a kinematic model for recrystallization and texture development in olivine polycrystals the interpretation of seismic anisotropy in the mantle requires a knowledge of the relationship between the lattice preferred orientation lpo of crystals and the convective flow field in order better to understand this link we present a model for the evolution of lpo in olivine aggregates that deform by both intracrystalline slip and dynamic recrystallization dynamic recrystallization depends on the dislocation density of the grains which is a function of the applied local stress grains with a large density of dislocations lower their bulk strain energy by nucleating strain free sub grains at a rate proportional to a dimensionless nucleation parameter grains with high energy are then invaded by grains with low energy by grain boundary migration at a rate proportional to a dimensionless grain boundary mobility m the value of is constrained by observed lpo patterns in experimentally deformed olivine aggregates and m is constrained by the temporal evolution of the strength of the lpo for m 125 75 and 3 the model predictions agree well with the experimental results numerical calculations of lpo using our model are significantly faster than those based on viscoplastic self consistent or equilibrium based theories making the model especially suitable for applications for complex convective flows

mantle flow in subduction systems the subslab flow field and implications for mantle dynamics 1 the character of the mantle flow field in subduction zones remains poorly understood despite its importance for our understanding of subduction dynamics in particular little attention has been paid to mantle flow beneath subducting slabs in order to identify processes that make first order contributions to the global pattern of subslab mantle flow we have compiled shear wave splitting measurements from subduction zones worldwide from previously published studies and estimated average splitting parameters for the subslab region globally the subslab region is overwhelmingly dominated by trench parallel fast splitting directions we tested for relationships between splitting delay time a measure of the strength of anisotropy and parameters that are indicators of tectonic processes such as trench migration velocity convergence velocity and obliquity age of subducting lithosphere and slab dip curvature seismicity and thickness we used several different plate motion models to describe plate and trench motions and evaluated the differences among the models we find that only one parameter namely the trench migration velocity in a pacific hot spot reference frame appears to correlate well with the strength of the subslab splitting signal this finding supports a model in which the mantle beneath subducting slabs is dominated by three dimensional flow induced by trench migration we explore several implications of our model for various aspects of mantle dynamics including the choice of a suitable reference frame s for mantle flow the existence of a thin decoupling zone between slabs and the subslab mantle and consequences for mass transfer between the upper and lower mantle

lower and middle cambrian acritarchs from northeastern poland abstract acid resistant organic walled microfossils acritarchs are reported from lower and middle cambrian rocks penetrated by borehole okuniew ig 1 in the western podlasie depression the study showed the presence of 11 acritarch species previously known from coeval rocks in northeast poland the assemblage from the okuniew borehole is compared with characteristic assemblages from other parts of the east european platform

sediment provenance changes in the western arctic ocean in response to ice rafting sea level and oceanic circulation variations since the last deglaciation two sediment piston cores were recovered from the chukchi alaskan 05jpc and canadian beaufort 02pc margins to investigate grain size geochemical and mineralogical compositions this allowed the reconstruction of changes in detrital sediment provenance and transport related to climate variability since the last deglaciation the end member modeling analyses of grain size indicate that sea ice and nepheloid transport and the mackenzie river sediment plume are major factors influencing sedimentation in the chukchi alaskan and canadian beaufort margins respectively unmixing of the sediment composition indicates that detrital sediments in core 02pc are derived mainly from the mackenzie river whereas sediments from core 05jpc are derived mainly from the mackenzie river during the deglaciation and include a mixture of holocene sediments from the bering strait mackenzie river and eurasian margin the dolomite rich ice rafted debris recorded in both cores could be related to the different phases of iceberg discharges from the amundsen gulf ice stream quartz and feldspar rich ice rafted debris dated at 13 and 10 6 ka cal bp before present are related to the lake agassiz outburst in core 02pc and meltwater discharge from the brooks range glaciers in core 05jpc detrital proxies in core 02pc support the hypothesis that large meltwater and iceberg discharges from the lake agassiz outburst to the arctic ocean and amundsen gulf ice stream may have triggered the younger dryas finally similar trends observed between sea level curves and our detrital proxy suggest that the sea level changes in the western arctic ocean have an important influence on the sediment dynamic during the early to middle holocene keywords bulk and clay mineralogy elemental geochemistry grain size distribution sediment provenance canadian beaufort sea chukchi sea

variability in flow and temperatures within mantle subduction zones 1 a series of laboratory experiments is used to model three dimensional aspects of flow in subduction zones and the consequent temperature variations in the slab and overlying mantle wedge the effects of longitudinal rollback and slab steepening components of motions are considered along with different thicknesses of the over riding lithosphere the results show that the style of plate sinking influences the evolution of subduction zones both in terms of the speed orientation and temperature of flow in the overlying wedge and in terms of the temperatures at the surface of the descending slab in the simplest case of longitudinal sinking without rollback motion velocities in the mantle wedge are 30 40 of the downdip plate speed return flow paths in the shallow mantle wedge are nearly horizontal for slow slab speeds and steepen with higher slab speeds and there is no mass flux around the edges of a slab segment of finite width rollback subduction leads to flow both around and beneath the sinking slab with larger velocities in the wedge up to 150 of the slab speed and flow focused toward the center of the plate segment rollback subduction in which either the trench migrates or the plate steepens with time induces shallow and steep wedge return flow trajectories respectively the thermal evolution of the plate is strongly influenced by sinking style and rate highest temperatures are along the edges of the slab for longitudinal sinking but along the centerline of the slab segment for rollback motion slab surface temperatures when scaled to the mantle are higher than those given by previous two dimensional numerical models and are consistent with recent observational and experimental data on melt compositions

shear wave polarization anisotropy and subduction induced flow in the mantle wedge of northeastern japan abstract shear wave polarization anisotropy in the central part of northeastern ne japan was investigated we analyzed s phases of 293 intermediate depth earthquakes recorded at 77 stations and obtained 1286 splitting parameters the obtained results show the systematic variation in splitting parameters across the arc the leading shear wave polarization directions fast direction obtained at stations in the western side of the study area are oriented nearly e w which is sub parallel to the direction of relative plate motion between the pacific plate and the north american okhotsk plate in contrast fast directions obtained at stations in the eastern side of the study area are oriented approximately n s stations in the western side have a larger average delay time compared to those in the eastern side these variations suggest that the nature of anisotropy is quite different between the eastern and the western sides both experimental studies on the deformation of olivine and numerical simulations of plate driven flow predict that the lattice preferred orientation of olivine causes the fast direction sub parallel to the flow direction for the conditions in the mantle wedge of ne japan we thus infer that the lattice preferred orientation of olivine generated by flow induced strain is the most likely candidate for anisotropy that produces splitting with e w fast direction three possible causes of anisotropy in the eastern side are considered deformation of water rich olivine in the mantle wedge trench parallel flow in the mantle wedge due to along strike dip variations in the slab and anisotropy in the crust and the slab we could not however determine which causes are dominant further systematic study is required to reveal the cause of anisotropy in the eastern side

