seismicity and stress distribution in the copiapo northern chile subduction zone using combined on and off shore seismic observations abstract historic as well as teleseismically and locally recorded seismicity of the subduction margin of copiapo northern chile is analysed an on and off shore temporary network was deployed during 2 months in the region in order to obtain a detailed definition of the wadati benioff zone an average dip angle of 20 along the subduction thrust interplate contact was determined this value is similar to that observed along the chilean margin suggesting that there are no variations in the dip angle of the nazca plate for depths less than 50 60 km locally recorded data do not provide seismic evidence to determine whether subhorizontal subduction exists north of latitude 28 s teleseismically recorded events exhibit a low rate of occurrence of inland earthquakes with a tendency for epicentres to be located off shore in contrast locally recorded seismicity reveals a homogeneous epicentral distribution without a predominance of off shore events along the subduction thrust contact there is a reverse stress regime with the greatest compression axis 1 horizontal and oriented perpendicular to the strike of the trench events located above the main decollement are in a normal stress regime with the minimum compressive stress axis 3 horizontally oriented parallel to the convergence direction the events located below the coupled interplate contact also lie in a normal faulting regime but the 3 axis plunges almost parallel to the subducting plate the intermediate depth fault plane solutions exhibit a substantial scatter suggesting a complicated faulting regime similar to that observed in previous studies of northern chile they are associated with a normal faulting stress regime in which 3 is oriented in a n14 e direction parallel to the strike of the trench high resolution bathymetric data and seismicity located above the thrust contact suggest that seamounts are being subducting

the flat to normal subduction transition study to obtain the nazca plate morphology using high resolution seismicity data from the nazca plate in central chile abstract data from 45 seismological stations mostly temporary were used to obtain an accurate data set of intraslab seismicity of the nazca subducted plate between 33 s and 35 s the interest zone located in the transition section where the nazca plate changes from flat slab north of 33 to normal slab south of that latitude in addition the study region is located where the active volcanic arc appears from a set of earthquakes which were relocated using a grid search multiple events algorithm we obtained the plate geometry from latitudes of 33 s to 34 5 s and from 60 km to 120 km in depth the obtained morphology shows notable similarity in its structure to maipo orocline revealing some possible strong connection between the overriding plate and the subducting plate we suggest that the subducted plate at the trench has been deformed in its shape consistently with the maipo orocline pattern and its deformation is observed below the interface zone our results are consistent with van keken et al 2011 models and based on this the seismicity located between 70 and 120 km is probably related with dehydration processes rather than mechanical processes from our precise earthquake locations we observed a complete lack of intraslab seismicity below 120 km depth this valuable finding can be used to better constrain thermal models for the subduction region of central chile

seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing water and hydrous minerals play a key role in geodynamic processes at subduction zones however there are few seismological constraints on the presence of water within the subducting plate audet et al model converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle their observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip this study models converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle the observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip water and hydrous minerals play a key part in geodynamic processes at subduction zones1 2 3 by weakening the plate boundary aiding slip and permitting subduction and indeed plate tectonics to occur4 the seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization5 6 7 however seismological observations bearing on the presence of water within the subducting plate itself are less well documented here we use converted teleseismic waves to obtain observations of anomalously high poisson s ratios within the subducted oceanic crust from the cascadia continental margin to its intersection with forearc mantle on the basis of pressure temperature and compositional considerations the elevated poisson s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values combined with observations of a strong negative velocity contrast at the top of the oceanic crust our results imply that the megathrust is a low permeability boundary the transition from a low to high permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization these results may have important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip

the nappe structure of the central northern calcareous alps and its disintegration during miocene tectonic extrusion a contribution to understanding the orogenic evolution of the eastern alps the classical concept of the nappe structure of the central northern calcareous alps nca is in contradiction with modern stratigraphic structural metamorphic and geochronological data we first perform a palinspastic restoration for the time before miocene lateral tectonic extrusion which shows good continuity of structures facies and diagenetic metamorphic zones we present a new nappe concept in which the tirolic unit practically takes the whole area of the central nca and is divided into three subunits nappes lower and upper tirolic subunit separated by the upper jurassic trattberg thrust and the metamorphic ultra tirolic unit the hallstatt iuvavic nappe s formed the highest unit but were completely destroyed by erosion after nappe stacking remnants of the hallstatt nappes are only represented by components of up to 1 km in size in middle upper jurassic radiolaritic wildflysch sediments hallstatt melange belonging to the tirolic unit destruction of the continental margin started in middle to upper jurassic time and prograded from the oceanic side towards the shelf the original substratum of the external nappes bavaric units of the nca was largely the austroalpine crystalline basement of the internal nappes tirolic units the weakly metamorphosed palaeozoic sequences greywacke zone and equivalents eocene movements caused limited internal deformation in the tirolic unit

mantle wedge anisotropy in southern tyrrhenian subduction zone italy from receiver function analysis abstract we constrain mantle wedge seismic structure in the southern tyrrhenian subduction zone italy using teleseismic receiver functions rf recorded at station cuc of the mednet seismographic network station cuc lies above the northern portion of the calabrian slab segment which is recognized from deep seismicity and tomographic imaging as a narrow laterally high arched slab fragment extending from the surface below calabria down to the transition zone to better define the descending slab interface and possible shear coupled flow in the mantle wedge above the slab we computed receiver functions from the p coda of 147 teleseismic events to analyze the back azimuth dependence of ps converted phases from interfaces beneath cuc we stack the rf data set with back azimuth to compute its harmonic expansion which relates to the effects of interface dip and anisotropy at layer boundaries the seismic structure constrained through the rf analysis is characterized in its upper part by a sub horizontal moho at about 25 km depth overlying a thin isotropic layer at top of mantle for the deeper part back azimuth variation suggests two alternative models each with an anisotropic layer between two dipping interfaces near 70 and 90 km depth with fast and slow symmetry axes respectively above the apennines slab although independent evidence suggests a north south strike for the slab beneath cuc the trend of the inferred anisotropy is 45 clockwise from north inconsistent with a simple downdip shear coupled flow model in the supra slab mantle wedge however complexities of flow and induced rock fabric in the tyrrhenian back arc may arise due to several concurring factors such as the arcuate shape of the apennines slab its retreating kinematics or slab edge effects

seismic signatures of a hydrated mantle wedge from antigorite crystal preferred orientation cpo abstract we report electron backscatter diffraction ebsd measurements of antigorite crystal preferred orientations cpos from 7 samples of antigorite schist from serpentinite melanges adjacent to the motagua fault system in central guatemala the cpos range from diffuse girdle to point distributions of 001 atg girdle distributions of 001 atg are not expected from deformation theory or experiments suggesting that they are a result of growth rather than deformation and may thus be topotactically related to the cpo of the olivine from which the antigorite grew the calculated seismic anisotropy ranges from 6 to 28 in v p and 5 to 33 in v s and is highest for samples with a point maximum of 001 atg for all samples the minimum v p corresponds to the pole to 001 atg and the maximum v p occurs within the foliation for samples with a clearly defined foliation trench parallel shear wave splitting observations for subduction zones can best be explained by a combination of olivine b type cpo and antigorite oriented with 001 parallel to the foliation only a relatively thin 20 km zone of hydrated mantle is required to explain the observed splitting times

