shear wave splitting in northeast venezuela trinidad and the eastern caribbean abstract we present new measurements of splitting of core shear wave phases sks skks pks recorded at seven stations in the eastern caribbean region six stations in northeastern venezuela and trinidad constituted the secasa92 temporary broadband array the seventh station sjg is installed on puerto rico we also attempted to measure source side splitting of s waves from earthquakes in the lesser antilles subducted slab but with little success mean splitting parameters fast polarization direction and delay time t at the stations are consistently east west trending and over 1 s respectively delay times at the secasa92 array are amongst the highest yet measured reaching 2 1 s and generally over 1 5 s we note an approximately linear decrease in delay times here from maxima at the northernmost stations to a minimum at the southernmost the secasa92 array is situated in the wide south america caribbean sa ca plate boundary zone and sjg is within the north america caribbean na ca plate boundary zone in both cases trends are parallel to the plate boundary zones locally we interpret these results to indicate that the sa ca plate boundary deformation extends to depths of at least 200 km and that south american crust lithospheric mantle and asthenosphere are coherently deformed the extreme delay times we measure are probably the result of transpression in the sa ca plate boundary and overriding of the lesser antilles slab by south america compressional flattening results in extremes of foliation development and dextral shearing yields a strong lineation in upper mantle olivine by contrast the delay time at sjg 1 2 s is near the global average and reflects the fact that the na ca plate boundary zone is purely sinistral or includes minor transtension we note that recent counterclockwise rotation of puerto rico does not apparently affect upper mantle deformation beneath the island we suggest that this rotation is crustal driven by far field sinistral shear and partly decoupled from the mantle shear our results are consistent with the trench parallel mantle flow hypothesis russo and silver 1994 science 263 1105 1111 and indicate that asthenospheric mantle flow beneath the caribbean plate is eastward and probably drives the plate s eastward motion

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

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

effect of stone content on water flow velocity over loess slope non frozen soil abstract stony soils are commonly found worldwide and are considerably studied for their hydrological characteristics and effect on soil erosion water flow velocity is an important parameter in understanding the effect of stone content on hydrodynamics and soil erosion in this study laboratory experiments were used to measure rill flow velocity by using electrolyte tracer method under different hydraulic conditions flow rates of 1 2 4 and 8 l min slope gradients of 5 10 15 and 20 and stone mass contents amounting to 0 10 20 and 50 nine sensors which were 1 m apart along the 8 m long rill were used to measure flow velocity by tracing solute transport measured flow velocity increased with slope gradient and flow rate the highest increase in flow velocity was measured from 15 to 20 which were also affected by flow rate effects of discharge rate on flow velocity presented the largest difference when flow rate increased from 2 l min to 8 l min at slope gradients higher than 5 the effects of different factors were quantified by a regression model with high accuracy of 0 99 maximum flow velocity of water was predicted at 15 23 of stone content flow velocity increased with 0 15 23 of stone content but decreased at higher values this study aims at further understanding the hydrodynamics of soil erosion and sediment transport behaviors in hillslopes with different stone contents to obtain information for quantifying soil erosion on stony slopes

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

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

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

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

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

active tectonics of the st elias orogen alaska observed with gps measurements 1 we use data from campaign and continuous gps sites in southeast and south central alaska to constrain a regional tectonic block model for the st elias orogen active tectonic deformation in the orogen is dominated by the effects of the collision of the yakutat block with southern alaska our results indicate that 37 mm yr of convergence is accommodated along a relatively narrow belt of n nw dipping thrust faults in the eastern half of the orogen with the present day deformation front running through icy bay and beneath the malaspina glacier near the bering glacier the collisional thrust fault regime transitions into a broad northwest dipping decollement as the yakutat block basement begins to subduct beneath the counterclockwise rotating elias block the location of this transition aligns with the gulf of alaska shear zone implying that the pacific plate is fragmenting in response to the yakutat collision our model indicates that the bering glacier region is undergoing internal deformation and could correspond to the final stage of offscraping and accretion of sediments from the yakutat block prior to subduction predicted block motions at the western edge of the orogen suggest that the crust is laterally escaping along the aleutian fore arc

