detection of lower mantle scatterers northeast of the marianna subduction zone using short period array data short period university of washington northern california and southern california seismic array records of a sequence of deep mariana subduction zone earthquakes show unexpected clear arrivals nearly 90 and 105 s following p we measure slowness arrival time and arrival azimuth of these phases relative to p using array techniques to find where they emanate in the mantle by combining slowness and back azimuth deviations and the move out with depth of the arrivals from different earthquakes we find their most probable origin is through s to p conversion at point scatterers in the mid to lower mantle at 25 7 n 148 2 e 1590 km and 25 5 n 151 0 e 1850 km respectively separated by 300 km these features indicate elastically distinct material in the middle of the lower mantle which if fragments of subducted slabs are not related to contemporary subduction they may represent upwelling convective material from the deep mantle or d but they are not purely thermal in nature as chemically distinct bodies they may represent potential reservoirs of the isotopic and trace element components recognized geochemically

small scale heterogeneity at the top of the lower mantle around the mariana slab abstract short period waveform data recorded at the western us seismic array for 14 mariana deep earthquakes near 19 n show two anomalous wave packets within about 30 s after the arrival of the direct p wave simply called p or p wave one at about 13 s called x1 phase and the other around 29 s called x2 phase we perform array analyses to locate the sources of these phases in the first step we measure arrival time slowness back azimuth and amplitude of these phases relative to p the amplitudes of the x1 and x2 phases correlate with each other for the individual events and vary more than an order of magnitude among the events which cluster in a region spanning less than 50 km the amplitude correlation suggests that the two waves have a similar origin and the large amplitude variation eliminates receiver side reverberation as the origin of the phases the x1 phase has a slowness and an arrival azimuth which are not distinguished from those of p waves across the entire array and has nearly constant delay times regardless of the focal depths from 567 to 605 km the x2 phase arrives as a more emergent wave packet with slowness and azimuth different from those of p in the second step we compute composite semblance coefficients for the cases of p to p and s to p single scattering near the foci the x1 and x2 phases are best interpreted as s to p scattered waves generated in the uppermost lower mantle north of the focal region based on three observations 1 highest composite semblance values 2 scatterer locations mutually consistent between the two event groups near the depths of 500 and 600 km and 3 a reasonable amount of elastic property anomalies required the x1 phase scatterer is determined at 19 8 0 3 n 145 7 0 3 e 710 30 km on the other hand the scatterer of the x2 phase appears to split into two objects one at 20 4 0 4 n 145 4 1 0 e 900 80 km and the other at 20 6 0 4 n 147 4 0 5 e 860 40 km although the geometries of the scatterers of x1 and x2 phases are not constrained horizontal or subhorizontal discontinuities are unlikely the changes in elastic properties associated with these heterogeneous objects probably occur within several kilometers according to their high efficiency at converting short period waves they are thus likely to represent sharp chemical variations in major element composition these objects are located within a thickened high velocity anomaly at the top of the lower mantle which has been determined by previous seismic studies a plausible tectonic interpretation of these objects is that they are fragments of former oceanic crust which are entrained in the pacific slab impinging on the more viscous lower mantle

kinematic considerations for mantle mixing recent experimental and computational studies tal and computational mixing studies in highly controlled show that islands u mixedrcgions that donot interactwith flows show that even seemingly inconsequential errors in the surrounding regions are ubiquitous features in chaotically computed velocity field of 3 d or time dependent flows can advecting fluids such is ands uitenaturall account for the geochemically inferred coexistence of apparently distinct long lived geochemical heterogeneity with relatively homo geneous regions of an actively convecting mantle these results also indicate that mixing patterns the set of islands and folds characterizing the large scale material advection are sensitive to small variations in the rheology of the fluid therefore interpretation of numerical simulations of mantle transport and mixing is less straightforward than currently supposed computational studies of analytic flow solutions with systematically introduced and controlled errors indi cate that mantle simulations are unlikely to accurately com pute individual trajectories for even moderate time but that tmjectory ensembles can be accuntely computed for long time significantly computations also indicate that mixing and transport resultsmay not evolve smoothly with increased rheological realism

