upper mantle anisotropy beneath the geoscope stations seismic anisotropy has been widely studied this last decade particularly by measuring splitting of vertically propagating core shear waves the main interest in this technique is to characterize upper mantle flow beneath seismic stations on the other hand the major restriction in this method is that a single station gives a single anisotropy measurement alternative methods have been developed in order to avoid this restriction an accurate determination of upper mantle seismic anisotropy beneath a seismic station may allow one by doing anisotropy correction to characterize remote or deeper anisotropy the geoscope network is ideal for this purpose because it is composed of a large set about 26 of high quality broadband seismometers globally distributed and because some of these stations have run for more than 10 years and most of them for more than 5 years we selected about 100 events at each site generally of magnitude mb 6 0 and we performed systematic measurements of the splitting parameters fast polarization direction and delay time t on sks skks and pks phases splitting on oceanic islands has been difficult to observe owing to the low quality of the signal but also perhaps owing to complex upper mantle structures beneath the stations station kip kipapa hawaii in the pacific is the only oceanic geoscope station with a clear anisotropy we determined well constrained splitting parameters for 10 of the 17 continental stations that may be explained by a single anisotropic layer the poor correlation between fast polarization directions and the absolute plate motion together with the apparent incoherence between the plate velocities and the observed delay times suggest that a simple drag induced asthenospheric flow alone fails to explain most of the observations for some stations located on or near major lithospheric structures tam tamanrasset algeria for instance we observe a good correlation between fast polarization directions and regional structures at station scz santa cruz california we found clear variations of the splitting parameters as a function of the event backazimuth compatible with two layers of anisotropy three stations can canberra hyb hyderabad india and ssb saint sauveur badole france seem to be devoid of detectable anisotropy

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

organic geochemical proxies of paleoceanographic paleolimnologic and paleoclimatic processes abstract organic matter constitutes a minor fraction of marine and freshwater sediments yet its important contribution to the sedimentary record can be used to reconstruct marine and continental paleoenvironments the organic matter content of sediments is the residue of past biota the amounts and types of organic matter present in sediments consequently reflect environmental conditions that impacted ecosystems at different past times general sources of the organic matter are inferred from bulk properties such as elemental compositions carbon and nitrogen stable isotope ratios rock eval pyrolysis data and organic petrography details of organic matter origins are refined by analyses of biomarker molecular compositions source changes are proxies for fluctuations in sea level oceanic surface currents and continental climates algal paleoproductivity rates are indicated by organic carbon mass accumulation rates and carbon and nitrogen stable isotopic compositions these parameters record past availability of nutrients and therefore are proxies of surface mixing in the oceans and amount of land runoff to lakes sea surface paleotemperatures are recorded by the number of carbon carbon double bonds in lipid biomarkers produced by marine algae larger proportions of the double bonds are proxies for the cooler surface waters that accompanied periods of global glaciation and intervals of enhanced upwelling the 13c and d values of plant organic matter record past concentrations of carbon dioxide in the atmosphere and changes in delivery of atmospheric moisture respectively diagenesis which causes the concentration and composition of organic matter in sedimentary settings to differ from those of the original biologically synthesized materials can bias organic geochemical paleoenvironmental records the magnitude of this potential source of misinformation must always be considered and evaluated comparison of multiple organic geochemical proxies of past conditions helps to compensate for the effects of diagenetic alterations and thereby to improve interpretations of paleoenvironmental change

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

evidence for ancient lithospheric deformation in the east european craton based on mantle seismic anisotropy and crustal magnetics abstract we present new shear wave splitting measurements performed at 16 stations on the east european craton and discuss their implications in terms of upper mantle anisotropy for this geophysically poorly known region previous investigations of mantle anisotropy in central europe have shown fast directions aligning smoothly with the craton s margin and various suggestions have been proposed to explain their origin such as asthenospheric flow or lithospheric frozen in deformation here we aim at investigating the continuation of this shear wave splitting pattern further to the east into the east european craton for the craton the interpretation appears to be less ambiguous than for central europe since several arguments support lithospheric anisotropy in this region 1 the large scale coherence within either of the four constituting blocks and the significant variations between the blocks on a small scale 2 the weak correlation with absolute plate motion vectors and 3 the good correlation between anisotropy and crustal features for which we use magnetic field alignments as a proxy rather good correlation of these magnetic features with seismic fast orientations strongly supports the idea of vertically coherent deformation throughout upper mantle and crust the observed splitting orientations thus reflect the last tectonic events of each block frozen in into the lithosphere for hundreds of millions of years

holocene vegetation and climate of the alashan plateau nw china reconstructed from pollen data a set of 55 recent pollen spectra from the alashan plateau and the qilian mountains qilianshan northwestern china has been analyzed the established relationships between the pollen spectra and modern vegetation and precipitation patterns of the studied area have been applied to 65 fossil pollen spectra from a 825 cm long sediment record collected in the topographic depression formerly occupied by eastern juyan palaeolake 41 89 n 101 85 e 892 m a s l several qualitative approaches e g indicator plant species artemisia chenopodiaceae and ephedra fragilis type s l ephedra distachya type pollen ratios and quantitative methods of vegetation and climate reconstruction have been tested with the surface pollen and then applied to the eastern juyan pollen record fossil pollen data reveal pronounced environmental changes in the studied area of the alashan plateau between ca 10 700 and 1700 cal yr b p desert taxa as chenopodiaceae nitraria calligonum reaumuria were abundant in the vegetation cover around the study site through the whole record pollen spectra dated to ca 10 700 5400 cal yr b p are characterised by highest values of chenopodiaceae e fragilis type and other desert indicating taxa suggesting rather dry climate most favourable conditions are reconstructed between 5400 and 3900 cal year b p on the basis of relative increase in abundance of artemisia pollen a return to dry conditions occurred at about 3900 cal yr b p the lake finally desiccated after 1700 cal yr b p reconstructed dry climate oscillations between 5900 and 5400 and 3100 cal yr b p correlate well with similar events found in the published records from northern and western china and central mongolia however eastern juyan record does not show the early holocene humidity maximum as suggested by the sedimentary data from monsoon influenced areas of china

a review of holocene solar linked climatic variation on centennial to millennial timescales physical processes interpretative frameworks and a new multiple cross wavelet transform algorithm article i nfo wereportontheexistenceandnature of holocene solar andclimaticvariationson centennialto millennial time scales we introduce a new solar activity proxy based on nitrate no3 concentration from the talos dome ice core east antarctica we also use a new algorithm for computing multiple cross wavelet spectra in time fre quency space that is generalized for multiple time series beyond two our results provide a new interpretive framework for relating holocene solaractivity variations on centennial to millennial timescales to co varying cli mate proxies drawn from a widespread area around the globe climatic proxies used represent variation in the north atlantic ocean western pacific warm pool southern ocean and the east asian monsoon regions our wavelet analysis identifies fundamental solar modes at 2300 yr hallstattzeit 1000 yr eddy and 500 yr unnamed periodicities leaves open the possibility that the 1500 1800 yr cycle may either be fundamental or derived and identifies intermediary derived cycles at 700 yr and 300 yr that may mark rectified responses of the atlantic thermohaline circulation to external solar modulation and pacing dating uncertainties suggest that the 1500 yr and 1800 yr cycles described in the literature may represent either the same or two separate cycles but in either case and irrespective too of whether it is a fundamental or derived mode in the sense of dima and lohmann 2009 the 1500 1800 yr periodicity is widely represented in a large number of paleoclimate proxy records it is obviously premature to reject possible links between changing solar activity at these multiple scales and the variations that are commonly observed in paleoclimatic records

holocene millennial scale vegetation changes in the yugu floodplain kongju area central south korea abstract to understand vegetation responses to past climate change on millennial timescales the holocene record of floodplain vegetation change in central south korea was reconstructed using the stable isotopic composition 13 c and pollen in buried soils low volume overbank deposition of fine grained alluvium by yugu stream over the past 10 000 years has produced a sequence that is well suited for 13 c analysis of soil organic matter the 13 c values varied between 25 and 20 during the holocene showing significant millennial scale fluctuations in five stages 11 000 8900 cal bp 8900 6100 cal bp 6100 3300 cal bp 3300 1200 cal bp and 1200 cal bp to present the abundance of c 4 plants estimated from a binary mixing model changed from approximately 17 57 during the stages from 3300 to 1200 cal bp and from 8900 to 6100 cal bp c 4 plant abundance and the percentage of arboreal pollen were high but the percentage of spores mainly moss and the freshwater algal sum were low indicating more arid environments temporal comparison of c 4 plant abundance change during the holocene with other reconstructed climate data from east asia suggests that millennial scale vegetation change or aridity change in central south korea was generally more synchronous with regional climate change during the late holocene than during the early to mid holocene this study suggests that the past vegetation in the yugu area has evolved in response to summer monsoon intensity in a non linear manner

