seismic imaging of subduction zone metamorphism combined analysis of high resolution seismic images of the alaska and cascadia subduction zones reveals where metamorphic fl uids are released both images show the subducted oceanic crust as a dipping low velocity layer with a clear termination depth however in alaska the crust is thicker 15 20 km compared to 8 km and terminates at greater depth 120 km com pared to 40 km than in cascadia based on metamorphic reaction estimates and geodynamic models we demonstrate that the termination depth corresponds to eclogitization of the crust triggered by dehydration of water bearing minerals and that the location of this reaction is dependent on the thermal structure of the subducted slab

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

model for the origin of the yakutat block an accreting terrane in the northern gulf of alaska a composite oceanic and continental terrane the yakutat block is currently colliding with and accreting to north america in the northern gulf of alaska marine geophysical data indicate that the block has moved with the pacific plate during pliocene and quaternary time but are indeterminant about its previous movement history a model that explains some observed geologic and structural features of the block is that it originated as a result of subduction of the kula farallon spreading center beneath north america in this model the spreading center that formed the basaltic basement of the yakutat block was offset from a spreading center to the south by a left lateral transform fault 400 500 km long subduction of the northern spreading center about 45 m y ago sliced off part of the north america continental margin attached it to the kula plate and initiated northward movement of the composite terrane as the yakutat block during eocene and oligocene time migration of the southern spreading center toward north america truncated the yakutat block on the south and left a fossil fracture zone that is now seen as the transition fault the southern spreading center subducted about 25 m y ago and the yakutat block has since traveled with the pacific plate to southern alaska

b type olivine fabric in the mantle wedge insights from high resolution non newtonian subduction zone models several hypotheses have been proposed to explain trench parallel shear wave splitting in the mantle wedge of subduction zones these include 3 d flow effects parallel melt filled cracks and b type olivine fabric we predict the distribution of btype and other fabrics with high resolution thermal and stress models of subduction zones a composite viscous rheology is used that incorporates wet diffusion creep wet dislocation creep and a stress limiting approximated peierls creep law rheological parameters for dislocation and diffusion creep are based on deformation experiments the effects of variable viscous coupling between the slab and mantle are explored using kinematic boundary conditions that change along the top of the slab two end member models are observed which depend on the depth of partial viscous coupling between the slab and mantle 1 deep partial coupling which gives rise to cold high stress conditions in the forearc mantle and high temperature low stress conditions in the arc and back arc mantle 2 full viscous coupling at the base of a 40 km conducting lid which is characterized by high temperature and low stress the case with deep partial coupling which produces a better match with attenuation tomography and heat flow shows a large region with suitable conditions for b type fabric in the forearc mantle and a rapid transition toward the back arc to conditions that are more suitable for a c or e type fabrics full viscous coupling gives rise to high temperature low stress conditions unsuitable for b type fabric our modeling predicts that the best candidates for regions with b type fabric are the forearc mantle and a cold 10 15 km layer above the slab d 2005 elsevier b v all rights reserved

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

surface motions and intraplate continental deformation in alaska driven by mantle flow the degree to which the lithosphere and mantle are coupled and contribute to surface deformation beneath continental regions remains a fundamental question in the field of geodynamics here we use a new approach with a surface deformation field constrained by gps geologic and seismicity data together with a lithospheric geodynamic model to solve for tractions inferred to be generated by mantle convection that 1 drive extension within interior alaska generating southward directed surface motions toward the southern convergent plate boundary 2 result in accommodation of the relative motions between the pacific and north america in a comparatively small zone near the plate boundary and 3 generate the observed convergence within the north american plate interior in the mackenzie mountains in northwestern canada the evidence for deeper mantle influence on surface deformation beneath a continental region suggests that this mechanism may be an important contributing driver to continental plate assemblage and breakup

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

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

seismicity of the wrangell and aleutian wadati benioff zones and the north american plate along the trans alaska crustal transect chugach mountains and copper river basin southern alaska the configuration of the aleutian and wrangell wadati benioff zones in southern alaska between longitudes 142 w and 148 w is determined from regionally recorded earthquakes from 1971 to 1986 the earthquakes are located with a simple velocity model calibrated with seismic refraction measurements conducted for the trans alaska crustal transect tact west of the transect meridian approximately 145 20 w and beneath the chugach mountains shocks in the depth range 25 60 km compose the shallow part of the northwest to north dipping aleutian zone this zone can be traced updip and seaward beneath prince william sound east of the transect meridian and north of 61 n latitude the weakly active north northeast dipping wrangell zone extends to a depth of at least 100 km beneath the wrangell volcanoes and along strike for at least 150 km the two zones appear to be continuous at least in the depth range 20 45 km and may define adjacent limbs of a buckle in the subducted oceanic lithosphere north of 61 n focal mechanisms in the shallow aleutian zone are compatible with a uniform stress field characterized by in plane least compressive stress directed toward the steeply dipping deeper part of the aleutian zone north of cook inlet stress orientation in the wrangell zone is unresolved because of the low level of seismicity no focal mechanisms corresponding to slip on a gently northward dipping megathrust have been observed along the transect meridian a change of focal mechanism with depth near the top of the dipping seismic zone is inferred to reflect the interplate boundary beneath the chugach mountains this boundary coincides with a band of strong reflectors imaged by tact vertical seismic profiling the overriding plate appears to be aseismic beneath the northern chugach mountains but minor shallow seismicity occurs beneath the copper river basin first motion patterns for these shallow shocks suggest the most compressive stress within the overriding plate is oriented subhorizontally and approximately north south

