identifying global seismic anisotropy patterns by correlating shear wave splitting and surface wave data we compare a global compilation of shear wave splitting measurements with azimuthal seismic anisotropy parameters inferred from surface wave tomography the currently available splitting dataset is taken from a novel comprehensive collection of available publications that is updated interactively online the comparison between the two types of data is made by calculating predicted splitting parameters from the anisotropic tomography model comparing these predicted splitting parameters with the observed ones we find a considerable correlation between the two datasets at global scale this result is noteworthy since such correlation did not seem to exist in previous studies the spatial resolution associated with the two types of methods is rather different while surface waves have good vertical resolution and poor lateral resolution of several hundreds of kilometers sks splitting measurements have good lateral but poor vertical resolution the correlation can be understood in light of recent propositions that anisotropy seen by sks splitting constrains mostly the upper mantle and therefore a similar depth region as surface waves the correlation also confirms the generally good quality of the shear wave measurements as well as that of the anisotropic tomography model

upper mantle anisotropy in se and central brazil from sks splitting evidence of asthenospheric flow around a cratonic keel abstract we present results of upper mantle anisotropy derived from measurements of core refracted shear wave splitting mainly sks phases recorded at 48 stations covering the major tectonic provinces in central and se brazil such as the tocantins province paraguay araguaia and brasilia belts between the amazon and sao francisco cratons the parana intracratonic basin the southern part of the sao francisco craton and the mantiqueira province with the coastal ribeira belt although the fast polarization directions vary across the region consistent orientations are observed over hundreds of kilometers the fast polarization directions tend to be close to the absolute plate motion given by the hot spot reference model hs3 nuvel1a however correlations with geological structures are also observed in the southern brasilia belt and in the ribeira belt respectively located sw and s of the sao francisco craton on the other hand in the northern tocantins province the fast shear wave direction n60 s is oblique to the sw ne trend of the geological units and faults and no anisotropy contribution from lithospheric sources can be clearly identified overall the fast polarization directions show a fan shaped pattern strongly suggesting asthenosperic flow around a thick and stiff keel in the southern part of the sao francisco craton consistent with the high velocity anomaly revealed by recent surface wave tomography the observed nw se directions in the southern part of the brasilia belt may also be interpreted as resulting from asthenospheric flow channeled between the sao francisco craton and a cratonic block beneath the parana basin the largest splitting delays observed in the southern brasilia belt and in the ribeira belt up to 2 4 s suggest contributions from both lithospheric and asthenospheric sources in those two areas our preferred model for the anisotropy causing the observed pattern of sks splitting consists of flow induced deformation in the asthenosphere caused by the absolute plate motion roughly towards w or wsw with flow channeled around a thick cratonic keel in the southern brasilia and in the ribeira fold belts the large delays and the parallelism with geological structures indicate additional anisotropy contribution from lithospheric sources

nucleosynthetic osmium isotope anomalies in acid leachates of the murchison meteorite abstract we present osmium isotopic results obtained by sequential leaching of the murchison meteorite which reveal the existence of very large internal anomalies of nucleosynthetic origin e 184 os from 108 to 460 e 186 os from 14 1 to 12 6 e 188 os from 2 6 to 1 6 e 190 os from 1 7 to 1 1 despite these large variations the isotopic composition of the total leachable osmium weighted average of the leachates is close to that of bulk chondrites this is consistent with efficient large scale mixing of os isotopic anomalies in the protosolar nebula the os isotopic anomalies are correlated and can be explained by the variable contributions of components derived from the s r and p processes of nucleosynthesis surprisingly much of the s process rich osmium is released by relatively mild leaching suggesting the existence of an easily leachable s process rich presolar phase or alternatively of a chemically resistant r process rich phase taken together with previous evidence for a highly insoluble s process rich carrier such as sic these results argue for the presence of several presolar phases with anomalous nucleosynthetic compositions in the murchison meteorite the s process composition of os released by mild leaching diverges slightly from that released by aggressive digestion techniques perhaps suggesting that the presolar phases attacked by these differing procedures condensed in different stellar environments the correlation between e 190 os and e 188 os can be used to constrain the s process 190 os 188 os ratio to be 1 275 0 043 such a ratio can be reproduced in a nuclear reaction network for a macs value for 190 os of 200 22 mbarn at 30 kev more generally these results can help refine predictions of the s process in the os mass region which can be used in turn to constrain the amount of cosmoradiogenic 187 os in the solar system and hence the age of the galaxy we also present evidence for extensive internal variation of 184 os abundances in the murchison meteorite a steep anti correlation is observed between e 184 os and e 188 os since 184 os is formed uniquely by the p process this anti correlation cannot be explained by variable addition or subtraction of s process os to average solar system material instead this suggests that p process rich presolar grains e g supernova condensates may be present in meteorites in sufficient quantities to influence the os isotopic compositions of the leachates nevertheless 184 os is a low abundance isotope and we cannot exclude the possibility that the measured anomalies for this isotope reflect unappreciated analytical artifacts

mantle flow under the western united states from shear wave splitting we show that sks splitting in the westernmost united states polarization of the fastest shear waves and splitting times including their back azimuthal dependence can be explained by a geodynamic model that includes a continuum mechanics description of plate motions and underlying asthenospheric circulation models that include a counterflow at depths of 300 km are preferred which may indicate a far field effect of the farallon slab anomaly sinking underneath the central continental united states this finding is broadly consistent with earlier suggestions and we demonstrate that a mechanically coupled system though with a strong viscosity contrast with depth is consistent with the data we explore the depth dependence of predicted anisotropy by means of computing seismogram synthetics and comparing synthetic splits with observations some patterns in the data including null observations are matched well linked models of geodynamic flow and mineral alignment in the mantle provide a means to test the relationship between strain and the saturation of texturing lower fabric saturation strains than for global models are preferred by the data which may reflect the relatively active tectonic setting and thin asthenosphere of the study region in general our results show that seismic anisotropy when interpreted jointly with mineral physics theories may be used to quantitatively constrain the spatial character of flow and the degree of force coupling at depth

complex upper mantle anisotropy in the pacific northwest evidence from sks splitting abstract we use a dense network of observations and an automated method of analysis to investigate complex patterns of seismic anisotropy in the pacific northwest we present sks splitting results for approximately 220 broadband seismic stations including 33 stations from the new wallowa2 array deployed between 2016 and 2018 in northeast oregon our data set contains approximately 3300 splitting measurements our anisotropy measurements indicate that anisotropy in the pacific northwest can be divided into two domains near the subduction zone anisotropy is perpendicular to the slab and inboard of the subduction zone anisotropy tends to parallel the velocity structure and is roughly oriented with absolute plate motion however splitting analysis performs poorly in northeast oregon and results are not consistent with uniform or layered anisotropy we argue that the complex splitting behavior in northern oregon is a result of spatially variable upper mantle anisotropy related to the complex upper mantle seismic structure interpreting the mantle anisotropy in this region requires methods not limited to standard ray theoretical shear wave splitting analysis

toroidal mantle flow through the western u s slab window the circular pattern of anisotropic fast axis orientations of split sks arrivals observed in the western u s cannot be attributed reasonably to either preexisting lithospheric fabric or to asthenospheric strain related to global scale plate motion a plume origin for this pattern accounts more successfully for the anisotropy field but little evidence exists for an active plume beneath central nevada we suggest that mantle flow around the edge of the sinking gorda juan de fuca slab is responsible for creating the observed anisotropy seismic images and kinematic reconstructions of gorda juan de fuca plate subduction have the southern edge of this plate extending from the mendocino triple junction to beneath central nevada and flow models of narrow subducted slabs produce a strong toroidal flow field around the edge of the slab consistent with the observed pattern of anisotropy this flow may enhance uplift extension and magmatism of the northern basin and range while inhibiting extension of the southern basin and range

anomalous mantle transition zone beneath the yellowstone hotspot track the origin of the yellowstone and snake river plain volcanism has been strongly debated the mantle plume model successfully explains the age progressive volcanic track but a deep plume structure has been absent in seismic imaging here i apply diffractional tomography to receiver functions recorded at usarray stations to map high resolution topography of mantle transition zone discontinuities the images reveal a trail of anomalies that closely follow the surface hotspot track and correlate well with a seismic wave speed gap in the subducting farallon slab this observation contradicts the plume model which requires anomalies in the mid mantle to be confined in a narrow region directly beneath the present day yellowstone caldera i propose an alternative interpretation of the yellowstone volcanism about 16 million years ago a section of young slab that had broken off from a subducted spreading centre in the mantle first penetrated the 660 km discontinuity beneath oregon and idaho and pulled down older stagnant slab slab tearing occurred along pre existing fracture zones and propagated northeastward this reversed polarity subduction generated passive upwellings from the lower mantle which ascended through a water rich mantle transition zone to produce melting and age progressive volcanism the mantle transition zone in the western united states is perturbed along a path that mirrors the line of the yellowstone hotspot track at the surface according to analysis of tomographic data

zirconium isotope evidence for incomplete admixing of r process components in the solar nebula abstract isotopic anomalies in mo and zr have recently been reported for bulk chondrites and iron meteorites and have been interpreted in terms of a primordial nucleosynthetic heterogeneity in the solar nebula we report precise zr isotopic measurements of carbonaceous ordinary and enstatite chondrites eucrites mesosiderites and lunar rocks all bulk rock samples yield isotopic compositions that are identical to the terrestrial standard within the analytical uncertainty no anomalies in 92zr are found in any samples including high nb zr eucrites and high and low nb zr calcium aluminum rich inclusions cais these data are consistent with the most recent estimates of