upper mantle flow beneath french polynesia from shear wave splitting summary upper mantle flow beneath the south pacific is investigated by analysing shear wave splitting parameters at eight permanent long period and broad band seismic stations and 10 broad band stations deployed in french polynesia from 2001 to 2005 in the framework of the polynesian lithosphere and upper mantle experiment plume despite the small number of events and the rather poor backazimuthal coverage due to the geographical distribution of the natural seismicity upper mantle seismic anisotropy has been detected at all stations except at tahiti where two permanent stations with 15 yr of data show an apparent isotropy the median value of fast polarization azimuths n67 5 w is parallel to the present pacific absolute plate motion direction in french polynesia apm n67 w this suggests that the observed sks fast polarization directions result mainly from olivine crystal preferred orientations produced by deformation in the sublithospheric mantle due to viscous entrainment by the moving pacific plate and preserved in the lithosphere as the plate cools however analysis of individual measurements highlights variations of splitting parameters with event backazimuth that imply an actual upper mantle structure more complex than a single anisotropic layer with horizontal fast axis a forward approach shows that a two layer structure of anisotropy beneath french polynesia better explains the splitting observations than a single anisotropic layer secondorder variations in the measurements may also indicate the presence of small scale lateral heterogeneities the influence of plumes or fracture zones within the studied area does not appear to dominate the large scale anisotropy pattern but may explain these second order splitting variations across the network

depth dependent mantle anisotropy below the san andreas fault system apparent splitting parameters and waveforms we measure apparent teleseismic shear wave splitting parameters at several stations of the berkeley digital seismic network bdsn and three temporary broadband stations of the university of california at santa cruz located near the san andreas fault system in northern and central california previously proposed anisotropic models for the region include a two layered structure and a structure with a constant degree of anisotropy and a horizontal fast axis that gradually rotates as a function of depth we significantly increase the number of observations and confirm the existence of depth dependent anisotropy beneath the san andreas fault system we also investigate the extent to which the enhanced data set can constrain the details of the depth dependence using theoretical expressions we determine a suite of two layer models that fit our observations and result in practically indistinguishable apparent splitting parameters the wide range of acceptable models indicates that apparent splitting parameters alone cannot constrain all four parameters that specify the two anisotropic layers upper and lower fast polarization directions and delay times synthetic seismograms for three very different two layer models chosen from our suite of acceptable models and a model with a gradually rotating fast axis show subtle waveform variations and apparent splitting parameters measured from these records do not overlap hence we conclude that waveforms indeed contain additional information about the anisotropic structure we incorporate waveform data by searching for the four splitting parameters of two layer models that minimize the splitting of several records at each station simultaneously the resulting models largely overlap with the models obtained using the theoretical expressions which indicates that the addition of waveform data is not sufficient to uniquely determine all four parameters of a two layer model constraining the upper fast polarization direction to be parallel to the relative motion vector of the pacific and north american plates n35 w allows the values of the other three model parameters to be uniquely determined the lower layer fast polarization direction beneath all stations is approximately e w and the delay times vary from 0 85 to 1 70 s the delay times in the upper layer are smaller and they vary between 0 50 and 1 25 s shear within the uppermost mantle due to the relative plate motion is a satisfactory explanation for the shallow anisotropic signature the sublithospheric e w fast directions may be caused by small scale convection within a slabless window or by motions of the lithosphere relative to the deeper mantle

mantle variation within the canadian shield travel times from the portable broadband archean proterozoic transect 1989 we report travel times from the archean proterozoic transect 1989 this type of data set recorded by a transect of portable broadband instruments allows us to make inferences about mantle structure in the region between the wyoming craton and the superior province of the canadian shield with station separations of 50 to 100 km and frequencies up to 5 hz the resolution of lateral changes is increased by nearly an order of magnitude over previous studies to a scale that allows us to study the relation between velocity variation in the continental upper mantle surface geology tectonic features and age provinces travel times of direct and later phases are obtained from waveform matching the values are corrected for crustal contribution and inverted for the vertical path upper mantle delay tum under each station as well as the azimuthal dependence of this quantity the prominent feature in the upper mantle delays tum is the variation by at least 1 5 s for s much of which occurs over a narrow zone of just a few hundred kilometers width this suggests a major lateral upper mantle transition which does not coincide with the surface geological edge of the canadian shield but is located within the shield this same transition is also observed in shear wave splitting delay times surprisingly however the p delays do not exhibit a corresponding variation we address this apparent contradiction and show how it may be explained in conjunction with anisotropy in the subcontinental lithosphere a simplified thermal model of the lithospheric transition zone in which temperature controls the degree of crystallographic alignment and thus seismic anisotropy predicts this phenomenon

multiple expansions of c4 plant biomass in east asia since 7 ma coupled with strengthened monsoon circulation the expansion of plants using the c4 photosynthetic pathway is one of the most prominent changes in the global ecosystem during the cenozoic era a significant late miocene expansion is well documented however the existence and cause of subsequent expansions are still not clear owing in part to the lack of long continuous climate vegetation records here we present two high resolution carbon isotope time series covering the past 7 m y derived from eolian deposits on the chinese loess plateau the data indicate three intervals of significant c4 plant expansions within the semiarid monsoonal region of east asia ca 2 9 2 7 ma 1 3 0 9 ma and 0 6 ma present these expansions covary with strengthened east asian summer monsoon circulation we conclude that in east asia large scale late miocene c4 c3 vegetation changes in semiarid areas have been primarily driven by warm seasonal precipitation and temperature variations associated with changes in monsoon circulation

shear wave splitting beneath the galapagos archipelago shear wave splitting measurements in the galapagos archipelago show a rapid change from consistently oriented anisotropy to no measurable anisotropy at the western edge of the archipelago delay times are 0 4 0 9 s and fast polarization directions are 81 109 e these directions are consistent with anisotropy resulting from shear of the asthenosphere by the overlying plate there is no indication of fossil lithospheric anisotropy in the plate spreading direction in contrast beneath the center of the archipelago the upper mantle is isotropic or weakly anisotropic the isotropic region coincides approximately with a volume of anomalously low upper mantle velocities suggesting that the presence of melt may weaken the effects of fabric on anisotropy or that melt preferred orientation generates a vertical fast polarization direction alternatively the complex flow field associated with a near ridge hotspot may result in apparent isotropy

regional climate driven c3 and c4 plant variation in the cheollipo area korea during the late pleistocene abstract marine isotope stage 3 mis3 spanning from 25 to 60 ka bp was a time of substantial environmental change that included changes in the global ice volume and atmospheric composition with resultant regional climate changes to understand past vegetation responses to such environmental changes in korea we estimated the abundance of c3 and c4 plants using the c n and carbon isotope ratios of sedimentary organic matter the mis3 sedimentary unit at the cheollipo site located in a western coastal area of korea is characterized by a dark to light gray colored sedimentary unit 3 05 to 5 05 m in elevation mis3 which is between an underlain oxidized yellowish sedimentary unit 5 05 m in elevation the estimated abundance of c4 plants varied between 60 and 10 during the mis3 the suborbital scale variation in the estimated percentage of c4 plants was similar to those reported in continental ice volume and the east asian monsoon intensity suggesting that c4 plants prevailed under colder and drier conditions intensified winter monsoon whereas c3 plants have favored a warmer and wetter climate intensified summer monsoon compared with high latitude regions e g chinese loess plateau and lake baikal where low summer temperature suppressed aridity and pco2 influences during the last glacial period vegetation change in cheollipo area may have more sensitively responded to aridity change which had been affected by the summer winter monsoon intensity the climate change in the western coastal area yellow sea side of korea during the late pleistocene may have been controlled by regional climate change coupled with continental ice volume change

shear wave splitting in the appalachians and the urals a case for multilayered anisotropy observations of shear wave splitting in the northeastern u s appalachians and in the foredeep of the urals vary significantly with the back azimuth and incidence angle of the incoming phase these variations suggest two or more layers within the upper mantle with different anisotropic properties synthetic seismograms for simple multilayered anisotropic structures show that shear wave splitting parameters tend to vary substantially with the direction of approach relying on a subset of back azimuth and incidence angle may strongly bias the model inferred especially if the observations are averaged on the other hand the azimuthal splitting pattern provides additional constraints on vertical or lateral variation of anisotropic properties in the earth using a new error estimation technique for splitting we find that individual measurements from broadband data have errors of the order of 3 7 for the fast direction and 0 1 0 2 s for the delay of split shear waves the azimuthal variation of splitting parameters is broadly consistent throughout the appalachian terranes in the northeast united states especially for two long running stations in the northeast united states hrv harvard massachusetts and pal palisades new york observations can be separated into two distinct populations with mean fast axis azimuths of n60 e 4 and n119 e 2 delay values within each population range from near zero to 1 s azimuthal splitting variation for aru arti russia in the foredeep of uralian mountains is characterized by sharp transitions between different groups of observations using synthetic seismograms in simple structures we develop one dimensional anisotropic models under stations hrv and aru the model for hrv includes two layers of anisotropic material under an isotropic crust with fast axis azimuths n53 e and n115 e for the bottom and the top layers respectively the model for the upper mantle under aru includes a layer with a fast axis at n50 e atop a layer with fast axis azimuth n90 e our modeling confirms the need for a layer of strong anisotropy with a slow axis of symmetry in the lower crust under aru reported by levin and park 1997a our results suggest that both urals and appalachians possess a relict anisotropy in the tectosphere associated with past continental collision and accretion underlain by anisotropy with orientation similar to the local absolute plate motion suggesting an asthenospheric component to the signal

