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

late eocene to early miocene ice sheet dynamics and the global carbon cycle paired benthic foraminiferal trace metal and stable isotope records have been constructed from equatorial pacific ocean drilling program site 1218 the records include the two largest abrupt 1 myr increases in the cenozoic benthic oxygen isotope record oi 1 in the earliest oligocene 34 ma and mi 1 in the earliest miocene 23 ma the paired mg ca and oxygen isotope records are used to calculate seawater 18o w calculated w suggests that a large antarctic ice sheet formed during oi 1 and subsequently fluctuated throughout the oligocene on both short 0 5 myr and long 2 3 myr timescales between about 50 and 100 of its maximum earliest oligocene size the magnitudes of these fluctuations are consistent with estimates of sea level derived from sequence stratigraphy the transient expansion of the antarctic ice sheet at mi 1 is marked in the benthic 18o record by two positive excursions between 23 7 and 22 9 ma each with a duration of 200 300 kyr bottom water temperatures decreased by 2 c over the 150 kyr immediately prior to both rapid 18o excursions however the onset of each of these phases of ice growth is synchronous within the resolution of the records with the onset of a 2 c warming over 150 kyr we suggest that the warming during these glacial expansions reflect increased greenhouse forcing prompted by a sudden decrease in global chemical weathering rates as antarctic basement silicate rocks became blanketed by an ice sheet this represents a negative feedback process that might have operated during major abrupt growth phases of the antarctic ice sheet

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

complex anisotropy in d beneath the eastern pacific from sks skks splitting discrepancies article i nfo although observations of seismic anisotropy in the lowermost mantle are abundant the mechanism which generates anisotropy in the d layer is not well understood most observational constraints on d anisotropy come from the splitting of shear waves that propagate nearly horizontally through the lowermost mantle however anisotropy in the d layer can also give rise to discrepancies in shear wave splitting for sks and skks phases for the same event station pair as these phases sample different regions of the lowermost mantle here i report observations of strongly discrepant sks skks splitting observed at broadband stations in western mexico and california in particular strong skks splitting with fast polarization directions near 60 and delay times up to 3 s is observed for a group of raypaths that sample a region of the d layer beneath the eastern pacific ocean a comparison of skks splitting with sks splitting observed from the same events as well as with sks splitting observed at a variety of backazimuths at each station suggests that the anomalous anisotropic structure is in the lowermost mantle my preferred model for the unusual anisotropic geometry in this region is large shear deformation in d at the edge of a region where slab material impinges upon the core mantle boundary resulting in lattice preferred orientation of lower mantle minerals

deformation in the lowermost mantle beneath australia from observations and models of seismic anisotropy observations of seismic anisotropy near the core mantle boundary may yield constraints on patterns of lowermost mantle flow we examine seismic anisotropy in the lowermost mantle beneath australia bounded by the african and pacific large low shear velocity provinces we combined measurements of differential splitting of sks skks and s scs phases sampling our study region over a range of azimuths using data from ten long running seismic stations observations reveal complex and laterally heterogeneous anisotropy in the lowermost mantle we identified two sub regions for which we have robust measurements of d associated splitting for a range of ray propagation directions and applied a forward modeling strategy to understand which anisotropic scenarios are consistent with the observations we tested a variety of elastic tensors and orientations including single crystal elasticity of lowermost mantle minerals bridgmanite post perovskite and ferropericlase tensors based on texture modeling in post perovskite aggregates elasticity predicted from deformation experiments on polycrystalline mgo aggregates and tensors that approximate the shape preferred orientation of partial melt we find that post perovskite scenarios are more consistently able to reproduce the observations beneath new zealand the observations suggest a nearly horizontal 100 axis orientation with an azimuth that agrees well with the horizontal flow direction predicted by previous mantle flow models our modeling results further suggest dominant slip on the 010 plane in post perovskite aggregates provide a good fit to the data but the solution is non unique our results have implications for the mechanisms of deformation and anisotropy in the lowermost mantle and for the patterns of mantle flow

widespread seismic anisotropy in earth s lowermost mantle beneath the atlantic and siberia deep inside the earth just above the core mantle boundary at around 2 700 km depth large scale mantle structures are assumed to play a key role for global geodynamic processes while unusual hot regions are attributed to feed rising mantle plumes and volcanic hotspots the accumulation of subducted lithospheric plates is associated with colder than average features in both environments the appearance of dynamic driven processes such as deformation and mantle flow can directly be inferred by the presence of seismic anisotropy however the geometries as well as the interactions of these massive anomalous structures with the surrounding mantle material are still under debate based on new seismic data from a dense and large aperture recording network in scandinavia we characterize the anisotropic signatures of two so far unexplored regions in the lowermost mantle by using observations of clearly discrepant sks skks shear wave splitting measurements thereby we can demonstrate that anisotropy is located along the northern edges of the large low shear velocity province beneath africa furthermore we recover an anisotropic structure in a region of fast seismic velocity underneath siberia which provides additional evidence for widespread deformation caused by a deeply subducted slab

laboratory and seismological observations of lower mantle isotropy we have carried out seismological and mineral physics investigations to identify the extent and origin of elastic anisotropy in the lower mantle based on observations of shear wave splitting we conclude that the lower mantle is effectively isotropic this result is surprising since the lower mantle is composed of elastically anisotropic minerals silicate perovskite mgo sio 2 that have been extensively strained by geologic processes to reconcile the seismic observations we have measured texture development during deformation and high temperature recrystallization of mantle silicates we show that these experiments provide a direct explanation for the shear wave splitting measurements the lower mantle appears isotropic because silicate perovskite maintains an isotropic texture during deformation and recrystallization

lowermost mantle anisotropy and deformation along the boundary of the african llsvp shear wave splitting of sk k s phases is often used to examine upper mantle anisotropy in specific cases however splitting of these phases may reflect anisotropy in the lowermost mantle here we present sks and skks splitting measurements for 233 event station pairs at 34 seismic stations that sample d beneath africa of these 36 pairs show significantly different splitting between the two phases which likely reflects a contribution from lowermost mantle anisotropy the vast majority of discrepant pairs sample the boundary of the african large low shear velocity province llsvp which dominates the lower mantle structure beneath this region in general we observe little or no splitting of phases that have passed through the llsvp itself and significant splitting for phases that have sampled the boundary of the llsvp we infer that the d region just outside the llsvp boundary is strongly deformed while its interior remains undeformed or weakly deformed

onset of cenozoic antarctic glaciation abstract this paper considers the wide range of evidence both direct and indirect for the onset of cenozoic antarctic glaciation it distinguishes two useful phases of antarctic glacial onset an initial phase of mountain glaciation from which ice streams occasionally and in isolated locations reached sea level and a subsequent phase of full glaciation with an ice sheet as large as today s extending everywhere to sea level according to direct evidence generally proximal from the continent itself or surrounding southern ocean the first of these occurred probably during the late eocene while the second developed at the eocene oligocene boundary indirect evidence mainly involving proxy measurements from dsdp and odp sites remote from the southern ocean suggests that middle and late eocene glaciations may have been full also ice sheets possibly even larger than today s but short lived and that the e o boundary onset differed from these mainly in producing a stable ice sheet in pursuing the notion of glacial onset we examined the direct record separately for east antarctica west antarctica and the antarctic peninsula the different sub ice topography and geographic positions of which suggest that their glacial histories could have been different the direct record for an initial middle or late eocene phase is very sparse only the rare occurrence of ird at southern ocean dsdp and odp sites suggests the possibility of early ice and all three regions include mountains that could have hosted such ice although the indirect record and climate modelling in combination suggest that full glaciation of each region was probably synchronous we find differences in the available direct evidence there is abundant evidence that east antarctica became fully glaciated in the earliest oligocene but certain evidence of glaciation of a similar age extending to sea level is sparse for the antarctic peninsula and is not found until the late oligocene for west antarctica high resolution direct evidence is required to resolve uncertainties in glacial history

possibility of a low p wave velocity layer in the outermost core from global smks waveforms abstract a global data set consisting of 1211 smks m 2 waveforms has been analyzed to investigate the radial seismic velocity structure around the core mantle boundary cmb band passed f 0 02 0 1 hz and stacked waveforms coincide with reflectivity synthetic ones for prem very well whereas those for other global models iasp91 ak135 and sp6 yield disagreements a preliminary waveform modeling for the outermost core with prem as initial structure results in a core surface layer with a p wave velocity of 7 95 km s at the core top and thickness of 90 km based on a 3 d global s wave velocity model smks phases collected in this study mainly pierce slightly high velocity regions at the base of the mantle and the predicted differential travel time residuals for s3ks skks are expected to be negative approximately 0 5 s for the averages however positive s3ks skks residuals of which average is 0 3 s are predominant in the stacked waveforms moreover a waveform modeling for the d structure can result in a 30 km thick layer with a 10 s wave velocity reduction at the mantle bottom ulvz in which waveform fitness for the part of mainly s3ks is improved on the other hand the waveform data is not well explained by the lowermost mantle structures with thickness of several hundred kilometers these suggest that the smks data is still affected by the structure of the lowermost several tens kilometers in the mantle that is not sufficiently modeled by a global tomography although the thin low s wave velocity model is not conclusive the possibility of a low p wave velocity layer in the outermost core is remained because the waveform fitness for the part of s4ks is improved by the combination of the ulvz and a 140 km thick layer with a 0 8 p wave velocity reduction at the core top