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

subduction zone anisotropy beneath corvallis oregon a serpentinite skid mark of trench parallel terrane migration 1 we studied the back azimuth dependence of ps converted phases at gsn station cor corvallis oregon using broadband p receiver functions computed from 602 teleseismic earthquakes the amplitudes and polarities of the transverse ps phases are largely two lobed which indicates anisotropy with a tilted symmetry axis a double peaked ps conversion at 4 5 6 5 s delay has the moveout of a dipping slab but is not consistent with simple deflection of the ps converted phase by the dipping interface a polarity flip on the transverse receiver function near north south back azimuth indicates an anisotropic symmetry axis aligned north south far from the convergence direction n68 e the ps phase is modeled using reflectivity synthetics with a highly anisotropic layer of depressed wave speed vp 6 0 6 5 km s near 40 km depth at the slab interface with the overriding north american plate one dimensional modeling suggests 10 anisotropy in the supraslab layer with a slow symmetry axis oriented n5 w at a 60 tilt from the vertical adjustments for the effect of slab interface dip on ps amplitude suggest a somewhat lower 7 anisotropy we infer a thin 7 km anisotropic detachment zone for the northward slippage of the siletz forearc terrane along the top of the descending slab a motion consistent with gps measurements and models of regional lithospheric dynamics serpentinite is a likely constituent for the deep anisotropic layer owing to its reduced vp and high poisson ratio the ductile rheology and hydrated composition of serpentinite make it a plausible lithology for a mechanical detachment zone

seismicity and shape of the subducted nazca plate an updated compilation of earthquake locations and focal mechanism solutions from the international seismological centre and preliminary determination of earthquakes is the basis of a comprehensive study of the geometry of the wadati benioff zone beneath western south america the new data support previous mapping of a sharp flexure rather than a tear in the subducted nazca plate beneath southern peru and provide evidence for a similar flexure in the southward transition from nearly horizontal subduction to a slab with 30 dip at latitude 33 s in contrast the transition from 30 slab dip beneath bolivia to a nearly horizontal dip in the region between 28 s 32 s is more gradual occurring over several hundred kilometers of along strike distance between 20 s and 32 s this southward flattening corresponds to a broadening of a horizontal benchlike part of the subducted plate formed between 100 and 125 km depth the transition in continental tectonic style near 27 s 28 s from a wide volcanically active plateau to a narrow nonvolcanic cordillera appears not to be associated with the main slab flattening which begins to the north of these latitudes but with a more abrupt change in curvature of the subducted slab from convex upward to concave upward immediately below the plate boundary interface the concept of gaussian curvature is applied to slab bending to explain how subduction geometry is affected by the shape of the south american plate we hypothesize that the polarity of vertical curvature in the subducting slab is governed by the orientation of lateral curvature of the plate margin focal mechanism solutions for intermediate and deep earthquakes are grouped by geographic region and inverted for the orientation and relative magnitudes of the principal stresses results of the inversion indicate that downdip extension dominates in the slab above 350 km while downdip compression dominates at greater depths

response of the mantle to flat slab evolution insights from local s splitting beneath peru the dynamics of flat subduction particularly the interaction between a flat slab and the overriding plate are poorly understood here we study the seismically anisotropic properties and deformational regime of the mantle directly above the peruvian flat slab we analyze shear wave splitting from 370 local s events at 49 stations across southern peru we find that the mantle above the flat slab appears to be anisotropic with modest average delay times 0 28 s that are consistent with 4 anisotropy in a 30 km thick mantle layer the most likely mechanism is the lattice preferred orientation of olivine which suggests that the observed splitting pattern preserves information about the mantle deformation we observe a pronounced change in anisotropy along strike with predominately trench parallel fast directions in the north and more variable orientations in the south which we attribute to the ongoing migration of the nazca ridge through the flat slab system

the moho in subduction zones abstract the moho in subduction zones exists in two distinct forms one associated with the subducting oceanic plate and second with the overriding plate the seismic expression of both forms is linked to the nature of a landward dipping low velocity zone lvz that has been detected in a majority of subduction zones about the globe and that approximately coincides with wadati benioff seismicity we review seismic studies that constrain the properties of the lvz in cascadia where it has been extensively studied for over a quarter century a model in which the lvz is identified with hydrated pillow basalts and sheeted dikes of oceanic crustal layer 2 is consistent with available geological and geophysical data and reconciles previously conflicting interpretations in this model the upper oceanic crust is hydrated through intense circulation at the ridge and becomes overpressured upon subduction as a result of metamorphic dehydration reactions combined with an impermeable plate boundary above and a low porosity gabbroic layer 3 below the resulting seismic velocity contrast approaching 50 for s waves significantly overwhelms that of a weaker underlying oceanic moho at greater depths oceanic crust undergoes eclogitization in a top down sense leading to gradual disappearance of the lvz the large volume change accompanying eclogitization is postulated to rupture the plate boundary allowing fluids to penetrate the cooled forearc mantle wedge pervasive serpentinization and free fluids reduce velocities within the wedge thereby diminishing erasing or even inverting the seismic contrast associated with the moho of the overriding plate this model is tested against observations of lvzs and forearc mantle structure worldwide

deep crustal roots beneath the northern apennines inferred from teleseismic receiver functions abstract teleseismic waveforms recorded by a regional array crossing the northern italian peninsula and northern corsica are analyzed using the receiver function technique to determine the first order crustal structure the receiver function approach is used to isolate receiver side ps conversions generated at the crust mantle boundary and any major velocity discontinuity beneath the stations we used the time delay between the direct p wave and the ps wave converted at the moho discontinuity to infer crustal thickness beneath the stations the crust mantle boundary is estimated at 25 1 km of depth in northern corsica 20 2 km beneath the elba island and 20 to 24 2 km beneath tuscany in the eastern portion of the array moho depth increases from 28 2 km beneath the adriatic coast to 49 3 km beneath the apennine chain in a distance of about 100 km a double ps conversion produced beneath the val tiberina graben to the west of the apennines corresponds to interfaces as deep as 20 2 and 52 2 km this observation supports the hypothesis of partial overlapping between the shallow tuscan moho and the deeper adriatic moho the westward deepening of the adriatic moho beneath the northern apennines can be explained by lithosphere delamination that has dragged downward the adriatic lower crust the deep crustal root estimated beneath the northern apennines indicates that this portion of the chain is isostatically overcompensated