a review of coastal palaeoclimate and relative sea level reconstructions using δ13c and c n ratios in organic material holocene reconstructions of relative sea level rsl and environmental change in the coastal zone have become progressively more numerous since the importance of assessing possible future sea level rises has become apparent traditionally this has been achieved using a combination of biological and physical indicators e g pollen diatoms foraminifera grain size etc in some circumstances microfossils can be rare or absent from holocene sediments and in these cases carbon isotope ratios 13c and organic carbon to total nitrogen ratios c n can be used because they have the potential to provide information as to the origin of organic material preserved in coastal environments the distinction in 13c and c n of the various sources of carbon to estuarine sediments has led to its wide use as a tracer of carbon pathways and storage in estuaries more recently these techniques have been applied to holocene sediments to decipher changes in palaeocoastal environments and thus position relative to sea level and or palaeoriver discharge which have direct relevance to palaeoclimate this paper reviews the studies that have utilised this technique and explores examples from a wide range of coastal environments saltmarshes estuaries lagoons isolation basins and fjords it also discusses the potential alteration of geochemical signatures as a result of decompositional processes organic matter decomposition has been shown to change sediment 13c and c n values and thus an understanding of the processes involved is necessary in order to have confidence in the palaeoenvironmental interpretation of holocene 13c and c n decompositional shifts in c n and particularly 13c can occur over a relatively short time period however it is the direction of change in 13c and c n rather than absolute values that is key for interpreting changes in relative sea level and thus such changes are commonly preserved

groundwater discharge to a gobi desert lake during mid and late holocene dry periods abstract high levels of lake eastern juyanze and a wetter climate in the gobi desert nw china were recorded between 5100 and 4100 cal year bp from ostracod distribution shell geochemistry sedimentology and palynological data after about 4100 cal year bp lake levels decreased and groundwater discharge became dominant source of water during short term regressive events at about 4000 3800 and 3500 cal year bp as suggested by a significant difference in modern river water and groundwater chemistry and the geochemical signal of fossil ostracod shells the overall trend towards lower lake levels culminated in repeated episodes of desiccation of lake eastern juyanze between 3200 and 2900 cal year bp individual regressive events as well as the rapid environmental fluctuations of lake eastern juyanze at about 3000 cal year bp were likely caused by mid to late holocene temperature fluctuations recorded by the dunde ice core yao t thompson l g 1992 trends and features of climatic changes in the past 5000 years recorded by the dunde ice core annals of glaciology 16 21 24 and shifts of the southeastern asian monsoon

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

middle holocene environmental change in central korea and its linkage to summer and winter monsoon changes to trace the surficial responses of lowlands to past climate change we investigated 13c in total organic carbon toc c n ratios magnetic susceptibility ms and silicon si intensity directly proportional to concentration in wetland sediments collected from the gimpo area of central korea covering 6600 4600 cal yr bp two organic layers with high toc negatively depleted 13ctoc values 27 to 29 low ms values and low si intensities were found at 6200 5900 and 5200 4800 cal yr bp respectively these middle holocene wet periods corresponded to relatively intensified summer monsoon and solar activity periods the intervening dry period 5900 5200 cal yr bp with high ms high si and low toc corresponded to an intensified dust activity interval and stronger winter monsoon this multi centennial climatic fluctuation of wet periods 6200 5900 cal yr bp and 5200 4800 cal yr bp and an intervening dry period 5900 5200 cal yr bp in central korea was more synchronous with climate change in the arid inner part of china than with that in south china suggesting possible strong high latitude driven climatic influences e g north atlantic cooling events during the middle holocene

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

crustal anisotropy beneath pacific ocean islands from harmonic decomposition of receiver functions crustal anisotropy beneath ocean islands can be attributed to preferentially aligned minerals cracks or dike structures stacked with harmonic weighting receiver functions from permanent ocean island stations display evidence of strong and distinct anisotropy parameters in the underlying crust and underplated layer we analyze data for 11 iris gsn stations in the pacific ocean we observe the prevalence of two lobed receiver function rf amplitude variations with back azimuth consistent with slow tilted axis anisotropy in most cases the anisotropy is accommodated in the underplated crust synthetic modeling of a representative station indicates that the strength of anisotropy of vp 10 and vs 5 is possible the strike direction of the inferred symmetry axis tends to align with plate motion with some scatter at stations in the northwest pacific i e kwaj tara and wake the strike direction of the symmetry axis aligns with plate motion at the time of volcano emplacement beneath station poha and the closest stations to the present day hawaiian hotspot alignment of the symmetry axis is almost orthogonal to the plate motion we attribute the crustal anisotropy to the preferred alignment of dike structures that transported asthenospheric magma toward the seafloor volcanic edifice our results suggest that the thermal plume origin for ocean islands must be supplemented by tectonic stress heterogeneities that allow magma to penetrate the lithosphere via fractures magma transport fractures should align normal to the least compressive direction which are predicted by theoretical models to align approximately with plate motion at the time of emplacement

the variation of summer monsoon precipitation in central china since the last deglaciation abstract two stalagmites c996 1 and c996 2 collected from the jiuxian cave in the shaanxi province in central china have been studied with u series dating and stable isotope analysis thirty eight 230 th dating results showed that the stalagmite c996 1 was continuously deposited through the last 8 5 ka bp thousand years before present present 1950 ad and c996 2 was deposited through the last 19 ka bp excluding two growth hiatuses between 8 3 and 5 7 ka bp and 15 4 and 11 9 ka bp with a relatively stable boundary condition we interpret the 18 o of speleothem calcite as most indicative of the amount of summer monsoon precipitation although temperature and other factors might have some minor impact the 18 o records show notable changes within the last glacial maximum lgm resembling other east asian monsoon records such as those from the hulu and sanbao caves suggesting that significant monsoonal climate changes occurred in eastern asia as far north as the qinling mountains during the lgm interval a comparison of our records to precisely dated contemporaneous speleothem records from other caves shows that the increasing trend of 18 o during the holocene commenced as early as 7 5 ka bp in the low latitude monsoonal area i e the hoti cave while at higher latitudes this shift occurred later such as 7 0 ka bp in the dongge cave 5 3 ka bp in the heshang cave 4 7 ka bp in the sanbao cave and 4 5 ka bp in the jiuxian cave these results imply an asynchronous change of the summer monsoon precipitation occurred in east asia during the holocene the asynchrony may be related to the responses of a coupled tropical and subtropical monsoon system to changes of the insolation and the differences in thermal forcing which result from the complex geographical configuration the variation of sea surface temperature sst in the western tropical pacific may also have important impacts on the summer monsoon precipitation changes in central and northern china because it affects the northwestern pacific subtropical high a monsoon front regulator more robust tests are needed to confirm this phenomenon and to evaluate the contribution of different factors in detail

lower temperature as the main cause of c4 plant declines during the glacial periods on the chinese loess plateau abstract the distribution of c3 and c4 plants changed in regionally contrasting ways during the last glacial period c4 plant expansion in low latitude africa and america coincided with c4 plant decreases in mesoamerica and the us great plains this c4 plant expansion has been attributed to lower pco2 and increased aridity and the decline in c4 plants is believed to have been caused by increased winter precipitation and lower temperatures however it is still difficult to generalize whether pco2 temperature or aridity was mainly responsible for c3 vs c4 vegetation changes during glacial periods paleoclimate conditions on the chinese loess plateau clp offer opportunities to further evaluate the role of temperature in the c3 vs c4 competition detailed climate records have shown that the clp region was drier and colder during glacial periods with aridity favoring c4 expansion and lower temperatures favoring c3 expansion here we present two high resolution compound specific carbon isotope records of n alkanes from the clp as vegetation biomarkers to estimate the relative abundance of c4 plants during the last two glacial interglacial transitions more negative 13c values during the glacial periods from two clp sites clearly show that in contrast with low latitude sites c4 plants declined this decrease in c4 abundance was caused by lower temperature despite the lower pco2 and increased aridity on the clp during glacial time

mantle flow beneath la reunion hotspot track from sks splitting if upper mantle anisotropy beneath fast moving oceanic plates is expected to align the fast azimuths close to the plate motion directions the upper mantle flow pattern beneath slow moving oceanic plates will reflect the relative motion between the moving plate and the underlying large scale convecting mantle in addition to the non correlation of the fast azimuths with the plate motion direction the flow and anisotropy pattern may be locally perturbed by other factors such as the upwelling and the sublithospheric spreading of mantle plumes investigating such plume lithosphere interaction is strongly dependent on the available seismological data which are generally sparse in oceanic environment in this study we take the opportunity of recent temporary deployments of 15 seismic stations and 5 permanent stations on the piton de la fournaise volcano the active locus of la re union hotspot and of 6 permanent stations installed along or close to its fossil track of about 3700 km in length to analyze azimuthal anisotropy detected by sks wave splitting and to decipher the various possible origins of anisotropy beneath the western indian ocean from about 150 good and fair splitting measurements and more than 1000 null splitting measurements we attempt to distinguish between the influence of a local plume signature and large scale mantle flow the large scale anisotropy pattern obtained at the sw indian ocean island stations is well explained by plate motion relative to the deep mantle circulation by contrast stations on la re union island show a complex signature characterized by numerous nulls and by fast split shear wave polarizations trending normal to the plate motion direction and obtained within a small backazimuthal window that cannot be explained by either a single or two anisotropic layers despite the sparse spatial coverage which precludes a unique answer we show that such pattern may be compatible with a simple model of sublithospheric spreading of la re union plume characterized by a conduit located at 100 200 km north of la re union island anisotropy beneath the new geoscope station in rodrigues island does not appear to be influenced by la re union plume spreading signature but is fully compatible with either a model of large scale deep mantle convection pattern and or with a channeled asthenospheric flow beneath the rodrigues ridge