stability metastability and elastic properties of a dense silica polymorph seifertite 1 dense silica polymorphs with sixfold coordinated si have been found in snc and lunar meteorites and may be important minerals for silica rich components in the lower mantle however the stable crystal structure in the lower mantle and properties of dense silica remain controversial under stable heating and quasi hydrostatic stress conditions we found that the cacl2 type undergoes a phase transition to the pbo2type seifertite at 130 140 gpa and 2500 k our data suggest that this phase transition occurs at a greater depth than the perovskite postperovskite transition in the lowermost mantle the molar volume measured at 1 bar is the smallest among the reported silica polymorphs therefore having the highest calculated density and in excellent agreement with recent first principles calculations the greater molar volume of seifertite found in the shergottite meteorite and previous experiments supports a metastable synthesis of the phase outside its stability field our data combined with the hugoniots of silica polymorphs also rule out the possibility of the formation of seifertite in the meteorite within its stability field we found very little change in bulk sound speed across the cacl2 type seifertite transition if shear wave velocity decreases at the transition to seifertite as suggested by some computational studies this silica transition may provide an alternative explanation for the discontinuities with a shear wave velocity decrease found at depths greater than the d discontinuity

seismic evidence for a chemical heterogeneity in the midmantle a strong and slightly dipping seismic reflector beneath the mariana subduction zone 1 a clear later phase 80 s after the direct p wave is observed in most of individual seismograms recorded by a short period seismometer network in japan j array from a cluster of deep earthquakes that occurred at the northern mariana subduction zone this phase 1 shows a p wave particle motion 2 arrives later from earthquakes with shallower focal depths 3 has a steeper incident angle than that of p wave and 4 shows a deviation of a few degrees in the arrival azimuth from that of p wave we interpret it as an s to p converted wave which takes off downward from the source and is reflected at a velocity discontinuity reflector below the earthquakes travel time inversion shows that the seismic reflector dips southwest by 20 at 24 25 n 144 75 e and at a depth of 1115 km with a lateral extension at least 100 100 km the location corresponds to the lower edge of a high velocity anomaly in global tomographic models amplitude and waveform analyses suggest a decrease of s wave velocity by 2 6 and an increase of density by 2 9 within the reflector there is almost no difference in p wave velocity 1 between the reflector and the surrounding mantle the estimated thickness of the reflector is 12 km these observations indicate that the observed seismic structure is more likely to be a chemical reservoir rather than a purely thermal anomaly the seismic reflector might be a piece of subducted oceanic crust as suggested by a previous study it also could be related to the break down of the d phase of dense hydrous magnesium silicates at midmantle pressure condition reported by recent mineral physics studies both scenarios imply that mechanical chemical segregation might occur within the subducted slab at midmantle condition

stability of anhydrous phase b experimental studies and implications for phase relations in subducting slab and the x discontinuity in the mantle 1 we have experimentally determined the equilibrium boundary of the reaction forsterite fo periclase per anhydrous phase b anh b at 900 1600 c and we used the retrieved gibbs free energy of formation of anh b to calculate the stability field of anh b stishovite sti with respect to the mg2sio4 polymorphs the results suggest the possibility of the reaction sequence wadsleytite anh b sti ringwoodite with increasing pressure at temperature below 600 c this might lead to an eye shaped splitting of the 410 km discontinuity within the interior of a sufficiently cold slab such as would prevail in a 140 myr old slab subducting at an angle of 60 with a rate of 14 cm yr e g tonga the equilibrium boundary of anh b sti with respect to the wadsleyite and ringwoodite has respectively positive and negative slope in the p t space the reaction of fo per to anh b seems to offer a viable explanation of the so called x discontinuity that has been observed at 275 345 km depth in several subcontinental and subduction zone environments it is suggested that periclase could form locally by the dissolution of silica in mantle fluid or precipitate from a hydrous melt that became enriched in mgo due to the incongruent melting of olivine at pressures of 6 8 gpa