shear wave velocities in the pampean flat slab region from rayleigh wave tomography implications for slab and upper mantle hydration 1 the pampean flat slab region located in central argentina and chile between 29 and 34 s is considered a modern analog for laramide flat slab subduction within western north america regionally flat slab subduction is characterized by the nazca slab descending to 100 km depth flattening out for 300 km laterally before resuming a more normal angle of subduction flat slab subduction correlates spatially with the track of the juan fernandez ridge and is associated with the inboard migration of deformation and the cessation of volcanism within the region to better understand flat slab subduction we combine ambient noise tomography and earthquake generated surface wave measurements to calculate a regional 3d shear velocity model for the region shear wave velocity variations largely relate to changes in lithology within the crust with basins and bedrock exposures clearly defined as low and high velocity regions respectively we argue that subduction related hydration plays a significant role in controlling shear wave velocities within the upper mantle in the southern part of the study area where normal angle subduction is occurring the slab is visible as a high velocity body with a low velocity mantle wedge above it extending eastward from the active arc where flat slab subduction is occurring slab velocities increase to the east while velocities in the overlying lithosphere decrease consistent with the slab dewatering and gradually hydrating the overlying mantle the hydration of the slab may be contributing to the excess buoyancy of the subducting oceanic lithosphere helping to drive flat slab subduction

p sh conversions in layered media with hexagonally symmetric anisotropy a cookbook reflectivity synthetic seismograms demonstrate that the type layering and orientation of 1 d anisotropy influences strongly the coda of teleseismic p waves at periods t 1 sec particularly p sh converted waves we assume the simplest form of anisotropy described by an elastic tensor with a symmetry axis of arbitrary orientation the resulting phase velocities vary as cos 2 with respect to that axis using three families of simple crustal models we compare the effects of an anisotropic surface layer with reverberations caused by both thick and thin layers of anisotropy at depth if anisotropy in the surface layer is significant the polarization of direct p can be distorted to generate a transverse component followed by ps and a prominent shear reverberation converted from direct p at the free surface if the anisotropic layer is buried the first and often the most prominent arrival on the transverse component is the p to sh conversion at its upper surface if the anisotropic layer is sufficiently thin p to sh conversions from its boundaries interfere to form a derivative pulse shape on the transverse component which could be mistaken as the signature of shear wave splitting if is horizontal compressional p and shear s anisotropy both produce similar waveform perturbations with four lobed azimuthal patterns suggesting that a weighted stack of p coda from different back azimuths would improve signal to noise for tilted between the horizontal and vertical however the effects of p and s anisotropy differ greatly the influence of p anisotropy on p to s conversion is greatest for a symmetry axis tilted at 45 to the vertical where its azimuthal pattern has two lobes rather than four combinations of p and s anisotropy typically lead to a composite azimuthal dependence in the p coda reverberations

a double seismic zone in the subducting juan fernandez ridge of the nazca plate 32 s central chile 1 the region of central chile offers a unique opportunity to study the links between the subducting juan fernandez ridge the flat slab the double seismic zone dsz and the absence of modern volcanism here we report the presence and characteristics of the first observed dsz within the intermediate depth nazca slab using two temporary seismic catalogs ovalle 1999 and chile argentina seismological measurement experiment the lower plane of seismicity lp is located 20 25 km below the upper plane begins at 50 km depth and merges with the lower plane at 120 km depth where the slab becomes horizontal focal mechanism analysis and stress tensor calculations indicate that the slab s state of stress is dominantly controlled by plate convergence and overriding crust thickness above 60 70 km depth the slab is in horizontal compression and below it is in horizontal extension parallel to plate convergence which can be accounted for by vertical loading of the overriding lithosphere focal mechanisms below 60 70 km depth are strongly correlated with offshore outer rise bend faults suggesting the reactivation of preexisting faults below this depth the large interplane distances for all nazca dszs can be related to the slab s unusually cold thermal structure with respect to its age since lps globally seem to mimic mantle mineral dehydration paths we suggest that fluid migration and dehydration embrittlement provide the mechanism necessary to weaken the rock and that the stress field determines the direction of rupture

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

elasticity of antigorite seismic detection of serpentinites and anisotropy in subduction zones abstract serpentinization of the mantle wedge is an important process that influences the seismic and mechanical properties in subduction zones seismic detection of serpentines relies on the knowledge of elastic properties of serpentinites which thus far has not been possible in the absence of single crystal elastic properties of antigorite the elastic constants of antigorite the dominant serpentine at high pressure in subduction zones were measured using brillouin spectroscopy under ambient conditions in addition antigorite lattice preferred orientations lpo were determined using an electron back scattering diffraction ebsd technique isotropic aggregate velocities are significantly lower than those of peridotites to allow seismic detection of serpentinites from tomography the isotropic v p v s ratio is 1 76 in the voigt reuss hill average not very different from that of 1 73 in peridotite but may vary between 1 70 and 1 86 between the voigt and reuss bonds antigorite and deformed serpentinites have a very high seismic anisotropy and remarkably low velocities along particular directions v p varies between 8 9 km s 1 and 5 6 km s 1 46 anisotropy and 8 3 km s 1 and 5 8 km s 1 37 and v s between 5 1 km s 1 and 2 5 km s 1 66 and 4 7 km s 1 and 2 9 km s 1 50 for the single crystal and aggregate respectively the v p v s ratio and shear wave splitting also vary with orientation between 1 2 and 3 4 and 1 3 and 2 8 for the single crystal and aggregate respectively thus deformed serpentinites can present seismic velocities similar to peridotites for wave propagation parallel to the foliation or lower than crustal rocks for wave propagation perpendicular to the foliation these properties can be used to detect serpentinite quantify the amount of serpentinization and to discuss relationships between seismic anisotropy and deformation in the mantle wedge regions of high v p v s ratios and extremely low velocities in the mantle wedge of subduction zones down to about 6 and 3 km s 1 for v p and v s respectively are difficult to explain without strong preferred orientation of serpentine local variations of anisotropy may result from kilometer scale folding of serpentinites shear wave splittings up to 1 1 5 s can be explained with moderately thick 10 20 km serpentinite bodies

upper mantle structure in the south central chilean subduction zone 30 to 36 s 1 regional p and s wave travel time data were used to obtain three dimensional seismic tomography models for vp vs and vp vs above the subducting slab in central chile and argentina in this region there is an abrupt change from a normal subduction geometry south of 33 s to a flat subduction geometry to the north we find low vp low vs and high vp vs ratios in the southern half of our study area directly beneath the modern active volcanic arc which we interpret as localized pockets of melt in the northern half of our study area above where the subducting nazca plate flattens at 100 km depth we find low vp high vs and low vp vs ratios these unusual results point to a lack of melt or hydrated mineralogies such as serpentine both of which are characterized by high vp vs values the only mantle rocks that have low vp vs and high vs are mg rich compositions such as dehydrated serpentinite or orthopyroxenite we suggest that significant portions of the mantle overlying the flat slab consist of orthopyroxenite formed by a transient fluxing of silica rich fluids such fluids may have come from sediments that were subducted during the initiation of flat subduction at this latitude at 10 ma this would imply that the hydration of mantle material above a flat slab can be a transient phenomenon which leaves little residual free water behind but significantly alters the mantle chemistry