chemical composition of the snowpack during the oasis spring campaign 2009 at barrow alaska 1 the chemical composition of the seasonal snowpack was determined close to barrow an arctic coastal location in northern alaska one hundred and twelve samples of different snow types including fresh snow surface hoar diamond dust blowing snow rounded snow grains and depth hoar were collected and analyzed for major sea salt components bromide and nitrate sodium chloride sulfate and potassium are mainly introduced into the snowpack by the deposition of sea salt while magnesium and calcium result from a combination of sea salt and dust sulfate was strongly depleted in most samples compared to other sea salt components this is attributed to the precipitation of mirabilite in newly formed sea ice and frost flowers that leads to an efficient fractionation of sulfate uptake of volatile but soluble species from the gas phase also contributed to the observed chloride sulfate and nitrate in the snow however for chloride and sulfate the input from the marine sources was overwhelming and the uptake from the gas phase was only visible in the samples with low concentrations like fresh snow diamond dust and surface hoar nitrate concentrations in the snowpack were less variable and for aged snow nitrate was related to the specific surface area of the snow indicating the adsorption of nitric acid can be an important nitrate source in the aged snow bromide was also introduced into the snowpack from marine sources but due to its high reactivity it was partly transferred back to the atmosphere in the form of reactive species the result of these processes was evident in bromide concentrations which were both enriched and depleted at the snowpack surface while deeper layers were mostly depleted blowing snow also exhibited a depleted bromide composition for all compounds except nitrate many depth hoar samples exhibited the greatest concentrations probably as a result of higher input earlier in the season as well as increases due to the sublimation of water during the metamorphism of the snow

anisotropy and mantle flow in the chile argentina subduction zone from shear wave splitting analysis 1 we examine shear wave splitting in teleseismic phases to observe seismic anisotropy in the south american subduction zone data is from the charge network which traversed chile and western argentina across two transects between 30 s and 36 s beneath the southern and northwestern parts of the network fast polarization direction j is consistently trench parallel while in the northeast j is trench normal the transition between these two zones is gradual we infer that anisotropy sampled by teleseismic phases is localized within or below the subducting slab we explain our observations with a model in which eastward nazca entrained asthenospheric flow is deflected by retrograde motion of the subducting nazca plate resulting southward flow through this area produces n s j observed in the south and northwest e w j result from interaction of this flow with the local slab geometry producing eastward mantle flow under the actively flattening part of the slab index terms 7203 seismology body wave propagation 7218 seismology lithosphere and upper mantle 8123 tectonophysics dynamics seismotectonics 8150 tectonophysics plate boundary general 3040 9360 information related to geographic region south america citation anderson m l g zandt e triep m fouch and s beck 2004 anisotropy and mantle flow in the chile argentina subduction zone from shear wave splitting analysis geophys res lett 31 l23608 doi 10 1029 2004gl020906

effect of three dimensional slab geometry on deformation in the mantle wedge implications for shear wave anisotropy shear wave splitting observations from many subduction zones show complex patterns of seismic anisotropy that commonly have trench parallel fast directions three dimensional flow may give rise to trench parallel stretching and provide an explanation for these patterns of seismic anisotropy along strike variations in slab geometry produce trench parallel pressure gradients and are therefore a possible mechanism for three dimensional flow in this study we quantify the effects of variable slab dip curved slabs oblique subduction and slab edges on flow geometry and finite strain in the mantle wedge of subduction zones temperature dynamic pressure velocity and strain are calculated with high resolution three dimensional finite element models these models include temperature and stress dependent rheology and parameterized slab and trench geometry thick layers 20 60 km with strong trench parallel stretching are observed in the mantle wedge when slab geometry involves a transition to slab dip less than 15 or strong curvature in the slab in these cases strong trench parallel stretching develops when flow lines have an oblique to trench normal orientation this suggests that trench parallel seismically fast directions may not indicate trench parallel flow lines in systems with large along strike variations an oblique component of stretching is confined to a 20 30 km layer above the slab in systems with oblique subduction the effects of slab edges include strong toroidal flow and focusing in the mantle near slab edges and trench parallel flow that extends 50 100 km into the core of the mantle wedge

seismic anisotropy reveals the long route of the slab through the western central mediterranean mantle abstract in the south eastern corner of the tyrrhenian basin in the central mediterranean sea a tight alignment of earthquakes along a well defined benioff zone marks one of the narrowest active trenches worldwide where one of the last fragment of the former tethys ocean is consumed seismic tomography furnishes snapshot images of the present day position of this slab and seismic anisotropy allows to reconstruct the past kinematics of the subduction process using seismic anisotropy fast directions as a proxy for the present and past mantle flow we look backward for the seismic traces of the slab motion through the western central mediterranean mantle from the starting locus of subduction toward its present day position the result of combining independent data sets provides a coherent pattern of anisotropy that illustrates an example of slab rollback from its initiation point to its present day position

toroidal mantle flow around the calabrian slab italy from sks splitting 1 sks wave birefringence is likely caused by ongoing and or fossil mantle flow splitting parameters in the southern tyrrhenian subduction zone define a quite complex pattern of fast directions with large delay times up to 2 7 s fast directions are prevalently trench parallel below the slab and rotate to trench normal at the western edge of the slab depicting a ring around it we interpret these anisotropic patterns also observed in laboratory experiment as toroidal mantle flow induced by rollback subduction

p wave deep velocity structure of the southern tyrrhenian subduction zone from nonlinear teleseismic traveltime tomography a new 3 d model for the p wave velocity structure of the southern tyrrhenian subduction zone stsz is determined from nonlinear inversion of relative arrival times of teleseismic events the data used in the imaging are the travel time residuals of both direct p and pk p df and secondary pp sp pcp pkp bc phases computed with respect to the global 1 d velocity model ak135 2308 teleseismic waveforms were collected for this study from 109 events recorded by the italian national seismic network rsnc during 1988 1998 the velocity perturbation field is reconstructed gradually by means an iterative sequence of linearized inversions incorporating a 3 d minimum travel time ray tracing the tomographic images reveal a broad high velocity zone dominating the pattern of lateral variations beneath the southern tyrrhenian sea and calabria this fast structure extends laterally for a maximum of 350 km from northern sicily to southern campania and vertically for at least 400 km from the uppermost mantle down to 500 km depth below 350 km the geometry of the depicted slab is characterized by horizontal deflection of the subducting lithosphere towards the central tyrrhenian basin

post depositional movement of methanesulphonic acid at law dome antarctica and the influence of accumulation rate a series of ice cores from sites with different snow accumulation rates across law dome east antarctica was investigated for methanesulphonic acid msa movement the precipitation at these sites up to 35 km apart is influenced by the same air masses the principal difference being the accumulation rate at the low accumulation rate w20k site 0 17 m ice equivalent msa was completely relocated from the summer to winter layer moderate movement was observed at the intermediate accumulation rate site 0 7 m ice equivalent dome summit south dss while there was no evidence of movement at the high accumulation rate de08 site 1 4 m ice equivalent the main dss record of msa covered the epoch ad 1727 2000 and was used to investigate temporal post depositional changes co deposition of msa and sea salt ions was observed in the surface layers outside of the main summer msa peak which complicates interpretation of these peaks as evidence of movement in deeper layers a seasonal study of the 273 year dss record revealed msa migration predominantly from summer into autumn in the up core direction but this migration was suppressed during the tambora 1815 and unknown 1809 volcanic eruption period and enhanced during an epoch 1770 1800 with high summer nitrate levels a complex interaction between the gradients in nss sulphate nitrate and sea salts which are influenced by accumulation rate is believed to control the rate and extent of movement of msa