seismic scatterers at the shallowest lower mantle beneath subducted slabs abstract data from a western united states short period seismic network are analyzed in order to investigate anomalous later phases within a time window from about 10 s to 120 s after direct p waves for deep and intermediate depth earthquakes in circum pacific subduction zones the anomalous phases are best interpreted as s to p scattered waves wavelengths of 10 km from heterogeneities in the shallow lower mantle depths 950 km several s to p scatterers where elastic properties of the rocks must substantially change within several kilometers are detected in the shallowest 300 km of the lower mantle beneath four circum pacific regions kuril bonin fiji and peru around most of the observed scatterers the seismic tomography models have delineated high seismic velocity anomalies which are associated with recently subducted pacific slab or nazca slab the most likely origin of the scatterers would be basalt which used to form the oceanic crust interestingly enough the majority of the scatterers are located near the bottom boundaries of the slabs given rather moderate degrees of folding and or buckling of the pacific and nazca slabs in the shallowest lower mantle at the study areas the existence of the oceanic crust associated with the most recent slab subduction at the scattering sites is unlikely alternative and more likely explanations include the presence of ancient basaltic rocks localized dehydration of hydrous minerals or a sharp boundary between different rock fabrics such as isotropic and anisotropic lower mantle rocks associated with mantle flow dragged by the slab subduction

calcium depletion halos and fe mn mg zoning around faceted plagioclase inclusions in garnet from a high grade pelitic gneiss

anisotropy of heterogeneity scale lengths in the lower mantle from pkikp precursors summary the anisotropy of heterogeneity scale lengths in the lower mantle is investigated by modelling its evect on the high frequency precursors of pkikp scattered by the heterogeneities although models having either an isotropic or an anisotropic distribution of scale lengths can fit the observed coda shapes of short period precursors the frequency content of broad band pkikp precursors favours a dominantly isotropic distribution of scale lengths precursor coda shapes are consistent with 1 per cent fluctuations in p velocity in the wavenumber band 0 05 0 5 km 1 extending to 1000 km above the core mantle boundary and with a d region open to circulation throughout the lower mantle the level of excitation of pkikp precursors observed in the frequency band 0 02 2 hz requires a power spectrum of heterogeneity that is nearly white or slowly increasing with wavenumber anisotropy of scale lengths may exist in a d layer having larger horizontal than vertical scale lengths and produce little or no detectable evects on pkikp precursors for p velocity perturbations as high as 3 per cent when averaged over a vertical scale of several kilometres and much higher when averaged over scales of hundreds of metres or less

spin transition of ferric iron in the nal phase implications for the seismic heterogeneities of subducted slabs in the lower mantle abstract al rich phases nal new hexagonal aluminous phase and cf calcium ferrite phase are believed to constitute 10 30 wt of subducted mid ocean ridge basalt morb in the earth s lower mantle in order to understand the effects of iron on compressibility and elastic properties of the nal phase we have studied two single crystal samples fe free na1 14mg1 83al4 74si1 23o12 and fe bearing na 0 71 mg 2 05 al 4 62 si 1 16 fe 2 0 09 fe 3 0 17 o 12 using synchrotron nuclear forward scattering nfs and x ray diffraction xrd combined with diamond anvil cells up to 86 gpa at room temperature a pressure induced high spin hs to low spin ls transition of the octahedral fe3 in the fe bearing nal is observed at approximately 30 gpa by nfs compared to the fe free nal the fe bearing nal undergoes a volume reduction of 1 0 1 2 a3 at 33 47 gpa as supported by xrd which is associated with the spin transition of the octahedral fe3 the fits of birch murnaghan equation of state b m eos to p v data yield unit cell volume at zero pressure v 0 183 1 1 a 3 and isothermal bulk modulus k t 0 233 6 gpa with a pressure derivative k t 0 3 7 2 for the fe free nal v 0 hs 184 76 6 a 3 and k t 0 hs 238 1 gpa with k t 0 hs 4 fixed for the fe bearing nal the bulk sound velocities v of the fe free and fe bearing nal phase are approximately 6 larger than those of al fe bearing bridgmanite and calcium silicate perovskite in the lower mantle except for the spin transition region where a notable softening of v with a maximum reduction of 9 4 occurs in the fe bearing nal at 41 gpa considering the high volume proportion of the nal phase in subducted morb the distinct elastic properties of the fe bearing nal phase across the spin transition reported here may provide an alternative plausible explanation for the observed seismic heterogeneities of subducted slabs in the lower mantle at depths below 1200 km