shear wave splitting beneath the snake river plain suggests a mantle upwelling beneath eastern nevada usa the snake river plain srp a 90 km wide topographic depression in southern idaho is a topographically anomalous feature in the western u s previous seismic studies focused on the northeastern srp to study its relationship with the yellowstone hotspot we present new teleseismic shear wave splitting data from six broadband seismic stations deployed along the axis of the srp from june 2000 to september 2001 we also analyze splitting at hlid a permanent station of the national seismic network located f100 km north of the plain splitting of individual teleseismic phases is consistent at all stations within 2r errors and we favor the interpretation of anisotropy with a single horizontal fast axis although a dipping axis interpretation is statistically permitted at two of the stations our station fast directions as well as shear wave splitting data from numerous other stations throughout the basin and range are best explained by a lattice preferred orientation of olivine due to horizontal shear along the base of the plate associated with the gravitational spreading of buoyant plume like upwelling material beneath eastern nevada into a southwestward flowing asthenosphere with respect to a fixed hotspot reference frame this parabolic asthenospheric flow paf model for the great basin is attractive because it explains the observed high elevations high mantle buoyancy low velocity anomaly beneath eastern nevada high heat flow and depleted geochemistry of some erupted basalts the lack of pliocene recent major volcanism in eastern nevada suggests that a significant amount of the buoyancy flux is due to compositional buoyancy our splitting station delay times vary in a way not predicted by the paf model and can be explained by a zone of aligned magma filled lenses and or partially molten dikes beneath the srp lithosphere a depleted olivine rich residuum underneath the sides of the eastern srp and or the effect of lateral lower crustal flow from beneath the esrp toward its adjacent flanks d 2004 elsevier b v all rights reserved

seismic anisotropy of oceanic upper mantle shear wave splitting methodologies and observations we develop methodologies to obtain accurate measurements of shear wave splitting and apply these techniques to examine the pattern of oceanic upper mantle anisotropy to obtain high quality estimates of receiver splitting at island stations we devise a stacking method that finds the optimum splitting parameters and 95 error region for teleseismic shear wave phases from a suite of earthquake events to obtain additional measurements in oceanic regions we develop techniques to measure splitting parameters and errors for ss phases that sample upper mantle anisotropy at their bounce points however we find the data are often of low resolution and anomalous characteristics are sometimes found that make splitting difficult to interpret ten second splitting of ss is observed across the banjo seismic array but we cannot unambiguously attribute this signal to mantle anisotropy at the bounce the receiver splitting methods are used to assess the adequacy of a two anisotropic layer model for the pacific region with fast polarization azimuth in the lithosphere oriented in the fossil spreading direction and in the asthenosphere oriented in the absolute plate motion direction this model has been proposed to explain surface wave data in the pacific ocean but our splitting results demonstrate that oceanic anisotropy patterns are more heterogeneous than would be predicted while the island splitting measurements could reflect the influence of individual hotspot upwellings hotspot effects do not appear to be universally dominant we propose that splitting observations alternatively indicate broad scale differences in the underlying character of oceanic upper mantle anisotropy associated with coherant patters of lithospheric structure and asthenospheric flow in particular splitting surface wave models and regional studies all support a model where lithospheric anisotropy throughout the south pacific has been erased or reoriented toward the absolute plate motion direction whereas more limited observations in the north pacific indicate that the fossil lithospheric signature appears to be preserved

east asian monsoon variability since the mid holocene recorded in a high resolution absolute dated aragonite speleothem from eastern china abstract paleoclimatic conditions associated with the east asian monsoon are reconstructed for the mid to late holocene using 18o values 819 measurements from a precisely dated 10 230th dates aragonite stalagmite from lianhua cave hunan province china 18o values 1 6 to 7 0 are interpreted to reflect the precipitation amount effect related to the strength of summer monsoonal circulation throughout this period the intensity of east asian summer monsoonal circulation declined in response to changes in insolation and a southward shift in the intertropical convergence zone itcz punctuating this first order trend are abrupt millennial to decadal scale fluctuations the most prominent of which occurred around 3337 5 yr bp and records a dramatic weakening in summer monsoonal circulation that coincides with the beginning of neoglacial conditions comparisons between the 18o values of stalagmite a1 and previously published data from stalagmite hs 4 from heshang cave and stalagmites d4 and da from dongge cave china demonstrate a similar regional response to changes in east asian monsoonal circulation several spectral analysis techniques and wavelet analysis were applied to these stalagmite records to evaluate the nature and character of cyclicity in the east asian monsoon during the holocene multi taper method mtm lomb scargle and single spectrum analysis ssa results reveal common periodicities that are unlikely to be artifacts of any single technique the periodicities expressed in the stalagmite 18o records occur at both solar and non solar frequencies and support previous interpretations that variability in monsoonal circulation responds to both solar forcing and internal climatic mechanisms wavelet analysis demonstrates that some of these frequencies are intermittent and recur at semi regular intervals which may indicate long term cyclicity that is unconstrained by the chronology of the record

shear wave splitting in the mantle of the pacific summary the bulk of previously known data on seismic anisotropy in the suboceanic mantle was obtained from observations of azimuthal variations of the surface wave and p velocities our study of azimuthal anisotropy in the mantle of the pacific is based on observations of shear wave splitting the idea of our method of measurement vinnik farra 1992 is to analyse evolution of the waveform of the long period multiple reflected scs and sscs phases as a function of the order of reflection the estimates of anisotropy correspond to circular regions with a radius of several hundred kilometres around the surface bounce points the technique was applied to the digital recordings of deep and intermediate events from the circum pacific seismic belt at seismograph stations in and around the pacific the estimates of polarization direction of the fast split wave and time lag of the slow wave are obtained for a few regions of the pacific basin marginal basins and the andes in the back arc extensional basins whose spreading has continued to the present time or ended later than about 15ma the inferred direction of extension in the mantle is close to that imprinted in the crust lau basin japan sea in the back arc basins whose active development ended 15 20 ma or earlier south fiji and parece vela basins the inferred directions of extension in the mantle are in strong disagreement with the crustal data in the central andes the region of plate collision and low angle subduction the inferred direction of flow in the mantle is perpendicular to the plate boundary contrary to the regularity previously found in several other collisional belts the fast direction of anisotropy in the mantle beneath the southwestern part of the 100ma old pacific plate is very close to the absolute plate motion direction the magnitude of anisotropy corresponds to the lowest values found in the continental mantle and is much lower than in most of the marginal basins and the andes the weakness of this presumably asthenospheric anisotropy in comparison with asthenospheric anisotropy under some old continental plates is probably related to a difference in upper mantle viscosities

reply to comment on regional climate driven c3 and c4 plant variation in the cheollipo area korea during the late pleistocene by j lim w h nahm j k kim d y yang palaeogeography palaeoclimatology palaeoecology 298 2010 370 377 abstract ouyang et al 2013 in this issue raise two concerns regarding carbon isotope analysis and its use for reconstructing past vegetation changes recently published by lim et al 2010 one issue questions the formation of sedimentary organic matter by competitive contribution of c 3 and c 4 plants and calculation of the relative contributions of c 3 and c 4 plants with a binary mixing model using their mean 13 c values the second issue questions the use of the 13 c value of flora from other regions or of the current flora as a universal reference to determine the composition of plants in the remote past pointing out the importance of understanding the exact assemblage of local plants in the past by complete sporo pollen assemblage analysis and molecular biomarker analysis in response to these concerns we provide a detailed description of the tracing process for determining the source of sedimentary organic matter using 13 c values and c n ratios and assess the suitability of calculating the relative contributions of c 3 and c 4 plants using a binary mixing model finally based on its advantages compared to other biological proxies we explain the utility of using 13 c toc values for reconstructing past vegetation changes

two distinct shear wave splitting directions in the northwestern deccan volcanic province 1 the northwestern deccan volcanic province in india has witnessed several tectonic episodes resulting in the formation of rift zones wide spread magmatism and deep seated faults that are host to some deadly intraplate earthquakes in this study we attempt to decipher the mantle deformation beneath the region using the sks splitting technique applied to high quality data from a regional network comprising 44 broadband seismic stations sited on diverse tectonic environments the first measurements of 461 312 sks and 149 skks splitting parameters from 117 earthquake sources reveal two major trends one coinciding with the absolute plate motion apm of the indian plate and the other roughly oriented in the e w direction forward modeling of the dependence of the splitting parameters on incoming polarization assuming horizontal axis of symmetry brings out two layers of anisotropy beneath the study region with the fast axis azimuths oriented n22 e in the bottom layer and n77 e in the top layer while stronger anisotropy t 1 2 s in the bottom layer is related to asthenospheric flow a weaker anisotropy t 0 5 s in the upper layer could represent anisotropy frozen in the lithosphere due to deformation related to widespread magmatism in this plume affected region