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

insight into craton evolution constraints from shear wave splitting in the north china craton the multi episodic tectonic activities from the precambrian to cenozoic including nucleus formation cratonic amalgamation and rejuvenation make the north china craton ncc an ideal natural laboratory for studying craton evolution spatial change in the upper deformation records is an important aspect for understanding cratonic formation and rejuvenation in this study we performed seismic shear wave splitting analysis using sks phases from 50 portable stations two different methodologies shear wave splitting measurement and amplitude analysis of transverse radial components produced mutually consistent splitting results these results showed that the seismic anisotropy beneath the ordos block can be divided into three subgroups reflecting the tectonic control combining these results with those from previous splitting studies in the eastern ncc we suggest that the proterozoic amalgamation generated the seismic anisotropy in the boundary zone between the ordos block and the trans north china orogen while the anisotropy in the eastern trans north china orogen and eastern ncc were possibly associated with the lithospheric rejuvenation during the late mesozoic to cenozoic

estimating crustal seismic anisotropy with a joint analysis of radial and transverse receiver function data summary we developed an integrated technique for estimating crustal anisotropy with a horizontal axis using radial r and transverse t receiver functions the technique includes computing three individual and one joint objective function jof and a reliability analysis of the estimated anisotropy the individual objective functions iofs are designed to 1 maximize the peak energy of the stacked r receiver function after a cosine moveout correction in the ps arrival time 2 to maximize the correlation of the radial receiver functions after a full correction of anisotropy or 3 to minimize the total energy of transverse receiver functions stacked after a removal of crustal anisotropy the jof was computed by a weighted average of the three iofs and the reliability analysis uses the principle that stacking coherent signals can lead to an increase of signal to noise ratio we applied the technique to synthetic receiver functions generated with 30 60 per cent white noise from a variety of anisotropic and heterogeneous models the synthetic tests indicate that the proposed technique has good capability to recover the input models despite the presence of random and other coherent noises such as those caused by inhomogeneous structures in the data the technique can always provide accurate estimates of crustal anisotropy we applied the technique to two permanent seismic stations in western china and found significant crustal anisotropy beneath one station located at the northern edge of the tibetan plateau the observed fast polarization direction at this station follows the direction of the maximum horizontal tensile stress suggesting that the observed seismic anisotropy is likely caused by mineral alignment in the lower crust the station situated in the sichuan basin on the other hand shows little to no seismic anisotropy which may suggest that the crust beneath the basin is nearly rigid with very little deformation the developed technique can be applied to any broadband seismic stations that have a good backazimuthal coverage of teleseismic events

observations of changing anisotropy across the southern margin of the african llsvp we present evidence for the presence of complex anisotropy in the lowermost mantle from 3 d waveform modelling of observed core diffracted shear waves that sample the southern edge of the african large low shear velocity province llsvp the anomalously strong amplitude of the sv component for the shear core diffracted phase at large distances indicates the presence of anisotropy we measure shear wave splitting parameters to determine which part of the elastic tensor is constrained by this particular data set the modelling is performed using the spectral element method the anisotropy is strong outside the llsvp weakens or rotates close to its boundary and appears to be absent inside the llsvp the presence of the llsvp margin may cause flow in the mantle to change direction the occurrence of strong anisotropy in the region of fast seismic velocities is compatible with lattice preferred orientation in post perovskite due to accommodation of flow through dislocation creep

shear wave splitting beneath southern korea and its tectonic implication abstract seismic anisotropy beneath the southern korean peninsula skp was investigated based on the scs waves of deep focused earthquakes recorded at 35 broadband stations the most striking feature of the observed splitting patterns is the preference of the fast directions nw se that are nearly parallel to the direction of the absolute plate motion in the region however the splitting patterns over the region show significant variation in splitting parameters indicating a complex anisotropic structure variations of the splitting directions and dissimilarity in the source domains of basaltic volcanisms suggest that the asthenospheric mantle flow since at least the late cenozoic cannot explain the seismic anisotropy beneath the region comparison to shear wave splitting measurements from eastern china revealed that the nw se fast direction of splitting measurements in the skp is close to that in the north china block ncb while the ne sw fast direction might be related to that in the south china block scb the shallow mantle lithosphere beneath the skp retains the fossil anisotropy amalgamated prior to the late paleozoic before the collision between the ncb and scb and the anisotropic structure was not completely realigned by the major orogenic events during the late paleozoic to mesozoic eras

sks splitting beneath transportable array stations in eastern north america and the signature of past lithospheric deformation seismic anisotropy in the upper mantle beneath continental interiors is generally complicated with contributions from both the lithosphere and the asthenosphere previous studies of sks splitting beneath the eastern united states have yielded evidence for complex and laterally variable anisotropy but until the recent arrival of the usarray transportable array ta the station coverage has been sparse here we present sks splitting measurements at ta stations in eastern north america and compare the measured fast directions with indicators such as absolute plate motion surface geology and magnetic lineations we find few correlations between fast directions and absolute plate motion except in the northeastern u s and southern canada where some stations exhibit variations in apparent splitting with backazimuth that would suggest multiple layers of anisotropy a region of the southeastern u s is dominated by null sks arrivals over a range of backazimuths consistent with previous work we document a pattern of fast directions parallel to the appalachian mountain chain suggesting a contribution from lithospheric deformation associated with appalachian orogenesis overall our measurements suggest that upper mantle anisotropy beneath the eastern united states is complex with likely contributions from both asthenospheric and lithospheric anisotropy in many regions

constraints on subduction geodynamics from seismic anisotropy 1 much progress has been made over the past several decades in delineating the structure of subducting slabs but several key aspects of their dynamics remain poorly constrained major unsolved problems in subduction geodynamics include those related to mantle wedge viscosity and rheology slab hydration and dehydration mechanical coupling between slabs and the ambient mantle the geometry of mantle flow above and beneath slabs and the interactions between slabs and deep discontinuities such as the core mantle boundary observations of seismic anisotropy can provide relatively direct constraints on mantle dynamics because of the link between deformation and the resulting anisotropy when mantle rocks are deformed a preferred orientation of individual mineral crystals or materials such as partial melt often develops resulting in the directional dependence of seismic wave speeds measurements of seismic anisotropy thus represent a powerful tool for probing mantle dynamics in subduction systems here i review the observational constraints on seismic anisotropy in subduction zones and discuss how seismic data can place constraints on wedge slab and sub slab anisotropy i also discuss constraints from mineral physics investigations and geodynamical modeling studies and how they inform our interpretation of observations i evaluate different models in light of constraints from seismology geodynamics and mineral physics finally i discuss some of the major unsolved problems related to the dynamics of subduction systems and how ongoing and future work on the characterization and interpretation of seismic anisotropy can lead to progress particularly in frontier areas such as understanding slab dynamics in the deep mantle

