault and fluid flow relationships links between tectonics and topography tectonic and erosional controls on exhumation and tectonic geomorphology this volume addresses the tectonic complexity and diversity of strike slip restraining and releasing bends with 18 contributions divided into four thematic sections 1 a topical review of fault bends and their global distribution 2 bends sedimentary basins and earthquake hazards 3 restraining bends transpressional deformation and basement controls on development 4 releasing bends transtensional deformation and fluid flow

age and origin of magmatism along the cenozoic red river shear belt china to decipher the geodynamic significance of cenozoic magmatism along the red river shear belt geochemical analyses u pb and rb sr dating and pb sr nd isotope tracing were undertaken zircon monazite titanite and a ti u oxide from foliated granitoid intrusions in the shear belt gneisses yield u pb emplacement ages of 33 1 0 2 2 31 9 0 3 25 8 0 2 and 24 7 0 2 ma and an age of 35 0 0 3 ma was obtained for the roughly 100 km long adjacent jinping phan si pang alkali granite together with our previous data the new ages suggest that magmatism and left lateral strike slip movements occurred coevally during latest eocene oligocene times from 33 to 22 ma the rb sr dating of muscovite and biotite from the northernmost gneisses indicates that cooling to 500 c occurred at 52 6 1 1 ma pre dating the onset of magmatism whereas further cooling to 300 c took place at 28 9 0 6 this shows that unroofing in the north took place almost 9 million years earlier than in the central gneiss segments of the shear zone geochemical data substantiate two types of magmas 1 amphibole bearing intrusions of alkaline trend which are derived from sources with isr 0 7065 0 7089 and i nd 3 7 to 6 6 2 leucogranitic layers and bodies having isr 0 7084 0 7354 and i nd 3 3 to 13 4 the former type of intrusion is found in both the gneisses and the adjacent unmetamorphosed cover rocks whereas leucogranites are restricted to the shear belt gneisses source signatures of the alkaline intrusions lie adjacent to the those of oib plotting at the lower end of the mantle array contamination of these melts by continental material seems to be very limited on the other hand the leucogranitic layers are essentially crustal derived but none of the them has country rock isotope signatures requiring melting of crust different from the actually exposed gneisses magma sources similar to those of ocean island basalt indicate magmatism to involve melting of light rare earth element and large ion lithophile element enriched mantle domains most likely present in the lithosphere underneath the region since lithospheric thickening or subduction can be ruled out to produce both types of magmas the presence of an important thermal anomaly is required which is coevally active with left lateral strike slip shear adiabatic decompression and melting within the rising anomaly is the most plausible mechanism to produce the mantle magmas which successively migrate through the crust to induce anatectic melting at 20 15 km crustal depth alkaline magmas largely dominate the volume of magmatism along the belt being continuously present in the shear zone for millions of years such lubrication potentially explains how very large amounts of displacement can be absorbed in surprisingly narrow shear zones such as the red river belt possibly also playing a role for where and when zones of plate scale lateral extrusion develop

exhumation of the ailao shan shear zone recorded by cenozoic sedimentary rocks yunnan province china 1 the role of strike slip faults such as the ailao shan shear zone in southwestern china in accommodating the india eurasia collision remains controversial cenozoic sedimentary rocks preserved along the northeast side of the ailao shan shear zone however record the nature and timing of exhumation of shear zone rocks field mapping shows that the lower strata consist of early oligocene lacustrine red mudstone and gypsum and the upper of late oligocene to miocene fluvial conglomerate with rare fine grained fossil leaf bearing intervals sediment derived from shear zone gneiss is present only in the upper part of the section our observations indicate deformation began in oligocene time with the onset of rapid exhumation by early miocene time consistent with existing geochronologic results exhumation must have ended before the formation of a regional low relief erosion surface and the onset of brittle deformation along the red river fault probably in early pliocene time rapid exhumation of the ailao shan shear zone has been interpreted to have occurred in transtension accommodated by a normal fault on the northeast margin of the shear zone however we observe consistent and pervasive shortening structures within the cenozoic strata including overthrusting and folding of the basin along faults defined by thick gouge zones within weak cenozoic lithologies deformation is likely syndepositional these structures are distinct from subvertical deformational features associated with the active red river fault our field data thus support mid cenozoic transpressional exhumation of the ailao shan shear zone this is consistent with regional evidence for widespread left lateral and compressional deformation in the region to the east of the eastern himalayan syntaxis during this time focused erosion along the red river established by late oligocene time may have contributed to deeper exhumation of the northeast part of the shear zone

global scale analysis of the mantle pds phases a global scale analysis of travel times of the mantle pds phases converted from p to s from the discontinuity at a depth d beneath the receiver is presented most of the data are related to p 410 s and p 660 s recorded by the broadband stations of geoscope incorporated research institutions for seismology iris and other networks in order to provide a good global scale coverage published measurements at 37 stations are complemented by our measurements at 45 stations lateral resolution of our data is many times higher than that of ss and its precursors previously used for similar purposes but we have fewer data in the oceans lateral variations of travel times of the major pds phases are strongly correlated with the lateral p and s velocity variations in the uppermost mantle as revealed by tomographic studies the scaling coefficient between the teleseismic s and p residuals as found from the variations of travel times of the major pds phases is around 3 7 for north america and around 3 3 for the rest of the world almost all the differential travel times t p 660 s t p 410 s are in the range of 1 s relative to the average value of 24 0 s if the lateral variations of the velocities within the mantle transition zone mtz can be neglected the corresponding variations of thickness of the mtz are in the range of 10 km relative to the standard depth of 250 km and do not show significant correlation with either velocities in the upper mantle above the mtz or with the variations of thickness of the mtz as found in some recent studies of precursors to ss there is a weak but marginally significant positive correlation between the variations of thickness of the mtz inferred from our data and the s velocity variations in the mtz as given by su et al 1994 this correlation suggests that both variables contain temperature dependent components lateral variability of the mtz is also manifested by extreme weakness of p 410 s at several locations this may indicate either anomalous topography on 410 km discontinuity or strong broadening of the otherwise sharp discontinuity at some stations there are arrivals that can be interpreted as p 520 s this interpretation implies that the strengh and or sharpness of 520 km discontinuity as well as its depth are laterally variable in a broad range