p wave tomography of the mantle under the alpine mediterranean area 1 we study the upper mantle p wave velocity structure below the euro mediterranean area down to 1000 km depth by seismic travel time tomography we invert summary residuals constructed with both regional and teleseismic first arrival data reported by the international seismological centre isc 1964 1995 introducing some alternative strategies in the travel time tomographic approach and a new scheme to correct teleseismic data for global mantle structure our high resolution model pm0 5 is parameterized with three dimensional 3 d linear splines on a grid of nodes with 0 5 spacing in both horizontal directions and 50 km vertical spacing we obtain about 26 root mean square rms reduction of residuals by inversion in addition to roughly 31 reduction after summary rays formation and selection sensitivity analyses are performed through several test inversions to explore the resolution characteristics of the model at different spatial scales the distribution of large scale fast anomalies suggests that two different stages of a convection process presently coexist in very close regions the mantle dynamics of western central europe is dominated by blockage of subducted slabs at the 660 km discontinuity and ponding of seismically fast material in the transition zone contrarily in the eastern mediterranean fast velocity material sinks into the lower mantle suggesting that the flow of the cold downwelling here is not blocked by the 660 km discontinuity on a smaller scale the existence of tears in the subducted slab lithospheric detachment all along both margins of the adriatic plate as proposed by some authors is not supported by our tomographic images

sks splitting measurements in the apenninic tyrrhenian domain italy and their relation with lithospheric subduction and mantle convection 1 we present a compilation of singular shear wave splitting measurements in the italian region we used the method of silver and chan 1991 to determine the splitting parameters of fast direction and delay time from broadband sks waveforms the stations analyzed were mainly deployed in temporary experiments a few are permanent mednet broadband stations the splitting parameters reveal the presence of strong seismic anisotropy in the mantle beneath italy the fast polarization directions define two anisotropic domains a tyrrhenian domain with e w prevalent fast directions and an apennines adriatic domain with nw se to nnw sse prevalent fast directions the stations along the boundary between these two main domains show fast polarization directions that vary for earthquakes coming from different back azimuths this variability is a sign of a complex upper mantle structure also shown by seismic tomography but allows us to constrain the depth of the anisotropic layer in the 100 250 km range at least in the boundary zone between the two domains data at station vsl sardinia suggest the presence of depth dependent anisotropic structure where the shallower signature is consistent with the tyrrhenian domain results and the deeper one is possibly related to the presence of a subhorizontal slab laying on the 670 km discontinuity we argue that the complex pattern of fast directions is strictly related to the upper mantle structure and is caused by asthenospheric flows induced by slab rollback and contemporary deformation of the overriding tyrrhenian plate

numerical simulations of mantle flow around slab edges abstract a simple kinematic dynamic model of mantle flow around the slab edge is constructed in order to understand the flow complexity there the flow velocity on the top and the small boundary region around the shallow plate boundary is kinematically imposed in order to achieve a subduction like feature and the flow in other part is dynamically calculated the geometry of the plate mimics the region around the junction of aleutian islands and kamchatka that are examples of the convergent transform fault plate boundaries in a simple model in which the overlying plate is almost stationary the lateral flow from the mantle under the subducting slab to the mantle under the neighboring plate is of minor importance once the slab penetrates into the high viscosity layer where the downward flow encounters the resistance similar situation was found when the trench is advancing that is the trench moves toward the overlying plate for the case with retreating trench that is the trench moves toward the subducting plate a lateral flow exists even after the slab penetrates into the high viscosity layer although its magnitude is significantly smaller than that of the plate velocity the presence of a low viscosity layer just beneath the subducting plate may promote the emergence of lateral flow a significant lateral flow is observed when the high temperature anomaly that is buoyant and low viscosity block carried by the movement of subducting plate approaches the slab these results may have important implications for the possible existence of trench parallel flow in the sub slab mantle

ion concentrations in ice wedges an innovative approach to reconstruct past climate variability abstract for atmospheric gases and aerosols snow and ice provide a useful archive of paleoenvironmental history in the northern hemisphere this archive has been largely limited to greenland ice but promising pioneering work has been done recently using chronology of an ice wedge in barrow northern alaska here we investigate past aerosols at the same ice wedge sampling site reconstructing the sea ice fluctuations in the adjacent beaufort sea during the bolling allerod ba and younger dryas yd periods we confirm the integrity of methanesulfonate ion ms concentrations in the biws as marine proxies and then find that the ice wedge has a high ms concentration through the beginning coldest yd periods the high ms concentration indicates that even during the coldest yd periods 12 900 12 700 yrbp the near shore region in the alaskan beaufort sea near barrow may not have been completely filled by permanent sea ice

southern ocean sea ice extent productivity and iron flux over the past eight glacial cycles sea ice and dust flux increased greatly in the southern ocean during the last glacial period palaeorecords provide contradictory evidence about marine productivity in this region but beyond one glacial cycle data were sparse here we present continuous chemical proxy data spanning the last eight glacial cycles 740 000 years from the dome c antarctic ice core these data constrain winter sea ice extent in the indian ocean southern ocean biogenic productivity and patagonian climatic conditions we found that maximum sea ice extent is closely tied to antarctic temperature on multi millennial timescales but less so on shorter timescales biological dimethylsulphide emissions south of the polar front seem to have changed little with climate suggesting that sulphur compounds were not active in climate regulation we observe large glacial interglacial contrasts in iron deposition which we infer reflects strongly changing patagonian conditions during glacial terminations changes in patagonia apparently preceded sea ice reduction indicating that multiple mechanisms may be responsible for different phases of co2 increase during glacial terminations we observe no changes in internal climatic feedbacks that could have caused the change in amplitude of antarctic temperature variations observed 440 000 years ago

frequency dependent anisotropy in wellington new zealand shear wave splitting measurements are made using teleseismic s scs and sks waveforms recorded at the gdsn broadband station snzo situated in south karori new zealand the average s and sks delay times are around 2 to 3 s among the highest in the world the average scs delay time is significantly smaller around 1 s this discrepancy appears to be due to differences in dominant frequency the most likely cause of frequency dependent anisotropy is oriented heterogeneities with a scale length much smaller than the wavelength the fast polarizations range between 21 and 79 with an average ne sw direction which is sub parallel to the trend of the local geologic structure and the strike of the hikurangi subduction zone azimuthal variations in delay time which cannot be explained by differences in period may be due to a dipping axis of symmetry or laterally varying anisotropy or a more complicated symmetry system