the polonez cove formation of king george island antarctica stratigraphy facies and implications for mid cenozoic cryosphere development the middle to late oligocene polonez cove formation exposed on south eastern king george island south shetland islands provides rare evidence of mid cenozoic west antarctic cryosphere evolution a revised lithostratigraphy and facies analysis and a review of the palaeoenvironmental significance of the formation are presented here the diamictite dominated basal member of the formation krakowiak glacier member records the presence and retreat of marine based ice on a shallow continental shelf five overlying members are recognized these consist of basaltic sourced sedimentary rocks and lavas and represent a variety of shoreface and shallow continental shelf environments in an active volcanic setting these units contain diverse reworked and ice rafted exotic clasts that become sparse towards the top of the formation suggesting a continuing but waning glacial influence new 40ar 39ar dates from interbedded lava flows indicate a late oligocene age 25 6 27 2 ma for the polonez cove formation but are slightly younger than skeletal carbonate sr isotope ages obtained previously 28 5 29 8 ma there is evidence for wet based subice conditions at the base of the polonez cove formation but no sedimentary facies to suggest substantial meltwater this may reflect a subpolar setting or may result from lack of preservation or a high energy depositional environment a northern antarctic peninsula south shetland islands provenance is probable for most non basaltic clasts but certain lithologies with possible origins in the transantarctic and ellsworth mountains also occur sparsely throughout the formation there is evidence to suggest that the presence of such far travelled clasts within subglacially deposited facies at the base of the formation reflects the advance of a local ice cap across marine sediments containing the clasts as ice rafted material the presence of these clasts suggests that extensive marine based ice drained into the southern weddell sea region and that a strong weddell sea surface current operated both before and during deposition of the polonez cove formation

seismic evidence links the subducted mongol okhotsk slab to deformation in d near the northeastern margin of the perm anomaly abstract numerical simulations have indicated that the large low shear velocity provinces llsvps dynamically interact with subducted slabs in d and slabs in turn play an important role in influencing the shape and location of the llsvps in this study we find evidence for the existence of the d discontinuity near the northeastern margin of the perm anomaly a smaller llsvp beneath eurasia by analyzing the scd the lower mantle triplication phases generated from two deep events occurring in the northwestern pacific subduction zone and recorded at stations in europe and turkey the d discontinuity in the north is characterized with a 2 velocity increase occurring at a height of 300 km above the core mantle boundary cmb whereas the discontinuity in the south is characterized with a 3 velocity increase occurring at a height of 260 km above the cmb the d discontinuity is most likely attributed to a phase transition from perovskite to post perovskite in conjunction with the presence of relic slab materials likely related to the subducted mongol okhotsk slab our observations along with previous seismic anisotropic detections provide a clue to dynamically link the subducted slab to deformation taking place near the edges of the llsvp

shear wave splitting and shear wave splitting tomography of the southern puna plateau fil calixto frank j university of missouri columbia department of geological sciences estados unidos

anisotropy in the deep earth abstract seismic anisotropy has been found in many regions of the earth s interior its presence in the earth s crust has been known since the 19th century and is due in part to the alignment of anisotropic crystals in rocks and in part to patterns in the distribution of fractures and pores in the upper mantle seismic anisotropy was discovered 50 years ago and can be attributed for the most part to the alignment of intrinsically anisotropic olivine crystals during large scale deformation associated with convection there is some indication for anisotropy in the transition zone particularly in the vicinity of subducted slabs here we focus on the deep earth the lower mantle and core where anisotropy is not yet mapped in detail nor is there consensus on its origin most of the lower mantle appears largely isotropic except in the last 200 300 km in the d region where evidence for seismic anisotropy has been accumulating since the late 1980s mostly from shear wave splitting measurements recently a picture has been emerging where strong anisotropy is associated with high shear velocities at the edges of the large low shear velocity provinces llsvps in the central pacific and under africa these observations are consistent with being due to the presence of highly anisotropic mgsio 3 post perovskite crystals aligned during the deformation of slabs impinging on the core mantle boundary and upwelling flow within the llsvps we also discuss mineral physics aspects such as ultrahigh pressure deformation experiments first principles calculations to obtain information about elastic properties and derivation of dislocation activity based on bonding characteristics polycrystal plasticity simulations can predict anisotropy but models are still highly idealized and neglect the complex microstructure of polyphase aggregates with strong and weak components a promising direction for future progress in understanding the origin of seismic anisotropy in the deep mantle and its relation to global mantle circulation is to link macroscopic information from seismology and microscopic information mineral physics through geodynamics modeling anisotropy in the inner core was proposed 30 years ago to explain faster p wave propagation along the direction of the earth s axis of rotation as well as anomalous splitting of core sensitive free oscillations there is still uncertainty about the origin of this anisotropy in particular it is difficult to explain its strength based on known elastic properties of iron as it would require almost perfect alignment of iron crystals indeed the strongly anomalous p travel times observed on paths from the south sandwich islands to alaska may or may not be due to inner core anisotropy and will need to be explained before consensus can be reached on the strength of anisotropy in the inner core and its origin

a two dimensional hybrid method for modeling seismic wave propagation in anisotropic media national science foundation of china 40504006 40434012 90814002 ear 0609717 chinese academy of sciences

complex seismic anisotropy at the border of a very low velocity province at the base of the earth s mantle 1 we constrain the anisotropy associated with a very low velocity province vlvp at the base of the earth s mantle using the sks and skks waves sampling the region our selected high quality data sets consist of 415 sks and 111 skks waveforms for 127 deep earthquakes recorded at distances between 90 and 150 by the seismic stations in three temporary broadband passcal seismic arrays the kaapvaal seismic array 1997 1999 the tanzania seismic array 1994 1995 and the ethiopia kenya seismic array 1999 2001 as well as the permanent stations in the global seismographic network these seismic data provide good sampling coverage for some portion of the vlvp and its surrounding areas our results show when the sks or skks phases sample the regions away from the border of the vlvp inside or outside the vlvp the apparent splitting parameters inferred from the sks phases are consistent with those inferred from the skks phases and their variations strongly correlate with seismic stations but not with the exit points at the core mantle boundary of these seismic phases however when the sks or skks phases sample near the border of the vlvp the apparent splitting parameters inferred from the sks phases and skks phases are different and their variations no longer correlate with seismic stations these features indicate that part of the shear wave splitting for the seismic data sampling the border of the vlvp has to originate from deep mantle most likely near the border of the vlvp we assume that the anisotropy in the shallow mantle beneath seismic stations has a horizontal hexagonal symmetry axis and infer the splitting parameters associated with the shallow anisotropy beneath the seismic stations using the sks and skks waveforms for the seismic data sampling the regions away from the border of the vlvp we then obtain the splitting parameters associated with the lowermost mantle anisotropy using the sks and skks waveforms corrected for the inferred shallow anisotropy beneath seismic stations assuming that the medium in the lowermost mantle also has a horizontal hexagonal symmetry axis our results reveal a complex anisotropy pattern near the border of the vlvp such a complex anisotropy pattern may be explained by lattice preferred orientation of anisotropic mantle aggregates aligned by a complex mantle flow near the vlvp margins the complex flow pattern near the vlvp margins may indicate strong interactions between the vlvp a compositional anomaly and the surrounding normal mantle and may provide an explanation to the concentration of some hot spots geographically near the borders of the vlvps in the lowermost mantle

the deformational journey of the nazca slab from seismic anisotropy s a and c m e are currently supported by australian research council grant de190100062

upper mantle anisotropy beneath the cameroon volcanic line and congo craton from shear wave splitting measurements summary the cameroon volcanic line cvl is an 1800 km long line of cenozoic volcanoes that does not show a chronological progression consistent with hotspot related volcanism we investigate seismic anisotropy to determine the upper mantle lattice preferred orientation and constrain the mantle flow pattern using a temporary array of 32 broad band seismographs deployed throughout cameroon between 2005 and 2007 along with two additional permanent seismographs in adjacent countries we determine the fast direction and lag time beneath each station by stacking sks and skks splitting measurements from multiple events the results indicate four regions with different splitting parameters the congo craton in southern cameroon and the garoua rift region in northeast cameroon have northeast southwest oriented fast directions and split times of about 1 s between the congo craton and the cvl in central cameroon the fast directions are variable and have small splitting times of 0 3 s or less along the cvl where previous studies show a strong slow velocity anomaly in the mantle the fast direction is oriented approximately north south with splitting times of about 0 7 s we interpret measurements from southern cameroon and northeast cameroon as indications of lattice preferred orientation frozen into the congo craton and subcontinental lithosphere related to relict plate motion and deformation the distinct pattern of splitting along the cvl suggests the existence of small scale convection in the asthenosphere related to the formation of the cvl perhaps driven by the adjacent cold edge of the congo craton