simultaneous inversion of mantle properties and initial conditions using an adjoint of mantle convection through the assimilation of present day mantle seismic structure adjoint methods can be used to constrain the structure of the mantle at earlier times i e mantle initial conditions however the application to geophysical problems is restricted through both the high computational expense from repeated iteration between forward and adjoint models and the need to know mantle properties such as viscosity and the absolute magnitude of temperature or density a priori we propose that an optimal first guess to the initial condition can be obtained through a simple backward integration sbi of the governing equations thus lessening the computational expense given a model with known mantle properties we show that a solution based on an sbi generated first guess has smaller residuals than arbitrary guesses mantle viscosity and the effective rayleigh number are crucial for mantle convection models neither of which is exactly known we place additional constraints on these basic mantle properties when the convection induced dynamic topography on earth s surface is considered within an adjoint inverse method besides assimilating present day seismic structure as a constraint we use dynamic topography and its rate of change in an inverse method that allows simultaneous inversion of the absolute upper and lower mantle viscosities scaling between seismic velocity and thermal anomalies and initial condition the theory is derived from the governing equations of mantle convection and validated by synthetic experiments for both one layer viscosity and two layer viscosity regionally bounded spherical shells for the one layer model at any instant of time the magnitude of dynamic topography is controlled by the temperature scaling while the rate of change of topography is controlled by the absolute value of viscosity for the two layer case the rate of change of topography constrains upper mantle viscosity while the magnitude of dynamic topography determines the temperature scaling lower mantle viscosity when upper mantle lower mantle density anomaly dominates the flow field this two stage scheme minimizes the tradeoff between temperature and lower mantle viscosity for both cases we show that the theory can constrain mantle properties with errors arising through the adjoint recovery of the initial condition for the two layer model this error is manifest as a tradeoff between the temperature scaling and lower mantle viscosity

subduction zone anisotropy beneath corvallis oregon a serpentinite skid mark of trench parallel terrane migration 1 we studied the back azimuth dependence of ps converted phases at gsn station cor corvallis oregon using broadband p receiver functions computed from 602 teleseismic earthquakes the amplitudes and polarities of the transverse ps phases are largely two lobed which indicates anisotropy with a tilted symmetry axis a double peaked ps conversion at 4 5 6 5 s delay has the moveout of a dipping slab but is not consistent with simple deflection of the ps converted phase by the dipping interface a polarity flip on the transverse receiver function near north south back azimuth indicates an anisotropic symmetry axis aligned north south far from the convergence direction n68 e the ps phase is modeled using reflectivity synthetics with a highly anisotropic layer of depressed wave speed vp 6 0 6 5 km s near 40 km depth at the slab interface with the overriding north american plate one dimensional modeling suggests 10 anisotropy in the supraslab layer with a slow symmetry axis oriented n5 w at a 60 tilt from the vertical adjustments for the effect of slab interface dip on ps amplitude suggest a somewhat lower 7 anisotropy we infer a thin 7 km anisotropic detachment zone for the northward slippage of the siletz forearc terrane along the top of the descending slab a motion consistent with gps measurements and models of regional lithospheric dynamics serpentinite is a likely constituent for the deep anisotropic layer owing to its reduced vp and high poisson ratio the ductile rheology and hydrated composition of serpentinite make it a plausible lithology for a mechanical detachment zone

intrinsic w nucleosynthetic isotope variations in carbonaceous chondrites implications for w nucleosynthesis and nebular vs parent body processing of presolar materials abstract the progressive dissolution of the carbonaceous chondrites orgueil ci1 murchison cm2 and allende cv3 with acids of increasing strength reveals correlated w isotope variations ranging from 3 5 e182w and 6 5 e183w in the initial leachate acetic acid to 60 e182w and 40 e183w in the leachate residue the observed variations are readily explained by variable mixing of s process depleted and s process enriched components one w s process carrier is sic however the observed anomaly patterns and mass balance considerations require at least on additional s process carrier possibly a silicate or sulfide the data reveal well defined correlations which provide a test for s process nucleosynthesis models the correlations demonstrate that current models need to be revised and highlight the need for more precise w isotope data of sic grains furthermore the correlations provide a mean to disentangle nucleosynthetic and radiogenic contributions to 182w e182wcorrected e182wmeasured 1 41 0 05 e183wmeasured e182wcorrected e182wmeasured 0 12 0 06 e184wmeasured a prerequisite for the successful application of the hf w chronometer to samples with nucleosynthetic anomalies the overall magnitude of the w isotope variations decreases in the order ci1 cm2 cv3 this can be interpreted as the progressive thermal destruction of an initially homogeneous mixture of presolar grains by parent body processing however not only the magnitude but also the w anomaly patterns of the three chondrites are different in particular leach step 2 that employs nitric acid reveals a s deficit signature for murchison but a s excess for orgueil and allende this could be the result of redistribution of anomalous w into a new phase by parent body alteration or the fingerprint of dust processing in the solar nebula given that the thermal and aqueous alteration of murchison is between the ci and cv3 chondrites parent body processing is probably not the sole cause for creating the different pattern small scale nebular redistribution of anomalous w may have played a role as well similar nebular processes possibly acted differently on specific carrier phases and elements resulting in the diverse nucleosynthetic signatures observed in planetary materials today

vertical mantle flow associated with a lithospheric drip beneath the great basin rapid surface uplift or subsidence and voluminous magmatic activity have often been ascribed to regional scale downwelling of lithospheric mantle however because lithospheric drips sinking plumes of cold and dense lithosphere are relatively small and transient features direct evidence of their existence has been difficult to obtain moreover the significant vertical mantle flow that should be associated with such structures has not been detected here we integrate seismic anisotropy data with tomographic images to identify and describe a lithospheric drip beneath the great basin region of the western united states the feature is characterized by a localized cylindrical core of cooler material with fast seismic velocities and mantle flow that rapidly shifts from horizontal to vertical our numerical experiments suggest that the drip can be generated by gravitational instability resulting from a density anomaly of as little as 1 and a localized temperature increase of 10 the drip tilts to the northeast opposite to the motion of the north american plate in the hotspot reference frame and thereby indicates northeast directed regional mantle flow seismic anisotropy data for the great basin region of the western united states coupled with tomographic images help delineate a northeast dipping lithospheric drip numerical experiments suggest that the drip could have formed owing to gravitational instability triggered by a density increase of about 1 and a temperature increase of about 10

origin of azimuthal seismic anisotropy in oceanic plates and mantle abstract seismic anisotropy is ubiquitous in the earth s mantle but strongest in its thermo mechanical boundary layers azimuthal anisotropy in the oceanic lithosphere and asthenosphere can be imaged by surface waves and should be particularly straightforward to relate to well understood plate kinematics and large scale mantle flow however previous studies have come to mixed conclusions as to the depth extent of the applicability of paleo spreading and mantle flow models of anisotropy and no simple globally valid relationships exist here we show that lattice preferred orientation lpo inferred from mantle flow computations produces a plausible global background model for asthenospheric anisotropy underneath oceanic lithosphere the same is not true for absolute plate motion apm models a 200 km thick layer where the flow model lpo matches observations from tomography lies just below the 1200 c isotherm of a half space cooling model indicating strong temperature dependence of the processes that control the development of azimuthal anisotropy we infer that the depth extent of shear and hence the thickness of a relatively strong oceanic lithosphere can be mapped this way these findings for the background model and ocean basin specific deviations from the half space cooling pattern are found in all of the three recent and independent tomographic models considered further exploration of deviations from the background model may be useful for general studies of oceanic plate formation and dynamics as well as regional scale tectonic analyses

origin of isotopic heterogeneity in the solar nebula by thermal processing and mixing of nebular dust we have investigated mo and w isotope compositions in acid leachates and an insoluble residue from the murchison carbonaceous chondrite the new data reveal variable enrichments of s and r process isotopes and demonstrate that several isotopically diverse presolar components are present in murchison the insoluble residue is enriched in s process mo and w evidently due to the enrichment of presolar sic grains in contrast mo and w released by leaching with weak acids are depleted in s process isotopes most likely reflecting the isotope composition of the homogenized portion of the protosolar nebula the mo and w isotope compositions of the different leach steps are broadly correlated as expected from s process nucleosynthesis theory indicating that mo and w are presumably hosted in the same carriers however at the bulk meteorite scale no nucleosynthetic w isotope anomalies have been identified except for ivb iron meteorites in spite of large mo isotope heterogeneities among the same samples this decoupling of mo and w isotopes in bulk meteorites may reflect physical mixing of varying proportions of isotopically diverse presolar components with a normal solar nebula component due to the high w mo ratio and near terrestrial w isotope composition of the latter such mixing has no measurable effect on w isotopes but results in large mo isotope variations alternatively thermal processes within the solar nebula imparted mo isotope heterogeneity on an initially homogeneous mixture of presolar dust while w was not affected removal of volatile mo oxides during the thermal destruction of fragile presolar components would have created isotopically heterogeneous reservoirs of nebular dust accretion of meteorite parent bodies from such variably processed dust would thus result in mo isotope heterogeneities at the bulk meteorite scale other elements such as os and w were not or only slightly affected because they were more refractory during thermal processing and therefore remained isotopically homogeneous thermal processing of presolar dust within the solar nebula can thus account for both isotope heterogeneities observed for some elements and the lack of such isotopic heterogeneity for other elements

molybdenum isotope anomalies in meteorites constraints on solar nebula evolution and origin of the earth the early evolution of the solar nebula involved substantial transport of mass resulting in mixing and homogenization of isotopically diverse materials that were contributed to the solar system from multiple stellar nucleosynthetic sources the efficiency of this mixing as well as its timescale can be quantified by determining nucleosynthetic isotope variations among meteorites and terrestrial planets here we present mo isotopic data for a wide range of samples including ca al rich inclusions chondrites and differentiated meteorites as well as martian and terrestrial samples most meteorites are depleted in s process mo relative to the earth and only the iab iiicd irons angrites and martian meteorites have terrestrial mo isotopic compositions in contrast most ca al rich inclusions are enriched in r process mo but one inclusion is characterized by a large s process deficit molybdenum isotopic anomalies in the bulk meteorites correlate with those in ru exactly as predicted from nucleosynthetic theory but no obvious correlation is apparent between mo and ni anomalies therefore s process mo and ru seem to be hosted in the same carrier which must be distinct from the carrier responsible for isotopic anomalies in the fe group elements ni cr ti furthermore the isotopic heterogeneity in mo and other elements contrasts with the isotopic homogeneity for hf and os indicating that different s process carriers once existed in the early solar nebula and that only some of these were heterogeneously distributed the mo isotopic anomalies of meteorites and their components decrease over time and with increasing size of the parent bodies providing evidence for a progressive homogenization of the solar nebula however the carbonaceous chondrites exhibit larger mo anomalies than expected for their age indicating that they received a greater portion of material from the outer solar system where homogenization was slow than other meteorite parent bodies and terrestrial planets compared to the meteorites earth is enriched in s process mo and must have accreted from material distinct from the meteorites combined mo and o isotopic data show that the composition of the earth cannot be reconstructed by any known combination of meteorites implying that meteorites may be inappropriate proxies for the isotopic composition of the bulk earth this is exemplified by the covariation of 92mo and 142nd anomalies in chondrites showing that the 142nd deficit of chondrites compared to the accessible earth may not unequivocally be interpreted as a signature of an early differentiation of the earth however further high precision isotopic data are needed to evaluate the role of chondrites in defining the isotopic composition of the earth