matrix rheology effects on reaction rim growth i evidence from orthopyroxene rim growth experiments the rim forming reaction quartz olivine orthopyroxene is used to investigate the effect of matrix rheology on rim growth rates orthopyroxene rim growth around olivine grains in quartz matrix is compared to rim growth around quartz grains in an olivine matrix at constant p t within one single capsule orthopyroxene rims grow faster around quartz clasts in olivine matrix than around olivine clasts in quartz matrix fourier transform infra red spectra indicate that the entire samples are water saturated because of water adsorption on the reactant grain surfaces the increased orthopyroxene growth rates in olivine matrix as opposed to quartz matrix are interpreted in terms of matrix rheology where in the two different matrix inclusion arrangements the olivine matrix behaves softer and the quartz matrix more rigid the strain energy associated with accommodation of the negative reaction volume is higher for the quartz than the olivine matrix and reduces the free energy that drives orthopyroxene rim growth growth textures in both kinds of orthopyroxene rims indicate that the diffusivity of mgo slightly exceeds the diffusivity of sio2 the relative mobility of mgo and sio2 at given p t fh2o seems to be controlled by energy minimization during orthopyroxene growth at the compressive ol opx interface our experiments provide evidence for two previously overlooked effects relevant to rim growth reactions in metamorphic rocks i diffusivity along chemical potential gradients to reaction sites is a function of rheology and ii the relative diffusivity of components during reaction rim or corona growth is a function of local volume changes at the rim s interfaces

dipping low velocity layer in the mid lower mantle evidence for geochemical heterogeneity data from western united states short period seismic networks reveal a conversion from an s to a p wave within a low seismic velocity layer greater than or equal to the 4 percent velocity difference compared to the surrounding mantle in the mid lower mantle 1400 to 1600 kilometers deep east of the mariana and izu bonin subduction zones the low velocity layer about 8 kilometers thick dips 30 to 40 southward and is at least 500 kilometers by 300 kilometers its steep dip large velocity contrast and sharpness imply a chemical rather than a thermal origin ancient oceanic crust subducted into the lower mantle is a plausible candidate for the low velocity layer because of its broad thin extent

density profiles of oceanic slabs and surrounding mantle integrated thermodynamic and thermal modeling and implications for the fate of slabs at the 660 km discontinuity abstract we have calculated the mineralogical properties of the earth s mantle and the lithological units constituting the subducting oceanic slabs within a wide range of p t conditions within the cao feo mgo al2o3 sio2 system except for the basalt top layer of a slab for which the system is extended to include na2o the mineralogical data are then converted using the appropriate p v t relations to bulk densities the calculated adiabatic density vs depth profile of the mantle between 200 and 725 km depths is in good agreement with geophysical and experimental data the density data of the different compositional units are combined with calculated thermal structures for a variety of slab mantle systems to construct equilibrium density profiles as a function of depth the mean equilibrium densities of the slabs within the transition zone 400 660 km depth are found to be 0 04 0 05 g cm3 greater than those of the ambient mantle within the same depth interval for the entire upper mantle density differences between slabs and ambient mantle are slightly less but the slabs still remain denser than the latter at 670 km depth slabs have lower density than the ambient lower mantle because of the commencement of perovskite forming reactions within the mantle and displacements of these reaction boundaries to higher pressures within the slabs as a consequence of their negative p t slopes if perovskite forming reactions within slabs are hindered for kinetic sluggishness then neutral buoyancy would be achieved when the slabs have penetrated 100 km into the lower mantle however using the available data on the kinetics of spinel to perovskite plus periclase reaction we conclude that the reaction would go to completion in a peru type young slab 41 myr and very likely also in a tonga type old slab 110 myr before these penetrated 100 km into the lower mantle thus slabs should always remain negatively buoyant and therefore continue to subduct through the lower mantle once it penetrates through the 660 km discontinuity despite a negative buoyancy force a slab could deflect at the top of the lower mantle 660 km because of factors resisting subduction namely viscosity jump low dip angle slab roll back and metastable persistence of olivine in cold slabs if published scale model experiments represent realistic approximations of the factors affecting plate subduction then according to our density data any slab with a dip angle of 40 50 would bend at the 660 km discontinuity if there is a viscosity jump of at least by a factor of 10 and trench migration the basalt top layer of a slab is denser than other slab components and the ambient mantle at all depths to 660 km and therefore should continue to sink into the lower mantle especially if a slab directly penetrates the 660 km barrier instead of peeling off in the transition zone to form a perched eclogite or piclogite layer as previously proposed the harzburgite layer which is sandwiched between denser basalt and lherzolite layers faces greater resistance to subduction especially in a young slab and thus could significantly contribute to the deformation of a slab near the 660 km discontinuity