elasticity of single crystal periclase at high pressure and temperature the effect of iron on the elasticity and seismic parameters of ferropericlase in the lower mantle

variability and origin of seismic anisotropy across eastern canada evidence from shear wave splitting measurements measurements of seismic anisotropy in continental regions are frequently interpreted with respect to past tectonic processes preserved in the lithosphere as fossil fabrics models of the present day sublithospheric flow often using absolute plate motion as a proxy are also used to explain the observations discriminating between these different sources of seismic anisotropy is particularly challenging beneath shields whose thick 200 km lithospheric roots may record a protracted history of deformation and strongly influence underlying mantle flow eastern canada where the geological record spans 3 ga of earth history is an ideal region to address this issue we use shear wave splitting measurements of core phases such as sks to define upper mantle anisotropy using the orientation of the fast polarization direction and delay time t between fast and slow shear wave arrivals comparison with structural trends in surface geology and aeromagnetic data helps to determine the contribution of fossil lithospheric fabrics to the anisotropy we also assess the influence of sublithospheric mantle flow via flow directions derived from global geodynamic models fast polarization orientations are generally ene wsw to ese wnw across the region but significant lateral variability in splitting parameters on a 100 km scale implies a lithospheric contribution to the results correlations with structural geologic and magnetic trends are not ubiquitous however nor are correlations with geodynamically predicted mantle flow directions we therefore consider that the splitting parameters likely record a combination of the present day mantle flow and older lithospheric fabrics consideration of both sources of anisotropy is critical in shield regions when interpreting splitting observations

upper mantle anisotropy beneath japan from shear wave splitting in this study we utilize data from 64 broadband seismic stations of the japanese f net network to investigate the threedimensional pattern of anisotropy in the subduction system beneath japan we have compiled a database of approximately 1900 high quality splitting measurements selected by visual inspection of over 25 000 records of s sks and skks phases at f net stations covering a wide range of incidence angles incoming polarization angles and backazimuths analysis of the variations of measured splitting parameters with these parameters allows us to consider complexities in structure such as multiple anisotropic layers dipping symmetry axes and small scale lateral variations in anisotropic properties here we focus on the presentation of the splitting measurements themselves a detailed interpretation in terms of tectonics and mantle flow is beyond the scope of this paper in the southern part of the f net array along the ryukyu arc we find that fast directions are consistently trench parallel with splitting times of 1 s or more moving northward along the array the measured splitting patterns become more complicated with significant variations in apparent splitting parameters that indicate complex anisotropic structure additionally measured fast directions vary significantly over short length scales and stations separated by less than 100 km often exhibit very different splitting behavior this increase in complexity of anisotropic structure coincides geographically with the complicated slab morphology of the subducting pacific plate at stations on hokkaido to the north and kyushu to the south we see some evidence that the fast direction of anisotropy may rotate from trench parallel close to the trench to subduction parallel further away from the trench which may correspond either to a change in stress conditions and or volatile content or to a change in flow regime 2005 elsevier b v all rights reserved

seismic anisotropy in the lower mantle a comparison of waveform splitting of sks and skks 1 we have carried out a global investigation on seismic anisotropy in the lower mantle by comparing the waveform splitting of sks and skks found at the same seismogram the two shear waves have very similar ray paths in the upper mantle but different ones in the lower mantle we collected a total of 104 sks skks waveform data recorded at 76 stations the sks skks data are first matched with a single anisotropic model 2 parameters the fast polarization direction and delay time t and two independent anisotropic models 4 parameters we then applied an f test to examine whether the 4 parameter models are better than the 2 parameter ones based on error improvement we found that the data from most of the stations can be explained by the simple 2 parameter models while this observation provides compelling evidence that the vast part of the lower mantle below 1000 km including the d region is transversely isotropic it is still arguable that the uppermost lower mantle and the transition may partly contribute the sks skks splittings we also found that the 4 parameter models provide a better fitting to the sks and skks splitting at 8 stations suggesting the existence of transverse anisotropy or anomalous dipping structures in some part of the lower mantle

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

seismic anisotropy in the lowermost mantle near the perm anomaly the lower mantle is dominated by two large structures with anomalously low shear wave velocities known as large low shear velocity provinces llsvps several studies have documented evidence for strong seismic anisotropy at the base of the mantle near the edges of the african llsvp recent work has identified a smaller structure with similar low shear wave velocities beneath eurasia dubbed the perm anomaly here we probe lowermost mantle anisotropy near the perm anomaly using the differential splitting of sks and skks phases measured at stations in europe we find evidence for lowermost mantle anisotropy in the vicinity of the perm anomaly with geographic trends hinting at lateral variations in anisotropy across the boundaries of the perm anomaly as well as across a previously unsampled portion of the african llsvp border our observations suggest that deformation is concentrated at the boundaries of both the perm anomaly and the african llsvp

seismic studies on the trans hudson orogen of western canada abstract the discovery of diamondiferous kimberlites in the late 1980 s within the central saskatchewan segment of the trans hudson orogen prompted an aggressive diamond exploration program in the region the presence of this apparently uncharacteristic mineralization within the proterozoic orogen was initially a puzzle recent seismic reflection and geochronology evidence however suggests that the glennie domain of the orogen may be cored by a microcontinental archean block while geochemical results imply that segments of the precambrian basement of southern saskatchewan mainly comprise archean crust which has been reworked by thermotectonism we present the initial results of two seismic programs designed to elucidate the lithospheric structure associated with the economically targeted segment of the orogen preliminary interpretation of wide angle reflection data along a 730 km north south profile in north central saskatchewan reveals a very complex structure with the crustal thickness varying from a maximum of 50 km to a minimum of 37 km over a distance of 200 km beneath the glennie domain a teleseismic feasibility study with an 8 station array with individual seismographs operating for 4 to 6 months supports and extends this image of complexity relative travel time differentials across a 500 km array are as large as 700 ms while key features of these delay times correlate with the crustal structure a significant portion of the variations must be assigned to deeper levels of the lithosphere receiver function analysis for crustal structure is shown to be feasible in this environment where the phanerozoic sediment thickness varies from 0 to 2 1 km and provides evidence for large variations 7 km in crustal thickness in the southwestern area of the province sks analysis at a single station also adds to the evidence which shows rapid variations in anisotropy within the orogen a teleseismic program with seventeen 3 component stations is now underway to clarify the nature of this complex crustal and upper mantle structure

overriding plate mantle wedge slab and subslab contributions to seismic anisotropy beneath the northern central andean plateau the central andean plateau the second highest plateau on earth overlies the subduction of the nazca plate beneath the central portion of south america the origin of the high topography remains poorly understood and this puzzle is intimately tied to unanswered questions about processes in the upper mantle including possible removal of the overriding plate lithosphere and interaction with the flow field that results from the driving forces associated with subduction observations of seismic anisotropy can provide important constraints on mantle flow geometry in subduction systems the interpretation of seismic anisotropy measurements in subduction settings can be challenging however because different parts of the subduction system may contribute including the overriding plate the mantle wedge above the slab the slab itself and the deep upper mantle beneath the slab here we present measurements of shear wave splitting for core phases sks skks pks and ssks local s and source side teleseismic s phases that sample the upper mantle beneath southern peru and northern bolivia relying mostly on data from the caught experiment we find evidence for seismic anisotropy within most portions of the subduction system although the overriding plate itself likely makes only a small contribution to the observed delay times average fast orientations generally trend roughly trench parallel to trench oblique contradicting predictions from the simplest two dimensional flow models and olivine fabric scenarios our measurements suggest complex layered anisotropy beneath the northern portion of the central andean plateau with significant departures from a two dimensional mantle flow regime

mid mantle anisotropy in subduction zones and deep water transport the earth s transition zone has until recently been assumed to be seismically isotropic increasingly however evidence suggests that ordering of material over seismic wavelengths occurs there but it is unclear what causes this we use the method of source side shear wave splitting to examine the anisotropy surrounding earthquakes deeper than 200 km in slabs around the globe we find significant amounts of splitting 2 4 s confirming that the transition zone is anisotropic here however there is no decrease in the amount of splitting with depth as would be the case for a metastable tongue of olivine which thins with depth suggesting this is not the cause the amount of splitting does not appear to be consistent with processes in the ambient mantle such as lattice preferred orientation development in wadsleyite ringwoodite or mgsio3 perovskite we invert for the orientation of several mechanisms subject to uncertainties in mineralogy and deformation and the best fit is given by updip flattening in a style of anisotropy common to hydrous phases and layered inclusions we suggest that highly anisotropic hydrous phases or hydrated layering is a likely cause of anisotropy within the slab implying significant water transport from the surface down to at least 660 km depth