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

new evidence for segmentation of the alaska subduction zone the purpose of this article is to provide additional evidence for segmentation of the subducted plate in alaska and to introduce a catalog of the relocated earthquakes for future studies we used the joint hypocenter determination method to relocate 14 099 subduction zone earthquakes that occurred from july 1988 to july 1998 and were located between 58 n and 65 n latitude the earthquake data were taken from the alaska earthquake information center catalog the selected earthquakes were divided into 16 blocks on the basis of their hypocentral locations and each block was relocated separately average epicenter shift was 3 8 km and average upward and downward depth shifts were 4 1 and 4 4 km respectively roughly the same number of earthquakes shifted upward 47 and downward 53 the overall change with respect to the initial locations is that the seismicity became more compact revealing details about the fine structure of the wadati benioff zone in particular we were able to identify more precisely the boundary between the kenai and mckinley blocks of the subducting plate in addition there is evidence for plate segmentation within the mckinley block manuscript received 15 november 2000

eocene basalts from the yakutat terrane evidence for the origin of an accreting terrane in southern alaska basalts from the yakutat terrane a composite oceanic and continental tectonostratigraphic terrane are chemically diverse large ion lithophile element lile depleted and lile enriched tholeiites interpreted as normal mid ocean ridge and oceanic island basalt that originated on seamounts near the kula farallon spreading center during the early to middle eocene coeval and geochemically similar basalts occurring sporadically in a linear belt from southern vancouver island to the southern oregon coast range are correlative with the yakutat terrane basalts both basaltic sequences were accreted to the continental margin during subduction of the kula farallon ridge and kula plate about 48 ma the yakutat basalts were emplaced along the coast of washington or british columbia the yakutat terrane probably originated during the neogene as a composite terrane when it was sliced off the continental margin and started to move northward along the queen charlotte fairweather transform fault toward its present location in southern alaska

estimating shear wave splitting parameters from broadband recordings in japan a comparison of three methods the goal of this study was to evaluate the performance of different splitting measurement techniques in the particularly complicated tectonic setting of subduction beneath japan we use data from the broadband japanese f net array and consider the methods of silver and chan 1991 levin et al 1999 and chevrot 2000 we find that the results generally agree well although discrepancies arise if the anisotropy beneath the station is more complex than the simple single layer an isotropic model often assumed in splitting studies a combination of multichannel and single record methods may serve as a powerful tool for recognizing complexities and for characterizing upper mantle anisotropy beneath a station

shear wave anisotropy beneath nicaragua and costa rica implications for flow in the mantle wedge 1 we present new shear wave splitting data from local events in costa rica and nicaragua recorded by the temporary july 2004 to march 2006 48 station tucan broadband seismic array observed fast polarization directions in the fore arc arc and back arc range from arc parallel to arc normal over very short distances 5 km when plotted at raypath midpoints making the direct interpretation of individual splitting measurements in terms of flow tenuous even when considering variations in the relationship between lattice preferred orientation and deformation e g b type dislocation creep in olivine therefore we tomographically invert the splitting measurements to find a three dimensional model of crystallographic orientation in the wedge we assume the elastic constants of olivine and orthopyroxene with hexagonal symmetry and use a damped iterative least squares approach to account for the nonlinear behavior of splitting when considering three dimensional ray propagation and distributions of anisotropy the best fitting model contains roughly horizontal arc parallel olivine 100 axes in the mantle wedge down to at least 125 km beneath the back arc and arc which we interpret to indicate along arc flow in the mantle wedge pb and nd isotopic ratios in arc lavas provide additional evidence for arc parallel flow and also constrain the direction northwest from costa rica to nicaragua and minimum flow rate 63 190 mm a with only slightly oblique subduction at 85 mm a of the relatively planar cocos plate the most likely mechanism for driving along arc transport is toroidal flow around the edge of the slab in southern costa rica generated by greater slab rollback in nicaragua two important implications of this arc parallel flow are the progressive depletion of the mantle source for arc lavas from costa rica to nicaragua and the possible need for significant decoupling between the wedge and downgoing plate