c4 plant productivity and climate co2 variations in south central texas during the late quaternary abstract a continuous record of organic carbon 13c from a buried soil sequence in south central texas demonstrates 1 strong coupling between marine and adjacent continental ecosystems in the late pleistocene as a result of glacial meltwater entering the gulf of mexico and 2 ecosystem decoupling in the holocene associated with a reduction of meltwater and a shift in global circulation patterns in the late pleistocene reduction in c4 plant productivity correlates with two well documented glacial meltwater pulses 15 000 and 12 000 14c yr b p indicating a cooler than present adjacent continental environment increased c4 production between 11 000 and 10 000 14c yr b p suggests that the younger dryas was a warm interval responding to the diversion of glacial meltwater away from the mississippi river with waning meltwater flow c4 productivity generally increased throughout the holocene culminating in peak warm intervals at 5000 and 2000 14c yr b p shifts in the abundances of c3 c4 plants through the late quaternary show no correlation to ecophysiological responses to atmospheric co2 concentration

shear wave splitting around the eifel hotspot evidence for a mantle upwelling summary we present the results of a shear wave splitting analysis across the eifel hotspot in westcentral europe our data set includes 18 permanent european broadband stations and 102 temporary eifel broadband and short period stations we observe variations in splitting at most permanent stations but are not able to model them reliably with unique dipping axis or two layer anisotropy models we prefer instead a single layer model with a horizontal fast axis to approximate the first order anisotropy which varies smoothly between stations we observe a first order parabolic pattern in fast polarization azimuth around the hotspot which suggests that a lattice preferred orientation lpo of olivine fast axes exists in the asthenosphere as a result of the interaction between the slowly wsw moving eurasian plate and a mantle upwelling beneath the eifel volcanic fields the minority of the variation not explained by this model correlates with rapid lateral changes in splitting and is interpreted as a result of additional anisotropy and or the effects of dynamic recrystallization associated with lpo development in the region of corner flow near the conduit our parabolic asthenospheric flow paf interpretation is consistent with eifel geological tomographic receiver function global absolute plate motion electrical conductivity anisotropy and geochemical studies as well as with splitting studies in the great basin and around hawaii we suggest that the eifel upwelling is sporadic and a result of a low excess upwelling temperature and or varying crustal stresses that periodically shift and facilitate eruption the paf pattern we observe neither suggests nor rules out anisotropy in the conduit associated with a wet eifel upwelling we use our optimum pa f model parameters to calculate a eurasian plate speed of 12 km ma 1 w hich is consistent with the recent hs3 nuvel1a speed of 19 14 km ma 1

anisotropy of the yellowstone hot spot wake eastern snake river plain idaho over the last 10 million years the yellowstone hot spot has passed beneath the eastern snake river plain both magmatically modifying the snake river plain crust and creating a wider wake like tectonic parabola of seismicity and uplift analysis of sks arrivals to a line array of 55 mostly broadband stations distribution across the tectonic parabola reveals a nearly uniform orientation of anisotropy with an average fast axis orientation of n64e the back azimuth of null splitting events is parallel to the measured fast axis suggesting that anisotropic material consists of a single layer splitting parameters are independent of backazimuth suggesting that anisotropy is constant beneath each station thus station averaged split parameters are representative of the anisotropy beneath the station station averaged split times range from 0 6 1 5 s and define a pronounced depression in split times centered about 80 km southeast of the axis of the snake river plain

sources and accumulation rates of organic carbon in an equatorial peat bog burundi east africa during the holocene carbon isotope constraints the relationship between organic carbon accumulation rates and 13c 12c ratios of total organic carbon toc was investigated in an highland peat bog core ru 3 from equatorial africa this core yielded a sequence spanning the last 14 kyr and was analysed with a 100 300 yr resolution for toc 13c values the holocene section shows contrasted toc accumulation regimes and toc 13c varying between 28 5 and 19 5 with a few very short isotopic excursions dated at ca 9 3 7 5 4 2 ka b p the organic carbon accumulation rates range from 2 to 20 mg c cm 2 yr 1 they increase when toc becomes more depleted in 13c notably between 12 and 9 8 ka b p 8 5 and 7 8 ka b p and after 1 6 ka b p periods of restricted carbon storage correspond to heavier toc accumulation at 9 3 and between 7 5 and 1 6 ka b p at the study site the variations can be related to variable c4 plant inputs and possibly to changes in the fractionation between co2 and the organic carbon in c3 vascular plants the ru 3 record indicates restricted carbon storage during the periods of increased contribution from c4 plants and or of decreased fractionation between co2 and organic carbon in c3 plants changes in toc 13c values in core ru 3 seem to match fluctuations of east equatorial african lakes high lake stands correspond to low 13c intervals and vice versa this points to indirect climatic forcing of 13c changes in intertropical peats

the history and variability of the east asian paleomonsoon climate abstract changes in the east asian paleomonsoon reflect interactions between the global atmosphere ocean land and ice systems and are also an expression of their combined effect within the boundary conditions imposed by the east asian continent and solar radiation the history of the east asian monsoon is an alternation between dominance by the dry cold winter and warm humid summer monsoons high resolution eolian sequences preserved in the chinese loess plateau reveal that the east asian monsoon may have commenced at least 7 2 ma ago they also provide evidence indicating that the pulsed uplift of the tibetan plateau at about 3 4 and 7 2 ma may have played an important role in inducing climate change the palaeoclimatic records of the last glacial cycle show high frequency variability on time scales of 1000 year or even shorter and instability of the east asian paleomonsoon system the high frequency variability could be due to a non linear response to orbital forcing or a result of the coupling processes between different components of the global system cold air activity in northern high latitudes the trans equatorial air streams from the southern hemisphere and possibly enso may have played an important role in east asian monsoon variability the synchroneity of all the palaeoclimatic events along the polar equator polar pep transect is still an open question correlation of limited palaeoclimatic records for the last 30 kyr obtained from east asia and australia suggests that the trans equatorial air streams driven by the monsoon and trade winds may have had an influence on opposite hemispheres

asynchronous holocene optimum of the east asian monsoon abstract the spatial and temporal distribution of summer monsoon precipitation or effective moisture during the holocene has been reconstructed on the basis of geological data including lake levels pollen profiles and loess paleosol records in addition the summer july precipitation increment effective precipitation and monsoon strength index have been obtained from numerical modeling experiments both geological data and numerical modeling indicate that the holocene optimum as defined by peak east asian summer monsoon precipitation was asynchronous in central and eastern china reaching a maximum at different times in different regions e g ca 10 000 8000 yr ago in northeastern china 10 000 7000 yr ago in north central and northern east central china ca 7000 5000 yr ago in the middle and lower reaches of the yangtze river and ca 3000 yr ago in southern china in southwestern china the maximum appeared ca 11 000 yr ago but probably was related to the maximum landward extension of the indian summer monsoon the regional shift in the maximum precipitation belt from northwest to southeast over the past 10 000 yr is interpreted as a response to changing seasonality related to orbital forcing of the climate the southeastward shift of the east asian summer monsoon maximum is consistent with the progressive weakening of the summer monsoon as the summer solar radiation anomaly decreased progressively through the holocene and the east asian monsoon index declined while the early maximum in southwestern china matches the maximum of the indian monsoon index

shear wave splitting in ethiopia precambrian mantle anisotropy locally modified by neogene rifting 1 twenty six broadband seismic stations in an areal array spanning 500 500 km across ethiopia were used for shear wave splitting studies our results show small to moderate delay times 0 5 1 7s with fast polarization azimuths sub parallel to the orientation of the east african rift nne ssw and also to the proterozoic tectonic fabric across the entire studied area our results imply ethiopian upper mantle anisotropy is controlled largely by the proterozoic accretion of the mozambique belt with possible minor effects within the rift due to aligned cracks or melt pockets parallel to the rift axis our observations are not consistent with anisotropy created by asthenospheric flow parallel either to the cenozoic extension direction nw se or to the modern absolute plate motion direction nnw sse or to asthenospheric radial flow from the afar plume

the non commutivity of shear wave splitting operators at low frequencies and implications for anisotropy tomography summary measurements of the splitting or birefringence of seismic shear waves constitute a powerful and popular technique for characterizing azimuthal anisotropy in the upper mantle the increasing availability of data sets from dense broad band seismic arrays has driven interest in the development of techniques for the tomographic inversion of shear wave splitting data and in comparing splitting measurements with anisotropic upper mantle models obtained from other constraints such as surface wave analysis two different theoretical approaches have been developed for predicting apparent shear wave splitting parameters fast direction and delay time for models that include multiple layers of anisotropy at depth which is useful for comparing azimuthally anisotropic surface wave models with shear wave splitting measurements these approaches differ in one key aspect which is whether or not the shear wave splitting operator can be treated as commutative in this paper we investigate the theoretical source of this discrepancy and show that at frequencies relevant to most studies of upper mantle anisotropy the term that results in the non commutivity of the shear wave splitting operator in the expressions for multiple layer splitting must be retained in contrast the quantity known as the splitting intensity which is closely related to the apparent fast direction and delay time does commute at these frequencies we illustrate these inferences with forward modelling examples and discuss their implications for the tomographic inversion of shear wave splitting measurements the comparison of surface wave models with shear wave splitting observations and the joint inversion of surface wave and shear wave splitting observations for upper mantle anisotropic models