sks splitting beneath alaska regional variability and implications for subduction processes at a slab edge abstract many aspects of subduction zone geodynamics remain poorly understood including the pattern of flow in the mantle surrounding subducting slabs because of the relationship between deformation and seismic anisotropy measurements of shear wave splitting in subduction zones can shed light on this pattern of flow while observations of splitting in subduction zones are numerous and robust the source and geodynamical implications of the anisotropy remain imperfectly constrained the alaska subduction zone associated with the subduction of the pacific plate beneath north america is a particularly complex tectonic setting dramatic lateral variations in slab morphology seismicity and volcanic character are present beneath the region and previous splitting observations and modeling studies have suggested the possibility of significant along strike mantle flow in the region here we present sks splitting measurements from more than 50 stations of the permanent broadband ak network located throughout alaska splitting patterns show significant regional variability but proximate stations often exhibit similar splitting behavior stations in southern alaska exhibit nearly trench normal fast directions while in southeasternmost alaska fast directions trend parallel to the plate boundary a group of stations in the eastern part of the array exhibit predominantly null splitting while stations to the northwest of this group tend to exhibit 1 s or more of splitting with fast directions that are roughly ne sw stations east of the kenai peninsula region exhibit complicated splitting patterns that suggest complex anisotropy we discuss a number of potential factors that may contribute to the complexity in splitting patterns observed in alaska including frozen anisotropic structure in the overriding plate lithosphere asthenospheric shear due to absolute plate motion trench parallel flow in the mantle wedge two dimensional entrained flow beneath the slab and mantle upwelling at the slab edge

evaluating contributions to sk k s splitting from lower mantle anisotropy a case study from station dbic cote d ivoire measurements of seismic anisotropy constitute a very important tool for examining patterns of flow and mineral properties in the earth s mantle a popular strategy for gaining insight into upper mantle processes is to examine the splitting of sk k s phases and interpret them in terms of upper mantle anisotropy and deforma tion in such studies any contribution to splitting from anisotropy in the lower mantle is usually ignored here we present measurements of sks and skks splitting at global seismograph network station dbic in the cote d ivoire which exhibits a very unusual pattern of shear wave splitting the splitting pattern is dominated by null measurements over a wide range of back azimuths with non null measurements found over a very limited back azimuthal range we also identified examples of dis crepant sks skks splitting for the same event station pair splitting at dbic has previously been interpreted in terms of upper mantle anisotropy but we argue that this splitting pattern can best be explained by an apparently isotropic upper mantle with a contribution from anisotropy in the lower mantle likely in the d layer using station dbic as a case study we discuss the potential pitfalls in interpreting sks splitting measurements and suggest a set of best practices to decrease the like lihood of misinterpreting shear wave splitting results at a seismic station online material summary of events used in the study with their associated splitting parameters

variations in sks splitting across western canada measurements of sks splitting parameters have been made at stations of the canadian national seismograph network for events over the period 1993 mid1994 here we report on results obtained from western stations in the northern cordillera the alignment of polarization directions with major transcurrent fault systems suggests that the crust and upper mantle are being deformed coherently splitting parameters in the southern cordillera however are oriented approximately perpendicular to the strike of major tectonic features notably the cascadia subduction zone this contrast in the character of fast polarization directions between the southern and northern cordillera is thought to reflect the change in the tectonic style of the adjacent plate margin from convergent along the southern b c coast to transform north of the triple junction one plausible explanation is that upper mantle anisotropy in the southern region is due primarily to mantle flow perpendicular to the margin involving upwelling at the juan de fuca ridge subduction along the coast and associated back arc asthenospheric flow beneath the b c interior

azimuthal anisotropy in the d layer beneath the caribbean 1 the lowermost mantle beneath central america has anisotropic seismic velocity structure manifested in shear wave splitting of signals from south american earthquakes recorded at north american broadband recording stations prior studies of deep mantle anisotropy in this region have characterized the structure as having vertical transverse isotropy vti which is sufficient to explain a general trend of early tangential sh component arrivals however vti models cannot quantitatively match systematic waveform complexities in the onset of many of the shear waves that graze this region after accounting for splitting effects of upper mantle anisotropy beneath the recording stations we model the corrected waveform data using full wave theory for mantle velocity models with an anisotropic d layer this is the first attempt to quantitatively model a large data set including azimuthal anisotropy in d the models include transverse isotropy with either a vertical or tilted symmetry axis the latter resulting in azimuthal anisotropy for some initial shear wave polarizations tilted transverse isotropy tti produces small reversed polarity arrivals on the sv components at the arrival time of sh consistent with the data geographical variations in the azimuth of the tti symmetry axis are indicated by the data the lack of azimuthal coverage prevents unique resolution of the tti orientation and also precludes distinguishing between tti and other azimuthal anisotropy structures such as that predicted for lattice preferred orientation of minerals nonetheless our modeling demonstrates the need for laterally varying anisotropic structure of more complex form than vti for this region

seismic anisotropy beneath stable continental interiors a robust knowledge of seismic anisotropy beneath the continents is essential to our understanding of plate tectonic theory as anisotropy provides a unique constraint on the character of past and present deformation in the lithosphere and sublithospheric mantle this review paper summarizes the range of techniques currently available to image seismic anisotropy with passive source seismic data and addresses current issues surrounding the observation and interpretation of continental seismic anisotropy to this end we present case studies for four regions where seismic anisotropy has been extensively investigated in recent years eastern north america the canadian shield australia and southern africa based on this full suite of results we infer that stable continental regions generally contain seismic anisotropy that is clearly located within both the lithosphere and the sublithospheric mantle usually to depths of at least 200 km and perhaps more an implication of these results is that tectonic plates are at most only partially coupled to the underlying mantle the results from these case studies also demonstrate that while remarkable progress in seismic anisotropy imaging has been achieved in recent years it is clear that much more work will be required to adequately understand the origin of continental seismic anisotropy we suggest that a more robust characterization of anisotropic parameters can only be achieved by integrating complementary seismic datasets and by incorporating constraints from key related datasets from mineral physics magnetotellurics gravity and geodesy

evidence for an anisotropic lower mantle beneath eastern asia comparison of shear wave splitting data of sks and p660s anisotropy in the lower mantle is studied by measuring the difference of the splitting parameter the delay time between two split shear waves between the two phases sks and p660s these two phases are the p to s conversion waves at the cmb and the 660 km discontinuity and therefore are affected by the accumulated anisotropy of whole and upper mantle respectively an approximately 0 4 sec time difference of value between sks and p660s is observed at the station hia a cdsn broadband seismic station located in central china this difference is supposed to be caused by the accumulated anisotropy in the lower mantle beneath the eastern asia region if the anisotropy is assumed to be distributed uniformly in the whole lower mantle or a lowermost 300 km d layer the resulted anisotropy will be 0 08 and 0 5 respectively however the real intensity of the existing anisotropy in the studied region may be much larger than the above values depending on its size and location in the lower mantle

sub slab mantle anisotropy beneath south central chile knowledge of mantle flow in convergent margins is crucial to unravelling both the contemporary geodynamics and the past evolution of subduction zones by analysing shear wave splitting in both teleseismic and local arrivals we can determine the relative contribution from different parts of the subduction zone to the total observed sks splitting providing us with a depth constraint on anisotropy we use this methodology to determine the location orientation and strength of seismic anisotropy in the south central chile subduction zone data come from the tipteq network deployed on the forearc during 2004 2005 we obtain 110 teleseismic sks and 116 local good quality shear wave splitting measurements sks average delay times are 1 3 s and local s delay times are only 0 2 s weak shear wave splitting from local phases is consistent with a shape preferred orientation spo source in the upper crust we infer that the bulk of shear wave splitting is sourced either within or below the subducting nazca slab sks splitting measurements exhibit an average north easterly fast direction with a strong degree of variation further investigation suggests a relationship between the measurement s fast direction and the incoming ray s back azimuth finite element geodynamic modelling is used to investigate the strain rate field and predicted lpo characteristics in the subduction zone these models highlight a thick region of high strain rate and strong s wave anisotropy with plunging olivine a axes in the sub slab asthenosphere we forward model the sub slab sourced splitting with a strongly anisotropic layer of thick asthenosphere comprising an olivine a axis oriented parallel to the direction of subduction the subducting lithosphere is not thick enough to cause 1 2 s of splitting therefore our results and subsequent models show that the nazca slab is entraining the underlying asthenosphere its flow causes it to be strongly anisotropic our observation has important implications for the controlling factors on sub slab mantle flow and the movement of asthenospheric material within the earth