the structure and sharpness of mg fe 2sio4 phase transformations in the transition zone abstract to calculate accurately the pressure interval and mineral proportions i e yields across the olivine to wadsleyite and wadsleyite to ringwoodite transformations requires a detailed knowledge of the non ideality of fe mg mixing in these mg fe 2 sio 4 solid solutions in order to constrain the activity composition relations that describe non ideal mixing fe mg partitioning experiments have been conducted between magnesiowustite and mg fe 2 sio 4 olivine wadsleyite and ringwoodite as a function of pressure at 1400 c using known activity composition relations for magnesiowustite the corresponding relations for the three polymorphs were determined from the partitioning data in all experiments the presence of metallic iron ensured redox conditions compatible with the earth s transition zone the non ideality of the mg fe 2 sio 4 solid solutions was found to decrease in the order w wadsleyite femg w ringwoodite femg w olivine femg these partitioning data were used along with published phase equilibria measurements for the mg 2 sio 4 and fe 2 sio 4 end member transformations to produce an internally consistent thermodynamic model for the mg 2 sio 4 fe 2 sio 4 system at 1400 c using this model the pressure interval of the olivine to wadsleyite transformation is calculated to be significantly smaller than previous determinations by combining these results with fe mg partitioning data for garnet the widths of transition zone phase transformations in a peridotite composition were calculated the olivine to wadsleyite transformation at 1400 c in dry peridotite was found to occur over a pressure interval equivalent to approximately 6 km depth and the mineral yields were found to vary almost linearly with depth across the transformation this transformation is likely to be even sharper at higher temperatures or could be significantly broader in wet mantle or in regions with a significant vertical component of mantle flow the entire range of estimated widths for the 410 km discontinuity 4 35 km could therefore be explained by the olivine to wadsleyite transformation in a peridotite composition over a range of quite plausible mantle temperatures and h 2 o contents the wadsleyite to ringwoodite transformation in peridotite mantle was calculated to take place over an interval of 20 km at 1400 c this transformation yield was also found to be near linear

sharp 400 km discontinuity from short period p reflections p wave data from the short period around 1 hz southern california seismic network reveal pre critical reflections from the 410 km discontinuity below the gulf of california appearing as a clear and continuous feature in the data to epicentral distances as small as 10 deconvolution and stacking techniques are used to extract these weak arrivals from the coda of earlier arriving p waves the reflections are used to estimate the width of the gradient zone near 410 km depth constraining the velocity jump across the discontinuity using critically refracted waves the velocity jump across the 410 km discontinuity is 6 2 5 which is in agreement with existing estimates of the velocity jump for this region the clear and uninterrupted branch of reflected waves suggests laterally homogeneous discontinuity properties on a scale of several hundred km synthetic seismogram modelling of the reflected waves using realistic models of the phase transition occurring at this depth shows that the transition must be as thin as 10 km with most of the velocity increase occurring over about 4 km or less for the reflections to be visible in short period data applying this result to recent thermodynamic models of upper mantle composition suggests that the high temperatures associated with the spreading centre continue down to at least 410 km depth

upper mantle velocity structure beneath the siberian platform summary we present a new velocity model for the continental upper mantle beneath central siberia based on observations of the 1982 rift deep seismic sounding dss profile three peaceful nuclear explosions pne were detonated to provide energy for the 2600 km long profile that extends from the yamal peninsula to the mongolian border se of lake baikal in this paper we model seismic recordings from the northernmost explosion since data from that shot shows unambiguous arrivals from the mantle transition zone discontinuities the analysis combines forward traveltime modelling and waveform matching using reflectivity synthetic seismograms our model for the lithosphere has velocities of 8 25 8 20 km s from the moho to 117km depth between 117 and 123km depth a strong velocity gradient 8 30 8 53 km s is required while a moderate gradient 8 53 8 55 km s l exists between 123 and 136km depth a low velocity zone from 136 to 210km depth terminates this phase arrival branch the gradient again rises between 210 and 233 km and depth culminating in a high gradient zone 8 63 8 80 km s between 233 and 235 km depth below the high gradient zone more moderate gradient 8 80 8 85 km s is required from 235 to 253 km depth terminating in a zone of lower velocity 8 62 8 64 km s from 253 to 400 km depth the upper mantle transition zone consists of two high gradient zones separated by a more moderate gradient the upper zone is best modelled as a 35 km thick velocity gradient 8 64 9 45 km s from 400 to 435 km depth the existence of the velocity gradient is based on the observation that arrivals from this feature can be identified starting at 1580 km range and rapidly become prominent with increasing distance a model with a first order discontinuity predicts significant arrivals at ranges closer than 1580 km our observations contain no compelling evidence for a 520 km discontinuity although a small discontinuity cannot be ruled out the lower transitionzone discontinuity is modelled as a 4 km thick gradient zone 10 25 10 62 km s i from 655 to 659 km depth