thermoluminescence dating of quaternary red sand beds a case study of coastal dunes in sri lanka red sand beds that occupy extensive tracts of coastal humid tropics have conventionally been considered undatable results of a first successful attempt to provide an absolute chronology to these sediments based on thermoluminescence dosimetry are presented along with a discussion on the importance of an understanding of ambient geochemistry in the application of the thermoluminescence technique the results on the sri lankan coastal region indicate that the sea level in the region may have been affected by regional tectonics during the past 70 000 years based on in situ finds of geometric microliths in the red sands a date of ca 25 000 years is ascribed to them suggesting them to be substantially older than believed so far

the lithophile trace elements in enstatite chondrites we report on the abundances of a selected set of lithophile trace elements namely rees y rb ba sr zr hf nb th u in a comprehensive suite of enstatite chondrites ec 13 eh and 11 el eh3 and el3 display only minor deviations from chondritic distributions for these elements in most metamorphosed ec a wide range of compositions is observed and suggests a mobility of many of the elements studied during the history of these rocks for example el6 chondrites exhibit light ree and nb depletions negative eu anomalies and positive y anomalies more important trace element fractionations are observed in metamorphosed eh like st marks rb depletion lap 02225 rb nb zr eu light ree depletions and galim b which displays large ba sr eu nb and light ree depletions leaching experiments were undertaken to investigate the contributions of sulfides in the whole rock budgets these phases control not only the ree budget but also important fractions of the other elements we studied these fractions strongly depend on the type of the rock eh or el and metamorphic grade for many elements the sulfide contributions increase with the metamorphic grades the trace element abundances of silicate residues are extremely variable negative sm and yb anomalies are observed in el3 and eh3 residues and are certainly the results of early nebular processes such anomalies are lacking in residues obtained with most metamorphosed ec underlining the importance of trace element redistributions during metamorphism in addition el6 residues display distinctive positive y anomalies that could be potentially ascribed to a less chalcophile behavior than ho in the conditions that prevailed during el metamorphism

mapping the distribution of weathered pleistocene wadi deposits in southern jordan using aster spot 5 data and laboratory spectroscopic analysis abstract in the arid regions of the levant ancient wadi fills act as a terrestrial sedimentary archive with a high potential for preserving archaeological findings this current study combines remote sensing with laboratory vis spectroscopy to investigate the spatial distribution of alluvial wadi fills in a small catchment in southern jordan due to its homogeneous sandstone geology the composition of the alluvial sediments is highly influenced by the local bedrock whilst fluvial relocation and surface weathering processes initiated a secondary alteration of dominant iron oxides fe 3 the differences in mineralogical composition of the sediments enable the detection and mapping of wadi deposits by remote sensing using different spectral combinations of aster and spot 5 satellite images additionally laboratory measurements of reflectance spectra were applied on selected surface samples and soil sections from the study area in order to verify the information derived from remote sensing and to quantify the degree of surface weathering and pedogenic processes the results show that an initial transformation from hematite to goethite is the dominant process related to the recent arid conditions in the study area furthermore it is possible to predict potential new archaeological finding areas using remote sensing techniques

anisotropy and vp vs in the uppermost mantle beneath the western united states from joint analysis of pn and sn phases pn and sn phases are valuable for resolving velocity structure in the mantle lid as they propagate horizontally right below the moho relatively few sn tomography attempts have been made compared to pn because sn is often highly attenuated or buried in p wave coda usarray has greatly increased data coverage for regional phases and both pn and sn are routinely picked by network analysts here we jointly invert pn and sn arrival time residuals with a modified time term analysis and a regularized tomography method and present new maps of crustal thickness uppermost mantle p velocity perturbations vp vs ratios and azimuthal anisotropy strength and orientation beneath the western united states the results indicate partially molten mantle below the snake river plain and the colorado plateau the seismic structure of the top 40 km of the mantle below the colorado plateau differs from that seen at greater depths in other studies such as surface wave or teleseismic body wave tomography whereas the snake river plain anomaly just below the moho is comparable to structures seen at about 200 km depth pn fast axes provide complementary information to sks shear wave splitting observations and our analysis indicates that in several regions in the western united states the orientation of azimuthal anisotropy changes with depth in the upper mantle however we have so far been unable to resolve shear wave splitting directly in sn waveforms which seem to be dominated by sn sv energy

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

a sharp cratonic lithosphere asthenosphere boundary beneath the american midwest and its relation to mantle flow abstract beneath the american midwest s to p sp converted wave imaging and multi mode surface wave tomography identify a north trending transition in seismic structure at 150 250 km depth to the east of this american midwest transition amt the lithosphere asthenosphere boundary lab is imaged as a 1 2 sp sv amplitude arrival at 200 240 km depth consistent with the depth of negative shear velocity and azimuthal anisotropy gradients imaged by surface wave tomography to the west of the amt sp conversions are much shallower at 150 190 km depth and are much weaker 0 7 or absent azimuthal anisotropy constrained by surface wave tomography also changes across the amt with stronger anisotropy to the east of the transition beneath the thicker lithospheric root we suggest that the seismic changes across the amt can be explained by considering the effects of asthenospheric flow beneath the leading edge of the thick lithospheric root the mantle flow is dominantly driven by the drift of the north america plate locally higher flow velocities are expected where the asthenosphere is forced to flow beneath the thicker root this mantle underflow could create a sharper seismic lab east of the amt via two effects first the local increase in flow velocities could steepen the thermal gradient at the base of the lithosphere and hence the isotropic velocity contrast second the increased strain rate along edge of the lithosphere could enhance the magnitude of azimuthal anisotropy our results suggest that seismically detectable lab sharpness variations could be used to constrain geographic variations in coupling between plates and mantle convection a secondary result is the image of a mid lithospheric discontinuity arrival at 80 110 km depth that is found primarily to the east of the amt this arrival is interpreted as produced by a layer of low velocity metasomatic minerals that have accumulated since the 1 8 ga creation of the lithosphere

the role of hydrous phases in the formation of trench parallel anisotropy evidence from rayleigh waves in cascadia 1 the cause of seismic anisotropy exhibiting trench parallel fast directions in subduction systems has been the subject of significant recent research we provide new constraints on the contributions of hydrous phases to seismic anisotropy from an unusually well localized region of trench parallel fast directions in rayleigh wave phase velocities near the cascade arc at 45 to 66 s periods we constrain the location of the anisotropic material to within or directly above the oceanic plate using the depth sensitivity of rayleigh waves as a function of frequency and the accurate slab imaging available for cascadia from scattered wave studies we infer that the likely source of trench parallel anisotropy is either a thin layer of sheared hydrous material directly above the slab or hydrated outer rise faults in the upper part of the subducting plate similar contributions to trench parallel anisotropy from hydrous phases are likely stronger in other subduction zones

mantle provinces under north america from multifrequency p wave tomography this is a survey of mantle provinces large scale seismic anomalies under north america from the surface down to 1500 1800 km depth the underlying p velocity model was obtained by multifrequency tomography a waveform based method that systematically measures and models the frequency dependence of teleseismic body waves a novel kind of three dimensional rendering technique is used to make the considerable structural complexities under north america accessible in the transition zone and below the north american mantle is dominated by seismically fast provinces which represent distinct subduction episodes of the farallon plate i attempt to date and interpret the various slab fragments by reconciling their present positions with paleotrench locations from plate tectonic reconstructions and with major geologic surface episodes differences in vertical sinking velocity have led to large vertical offsets across adjacent coeval slabs some of the mantle provinces have not been discussed much previously including 1 a seismically slow blanket overlying the oldest farallon subduction along the eastern continental margin 2 a transition zone slab coeval with the laramide orogeny ca 80 60 myr which i discuss in analogy to the stagnant slab subduction style commonly found in the western pacific today 3 the lower mantle root of present day cascadia subduction which may have started out as intraoceanic subduction 4 a lower mantle slab under arizona and new mexico the last material to subduct before strike slip motion developed along the san andreas boundary and 5 two narrow plate tears thousands of kilometers long one of which is the subducted conjugate of the mendocino fracture zone

shear wave splitting continental keels and patterns of mantle flow in this study we investigated the origin of seismic anisotropy in the mantle beneath north america in particular we evaluated whether shear wave splitting patterns in eastern north america are better explained by anisotropy caused by lithospheric deformation anisotropy due to mantle flow beneath the lithosphere or a combination of both we examined new measurements of shear wave splitting from the missouri to massachusetts broadband seismometer array moma the north american mantle anisotropy and discontinuity experiment nomad as well as splitting parameters from several previous studies we developed a simple finite difference model that approximates mantle flow around a complex three dimensional continental lithospheric keel to evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern north america we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model our results indicate that a significant portion of observed shear wave splitting in eastern north america can be explained by mantle flow around the continental keel however shear wave splitting patterns in a few regions of eastern north america indicate that a component of lithospheric anisotropy must exist particularly in regions containing the largest keel thicknesses for eastern north america as well as for splitting observations in australia europe and south america we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow

the origin of oldhamite in unequilibrated enstatite chondrites abstract we report the results of an ion probe study of rare earth elements rees in six unequilibrated enstatite chondrites oldhamite is confirmed as the major but not the only ree carrier in these meteorites data are also provided for other minerals enstatite niningerite and alabandite that carry detectable amounts of rees although the variety of ree patterns in oldhamite is not as extreme as in hibonite there are seven distinct ree patterns four of which form a continuum that characterize most oldhamite grains from e chondrites two of these patterns f and g were each found in only one weathered grain there is no correlation between ree pattern and oldhamite petrologic setting our trace element data are used to evaluate earlier work and a number of elements zr y sc rees sr se and probably sb and cs including both refractory and volatile species under oxidizing conditions are found enriched in oldhamite a comparison of ree patterns with the results of equilibrium condensation calculations indicates that nebular processes were important in determining the composition of the oldhamite precursors whereas metamorphic processes also affected oldhamite compositions enrichments of chalcophiles are not necessarily due to interaction of oldhamite with nebular gas at low temperatures as previously suggested oldhamite like hibonite was formed from melts whose trace element compositions were determined by high temperature transient events in the solar nebula