scale limits of free silica seismic scatterers in the lower mantle abstract lower mantle seismic scatterers are often interpreted in terms of subducted basaltic crust which should contain free silica phases that are seismically fast however free silica phases are unstable in a peridotite lower mantle as they react with ferropericlase magnesiowustite to form silicate perovskite nonetheless conservative estimates and recent measurements of diffusion coefficients in silicate perovskite suggest that free silica phases may persist as polycrystalline armored relics in lower mantle peridotite at length scales equivalent to those observed for seismic scatterers simple numerical simulations also suggest that polycrystalline silica may persist within subducted metabasalts

phase transition and density of subducted morb crust in the lower mantle abstract phase relations mineral chemistry and density of a natural mid oceanic ridge basalt morb composition were investigated up to 134 gpa and 2300 k by a combination of in situ x ray diffraction measurements and chemical analyses using transmission electron microscope tem results demonstrate that the morb composition consists of mgsio 3 rich perovskite stishovite casio 3 perovskite and cafe 2 o 4 type al phase in the upper part of the lower mantle the most abundant mineral of mgsio 3 rich perovskite undergoes phase transition to a cairo 3 type post perovskite phase above 110 gpa and 2500 k post perovskite phase is similar in composition to perovskite except considerably high na 2 o content stishovite transforms to cacl 2 type sio 2 phase above 62 gpa and 2000 k and further to pbo 2 type phase above 110 gpa pbo 2 type sio 2 phase includes large amount of al 2 o 3 which significantly expands its stability relative to cacl 2 type phase phase transition of casio 3 perovskite from tetragonal to cubic was also observed with increasing temperature cafe 2 o 4 type al phase is stable to the bottom of the mantle the density of morb crust was calculated using volume data combining with measured chemical compositions and calculated mineral proportions the former morb crust is denser than the average lower mantle at all depths greater than 720 km contrary to earlier predictions the subducted basaltic crust may have accumulated at the base of the mantle

seismic evidence for a 920 km discontinuity in the mantle analysis of hundreds of seismograms from a seismic array in the japanese islands reveals a seismic discontinuity at a depth of about 920 km in the mantle beneath the tonga subduction zone this discontinuity is also evident beneath subduction zones in the japan and flores seas the discontinuity probably represents either a change in mantle chemical composition or the signature of the subducted slab s garnet layer

mid mantle scatterers near the mariana slab detected with a double array method we use a cluster of deep earthquakes the fore and aftershocks of the 23 aug 1995 mw 7 1 18 8n 145 3e h 595 km event in the mariana subduction zone as source array to locate where waves scatter near the deep slab in order to obtain the high precision location needed in this context we first relocate the events with a modified master event technique using the double beam method on the relocated events and data from the warramunga array in australia we identified two non standard s to p conversions arriving about 20 5 and 24 s after the p wave the corresponding scatterers are approximately located at a depth of 790 km 720 km and a distance of 0 9 to the southeast 2 3 to the southwest of the main shock the scatterers positions correspond to the location of lithosphere subducted below the 660 km discontinuity

locating scatterers in the mantle using array analysis of pkp precursors from an earthquake doublet abstract well separated individual pkp precursors observed at the yellowknife seismic array yk for a high quality doublet of earthquakes provide an opportunity to study the location of the corresponding scatterers and assess the stability of the location estimation based on the comparison of the waveforms of stacked individual precursors and those of pkikp phases we are able to determine that most of these precursors originate from scattering of the pkpbc rather than the pkpab branch above the b caustic on the receiver side this allows a reliable location of the scatterers in the lower mantle their depths range from 2890 km the cmb to 2270 km scattering angles range from 45 8 to 16 0 and surface projections range from southern ontario to northern saskatchewan in canada these locations are associated with transitions from slow to fast velocities in mantle tomographic models and follow the expected general dip direction of fossil slabs under north america this suggests that the subducted slab remnants under north america have retained their compositional signature the fact that we can essentially treat these scatterers as reflections from plane boundaries suggests that the remnant fragments of slab may be spatially extended which should be confirmed using broadband data average differences in measured slowness and back azimuth of the doublet precursors are as small as 0 08 s deg and 1 4 deg respectively our study indicates that it may be possible to locate such scatterers using single earthquakes and small aperture seismic arrays