bimodal grain size distribution of aeolian quartz in a maar of cheju island korea during the last 6500 years its flux variation and controlling factor 1 aeolian quartz chemically isolated from maar sediments in cheju island korea showed bimodal grain size distributions after partitioning into fine and coarse components fine and coarse quartz fluxes fqf and cqf were calculated the average median grain size of fine quartz was 3 m and the average fqf was 0 5 mg cm2 yr with a range of 0 1 1 6 mg cm2 yr during the last 6500 years the average cqf was 3 mg cm2 yr with a range of 1 5 5 mg cm2 yr and its average median grain size was 15 m comparisons of our results with the present aeolian dust activities and climate changes in dust source areas suggest that fqf was influenced by the pathway of the westerlies between taklimakan desert and cheju island while cqf was affected by the aridity in the source regions including the gobi desert in southern mongolia and the adjoining gobi and sand deserts in china during the last 6500 years

comment on shear wave splitting to test mantle deformation models around hawaii by kristoffer t walker gotz h r bokelmann and simon l klemperer

reply to shear wave splitting to test mantle deformation models around hawaii by vinnik et al index terms 7218 seismology lithosphere and upper mantle 8120 tectonophysics dynamics of lithosphere and mantle general 8121 tectonophysics dynamics convection currents and mantle plumes 8130 tectonophysics heat generation and transport 8162 tectonophysics rheology mantle citation walker k t g h r bokelmann and s l klemperer reply to shear wave splitting to test mantle deformation models around hawaii by vinnik et al geophys res lett 30 13 1676 doi 10 1029 2002gl016712 2003

summer monsoon intensity controls c4 c3 plant abundance during the last 35 ka in the chinese loess plateau carbon isotope evidence from bulk organic matter and individual leaf waxes the natural abundance of c3 and c4 plants is affected by multiple environmental factors including temperature moisture balance and atmospheric pco2 the relative importance of these factors is a subject of considerable debate and may vary in different natural ecosystems previous studies generally focus on single loess sequences in the chinese loess plateau and conflicting conclusions on c4 c3 have been reached when studying carbon isotope ratios in carbonate and organic matter in this paper we report a comprehensive carbon isotopic characterization of total organic carbon toc and individual higher plant leaf waxes from five loess sequences spanning the last 35 ka from the chinese loess plateau clp the five coring sites encompass large gradients of annual mean temperature 9 2 13 9 c and precipitation 402 673 mm allowing us to assess the controlling mechanisms on c4 c3 plant ratios glacial interglacial sequences provide carbon isotope data for comparison with other climatic and environmental proxies such as lithology and magnetic susceptibility our results demonstrate that increased c4 c3 ratios are positively correlated with higher temperature and increased summer rainfall which characterize stronger summer monsoon in all five sites we conclude c4 abundance increases from the last glacial to the holocene in response to greater monsoon activity and that the c4 expression is suppressed in the cold and drier intervals

seismic anisotropy and global geodynamics for many years seismic anisotropy was often neglected mostly because of the inherent heavy mathematical and computational tools needed to describe and model its effects on seismic waves the usual basic knowledge about propagation in isotropic media cannot easily apply to anisotropic media where new phenomena come up such as birefringence or shear wave splitting or difference between directions of propagation of phase velocity and of group velocity consequently geophysicists often claimed that it was a second order effect and considered the earth as isotropic this hypothesis was assumed to be a good approximation because of the random orientation of crystals in most parts of the earth and of the random sampling of anisotropic regions by seismic rays this assumption furthermore made easier the description of wave propagation as well as the parameterization of media in inverse problems an isotropic elastic medium can be described by two independent elastic parameters and lame parameters but the simplest anisotropic medium transverse isotropy with a vertical symmetry axis requires 5 independent parameters love 1927 anderson 1961 to date seismic observations have been explained in terms of isotropic and often thermal lateral heterogeneities ignoring manifestations of anisotropy however since the 1960s it was recognized that most parts of the earth are not only laterally heterogeneous but also anisotropic and that seismic anisotropy provides a simple explanation of different observational data seismic anisotropy contrary to isotropy is reflecting some inherent

testing oceanic subduction and convective removal models for the gibraltar arc seismological constraints from dispersion and anisotropy we discuss mantle structure and dynamics under the gibraltar arc for that region a large variety of geodynamic models has been proposed and there was up to recently no consensus on which of these models is the best one to describe the geodynamic situation of the region the key to distinguishing the different models probably lies in the upper mantle and we propose two specially adapted seismological techniques that help to distinguish each of the two models that are more frequently invoked these are on one hand the subduction rollback sr model and on the other continental delamination or convective removal cdcr the first technique that we use is based on observations of dispersed p wavetrains that are expected for certain ray directions observations made on the gibraltar arc suggest the presence of subducted oceanic lithosphere under the alboran sea the second technique is based on shear wave splitting of sks phases and it allows us to look for the radially oriented flow that must be present in the mantle beneath the alboran sea for a cdcr mechanism there are however no indications of any radial flow up to this point both approaches therefore suggest that subduction rollback is the best model to explain the complex geodynamic situation under the region

a tertiary asthenospheric flow beneath the southern french massif central indicated by upper mantle seismic anisotropy and related to the west mediterranean extension abstract upper mantle flow beneath the french massif central is investigated using teleseismic shear wave splitting induced by seismic anisotropy about 25 three component stations short period intermediate and broadband were installed during the period 1998 1999 in the southern massif central from the clermont ferrand volcanic area to the mediterranean sea teleseismic shear waves sks skks and pks were used to determine the splitting parameters the fast polarization direction and the delay time delay times ranging between 0 7 and 1 5 s have been observed at most of the sites the azimuths of the fast split shear waves trend homogeneously nw se in the southern massif central suggesting a homogeneous mantle flow beneath this area the observed nw se direction differs from the n100 e pyrenean anisotropy further south it does not appear to be correlated to hercynian structures nor to the present day motion of the plate but is well correlated to the tertiary extension direction we propose that the opening of the western mediterranean induced by the rotation of the corsica sardinia lithospheric block and the roll back to the se of the tethys slab may have generated a large asthenospheric mantle flow beneath the southern massif central and a deflection of the up going plume centered beneath the northern massif central toward the se

relationship between environmental change on geoje island southern coast of korea and regional monsoon and temperature changes during the late holocene abstract to reconstruct environmental change during the late holocene we investigated sedimentary 13 c values from an exposed section on geoje island on the southern coast of korea the 13 c values ranged from 24 3 to 21 2 and were divided into five stages based on the average value 22 5 a comparison with the regional climate index revealed that wet periods linked to intensified summer monsoon conditions occurred at stage 1 4500 3600 cal bp stage 3 1350 900 cal bp and stage 5 300 cal bp to present dry intervals were found at stage 2 3600 1350 cal bp and stage 4 900 300 cal bp we found two different vegetational responses to changes in regional temperature and summer monsoons stage 3 corresponding to the mwp was characterized by an association between the 13 c values on the island and the regional temperature but stage 4 including the lia interval did not show such a relationship furthermore based on comparison with the summer monsoon index this study suggested that on the southern coast of korea vegetational change was controlled by an mwp like mode during the warm and wet periods following regional climate change whereas a lia like mode showing independent climate change was dominant during the cold and dry periods this difference in vegetational changes along the southern coast of korea and regional mainly terrestrial signal based climate change may be attributed to the influence of sea surface temperature changes in the tropical ocean through the transport of warming seawater by the kuroshio and tsushima currents on the east asian coastal area including this study area than on the inner part of the east asian continent

seismic anisotropy and mantle flow from the great basin to the great plains western united states shear wave splitting and p sks and s travel time residuals are calculated for teleseismic arrivals recorded on the colorado plateau great basin program for array seismic studies of the continental lithosphere passcal portable broadband seismic deployment and for permanent stations in the western united states little shear wave splitting is observed for broadband recordings in the northern colorado plateau the rocky mountains or the central great basin the transition between the colorado plateau and the great basin is marked by moderate shear wave splitting 1 0 s and unusually late teleseismic phase arrivals this suggests material with a higher content of mantle melt or volatiles than regions to either side splitting in the transition between the colorado plateau and great basin is part of a pattern of fast polarizations that align in a semicircle surrounding a central great basin region of null no splitting measurements away from the california plate boundary splitting to the north and south of our study region aligns roughly parallel to the absolute plate motion of the north american plate no simple spatial relation of splitting with geological and geophysical features such as mountain ranges velocity anomalies gravity magnetics or heat flow is evident in most of the western united states however splitting in the great basin is compatible with asthenospheric flow the smallest shear wave splitting delay times coincide with the eureka low in heat flow also having low s velocity at 300 km depth we suggest that the circumferential pattern of fast polarization directions ringing a central region of nulls in the great basin is caused by mantle flow by the interaction of mantle up welling and the absolute motion of the north american plate