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

mapping p wave azimuthal anisotropy in the crust and upper mantle beneath the united states abstract much progress has been made on revealing seismic structure and mantle dynamics beneath the united states us with the earthscope usarray project seismic anisotropy revealed by shear wave splitting studies provides important constraints on constructing geodynamic models with regard to the seismic images but the shear wave splitting observations have poor vertical resolution and so their interpretations are often not unique in this work we used a large number of arrival time data from local and distant earthquakes recorded by the usarray to determine the first p wave azimuthal anisotropy tomography of the crust and upper mantle beneath the us our results show that fast velocity directions fvds in the lithosphere under the tectonically active areas correlate well with the surface tectonic features suggesting that the p wave anisotropy mainly reflects the present deformation a circular pattern of the fvds centered in the great basin is revealed which is well consistent with the specific circular shear wave splitting observations there suggesting that the anisotropy occurs in the crust and uppermost mantle in contrast beneath the stable cratonic region the fvds revealed by this study differ from the shear wave splitting observations but consistent with the features of gravity and magnetic anomalies indicating that the p wave fvds mainly reflect the fossil anisotropy in the lithosphere whereas the s wave splitting observations mainly reflect the significant anisotropy in the asthenosphere the present results shed new light on the seismic anisotropy in the crust and upper mantle and provide new constraints on constructing geodynamic models beneath the us

samarium neodymium chronology and rubidium strontium systematics of an allende calcium aluminum rich inclusion with implications for 146sm half life abstract calcium aluminum rich inclusions cais are primitive objects that formed within the protoplanetary disk surrounding the young sun recent pb pb chronologic studies have demonstrated that cais are the oldest solar system solids crystallizing 4567 ma ago amelin et al 2002 connelly et al 2012 the isotope systematics of cais therefore provide critical insight into the earliest history of the solar system although sm nd and rb sr geochronometers are highly effective tools for investigating cosmochemical evolution in the early solar system previous studies of cais have revealed evidence for isotopically disturbed systems here we report new age data for allende cai al3s4 derived from both the long lived 147sm 143nd and short lived 146sm 142nd isotopic systems the 147sm 143nd chronometer yields an age of 4560 34 ma that is concordant with 207pb 206pb ages for cais and indicates that the sm nd system was not significantly disturbed by secondary alteration or nucleosynthetic processes the slope of the 146sm 142nd isochron defines the solar system initial 146sm 144sm of 0 00828 0 00044 this value is significantly different from the value of 0 0094 determined by kinoshita et al 2012 ages recalculated from all published 146sm 142nd isochron data using the traditional 103 ma half life and the initial 146sm 144sm value determined here closely match pb pb and 147sm 143nd ages determined on the same samples in contrast ages recalculated using the 68 ma half life determined by kinoshita et al 2012 and either of the initial 146sm 144sm values are often anomalously old this is particularly true for the youngest samples with 146sm 142nd isochron ages that are most sensitive to the choice of 146sm half life used in the age calculation in contrast to the sm nd isotope system the rb sr system is affected by alteration but yields an apparent isochron with a slope corresponding to a much younger age of 4247 110 ma although the rb sr system in cais appears to be disturbed the initial 87sr 86sr value determined from the isochron is 0 698942 0 000008 and closely approximates estimates of the initial solar system value although this isochron may be a mixing line it might also record alteration on the allende parent body in which rb was added to the al3s4 cai that was initially largely devoid of rb

strong seismic anisotropy in the deep upper mantle beneath the cascadia backarc constraints from probabilistic finite frequency sks splitting intensity tomography abstract the volcanically active high lava plains hlp region is a striking tectonomagmatic feature of eastern oregon in the backarc of the cascadia subduction zone it features young

the role of a mantle plume in the formation of yellowstone volcanism the origin of the yellowstone volcanic province remains debated proposed hypotheses involve either a mantle plume or not recent tomographic images allow a quantitative evaluation of the plume hypothesis and its interaction with the farallon slabs using 4 d geodynamic models with data assimilation we find that the slab is always in the way of the initially rising plume and that the plume could reach the surface only through the broken slab hinge at 15 ma for most of the time the sinking slabs dominate the mantle flow and prohibit upwelling we find that a plume that satisfies the present mantle image beneath yellowstone fails to predict both voluminous hot materials at shallow depths beneath the western u s and the age migration of the hot spot tracks we suggest that a plume is likely to have much less influence on the yellowstone volcanism than previously thought

the rate of iron oxide enrichment in arid zone alluvial fan soils tunisian southern atlas measured by mineral magnetic techniques abstract rates of iron oxide enrichment of soils on a segmented alluvial fan in southern tunisia are examined mineral magnetic and chemical characteristics of soils developed on two dated relict segments and on contemporary alluvium suggest that the rate of fe oxide enrichment was more rapid in the holocene 2 6 10 8 g g 1 yr 1 over the last 5000 years than in the pleistocene 1 91 10 9 g g 1 yr 1 over the previous 45 000 years thus fe oxide concentration may be used to determine an ordinal chronology of some geomorphological surfaces in drylands but rates of enrichment cannot be assumed to be uniform over time

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

complex and variable crustal and uppermost mantle seismic anisotropy in the western united states deformation in the crust and mantle measured using seismic anisotropy is poorly constrained in the western united states because of inconsistencies in the existing data a three dimensional model that reconciles these discrepancies reveals that seismic anisotropy in the crust correlates with different geological provinces but is unrelated to anisotropy in the underlying mantle

experimental deformation of olivine single crystals at mantle pressures and temperatures abstract deformation experiments were carried out in a deformation dia high pressure apparatus d dia on oriented san carlos olivine single crystals at pressure p ranging from 3 5 to 8 5 gpa temperature t from 1373 to 1673 k and in poor water condition oxygen fugacity fo2 was maintained within the olivine stability field and contact with enstatite powder ensured an orthopyroxene activity aopx 1 two compression directions were tested promoting either 1 0 0 slip alone or 0 0 1 slip alone in 0 1 0 crystallographic plane here called respectively a slip and c slip constant applied stress and specimen strain rates e were monitored in situ using time resolved x ray synchrotron diffraction and radiography respectively transmission electron microscopy tem investigation of run products revealed that dislocation creep was responsible for sample deformation comparison of the obtained high p deformation data with the data obtained at room p by bai et al bai q mackwell s l kohlstedt d l 1991 high temperature creep of olivine single crystals 1 mechanical results for buffered samples journal of geophysical research 96 2441 2463 on identical materials deformed at comparable t fo2 aopx conditions allowed quantifying the p effect on a slip and c slip rheological laws a slip transition with increasing pressure from dominant a slip to dominant c slip is documented a slip appears sensitive to pressure which translates into the high activation volume v a 12 4 c m 3 mol in the corresponding rheological law while pressure has little effect on c slip with v c 3 4 c m 3 mol these results may explain the discrepancy between olivine low p and high p deformation data which has been debated in the literature for more than a decade

d rex a program for calculation of seismic anisotropy due to crystal lattice preferred orientation in the convective upper mantle summary models of development of lattice preferred orientation lpo of crystals aggregates in convec tive flow are necessary to interpret the anisotropic seismic signature of the earth s upper mantle for that purpose we previously developed a model of lpo evolution in olivine aggregates by plastic deformation and dynamic recrystallization by subgrain rotation and grain boundary migration this paper presents a refined version of that model called d rex for dynamic recrystallization induced lpo a public version of which is made available on our web site the code displays two new features 1 enstatite is incorporated in the aggregates and 2 grain boundary sliding gbs of small grains is taken into account enstatite is incorporated on the assumption of no direct interaction with olivine the fast a axis of enstatite grains tend to be parallel to the slow c axis of olivine which dilutes the total anisotropy grain boundary sliding is included using a threshold dimensionless volume fraction defined as the ratio of the initial size of the grains over the size for which gbs is the dominant mechanism of deformation grains with a dimensionless volume smaller than do not rotate by plastic deformation and their strain energy is set to zero comparison with torsion experiments at very large strain constrains the threshold dimensionless volume to 0 3 0 1 the incorporation of grain boundary sliding prevents the lpo from becoming singular at large strains and yields more realistic predictions our kinematic formalism and the model s semi analytical character insures that it is fast robust and stable it can be applied efficiently to arbitrary 3 d convective flows

usarray observations of quasi love surface wave scattering orienting anisotropy in the cascadia plate boundary 1 quasi love ql surface wave anomalies are observed along multiple great circle paths crossing the pacific and through the mid ocean ridge and subduction zone associated with the juan de fuca plate the long period ql waves observed on the usarray component of earthscope arrive immediately after the fundamental love wave arrival suggesting that the ql waves were generated close to usarray and near the plate boundary the dense usarray network allows the analysis of the azimuthal dependence of the ql surface wave scattering to determine the horizontal anisotropic axes of symmetry in earth s upper mantle we focus on the location 42 5 n and 127 5 w just offshore of the southern oregon coastline near the gorda ridge and we analyze the ql scattering along 14 different great circle paths crossing through this location we low pass filter the data to suppress overtones and crustal effects ql amplitude and polarity constrains an anisotropic axis of symmetry that correlates well with the hot spot referenced juan de fuca plate motion and regional shear wave splitting ql observations at multiple periods 50 75 100 150 and 200 s suggest a maximum love to rayleigh scattering at 100 s period which is consistent with anisotropic lateral gradients in the asthenosphere beneath the juan de fuca and gorda plates our observations do not follow the predictions of slab rollback and suggest the entrainment of asthenosphere with the overriding juan de fuca and gorda plates

depth dependent azimuthal anisotropy in the western us upper mantle abstract we present the results of a joint inversion of long period seismic waveforms and sks splitting measurements for 3d lateral variations of anisotropy in the upper mantle beneath the western us incorporating recent datasets generated by the usarray deployment as well as other permanent and temporary stations in the region we find that shallow azimuthal anisotropy closely reflects plate motion generated shear in the asthenosphere in the shallow upper mantle 70 150 km depth whereas at depths greater than 150 km it is dominated by northward and upward flow associated with the extension of the east pacific rise under the continent constrained to the east by the western edge of the north american craton and to the north by the presence of the east west trending subduction zone in particular the depth integrated effects of this anisotropy explain the apparent circular pattern of sks splitting measurements observed in nevada without the need to invoke any local anomalous structures