crustal thickness variation in south central alaska crustal thicknesses have been determined by receiver function analysis of broadband teleseismic waveforms recorded during the broadband experiment across the alaska range beaar typical crust beneath the northern lowlands is 26 km thick while beneath the mountains it is 35 45 km thick the transition from thick to thin crust coincides with the location of the hines creek fault a major tectonostratigraphic boundary crustal thicknesses determined by receiver functions agree with those predicted from topography assuming airy type isostasy suggesting that the alaska range is compensated by its crustal root north of the range however the crust is systematically thinner than predicted by simple airy isostasy a crustal density contrast of 4 6 across the hines creek fault 2700 kg m 3 to the north and 2830 kg m 3 to the south explains the observed difference between the crustal thicknesses predicted by simple airy isostasy and the crustal thicknesses determined by receiver function analysis

development of mantle seismic anisotropy during subduction induced 3 d flow 1 the dynamics of subduction can be indirectly constrained by studying the induced mantle flow however inferring the circulation of the mantle around subducting plates from the interpretation of shear wave splitting patterns remains elusive we calculated the strain induced lattice preferred orientation lpo developed in 3 d models of subduction where retreat motions are maximized and found that in the mantle layer entrained with the downgoing slab the seismic anisotropy is trench perpendicular and becomes trench parallel deeper where the toroidal flow accommodates slab retreat synthetic sks splitting shows that in the fore arc slab rollback favors trench parallel polarization of the fast shear wave component while plate advance enhances trench perpendicular seismic anisotropy the interference between these two competing mechanisms yields subslab delay times of 0 5 to 1 3 sec comparable with those observed at most natural subduction zones the magnitude of the subslab trench parallel splitting is independent of the rate at which the slab migrates instead it is proportional to the amount of retreat

subduction geometry beneath south central alaska and its relationship to volcanism the southern alaskan margin captures a transition between compression and strike slip dominated deformation accretion of the over thickened yakutat terrane termination of aleutian arc magmatism and the enigmatic wrangell volcanic field the extent of subduction and mantle structure below this region is uncertain with important implications for volcanism we present compressional and shear wave mantle velocity models below south central alaska that leverage a new seismometer deployment to produce the most complete image of the subducting pacific yakutat plate to date we image a steeply dipping slab extending below central alaska to 400 km depth which abruptly terminates east of 145 w there is no significant slab anomaly beneath the nearby wrangell volcanoes a paucity of volcanism is observed above the subducting yakutat terrane but the slab structure below 150 km depth and wadati benioff zone here are similar to those along the aleutian alaska arc features of the mantle wedge or overlying lithosphere are thus responsible for the volcanic gap

impact of aseismic transients on the estimation of aftershock productivity parameters abstract the epidemic type aftershock sequence etas model has been shown to describe successfully the statistical seismicity properties if earthquake triggering is related to tectonic forcing and earthquake induced stress changes however seismicity is locally often dominated by stress changes related to transient aseismic processes to avoid erroneous parameter estimations leading to biased forecasts it is important to account for those transients we apply a recently developed iterative algorithm based on the etas model to identify the time dependent background and etas parameters simultaneously we find that this procedure works well for synthetic data sets if catalog errors are appropriately considered however ignoring the time dependence leads to significantly biased parameter estimations in particular the value describing the magnitude dependence of the triggering kernel can be strongly underestimated if transients are ignored low values have been previously found for swarm activity for which transient aseismic processes are expected to play a major role these observed anomalously low values might thus indicate the importance of transient forcing rather than being due to differences in the earthquake earthquake trigger mechanism to explore this we apply the procedure systematically to earthquake clusters detected in southern california and to earthquake swarm activity in vogtland western bohemia while low values are mostly shown to be a consequence of catalog errors and time dependent forcing but not related to different earthquake earthquake interaction mechanisms some significant low values are observed in high heat flow areas in california confirming the existence of thermal control on earthquake triggering

subduction of oceanic asthenosphere evidence from sub slab seismic anisotropy 1 the oceanic asthenosphere is characterized as a low viscosity channel down to 200 300 km depth separating the cold lithosphere from above and it is intimately linked to a layer of low seismic velocity and prominent seismic anisotropy observed globally beneath ocean basins while subduction of tectonic plates in convergent margins is well recognized the fate of oceanic asthenosphere remains enigmatic we demonstrate that subduction of the oceanic asthenosphere characterized by weak azimuthal anisotropy and strong radial anisotropy explains the essence of sub slab shear wave splitting patterns where the fast splitting direction changes from predominantly trench parallel or sub parallel under relatively steep subduction zones to frequently trench normal under shallow subduction zones to explain the observed splitting time the thickness of the subducted asthenosphere is estimated to be 100 50 km