seismic evidence for small scale heterogeneity throughout the earth s mantle details in the amplitude structure of short period precursors to the seismic core phase pkp can be used to constrain the depth extent of mantle scattering the simplest model consistent with the observations invokes small 8 km weak r m s velocity perturbations of 1 random heterogeneities uniformly distributed throughout the mantle the data do not support previously proposed models that place an increase in the concentration of scattering sites near the base of the mantle and instead place an upper limit on the amplitude of any short wavelength topography at the core mantle boundary

morb in the lowermost mantle beneath the western pacific evidence from waveform inversion abstract we study the fine structure of the lowermost several hundred km of the large low shear velocity province llsvp beneath the western pacific by inverting the transverse components of long period 20 200 s broadband seismic waveforms we find a low s velocity zone in the depth range from about 2550 2750 km with the greatest decrease relative to prem of about 0 2 km s at depths of about 2650 2700 km this suggests the presence of significant amounts of mid ocean ridge basalt morb for which the phase transitions from pv to ppv and from cacl 2 type to pbo 2 type sio 2 result in a decrease in s velocities we also find an increase in the s velocity relative to this minimum of about 0 15 km s peaking in the depth range from about 2750 2800 km which is consistent with the phase transition from pv to ppv expected for average composition models such as pyrolite

advancement in strong motion analysis by the use of evolutionary power spectra abstract illustrated by strong motion records from the michoacan earthquake 19 september 1985 and the loma prieta earthquake 17 october 1989 the application of the evolutionary power spectrum as a tool in strong motion analysis is demonstrated it is shown that a strong motion record is not a realisation of a simple stationary stochastic process it is a non stationary stochastic process which can be explained and modeled by some basic parameters and through the separation of the seismic process into stationary stochastic sub processes which are due to the dominant wave types

search for seismic discontinuities in the lower mantle summary indications of lower mantle discontinuities have been debated for decades but still little is known about their properties and their origins are enigmatic in our study broad band recordings of deep events are examined for the presence of signals from the lower mantle discontinuities with a novel technique we deconvolve vertical component of the p wave coda in the period range around 10 s by the s waveform and stack many deconvolved traces with moveout time corrections in synthetic seismograms for an earth model without lower mantle discontinuities the strongest signal thus detected in the time window of interest is often s 410 p phase generated as s and reflected as p from the 410 km discontinuity above the source in actual seismograms there are other phases that can be interpreted as converted from s to p at discontinuities in the lower mantle beneath the seismic source we summarize the results of processing the seismograms 1 of deep events in sunda arc at seismograph stations in east asia 2 deep kermadec fiji tonga events at the j array and freesia networks in japan and stations in east asia and 3 deep events in the northwest pacific region mariana izu bonin and the japan arc recorded at stations in north america in our data there are indications of discontinuities near 860 880 1010 1120 1170 1250 and 1670 1800 km depths the clearest signals are obtained from the discontinuity at a depth of 1200 km we argue that the 900 1200 and 1700 km discontinuities are global but laterally variable in both depth and strength seismic stratification of the lower mantle may have bearings on the patterns of subduction as revealed by tomographic models

lower mantle scattering profiles and fabric below pacific subduction zones abstract the earth s mantle is compositionally evolving principally through the process of mid ocean ridge basalt production 95 by lithospheric area but also by way of arc magmatism 5 and continental growth reymer a schubert g 1984 phanerozoic addition rate to the continental crust and crustal growth tectonics 3 63 77 rowley d b 2002 rate of plate creation and destruction 180 ma to present geol soc am bull 114 927 933 convection eventually must mix these products back into the mantle but the details of the mixing process are unknown we report here new observational data documenting the location of differentiated material in the present day lower mantle and scattering power profiles with depth small scale 10 km objects which efficiently scatter short period s waves are present in the mantle lying below the pacific ocean rim at depths between 1100 and 1800 km we also observe a reduction in scattering power with depth indicating that the objects represent bodies richer in differentiated basalt than the surrounding mantle the scattering objects appear to be deformed to various degrees and usually form clusters separated from each other by a distance of 100 to 200 km the clusters probably represent folds of the order of 100 km probably corresponding to early stage of mixing process because the scattering power does not increase with depth neither does a permanent barrier to flow exist in the mid mantle nor does the intrinsic density of basalt prevent re entrainment into mantle flow from the bottom of the mantle