upper mantle seismic anisotropy as a constraint for mantle flow and continental dynamics of the north american plate abstract the alignment of intrinsically anisotropic olivine crystals under convection is typically invoked as the cause of the bulk of seismic anisotropy inferred from shear wave splitting sws this provides a means of constraining the interplay between continental dynamics and the deep mantle in particular for densely instrumented regions such as north america after usarray there a comparison of fast orientations from sws with absolute plate motions apm suggests that anisotropy is mainly controlled by plate motions however large regional misfits and the limited realism of the apm model motivate us to further explore sws based anisotropy if sws is estimated from olivine alignment in mantle circulation instead plate driven flow alone produces anisotropy that has large misfits with sws the addition of large scale mantle density anomalies and lateral viscosity variations significantly improves models although a strong continental craton is essential varying its geometry does however not improve the plate scale misfit moreover models based on higher resolution tomography degrade the fit indicating issues with the flow model assumptions and or a missing contributions to anisotropy we thus compute a lithospheric complement to achieve a best fit joint representation of asthenospheric and frozen in lithospheric anisotropy the complement shows coherent structure and regional correlation with independently imaged crustal and upper mantle anisotropy dense sws measurements therefore provide information on depth dependent anisotropy with implications for tectonics but much remains to be understood about continental anisotropy and its origin

segmentation of the farallon slab abstract recent tomography images reveal a complex 3d mantle structure beneath western united states with feature morphology varying rapidly with depth by assimilating plate motion history paleo age of sea floor and paleo geography of plate boundaries in a 3 d numerical model we simulate the farallon juan de fuca subduction during the past 40 ma we find that the highly segmented upper mantle structure of western u s is a direct result of the farallon subduction we show that the tilted horseshoe shaped fast seismic anomaly beneath nevada and utah at 300 600 km depth range is in fact a segment of curled slab subducted since 15 ma and the shallower linear slab beneath the cascades is younger than 5 ma the distinct morphology between these two parts of the subduction system indicates the strong influence of the fast trench rollback since 20 ma the northward migrating jf pa na triple junction and the toroidal flow around slab edges the observed mantle structures are used to constrain the rheology of the upper mantle through matching the shape depth and location of modeled subducted slab segments the inferred viscosity for the asthenosphere is 5 10 19 pa s and those for the transition zone and lower mantle are 1 5 10 21 pa s and 2 10 22 pa s respectively the slab is found to be about 2 orders of magnitude stronger than the ambient mantle above 410 km depth but of similar order of magnitude viscosity in the transition zone

depth constraints on azimuthal anisotropy in the great basin from rayleigh wave phase velocity maps abstract we present fundamental mode rayleigh wave azimuthally anisotropic phase velocity maps obtained for the great basin region at periods between 16 s and 102 s these maps offer the first depth constraints on the origin of the semi circular shear wave splitting pattern observed in central nevada around a weak azimuthal anisotropy zone a variety of explanations have been proposed to explain this signal including an upwelling toroidal mantle flow around a slab lithospheric drip and a megadetachment but no consensus has been reached our phase velocity study helps constrain the three dimensional anisotropic structure of the upper mantle in this region and contributes to a better understanding of the deformation mechanisms taking place beneath the western united states the dispersion measurements were made using data from the usarray transportable array at periods of 16 s and 18 s which mostly sample the crust we find a region of low anisotropy in central nevada coinciding with locally reduced phase velocities and surrounded by a semi circular pattern of fast seismic directions away from central nevada the fast directions are n s in the eastern great basin nw se in the walker lane region and they transition from e w to n s in the northwestern great basin our short period phase velocity maps combined with recent crustal receiver function results are consistent with the presence of a semi circular anisotropy signal in the lithosphere in the vicinity of a locally thick crust at longer periods 28 102 s which sample the uppermost mantle isotropic phase velocities are significantly reduced across the study region and fast directions are more uniform with an e w fast axis the transition in phase velocities and anisotropy can be attributed to the lithosphere asthenosphere boundary at depths of 60 km we interpret the fast seismic directions observed at longer periods in terms of present day asthenospheric flow driven deformation possibly related to a combination of juan de fuca slab rollback and eastward driven mantle flow from the pacific asthenosphere our results also provide context to regional sks splitting observations we find that our short period phase velocity anisotropy can only explain 30 of the sks splitting times despite similar patterns in fast directions this implies that the origin of the regional shear wave splitting signal is complex and must also have a significant sublithospheric component

shear wave splitting and mantle flow beneath the colorado plateau and its boundary with the great basin shear wave splitting measurements from sks and skks phases show fast polarization azimuths that are subparallel to north american absolute plate motion within the central rio grande rift rgr and colorado plateau cp through to the western rim of the cp with anisotropy beneath the cp and central rgr showing a remarkably consistent pattern with a mean fast azimuth of 40 6 e of n approaching the rim from the southeast fast anisotropic directions become north northeast south southwest nne ssw rotate counter clockwise to north south in the cp gb transition and then to nnw sse in the western great basin gb this change is coincident with uppermost mantle s wave velocity perturbations that vary from 4 beneath the western cp and the eastern edge of the marysvale volcanic field to about 8 beneath the gb corresponding delay times average 1 5 sec beneath the central cp decrease to approximately 0 8 sec near the cp gb transition and increase to about 1 2 sec beneath the gb for the central cp we suggest anisotropy predominantly controlled by north american plate motion above the asthenosphere the observed pattern of westward rotating anisotropy from the western cp through the cp gb transition may be influenced to asthenospheric flow around a cp lithospheric keel and or by vertical flow arising from edge driven small scale convection the anisotropic transition from the cp to the gb thus marks a first order change from absolute plate motion dominated lithosphere asthenosphere shear to a new regime controlled by regional flow processes the nnw sse anisotropic fast directions of split sks waves in the eastern gb area are part of a broad circular pattern of seismic anisotropic fast direction in the central gb that has recently been hypothesized to be due to toroidal flow around the sinking juan de fuca gorda slab

fluvio pedogenic processes in an ephemeral stream channel nahal yael southern negev israel abstract a detailed stratigraphic survey conducted in the alluvial fill of the channel of nahal yael a small catchment in the southern negev desert indicates the existence of a continuous compacted red colored unit at an average depth of 50 cm beneath the surficial grey non cohesive alluvium granulometric distinction between the grey and the red alluvium is evident only within the fine

nucleosynthetic nd isotope anomalies in primitive enstatite chondrites abstract we carried out stepwise dissolutions of four primitive enstatite chondrites ec belonging to the eh subgroup large nd isotope anomalies are found in the most refractory phases dissolved using strong acids residues are characterized by excesses in 142nd and deficits in 145nd 148nd and 150nd isotopes the nd anomalies measured in the alha77295 residue are even greater than those measured in the murchison carbonaceous chondrite cc using a similar analytical technique qin et al 2011 once corrected for a common sm nd evolution the 142nd excess in the alha77295 residue is equal to 700 ppm relative to the terrestrial standard value the nd isotope patterns measured in ec and cc residues can be adjusted to coincide by adding a small amount of an s process rich carrier phase such as sic and 0 075 is required to fit the alha7795 residue small isotope differences still persist between these residues even if they can be considered similar within error in enstatite chondrites residues have a deficit in 150nd similar to or smaller than that measured in 148nd whereas in sic extracted from carbonaceous chondrites or in whole rock the deficit in 150nd is always greater than that in 148nd moreover in a binary 142nd 148nd diagram the best fit lines obtained for leachates and residues from carbonaceous chondrites and enstatite chondrites have slightly different slopes for the same 148nd 144nd ratio the anomalous component in an enstatite chondrite has a higher 142nd 144nd ratio compared to carbonaceous chondrites a feature already observed at the whole rock scale our results suggest that different chondrite groups sample different reservoirs of presolar grains formed in different environments assuming that the carrier of this anomalous component measured in residues of enstatite chondrites are sic our results may suggest that different meteorite parent bodies sample reservoirs of presolar sic formed in different stellar environments this could explain why alha77295 the sample which is the most enriched in presolar grains has a bulk 142nd isotope composition similar to the terrestrial value further investigation of enstatite chondrites is needed to test whether the isotope composition of the most refractory phases is similar to that measured in carbonaceous chondrites and in particular the 144sm that is a p process isotope only finally this study highlights the difficulty of interpreting the 142nd excess in terrestrial samples relative to chondrites since incomplete mixing of nucleosynthetic material in the solar nebula creates significant 142nd variation as shown by alha77295

the anatomy of uppermost mantle shear wave speed anomalies in the western u s from surface wave amplification abstract we build swus amp a three dimensional shear wave speed model of the uppermost mantle of the western u s using rayleigh wave amplification measurements in the period range of 35 125 s from teleseismic earthquakes this represents the first ever attempt to invert for velocity structures using rayleigh wave amplification data alone we use over 350 000 rayleigh wave amplitude measurements which are inverted using a monte carlo technique including uncertainty quantification being a local seismic observable rayleigh wave amplification is little affected by path averaged effects and in principle has stronger depth resolution than classical seismic observables such as surface wave dispersion data swus amp confirms shallow mantle heterogeneities found in previous models in the top 100 km of the mantle we observe low velocity anomalies associated with yellowstone and the basin range province as well as a fast velocity anomaly underneath the colorado plateau where a strong velocity gradient at its edges shows a drastic contrast with its surroundings swus amp also gives additional insights into the current state of the uppermost mantle in the region we image a high velocity anomaly beneath the high topography wyoming province with a maximum depth extent of about 150 170 km which is shallower than in previous tomographic models and resolves previous inconsistencies with geological information beneath the snake river plain a finger like low velocity anomaly dips to the west suggesting lateral flow in the region below about 150 km depth swus amp shows a north south dichotomy in shear wave speed structure with the northern region showing mostly high velocity anomalies whereas the southern region shows low velocity anomalies this is consistent with the continuous subduction history of the western u s and with the recent extension and uplift of the southern region