the southern tyrrhenian subduction zone deep geometry magmatism and plio pleistocene evolution abstract we report on a high resolution vp vp vs and qp model of the southern tyrrhenian subduction zone obtained by the inversion of p and s wave arrival times and t values from intraslab seismicity the arcuate shape of the southern apennines calabrian arc sicilian maghrebides is perfectly mirrored by two rather continuous low and high vp bands lying beneath the belt system at ca 25 and 100 km respectively between 100 and 300 km two independent high vp slabs lie beneath the neapolitan region and the southern tyrrhenian sea separated by unperturbed mantle we suggest that the ca 150 km wide slab window beneath the southern apennines opened after a tear occurring within a composite subduction system formed by the apulian continental lithosphere and the ionian oceanic slab the abrupt slab rupture induced ultrafast southeastward retreat of the ionian slab and the 19 cm yr spreading of the back arc oceanic marsili basin between ca 2 1 and 1 6 ma ago the 25 km low vp zone beneath the arc denotes continental upper crustal rocks below the chain its striking continuity requires a unique orogenic wedge at 25 km depth below the southern apennines the calabrian arc and the sicilian maghrebides the alternative explanation would imply the ubiquitous occurrence of autochthonous lower plate rocks at 25 km depth i e a puzzling autochthonous continental calabria the ionian slab beneath calabria shows high vp high qp and low vp vs anomalies typical of old oceanic lithosphere intermediate depth seismicity is concentrated within its thin oceanic crust suggesting the occurrence of vigorous metamorphism the slab dehydration promotes the melting of the overlying mantle as testified by high vp vs and low qp anomalies between the slab and the aeolian magmatic arc

sks splitting in southern italy anisotropy variations in a fragmented subduction zone abstract in this paper we present a collection of good quality shear wave splitting measurements in southern italy in addition to a large amount of previous splitting measurements we present new data from 15 teleseisms recorded from 2003 to 2006 at the 40 stations of the cat scan temporary network these new measurements provide additional constraints on the anisotropic behaviour of the study region and better define the fast directions in the southern part of the apulian platform for our analysis we have selected well recorded sks phases and we have used the method of silver and chan to obtain the splitting parameters the azimuth of the fast polarized shear wave and delay time t shear wave splitting results reveal the presence of a strong seismic anisotropy in the subduction system below the region three different geological and geodynamic regions are characterized by different anisotropic parameters the calabrian arc domain has fast directions oriented nne ssw and the southern apennines domain has fast directions oriented nnw sse this rotation of fast axes following the arcuate shape of the slab is marked by a lack of resolved measurements which occurs at the transition zone between those two domains the third domain is identified in the apulian platform here fast directions are oriented almost n s in the northern part and nne ssw to ene wsw in the southern one the large number of splitting parameters evaluated for events coming from different back azimuth allows us to hypothesize the presence of a depth dependent anisotropic structure which should be more complicated than a simple 2 layer model below the southern apennines and the calabrian arc domains and to constrain at 50 km depth the upper limit of the anisotropic layer at least at the edge of southern apennines and apulian platform we interpret the variability in fast directions as related to the fragmented subduction system in the mantle of this region the trench parallel observed in calabrian arc and in southern apennines has its main source in the asthenospheric flow below the slab likely due to the pressure induced by the retrograde motion of the slab itself the pattern of in the apulian platform does not appear to be the direct result of the rollback motion of the slab whose influence is limited to about 100 km from the slab the anisotropy in the apulian platform may be related to an asthenospheric flow deflected by the complicated structure of the adriatic microplate or may also be explained as frozen in lithospheric anisotropy

constraints on mantle circulation around the deforming calabrian slab 1 combining tectonics with seismological and geochemical data we reconstruct the deformation history of the presently narrow calabrian slab and the path of mantle circulation during the last 10 ma we show that during the slab deformation the mantle laterally flowed inside the back arc region permitting its retrograde motion and giving a seismological and volcanological record after 1 2 myr

ice core evidence for antarctic sea ice decline since the 1950s the instrumental record of antarctic sea ice in recent decades does not reveal a clear signature of warming despite observational evidence from coastal antarctica here we report a significant correlation p

stress strain and b type olivine fabric in the fore arc mantle sensitivity tests using high resolution steady state subduction zone models 1 shear wave splitting observations at several subduction systems show trench parallel fast directions in the fore arc mantle the presence of b type olivine fabric in the mantle wedge may provide an explanation for this pattern of anisotropy under low temperature and hydrated conditions sensitivity tests are shown that provide insights into the distribution and magnitude of b type fabric using two dimensional high resolution kinematic dynamic subduction zone models these models include a wet olivine rheology and a simulated aseismic creep zone in the fore arc mantle with parameterized viscous coupling between the slab and mantle wedge the calculated thermal structure is in reasonable agreement with heat flow observations and q tomography for small amounts of coupling and moderate amounts of shear heating in the seismogenic and aseismic creep zones these small amounts of viscous coupling give rise to a slowly flowing fore arc mantle with thermal stress and strain conditions suitable for significant b type fabric development the minimum shear stress necessary to drive geologically significant flow in the predicted b type region is sensitive to the magnitude of shear heating along the aseismic creep zone and ranges from less than 100 to 300 mpa the minimum time required to generate sufficient finite strain for fabric development in the b type region is around 10 myr depending on the amount of coupling and shear heating the predicted distribution of olivine fabric is consistent with a pattern of shear wave anisotropy with trench parallel fast polarization directions above the fore arc and a rapid rotation to trench perpendicular in the arc and back arc

separation of melting and environmental signals in an ice core with seasonal melt we examine the impact of melt water percolation on the soluble ion chemical record from the lomonosovfonna ice core principle component analysis shows that melting produces only simple changes bet

lateglacial and holocene isotopic and environmental history of northern coastal alaska results from a buried ice wedge system at barrow barrow the northernmost point in alaska is one of the most intensively studied areas in the arctic however paleoenvironmental evidence is limited for northern alaska for the lateglacial holocene transition for a regional paleoenvironmental reconstruction we investigated a permafrost ice wedge tunnel near barrow alaska the studied site was first excavated in the early 1960s and intercepts a buried ice wedge system at 3e6 m depth below the surface a multi methodological approach was applied to this buried ice wedge system and the enclosing sediments which in their combination give new insight into the late quaternary environmental and climate history results of geochronological sedimentological cryolithological paleoecological isotope geochemical and microbiological studies reflect different stages of mid to late wisconsin mw to lw allerod ad younger dryas yd preboreal pb and late holocene paleoenvironmental evolution the lwage of the site is indicated by ams dates in the surrounding sediments of 21 7 kyr bp at the lateral contact of the ice wedge system as well as 39 5 kyr bp below the ice wedge system it is only recently that in this region stable isotope techniques have been employed i e to characterize different types of ground ice the stable isotope record oxygen 18o hydrogen d of two intersecting ice wedges suggests different phases of the northern alaskan climate history from ad to pb with radiocarbon dates from 12 4 to 9 9 kyr bp ranging from 14 8 to 10 6 kyr cal bp stable isotope geochemistry of ice wedges reveals winter temperature variations of the lateglacial holocene transition including a prominent yd cold period clearly separated from the warmer ad and pb phases yd is only weakly developed in summer temperature indicators such as pollen for the northern alaska area and by consequence the yd cold stadial was here especially related to the winter season this highlights that the combination of winter and summer indicators comprehensively describes the seasonality of climate relevant processes in discrete time intervals the stable isotope record for the barrow buried ice wedge system documents for the first time winter climate change at the lateglacial holocene transition continuously and at relatively high likely centennial resolution