depth variation of the mid mantle seismic discontinuity short period array seismograms of deep events that occurred in the indonesia japan and izu bonin arcs are stacked and beam formed to identify the near source s p converted waves that result from the mantle transition discontinuities most of the resulting images reveal the existence of a mid mantle seismic discontinuity 920 km discontinuity in these regions of the 15 events analyzed three that occurred at the western end of the indonesia arc show clear s p arrivals observable even in individual seismograms the mid mantle discontinuity is characterized by large depth variation 900 1080 km and velocity contrast variation in different subduction zones especially the depth variation of the mid mantle discontinuity beneath the indonesia arc where the discontinuity deepens from 940 km at the eastern end to 1080 km at the western end appears to be well correlated with the location of the high velocity anomalies in recent tomographic models however the mid mantle discontinuity cannot be simply coincided with the bottom of the high velocity anomalies because a velocity increase at the discontinuity is observed from the waveform analysis

slabs and shear wave reflectors in the midmantle 1 an extensive analysis of multiple scs reverberation data sets sampling regions of active or recent subduction reveals evidence of midmantle reflections apparently stemming from fragments of subducted slabs or from the interaction between subducted slabs and the surrounding mantle transition zone reflectors are detected beneath the southwest pacific ocean and melanesia with mean depths of 850 and 1100 km in the majority of source receiver corridors crossing the study area the observations are most spatially coherent beneath the coral and tasman seas coincident observations of the two reflectors along most seismic corridors suggests but does not mandate the existence of two distinct reflectors rather than bimodal depth distribution of a single reflector or distributed scatterers beneath north america reflectors at depths of 1380 and 1530 km are seen in the midcontinent region further east the reflections are shallower with depths near 940 and 1130 km unlike the southwest pacific the reflectors are not paired in any of the individual source receiver corridors this and the depth variability of the observations indicate that the reflector or reflectors in the americas is are fragmented from west to east transecting source receiver corridors the impedance contrasts of these features rival that of the 660 km discontinuity suggesting that individual fragments of the reflecting surfaces must be relatively continuous and flat from north to south along individual corridors to maintain a strong apparent impedance contrast the reflections in both study areas are unlikely to be the result of slabs interacting with a chemical boundary layer or small scale scatterers within the midmantle more likely these reflectors result from a pressure temperature dependent phase transition within or around subducting slabs

u th pb dating of collision in the external alpine domains urseren zone switzerland using low temperature allanite and monazite article i nfo dating metamorphism in external domains of orogenic belts is a challenging task due to the few chronometers available and common inheritance or disturbance of the isotopic systems at low temperature metamorphic con ditions in the external domains of the central alps urseren zone the occurrence of distinct populations of monazite and allanite in one single outcrop offers a unique chance to evaluate the potential of these two ree minerals to date successive metamorphic stages within a collisional cycle the studied outcrop locality of garvera exposes mesozoic metasediments that were metamorphosed under greenschist facies conditions t b 450 c under these conditions ree mineral grains or domains are typically small 5 50 m and in this case also occur as porphyroblast in pelites at the contact with carbonate layers or veins allanite grains n200 m based on the texture and mineral assemblages ree mineral growth conditions were attributed to be detrital mnz1 syn kinematic mnz2 and aln1 post kinematic at the thermal peak aln2 and retrograde during late stage deformation mnz3 to constrain temporally the successive crystallisation conditions ree minerals have been dated using shrimp ion microprobe u th pb analyses show that the alpine ree minerals have a high th u and while most of the 206 pb is non radiogenic f206 that can reach up to 99 for allanite th pb ages can be obtained using 206 pbc corrected th pb isochron the age of different generations of allanite and monazite constrains the timing of successive collisional stages for the prograde accretion at 22 5 1 5 ma mnz1 and aln1 thermal peak at 19 3 2 0 ma aln2 and late stage hydrothermal veining at13 6 1 4 ma mnz3 these novel geochronological data confirmtheregional diachroneity throughthe cen tral alps with younger ages burial thermal peak and exhumation toward the external domains the similarity betweenmetamorphicagesofsamplestakenintheurserenzoneandthenearbycrystallinemassifssuggeststhat themetasedimentarycoveroftheurserenzoneremainedcloselyjuxtaposedtotheexternalcrystallinebasement during the alpine collision cycle the monazite age of the late stage vein testifies for active tectonic deformation at 13 6 1 5 ma which likely played a crucial control on the exhumation of the external massifs in the middle miocene