mantle flow in the rivera cocos subduction zone summary western mexico where the young and small rivera plate and the adjacent large cocos plate are subducting beneath the north american plate is a unique region on earth where tearing of subducting oceanic plates as well as fragmentation of the overriding continental plate is occurring today characterizing the mantle flow field that accompanies the subduction of the rivera and adjacent cocos plates can help to clarify the tectonics and magma genesis of this young plate boundary here we report observations of seismic anisotropy as manifested by shear wave splitting derived from local s and teleseismic sks data collected by the mapping rivera subduction zone array that was deployed from 2006 january to 2007 june in southwestern mexico and from data collected by two of mexico s servicio sismologico nacional stations sks and local s wave splitting parameters indicate that the fast directions of the split sks waves for stations that lie on the central and southern jalisco block are approximately trench normal following the convergence direction between the rivera plate and jalisco block s wave splitting from slab events show a small averaged delay time of 0 2 s for the upper 60 km of the crust and mantle therefore the main source of anisotropy must reside in the entrained mantle below the young and thin rivera plate trench oblique fast sks split directions are observed in the western edge of the rivera plate and the western parts of the cocos slab the curved pattern of fast sks split directions in the western jalisco block and beneath the rivera cocos slab gap indicates 3 d toroidal mantle flow around the northwestern edge of the rivera slab and the rivera cocos gap which profoundly affect the finite strain field in the northwestern edge of the rivera slab and the mantle wedge both the tomographic images and shear wave splitting results support the idea that the rivera and western cocos plates not only moved in a downdip direction but also have recently rolled back towards the trench and that the colima rift is intimately related to the tearing between the rivera and cocos plates

shear wave splitting and implications for mantle flow beneath the melt region of the east pacific rise shear wave splitting across the fast spreading east pacific rise has been measured from records of sks and skks phases on the ocean bottom seismometers of the mantle electromagnetic and tomography melt experiment the direction of fast shear wave polarization is aligned parallel to the spreading direction delay times between fast and slow shear waves are asymmetric across the rise and off axis values on the pacific plate are twice those on the nazca plate splitting on the pacific plate may reflect anisotropy associated with spreading induced flow above a depth of about 100 km as well as a deeper contribution from warm asthenospheric return flow from the pacific superswell region

evidence for shear wave anisotropy in the mantle wedge beneath south central alaska we investigate shear wave polarization and splitting for more than 200 earthquakes with hypocentral depths ranging from 20 to 180 km beneath southcentral alaska data were recorded at three sites stations skn aui and red a preferred polarization parallel to the strike of the wadati benioff zone wbz n25 e is found for a majority of events however some regions show systematic differences in polarization for a subset of nine events we compute focal mechanisms and find no correlation between the theoretical shear wave polarization caused by the fault plane orientation and the observed polarization delays of the slow versus the fast s wave range from less than 0 05 to 0 25 sec for station skn and up to 0 4 sec for stations aui and red a statistically significant correlation of delay with depth is found r 0 78 with the largest delays corresponding to the deepest events this indicates that the source of the anisotropy cannot be located in the crust based on the ray path geometry the source cannot be solely located in the subducting slab either and must therefore originate in the mantle wedge our results for the subduction zone beneath alaska are consistent with similar studies in other subduction zones that describe slab parallel anisotropy in the mantle wedge

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

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

seismic imaging of massive sulfide deposits part iii borehole seismic imaging of near vertical structures surface seismic profiling is well suited for geologic provinces characterized by low to moderate dip but it is less effective where dips exceed 65 degrees due to inherent limitations of the recording geometry and processing algorithms vertical seismic profiling vsp a method used mainly for hydrocarbon exploration in sedimentary basins overcomes this limitation by utilizing deep boreholes as an alternative acquisition datum here we evaluate the effectiveness of this method for imaging massive sulfide deposits associated with folded and steeply dipping volcanic units near the kidd creek orebody in ontario canada small 150 225 g explosive charges placed in a water filled pit provided a consistent energy source with sufficient band width and signal penetration to image the target zone to a depth of at least 1 0 km after preliminary processing the vertical seismic profile record sections were mapped into cross sectional format using a transformation based on ray tracing this transformation facilitates the extrapolation of subsurface contacts updip from borehole intersections and the resulting images delineate several stratigraphic contacts in addition to a prominent seismic reflection from a massive sulfide body