seismological constraints on the ultralow velocity zones in the lowermost mantle from core reflected waves abstract analyses of reflected seismic phases pcp and scp from the core mantle boundary cmb enable us to constrain the physical properties of the ultralow velocity zones ulvzs at the base of the mantle the cmb region beneath the western and southern pacific is investigated in detail by using a large amount of waveform data recorded by the short period array stations clear arrivals of postcursor to scp scsp are detected from several event receiver pairs which sample the cmb regions beneath philippine kalimantan and east of australia indicating the existence of ulvzs under the regions no scp precursors sdp and spcp are found in our data set except for a few events with a large epicentral distance over 45 no additional arrivals related to pcp are found in the region where scsp is observed in the scp wavelet elastic properties of the ulvz are precisely constrained by waveform modeling of both pcp and scp which sample the cmb region under philippine kalimantan different ulvz structures are found between the northeastern and southwestern parts of the philippine kalimantan the ratio of shear to compressional velocity reductions is 2 0 2 5 and the ratio of density to shear velocity changes is 0 5 to 0 33 in the northeastern part while minimal changes in compressional velocity and large shear velocity decrease in the southwestern part are found a certain degree of density increases and shear velocity decreases are also observed in the ulvz beneath east of australia increase in density and decrease in shear velocity throughout our observation imply that the iron incorporation into the lowermost mantle minerals may generate the ulvz anomaly however other mechanisms such as partial melting can explain the structural difference between the northeastern and southwestern parts of the ulvz beneath the philippine kalimantan region

quaternary geochronology and distribution of mammuthus on the colorado plateau there are 41 known localities containing mammoth remains from the colorado plateau 24 in arizona 12 in utah 3 in new mexico and 2 in colorado of the 41 localities 13 32 arizona and utah only have yielded radiometric dates sup 14 c and u th 10 77 of these have been the result of the authors investigations the four youngest radiocarbon dates produce a weighted average date of approximately 11 270 plus minus 65 yr b p the youngest directly aged mammoth remains on the colorado plateau mammoth remains are recovered predominantly in alluvial regimes in addition to alcove cave and spring deposits no direct association of mammuthus and the clovis hunters has been reported from the colorado plateau dietary intake recorded in dung remains included predominantly graminoids in addition to various woody shrubs and trees that currently grow at higher elevations on the colorado plateau

upper and lower mantle anisotropy inferred from comprehensive sks and skks splitting measurements from india abstract in this study we investigate the upper mantle anisotropy beneath india using high quality sks and skks waveforms from 382 teleseismic earthquakes recorded at 119 broadband seismic stations in addition we present evidence for anisotropy in the d layer beneath southeast asia using sks and skks splitting discrepancies on the same seismogram during this exercise we obtain 200 new splitting measurements from 35 stations recently deployed in the indo gangetic plains igp central india and northeast india while the delay times between the fast and slow axes of anisotropy t range from 0 3 to 1 7 s the fast polarization azimuths at a majority of stations in the igp and central india coincide with the absolute plate motion of india implying shear at the base of the lithosphere as the dominant mechanism for forging anisotropy however stations in ne india reveal fast polarization azimuths mainly in the ene wsw direction suggestive of lithospheric strain induced by the ongoing indo eurasian collision our analysis for d anisotropy yielded a total of 100 sks skks pairs which can be categorized into those exhibiting i null measurements for one phase and significant splitting for the other phase ii null measurement for both the phases iii significant splitting for both the phases a pair is considered to be anomalous if the splitting difference between sks and skks is 0 5 s and the individual split time is 0 5 s using this criterion we obtain 12 measurements under category iii and 9 under category i that show a null measurement for sks and large splitting for the skks phase further we quantify the strength of the lower mantle anisotropy by correcting the skks measurement for the upper mantle anisotropy obtained by the sks phase on the same seismogram the sks delay times are found to be consistently less than skks times suggesting that the sks phases do not capture the lower mantle anisotropy in comparison to their skks counterparts seven coherent measurements thus obtained reveal measurable d anisotropy with fast polarization azimuths oriented mainly in the ene wsw direction these results suggest presence of a large region of deformation in the lowermost mantle beneath southeast asia a possible model for anisotropy in these regions could be the presence of slab material that pounded upon the core mantle boundary cmb and is experiencing large shear deformation resulting in lattice preferred orientation lpo of the lower mantle van der hilst and karason 1999 long 2009 the other possibility is the phase transformation from mgsio3 perovskite to a more stable post perovskite phase under favorable conditions which results in lpo of the lower mantle

bedmap a new ice thickness and subglacial topographic model of antarctica measurements of ice thickness on the antarctic ice sheet collected during surveys undertaken over the past 50 years have been brought together into a single database from these data a seamless suite of digital topographic models have been compiled for antarctica and its surrounding ocean this includes grids of ice sheet thickness over the grounded ice sheet and ice shelves water column thickness beneath the floating ice shelves bed elevation beneath the grounded ice sheet and bathymetry to 60 s including the sub ice shelf cavities these grids are consistent with a recent high resolution surface elevation model of antarctica while the digital models have a nominal spatial resolution of 5 km such high resolution is justified by the original data density only over a few parts of the ice sheet the suite does however provide an unparalleled vision of the geosphere beneath the ice sheet and a more reliable basis for ice sheet modeling than earlier maps the total volume of the antarctic ice sheet calculated from the bedmap grid is 25 4 million km3 and the total sea level equivalent derived from the amount of ice contained within the grounded ice sheet is 57 m comprising 52 m from the east antarctic ice sheet and 5 m from the west antarctic ice sheet slightly less than earlier estimates the gridded data sets can be obtained from the authors

geoarchaeological and paleohydrological evidence for a clovis age drought in north america and its bearing on extinction abstract at the murray springs clovis site in southeastern arizona stratigraphic and geomorphic evidence indicates that an abnormally low water table 10 900 yr b p was followed soon thereafter by a water table rise accompanied by the deposition of an algal mat the black mat that buried mammoth tracks clovis artifacts and a well this water table fluctuation correlates with pluvial lake fluctuations in the great basin during and immediately following clovis occupation of that region many elements of pleistocene megafauna in north america became extinct during the dry period oxygen isotope records show a marked decrease in 18o correlated with the younger dryas cold dry event of northern europe which ended 10 750 yr b p essentially the same time as the water table began to rise in southeastern arizona clovis hunters may have found large game animals easier prey when concentrated at water holes and under stress if so both climate and human predation contributed to pleistocene extinction in america

mantle anisotropy beneath the earth s mid ocean ridges observations of seismic anisotropy at oceanic spreading centres offer insights into mid ocean ridge processes and the formation of new plates here remote observations of seismic anisotropy beneath mid ocean ridges are made using measurements of source side shear wave splitting over 100 high quality measurements are made using earthquakes that occur near mid ocean ridges and transform faults but are observed at teleseismic distances in general for off axis ridge events the polarisation of fast shear waves phi prime prime is approximately parallel to the spreading direction nearer the ridge lesssim 50 km phi prime prime becomes more scattered and is often ridge parallel delay times delta t tend to increase from 1 s near the ridge axis to sim 3 s further away slow spreading regions gakkel and southwest indian ridges show smaller amounts of splitting than faster spreading centres at transform zones the pattern is more complex coverage beneath the east pacific rise is especially good and we observe a systematic increase in delay times in s wave splitting measurements compared to previous sks splitting observations made at ocean bottom seismometers one compatible explanation is the presence of horizontally aligned connected layers of melt at depth with an inclusion density which increases near the top of the layer this is also compatible with other observations of the lab discontinuity and surface wave derived measurements of radial anisotropy