p and s wave anisotropy and shear wave splitting at pressure and temperature in possible mantle rocks and their relation to the rock fabric the seismic properties and their directional dependence have been measured in three potentially mantle derived rocks peridotite ivrea zone italy lherzolitic xenolith massif central france serpentinite the experimental data include the measurement of elastic wave velocities and densities at confining pressures up to 600 mpa and temperatures up to 600 c 600 mpa and the determination of velocity anisotropy and shear wave splitting the petrophysical data can be related to mineral composition and to microstructural characteristics of the rocks macroscopic anisotropy of p and s wave velocities is strongly controlled in the peridodites by the olivine and in the serpentinite by the serpentine antigorite lattice preferred orientation lpo respectively shear wave splitting was found to be an important characteristic of the anisotropic rocks the study shows that for the interpretation of shear wave splitting the direction of wave propagation in relation to the lpos of the major minerals and to the symmetry of their lattice fabric is of great importance in the ivrea peridotite exhibiting rhombic symmetry there is practically no shear wave splitting to be observed for s waves propagating parallel to the olivine 100 axes and the maximum shear wave splitting is not coupled with the direction of highest p wave velocity in the transversely isotropic massif central xenolith shear waves do not split when propagating parallel to the olivine 010 maximum coinciding with the direction of the radial symmetry axis of the olivine fabric the experimental data are principally in accordance with velocity calculations based on the measured olivine lpo and the olivine single crystal stiffness coefficients implications for the interpretation of seismic data are discussed

the importance of serpentinite mylonites for subduction and exhumation of oceanic crust abstract in the ultramafic erro tobbio unit voltri massif western alps a set of overprinting structures in serpentinite mylonites is related to alpine subduction to about 80 km depth and to subsequent exhumation antigorite mylonites are cut by en echelon olivine veins which in turn are dissected by multiple sets of shear bands containing olivine and titanian clinohumite the transition from olivine free to olivine bearing structures indicates recrystallization during prograde metamorphism all structures display the same top to the nw kinematics providing evidence for a continuous non coaxial deformation the serpentinite mylonites surround km scale bodies of pre alpine peridotite which show only minor alpine overprint this indicates that during subduction related deformation recrystallization and fluid flow were strongly localized within serpentinite mylonites olivine bearing discontinuous shear planes with top to the se sense of movement crosscut the prograde structures the inversion of shear sense suggests a change in position of the serpentinites relative to the downgoing slab i e from the subducted slab to the upper plate during accretion thus the shear sense inversion marks the change from burial to exhumation of the serpentinites the low density of antigorite serpentinites 2 75 g cm3 causes strong buoyancy thus providing a mechanism for the exhumation of deeply subducted rocks it is suggested that serpentinites may act as carriers for the uprise of eclogite bodies which have higher densities than the peridotitic upper mantle

upper mantle anisotropy beneath peru from sks splitting constraints on flat slab dynamics and interaction with the nazca ridge a r t i c l e i n f o a b s t r a c t the peruvian flat slab is by far the largest region of flat subduction in the world today but aspects of its structure and dynamics remain poorly understood in particular questions remain over whether the relatively narrow nazca ridge subducting beneath southern peru provides dynamic support for the flat slab or it is just a passive feature we investigate the dynamics and interaction of the nazca ridge and the flat slab system by studying upper mantle seismic anisotropy across southern peru we analyze shear wave splitting of sks ssks and pks phases at 49 stations distributed across the area primarily from the peru lithosphere and slab experiment pulse we observe distinct spatial variations in anisotropic structure along strike most notably a sharp transition from coherent splitting in the north to pervasive null non split arrivals in the south with the transition coinciding with the northern limit of the nazca ridge for both anisotropic domains there is evidence for complex and multi layered anisotropy to the north of the ridge our ks splitting measurements likely reflect trench normal mantle flow beneath the flat slab this signal is then modified by shallower anisotropic layers most likely in the supra slab mantle but also potentially from within the slab to the south the sub slab mantle is similarly anisotropic with a trench oblique fast direction but widespread nulls appear to reflect dramatic heterogeneity in anisotropic structure above the flat slab overall the regional anisotropic structure and thus the pattern of deformation appears to be closely tied to the location of the nazca ridge which further suggests that the ridge plays a key role in the mantle dynamics of the peruvian flat slab system

double seismic zone of the nazca plate in northern chile high resolution velocity structure petrological implications and thermomechanical modeling this paper presents an interdisciplinary study of the northern chile double seismic zone first a high resolution velocity structure of the subducting nazca plate has been obtained by the tomodd double difference tomography method the double seismic zone dsz is observed between 80 and 140 km depth and the two seismic planes is 20 km apart then the chemical and petrologic characteristics of the oceanic lithosphere associated with this dsz are deduced by using current thermal petrological seismological models and are compared to pressure temperature conditions provided by a numerical thermomechanical model our results agree with the common hypothesis that seismicity in both upper and lower planes is related to fluid releases associated with metamorphic dehydration reactions in the seismic upper plane located within the upper crust these reactions would affect material of basaltic morb composition and document different metamorphic reactions occurring within high p 2 4 gpa and low t 130 km lawsonite amphibole eclogite conditions the lower plane lying in the oceanic mantle can be associated with serpentinite dehydration reactions the vp and vs characteristics of the region in between both planes are consistent with a partially 25 30 vol antigorite 0 10 vol brucite and 4 10 vol chlorite hydrated harzburgitic material discrepancies persist that we attribute to complexities inherent to heterogeneous structural compositions while various geophysical indicators evidence particularly cold conditions in both the descending nazca plate and the continental fore arc thermomechanical models indicate that both seismic planes delimit the inner slab compressional zone around the 400 c 50 c isotherm lower plane earthquakes are predicted to occur in the slab s flexural neutral plane where fluids released from surrounding metamorphic reactions could accumulate and trigger seismicity fluids migrating upward from the tensile zone below could be blocked in their ascension by the compressive zone above this plane thus producing a sheeted layer of free fluids or a serpentinized layer therefore earthquakes may present either downdip compression and downdip tensile characteristics numerical tests indicate that the slab s thermal structure is not the only factor that controls the occurrence of inner slab compression 1 a weak ductile subduction channel and 2 a cold mantle fore arc both favor inner slab compression by facilitating transmission of compressional stresses from the continental lithosphere into the slab 3 decreasing the radius of curvature of the slab broadens the depth of inner slab compression whereas 4 decreasing upper plate convergence diminishes its intensity all these factors indicate that if dszs indeed contour inner slab compression they cannot be linked only to slab unbending but also to the transmission of high compressional stresses from the upper plate into the slab

three dimensional models of p wave velocity and p to s velocity ratio in the southern central andes by simultaneous inversion of local earthquake data the pisco 94 proyecto de investigation sismologica de la cordillera occidental 1994 seismological network of 31 digital broad band and short period three component seismometers was deployed in northern chile between the coastal cordillera and the western cordillera more than 5300 local seismic events were observed in a 100 day period a subset of high quality p and s arrival time data was used to invert simultaneously for hypocenters and velocity structure additional data from two other networks in the region could be included the velocity models show a number of prominent anomalies outlining an extremely thickened crust about 70 km beneath the forearc region an anomalous crustal structure beneath the recent magmatic arc western cordillera characterized by very low velocities and a high velocity slab a region of an increased vp vs ratio has been found directly above the wadati benioff zone which might be caused by hydration processes a zone of lower than average velocities and a high vp vs ratio might correspond to the asthenospheric wedge the upper edge of the wadati benioff zone is sharply defined by intermediate depth hypocenters while evidence for a double seismic zone can hardly be seen crustal events between the precordillera and the western cordillera have been observed for the first time and are mainly located in the vicinity of the salar de atacama down to depths of about 40 km