the 410 km depth discontinuity a sharpness estimate from near critical reflections the abrupt increase in seismic velocity and density about 410 km below the surface of the earth occurs at the depth where a phase change in olivine is expected akaogi et al 1989 katsura and ito 1989 ringwood 1969 we have assembled recordings from 22 short period seismometers in washington and british columbia of 35 nuclear explosions in nevada these data reveal relatively noise free reflections from the 410 discontinuity most of the transition is completed within a depth interval of less than 10 km under the western united states this result adds to the accumulating seismic evidence of a fairly sharp 410 transition

single crystal elasticity of ringwoodite to high pressures and high temperatures implications for 520 km seismic discontinuity abstract the single crystal elastic moduli of spinel structured mg 0 91 fe 0 09 2 sio 4 ringwoodite were measured to 16 gpa at room temperature and to 923 k at ambient pressure by brillouin spectroscopy a third order finite strain equation of state yields pressure derivatives of 4 1 2 and 1 3 1 for the adiabatic bulk modulus k s and shear modulus respectively the pressure derivatives of aggregate elastic moduli for all three polymorphs and of mg fe 2 sio 4 in the mg rich end are equal within the mutual experimental uncertainties with k s p t 4 1 4 3 and p t 1 3 1 4 a linear decrease of the elastic moduli and sound velocities with temperature adequately describes the data the temperature derivatives of k s and are 0 021 and 0 016 gpa k respectively an extrapolation of our data to transition zone pressures and temperatures indicates that the shear and compressional impedance contrasts associated with mg fe 2 sio 4 mg fe 2 sio 4 transition are sufficient to produce a visible discontinuity at 520 km depth even with only a moderate 30 50 amount of olivine moreover the partitioning of fe between mg fe 2 sio 4 and mg fe 2 sio 4 and majoritic garnet will produce a relatively sharp seismic discontinuity in the order of 10 15 km even when the latent heat of reaction is taken into account the visibility of the discontinuity in regional and global studies could be inhibited by large topography that would accompany any local variations in temperature or fe content the high v p contrasts observed in some localities are difficult to explain by the transition and are suggestive of a chemical boundary this is consistent with chemical heterogeneity in the transition zone probably related to subduction one of the possible explanations for high velocity contrasts near 520 km is a buried moho a boundary between garnetite former oceanic crust and mg fe 2 sio 4 enriched former peridotite layers

tertiary diachronic extrusion and deformation of western indochina structural and 40ar 39ar evidence from nw thailand the wang chao and three pagodas fault zones cut the western part of the indochina block and run parallel to the red river fault evidence of intense ductile left lateral shear is found in the lansang gneisses which form a 5 km wide elongated core along the wang chao fault zone dating by 40 ar 39 ar shows that such deformation probably terminated around 30 5 ma the wang chao and three pagodas faults offset the north striking lower mesozoic metamorphic and magmatic belt of northern thailand 40 ar 39 ar results suggest that this belt suffered rapid cooling in the tertiary probably around 23 ma these results imply that the extrusion of the southwestern part of indochina occurred in the upper eocene lower oligocene it probably induced rifting in some basins of the gulf of thailand and in the malay and mekong basins in the oligo miocene the continuing penetration of india into asia culminated with the extrusion of all of indochina along the ailao shan red river fault this occurred concurrently with the onset of e w extension more to the south plotting in a geographical reference frame the diachronic time spans of movement on left lateral faults east and southeast of tibet implies that the northward movement of the indian indenter successively initiated new strike slip faults located farther and farther north along its path

detecting seismic anisotropy above the 410 km discontinuity using reflection coefficients of underside reflections abstract we investigate the effect of various types of deformation mechanisms on the reflection coefficients of p and s waves underside reflections off the 410 km discontinuity to develop a diagnostic tool to detect the style of deformation at boundary layers we calculate the reflection coefficient for p and sh underside reflections using velocity perturbations resulting from aligned minerals above the 410 km discontinuity for different deformation scenarios the results show that in the case of an anisotropic olivine layer above an isotropic wadsleyite layer the p wave reflection coefficient amplitudes are only slightly influenced by the joint effect of angle of incidence and the strength of imposed deformation without any polarity reversal and for all deformation styles for the sh wave underside reflections the incidence angle for which a polarity reversal occurs changes with distance for all scenarios and in addition changes with azimuth for shear deformation scenarios these differences in amplitude and polarity patterns of reflection coefficients of different deformation geometries especially for s wave at shorter distances potentially provide a possibility to distinguish between different styles of deformation at a boundary layer we also show a first test using currently available elastic constants of anisotropic wadsleyite beneath anisotropic olivine

structural development of the mid tertiary doi suthep metamorphic complex and western chiang mai basin northern thailand abstract the northern western ranges of thailand contain mylonitic gneisses of the doi suthep metamorphic complex dsmc near chiang mai mylonitic foliations outline a southeast trending dome and stretching lineations trend n80 w low grade metasedimentary rocks and tertiary fanglomerates flank the dome on the east and widespread chloritic breccia marks the intervening doi suthep detachment fault we interpret that the metamorphic complex developed between triassic and early miocene with detachment faulting mylonitization and uplift during oligocene to miocene within the dsmc dikes of mutually cross cutting hence coeval granitic pegmatite and aplite cut mylonitic orthogneiss although the contacts of the dikes are sharp the dikes contain a mylonitic fabric that parallels the fabric in the surrounding mylonitic orthogneiss thus we infer that intrusion occurred during mylonitization east of doi suthep tertiary fanglomerates of the mae rim formation overlie the doi suthep detachment fault variable strikes with dips of 10 50 and several outcrop scale fold hinges suggest that the mae rim formation is deformed into a set of non cylindrical folds numerous variably striking high angle normal faults suggest either multiple deformations or deformation in a three dimensional strain field the discovery of an intraformational angular unconformity confirms that the mae rim formation accumulated during folding