wrangell benioff zone southern alaska the first unequivocal evidence for the existence of a benioff zone that may be related to the quaternary wrangell volcanoes comes from a set of 86 well located hypocenters for earthquakes smaller than about magnitude 4 in the region between lat 61 and 62 5 n and between long 142 and 145 5 w about half of the earthquakes occur at depths of 25 km or less below 40 km a clearly defined north northeast dipping zone of seismicity here termed the wrangell benioff zone extends to a depth of about 85 km and continues for about 115 km along strike subparallel to the volcanic trend the western end of the zone may be offset from the northern end of the much more active aleutian benioff zone where the benioff zone shoals to 30 to 40 km it becomes nearly horizontal and cannot be clearly distinguished from upper plate seismicity it is uncertain whether the subducted plate segment that contains the wrangell benioff zone is structurally part of the pacific plate or the yakutat block

seismic velocity structure and anisotropy of the alaska subduction zone based on surface wave tomography southcentral alaska is a complex tectonic region that transitions from subduction of pacific crust to flat slab subduction and collision of overthickened yakutat crust because much of the yakutat crust has been subducted seismic imaging is needed in order to understand the crustal and upper mantle structural framework for this active tectonic setting here we use teleseismic rayleigh waves to image large scale variations in shear wave structure our imaging technique employs a two plane wave representation with finite frequency sensitivity kernels our 3 d isotropic model reveals several features the subducting pacific yakutat slab slow wave speeds characterizing the onshore yakutat collision zone slow wave speeds of the wrangell subduction zone and a deep tomographic contrast at the eastern edge of the pacific yakutat slab we produce anisotropic phase velocity maps that exhibit variations in the fast direction of azimuthal anisotropy these maps show the dominance of the yakutat slab on the observed pattern of anisotropy west of the yakutat slab the fast directions are approximately aligned with the plate convergence direction in the region of the yakutat slab the pattern is more complicated along the margins of the slab the fast directions are roughly parallel to the margins we identify notable differences and similarities with published sks splitting measurements integrative modeling using 3 d anisotropy models and different seismic measurements will be needed in order to establish a detailed 3 d anisotropic velocity model for alaska this study provides a large scale starting point for such an effort

the magnitude symmetry and origin of upper mantle anisotropy based on fabric analyses of ultramafic tectonites summary seismic anisotropy within the upper mantle originates from the preferred orientation of highly anisotropic single crystals the symmetry and magnitude of anisotropy depend upon 1 the volume percentages of the minerals constituting the upper mantle 2 the degree and symmetry of preferred orientation of each mineral and 3 the alignment of the minerals crystallographic axes relative to one another the nature of upper mantle anisotropy can be examined by studying mineral orientations within ultramafic rocks which were once part of the mantle petrofabric data for olivine and pyroxene have been used to obtain velocity anisotropy patterns over large regions of ultramafic rocks from the samail ophiolite oman the troodos ophiolite cyprus the bay of islands ophiolife newfoundland the twin sisters ultramafic washington usa the dun mountain ophiolite new zealand the red hills ophiolite new zealand and the red mountain ophiolite new zealand the compressional wave anisotropy calculated for these massifs ranges from 3 to 8 per cent in excellent agreement with observed seismic anisotropy in the upper continental and oceanic mantle the symmetry varies from orthorhombic to axial with the axial symmetry axis corresponding to the olivine a axes maxima and subparallel to spreading directions in oceanic upper mantle pyroxene a b and c axes maxima generally parallel olivine b c and a axes respectively and anisotropy decreases with increasing pyroxene content shear wave splitting is predicted for all propagation directions within the upper mantle symmetry is also orthorhombic or axial with the minimum difference in velocity between the two shear waves parallel to the maximum compressional wave velocity

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

new constraints on tectonics of interior alaska earthquake locations source mechanisms and stress regime we used the joint hypocenter determination jhd method to relocate 3611 crustal earthquakes that occurred from 1988 to 1999 in central alaska the new earthquake locations provide more details on the structure of the kantishna cluster and better locations for the aftershock sequence of the 29 november 2000 m l 5 6 earthquake in the minto flats seismic zone the jhd locations for the aftershocks of the 1995 m w 6 0 minto flats earthquake and 22 october 1996 m w 5 8 earthquake near the denali fault are also available a catalog of 196 fault plane solutions consisting of the moment tensor solutions for the earthquakes with magnitude 4 0 or above and p wave first motion solutions for the earthquakes with magnitude 3 4 and above that occurred from 1988 to 2000 was composed moment tensor solutions were calculated using regional broadband data this catalog was used to calculate principal stress orientations in the crust the stress orientations change across central alaska in particular the maximum principal stress orientation rotates clockwise from se nw to ssw nne direction as one moves from west to east these stress orientations are consistent with the stress field transferred from the plate convergence in southern alaska in addition we tested different velocity structures in the moment tensor inversion procedure to identify velocity models for calculating the green s functions the moment tensor inversion study shows that it is possible to obtain a reliable moment tensor solution for moderate sized earthquakes m l 4 using three component records from a single broadband station when the epicentral distances are between 50 and 300 km manuscript received 8 june 2001