seismic investigations of the o higgins seamount group and juan fernandez ridge aseismic ridge emplacement and lithosphere hydration the o higgins seamount group is a cluster of volcanic domes located 120 km west of the central chilean trench on the crest of the juan fernandez ridge this aseismic hot spot track is subducting under south america triggering a belt of intraslab earthquake hypocenters extending about 700 km inland the juan fernandez ridge marks the southern boundary of a shallow subduction segment subduction of oceanic basement relief has been suggested as a cause for the flat slab segments characterizing the andean trench system the juan fernandez ridge however shows only moderate crustal thickening inadequate to cause significant buoyancy in 2001 wide angle seismic data were collected along two perpendicular profiles crossing the o higgins group we present tomographic images of the volcanic edifices and adjacent outer rise trench environment which indicate a magmatic origin of the seamounts dominated by extrusive processes high resolution bathymetric data yield a detailed image of a network of syngenetic structures reactivated in the outer rise setting a pervasive fault pattern restricted to the hot spot modified lithosphere coincides with anomalous low upper mantle velocities gained from a tomographic inversion of seismic mantle phases reduced uppermost mantle velocities are solely found underneath the juan fernandez ridge and may indicate mineral alterations enhanced buoyancy due to crustal and upper mantle hydration may contribute an additional mechanism for shallow subduction which prevails to the north after the southward migration of the juan fernandez ridge

pressure temperature estimates of the lizardite antigorite transition in high pressure serpentinites serpentine minerals in natural samples are dominated by lizardite and antigorite in spite of numerous petrological experiments the stability fields of these species remain poorly constrained this paper presents the petrological observations and the raman spectroscopy and xrd analyses of natural serpentinites from the alpine paleo accretionary wedge serpentine varieties are identified from a range of metamorphic pressure and temperature conditions from sub greenschist p 20 kbar t 460 c along a subduction geothermal gradient we used the observed mineral assemblage in natural serpentinite along with the tmax estimated by raman spectroscopy of the carbonaceous matter of the associated metasediments to constrain the temperature of the lizardite to antigorite transition at high pressures we show that below 300 c lizardite and locally chrysotile are the dominant species in the mesh texture between 320 and 390 c lizardite is progressively replaced by antigorite at the grain boundaries through dissolution precipitation processes in the presence of sio2 enriched fluids and through a solid state transition in the cores of the lizardite mesh above 390 c under high grade blueschist to eclogite facies conditions antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460 c

magmatic evidence for neogene lithospheric evolution of the central andean flat slab between 30 s and 32 s abstract geochemical data from andean miocene magmas erupted through the now volcanically inactive flat slab between 30 s to 32 s coupled with geological and geophysical data illuminate details on magmatic and continental lithospheric evolution over a progressively shallowing subduction zone pb sr and nd isotopic and trace element data show that main cordilleran precordilleran and pampean magmas were contaminated in both the mantle and crust and that the nature of the contaminants varied in time and space reflecting tectonic events contaminants included both enriched high lil element high sr and pb and low nd isotopic ratios and depleted low lile element low sr and pb and high nd isotopic ratios types in the western region main cordilleran earlier miocene lavas had contaminants with less enriched signatures than later miocene lavas progressive enrichment is attributed to a increasing amounts of sediment and tectonically eroded crust being subducted into the mantle wedge and b contamination in a thickening main cordilleran lower crust whose composition was progressively enriched this enrichment occurred through addition of upper crust by an intracrustal mixing process driven by a propagating wedge tip associated with westward wedging heating and deformation of crust from beneath the shortening precordillera thrust belt to the east further east magmas erupted through back arc crust have more depleted signatures those erupted in the central part through the evolving precordilleran thrust belt were contaminated by an older thinner grenville 1100 ma basement whose depleted signature is unique among central andean terranes late miocene pocho lavas erupted further east in conjunction with uplift of the sierras pampeanas show enrichment through time arguably these magmas could contain a component mechanically removed from the base of the thinning continental lithosphere to the west and progressively incorporated into the convecting asthenosphere first order mass balance calculations comparing the lithospheric geometry proposed for 20 ma with that today suggests that 60 of the lithosphere within 600 km of the trench at 20 ma has been lost this lithospheric loss is required to accommodate simultaneous shallowing of the oceanic slab and thickening and shortening of the crust continental lithosphere recycled into the asthenosphere could contribute to the sources of the enriched em1 and em2 mantle components in oceanic island basalts

anomalous seismic structure beneath the klyuchevskoy group kamchatka 1 the klyuchevskoy group is among the most active volcanic features on earth yet its position within the kamchatka subduction zone is hard to explain with a simple tectonic mechanism the geochemistry of klyuchevskoy group lavas is typical for volcanic products resulting from flux melting in the mantle wedge yet the depth to the subducting pacific plate beneath it is much larger than the average depth of volatile release in subduction zones we present new seismological constraints on the upper mantle structure beneath the klyuchevskoy group and identify a planar dipping seismic feature in the mantle wedge which is 100 km deep and appears to be sharply bounded we hypothesize that this upper mantle structure may be the true source of melts erupting at the klyuchevskoy group and that the presence of this feature may explain both the extraordinary productivity of the klyuchevskoy group and its unusual location

high pressure elasticity of serpentine and seismic properties of the hydrated mantle wedge 1 a subset of the single crystal elastic moduli of natural antigorite has been measured using brillouin scattering at high pressure up to 9 gpa aggregate properties and axial compressibilities are in good agreement with equation of state results from x ray diffraction stiffness along the c axis increases becoming close to those within the silicate layer near 7 gpa a slight discontinuity in the evolution of the elastic moduli near 7 gpa is associated with a phase transition raman spectroscopy shows that the transition does not occur in subducting slabs except in the core of the coldest slabs where serpentine may be subducted to depths of about 200 km varying temperature and pressure has limited effects on the interpretation of seismic velocities principally because of the limited depth range of serpentine stability mostly above 100 km depth and also due to the compensating effects of pressure and temperature that maintain velocity variations well within the uncertainties and statistical variability of seismological studies serpentinites are excellent candidates for explaining low velocities in the hydrated mantle wedge inversion of the moho and thin anisotropic low velocity layers at the plate interface serpentinite layers provide an alternative explanation to fluid saturated oceanic crust for explaining these phenomena and account for a transition to an aseismic decoupled plate interface

internal deformation of the subducted nazca slab inferred from seismic anisotropy subducting oceanic plates are often considered as cold rigid slabs analysis of seismic anisotropy in the subducted nazca plate beneath peru suggests that the plate has deformed internally during subduction