an olivine fabric database an overview of upper mantle fabrics and seismic anisotropy we present a unique database of 110 olivine petrofabrics and their calculated seismic properties the samples come from a variety of the upper mantle geodynamic environments ophiolites subduction zones and kimberlites with a wide range of micro structures a phenomenological relationship is established between p and s wave seismic anisotropy and the degree of crystal alignment fabric strength seismic anisotropy increases rapidly at low fabric strength before reaching a near saturation level of 15 to 20 for p waves and 10 to 15 for s waves despite a large variation in the symmetry of fabric patterns the average seismic properties of the different fabric micro structural and geodynamic settings have similar anisotropies in both magnitude and symmetry hence it would seem possible to determine some measure of fabric strength from seismic anisotropy if the dimensions of the anisotropic region are known but not geodynamic environment or details of the petrofabric pattern a simple pattern of seismic anisotropy characterises the average sample of the database which has the following features the polarisation plane of the fastest s wave is parallel or sub parallel to the foliation plane the maximum shear wave splitting is parallel to the y structural direction in foliation plane and normal to the lineation the maximum of the p wave velocity is parallel to the high concentration of 100 axes which is sub parallel to the lineation the 100 orientation distribution has the greatest influence on the p wave seismic anisotropy the 100 and 001 orientation distributions have the greatest influence on the symmetry of s wave anisotropy although the magnitude of anisotropy is influenced by the distribution of all three principal axes although the database only contains olivine petrofabrics this statistical study clearly shows that seismic anisotropy can be used to deduce the orientation of the structural frame in the upper mantle 1998 elsevier science b v all rights reserved

extending alaska s plate boundary tectonic tremor generated by yakutat subduction the tectonics of the eastern end of the alaska aleutian subduction zone are complicated by the inclusion of the yakutat microplate which is colliding into and subducting beneath continental north america at near pacific plate rates the interaction among these plates at depth is not well understood and further east even less is known about the plate boundary or the source of wrangell volcanism the drop off in wadati benioff zone wbz seismicity could signal the end of the plate boundary the start of aseismic subduction or a tear in the downgoing plate further compounding the issue is the possible presence of the wrangell slab which is faintly outlined by an anemic eastward dipping wbz beneath the wrangell volcanoes in this study i performed a search for tectonic tremor to map slow plate boundary slip in south central alaska i identified 11 000 tremor epicenters which continue 85 km east of the inferred pacific plate edge marked by wbz seismicity the tremor zone coincides with the edges of the downgoing yakutat terrane and tremors transition from periodic to continuous behavior as they near the aseismic wrangell slab i interpret tremor to mark slow semicontinuous slip occurring at the interface between the yakutat and north america plates the slow slip region lengthens the megathrust interface beyond the wbz and may provide evidence for a connection between the yakutat slab and the aseismic wrangell slab

reconciling surface plate motions with rapid three dimensional mantle flow around a slab edge a central axiom of the theory of plate tectonics is that the surface plates are coupled to and carried along by convection in the earth s mantle the situation is rather different at subduction zones however margarete jadamec and magali billen now present calculations that quantify this variance they use three dimensional numerical models of buoyancy driven deformation with realistic slab geometry for the alaska subduction transform system to determine the surrounding mantle flow field they show that near a subduction zone edge mantle flow velocities can have magnitudes of greater than ten times the surface plate motions and that the complex mantle flow field is consistent with observations from seismic anisotropy these results imply local rapid transport of geochemical signatures through subduction zones the direction of tectonic plate motion at the earth s surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally implying large scale coupling between the mantle and surface plates but the fit is poor at subduction zones here three dimensional numerical models of buoyancy driven deformation for the alaska subduction transform system are used to find the origin of the regional decoupling of flow the results imply local rapid transport of geochemical signatures through subduction zones the direction of tectonic plate motion at the earth s surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally suggesting large scale coupling between the mantle and the surface plates1 2 the fit is typically poor at subduction zones however where regional observations of seismic anisotropy suggest that the direction of mantle flow is not parallel to3 4 5 6 7 and may be several times faster than6 plate motions here we present three dimensional numerical models of buoyancy driven deformation with realistic slab geometry for the alaska subduction transform system and use them to determine the origin of this regional decoupling of flow we find that near a subduction zone edge mantle flow velocities can have magnitudes of more than ten times the surface plate motions whereas surface plate velocities are consistent with plate motions8 and the complex mantle flow field is consistent with observations from seismic anisotropy5 the seismic anisotropy observations constrain the shape of the eastern slab edge and require non newtonian mantle rheology the incorporation of the non newtonian viscosity9 10 results in mantle viscosities of 1017 to 1018 pa s in regions of high strain rate 10 12 s 1 and this low viscosity enables the mantle flow field to decouple partially from the motion of the surface plates these results imply local rapid transport of geochemical signatures through subduction zones and that the internal deformation of slabs decreases the slab pull force available to drive subducting plates

anisotropy and flow in pacific subduction zone back arcs we have obtained constraints on the strength and orientation of anisotropy in the mantle beneath the tonga southern kuril japan and izu bonin subduction zones using shear wave splitting in s phases from local earthquakes and in teleseismic core phases such as sks the observed splitting in all four subduction zones is consistent with a model in which the lower transition zone 520 660 km and lower mantle are isotropic and in which significant anisotropy occurs in the back arc upper mantle the upper transition zone 410 520 km beneath the southern kurils appears to contain weak anisotropy the observed fast directions indicate that the geometry of back arc strain in the upper mantle varies systematically across the western pacific rim beneath izu bonin and tonga fast directions are aligned with the azimuth of subducting pacific plate motion and are parallel or sub parallel to overriding plate extension however fast directions beneath the japan sea western honshu and sakhalin island are highly oblique to subducting plate motion and parallel to present or past overriding plate shearing models of back arc mantle flow that are driven by viscous coupling to local plate motions can reproduce the splitting observed in tonga and izu bonin but further three dimensional flow modeling is required to ascertain whether viscous plate coupling can explain the splitting observed in the southern kurils and japan the fast directions in the southern kurils and japan may require strain in the back arc mantle that is driven by regional or global patterns of mantle flow