the thickness of upper mantle discontinuities as inferred from short period j array data the thickness of upper mantle discontinuities at depths around 410 km and 660 km provides a critical clue to our understanding of deep earth since the amplitude of reflected or s p converted waves is controlled by the thickness of discontinuities short period studies of such waves may give a fine constraint on the thickness of upper mantle discontinuities in this study we analysed short period and vertical waveform data from 14 deep events in the fiji tonga region the data were obtained from j array a short period and large aperture seismic array in japan with slant stack analyses of these data we detected short period reflections or s to p conversions from 410 km and 660 km discontinuities near sources which implies that both discontinuities are quite sharp to directly measure the frequency content of these waves linear slant stacks were made with various frequency pass bands 0 2 0 5 0 5 1 0 and 1 0 2 0 hz synthetic traces were generated for the models with various thicknesses of the 410 km and the 660 km discontinuities comparison of slant stacks with synthetic traces indicates that the thickness of both discontinuities is at most 5 km

topography of the 660 km seismic discontinuity beneath izu bonin implications for tectonic history and slab deformation we analyze the p wave codas of 65 paths from deep northwestern pacific earthquakes recorded by arrays of stations in germany the western united states india and turkmenistan we identify a phase resulting from a near source s to p conversion at a nearly horizontal discontinuity ranging in depth from 650 to 730 km which we interpret as a thermally depressed spinel to perovskite and magnesiowustite phase transition we migrate these data along with 39 more from wicks and richards 1993 accounting for three dimensional ray bending by the sloping discontinuity to produce a high resolution topography map of the 660 km discontinuity in the izu bonin region assuming an equilibrium phase transition we interpret the discontinuity depth in terms of local temperatures the slab if defined by a thermal anomaly greater than 400 k is only about 100 km thick near 28 n suggesting the slab is penetrating into the lower mantle with little or no advective thickening farther to the north however cold material appears spread out over a wide region consistent with the slab having been laid down flat on the 660 km discontinuity as the trench retreated 2000 km eastward both the narrow slab to the south and the flat lying slab to the north are consistent with recent high resolution tomographic images the depression to 745 km along the arc is consistent with a maximum thermal anomaly of about 1100 k along the entire arc the depression occurs directly beneath the deepest earthquakes even where seismicity is dipping at 45 and stops at 450 km depth suggesting that the slab steepens to a vertical dip at the deepest seismicity this change to a vertical orientation suggests that the slab loses strength temporarily through a physical process which causes the seismicity to increase dramatically and then abruptly cease

receiver function imaging of upper mantle complexity beneath the pacific northwest united states abstract small scale topographic variations on the upper mantle seismic discontinuities provide important constraints on the thermal influences of upwellings and downwellings in geodynamically complex regions subduction of the juan de fuca plate and other tectonic processes dominating the pacific northwest united states in the cenozoic involve massive thermal flux that likely result in an upper mantle that has strong 3 d temperature variations we address the interaction of such processes in the region using receiver functions to image the upper mantle seismic discontinuities at 410 and 660 km we utilized over 15 000 high quality receiver functions gathered from 294 teleseismic earthquakes recorded at 277 regional broadband seismic stations primarily those of the earthscope usarray transportable array we find the average depths of the discontinuities to be 412 km and 658 km respectively with no obvious 520 km discontinuity detected the peak to peak range is greater on the 410 than the 660 suggesting the possibility of more significant regional dynamic processes at upper mantle depths our results are not consistent with a mantle plume below central oregon in the high lava plains region our observation of a thinner transition zone beneath the western snake river plain region however is consistent with a regional increase in mantle temperatures perhaps due to either asthenospheric flow from beneath and around the southern edge of the juan de fuca plate or to vertical flow in the form of regional mantle upwelling related to the snake river plain yellowstone hotspot track further our results are not consistent with a simple subducting juan de fuca slab morphology but rather suggest similar levels of significant complexity in slab structure found by recent regional tomographic studies we find evidence for a thickened and therefore cooler mantle transition zone beneath the wallowa idaho batholith region consistent with tomographic models which suggest broad scale increased seismic velocities to transition zone depths we speculate that this region may be the location of mantle downwelling at larger scales than previously proposed

topography of the transition zone seismic discontinuities phase transformations in mantle mineralogies probably cause the transition zone seismic discontinuities nominally at 410 520 and 660 km depth thermodynamic principles govern phase transformations making them sensitive to changes in the mantle s ambient conditions through the thermodynamic response changes in temperature or composition shift the transformation to a different pressure creating topography on a level discontinuity with this use as an exploratory tool for the mantle in mind we review the basic seismological and phase equilibrium concepts underlying the detection and interpretation of the seismic wave arrivals associated with the transition zone discontinuities reviewing the evidence for and against the phase transition model we conclude that it is viable and describe how discontinuities have been and can be used to probe the physical and chemical state of the transition zone