shear wave splitting in the mantle ps phases distribution with depth of mantle anisotropy is uncertain to better constrain this distribution we analyse records of the mantle ps phases at the grf array in germany two components of these phases sv and t are detected by stacking teleseismic records of events well distributed in azimuth and distance the detected t component signals are generated not so much by splitting of sv as by conversion from p to sh at the mantle discontinuities to reconcile these observations with previously known observations of splitting in sks at least three anisotropic layers are required the data favour a model with nne oriented fast velocity at the top of the mantle this is consistent with the results of other seismic studies beneath this layer the fast direction of anisotropy changes by around 90 our analysis suggests as well that a layer at the base of the mantle transition zone mtz is anisotropic this anisotropy might arise in the convective boundary layer between the upper and the lower mantle

lowermost mantle anisotropy beneath the northwestern pacific evidence from pcs scs sks and skks phases 1 in this study we utilize data from broadband seismic stations of the japanese f net network to investigate anisotropic structure in the lowermost mantle beneath the northwestern pacific the comparison of shear wave splitting from phases that have similar paths in the upper mantle but different paths in the lowermost mantle such as pcs scs or sks skks can yield constraints on anisotropy in the d region we measured splitting for sks skks pcs and scs phases at f net stations and compared the measured splitting parameters to previously published estimates of upper mantle anisotropy beneath the region we observed many examples of discrepant sks skks splitting and splitting of scs pcs sks and skks phases that does not agree well with the known upper mantle anisotropy beneath individual stations the most likely explanation for these discrepancies is a contribution to splitting from anisotropy in the lowermost mantle in particular for sks skks phases that sample the lowermost mantle just to the south and east of the kamchatka peninsula we observed generally n s fast directions with delay times between 0 5 and 1 5 s these data suggest the presence of a fairly large coherent region of deformation in the lowermost mantle beneath the northwestern pacific our preferred model for these observations is that solid state flow at the base of the mantle induces a lattice preferred orientation of lowermost mantle minerals generating a seismically anisotropic fabric

two anisotropic layers in central orogenic belt of north china craton abstract we present evidence of two layers of anisotropy in earth s upper mantle beneath the central orogenic belt of the north china craton from teleseismic shear wave splitting analysis a one layer model is in disagreement with the well constrained azimuth dependent splitting parameters for stations in the central orogenic belt however two layers of anisotropy improve the fit to the data at the bottom layer the fast polarization directions are subparallel to the absolute plate motion indicating the presence of the current strain within the subcratonic asthenosphere related to the interactions between the surrounding plates in contrast at the top layer the fast polarization directions are consistent with the strikes of the local surface faults confirming a combined contribution of the past orogenies in the lithosphere

seismic anisotropy in the lowermost mantle beneath north america from sks skks splitting intensity discrepancies abstract we examined sks skks splitting intensity discrepancies for phases that sample the lowermost mantle beneath north america which has previously been shown to exhibit seismic anisotropy using other analysis techniques we examined data from 25 long running seismic stations along with 244 stations of the temporary usarray transportable array located in the eastern southeastern and western u s we identified 279 high quality sks skks wave pairs that yielded well constrained splitting intensity measurements for both phases of the 279 pairs a relatively small number 15 exhibited discrepancies in splitting intensity of 0 4 s or greater suggesting a contribution to the splitting of one or both phases from anisotropy in the lowermost mantle because only a small minority of sk k s phases examined in this study show evidence of being affected by lowermost mantle anisotropy the traditional interpretation that splitting of these phases primarily reflects anisotropy in the upper mantle directly beneath the stations is appropriate the discrepant pairs exhibited a striking geographic trend sampling the lowermost mantle beneath the southern u s and northern mexico while other regions were dominated by non discrepant pairs we carried out ray theoretical modeling of simple anisotropy scenarios that have previously been suggested for the lowermost mantle beneath north america invoking the alignment of post perovskite due to flow induced by the impingement of the remnant farallon slab on the core mantle boundary we found that our measurements are generally consistent with this model and with the idea of slab driven flow but relatively small scale lateral variations in the strength and or geometry of lowermost mantle anisotropy beneath north america are also likely present

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

air blast produced by the meteor crater impact event and a reconstruction of the affected environment using scaling relationships determined from nuclear explosions the radial extent of the air blast produced by the meteor crater impact event is estimated the wind velocity at a distance of 5 crater radii 3 km from the point of impact should have exceeded 2000 km h hurricane force winds would have existed as far away as 20 to 40 km depending on the exact explosive energy of the impact event to determine how this event may have affected the environment surrounding the crater the topography vegetation and animal life that existed at the time of the impact are reconstructed for example if the coniferous woodlands were 100 m lower than they are presently and they had moved farther out onto the plains then the air blast would have flattened trees within a 16 to 22 km radius of the point of impact and damaged them over an area of 4100 to 8500 km2 the distance over which the damage occurred may have been up to 2 larger in some directions around the crater because of additional effects produced by the ballistic shock wave unfortunately since the trajectory of the projectile is not well known the direction of the ballistic shock wave effects cannot yet be determined

geological record and reconstruction of the late pliocene impact of the eltanin asteroid in the southern ocean in 1995 an expedition on board the research vessel fs polarstern explored the impact site of the eltanin asteroid in the southern ocean the only known asteroidimpact into a deep ocean basin analyses of the geological record of the impact region place the event in the late pliocene 2 15 myr and constrain thesize of the asteroid to be 1 km the explosive force inferred for this event places it at the threshold of impacts believed to have global consequences and its studyshould therefore provide a baseline for the reconstruction and modelling of similar events which are common on geological timescales

synthetic seismic anisotropy models within a slab impinging on the core mantle boundary the lowermost few hundreds of kilometres of the earth s mantle are elastically anisotropic seismic velocities vary with direction of propagation and polarization observations of strong seismic anisotropy correlate with regions where subducted slab material is expected in this study we evaluate the hypothesis that crystal preferred orientation cpo in a slab as it impinges on the core mantle boundary is the cause of the observed anisotropy next we determine if fast polarization directions seen by shear waves can be mapped to directions of geodynamic flow this approach is similar to our previous study performed for a 2 d geodynamic model in this study we employ a 3 d geodynamic model with temperature dependent viscosity and kinematic velocity boundary conditions defined at the surface of the earth to create a broad downwelling slab tracers track the deformation that we assume to be accommodated by dislocation creep we evaluate the models for the presence of perovskite or post perovskite and for different main slip systems along which dislocation creep may occur in post perovskite 100 010 and 001 resulting in four different mineralogical models of cpo combining the crystal pole orientations with single crystal elastic constants results in seismically distinguishable models of seismic anisotropy the models are evaluated against published seismic observations by analysing different anisotropic components the radial anisotropy the splitting for sub vertical phases i e azimuthal anisotropy and the splitting for subhorizontal phases the patterns in radial anisotropy confirm our earlier results in 2 d observations of radial anisotropy and splitting in subhorizontal phases are mostly consistent with our models of post perovskite with 010 slip and 001 slip our model of 001 slip predicts stronger splitting than for 010 slip for horizontally propagating phases in all directions the strongest seismic anisotropy in this model occurs where the slab impinges on the core mantle boundary the azimuthal anisotropy pattern for 001 slip shows fast axis directions at the edges of the slab sub parallel to flow directions suggesting horizontal flows may be mapped out in the lowermost mantle using seismic observations

a potential post perovskite province in d beneath the eastern pacific evidence from new analysis of discrepant sks skks shear wave splitting

antarctic peninsular cryosphere early oligocene c 30 ma initiation and a revised glacial chronology strontium isotope stratigraphy provides a chronology for cenozoic cryogenic strata in the northern antarctic peninsula and allows an assessment of diachronism in onset of glacial conditions between east and west antarctica the earliest observed event on the antarctic peninsula was late early oligocene 29 8 0 6 ma at least c 4 ma later than in east antarctica with a second in the early early miocene 22 6 0 4 ma both glacials and intervening interglacial were continent wide phenomena two late neogene glaciations are distinguished early late miocene 9 9 0 97 ma hobbs glacier jones mountains and latest miocene alexander island a further late neogene glacial deposit weddell formation can be constrained only as c 10 ma

lowermost mantle flow at the eastern edge of the african large low shear velocity province abstract observations of seismic anisotropy in the lowermost mantle are plentiful but their interpretation in terms of mantle flow remains challenging here we interrogate the anisotropic structure of the lowermost mantle beneath the afar region just outside the edge of the african large low shear velocity province using a combination of shear wave splitting techniques applied to phases propagating at five distinct raypath orientations we then model the resulting data set by testing various candidate mechanisms for anisotropy the observations are best fit by a model that invokes the lattice preferred orientation lpo of post perovskite with the 100 crystallographic axis oriented either nearly vertically or highly obliquely to the horizontal plane plausible corresponding mantle flow scenarios involve a significant vertical flow component which suggests that the african large low shear velocity province edge may deflect ambient mantle flow upwards or may be associated with a sheet like upwelling