seismic anisotropy under central alaska from sks splitting observations 1 seismic anisotropy under central alaska is studied using shear wave splitting observations of sks waves recorded on the broadband experiment across the alaska range beaar 1999 2001 splitting results can be divided into two distinct regions separated by the 70 km contour of the subducting pacific plate waves that travel through the thicker mantle wedge show fast directions that are parallel to the strike of the slab these slab parallel directions appear to indicate along strike flow in the mantle wedge and splitting delay times increase with path length in the mantle wedge suggesting anisotropy of 7 9 0 9 the region of along strike flow corresponds to high seismic attenuation and hence high temperatures along strike flow here may be driven by secular shallowing of the slab driven by subduction of buoyant yakutat terrane crust or by toroidal flow around the east end of the aleutian slab waves traveling southeast of the 70 km contour sample the pacific plate and the nose of the mantle wedge they show fast directions that parallel the direction of plate motion these fast directions are most likely due to flow under the subducting pacific plate and or anisotropy within the subducting pacific lithosphere the high splitting delay times 0 8 1 7 s associated with these convergence parallel directions cannot be produced in the mantle wedge which is 10 30 km thick here thus anisotropy shows a sharp 90 change in fabric associated with the onset of high temperature wedge flow

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 beneath the shumagin islands segment of the aleutian alaska subduction zone lithospheric strain and mantle flow in subduction zone back arcs reflect the fundamental dynamics of lithospheric subduction in this study we inferred possible strain geometries in the mantle beneath the eastern aleutians by mapping seismic anisotropy with shear wave splitting we analyzed s phases of local events recorded by the shumagin seismic network and obtained splitting parameters fast direction and splitting time t and their 95 confidence regions to avoid complexity from converted phases data were restricted to paths at incidence angles less than 32 and to events deeper than 50 km the s phases sample the forearc and back arc of the shumagin segment from 159 7 w to 163 8 w at depths up to 216 km in our data set of 84 local events 44 of 127 s phases yielded constrained splitting parameters with consistent fast directions that are approximately parallel to the volcanic arc roughly 60 the remaining 83 s phases produced null measurements and the fast directions inferred from these phase polarizations agree with the fast axis alignment in the constrained measurements splitting times vary from 0 1 to 0 35 s and show an overall increase with hypocentral depth observed splitting parameters are well matched by predicted values for olivine rich mantle wedge models with 1 sv anisotropy where the olivine b axis is arc orthogonal and the a axis is vertical no arc parallel strain or arcparallel no vertical strain these results are consistent with mantle wedge strain models in which arc normal compression is accompanied by arc parallel or vertical shearing or extension

olivine fabric transitions and shear wave anisotropy in the ryukyu subduction system recent shear wave splitting measurements from the fore arc region of the ryukyu subduction system show large magnitude 0 3 1 6 s trenchparallel splitting in both local and teleseismic phases the similarity of splitting parameters associated with shallow local s and teleseismic phases suggests that the source of anisotropy is located in the fore arc mantle one explanation for this pattern of shear wave splitting involves a transition from commonly observed high temperature olivine fabrics with flow parallel seismically fast directions to a flow normal b type olivine fabric in the cold fore arc mantle of the ryukyu wedge we test the b type fabric hypothesis by comparing observed splitting parameters to those predicted from geodynamic models that incorporate olivine fabric development the distribution of olivine fabric is calculated with high resolution thermomechanical models of the ryukyu subduction zone that include realistic slab geometry and an experimentally based wet olivine rheology we conclude that b type fabric can explain the magnitude and trench parallel orientation of deep local s phases that sample the core of the forearc mantle however our calculations show that b type fabric alone cannot account for large magnitude trench parallel splitting associated with teleseismic phases that sample the shallow tip of the fore arc mantle alternative models for trench parallel teleseismic splitting in the shallow tip of the fore arc mantle involve the addition of crustal or slab anisotropy and highly anisotropic foliated antigorite serpentinite 2008 elsevier b v all rights reserved