evidence for serpentinization of the forearc mantle wedge along the nicoya peninsula costa rica 1 characterizing the hydration state of the forearc mantle wedge yields valuable information on frictional stability at the downdip edge of subduction megathrusts simultaneous inversion of p and s wave arrival times collected as part of the costa rica seismogenic zone experiment yields 1d and 3d p and s wave velocity models vp and vs for the nicoya peninsula segment of the middle america trench nicoya peninsula 1d velocity models show similar velocity gradients to country wide 1d velocity models from 5 30 km depth but diverge at expected moho depths due to proximity to the subducting cocos plate 3d vp values range from 7 2 7 6 km sec in the forearc mantle wedge receiver functions computed at global seismic network station jts in northwestern costa rica confirm these vp values yield vp vs of 1 85 and place the continental moho at 36 4 km depth vp and vp vs are consistent with 15 25 serpentinization of the forearc mantle wedge

subduction of water into the mantle history of an alpine peridotite the erro tobbio peridotite western alps is a slice of subcontinental mantle that underwent pre jurassic passive rifting followed by sea floor hydration and cretaceous alpine subduction analysis of its extension and subduction related structures and metamorphism suggest the dip below the adria plate of both the extensional detachments and the subduction plane high pressure boudinage structures of rigid metarodingite within ductile antigorite serpentinite prove that antigorite survived subduction and remained stable at eclogite facies conditions subduction of serpentinized peridotites from ophiolite slabs is here considered the most effective mechanism of bringing water to great depth within the mantle

the fate of the downgoing oceanic plate insight from the northern cascadia subduction zone abstract in this study we use teleseismic receiver function analysis to image the seismic structure of the juan de fuca oceanic plate during its subduction beneath the north american plate seismic data have been recorded at 58 seismic stations deployed along the northern cascadia subduction zone harmonic decomposition of the receiver function data set along a trench normal profile allows us to image both the isotropic and the anisotropic structure of the plate slab our images highlight the presence of a highly anisotropic region at 40 70 km depths across the cascadia subduction zone the detected seismic anisotropy is interpreted to be related to both metamorphic facies e g blueschists and fluid released during the dehydration of the subducting mantle the processes of dehydration and metamorphism produce the variations of the seismic properties within each lithologic unit that constitutes the subducted slab i e basalts gabbro layer and upper mantle as the oceanic plate sinks in the upper mantle such variations make it almost impossible to recognize the plate boundary as a characteristic velocity jump at depth neither positive nor negative along the cascadia subduction zone based on the comparative interpretation of both the isotropic and the anisotropic structures retrieved we propose a 4 stage model of the evolution of the juan de fuca oceanic plate during its subduction beneath the north american plate

improving gas hydrate saturation estimates using p wave velocity log data by incorporating xrd data for detailed matrix mineralogy definition logging while drilling lwd and wire line wl velocity data from the second korean ulleung basin gas hydrate drilling expedition in 2010 ubgh2 were used to predict gas hydrate saturations using the effective medium theory emt at four representative sites we used sites ubgh2 10 and ubgh2 6 where gas hydrate occurs mostly within strata bound sandy turbidite layers sites ubgh2 2 1 and ubgh2 2 2 are used to investigate a typical cold vent setting and fracture dominated systems we first use a new technique to correct the lwd velocity data within the shallower portions of the borehole typically upper 60 100 m below seafloor mbsf where the nature of soft sediments often results in erroneous low velocity readings of the lwd tool around values between 1350 m s and 1400 m s using the vertical seismic profile vsp data a smooth background velocity trend is defined onto which the lwd data is projected after this correction the lwd velocity data of all four sites were used to estimate gas hydrate saturation values using the emt for a 2 phase mineral assemblage of 90 clay and 10 quartz only as well as a detailed mineralogy based on the core xrd data both these estimates are then compared to the estimates from the electrical resistivity data using an empirical archie analysis the results from emt and archie are on average the same when gas hydrate occurs as a pore filling medium within thick sediment layers of homogenous nature such as that encountered at site ubgh2 10 where saturations are about 60 of the pore space in an interval of 124 128 mbsf if gas hydrate occurs in thin sand layers where the thickness of the layer is at or smaller than the vertical resolution of the velocity logging tool archie based results are higher than the emt based results due to the higher resolution of the resistivity logging tool in cold vent settings with fracture dominated gas hydrate settings such as that investigated at sites ubgh2 2 1 and 2 2 2 archie based results are non reliable and strongly over estimated due to the anisotropic effect of near vertical hydrate filled fractures in such cold vent settings velocity based techniques using the emt are the preferred method to estimate the gas hydrate saturations at site ubgh2 2 1 hydrate saturations are on average 5 10 of the pore space and even lower at site ubgh2 2 2 less than 5 on average

multiple layers of seismic anisotropy and a low velocity region in the mantle wedge beneath japan evidence from teleseismic receiver functions 1 a complete characterization of seismic anisotropy can yield powerful constraints on mantle flow and deformation this is particularly important for the mantle wedge above subducting slabs where the geometry of mantle flow remains poorly understood we seek to better characterize the geometry and strength of anisotropy in the mantle wedge beneath northeast honshu and hokkaido both of which overlie the subducting pacific plate previous studies indicate that upper mantle anisotropy in the japan subduction zone is highly complex and exhibits dramatic spatial variations to provide complementary constraints on the along strike variations in anisotropy we analyze teleseismic receiver functions from stations of the broadband f net array using the multitaper correlation receiver function estimator backazimuthal variations in p to sh converted energy provide clear evidence for complicated anisotropic structure in the mantle wedge beneath northeast honshu and hokkaido in northeast honshu forward modeling of receiver functions using synthetic seismograms suggests the presence of an anisotropic layer in the forearc mantle wedge above the subducting slab and a second anisotropic layer beneath the crust of the overriding plate we also see evidence for a region of low isotropic velocity in the central part of the wedge beneath ne honshu comparisons between transverse component receiver functions at stations located in ne honshu and hokkaido reveal striking differences providing further evidence for along strike variation in anisotropic structure in the mantle wedge beneath japan

a lithospheric velocity model for the flat slab region of argentina from joint inversion of rayleigh wave phase velocity dispersion and teleseismic receiver functions fil ammirati jean baptiste ministerio de ciencia tecnologia e innovacion productiva agencia nacional de promocion cientifica y tecnologica argentina universidad nacional de san juan facultad de ciencias exactas fisicas y naturales departamento de geofisica y astronomia argentina

serpentine mineral replacements of natural olivine and their seismic implications oceanic lizardite versus subduction related antigorite we report on microstructural data obtained by optical microscopy and transmission electron microscopy concerning the crystallographic relationships of serpentine minerals with their host olivine in two contrasting situations in the first case mesh textured lizardite liz is developed in a standard 60 serpentinized oceanic harzburgite from the oman ophiolite where olivine converts to columnar lizardite the joined columns are perpendicular to the basal plane 001 liz corresponding to the pseudofibres observed optically the plane 001 liz is locally parallel to the narrow boundary ol liz thus column orientations register the interface of serpentinization the ol liz relationships are not strictly topotactic but reflect preferred cracking orientations in olivine parallel to 010 ol in the second case antigorite atg develops in a rare sample of antigorite schist in a kimberlite from moses rock colorado plateau representative of a suprasubduction zone mantle wedge high resolution transmission electron microscopy hrtem images along 010 atg show domains of very regular modulation with a 43 center dot 5 a wavelength m 17 where m is the number of silicate tetrahedra along the wave with few defects indicative of hp ht antigorite and also heavily kinked regions as fingerprints of strong tectonic shear tem imaging and electron diffraction patterns reveal two topotactic relationships between antigorite and olivine 100 atg 010 ol and atg ol the planes in contact are 001 atg 100 ol and 001 atg 010 ol respectively the 010 atg 001 ol and antigorite lamellae are parallel to the forsterite b axis in both cases the topography of olivine serpentine interfaces is controlled by open fluid pathways along microcracks oriented according to the anisotropy of the olivine aggregate in the cases studied the serpentine aggregate exhibits a preferred orientation inherited from that of the peridotite these results have some relevance to the seismic anisotropy of serpentinized mantle anisotropy of propagation of seismic waves as a result of the olivine fabric is maintained and reinforced with the development of lizardite conversely the development of antigorite produces a trench parallel fast s wave polarization and an anisotropy that is lowered at low degrees of serpentinization and then increased with increasing serpentinization