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

insights into the structure and dynamics of the upper mantle beneath bass strait southeast australia using shear wave splitting abstract we investigate the structure of the upper mantle using teleseismic shear wave splitting measurements obtained at 32 broadband seismic stations located in bass strait and the surrounding region of southeast australia our dataset includes 366 individual splitting measurements from sks and skks phases the pattern of seismic anisotropy from shear wave splitting analysis beneath the study area is complex and does not always correlate with magnetic lineaments or current n s absolute plate motion in the eastern lachlan fold belt fast shear waves are polarized parallel to the structural trend n25e further south fast shear wave polarization directions trend on average n25 75e from the western tasmania terrane through bass strait to southern victoria which is consistent with the presence of an exotic precambrian microcontinent in this region as previously postulated stations located on and around the neogene quaternary newer volcanics province in southern victoria display sizeable delay times 2 7 s these values are among the largest in the world and hence require either an unusually large intrinsic anisotropy frozen within the lithosphere or a contribution from both the lithospheric and asthenospheric mantle in the eastern tasmania terrane nearly all observed fast directions are approximately nw se although part of our data set strongly favours anisotropy originating from fabric frozen in the lithospheric mantle a contribution from the asthenospheric flow related to the present day plate motion is also required to explain the observed splitting parameters we suggest that deviation of asthenospheric mantle flow around lithospheric roots could be occurring and so variations in anisotropy related to mantle flow may be expected alternatively the pattern of fast polarisation orientations observed around bass strait may be consistent with radial mantle flow associated with a plume linked to the recently discovered cosgrove volcanic track however it is difficult to characterise the relative contributions to the observed splitting from the lithospheric vs asthenospheric upper mantle due to poor backazimuthal coverage of the data

upper mantle anisotropy beneath the seychelles microcontinent 1 the seychelles plateau is a prime example of a microcontinent yet mechanisms for its creation and evolution are poorly understood recently acquired teleseismic data from a deployment of 26 stations on 18 islands in the seychelles are analyzed to study upper mantle seismic anisotropy using sks splitting results strong microseismic noise is attenuated using a polarization filter results show significant variation in time delays t 0 4 2 4 s and smooth variations in orientation 15 69 where is the polarization of the fast shear wave the splitting results cannot be explained by simple asthenospheric flow associated with absolute plate motions recent work has suggested that anisotropy measurements for oceanic islands surrounding africa can be explained by mantle flow due to plate motion in combination with density driven flow associated with the african superswell such a mechanism explains our results only if there are lateral variations in the viscosity of the mantle it has been suggested that the seychelles are underlain by a mantle plume predictions of flow induced anisotropy from plume lithosphere interaction can explain our results with a plume possibly impinging beneath the plateau finally we consider lithospheric anisotropy associated with rifting processes that formed the seychelles the large variation in the magnitude of shear wave splitting over short distances suggests a shallow source of anisotropy fast directions align parallel to an area of transform faulting in the amirantes farther from this area the orientation of anisotropy aligns in similar directions as plate motions this supports suggestions of transpressive deformation during the opening of the mascarene basin

models of lithosphere and asthenosphere anisotropic structure of the yellowstone hot spot from shear wave splitting 1 teleseismic shear wave splitting measured at 56 continuous and temporary seismographs deployed in a 500 km by 600 km area around the yellowstone hot spot indicates that fast anisotropy in the mantle is parallel to the direction of plate motion under most of the array the average split time from all stations of 0 9 s is typical of continental stations there is little evidence for plume induced radial strain suggesting that any contribution of gravitationally spreading plume material is undetectably small with respect to the plate motion velocity two stations within yellowstone have splitting measurements indicating the apparent fast anisotropy direction is nearly perpendicular to plate motion these stations are 30 km from stations with parallel to plate motion the 70 rotation over 30 km suggests a shallow source of anisotropy however split times for these stations are more than 2 s we suggest melt filled stress oriented cracks in the lithosphere are responsible for the anomalous orientations within yellowstone stations southeast of yellowstone have measurements of oriented nnw to wnw at high angles to the plate motion direction the archean lithosphere beneath these stations may have significant anisotropy capable of producing the observed splitting

calculating splitting parameters for plume type anisotropic structures of the upper mantle summary we apply a forward propagator method to calculate shear wave splitting parameters for general 3 d weakly anisotropic upper mantle structures the approach is valid under the assumption that the ray paths of the two quasi shear qs waves can be approximated by a common reference ray within an isotropic background medium along the reference ray the incremental splitting is expressed in terms of qs polarizations and slownesses in the direction of propagation using a ray theory ansatz we show that the resulting shear wave seismogram is equivalent to the coupling ray theory of coates chapman 1990 under the assumption of smoothly varying anisotropy here we use the forward propagator method to calculate apparent shear wave splitting parameters that can be compared directly with observations of teleseismic shear wave splitting a comparison with finite difference calculations is used to assess the validity of the method as an application we consider shear wave splitting due to a plume type anisotropic upper mantle structure the anisotropy is assumed to result from the preferred alignment of orthorhombic olivine we show examples of waveforms and splitting parameters as functions of backazimuth and angle of incidence anisotropic effects for sks are weak at stations near the central upwelling due to the combined effects of initial polarization and olivine a axis orientation here the splitting parameters are irregular and fast axis directions may vary by up to 70 depending on the vertical incidence angle at larger distances splitting parameters slowly converge towards values expected for homogeneous media the results suggest that direct evidence for mantle plumes from shear wave splitting is more likely to come from obs stations at some distance away from the central upwelling at shallower incidence the apparent splitting parameters for s exhibit a 90 periodicity as functions of the initial polarization of the incident shear wave provided that the ray path is kept fixed our results show that this is a general characteristic of shear wave splitting in inhomogeneous anisotropic media the effect can be used to identify complex anisotropic regions within the earth s mantle

seismic anisotropy and mantle flow seismic anisotropy in the mantle is primarily due to deformation induced lattice preferred orientation lpo of olivine crystals the aim of this paper is to better understand how such lpo is produced by flow and to learn whether observations of seismic anisotropy can be used to infer the local direction of flow within the mantle the basis of the work is a two dimensional theory which describes how lpo evolves during an arbitrary time dependent deformation ribe 1989 i investigate the evolution of lpo beneath ocean ridges above subducted slabs and in thermal convective flows the ocean ridge model predicts that the seismic anisotropy produced at a fast spreading ridge should be greater in magnitude than that produced at a slow spreading ridge and that the fast axis should be more nearly horizontal the subduction zone model predicts that strong seismic anisotropy should be produced above subducted slabs with the fast axis aligned with the slab regardless of the subduction angle the evolution of lpo in typical convective flows is extremely complex and depends strongly on such factors as the mode of heating and the surface boundary conditions in general there is no simple relation between the lpo and the local flow direction in any of the models considered the attempt to use seismic anisotropy to map flow patterns on a global scale is therefore probably unwarranted however the lpo does become aligned with the flow direction when the flow is a progressive simple shear such a flow occurs in limited portions of the mantle beneath the lithosphere at a fast spreading ridge above subducted slabs and in boundary layers beneath rigid surfaces such as continents observations of seismic anisotropy may therefore indicate the sense of shear at shallow depths in the mantle

palaeomagnetism rock magnetism and ams of the cabo magmatic province ne brazil and the opening of south atlantic summary reconstruction of the south atlantic opening has long been a matter of debate and several models have been proposed one problem in tracing properly the atlantic history arises from the existence of a long interval without geomagnetic reversals the cretaceous normal superchron for which ages are difficult to assign palaeomagnetism may help in addressing this issue if high quality palaeomagnetic poles are available for the two drifting continental blocks and if precise absolute ages are available in this work we have investigated the cabo magmatic province northeastern brazil recently dated at 102 1 ma zircon fission tracks ar 39 ar 40 all volcanic and plutonic rocks showed stable thermal and af demagnetization patterns and exhibit primary magnetic signatures ams data also support a primary origin for the magnetic fabric and is interpreted to be contemporaneous of the rock formation the obtained pole is located at 335 9 e 87 9 s n 24 a95 2 5 k 138 and satisfies modern quality criteria resulting in a reference pole for south america at 100 ma this new pole also gives an insight to test and discuss the kinematic models currently proposed for the south atlantic opening during mid cretaceous

holocene slip rate of the wasatch fault zone utah from geodetic data earthquake cycle effects 1 gps data define a broad zone of present day deformation in the eastern basin and range province western us using finite element models with elastic upper crust over viscoelastic lower crust upper mantle and incorporating earthquake cycle effects we show that these data are consistent with a model whereby most contemporary fault slip is focused on the wasatch fault zone modeled rates of horizontal extension are 3 0 4 5 mm yr in agreement with holocene geologic data the models are non unique in part because much of the wasatch fault is in the late stages of the earthquake cycle when surface velocity gradients across the fault are low

a high resolution absolute dated holocene and deglacial asian monsoon record from dongge cave china abstract we present a continuous record of the asian monsoon over the last 16 ka from 18o measurements of stalagmite calcite over 900 oxygen isotopic measurements providing information on shifts in monsoon precipitation are combined with a chronology from 45 precise 230th dates 18o and therefore asian monsoon intensity generally follows changes in insolation although changes in 18o are generally accommodated in abrupt shifts in contrast to smoothly varying insolation indicating that threshold effects may be important 18o decreased dramatically 3 at the start of the holocene 11 5 ka and remained low for 6 ka four positive 18o events centered at 11225 97 yr bp 1 05 10880 117 yr bp 1 15 9165 75 yr bp 1 4 and a double event centered at 8260 64 yr bp 1 1 and 8080 74 yr bp 1 0 punctuated this period of high monsoon intensity all four events correlate within error with climate changes in greenland ice cores thus the relationship between the asian monsoon and the north atlantic observed during the glacial period appears to continue into the early holocene in addition three of the four events correlate within error with outburst events from lake agassiz the decline of monsoon intensity in the mid late holocene is characterized by an abrupt positive shift in 18o which occurs at 3550 59 yr bp 1 1 in 100 yr in addition the holocene is punctuated by numerous centennial and multi decadal scale events amplitudes 0 5 to 1 up to half the amplitude of the glacial interstadial events seen in the last glacial period thus holocene centennial and multi decadal scale monsoon variability is significant although not as large as glacial millennial scale variability the monsoon shows a strong connection with northern south american hydrological changes related by changes in itcz position spectral analysis of the 18o record shows significant peaks at solar periodicities of 208 yr and 86 yr suggesting variation is influenced by solar forcing however there are numerous other significant peaks including peaks at el nino frequencies observed for high resolution portions of the record between 8110 and 8250 yr which suggest that changes in oceanic and atmospheric circulation patterns in addition to those forced by solar changes are important in controlling holocene monsoon climate in addition for this high resolution portion we observe a distinctive biennial oscillation of the asian monsoon which has been associated with the tropospheric biennial oscillation tbo