evolution of deformation styles at a major restraining bend constraints from cooling histories mae ping fault zone western thailand the c 500 km long mae ping fault zone trends nw se across thailand into eastern myanmar and has probably undergone in excess of 150 km sinistral motion during the cenozoic a large c 150 km long restraining bend in this fault zone lies on the western margin of the chainat duplex the duplex is a low lying region dominated by north south trending ridges of mesozoic and palaeozoic sedimentary metamorphic and igneous rocks flanked by flat post rift basins of pliocene recent age to the north and south a review of published cooling age data plus new apatite and zircon fission track results indicates that significant changes in patterns of exhumation occurred along the fault zone with time oldest uplift and erosion eocene occurred in the umphang gneiss region west of an inferred thrust dominated restraining bend setting from 36 ma to 30 ma exhumation was strongest north of the duplex along the nw se trending segment of the fault zone at the northern exiting bend of the chainat duplex this region of the fault zone is characterized by a mid crustal level shear zone 5 6 km wide lan sang gneisses that passes to the nw into an apparent strike slip duplex geometry the deformation is interpreted to have occurred during passage around the northern restraining bend which resulted in vertical thickening uplift erosion and extensional collapse of the northern side of the shear zone this concentration of deformation at the bends at the ends of the restraining bend is thought to be a characteristic of strike slip dominated restraining bends following late oligocene early miocene extension there is apatite fission track evidence for 22 18 ma exhumation in the chainat duplex that coincides with a phase of inversion in the phitsanulok basin to the north the miocene recent history of the chainat duplex is one of minor sinistral and dextral displacements related to a rapidly evolving stress field influenced by the numerous tectonic reorganizations that affected se asia during that time

upper mantle and lithospheric heterogeneities in central and eastern europe as observed by teleseismic receiver functions summary data from 90 permanent broad band stations spread over central and eastern europe were analysed using ps receiver functions to study the crustal and upper mantle structure down to the mantle transition zone receiver functions provide valuable information on structural features which are important for the resolution of european lithospheric dynamics moho depths vary from less than 25 km in extensional areas in central europe to more than 50 km at stations in eastern europe craton and beneath the alpine carpathian belt a very shallow moho depth can be observed at stations in the upper rhine graben area ca 25 km whereas for example stations in the sw bohemian massif show a significantly deeper moho interface at a depth of 38 km vp vs ratios vary between 1 60 and 1 96 and show no clear correlation to the major tectonic units thus probably representing local variations in crustal composition delayed arrivals of converted phases from the mantle transition zone are observed at many stations in central europe whereas stations in the cratonic area show earlier arrivals compared with those calculated from the iasp91 earth reference model differential delay times between the p410s and p660s phases indicate a thickened mantle transition zone beneath the eastern alps the carpathians and the northern balkan peninsula whereas the transition zone thickness in eastern and central europe agrees with the iasp91 value the thickening of the mantle transition zone beneath the eastern alps and the carpathians could be caused by cold deeply subducted oceanic slabs

mantle reflectivity structure beneath oceanic hotspots summary this study applies high resolution radon transform to a large set of ss precursors and explores the mantle reflectivity structure beneath 17 potentially deep rooted hotspots the combined reduced time and ray parameter p information effectively constrains the depth spatial distribution and sharpness of upper mid mantle reflectors the olivine to wadsleyite phase boundary is deeper than the ocean and global averages and produces a dominant p domain signal laterally coherent observations of the deep 410 km seismic discontinuity thin upper mantle transition zone and weak absent 520 km reflector beneath hotspots make compelling arguments for large scale hot thermal anomalies in the top 400 600 km of the mantle on the other hand a relatively flat and weak reflector at 653 km is inconsistent with ringwoodite to silicate perovskite magnesiowustite transformation at temperatures greater than 2000 k the lack of a negative correlation between topography and temperature implies 1 average or below average temperatures at 600 700 km depths or 2 high temperatures and a dominating majorite garnet to ca perovskite phase transformation the proper choice between these two scenarios will directly impact the origin and depth of mantle plumes beneath hotspots we further identify lower mantle reflectors at 800 950 and 1100 1350 km depths beneath a number of the hotspots their presence implies that the chemistry and thermodynamics of the mid mantle may be more complex than suggested by seismic tomography

velocity structure of the upper mantle discontinuities beneath north america from waveform inversion of broadband seismic data using a genetic algorithm we present a velocity structure of the mantle discontinuities beneath the united states from a full waveform inversion of broadband seismic data the data consist of 10 moderate earthquakes in the western united states that were recorded on 13 broadband seismometers across the united states operated by the u s national seismic network a total of 36 seismograms with distance ranging from 1150 to 2700 km were used a full waveform inversion based on two dimensional modeling and one dimensional inversion methods using a genetic algorithm was adopted the inversion method is objective provides error bounds on the model and does not require any a priori information about these discontinuities we find that the upper discontinuity is 10 km thick from 395 to 405 km where the velocity increases from 8 80 to 9 15 km s 1 with a velocity gradient of 0 035 s 1 the lower discontinuity is 5 km thick occurring over a depth range of 650 655 km where the velocity increases from 10 20 to 10 70 km s 1 our results are in agreement with previous studies of the lower discontinuity but do not agree for a sharp 5 km upper mantle discontinuity

the elusive coldfinger the earth s continents can extend deep into the mantle and these continental roots are thought to be cooler than the surrounding mantle material so they could lower mantle temperature and even induce downwelling but a seismological study across eastern north america finds that there at least these features of a continental coldfinger are absent

seismic discontinuities in the mediterranean mantle abstract layering in the upper and lower mantle across the mediterranean has been determined using p to s converted phases identified through receiver function analysis conversion of the receiver function s time axis to depth was based on local 1d s velocity models the 1d velocity model for each station was extracted from a three dimensional s velocity model for the mediterranean region eav03 enhanced with crustal structure derived from receiver function analysis under the mediterranean region we observed a mantle transition zone thickness of 261 10 km on average which agrees with a dominance of high velocities imaged in tomographic models at these depths a thick mantle transition zone 270 km was observed in regions with ongoing or past subduction eastern spain southern italy southern greece and the north western african coast conversions from the 410 km and 660 km discontinuities were clearly observed for most stations amplitudes of both discontinuities are anomalously large throughout the mediterranean region 4 9 and 5 7 respectively we observed significant conversions from the 520 km discontinuity the lower mantle showed strong lateral variations resulting in spatially incoherent converted phases from discontinuities near 860 km between 900 km and 1200 km and 1320 km depth