crustal structure and moho geometry beneath the northern apennines italy 1 we used receiver functions rfs from broad band seismic stations to investigate the crustal structure of the northern apennines italy additionally we use data obtained in this study to provide initial constraints for a map of the moho depth of italy ten stations were deployed along a transect n75 e during the 1994 geomodap project rf analysis shows the presence of lateral variations in the crust we observed patterns of symmetric and anti symmetric converted phases from radial and tangential rfs vs back azimuth these patterns can be explained by the presence of dipping interfaces and or anisotropy within the crust we then inverted rfs following the inversion scheme proposed by sambridge 1999 the results show the presence of s velocity inversions in the lower crust beneath the apennines the upwelling of moho in the tyrrhenian area and its progressive deepening from the tyrrhenian sea toward the adriatic coast

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

receiver function study of the cascadia megathrust evidence for localized serpentinization 1 we characterize the crustal and upper mantle structure of the cascadian fore arc using receiver function analysis of data from long running observatories located along the north american margin in oregon and washington we identify coherent sv polarized phases consistent with p s conversion from the top of the subducting juan de fuca plate throughout the area west of the volcanic arc in many instances we also find evidence for deeper features likely associated with the subducted oceanic crust at many locations sh polarized energy in receiver functions is associated with the surface of the subducting plate these sh polarized phases display significant directional variation and show clear polarity reversals consistent with the presence of seismic anisotropy previously a layer of serpentinized material within the megathrust has been proposed to explain an anisotropic low velocity high poisson s ratio layer identified by receiver function analysis at corvallis oregon global seismic network station cor our new results suggest that this feature is widespread in cascadia but the specific receiver function signature varies this anisotropic supraslab layer is supported by receiver function estimates beneath seismic stations which similar to cor are 100 km from the coast and 40 km above the slab surface however the orientation of rock fabric implied by the anisotropy varies greatly from station to station modeling with synthetic seismograms supports the existence of serpentinite beneath station gnw green mountain washington closer to the coast where the slab is shallower an anisotropic layer is likely beneath some stations but the evidence is less conclusive we propose that the anisotropic signature beneath the inland stations is related to serpentinization of the mantle wedge at greater depths best explained by the presence of the polymorph antigorite hypothetically suctioned upward by slab rollback a deformed sliver of serpentinite rich rock above the slab interface would likely display the observed large spatial variation in anisotropic symmetry axis within the layer consistent with our observations

structural features of the subducting slab beneath the kii peninsula central japan seismic evidence of slab segmentation dehydration and anisotropy 1 seismic evidence of lithospheric slab segmentation dehydration and anisotropic rocks beneath the kii peninsula central japan is detected with receiver function rf analyses by stacking radial and transverse rfs with moveout perturbations we estimate the downdip azimuth dip angle and depth of the slab moho beneath each seismic station comparing the downdip azimuth to the slab convergence azimuth we can classify the slab into three segments the segments correspond to both the fault zones of past megathrust earthquakes and to the spatial distribution of intraslab seismicity the slab moho lies just above the intraslab seismicity in the southwestern kii peninsula slab moho lies deeper at the eastern part of the peninsula at the bottom of seismicity this suggests that earthquakes occur only within the slab mantle under the southern kii peninsula the dip angle of slab moho becomes steeper at 40 to 50 km depth perhaps caused by dehydration transformation of basalt to eclogite at some stations located in the western part of the kii peninsula rf estimated dip angle of the interface is much steeper than that estimated by spatial trends in seismicity since the local seismicity distribution reflects well the larger scale geometry of the slab the rf estimated dip angle may imply anisotropic wave speeds within strongly sheared and or hydrated media around the slab moho these detailed features of the slab may be one key in distinguishing the source regions of megathrust earthquakes in the nankai region

relationship between anisotropy of p and s wave velocities and anisotropy of attenuation in serpentinite and amphibolite velocities and q values of p and s waves as functions of pressure and temperature at 100 and 600 mpa are presented for a serpentinite and an amphibolite both rocks exhibit a strong lattice preferred orientation lpo of the major mineral phases antigorite and hornblende respectively velocities and q values increase with pressure the rate of increase is different in the three orthogonal directions normal and parallel to foliation and lineation and closely related to progressive closure of microcracks increasing temperature decreases velocities and q values only slightly as long as thermal cracking is prevented by the applied confining pressure substantial anisotropy of velocities and q in p and s waves is observed in both rocks but is found to be different in origin anisotropy of p and s wave velocities is highest at low pressure and basically caused by constructive interference of effects related to oriented microcracks and to the lpo of major minerals increasing confining pressure decreases velocity anisotropy at a smaller and smaller rate the residual anisotropy of p and s wave velocities shear wave splitting at high confining pressure is mainly a result of preferred mineral orientation by contrast anisotropy of q is very low at low confining pressure and markedly enhanced as pressure is increased at high confining pressure substantial anisotropy of q in p waves is apparent but reversed from that of p wave velocities qp is highest in the direction normal to the foliation plane whereas vp and vs is lowest in this direction the generation of a pronounced anisotropy of qp by increasing pressure is due to a directionally dependent increase of contact areas on the oriented grain boundaries of the platy minerals defining the foliation the increase of q with pressure in the direction normal to foliation is mainly caused by the decrease of energy loss due to compressive strain relative to shear strain the reverse is true for the x and y directions serpentinite and x direction amphibolite parallel to the foliation plane

an inverted continental moho and serpentinization of the forearc mantle volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back arc region but the forearc mantle in the corner of the wedge is thought to be significantly cooler here we explore the structure of the mantle wedge in the southern cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers we find very low shear wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an inverted continental moho which reverts to normal polarity seaward of the cascade arc this observation provides compelling evidence for a highly hydrated and serpentinized forearc region consistent with thermal and petrological models of the forearc mantle wedge this serpentinized material is thought to have low strength and may therefore control the down dip rupture limit of great thrust earthquakes as well as the nature of large scale flow in the mantle wedge