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

shear wave splitting from local events beneath the ryukyu arc trench parallel anisotropy in the mantle wedge we present shear wave splitting measurements from local slab earthquakes at eight seismic stations of the japanese f net array located in the ryukyu arc we obtained high quality splitting measurements for 70 event station pairs and found that the majority of the measured fast directions were parallel to the strike of the trench and perpendicular to the convergence direction splitting times for individual measurements ranged from 0 25 to 2 s most values were between 0 75 and 1 25 s both the fast directions and the split times were similar to results for teleseismic s k ks and s wave splitting at the same stations which suggests that the anisotropy is located in the mantle wedge above the slab we considered several mantle deformation scenarios that would result in predominantly trench parallel fast directions and concluded that for the ryukyu subduction system the most likely explanation for the observations is corner flow in the mantle wedge combined with b type olivine fabric in this model the flow direction in the wedge is perpendicular to the trench but the fast axes of olivine crystals tend to align perpendicular to the flow direction resulting in trench parallel shear wave splitting

tidal energy fluxes and dissipation on the european continental shelf the spatial distribution of the energy flux energy dissipation current magnitude and surface elevation amplitude of the major tidal constituents over the northwest european shelf are examined in detail these distributions are obtained from the harmonic analysis of a 6 month three dimensional simulation of 28 tidal constituents the model currents are validated by a comparison of computed tidal harmonics against those derived from a harmonic analysis of up to 278 current time series a similar comparison is performed for tidal elevations based on 257 tide gauges calculations show that for the most significant tidal constituents e g m2 s2 and k2 there is a major energy flux in the deep water along the shelf edge off the northwest of scotland with some energy leaking onto the shelf and into the north sea a second source of energy is across the shelf edge at the southern end of the celtic sea with this energy flux propagating into the irish sea and southern north sea where the majority of the energy is dissipation in the shallow regions significantly different distributions are found for the diurnal and shallow water constituents and the spatial distributions of energy flux and dissipation of the various constituents are considered the accuracy of separating tidal current harmonics using model data of less than a synodic period is examined with reference to the s2 and k2 tide calculations suggest that the accuracy of computed currents comparable to those obtained from observations can be obtained from an analysis of a 60 day period compared with the synodic period for s2 and k2 of 182 days a significant saving in computer time

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

mapping the megathrust beneath the northern gulf of alaska using wide angle seismic data in the northern gulf of alaska and prince william sound we have used wide angle seismic reflection refraction profiles earthquake studies and laboratory measurements of physical properties to determine the geometry of the prince william and yakutat terranes the aleutian megathrust and the subducting pacific plate in this complex region the yakutat terrane is underthrust beneath the prince william terrane and both terranes are interpreted to be underlain by the pacific plate wide angle seismic reflection refraction profiles recorded along five seismic lines are used to unravel this terrane geometry modeled velocities in the upper crust of the prince william terrane to 18 km depth agree closely with laboratory velocity measurements of orca group phyllites and quartzofeldspathic graywackes the chief components of the prince william terrane to hydrostatic pressures as high as 600 mpa 6 kbar a landward dipping reflector at depths of 16 24 km is interpreted as the base of the prince william terrane this reflector corresponds to the top of the wadati benioff zone seismicity and is interpreted as the megathrust immediately beneath the megathrust is a 4 km thick 6 9 km s refractor which we infer to be the source of a prominent magnetic anomaly and which is interpreted by us and previous workers to be gabbro in eocene age oceanic crust of the underthrust yakutat terrane wide angle seismic data magnetic anomaly data and tectonic reconstructions indicate that the yakutat terrane has been underthrust beneath the prince william terrane for at least a few hundred kilometers wide angle seismic data are consistent with a 9 to 10 landward dip of the subducting pacific plate beneath the outer shelf and slope distinctly different from the inferred average 3 to 4 dip of the overlying 6 9 km s refractor and the wadati benioff seismic zone beneath the inner shelf our preferred interpretation of the geophysical data is that one composite plate composed of the pacific plate of a fairly uniform thickness and the yakutat plate of varying thickness is subducting beneath southern alaska

shear wave birefringence in wedge shaped anisotropic regions summary a popular method for studies of seismic wave speed anisotropy within the earth is based on observations of birefringence in subvertically propagating shear waves a well recognized shortcoming of the method is that individual birefringence measurements commonly interpreted as a path average of anisotropy do not reveal the potential complexity of anisotropic structure consequently there is a necessary ambiguity in the interpretation of observed birefringence parameters fast polarization directions and delays between components in this paper we explore the influence of observational geometry and the configuration of an anisotropic region on the birefringence parameters we use full wavefield simulations in 3 d models to access these influences in a subduction zone setting our primary findings are that a modest wavefield complications associated with the non planar geometry of a subduction zone can significantly bias the determination of the fast polarization and time delay associated with shear wave birefringence b shear wave splitting associated with flow in the mantle wedge near the trench e g the forearc is likely to be weak owing to short path lengths through an anisotropic region and c popular splitting estimators are vulnerable to waveform distortions caused by s wave interaction with the dipping slab interface we show that simultaneous interpretation of numerous birefringence observations from a variety of propagation directions is a more reliable way to constrain parameters of anisotropy at depth