a shorter 146sm half life measured and implications for 146sm 142nd chronology in the solar system radiometric dating relies on measuring the abundance of long lived radionuclides relative to the abundance of their radiogenic decay products a process determined by the original radionuclide s half life for primordial radionuclides that decay slowly such as 146sm decaying to 142nd this method provides the timing of some of the earliest processes in solar system history using accelerator mass spectrometry kinoshita et al p 1614 1 provide a revised estimate for the 146sm half life of 68 7 million years which is 30 shorter than the previously accepted value this shorter half life suggests that reductions need to be made in the estimated ages for differentiation of earth s mantle and the solidification of the moon s magma ocean and for other more recent processes 1 lookup doi 10 1126 science 1215510

western u s seismic anisotropy revealing complex mantle dynamics abstract the origin of the complex pattern of sks splitting over the western united states u s remains a long lasting debate where a model that simultaneously matches the various sks features is still lacking here we present a series of quantitative geodynamic models with data assimilation that systematically evaluate the influence of different lithospheric and mantle structures on mantle flow and seismic anisotropy these tests reveal a configuration of mantle deformation more complex than ever envisioned before in particular we find that both lithospheric thickness variations and toroidal flows around the juan de fuca slab modulate flow locally but their co existence enhances large scale mantle deformation below the western u s the ancient farallon slab below the east coast pulls the western u s upper mantle eastward spanning the regionally extensive circular pattern of sks splitting the prominent e w oriented anisotropy pattern within the pacific northwest reflects the existence of sustaining eastward intrusion of the hot pacific oceanic mantle to beneath the continental interior from within slab tears below oregon to under the snake river plain and the yellowstone caldera this work provides an independent support to the formation of intra plate volcanism due to intruding shallow hot mantle instead of a rising mantle plume

analyses of iron oxide assemblages within namib dune sediments using high field remanence measurements 9 t and mossbauer analysis

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

old sm nd ages for cumulate eucrites and redetermination of the solar system initial 146sm 144sm ratio article i nfo sm 142 nd systematics short lived 146 sm 142 nd and long lived 147 sm 143 nd chronometers have been measured in three cumulate eucrites binda moore county and moama the two major mineral phases plagioclase and pyroxene present in these achondrites are characterized by a wide range of sm nd ratios that allows well resolved sm nd isochrons this group of meteorites thus is suitable to better constrain the initial 146 sm 144 sm ratio of the solar system binda and moore county give concordant ages of 4544 88 and 4542 85 ma respectively with initial 143 nd 144 nd ratios slightly higher to within error of chondritic these ages are in agreement with most of the radiometric ages determined on basaltic eucrites a best estimate for the solar system initial 146 sm 144 sm ratio is obtained using the five point regression line determined for binda the 146 sm 144

nucleosynthetic components of the early solar system inferred from ba isotopic compositions in carbonaceous chondrites abstract the ba isotopic composition of primitive meteorites is one of the more useful tracers to study nucleosynthetic processes in the early solar system because the seven stable ba isotopes consist of p r and s process nucleosynthetic components and possible decay products from presently extinct 135 cs ba isotopic analyses were performed on acid leachates from five carbonaceous chondrites murchison cm2 sayama cm2 allende cv3 isuna co3 and maralinga ck4 and one basaltic achondrite juvinas eucrite the ba isotopic data of acid treatment fractions in carbonaceous chondrites suggest the presence of independent nucleosynthetic components for s and r processes in the solar system the isotopic patterns from the acid residual fractions of two cm2 meteorites murchison and sayama show an enrichment of s process isotopes which is similar to that of presolar sic grains on the other hand their acid leachates have an excess of r process components among the ba isotopes in many cases of carbonaceous chondrites other than cm meteorites the excess of r process isotopes is too small for a detailed discussion about the nucleosynthetic contribution this may be interpreted as metamorphic destruction of presolar grains in meteorite matrices associated with petrographic type meteorites juvinas shows no isotopic anomalies of ba because the isotopic record in the early solar system was reset possibly by magmatic processes in the parent body

patterns in seismic anisotropy driven by rollback subduction beneath the high lava plains received 24 march 2011 revised 25 may 2011 accepted 31 may 2011 published 8 july 2011 1 we present three dimensional laboratory modeling of the evolution of finite strain and compare these to shear wave splitting observations in the northwest u s under the high lava plains hlp we show that relationships between mantle flow and anisotropy are complicated in subduction zones and factors such as initial orientation of the olivine fast axis style of subduction and time evolving flow are important due to increased horizontal shear systems with a component of rollback subduction have simple trench normal strain alignment within the central region of the backarc mantle wedge while those with more simple longitudinal sinking are often variable and complex in the hlp splitting observations are consistent with rollback driven laboratory results citation druken k a m d long and c kincaid 2011 patterns in seismic anisotropy driven by rollback subduction beneath the high lava plains geophys res lett 38 l13310 doi 10 1029 2011gl047541

topographic evolution of the western united states since the early miocene abstract the origin of the high topography within the western united states has been attributed to either crustal lithospheric isostasy or dynamic topography but their relative contributions remain unconstrained here we investigate this problem using gravity residual topography and geodynamic modeling we first evaluate two end member scenarios of isostatic balance crustal isostasy and lithospheric isostasy both cases lead to prominent negative mantle residual gravity within the tectonically active western u s and unrealistic crustal lithospheric density structures requiring the presence of low density mantle underneath the negative mantle residual gravity is consistent with both the estimated positive residual topography and calculated dynamic uplift due to the presence of hot asthenospheric mantle underneath geodynamic modeling further reveals that this landward migrating dynamic uplift originates from the eastward intrusion of the hot pacific mantle through tears and edges of the juan de fuca slab since middle miocene the estimated paleotopography maps by combining dynamic topography and lithosphere isostasy over the western u s are consistent with several observational constraints including episodic uplifts of the sierra nevada post mid miocene uplift of the idaho batholith the sustaining subsidence within most of the b r and the largely stable topography of central colorado plateau since 20 ma

the origin and significance of boxwork clay coatings on dune sand grains from the nebraska sand hills usa abstract aeolian dune sands of late pleistocene to late holocene age in the nebraska sand hills are widely characterized by the development of partial clay coatings which form connected ridges and boxwork structures on the grain surfaces the proportion of sand grains with clay coatings typically shows considerable variability with depth in any given section distinct bands of sand enriched in fines are quite common especially just below the present or buried dune surfaces the morphology texture and mineralogical composition of clay coatings from two sites the merritt reservoir megabarchan and the dismal river ranch section have been investigated using a combination of sem x ray microanalysis xrd and coulter multisizer analysis in order to determine their mode of formation and environmental significance based on the analytical results and field observations it is concluded that the boxwork clay coatings formed in the vadose zone mainly as a result of precipitation and surface runoff induced infiltration of clay and very fine silt from the dune surface the main source of fines is concluded to be fine grained allochthonous aeolian dust derived principally from the high plains or more distal sources

subduction dynamics and the origin of andean orogeny and the bolivian orocline the building of the andes results from the subduction of the oceanic nazca plate underneath the south american continent however how and why the andes and their curvature the bolivian orocline formed in the cenozoic era 65 5 million years myr ago to present despite subduction continuing since the mesozoic era 251 0 65 5 myr ago is still unknown three dimensional numerical subduction models demonstrate that variations in slab thickness arising from the nazca plate s age at the trench produce a cordilleran morphology consistent with that observed the age dependent sinking of the slab in the mantle drives traction towards the trench at the base of the upper plate causing it to thicken thus subducting older nazca plate below the central andes can explain the locally thickened crust and higher elevations here we demonstrate that resultant thickening of the south american plate modifies both shear force gradients and migration rates along the trench to produce a concave margin that matches the bolivian orocline additionally the varying forcing along the margin allows stress belts to form in the upper plate interior explaining the widening of the central andes and the different tectonic styles found on their margins the eastern and western cordilleras the rise of the central andes and orocline formation are directly related to the local increase of nazca plate age and an age distribution along the margin similar to that found today the onset of these conditions only occurred in the eocene epoch this may explain the enigmatic delay of the andean orogeny that is the formation of the modern andes

influence of trench width on subduction hinge retreat rates in 3 d models of slab rollback subduction of tectonic plates limited in lateral extent and with a free trailing tail i e free subduction is modeled in a three dimensional 3 d geometry the models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength width and thickness we investigate the time evolution of this progressive rollback subduction measure the accompanying return flow in the upper mantle and quantify the plate kinematics due to the 3 d geometry flow is allowed to accompany slab rollback around the lateral edges of the slab the toroidal component as opposed to 2 d geometry where material is forced to flow underneath the slab tip the poloidal component a simple force balance is provided which relates the speed of backward trench migration to the resistive forces of generating flow and weakening the plate our results indicate most of the gravitational energy of the system i e the negative buoyancy of the subducting slab is converted into a toroidal flow 69 a much smaller amount goes into weakening the plate 18 and the remaining amount goes into driving flow parallel to displacement of the slab 13 for the trench widths w we investigate 1500 km a maximum trench retreat rate occurs for trenches 600 km wide which is attributed to the interaction between a plate of finite width and the induced flow which has a lengthscale in the horizontal direction these numerical results quantitatively agree with comparable 3 d laboratory experiments using analogue models with a purely viscous plate material schellart 2004a 2004b including correlations between increasing retreat rate with increasing plate thickness trench width for maximum retreat rate 500 km and estimated amount of slab buoyancy used to drive rollback induced flow 70 several implications for plate tectonics on earth result from these models such as rollback subduction providing a physical mechanism for ephemeral slab graveyards situated above the more viscous lower mantle and endothermic phase transition prior to a flushing event into the lower mantle mantle avalanche

distinctive upper mantle anisotropy beneath the high lava plains and eastern snake river plain pacific northwest usa 1 the pacific northwest pnw has experienced voluminous intraplate volcanism over the past 17 ma beginning with the steens columbia river flood basalts and continuing with the still ongoing volcanism in the high lava plains hlp and eastern snake river plain srp here we present two complementary datasets sks splitting and rayleigh wave phase velocity anisotropy that place constraints on the anisotropic structure of the upper mantle beneath the hlp and srp regions beneath the hlp sks phases reveal dominantly e w fast splitting directions and large up to 2 7 s delay times with pronounced lateral variations in t lateral and depth variability in the strength of anisotropy beneath the hlp is also evident from rayleigh wave dispersion beneath the srp sks splitting delay times are much smaller 0 5 s and surface wave observations suggest a region of upper mantle anisotropy 50 150 km depth with a geometry that deviates significantly from the generally plate motion parallel fast directions observed just outside of the srp beneath the hlp the geometry of the anomalously strong anisotropy is similar to the anisotropy in the deeper parts of the upper mantle resulting in constructive interference and large sks splitting delay times beneath the srp the geometry of the anomalous anisotropic region in the shallow mantle is different resulting in destructive interference and reduced sks splitting delay times we discuss several possible explanations for these observations including variations in olivine lattice preferred orientation lpo strength transitions in olivine fabric type and a contribution from aligned partial melt