seismological evidence for the existence of serpentinized wedge mantle serpentinized peridotite is detected seismologically by mapping poisson s ratio beneath the kanto district central japan because serpentinite has a higher poisson s ratio than other rocks in the crust and upper mantle we determine three dimensional p and s wave velocity models using travel time tomography and then construct a three dimensional map of poisson s ratio the models show low velocity anomalies near the upper boundary of the slab in a depth range from 20 to 70 km in the low velocity region there is an area with poisson s ratio greater than 0 3 at depths of 20 45 km which we attribute to serpentinized peridotite the rest of the low velocity region is interpreted as descending oceanic crust with a poisson s ratio of about 0 25 the high poisson s ratio area is associated with low seismicity and weak interplate coupling which is consistent with the ductility of serpentinite

mapping seismic anisotropy using harmonic decomposition of receiver functions an application to northern apennines italy 1 isotropic and anisotropic seismic structures across the northern apennines italy subduction zone are imaged using a new method for the analysis of teleseismic receiver functions rfs more than 13 000 p wave coda of teleseismic records from the 2003 2007 retreating trench extension and accretion tectonics retreat experiment are used to provide new insights into a peculiar subduction zone between two continental plates that is considered a focal point of mediterranean evolution a new methodology for the analysis of receiver functions is developed which combines both migration and harmonic decomposition of the receiver function data set the migration technique follows a classical common conversion point scheme and helps to focus on a crucial depth range 20 70 km where the mantle wedge develops harmonic decomposition of a receiver function data set is a novel and less explored approach to the analysis of p to s converted phases the separation of the back azimuth harmonics is achieved through a numerical regression of the stacked radial and transverse receiver functions from which we obtain independent constraints on both isotropic and anisotropic seismic structures the application of our method to the retreat data set succeeds both in confirming previous knowledge about seismic structure in the area and in highlighting new structures beneath the northern apennines chain where previous studies failed to clearly retrieve the geometry of the subducted interfaces we present our results in closely spaced profiles across and along the northern apennines chain to highlight the convergence of the tyrrhenian and the adriatic microplates which differ in their crustal structure where the adriatic microplate subducts beneath tuscany and the tyrrhenian sea a signature of the dipping adriatic moho is clearly observed beneath the tyrrhenian moho in a large portion of the forearc region in the area where the two mohos overlap our new analysis reveals the presence of an anisotropic body above the subducted moho there is a strong ps converted phase with anisotropic characteristics from the top of the adriatic plate to a depth of at least 80 km because the ps conversion occurs much deeper than similar ps phases in cascadia and japan dehydration of oceanic crust seems unlikely as a causative factor rather the existence of this body trapped between the two interfaces supports the hypothesis of lower crustal delamination in a postsubduction tectonic setting

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

horizontal subduction zones convergence velocity and the building of the andes we discuss the relationships between andean shortening plate velocities at the trench and slab geometry beneath south america although some correlation exists between the convergence velocity and the westward motion of south america on the one hand and the shortening of the continental plate on the other hand plate kinematics neither gives a satisfactory explanation to the andean segmentation in general nor explains the development of the bolivian orocline in paleogene times we discuss the cenozoic history of horizontal slab segments below south america arguing that they result from the subduction of oceanic plateaus whose effect is to switch the buoyancy of the young subducting plate to positive we argue that the existence of horizontal slab segments below the central andes during eocene oligocene times and below peru and north central chile since pliocene resulted 1 in the shortening of the continental plate interiors at a large distance from the trench 2 in stronger interplate coupling and ultimately 3 in a decrease of the trenchward velocity of the oceanic plate present day horizontal slab segments may thus explain the diminution of the convergence velocity between the nazca and south american plates since late miocene

global prevalence of double benioff zones double benioff zones provide opportunities for insight into seismogenesis because the underlying mechanism must explain two layers of deep earthquakes and the separation between them we characterize layer separation inside subducting plates with a coordinate rotation to calculate the slab normal distribution of earthquakes benchmark tests on well established examples confirm that layer separation is accurately quantified with global seismicity catalogs alone global analysis reveals double benioff zones in 30 segments including all 16 subduction zones investigated with varying subducting plate ages and stress orientations which implies that they are inherent in subducting plates layer separation increases with age and is more consistent with dehydration of antigorite than chlorite

continental and oceanic crustal structure of the pampean flat slab region western argentina using receiver function analysis new high resolution results summary the pampean flat slab of central chile and argentina 30 32 s has strongly influenced cenozoic tectonics in western argentina which contains both the thick skinned basement cored uplifts of the sierras pampeanas and the thin skinned andean precordillera fold and thrust belt in this region of south america the nazca plate is subducting nearly horizontally beneath the south american plate at 100 km depth to gain a better understanding of the deeper structure of this region including the transition from flat to normal subduction to the south three iris passcal arrays of broad band seismic stations have been deployed in central argentina using the dense siembra array combined with the broader charge and esp arrays the flat slab is imaged for the first time in 3 d detail using receiver function rf analysis a distinct pair of rf arrivals consisting of a negative pulse that marks the top of the oceanic crust followed by a positive pulse which indicates the base of the oceanic crust can be used to map the slab s structure depths to moho and oceanic crustal thicknesses estimated from rf results provide new more detailed regional maps an improved depth to continental moho map shows depths of more than 70 km in the main cordillera and 50 km in the western sierras pampeanas that shallow to 35 km in the eastern sierras pampeanas depth to moho contours roughly follow terrane boundaries offshore the hotspot seamount chain of the juan fernandez ridge jfr is thought to create overthickened oceanic crust providing a mechanism for flat slab subduction by comparing synthetic rfs based on various structures to the observed rf signal we determine that the thickness of the oceanic crust at the top of the slab averages at least 13 19 km supporting the idea of a moderately overthickened crust to provide the additional buoyancy for the slab to remain flat the overthickened region is broader than the area directly aligned with the path of the jfr however and indicates along with the slab earthquake locations that the flat slab area is wider than the jfr volcanic chain observed in the offshore bathymetry further rfs indicate that the subducted oceanic crust in the region directly along the path of the subducted ridge is broken by trench parallel faults one explanation for these faults is that they are older structures within the oceanic crust that were created when the slab subducted alternatively it is possible that faults formed recently from tectonic underplating caused by increased interplate coupling in the flat slab region

cenozoic tectonics of the high andes of west central argentina 30 36 s latitude abstract the structure of the central andes shows three distinctive segments characterized by different geometries these geometries are superimposed on the present large scale plate tectonic setting characterized by distinct subduction segments the northern la ramada segment is a thick skinned fold and thrust belt formed by tectonic inversion of a late triassic rift the central aconcagua segment consists of a thin skinned fold and thrust belt while the southern malargue segment like the first one is a thick skinned fold and thrust belt developed by tectonic inversion of a late triassic early jurassic rift system during late cenozoic times the amount of shortening gradually decreases from north to south as indicated by the crustal roots of the central andes the different geometries along the principal cordillera controlled the abrupt changes in the shortening among segments the structure of precordillera and sierras pampeanas has also been considered in order to account for the total shortening in the la ramada segment the main shortening occurred in the precordillera in the aconcagua segment in the principal cordillera while in the malargue segment the shortening is widely distributed in a broader principal cordillera because south of the flat slab subduction segment the precordillera and sierras pampeanas are missing