tectonic evolution of the mogok metamorphic belt burma myanmar constrained by u th pb dating of metamorphic and magmatic rocks 1 the mogok metamorphic belt mmb extends for over 1500 km along the western margin of the shan thai block from the andaman sea north to the eastern himalayan syntaxis previous geochronology has suggested that a long lasting jurassic early cretaceous subduction related event resulted in emplacement of granodiorites and orthogneisses 171 120 ma and a poorly constrained tertiary metamorphic event on the basis of new u pb isotope dilution thermal ionization mass spectrometry and u th pb laser ablation multicollector inductively coupled plasma mass spectrometer geochronology presented here we propose two tertiary metamorphic events affected the mmb in burma the first was a paleocene event that ended with intrusion of crosscutting postkinematic biotite granite dikes at 59 ma a second metamorphic event spanned late eocene to oligocene at least from 37 possibly 47 to 29 ma this resulted in the growth of metamorphic monazite at sillimanite grade growth of zircon rims at 47 43 ma sillimanite muscovite replacing older andalusite and synmetamorphic melting producing garnet and tourmaline bearing leucogranites at 45 5 0 6 ma and 24 5 0 7 ma these data imply high temperature sillimanite muscovite metamorphism peaking at 680 c and 4 9 kbar between 45 and 33 ma to around 606 656 c and 4 4 4 8 kbar at 29 3 0 5 ma the later metamorphic event is older than 24 5 0 3 ma the age of leucogranites that crosscut all earlier fabrics our structural and geochronological data suggest that the mmb links north to the unexposed middle or lower crust rocks of the lhasa terrane south tibet and east to high grade metamorphic core complexes in northwest thailand

topography on the 410 km seismic velocity discontinuity near subduction zones from stacking of ss sp and pp precursors we stack the teleseismic depth phases ss sp and pp produced by deep focus earthquakes to image precursory arrivals that result from near source underside reflections off the 410 km seismic velocity discontinuity hereinafter referred to as the 410 and use differential time measurements between these phases and their precursors to compute discontinuity depths near seven subduction zones around the pacific ocean margin we begin by selecting seismograms with high quality depth phase arrivals recorded by several long period networks between the years 1976 and 1996 filtering the waveforms and stacking them along theoretical travel time curves reveals clear precursors which vary in shape and timing we compute confidence levels to evaluate the reliability of the observed precursory features using a bootstrap method that randomly resamples the seismograms prior to stacking we measure the differential travel time between the reference pulse and the precursor using a cross correlation technique and convert this time to an apparent discontinuity depth using the isotropic preliminary reference earth model prem at 25 s period corrected to an oceanic crustal thickness the lateral resolution of our long period stacks for 410 topography is limited compared to that sometimes achieved in short period analyses but is much higher than that obtained from global ss precursor studies for most subduction zones the results indicate little change in the average depth to the 410 km discontinuity in the local areas sampled by the precursor bounce points compared to broad regional depths inferred from ss precursor results this implies that any large variations in depth to the 410 km discontinuity near subduction zones are limited to a narrow zone within the slab itself where they may be difficult to resolve with long period data coverage for the tonga and peru chile subduction zones is sufficiently dense that we can observe lateral variations in 410 depths in tonga the results suggest depth variations perpendicular to the slab of up to 33 km after correcting for probable lateral heterogeneity in velocity above 400 km depth and variations parallel to the slab orientation as large as 13 km the cross slab variation is consistent with the elevation of olivine phase transformations in cold regions the variation along strike suggests a more complex thermal heterogeneity that may be related to the subduction history of this region we see evidence for additional reflectors above the 410 in some of the waveform stacks but the inconsistency and weak amplitude of these features preclude definitive interpretations

seismic signatures of detached lithospheric fragments in the mantle beneath eastern himalaya and southern tibet abstract in this study we investigate the mantle transition zone mtz structure beneath eastern himalaya and southern tibet using 9000 high quality receiver functions from 96 broadband stations spanning the entire region the pervasive early arrival times 1 7 s compared to iasp91 of the p to s conversions from the 410 km discontinuity are attributed to the high shear wave velocities associated with the subducted indian and asian lithosphere s global and regional shear wave velocity models obtained from seismic tomography evince such high velocity anomalies both in the uppermost mantle and within the mtz in this complex collision environment in contrast the conversions from the 660 km discontinuity are either normal or delayed up to 1 s providing evidences for a thickened mtz beneath most of the study region although presence of water in the mtz can result in such a thickening issues like 1 low amplitudes of the p410s and p660s conversions 2 small dependence of their amplitudes with frequency 3 absence of a detectable low velocity layer atop 410 and 4 lack of evidence for a 520 km discontinuity preclude such an interpretation instead we attribute this thickening to lowered temperatures affected by the possible presence of detached cold and dense lithospheric slabs within the mtz such a detachment might have been facilitated either by convection or gravity removal of the lithosphere thickened due to continued subduction of the indian plate since the mesozoic

seismic evidence for water deep in earth s upper mantle water in the deep upper mantle can influence the properties of seismic discontinuities in the mantle transition zone observations of converted seismic waves provide evidence of a 20 to 35 kilometer thick discontinuity near a depth of 410 kilometers most likely explained by as much as 700 parts per million of water by weight