permafrost evidence for severe winter cooling during the younger dryas in northern alaska 1 the younger dryas cold event a rapid reversion to glacial climate conditions at the pleistocene holocene transition has generally been attributed to the release of meltwater from the laurentide ice sheet to the north atlantic or arctic oceans the reaction of the north pacific region to this shutdown of the thermohaline circulation in the north atlantic during younger dryas is little understood in this paper we present the first radiocarbon dated centennial scale stable water isotope record from permafrost in northern alaska this late glacial winter climate reconstruction from barrow ice wedges demonstrates the existence of a younger dryas cold event formerly believed to be reduced or absent in this area our stable isotope data display a gradual change of the atmospheric moisture source conditions during the younger dryas likely associated with the successive opening of the bering strait

seismic anisotropy reveals focused mantle flow around the calabrian slab southern italy 1 sks splitting at the calabrian subduction zone with delay times t up to 3s reveals the presence of a strong anisotropic fabric fast directions are oriented nne ssw in the calabrian arc c a and rotate nnw sse to the north following the arcuate shape of the subducting plate we interpret the trench parallel as local scale mantle flow driven by the retrograde motion of the slab the absence of trench perpendicular below the southern apennines s a excludes the presence of a seismically detectable return flow at its ne edge this may be due to the relative youth and limited width of the s a slab tear a possible return flow is identified farther north at the boundary of the s a and central apennines different and weaker anisotropy is present below the apulian platform a p this implies that the influence of the slab rollback in the sub slab mantle is limited to less then 100 km from the slab

phytoplankton dynamics from the cambrian explosion to the onset of the great ordovician biodiversification event a review of cambrian acritarch diversity most early palaeozoic acritarchs are thought to represent a part of the marine phytoplankton and so constituted a significant element at the base of the marine trophic chain during the cambrian explosion and the subsequent great ordovician biodiversification event cambrian acritarch occurrences have been recorded in a great number of studies in this paper published data on cambrian acritarchs are assembled in order to reconstruct taxonomic diversity trends that can be compared with the biodiversity of marine invertebrates we compile a database and calculate various diversity indices at global and regional i e gondwana or baltica scales the stratigraphic bins applied are at the level of the ten cambrian stages or of fourteen commonly used biozones in a somewhat higher resolved scheme our results show marked differences between palaeogeographical regions they also indicate limitations of the data and a potential sampling bias as the taxonomic diversity indices of species are significantly correlated with the number of studies per stratigraphic bin the total and normalized diversities of genera are not affected in the same way the normalized genus diversity curves show a slow but irregular rise over the course of the cambrian these also are the least biased a radiation of species and to a lesser extent of genera in the lower cambrian series 2 appears to mirror the cambrian explosion of metazoans this radiation not evident on gondwana is followed by a prominent low in species diversity in the upper series 3 and lower furongian highest diversities are reached globally and on both baltica and gondwana in the uppermost cambrian stage 10 more precisely in the peltura trilobite zone preceding a substantial phase of acritarch species extinction below and at the cambrian ordovician boundary nearly all the genera present in stage 10 survived into the ordovician the forms that emerged during the cambrian therefore became the foundation for the more rapid radiation of acritarchs during the great ordovician biodiversification event

anisotropic structure under a back arc spreading region the taupo volcanic zone new zealand 1 we investigate anisotropy surrounding a continental back arc spreading region the central volcanic region cvr in the north island of new zealand and present a simple method for spatial averaging to examine heterogeneous anisotropy s phases from local earthquakes yield consistent trench parallel fast directions in the southern compressional region but varied directions in the northern extensional region in the forearc east of the cvr is nearly trench parallel suggesting shear in the mantle or trench parallel flow in the western cvr local phases from 50 to 80 km depth give parallel to extension 120 and phases from 150 to 220 km depth yield similar 150 these results suggest asthenospheric flow with olivine a axes oriented in the extension direction down to 100 150 km depth in the eastern cvr local phases show mixed suggesting that it is a transition between the western cvr and the forearc trench parallel for shallow local events within the cvr may be caused by fluid filled cracks trench parallel for deeper local events and for sks phases can be explained by standard olivine fabrics developed by lithospheric shearing by trench parallel flow and by fossil anisotropy in the subducting oceanic lithosphere alternatively in the cvr they could be caused in part by hydrous minerals at the slab mantle upper interface which could cause olivine a axes to orient parallel to the trench due to a maximum shear stress oriented perpendicular to the trench along the slab we suggest that trench perpendicular flow in the extensional region helps to drive fluids away from the slab while in the compressional region trench parallel flow fails to distribute the fluids explaining the location of changes in geophysical properties

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

seismic characterization of mantle flow in subduction systems can we resolve a hydrated mantle wedge this study provides new constraints on the resolvability of mantle flow in subduction zone settings as inferred by observations of seismic anisotropy we are motivated by the broad range of shear wave splitting observations in subduction systems that suggest complex flow geometries changes in the deformation state of mantle minerals or a combination of both while shear wave splitting fast polarization directions are typically interpreted as a proxy for flow or maximum finite extension experimental studies suggest that olivine slip systems change under higher stress and hydration states conditions likely appropriate for subduction systems in this study we predict shear wave splitting as a result of mantle silicate lattice preferred orientation development in steady state two dimensional mantle flow models using a textural development theory that incorporates the combined effects of intracrystalline slip and dynamic recrystallization we utilize the resulting textures to predict shear wave splitting for populations of seismic raypaths traversing the model within the subduction zone mantle wedge the results of our study demonstrate that combined observations of variations in fast polarization directions and splitting times make it possible to resolve a shift from anhydrous to hydrous mantle insubduction zone settings provided very good sampling of the mantle wedge our models are generally consistent with observed splitting variations for several subduction zones with dense data sampling including tonga japan and kamchatka the implications of our work suggest that provided adequate data sampling shear wave splitting measurements can provide the necessary information to evaluate potential competing effects between variations in mantle flow direction and changes in the stress and hydration states of subduction zone mantle wedges

three dimensional p wave attenuation and velocity upper mantle tomography of the southern apennines calabrian arc subduction zone 1 we propose a 3 d crust upper mantle seismic attenuation qp model of the southern apennines calabrian arc subduction zone together with a 3 d velocity vp model the qp model is calculated from relative t using the spectral ratio method and the vp from traveltime data the final data set used for the inversion of the vp model consists of 2400 traveltime arrivals recorded by 34 short period stations that are part of the italian national seismic network and for the qp model 2178 pn phases recorded by a subset of 32 stations traveltimes and waveforms come from 272 intermediate depth calabrian slab events this 3 d model of attenuation together with the 3 d velocity model improves our knowledge of the slab mantle wedge structure and can be a starting point in determining the physical state of the asthenosphere i e its temperature the presence of melt and or fluids and its relation to volcanism found in the study area main features of the qp and vp models show that the mantle wedge slab in particular the area of highest attenuation is located in a volume underlying the marsili basin the existence and shape of this main low qp and low vp anomaly points to slab dehydration and fluid material flow a process that may explain the strong geochemical affinities between the subduction related magmas from stromboli and vesuvius other interesting features in the models are strong lateral variations in qp and vp that are put in relation with known important tectonic structures and volcanic centers in the area