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

reddening of dune sands evidence from southeast india thin section sem and tem analyses have indicated that the red teri sands of southeast india have formed from weathering in situ of coastal dune sands the data does not support a detrital origin for the red sands c14 dates indicate that dune deposition and weathering to a maximum depth of 10 m leading to the formation of a hematite koalinite and illite rich matrix has taken place rapidly over the past 25 000 years it is postulated that garnet was a major source of hematite ferric oxide

silicon isotope evidence against an enstatite chondrite earth the compositions of earth materials are strikingly similar to those of enstatite chondrite meteorites in many isotope systems although this suggests that earth largely accreted from enstatite chondrites definitive proof of this model has been lacking by comparing the silicon si isotope signatures of several extraterrestrial materials with terrestrial samples we show that they cannot be explained by core formation scenarios involving a bulk earth of enstatite chondrite composition si isotope similarities between the bulk silicate earth and the moon preclude the existence of a hidden reservoir in the lower mantle a necessary condition of the enstatite chondrite model and require an equilibrium process after the moon forming impact a three end member chondritic mixing model for earth reconciles the si isotope similarities between enstatite chondrites and earth

p and s wave tomography of the mantle beneath the united states mantle seismic structure beneath the united states spanning from the active western plate margin to the passive eastern margin was imaged with teleseismic p and s wave traveltime tomography including usarray data up to may 2014 to mitigate artifacts from crustal structure 5 40 s rayleigh wave phase velocities were used to create a 3 d starting model major features of the final p and s models include two distinct low velocity anomalies at depths of 60 300 km beneath the central and northern appalachians and passive margin the central appalachian low velocity anomaly coincides with eocene basaltic magmatism and the northern anomaly is located along the cretaceous track of the great meteor hot spot at depths of 300 700 km beneath the central and eastern u s large high velocity anomalies are inferred to be remnants of the farallon slab that subducted prior to 40 ma during the laramide orogeny

a hybrid approach to data assimilation for reconstructing the evolution of mantle dynamics quantifying past mantle dynamic processes represents a major challenge in understanding the temporal evolution of the solid earth mantle convection modeling with data assimilation is one of the most powerful tools to investigate the dynamics of plate subduction and mantle convection although various data assimilation methods both forward and inverse have been created these methods all have limitations in their capabilities to represent the real earth pure forward models tend to miss important mantle structures due to the incorrect initial condition and thus may lead to incorrect mantle evolution in contrast pure tomography based models cannot effectively resolve the fine slab structure and would fail to predict important subduction zone dynamic processes here we propose a hybrid data assimilation approach that combines the unique power of the sequential and adjoint algorithms which can properly capture the detailed evolution of the downgoing slab and the tomographically constrained mantle structures respectively we apply this new method to reconstructing mantle dynamics below the western u s while considering large lateral viscosity variations by comparing this result with those from several existing data assimilation methods we demonstrate that the hybrid modeling approach recovers the realistic 4 d mantle dynamics the best

lu hf and sm nd isotopic systematics in chondrites and their constraints on the lu hf properties of the earth sm nd and lu hf isotopic data are presented for 19 chondritic meteorites six carbonaceous chondrites five l chondrites seven h chondrites and a single enstatite chondrite the primary goal of the study is to better define the bulk silicate earth bse reference values for hf isotopes except for one sample with lower sm nd the sm nd data define a cluster around the accepted reference values for chondrites and terrestrial planets giving a mean 147sm 144nd of 0 1960 0 0005 and a mean 143nd 144nd of 0 512631 0 000010 uncertainties are two standard errors it seems appropriate to retain the presently accepted sm nd reference parameters 147sm 144nd 0 1966 and 143nd 144nd 0 512638 when fractionation corrected to 146nd 144nd 0 7219 lu hf isotopic data are not clustered but spread along an approximate 4 5 ga isochron trend with a range of 176lu 177hf from 0 0301 to 0 0354 the data are similar to many of the samples of chondrites presented by bizzarro et al nature 421 2003 931 but lack the range to lower lu hf shown by those authors our chondrite data define a regression line of 4 44 0 34 ga when 1 867 10 11 year 1 is used for the decay constant of 176lu science 293 2001 683 earth planet sci lett 219 2004 311 324 combining our data with the main population of analyses from bizzarro et al nature 421 2003 931 yields 4 51 0 24 ga unless samples of eucrite meteorites and deviating replicates of chondrites with 176lu 177hf less than 0 030 are employed no combination of the main population of chondrite lu hf data yields a regression with sufficiently low error to constrain the decay constant of 176lu sample heterogeneity seems to hinder the acquisition of reproducible lu hf analyses from small manually ground pieces of chondrites and we suggest that analysis of powders prepared from large volumes of meteorite will be needed to adequately characterize the lu hf isotope systematics of chondritic reservoirs and of bse our results for carbonaceous chondrites show higher average 176lu 177hf and 176hf 177hf than ordinary chondrites and the mean of carbonaceous chondrites also coincides with replicate analyses of a powder representing a large volume of meteorite the allende powder from the smithsonian institution use of the carbonaceous chondrite mean for bse lu hf characteristics results in a bse hf nd point that lies well within the array of terrestrial compositions and leads to plausible initial ehf values for precambrian rocks an improved objective resolution of meteorite data and of meteoritic models for the earth needs to occur before bse can be established for lu hf

mantle dynamics and seismic anisotropy abstract observations of seismic anisotropy yield some of the most direct constraints available on both past and present day deformation in the earth s mantle insight into the character of mantle flow can also be gained from the geodynamical modeling of mantle processes on both global and regional scales we highlight recent progress toward understanding mantle flow from both observations and modeling and discuss outstanding problems and avenues for progress particularly in the integration of seismological and geodynamical constraints to understand seismic anisotropy and the deformation that produces it to first order the predictions of upper mantle anisotropy made by global mantle circulation models match seismological observations well beneath the ocean basins but the fit is poorer in regions of greater tectonic complexity such as beneath continental interiors and within subduction systems in many regions of the upper mantle models of anisotropy derived from surface waves are seemingly inconsistent with shear wave splitting observations which suggests that our understanding of complex anisotropic regions remains incomplete observations of anisotropy in the d layer hold promise for improving our understanding of dynamic processes in the deep earth but much progress remains to be made in characterizing anisotropic structure and relating it to the geometry of flow geochemical heterogeneity or phase transitions major outstanding problems related to understanding mantle anisotropy remain particularly regarding the deformation and evolution of continents the nature of the asthenosphere subduction zone geodynamics and the thermo chemical state of the lowermost mantle however we expect that new seismological deployments and closer integration of observations with geodynamical models will yield rapid progress in these areas

simulation of late cenozoic south american flat slab subduction using geodynamic models with data assimilation abstract the formation mechanisms of flat slabs in south america remain unclear to quantitatively evaluate the earlier proposed mechanisms we simulate the post 100 ma subduction history below south america using 4 d geodynamic models by progressively incorporating plate kinematics seafloor ages and key tectonic features including the buoyant oceanic crust continental cratons oceanic plateaus i e the inferred inca plateau subducting nazca ridge and juan fernandez ridge as well as deformable trench profiles according to recent geological reconstructions we find that in the absence of an overriding plate and subducting buoyancy features the seafloor age affects slab dip angle by controlling the slab s mechanical strength i e the resistance to bending and negative buoyancy integrated positive density anomaly that enhances bending our models show that slab strength dominates its buoyancy at age 30 ma and the opposite for younger ages the existence of a thick overriding plate reduces the slab dip by increasing dynamic suction and individual cratonic roots further lead to along trench variations of dip angle reduction while dynamic suction from the overriding plate generates a permanent reduction of the long wavelength slab dip angle it is the final addition of subducting oceanic plateau and aseismic ridges that produces the transient and localized flat slabs as observed these results suggest that all mechanisms except the buoyancy features affect the slab dip only at large spatial scales our best fit model with all the above tectonic features included provides a good match to both the upper mantle benioff zones and the temporal evolution of volcanic arcs since the mid miocene the imperfect match of the peruvian flat slab is likely associated with the uncertain 3 d configuration of the amazonian craton

on the relationship between azimuthal anisotropy from shear wave splitting and surface wave tomography 1 seismic anisotropy provides essential constraints on mantle dynamics and continental evolution one particular question concerns the depth distribution and coherence of azimuthal anisotropy which is key for understanding force transmission between the lithosphere and asthenosphere here we reevaluate the degree of coherence between the predicted shear wave splitting derived from tomographic models of azimuthal anisotropy and that from actual observations of splitting significant differences between the two types of models have been reported and such discrepancies may be due to differences in averaging properties or due to approximations used in previous comparisons we find that elaborate full waveform methods to estimate splitting from tomography yield generally similar results to the more common simplified approaches this validates previous comparisons and structural inversions however full waveform methods may be required for regional studies and they allow exploiting the back azimuthal variations in splitting that are expected for depth variable anisotropy applying our analysis to a global set ofskssplitting measurements and two recent surface wave models of upper mantle azimuthal anisotropy we show that the measures of anisotropy inferred from the two types of data are in substantial agreement provided that the splitting data is spatially averaged so as to bring it to the scale of long wavelength tomographic models and reduce spatial aliasing observed and tomography predicted delay times are significantly correlated and global angular misfits between predicted and actual splits are relatively low regional anisotropy complexity notwithstanding our findings imply that splitting and tomography yield a consistent signal that can be used for geodynamic interpretation