origin of localized fast mantle flow velocity in numerical models of subduction 1 the origin of localized fast mantle flow near subduction zones found in regional jadamec and billen 2010 2012 and global stadler et al 2010 alisic et al 2010 numerical models using non newtonian rheology is explored using 3d models with simple geometry two suites of models using both a free slab and a slab attached to a lithospheric layer are presented to determine 1 the origin of high magnitudes of mantle flow 50 cm yr and 2 focusing of flow within the mantle wedge with mantle wedge velocities up to two times higher than slab sinking velocities the rheology in the models is either newtonian with a prescribed low viscosity wedge lvw or non newtonian wherein low viscosity regions form in response to high stress surrounding the sinking slab the results show that there are two aspects of the models that lead to localized fast velocities reduction in the mantle viscosity surrounding the sinking slab leads to faster overall flow rates induced by the negative buoyancy of the sinking slab and geometric effects that are caused by lateral variations in viscosity including retrograde pivoting motion of the slab

seismic anisotropy in the hellenic subduction zone effects of slab segmentation and subslab mantle flow abstract the segmentation and differentiation of subducting slabs have considerable effects on mantle convection and tectonics the hellenic subduction zone is a complex convergent margin with strong curvature and fast slab rollback the upper mantle seismic anisotropy in the region is studied focusing at its western and eastern edges in order to explore the effects of possible slab segmentation on mantle flow and fabrics complementary to new sks shear wave splitting measurements in regions not adequately sampled so far the source side splitting technique is applied to constrain the depth of anisotropy and to densify measurements in the western hellenic arc a trench normal subslab anisotropy is observed near the trench in the forearc domain source side and sks measurements reveal a trench parallel pattern this indicates subslab trench parallel mantle flow associated with return flow due to the fast slab rollback the passage from continental to oceanic subduction in the western hellenic zone is illustrated by a forearc transitional anisotropy pattern this indicates subslab mantle flow parallel to a ne sw smooth ramp that possibly connects the two subducted slabs a young tear fault initiated at the kefalonia transform fault is likely not entirely developed as this trench parallel anisotropy pattern is observed along the entire western hellenic subduction system even following this horizontal offset between the two slabs at the eastern side of the hellenic subduction zone subslab source side anisotropy measurements show a general trench normal pattern these are associated with mantle flow through a possible ongoing tearing of the oceanic lithosphere in the area although the exact geometry of this slab tear is relatively unknown sks trench parallel measurements imply that the tear has not reached the surface yet further exploration of the hellenic subduction system is necessary denser seismic networks should be deployed at both its edges in order to achieve a more definite image of the structure and geodynamics of this area

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

seismic anisotropy above and below the subducting nazca lithosphere in southern south america 1 the goal of this study is to better constrain anisotropy and mantle flow above and below the nazca slab from 28 s to 42 s through modeling of shear wave splitting in local s sks and skks sk k s phases comparisons of local s splitting times and path lengths in the slab mantle wedge and upper plate indicate that splitting times for arc and back arc stations are consistent with anisotropy in the mantle wedge but long slab paths to fore arc stations imply that slab anisotropy is also significant sk k s shear wave splitting observations and models for sub slab anisotropy show that significant anisotropy is present below the slab and that the orientation of sub slab anisotropy sometimes differs from anisotropy above the slab anisotropy both above the slab and below the slab in the south american subduction zone is consistent with mantle flow that is driven by a combination of entrainment with downgoing slab motion and flow complexity related to variations in slab shape and slab rollback

seismic attenuation and mantle wedge temperatures in the alaska subduction zone 1 anelastic loss of seismic wave energy or seismic attenuation 1 q provides a proxy for temperature under certain conditions the q structure of the upper mantle beneath central alaska is imaged here at high resolution an active subduction zone where arc volcanism is absent to investigate mantle thermal structure the recent broadband experiment across the alaska range beaar provides the first dense broadband seismic coverage of this region the spectra of p and sh waves for regional earthquakes are inverted for path averaged attenuation operators between 0 5 and 20 hz along with earthquake source parameters these measurements fit waveforms significantly better when the frequency dependence of q is taken into account and in the mantle frequency dependence lies close to laboratory values inverting these measurements for spatial variations in q reveals a highly attenuating wedge with q 500 assuming frequency dependence comparison with p results shows that attenuation in bulk modulus is negligible within the low q wedge as expected for thermally activated attenuation mechanisms bulk attenuation is significant in the overlying crust and subducting plate indicating that q must be controlled by other processes the shallowest part of the wedge shows little attenuation as expected for a cold viscous nose that is not involved in wedge corner flow overall the spatial pattern of q beneath alaska is qualitatively similar to other subduction zones although the highest wedge attenuation is about a factor of 2 lower the q values imply that temperatures exceed 1200 c in the wedge on the basis of recent laboratory based calibrations for dry peridotite these temperatures are 100 150 c colder than we infer beneath japan or the andes possibly explaining the absence of arc volcanism in central alaska