the influence of plate motions on three dimensional back arc mantle flow and shear wave splitting both the polarization direction of the fast shear waves and the types of deformation within overriding plates vary between the back arc basins of western pacific subduction zones the goal of this study is to test the possibility that motions of the overriding plates may control the patterns of seismic anisotropy and therefore the observed shear wave splitting we calculated three dimensional models of viscous asthenospheric flow driven by the motions of the subducting slab and overriding plates shear wave splitting was calculated for polymineralic anisotropy within the back arc mantle wedge assuming that the anisotropy was created by flow induced strain predicted splitting may differ substantially depending on whether anisotropy is computed directly using polycrystalline plasticity models or is based on the orientation of finite strain a parameter study shows that both finite strain and textural anisotropy developed within three dimensional plate coupled asthenospheric flow models are very heterogeneous when back arc shearing occurs within the overriding plate therefore predicted shear wave splitting varies strongly as a function of plate motion boundary conditions and with ray path through the back arc asthenosphere flow models incorporating plate motion boundary conditions for the tonga southern kuril and eastern aleutian subduction zones produce splitting parameters consistent with observations from each region trench parallel flow generated by small variations in the dip of the subducting plate may be important in explaining observed fast directions of anisotropy sampled within the innermost corner of the mantle wedge

trench parallel flow and seismic anisotropy in the mariana and andean subduction systems shear wave splitting measurements above the mantle wedge of the mariana1 and southern andean2 3 subduction zones show trench parallel seismically fast directions close to the trench and abrupt rotations to trench perpendicular anisotropy in the back arc these patterns of seismic anisotropy may be caused by three dimensional flow associated with along strike variations in slab geometry1 2 3 4 5 the mariana and andean subduction systems are associated with the largest along strike variations of slab geometry observed on earth6 7 and are ideal for testing the link between slab geometry and solid state creep processes in the mantle here we show with fully three dimensional non newtonian subduction zone models that the strong curvature of the mariana slab and the transition to shallow slab dip in the southern andes give rise to strong trench parallel stretching in the warm arc and warm back arc mantle and to abrupt rotations in stretching directions that are accompanied by strong trench parallel stretching these models show that the patterns of shear wave splitting observed in the mariana and southern andean systems may be caused by significant three dimensional flow induced by along strike variations in slab geometry

major and trace elements δ13c and polycyclic aromatic hydrocarbons in the late pleistocene ice wedges a case study of batagay yedoma central yakutia abstract we studied the ionic composition of ice the content of suspended and dissolved trace and major elements in ice values of 13 and the composition of pahs of inclusions in the late pleistocene syngenetic ice wedges of the batagay yedoma for the first time the complex geochemical study of batagay megaslump was carried out the ice of the upper and lower complex of different ages was studied mineralisation of the late pleistocene ice wedges of batagay yedoma ranges from 66 56 to 424 8 mg l 1 from ultrafresh to desalinated the ionic composition of the ice is bicarbonate calcium and corresponds to snow formed under the influence of continental air masses the proportion of ca contained in dissolved form is not more than 20 30 as for the other elements the proportion of mg k na al fe mn sr ba in suspended form is greater than 90 the content of trace elements in inclusions in ice wedges and soils is equal which confirms the pedogenic origin of the inclusions in ice according to our data values of 13 in lipids of the pedogenic material in ice decrease with depth the lowest values values of 13 less than 30 were observed in the ice wedge of the upper complex at a depth of 7 4 8 6 m the increase of 13c values of lipids from pedogenic material in ice wedges with depth is due to the landscape change from grassland ecosystems to forest ecosystems and may also be influenced by the increased frequency of fires that occurred during the transition to forest ecosystems and the disappearance of large herbivores

late vendian early palaeozoic tectonic evolution of the baltic basin regional tectonic implications from subsidence analysis abstract subsidence analysis was performed on 43 boreholes penetrating the upper vendian lower palaeozoic sedimentary succession of the baltic basin the results were related to lithofacial and structural data to elucidate subsidence mechanisms and the regional tectonic setting of basin development tectonic subsidence patterns are consistent throughout the basin for the time period studied an extensional tectonic subsidence event possibly of two phases is indicated from the late vendian to the beginning of the middle cambrian this event is seen in the southwestern part of the baltic basin peri tornquist zone until the earliest cambrian after which it is also observed in the sw ne trending baltic depression part of the basin basin development during this time is interpreted as recording the latest stages of break up of the precambrian super continent rodinia and ultimately the formation of the tornquist sea the late middle cambrian to middle ordovician tectonic subsidence pattern of the baltic basin is characteristic of post rift thermal subsidence of the newly formed passive continental margin of baltica developed along its southwestern edge a gradual increase in subsidence rate is observed from the middle late ordovician and throughout the silurian particularly for ludlow and pridoli times creating subsidence curves with convex shapes typical of foreland basin development the rate of late silurian tectonic subsidence increases significantly towards the southwest margin of the baltic basin adjacent to the present location of the north german polish caledonides the baltic basin therefore appears to have developed primarily as a flexural foreland basin during silurian oblique collision of baltica and eastern avalonia a foreland setting is supported by the influx of distal turbidites into the basin from southwest sources in the late silurian compressional deformation structures of early devonian lochkovian age are seen in seismic sections in the central part of the baltic basin lithuania these together with a change in subsidence pattern mark the end of the caledonian stage of basin development of the baltic basin

deglacial floods in the beaufort sea preceded younger dryas cooling a period of cooling about 13 000 years ago interrupted about 2 000 years of deglacial warming known as the younger dryas yd the event is thought to have resulted from a slowdown of the atlantic meridional overturning circulation in response to a sudden flood of laurentide ice sheet meltwater that reached the nordic seas oxygen isotope evidence for a local source of meltwater to the open western north atlantic from the gulf of st lawrence has been lacking here we report that the eastern beaufort sea contains the long sought signal of 18o depleted water beginning at 12 94 0 15 thousand years ago oxygen isotopes in the planktonic foraminifera from two sediment cores as well as sediment and seismic data indicate a flood of meltwater ice and sediment to the arctic via the mackenzie river that lasted about 700 years the minimum in the oxygen isotope ratios lasted 130 years we suggest that the floodwater travelled north along the canadian archipelago and then through the fram strait to the nordic seas where freshening and freezing near sites of deep water formation would have suppressed convection and caused the yd cooling by reducing the meridional overturning

shear wave anisotropy beneath the andes from the banjo seda and pisco experiments we present the results of a detailed shear wave splitting analysis of data collected by three temporary broadband deployments located in central western south america the broadband andean joint experiment banjo a 1000 km long east west line at 20 s and the projecto de investigacion sismologica de la cordillera occidental pisco and seismic exploration of the deep altiplano seda deployed several hunderd kilometers north and south of this line we determined the splitting parameters fast polarization direction and t splitting delay time for waves that sample the above and below slab regions teleseismic ks and s scs waves from local deep focus events as well as s waves from intermediate focus events that sample only the above slab region all but one of the ks stacks for the banjo stations show e w fast directions with t varying between 0 4 and 1 5 s however for ks recorded at most of the seda and pisco stations and for local deep focus s events north and south of banjo there is a rotation of to a more nearly trench parallel direction the splitting parameters for above slab paths determined from events around 200 km deep to western stations yield small delay times 0 3 s and n s fast polarization directions assuming the anisotropy is limited to the top 400 km of the mantle olivine stability field these data suggest the following spatial distribution of anisotropy for the above slab component as one goes from east where ks reflects the above slab component to west changes from e w to n s and delay times are substantially reduced this change may mark the transition from the brazilian craton to actively deforming e w shortening andean mantle we see no evidence for the strain field expected for either corner flow or shear in the mantle wedge associated with relative plate motion the small delay times for above slab paths in the west require the existence of significant spatially varying below slab anisotropy to explain the ks results the implied anisotropic pattern below the slab is not easily explained by a simple model of slab entrained shear flow beneath the plate instead flow induced by the retrograde motion of the slab in combination with local structural variations may provide a better explanation