global mantle flow and the development of seismic anisotropy differences between the oceanic and continental upper mantle 1 viscous shear in the asthenosphere accommodates relative motion between earth s surface plates and underlying mantle generating lattice preferred orientation lpo in olivine aggregates and a seismically anisotropic fabric because this fabric develops with the evolving mantle flow field observations of seismic anisotropy can constrain asthenospheric flow patterns if the contribution of fossil lithospheric anisotropy is small we use global viscous mantle flow models to characterize the relationship between asthenospheric deformation and lpo and compare the predicted pattern of anisotropy to a global compilation of observed shear wave splitting measurements for asthenosphere 500 km from plate boundaries simple shear rotates the lpo toward the infinite strain axis isa the lpo after infinite deformation faster than the isa changes along flow lines thus we expect the isa to approximate lpo throughout most of the asthenosphere greatly simplifying lpo predictions because strain integration along flow lines is unnecessary approximating lpo with the isa and assuming a type fabric olivine a axis parallel to isa we find that mantle flow driven by both plate motions and mantle density heterogeneity successfully predicts oceanic anisotropy average misfit 13 continental anisotropy is less well fit average misfit 41 but lateral variations in lithospheric thickness improve the fit in some continental areas this suggests that asthenospheric anisotropy contributes to shear wave splitting for both continents and oceans but is overlain by a stronger layer of lithospheric anisotropy for continents the contribution of the oceanic lithosphere is likely smaller because it is thinner younger and less deformed than its continental counterpart

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

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

lithospheric discontinuity structure in alaska thickness variations determined by sp receiver functions we present the first broad scale image of lithospheric thickness across the major tectonic domains of alaska based on s wave receiver functions and joint interpretation with the potential field seismic velocity and heat flow measurements thus we provide context for the distribution of strain throughout the alaskan orocline in the north below the brooks range a 130 km thick lithosphere is resolved consistent with the presence of strong lithosphere that deflects strain to the south into central and southern alaska in southern alaska beneath the chugach and st elias mountains multiple interfaces are present and we interpret a thinner 80 90 km north american lithosphere above a deeper interface that represents the base of the yakutat microplate thereby extending it to the area below the wrangell volcanic field and st elias mountains immediately north of the e w striking denali fault shallow negative conversions 80 km denote thin lithosphere in the greater back arc region where heat flow is observed to be high thin lithosphere in eastern alaska and adjacent yukon territory coincides with the occurrence of inboard crustal seismicity and may be indicative of transmitted compression caused by the collision of the yakutat microplate relatively thin lithosphere 90 km south of the arctic alaska domain that is deforming throughout the alaskan orocline may result from lithospheric thinning associated with guided deformation expansion of this model using the upcoming transportable array will be critical to establish lateral continuity or lack thereof of lithospheric structure and directly discriminate between existing regional deformation models

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

alaska megathrust 2 imaging the megathrust zone and yakutat pacific plate interface in the alaska subduction zone we image the slab underneath a 450 km long transect of the alaska subduction zone to investigate 1 the geometry and velocity structure of the downgoing plate and their relationship to slab seismicity and 2 the interplate coupled zone where the great 1964 earthquake mw 9 2 exhibited the largest amount of rupture the joint teleseismic migration of two array data sets based on receiver functions rfs reveals a prominent shallow dipping low velocity layer at 25 30 km depth in southern alaska modeling of rf amplitudes suggests the existence of a thin layer vs of 2 1 2 6 km s that is 20 40 slower than underlying oceanic crustal velocities and is sandwiched between the subducted slab and the overriding plate the observed megathrust layer with vp vs of 1 9 2 3 may be due to a thick sediment input from the trench in combination with elevated pore fluid pressure in the channel our image also includes an unusually thick low velocity crust subducting with a 20 dip down to 130 km depth at 200 km inland beneath central alaska the unusual nature of this subducted segment results from the subduction of the yakutat terrane crust our imaged western edge of the yakutat terrane aligns with the western end of a geodetically locked patch with high slip deficit and coincides with the boundary of aftershock events from the 1964 earthquake it appears that this sharp change in the nature of the downgoing plate could control the slip distribution of great earthquakes on this plate interface

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