survey of precursors to p p fine structure of mantle discontinuities 1 the jumps in seismic velocity and density near 410 and 660 km depths have been postulated to mark phase changes in a locally chemically homogeneous mantle seismic estimates of upper mantle discontinuity thickness and lateral variability have varied posing difficulties for assessing models with chemical and phase transition layering observations of underside high frequency 1 hz p wave reflections from the 410 and 660 km discontinuity that precede the p p phase provide unique constraints on the physical properties of these dynamically critical features we have collected high quality recordings of p p and its precursors from three sources the california seismic network the large aperture seismic array in montana and the global seismic network the array and regional network data have great sensitivity to weak p p precursors whereas the global network recordings have good geographical distribution of bounce points the only consistently observed reflections originate from depths near 410 km and 660 km the reflector observed near 520 km depth with longer period waves which has been interpreted as the olivine phase transition is too small in impedance contrast or too thick to reflect 1 hz p waves data from all three networks indicate that the 1 hz p waves from the 660 are bigger than those from the 410 although large variability in p p and its precursors amplitudes prevents strong conclusions we show that the p p precursor amplitudes travel time dispersion and frequency content are consistent with a sharp 2 km 660 and a more diffuse 410 consistent with mineral physics predictions

reflection properties of phase transition and compositional change models of the 670 km discontinuity seismic observations of the amplitude ratio amplitude p 670p amplitude p p suggest a reflection coefficient r 7 13 for reflections from the underside of the 670 km discontinuity this can be compared with calculated reflection coefficients to constrain the nature of the mantle transition region velocity and density profiles through the transition region were calculated for olivine pyroxene garnet and olivine garnet model mineral assemblages for models with identical upper and lower mantle compositions the computed reflection coefficients from the phase transitions pertinent to the 670 km discontinuity are less than 0 5 these phase transitions produce their maximum r if they are effectively discontinuous which gives r 2 3 except for the garnet to perovskite transformation which gives r 8 in general given realistic impedance contrasts across a phase transition the change in material properties must occur over a region less than 3 km thick for r to be of the order of 5 a discontinuous change in chemical composition at 670 km depth coupled with a change in phase can also produce a reflection coefficient of the observed magnitude r 2 3 for a first order change in phase at 670 km with different iron concentrations on each side of the discontinuity and r 4 6 for cases with a discontinuous increase in silica content based on our analysis the thermal and chemical boundary layers which would occur across such a chemical discontinuity would probably have negligible effects on the reflection coefficient thus although limited by considerable uncertainties the mineralogical and seismic data suggest a two layered model of the mantle with a discontinuous change in both phase and composition at 670 km

zircon and monazite upb dating of the doi inthanon core complex northern thailand implications for extension within the indosinian orogen abstract the doi inthanon metamorphic core complex occupies a prominent central position within the northwestern thai gneiss belt which has been inferred to be as old as precambrian and as young as permo triassic u pb dating of zircon from core orthogneisses of the complex suggests that the orthogneisses were derived from granitic protoliths with essentially the same late triassic early jurassic age as the mae cham pluton 205 3 4 ma to the west of the doi inthanon complex monazite ages from the orthogneisses indicate that low pressure and high temperature metamorphism and migmatitization occurred in the late cretaceous 84 72 ma limited published radiometric age data from elsewhere in the migmatitic gneiss belt suggest that widespread minor plutonism accompanied this thermal event a younger u pb data of 26 8 0 5 ma for zircon and monazite from the mae klang granite which intruded mylonitic gneisses on the eastern side of the doi inthanon complex indicates that there was also significant plutonism in the late oligocene these u pb ages constrain the development of the complex and hence of the migmatitic gneiss belt as a whole to an interval between late cretaceous and early miocene ductile shearing uplift and tectonic denudation via detachment faulting chronologically overlap the initiation of extensional basin development to the east and southeast of the gneiss belt and imply a genetic connection thus oligocene early miocene thermal rifting from the gulf of thailand northward into mainland thailand may have been the direct product of crustal extension rather than the transtensional product of regional strike slip faulting

subduction zone thermal structure and mineralogy and their relationship to seismic wave reflections and conversions at the slab mantle interface seismological studies of the interface between downgoing slabs of subducted lithosphere and the overriding mantle provide constraints against which models of mantle and slab temperature and bulk composition can be tested these studies employing waves reflected and converted at the slab mantle interface suggest that a velocity contrast of 5 10 occurs over a distance of 10 20 km to depths of at least 350 km the authors have investigated whether such sharp contrasts can result form thermal structure compositional differences phase changes or other effects such as preferred mineral orientation using a mantle and slab temperature model a petrologic model of mantle and slab mineralogy and a data base of elastic properties of mantle and slab phrases theoretical seismic velocities were calculated and compared with the observed velocity contrast and boundary thickness constraints at 9 6 gpa and 1000c conditions modeled at the slab mantle interface at approximately 280 km depth cooler temperatures in the slab interior produce at constant composition only approximately a 1 75 v sub p increase into the slab in any of the peridotitic and eclogitic mineralogies investigated variation in v sub p among the different bulk compositions is only approximately 0 5 these contrasts are substantially lower than those obtained from the seismological studies andmore indicate that temperature and bulk compositional differences between mantle and slab are inadequate to cause the observed velocity contrasts they propose that phase transformations in mantle and slab mineralogies notably elevation of the olivine spinel phase change normally occurring at 400 km depth are important factors in these sharp contrasts other contributing effects may include preferential mineral orientation near the slab surface and at shallower depths the presence of partial melt or hydrous fluid evolved from dehydration reactions along the interface less