cambrian evolutionary radiation context correlation and chronostratigraphy overcoming deficiencies of the first appearance datum fad concept abstract use of the first appearance datum fad of a fossil to define a global chronostratigraphic unit s base can lead to intractable correlation and stability problems fads are diachronous they reflect species evolutionary history dispersal biofacies preservation collection and taxonomy the cambrian evolutionary radiation is characterised by diachronous fads biofacies controls and provincialism of taxa and ecological communities that confound a stable lower cambrian chronostratigraphy cambrian series and stage definitions require greater attention to assemblage zone successions and non biostratigraphic particularly carbon isotope correlation techniques such as those that define the ediacaran system base a redefined basal cambrian trichophycus pedum assemblage zone lies above the highest ediacaran type biotas vendobionts putative metazoans and calcareous problematica such as cloudina and the basal asteridium tornatum comasphaeridium velvetum zone acritarchs this definition and the likely close correspondence of evolutionary origin and local fad of t pedum preserves the fortune head newfoundland gssp of the cambrian base and allows the presence of sub cambrian branched ichnofossils the sub tommotian equivalent base of stage 2 a suggested laolinian stage should be defined by the i l4 zhuce 13 c positive peak bracketed by the lower ranges of watsonella crosbyi and aldanella attleborensis molluscs and the skiagia ornata fimbrioglomerella membranacea zone acritarchs the w crosbyi and a attleborensis fads cannot define a stage 2 base as they are diachronous even in the newfoundland type w crosbyi zone the series 2 base cannot be based on a species fad owing to the provincialism of skeletalised metazoans in the terreneuvian series 2 boundary interval and global heterochrony of the oldest trilobites a series 2 and stage 3 a suggested lenaldanian series and zhurinskyan stage new gssp base is proposed at the siberian lower atdabanian 13 c iv peak which correlates into south china avalonia and morocco and assigns the oldest trilobites to the terminal terreneuvian series

a laboratory model of subduction zone anisotropy abstract measurements of split seismic shear waves near pacific subduction zones show that fast polarization directions are primarily trench parallel in the seaward side mantle beneath a subducting slab and variable in the mantle wedge beneath back arc basins it is generally accepted that the azimuth of the fast polarization direction defines the average orientation of the crystallographic a axis in olivine here we use laboratory experiments to determine the orientation of the olivine a axis in the upper mantle in the vicinity of a subducting slab a laboratory analog for olivine a axis motion during creep deformation is developed using small cylinders whiskers suspended in a viscous fluid subduction beneath a stationary overriding plate variable dip angles and realistic rollback and down dip plate kinematics are examined plate rollback produces strong plate parallel whisker alignments for all dip angles examined alignment in the wedge occurs in three layers a layer of dip parallel alignment above the downgoing plate a layer of subhorizontal alignment beneath the overriding plate and a core of intermediate orientations the amount of alignment beneath the overriding plate and the wedge core decreases with increasing dip angle our results indicate that olivine a axis orientation in the seaward side mantle is controlled by the amount of slab rollback and that orientations in the mantle wedge depend upon slab dip angle

a stratigraphic framework for abrupt climatic changes during the last glacial period based on three synchronized greenland ice core records refining and extending the intimate event stratigraphy due to their outstanding resolution and well constrained chronologies greenland ice core records provide a master record of past climatic changes throughout the last interglacial glacial cycle in the north atlantic region as part of the intimate integration of ice core marine and terrestrial records project protocols have been proposed to ensure consistent and robust correlation between different records of past climate a key element of these protocols has been the formal definition and ordinal numbering of the sequence of greenland stadials gs and greenland interstadials gi within the most recent glacial period the gs and gi periods are the greenland expressions of the characteristic dansgaard oeschger events that represent cold and warm phases of the north atlantic region respectively we present here a more detailed and extended gs gi template for the whole of the last glacial period it is based on a synchronization of the ngrip grip and gisp2 ice core records that allows the parallel analysis of all three records on a common time scale the boundaries of the gs and gi periods are defined based on a combination of stable oxygen isotope ratios of the ice 18o reflecting mainly local temperature and calcium ion concentrations reflecting mainly atmospheric dust loading measured in the ice the data not only resolve the well known sequence of dansgaard oeschger events that were first defined and numbered in the ice core records more than two decades ago but also better resolve a number of short lived climatic oscillations some defined here for the first time using this revised scheme we propose a consistent approach for discriminating and naming all the significant abrupt climatic events of the last glacial period that are represented in the greenland ice records the final product constitutes an extended and better resolved greenland stratotype sequence against which other proxy records can be compared and correlated it also provides a more secure basis for investigating the dynamics and fundamental causes of these climatic perturbations

influence of convergent plate boundaries on upper mantle flow and implications for seismic anisotropy shear wave splitting observations in the region of the upper mantle enveloping subduction zones have been interpreted as showing extensive regions of trench parallel flow despite the difficulty of reconciling such behavior with a sound model based on the forces that drive mantle motion to gain insight into the observations we systematically investigate flow patterns around the cold downwelling sheets associated with consumed plate material in a three dimensional numerical mantle convection model first we compare results from calculations employing prescribed plate geometries and kinematic plate velocities where the convergent plate boundary morphology is varied while keeping the plate velocity and convective parameters fixed subsequently we examine the flow around sheet like downwellings in a number of convection calculations featuring dynamically evolving plate velocities all of the calculations include thick viscous plates and a stratified mantle viscosity in all of the models examined we find that at mid upper mantle depths flow directions no longer align with plate motion and the influence of buoyancy driven downwellings clearly dominates flow solutions in the first models analyzed a pair of plates are included in the calculations and the large scale flow is generally roll like in the final model we investigate the interaction of four plates and a plate geometry characterized by triple junctions we examine a sequence from this calculation that features a triple junction of convergent boundaries in this model large scale flow characterized by convection rolls is superseded by a complex flow solution where flow in the mid upper mantle neither aligns uniformly with the plate motion nor necessarily follows the forcing associated with local buoyancy sources in this setting upper mantle flow in the vicinity of the sheet like downwellings featured in the solution moves orthogonal obliquely and even parallel to different sections of the convergent plate boundaries in the latter case our calculations of the deformation of a fixed volume parcel of upper mantle material suggest that an olivine lattice preferred orientation should develop that would result in a fast polarizing direction for seismic shear waves parallel to the slab our findings have implications for the interpretation of flow in the upper mantle based on seismic anisotropy