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

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

the zone of influence of the subducting slab in the asthenospheric mantle due to the multi disciplinary nature of combined geodynamics and shear wave splitting studies there is still much to be understood in terms of isolating the contributions from mantle dynamics to the shear wave splitting signal even in a two dimensional 2d mantle flow framework this paper investigates the viscous flow development of lattice preferred orientation lpo and predicted shear wave splitting for a suite of buoyancy driven subduction models to shed light on the nature of the slab driven asthenospheric flow field the models show that the spatial dimensions of the slab driven zone of influence in the mantle lpo fabric and resulting synthetic splitting are sensitive to the slab dip and slab strength with the maximum lpo fabric development occurring for models with a moderate initial slab dip and weaker slab the models show that olivine lpo fabric in the asthenosphere generally increases in alignment strength with increased proximity to the slab but can be transient and spatially variable on small length scales the results suggest lpo formed during initial subduction may persist into the steady state subduction regime the vertical flow fields in the asthenosphere can produce shear wave splitting variations with back azimuth that deviate from the predictions of uniform trench normal anisotropy a result that bears on the interpretation of complexity in shear wave splitting observed in real subduction zones furthermore the models demonstrate that the corner flow paradigm should not be equated with a 2d subduction framework

crustal structure across the central alaska range anatomy of a mesozoic collisional zone a first order process in the growth of continents is the collision and accretion of terranes against continental margins collision leads to the formation of a suture zone between the accreted terrane and the former continental margin new insights on the suturing process are observed from two receiver function transects across the mesozoic alaska range suture zone three distinct crustal sections are identified from observations of crustal thickness intracrustal discontinuities and vp vs a northern section with 27 km thick crust of felsic to intermediate composition a central section that is 37 km thick that exhibits intracrustal discontinuities and has felsic to intermediate composition and a southern section that is 30 km thick and has a more mafic composition we interpret these sections to correspond with the former continental margin yukon composite terrane the suture zone proper and the allochthonous oceanic terrane wrangellia composite terrane the boundary between the yukon composite terrane and the suture zone appears to be a subhorizontal discontinuity that accommodated underthrusting of crust from the suture zone beneath the former continental margin the boundary between the suture zone and the wrangellia composite terrane in contrast appears to be a relatively discrete vertical boundary the observed variability in the crust across the alaska range suture zone is likely controlled by the differing compositions of the terranes involved which influences how each section responds to precollisional syncollisional and postcollisional deformation

unusual mantle poisson s ratio subduction and crustal structure in central alaska 1 subduction zones alter the upper plate in a variety of ways including metasomatism via slab derived fluids and accretion of exotic terranes these processes should produce distinctive seismic signatures as seismic velocities are sensitive to compositional variations a new method is developed to estimates depths to interfaces and poisson s ratio vp vs ratio for dipping layered structures by stacking teleseismic receiver functions it also estimates crustal vp vs and thickness using this method we analyze a broadband data set from the central alaska subduction zone to seek evidence for mantle wedge composition crustal thickness varies from 27 to 45 km and crustal vp vs indicates an intermediate to mafic composition expected for these accreted island arc terranes inversions for mantle structure confirm the presence of a 15 20 km thick low velocity zone atop of the downgoing plate to 130 km depth perhaps subducted crust of an exotic terrane vp vs in the mantle wedge allows for 15 15 serpentinization where the slab is 80 km deep and the wedge is cold where the slab is deeper vp vs for the mantle wedge is unusually low 1 7 this value is lower than predicted for any common mantle mineral and may indicate significant quantities of quartz provided that past physical properties measurements are not grossly in error these results suggest that the mantle wedge in subduction zones can occasionally differ substantially in bulk composition from normal subcrustal mantle perhaps because collisions of exotic terranes cause tectonic mixing of crustal material into the mantle

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

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

complex anisotropy beneath the peruvian flat slab from frequency dependent multiple phase shear wave splitting analysis flat or shallow subduction is a relatively widespread global occurrence but the dynamics remain poorly understood in particular the interaction between flat slabs and the surrounding mantle flow has yet to be studied in detail here we present measurements of seismic anisotropy to investigate mantle flow beneath the peruvian flat slab segment the largest present day region of flat subduction we conduct a detailed shear wave splitting analysis at a long running seismic station nna located near lima peru we present measurements of apparent splitting parameters fast direction and delay time t for sks scs and local s phases from 80 events we observe well defined frequency dependence and backazimuthal variability indicating the likely presence of complex anisotropy forward modeling the observations with two or three layers of anisotropy reveals a likely layer with a trench normal fast direction underlying a layer with a more trench oblique to trench subparallel fast direction in order to further constrain the anisotropic geometry we analyzed the source side splitting from events originating within the slab measured at distant stations beneath the flat slab segment we found trench normal fast splitting directions in the subslab mantle while within the dipping portion of the slab further to the east likely trench subparallel anisotropy within the slab itself this subslab pattern contradicts observations from elsewhere in south america for normal i e more steeply dipping slab conditions it is similar however to inferences from other shallowly dipping subduction zones around the world while there is an apparent link between slab dip and the surrounding mantle flow at least beneath peru the precise nature of the relationship remains to be clarified