linear partitioning theory applied to the earth s 410 km discontinuity the theory of linear partitioning in binary solutions is here extended to solid solid equilibria in p x space at constant t with general rules for formulating the linear relationship when the interphase partition coefficient formulated for the fe endmember is linear with the composition of the numerator phase the exchange coefficient kd is constant and vice versa for olivine wadsleyite in p x the four best experimental brackets in the literature give kd 0 56 0 02 1 s d at 1600 c the s s transition at xfe 0 1 is 2 8 kb or about 8 km wide for pure olivine half the olivine is converted to wadsleyite at 0 6 the p interval however a transition zone lid 4 km thick is consistent with a multiphase mantle in which the presence of a non transforming phase e g pyroxene cuts the effective transformation window in half as recently shown by l stixrude

seismic velocity and density jumps across the 410 and 660 kilometer discontinuities the average seismic velocity and density jumps across the 410 and 660 kilometer discontinuities in the upper mantle were determined by modeling the observed range dependence in long period seismic wave arrivals that reflect off of these interfaces the preliminary reference earth model prem is within the computed 95 percent confidence ellipse for the 410 km discontinuity but outside the allowed jumps across the 660 kilometer discontinuity current pyrolite mantle models appear consistent with the constraints for the 410 kilometer discontinuity but overpredict amplitudes for the 660 kilometer reflections the density jump across the 660 kilometer discontinuity is between 4 and 6 percent below the prem value of 9 3 percent commonly used in mantle convection calculations

intracrustal detachment within zones of continental deformation geologic mapping of active to recently active geologic structures in panamint valley western united states and in the haiyuan region and northern tibetan plateau china suggests detachment in the middle and lower crust on a scale of tens of kilometres to at least several hundred kilometres detachment occurs similarly in predominantly extensional panamint valley and in predominantly compressional china environments it involves structures with displacements of more than 10 km and displacement rates of more than 3 mm yr perhaps more than 10 mm yr the steeply dipping strike slip faults present in all three areas hunter mountain fault haiyuan fault and altyn tagh fault zone terminate in zones of extension or compression and geometric relations indicate that all structures including strike slip faults are thin skinned and restricted to the upper crust in panamint valley and in the haiyuan region deformation within these systems can be reconstructed in three dimensions displacement on the strike slip faults is absorbed by extension or compression occurring at the termination of the faults so strike slip displacement is roughly equal to and in the same direction as shortening or extension we propose that left slip on the altyn tagh fault zone in northern tibet is similarly absorbed by shortening southeast of the fault zone within the qaidam basin and the nan shan

velocity anomalies and spatial distributions of physical properties in horizontally lying slabs beneath the northwestern pacific region abstract we calculated velocity anomalies by constructing thermal structural models of horizontally lying slabs beneath kuriles central japan northern izu bonin and central izu bonin arcs the values of v p and v s anomalies become maximum in the depth range of 100 to 200 km within the slabs reaching 5 to 6 4 for v p anomalies and 7 to 8 5 for v s anomalies the calculated v s anomalies are larger than the v p anomalies and the ratio of the former to the latter is 1 2 to 1 5 within the slab this suggests that v s anomalies are more sensitive to temperature variation than v p anomalies indicating that the existence of the horizontally lying slabs is detectable in more detail by using s wave we also calculated spatial distributions of physical properties in the horizontally lying slabs beneath the four regions in the calculation we assumed that the material of the slabs consists of olivine mg 0 89 fe 0 11 2 sio 4 the estimated values of physical properties within the slabs are larger than those in the surrounding upper mantle except for poisson s ratio while the values within the metastable wedge of olivine become smaller this is most remarkable within the slab beneath the central izu bonin arc due to the lowest temperature among the four regions the horizontally lying slab above the 660 km seismic discontinuity beneath the central izu bonin arc is denser up to 60 kg m 3 and the slab within the metastable wedge of olivine is lighter up to 120 kg m 3 the values of the physical properties except for poisson s ratio decrease with increasing the horizontal distances of the horizontally lying portions of the slabs this is due to thermal assimilation which is largely dependent on moving velocity of the slabs

oligo miocene granitic magmatism in central vietnam and implications for continental deformation in indochina two granitoid intrusions within the bu khang extensional complex in central vietnam have been dated by u pb and rb sr geochronology a monazite u pb age of 26 0 0 2 2 myr was obtained for the bu khang pluton and 23 7 1 6 1 7 myr for monazite allanite and zircon from the dai loc intrusion these ages date crystallization of magmas previously assigned precambrian to devonian rb sr analyses of k feldspar and biotite fractions from the samples yield ages of 19 8 0 6 2 myr and 19 6 0 5 myr respectively the thermal history recorded by the different geochronometers implies an average exhumation rate of 2 mm yr 1 corresponding to 9 km of unroofing magmatism was either i induced passively by lithospheric thinning driven by changes in regional tectonic stresses or ii triggered actively by an ascending plume tertiary exhumation and magmatism documented elsewhere in indochina e g ailao shan red river and wang chao shear zones favours a regional tectonic cause for extension and granitoid magmatism in the bu khang complex on the other hand the presence of an upwelling thermal anomaly since at least 35 ma causing mantle melting below indochina is supported by shear wave velocity variations in the mantle and source geochemistry of both the bu khang plutons and the red river belt intrusions in either case tertiary exhumation of the bu khang complex can account for previously undocumented ne sw directed extension which is required in northern vietnam to account for structural changes related to the opening of the south china sea