eolian quartz flux variations in cheju island korea during the last 6500 yr and a possible sun monsoon linkage we have obtained a high resolution sedimentary record covering the last 6500 yr from a maar in cheju island korea in order to reconstruct the history of variations in the eolian quartz flux eqf and hence asian dust the long term variation of eqf reveals three intervals a period of high eqf 4000 2000 cal yr b p and two periods of low eqf 6500 4000 cal yr b p and 2000 cal yr b p to present which have been affected by the east asian monsoon due to insolation change and the cold air activity in high latitudes correlated with polar high pressure systems this long term variation is superimposed by millennial and centennial scale fluctuations with periodicities of 1137 739 214 162 137 127 and 111 yr implying drier conditions in the source areas in china the detrended eqf record correlates visually and statistically cross spectral analysis with the atmospheric d 14 c record solar proxy the centennial scale variability in eqf may be affected

holocene variability of the east asian summer monsoon from chinese cave records a re assessment oxygen isotope records from stalagmites in caves in southern china interpreted as proxy rainfall records reflecting the intensity of the east asian summer monsoon indicate gradual monsoon weakening for the last 9000 years as also documented for the indian monsoon coupled with high precision dating the speleothem proxy records have been used to test monsoon links with orbital forcing solar changes iceberg discharges in the north atlantic ocean currents and atmospheric methane however these benchmark cave records do not match other published dated e asian proxy rainfall record specifically here independently calibrated rainfall records from loess palaeosol magnetic properties and cave oxygen isotope intercomparisons which show variable e asian monsoon intensity through the entire holocene the strong correlation of the cave records with the extraregional indian monsoon record yet their mismatch with these other dated chinese rainfall records might be reconciled if the speleothem isotope variations reflect not changes in holocene rainfall amount but in rainfall source declining holocene influence of isotopically lighter indian monsoon sourced moisture over china would have resulted in increasing proportions of isotopically heavier rainfall sourced from the more oceanic e asian monsoon individual speleothems may thus regionally record holocene changes in indian monsoon intensity and isotopic influence conversely the other chinese proxy records described here reflect changes in rainfall amount and thus in e asian summer monsoon intensity for the holocene the e asian and the indian monsoon responses to orbital forcing are likely to have differed specifically due to e asian internal feedbacks and the seasonal contrasts between the two monsoon systems

shear wave splitting to test mantle deformation models around hawaii seismic anisotropy allows us to study mantle deformation and it can thus help to constrain mantle flow in the vicinity of hotspots hypotheses for the cause of seismic anisotropy in this environment include the parabolic asthenospheric flow paf model radial flow from a mantle plume impinging on a moving lithosphere is dragged by the plate in the direction of absolute plate motion in map view this gives a parabolic pattern of flow opening in the direction of plate motion we present new shear wave splitting observations from land and ocean stations around the hawaiian islands that can be explained by the parabolic flow model the observations suggest asthenospheric anisotropy under the hawaiian islands which may be explained if dislocation creep persists to deeper depths there than in other regions perhaps due to the higher temperatures near hotspots

insights into mantle structure and flow beneath alaska based on a decade of observations of shear wave splitting sks shear wave splitting measurements from three program for array seismic studies of the continental lithosphere experiments broadband experiment across the alaska range alaska receiving cross transect of the inner core and multidisciplinary observations of subduction which form a north south transect across alaska show a remarkably simple pattern of two large anisotropy domains in the northern domain extending from the 70 km contour of the subducting pacific plate north to the arctic ocean fast directions are consistently in the ne sw direction these directions are essentially parallel to the absolute plate motion direction in northern alaska and parallel to the strike of the subducting plate above the mantle wedge suggesting that they represent some combination of plate scale asthenospheric flow in the upper mantle and flow along the subducting plate in the mantle wedge a strong wedge component beneath the alaska range is required to explain systematics of splitting delay times in the southern domain which extends south from the 70 km depth contour to the subducting plate fast directions are in the nw se direction a 90 rotation from the northern domain these fast directions are parallel to the dip of the subducting plate in the direction of convergence and represent entrained flow beneath the subducting slab the pacific plate absolute motion approximately parallels local convergence two major factors seem to control flow in these regions absolute plate motion in the north and the subduction of the pacific plate in the south although both subduction driven wedge flow and absolute plate motion contribute to the southern part of the northern regime

seismic anisotropy of northeastern algeria from shear wave splitting analysis abstract there are few studies of internal deformation under northern africa here we present such a study we analyze teleseismic shear wave splitting for northeast algeria to improve our knowledge of lithospheric and asthenospheric deformation mechanisms in this region we study waveform data generated by tens of teleseismic events recorded at five recently installed broadband bb stations in algeria these stations cover an area 2 across extending from the tellian geological units in the north to the saharan atlas units in the south analysis of sks wave splitting results insignificant spatial variations in fast polarization orientation over a scale length of at most 100 km the seismic anisotropy shows three clear spatial patterns a general ene wsw orientation is observed under the stations in the north this polarization orientation follows the direction of the tell atlas mountain chain which is perpendicular to the convergence direction between africa and eurasia delay times vary significantly across the region between 0 6 and 2 0 s at several stations there is an indication of a wnw ese polarization orientation which is apparently related to a later geodynamic evolutionary phase in this region a third pattern of seismic anisotropy emerges in the south with an orientation of roughly n s we discuss these observations in light of geodynamic models and present day geodetic motion

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

three dimensional modeling of plume lithosphere interaction in order to understand better the dynamics of hot spots such as hawaii we present a three dimensional numerical model for the interaction of a thermal plume with a moving lithosphere the model domain is a rectangular box filled with fluid whose viscosity depends upon temperature and pressure the lithosphere is represented by a layer of cold highly viscous fluid moving with an imposed horizontal velocity u in the x direction and a thermal plume is generated by a circular temperature anomaly on the bottom of the box the steady flow is determined numerically using a hybrid spectral finite difference technique the flow is characterized by a stagnation streamline of width y x1 5 that represents the edge of the spreading plume material we illustrate the detailed behavior of the model using the example of the hawaiian plume our best fitting hawaiian model is obtained by adjusting the plume buoyancy flux b until the predicted topography anomaly matches the observed hawaiian swell topography we find b 4100 kg s 1 which implies a plume radius of 90 km for an assumed plume mantle temperature contrast of 300 c the predicted topography is supported primarily by density anomalies beneath the lithosphere and cannot be explained by lithospheric erosion we therefore conclude that the classical lithospheric reheating model is unable to account for hotspot swells the horizontal flux of buoyancy associated with the swell exceeds b by up to 80 suggesting that current estimates of b for mantle plumes are too high empirically derived scaling laws for the width of the stagnation streamline and for the topography anomaly exhibit power law dependencies on b and u that agree well with those predicted by the refracted plume model of olson 1990 the principal weakness of the model is that the predicted geoid topography ratio of 0 010 for the hawaiian swell is about twice the observed value of 0 004 0 006

sks splitting in the western indian ocean from land and seafloor seismometers plume plate and ridge signatures we present sks splitting measurements in the western indian ocean recorded on 20 land and 57 seafloor seismometers deployed by the rhum rum experiment reunion hotspot and upper mantle reunions unterer mantel we discuss our splitting observations within their geodynamic settings and compare them to sks splitting parameters predicted from an azimuthally anisotropic rayleigh wave tomography model that includes the rhum rum data we find that anisotropic directions poorly correlate with the present day motion of the somali plate which at 2 6 cm yr may be too slow to cause strongly sheared fabric in the asthenosphere fast split directions between la reunion and the central indian ridge cir trend e w and provide strong first seismological evidence for near horizontal flow in the asthenosphere that connects the reunion mantle upwelling with the cir supporting a long standing hypothesis on plume ridge interaction in the vicinity of the reunion hotspot we observe a seismic anisotropy pattern indicative of a parabolic asthenospheric flow controlled by the reunion mantle upwelling and its consecutive asthenospheric spreading we furthermore observe ridge normal along the cir and ridge parallel along the southwest indian ridge swir both mainly attributed to asthenospheric mantle flows in the mozambique channel between east africa and madagascar we attribute e w trending to frozen lithospheric structures recording the paleo orientation of the spreading ridges that enabled madagascar s separation away from africa based on the synopsis of this and previous sks splitting studies at mid ocean ridges we propose that ridge normal may develop at fast and intermediate spreading ridges e g cir and east pacific rise and ridge parallel could be characteristic to slow spreading ridges e g swir mid atlantic ridge and the paleo ridges in the mozambique channel