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

anisotropy patterns in the subducting lithosphere and in the mantle wedge a case study the southern italy subduction system 1 in this study we present a collection of high quality s wave splitting measurements in the southern italy subduction system we analyzed 76 deep earthquakes located within the descending slab using the method of silver and chan 1991 to determine the splitting parameters and t the local deep earthquakes allow us to analyze raypaths primarily sampling the slab and the wedge above it mainland calabria is an outcropping forearc enabling us to sample rays that propagate up the slab s wave splitting parameters show a complex pattern of anisotropy with variable fast directions and with delay times ranging from 0 1 s to 2 2 s we compared local s wave splitting data with sks results at the same stations and we found that the average t is very different 1 8 s for sks and 0 5 s for s we found consistency between averaged s fast directions and the sks splitting measurements including a pattern of toroidal mantle flow at the sw edge of the slab the s wave splitting parameters show frequency dependent behavior that we attribute to the presence of small scale anisotropic heterogeneities comparison of s splitting measurements to p wave velocity anomaly at 100 200 km depth shows that where the rays primarily sample the slab the delay times are small in contrast where s rays sample the mantle wedge the delay times are quite high this t pattern depicts the slab as a weakly anisotropic region and suggests that the main source of anisotropy in the subduction zone is the surrounding asthenosphere

tomographic constraints on the geodynamic evolution of the italian region in this paper we present p wave tomographic images of the mantle beneath italy obtained by inverting 6000 teleseismic p and pkp wave arrival times accurately repicked recorded in the time period 1988 1994 by the stations of the national seismic network of the istituto nazionale di geofisica we pay great attention in the data selection and picking procedure of seismic phases to obtain a very high quality data set the data were inverted with the well established aki christofferson husebye tomographic technique different reference models and residuals computation have been tried to verify the stability of the results the high quality of the repicked arrival times allows us to enhance the definition of the deep structures beneath both the alps and the apennines looking for their lateral and vertical continuity down to 800 km depth the main finding of this study is a continuous high velocity body located between 250 and 670 km depth beneath the entire apenninic system dipping toward the tyrrhenian area which continues upward segmented in two main anomalies in the northern apenninic and the calabrian arcs we interpret this high velocity feature as the subducted oceanic lithosphere between the eurasian and african plates dipping down to the upper lower mantle boundary beneath the tyrrhenian sea the retrieved images of the lithosphere subducting beneath apennines are reliable in terms of thickness about 80 90 km and p wave velocity contrast 2 4 higher than the normal mantle furthermore our tomographic images which focus on the deep geometry and continuity of the velocity structures provide new keys to understanding the geodynamic evolution of the italian region the segmentation of the high velocity slab upward suggests a complex evolution of the arc trench system and the initially continuous subduction of the ionian adriatic plate progressively developed in subordinate arcs probably due to lateral heterogeneity of the subducting lithosphere

subduction rollback slab breakoff and induced strain in the uppermost mantle beneath italy differences in seismic anisotropy revealed by split sks waves that traverse the upper mantle beneath the italian region reveal four areas of internally coherent anisotropic strength providing evidence for mantle strain partitioning when compared to uppermost mantle structure imaged by tomography the sequence of these areas displays a straightforward parallelism under proper assumptions the correspondence between areas of coherent splitting degree and areas of coherent velocity perturbations offers new insights into the late evolution of subduction in italy and their effect on mantle strain

geophysics going with the mantle flow measurements of directional travel speed of seismic waves constrain flow in the upper mantle laboratory experiments suggest that high pressure can change the mantle s mineral alignment leading to a 90 offset in the direction of the fastest seismic waves

organic walled microfossils from the chert phosphorite member tal formation precambrian cambrian boundary india the chert member also designated chert phosphorite member of the tal formation in the lesser himalaya contains well preserved organic walled microfossil assemblage the assemblage includes small 25 m acanthomorphic acritarchs sphaeromorphic acritarchs cyanobacterial filaments sponge spicules and thallophytic remains identified as micrhystridium lanceolatum m dissimilare m tornatum m lanatum cf lophosphaeridium sp paracymatiosphaera irregularis cymatiosphaera sp estiastra minima leiosphaeridia sp siphonophycus robustum thalloidal fragments and hexactinellid sponge spicules the present assemblage rich in small acanthomorphs appears to be best developed in upper nemakit daldynian beds based on acritarch biostratigraphy the present work suggests a nemakit daldynian to early tommotian age for the chert member which is in agreement with the formerly proposed age based mainly on small shelly fossils the assemblage is correlatable to the precambrian cambrian boundary microbiotic assemblage from china central asia and east european platform the assemblage shows low diversity of species the characteristic of inshore biota

the mantle flow field beneath western north america although motions at the surface of tectonic plates are well determined the accompanying horizontal mantle flow is not we have combined observations of surface deformation and upper mantle seismic anisotropy to estimate this flow field for western north america we find that the mantle velocity is 5 5 1 5 centimeters per year due east in a hot spot reference frame nearly opposite to the direction of north american plate motion west southwest the flow is only weakly coupled to the motion of the surface plate producing a small drag force this flow field is probably due to heterogeneity in mantle density associated with the former farallon oceanic plate beneath north america

shear wave splitting beneath the southwestern kurile arc and northeastern japan arc a new insight into mantle return flow 1 shear wave splitting in the southwestern part of the kurile arc and the northeastern ne japan arc is investigated using the waveforms from local earthquakes for both arcs observed shear wave splitting shows clear evidence for a striking rotation of fast direction across the arc suggesting the different feature of anisotropy between the fore arc and back arc sides trench parallel fast directions are observed in the fore arc side which is consistent with the anisotropy expected from the deformation of the b type olivine fabric as well as the anisotropy in the slab fast directions observed in the back arc side show approximately e w and ese wnw in the ne japan arc and n s in the southwestern kurile arc which are characterized by the local maximum dip direction of the subducted pacific plate not by the direction of plate convergence this relationship suggests that the direction of the mantle return flow is governed by the local slab geometry

so42 minimum in summer snow layer at dome fuji antarctica and the probable mechanism 1 by chemically analyzing snow samples at dome fuji antarctica we found that the snowdrift deposited just after snowfall and the snow layer down to 3 4 m have summer minima in the non sea salt nss so42 and na concentrations and summer maxima of cl na such a summer nss so42 minimum in either snowdrift or the snow layer has not been reported at any other site in antarctica and arises in spite of the known maximum in the nss so42 concentration in aerosol at dome fuji in summer we then did laboratory experiments to better understand the phenomenon the results supported the following mechanism for the summer nss so42 minimum in the snowdrift and snow layer in summer water vapor sublimates from within the snow in the daytime and condenses on the surface as frost in the nighttime resulting in a dilution of the nss so42 concentration this sublimation condensation process likely occurs at other cold inland regions in addition the results might be useful for obtaining a high resolution dating method for dome fuji deep ice cores by counting the number of layers with low nss so42 concentration

on the post 25 ma geodynamic evolution of the western mediterranean during the neogene and quaternary western mediterranean geodynamics were dominated by the eastward migration of the apenninic arc and associated back arc basins the migration was controlled by retreat of the apenninic slab and was associated with boudinage of the lithosphere in the back arc area palaeo reconstruction of the kinematics of the arc suggests about 775 km of migration from the late oligocene to present along a transect from the gulf of lions to calabria a maximum of 135 km of n s converge occurred between africa and europe during the same time span the western mediterranean was thus mainly shaped by the migration of the slab related to west directed subduction it is hypothesized that minor n s convergence deformed the arc but was not the cause of its formation 1998 elsevier science b v all rights reserved