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

scs wave splitting of deep earthquakes around japan summary scs wave splitting of five deep earthquakes in subduction zones near japan is investigated using horizontal seismograms recorded al jma stations for each earthquake we clearly observe uniform scs wave splitting in all stations over japan especially for the events located south of honshu in 1982 1984 and 1993 however the directions of fast polarized waves of these events differed by a maximum of about 50 from one another the orientation of fast polarized waves in the 1982 event was nnw sse those in the two later events wnw ese we also recognize this discrepancy in the results of the analysis of the 1971 sea of okhotsk event reported by fukao 1984 the sakhalin islands event in 1990 reveals a linear particle motion without such a change in direction of the second arrivals implying no anisotropy these observations are interpreted as indicating an anisotropic region within the slab near the earthquake sources but not beneath the receivers since the orientations of fast polarized waves recorded at each station are not common to all the earthquakes furthermore we consider that anisotropy exists non uniformly within the slab the event in 1982 which occurred in almost the same area as those in 1984 and 1993 showed a fast direction different from the events in 1984 and 1993 the 1982 event was 179 km deep but the two later events were at 398 km and 360 km respectively the fast direction observed from the 1982 event is parallel to the fossil plate motion whereas those from the events in 1984 and 1993 are parallel to the compression axis within the subducting slab the depth of 400 km is a phase boundary where olivine changes to spinel we consider that the most likely cause of the change in anisotropy direction is the re orientation of crystals associated with the phase change of olivine to spinel due to subduction of the slab

a review of quaternary proboscideans from alberta canada here we present a summary of the largest museum collection of quaternary proboscideans known from alberta housed at the royal alberta museum the 378 specimens reported here come from localities across the province highlighting the broad distribution of proboscideans in southwestern canada during the late pleistocene despite the isolated nature of most individual specimen records the cumulative record provides a broad sense of the chronologic distribution of late pleistocene proboscideans in a large portion of southwestern canada new and summary radiocarbon data indicate that the sparse record of pre lgm mastodon mammut americanum is limited to 40 700 3000 yr bp suggesting a pattern of pre lgm extirpation previously observed at more northern latitudes mammoths mammuthus spp occur in greater numbers and have a more lengthy late pleistocene record characterized by a temporal gap related to the last glacial maximum lgm

anisotropy in subduction zones insights from new source side s wave splitting measurements from india this study presents 106 splitting and 40 null measurements of source side anisotropy in subduction zones utilizing direct s waves registered at two stations sited on the indian continent which show null shear wave splitting measurements for sks phases our results suggest that trench parallel anisotropy is dominant beneath the philippines mariana izu bonin and edge of the java slab while plate motion parallel anisotropy is observed beneath the solomon aegean japan and java slabs results from kuril and aleutian regions reveal trench oblique anisotropy we chose to interpret these observations primarily in terms of mantle flow beneath a subduction zone while the two dimensional 2d slab entrained flow model offers a simple explanation for trench normal fast polarization azimuths fpa the trench parallel fpa can be reconciled by extension due to slab rollback the model that invokes age of the subducting lithosphere can explain anisotropy in the sub slab derived from rays recorded at the updip stations however when downdip stations are used contributions from the slab and supra slab need to be considered in japan anisotropy in the sub slab mantle shallower than 300 km might be associated with trench parallel mantle flow resulting in the alignment of fpa in the same direction anisotropy in the deeper part above the transition zone is probably associated with 2d flow resulting in trench normal fpa anisotropy in the mariana trench might be associated with trench parallel mantle flow in the supra slab region with similar deformation in the upper mantle and the transition zone

constraints on upper mantle anisotropy surrounding the cocos slab from sk k s splitting 1 sks and skks splitting observations are used to constrain the pattern of mantle flow in the central american subduction zone beneath costa rica and nicaragua after removing the effects of shallow wedge anisotropy on sk k s waveforms a best fitting model of anisotropy beneath the cocos plate and in the deeper mantle wedge is determined fast polarization directions and model symmetry axis orientations in both regions as well as the shallow wedge are dominated by roughly arc parallel azimuths and therefore are not consistent with sublithospheric mantle flow entrained by the subducting cocos plate or simple two dimensional corner flow in the wedge in conjunction with geochemical data and local s splitting tomography the sk k s splitting observations and anisotropy models are consistent with flow to the wnw within the mantle wedge on the caribbean side of the cocos plate possibly drawn through a slab window beneath panama and southern costa rica anisotropy in the pacific mantle beneath the cocos plate is also best explained by flow with a component that is roughly parallel to the strike of the slab although the absolute direction of this flow is not uniquely constrained

how to make robust splitting measurements for single station analysis and three dimensional imaging of seismic anisotropy summary we quantify errors on splitting intensity measurements produced by incoherent noise on horizontal components through a detailed analysis of synthetic seismograms and of sks and skks waves recorded by four permanent broad band stations we find that these errors are quite significant even on high quality records because the measurement procedure involves the comparison of radial and transverse components which are both contaminated by noise to decrease the level of noise in the data it is thus recommended to average splitting intensities from waves coming from the same backazimuth alternatively one can analyse stacked radial and transverse components after applying a wiener filter which standardizes the waveforms this is found to be equivalent because the measure of splitting intensity is a linear process the utilization of wiener filters reduces variations of apparent splitting resulting from differences in frequency content of the waveforms it is possible to determine splitting parameters splitting delays and fast directions from azimuthal variations of splitting intensity at a particular station tests on synthetic seismograms demonstrate that this approach gives unbiased and robust estimates of splitting parameters in contrast to the silver and chan method which only works on the rare records that have very large signals on their transverse components the wolfe and silver method does not suffer from this limitation and gives the same results as the multichannel splitting intensity method analysis of splitting intensities at stations bdfb bgca can and erm gives fast directions that are generally in excellent agreement with those determined with the wolfe and silver method however at some stations azimuthal variations of splitting intensities reveal features that are not well captured by the sinusoidal variation representing the average anisotropy beneath the station this suggests that smallscale lateral variations of anisotropy are present in the subcontinental mantle which remain to be imaged in detail

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

frequency dependent shear wave splitting and heterogeneous anisotropic structure beneath the gulf of california region abstract the gulf of california region has undergone a major evolution over the past 15 ma since subduction of the now extinct farallon plate ceased and transform motion began leading to the initiation of rifting and the opening of the gulf of california 5 ma ago the character of dynamic processes in the upper mantle beneath the region remains poorly understood but constraints on seismic anisotropy in the upper mantle can shed light on contemporary mantle flow processes in order to characterize more fully the anisotropic structure of the upper mantle beneath the region i present measurements of sks splitting at 14 broadband stations of the nars baja array at periods between 8 and 50 s the measured splitting parameters t at nars baja stations exhibit a high degree of complexity with dramatic lateral variations significant backazimuthal variations at individual stations and a large number of well constrained null measurements over a range of backazimuths the dependence of splitting parameters on frequency is evaluated by applying a series of bandpass filters to high signal to noise sks arrivals i find that while fast directions are generally insensitive to the sks frequency content delay times are frequency dependent at several stations the measurements presented in this study indicate that the anisotropic structure of the upper mantle beneath the gulf of california region is complex and exhibits a high degree of both lateral and vertical heterogeneity the observed splitting is consistent with a scenario in which a large scale reorganization of the mantle flow field has been inhibited by the presence of a stalled fragment of oceanic lithosphere associated with farallon subduction the pattern of mantle flow appears to be controlled by more localized processes likely including small scale buoyant upwelling associated with partial melting beneath the central part of the gulf of california rift