receiver function study of the cascadia megathrust evidence for localized serpentinization 1 we characterize the crustal and upper mantle structure of the cascadian fore arc using receiver function analysis of data from long running observatories located along the north american margin in oregon and washington we identify coherent sv polarized phases consistent with p s conversion from the top of the subducting juan de fuca plate throughout the area west of the volcanic arc in many instances we also find evidence for deeper features likely associated with the subducted oceanic crust at many locations sh polarized energy in receiver functions is associated with the surface of the subducting plate these sh polarized phases display significant directional variation and show clear polarity reversals consistent with the presence of seismic anisotropy previously a layer of serpentinized material within the megathrust has been proposed to explain an anisotropic low velocity high poisson s ratio layer identified by receiver function analysis at corvallis oregon global seismic network station cor our new results suggest that this feature is widespread in cascadia but the specific receiver function signature varies this anisotropic supraslab layer is supported by receiver function estimates beneath seismic stations which similar to cor are 100 km from the coast and 40 km above the slab surface however the orientation of rock fabric implied by the anisotropy varies greatly from station to station modeling with synthetic seismograms supports the existence of serpentinite beneath station gnw green mountain washington closer to the coast where the slab is shallower an anisotropic layer is likely beneath some stations but the evidence is less conclusive we propose that the anisotropic signature beneath the inland stations is related to serpentinization of the mantle wedge at greater depths best explained by the presence of the polymorph antigorite hypothetically suctioned upward by slab rollback a deformed sliver of serpentinite rich rock above the slab interface would likely display the observed large spatial variation in anisotropic symmetry axis within the layer consistent with our observations

western us volcanism due to intruding oceanic mantle driven by ancient farallon slabs the origin of late cenozoic intraplate volcanism over the western united states is debated one important reason is the lack of a clear understanding of the mantle dynamics during this volcanic history here we reconstruct the mantle thermal states beneath north america since 20 million years ago using a hybrid inverse geodynamic model with data assimilation the model simultaneously satisfies the past subduction kinematics present mantle tomographic image and the volcanic history we find that volcanism in both the yellowstone volcanic province and the basin and range province corresponds to a similar eastward intruding mantle derived from beneath the pacific ocean and driven mostly by the sinking farallon slab below the central eastern united states the hot mantle that forms the columbia river flood basalt and subsequent yellowstone newberry hotspot tracks first enters the western united states through tears within the juan de fuca slab subsequent coexistence of the westward asthenospheric flow above the retreating juan de fuca slab and eastward propagating mantle beyond the back arc region reproduces the bifurcating hotspot chains a similar but weaker heat source intrudes below the basin and range around the southern edge of the slab and can explain the diffuse basaltic volcanism in this region according to our models the putative yellowstone plume contributes little to the formation of the yellowstone volcanic province volcanism in the western us may result from warm oceanic mantle beneath the pacific ocean being drawn eastwards by mantle flow induced by the sinking of farallon slabs according to numerical model simulations

seismic anisotropy around subduction zones insights from three dimensional modeling of upper mantle deformation and sks splitting calculations 1 inferring the circulation of the mantle around subducting plates from surface measurements of shear wave splitting patterns remains to date elusive to assist the interpretation of the seismic signal and its relation with the mantle circulation pattern we present a new methodology to compute the seismic anisotropy directly from the flow in the upper mantle of 3 d numerical models of earth like subduction this computational strategy accounts for the non steady state evolution of subduction zones yielding mantle fabrics that are more consistent with the deformation history than previously considered in the subduction models a strong mantle fabric develops throughout the upper mantle with a magnitude of the anisotropy that is proportional to the amount of subduction and is independent of the subduction rate the sub slab upper mantle is characterized by two domains with different fabrics at shallow depth the mantle entrained with the subducting slab develops trench perpendicular directed anisotropy due to simple shear deformation while in the deeper mantle slab rollback induces pure shear deformation causing trench parallel extension and fast seismic directions subducting plate advance favors the development of the fabric in the entrained mantle domain while slab retreat increases the trench parallel anisotropy in the deeper upper mantle in the deeper domain the strength of the fabric is proportional to the horizontal divergence of the flow and weakens from the slab edges toward the center as such strong trench parallel anisotropy forms below retreating and relatively narrow slabs or at the margins of wider plates the synthetic sks splitting patterns calculated in the fore arc are controlled by the magnitude of the anisotropy in the upper domain with trench perpendicular fast azimuths in the center of large plates and trench parallel toward the plate edges instead above relatively narrow retreating slabs 600 km and low subduction partitioning ratio spr azimuths are trench parallel due to the strong anisotropy in the lower sub slab domain in all models the anisotropy in the back arc and on the sides of the subducting plate is respectively trench perpendicular and sub parallel to the return flow at depth results from our regional scale models may help to infer the flow and composition of the upper mantle by comparison with the wide range of subduction zones seismic data observed globally

subduction controlled mantle flow and seismic anisotropy in south america abstract seismic anisotropy records both the past and present deformation inside the solid earth in the mantle seismic anisotropy is mainly attributed to the lattice preferred orientation lpo of mineral fabrics caused by the shear deformation due to mantle flow however contributions from different tectonic processes remain debated and a single geodynamic model that simultaneously explains the observed mantle structures and various seismic anisotropy measurements is still lacking here we present a model for the cenozoic subduction history in south america using a geodynamic simulation constrained by both past plate reconstructions and present mantle seismic structures with a recently developed software package drexs we further predict azimuthal seismic anisotropy at different depths and generate synthetic shear wave splitting sws measurements using the resulting mantle flow our results provide a good match to both depth dependent surface wave anisotropy and various land based sws records we find that the dominant control on seismic anisotropy in south america comes from subduction induced mantle flow where anisotropy below the subducting nazca plate aligns with plate motion induced couette flow and that below the overriding south american plate follows slab induced poiseuille flow this large scale mantle flow can be diverted by secondary slabs such as that below the antilles subduction zone in contrast the contribution to sws from fossil continental anisotropy and from the effects due to mantle flow modulation by lithosphere thickness variation are minor upper mantle fast seismic anomalies beneath the southern atlantic margin should have close to neutral buoyancy in order to satisfy the observed seismic anisotropy

ree and actinide microdistribution in sahara 97072 and alha77295 eh3 chondrites a combined cosmochemical and petrologic investigation abstract we report the results of rare earth elements rees and u th inventory of individual minerals oldhamite enstatite and niningerite in two of the most unequilibrated and primitive eh3 known so far alha77295 and sahara 97072 under the highly reducing condition that prevailed during the formation of enstatite chondrites rees are mainly chalcophile and concentrated in oldhamite the study is guided by detailed petrographic investigations of the individual minerals in chondrules complex sulfide metal clasts and enstatite dominated matrices we developed two textural parameters in order to resolve the evolution of oldhamite condensates and their residence in the solar gas prior to their accretion in the individual objects or in matrices and relate these textural features to the measured ree patterns of the individual oldhamite crystals these textural parameters are the crystal habit of oldhamite grains idiomorphic or anhedral and their host assemblages ree concentrations were measured by sims and la icpms oldhamite grains display ree enrichments 10 100 ci four types of ree patterns are encountered in oldhamite in alha77295 in general the ree distributions cannot be assigned to a specific oldhamite bearing assemblage the most represented ree pattern is characterized by both slight to large positive eu and yb anomalies and is enriched in light rees relative to heavy rees this pattern is present in 97 of oldhamite in sahara 97072 suggesting a different source region in the reduced part of the nebula or different parental eh asteroids for the two eh3 chondrites different parental asteroids are also supported by mgs fes zoning profiles in niningerite grains adjacent to troilite revealing both normal and reverse zoning trends and different mns contents the observed homogeneity of ree distribution in oldhamite grains in sahara 97072 is not related to the mild metamorphic event identified in this meteorite that caused breakdown of the major k and rb bearing sulfide djerfisherite ree concentrations in enstatite range between 0 2 and 8 ci hence enstatite is an important ree host next to oldhamite most patterns are characterized by negative eu and yb anomalies niningerites are negligible contributors to bulk eh3 ree inventory average positive eu and yb anomalies observed in most oldhamite are complimentary to the negative ones in enstatite thus explaining the flat patterns of the bulk meteorites the condensation calculations based on cosmic abundances predict that the first oldhamite condensates should have flat ree patterns with eu and yb depletions since eu and yb condense at lower temperature than other ree however this pattern is seen in enstatite our findings are at odds with the predicted negative eu and yb anomalies in oldhamite earliest condensates from a closed system in a reduced solar source our petrographic mineral chemistry and ree abundances of oldhamite enstatite and niningerite discards an origin of oldhamite by impact melting rubin et al 2009 our results do not support in first order the scenario of the incorporation of ree in the earth s core to explain 142nd excess in terrestrial samples relative to chondrites because oldhamite is the major ree carrier phase and has super chondritic sm nd ratios

lead isotopes behavior in the fumarolic environment of the piton de la fournaise volcano reunion island abstract the recent activity of the piton de la fournaise volcano offers a rare opportunity to address the issue of pb isotope behavior in volcanic fumaroles as the composition of the degassing source is accurately and precisely known gas sublimates formed between 2007 and 2011 at temperature ranging from 400 to ca 100 c include na k sulfate aphthitalite ca cu sulfate e g gypsum na sulfate thenardite ca mg al fe fluoride e g ralstonite and native sulfur the high temperature deposits show trace element patterns typical of volcanic gas with pb concentration up to 836 ppm while the low temperature deposits are depleted in most volatile elements pb only for low temperature fluoride samples do pb isotope compositions plot significantly outside the field of lavas the isotopic shift is ascribed to leaching ubiquitous unradiogenic phases e g sulfides by acidic gas condensates the similarity in pb isotope signature between lavas and sublimate samples more representative of the gas phase sulfates indicates that the net fractionation of pb isotopes resulting from volatilization and condensation processes is smaller than the precision of pb isotope measurements better than 60 ppm a m u the absence of net fractionation could result from negligible isotope fractionation during pb volatilization followed by extensive condensation of gaseous pb with possibly significant isotopic fractionation at this stage although this scenario has to be refined by more direct measurement of the gas phase and its general applicability tested it suggests that a small fraction resolving very small isotopic differences between magmas and their gaseous products remains an analytical challenge high precision pb isotope measurements rest not only on instrumental performance but also on high yield chemistry as pb isotopes drastically fractionate 800 ppm a m u upon elution on anionic resin for 50 pb recovery the estimated isotopic bias is plus or minus 60 80 ppm a m u depending on which of the early isotopically light or late isotopically heavy pb fraction is lost