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H R Wenk - One of the best experts on this subject based on the ideXlab platform.
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Seismic Anisotropy of serpentinite from val malenco italy
Journal of Geophysical Research, 2015Co-Authors: H Kern, T Lokajicek, T Svitek, H R WenkAbstract:Serpentinites, deformed in mantle subduction zones, are thought to contribute significantly to Seismic Anisotropy of the upper mantle and have therefore been of great interest with studies on deformation, preferred orientation, and elastic properties. Here we present a combined study of a classical sample from Val Malenco, Italy, investigating the microstructure and texture with state-of-the art synchrotron X-ray and neutron diffraction methods and measuring ultrasonic velocities both with a multi-anvil apparatus and a novel instrument to measure 3-D velocities on spheres. Both, results from diffraction methods and velocity measurements are compared, discussing advantages and disadvantages. From spherical velocities, elastic tensor properties can be derived by inversion. Also from quantitative texture measurements, elastic properties can be modeled by self-consistent averaging. Good agreement between the velocity and microstructural models is observed.
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Seismic Anisotropy of serpentinite from val malenco italy
Journal of Geophysical Research, 2015Co-Authors: H Kern, T Lokajicek, T Svitek, H R WenkAbstract:© 2015. American Geophysical Union. All Rights Reserved. Serpentinites, deformed in mantle subduction zones, are thought to contribute significantly to Seismic Anisotropy of the upper mantle and have therefore been of great interest with studies on deformation, preferred orientation, and elastic properties. Here we present a combined study of a classical sample from Val Malenco, Italy, investigating the microstructure and texture with state-of-the art synchrotron X-ray and neutron diffraction methods and measuring ultrasonic velocities both with a multi-anvil apparatus and a novel instrument to measure 3-D velocities on spheres. Both, results from diffraction methods and velocity measurements are compared, discussing advantages and disadvantages. From spherical velocities, elastic tensor properties can be derived by inversion. Also from quantitative texture measurements, elastic properties can be modeled by self-consistent averaging. Good agreement between the velocity and microstructural models is observed.
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synthetic Seismic Anisotropy models within a slab impinging on the core mantle boundary
Geophysical Journal International, 2014Co-Authors: A K Mcnamara, Sanne Cottaar, Barbara Romanowicz, H R WenkAbstract:The lowermost few hundreds of kilometres of the Earth's mantle are elastically anisotropic; Seismic velocities vary with direction of propagation and polarization. Observations of strong Seismic Anisotropy correlate with regions where subducted slab material is expected. In this study, we evaluate the hypothesis that crystal preferred orientation (CPO) in a slab, as it impinges on the core–mantle boundary, is the cause of the observed Anisotropy. Next, we determine if fast polarization directions seen by shear waves can be mapped to directions of geodynamic flow. This approach is similar to our previous study performed for a 2-D geodynamic model. In this study, we employ a 3-D geodynamic model with temperature-dependent viscosity and kinematic velocity boundary conditions defined at the surface of the Earth to create a broad downwelling slab. Tracers track the deformation that we assume to be accommodated by dislocation creep. We evaluate the models for the presence of perovskite or post-perovskite and for different main slip systems along which dislocation creep may occur in post-perovskite [(100),(010) and (001)]—resulting in four different mineralogical models of CPO. Combining the crystal pole orientations with single crystal elastic constants results in Seismically distinguishable models of Seismic Anisotropy. The models are evaluated against published Seismic observations by analysing different anisotropic components: the radial Anisotropy, the splitting for (sub-)vertical phases (i.e. azimuthal Anisotropy), and the splitting for subhorizontal phases. The patterns in radial Anisotropy confirm our earlier results in 2-D. Observations of radial Anisotropy and splitting in subhorizontal phases are mostly consistent with our models of post-perovskite with (010)-slip and (001)-slip. Our model of (001)-slip predicts stronger splitting than for (010)-slip for horizontally propagating phases in all directions. The strongest Seismic Anisotropy in this model occurs where the slab impinges on the core–mantle boundary. The azimuthal Anisotropy pattern for (001)-slip shows fast axis directions at the edges of the slab (sub-)parallel to flow directions, suggesting horizontal flows may be mapped out in the lowermost mantle using Seismic observations.
David Mainprice - One of the best experts on this subject based on the ideXlab platform.
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Deformation, crystal preferred orientations, and Seismic Anisotropy in the Earth's D '' layer
Earth and Planetary Science Letters, 2018Co-Authors: Andrea Tommasi, Patrick Cordier, Alexandra Goryaeva, Philippe Carrez, David MainpriceAbstract:We use a forward multiscale model that couples atomistic modeling of intracrystalline plasticity mechanisms (dislocation glide ± twinning) in MgSiO3 post-perovskite (PPv) and periclase (MgO) at lower mantle pressures and temperatures to polycrystal plasticity simulations to predict crystal preferred orientations (CPO) development and Seismic Anisotropy in D″. We model the CPO evolution in aggregates of 70% PPv and 30% MgO submitted to simple shear, axial shortening, and along corner-flow streamlines, which simulate changes in flow orientation similar to those expected at the transition between a downwelling and flow parallel to the core–mantle boundary (CMB) within D″ or between CMB-parallel flow and upwelling at the borders of the large low shear wave velocity provinces (LLSVP) in the lowermost mantle. Axial shortening results in alignment of PPv [010] axes with the shortening direction. Simple shear produces PPv CPO with a monoclinic symmetry that rapidly rotates towards parallelism between the dominant [100](010) slip system and the macroscopic shear. These predictions differ from MgSiO3 post-perovskite textures formed in diamond-anvil cell experiments, but agree with those obtained in simple shear and compression experiments using CaIrO3 post-perovskite. Development of CPO in PPv and MgO results in Seismic Anisotropy in D″. For shear parallel to the CMB, at low strain, the inclination of ScS, Sdiff, and SKKS fast polarizations and delay times vary depending on the propagation direction. At moderate and high shear strains, all S-waves are polarized nearly horizontally. Downwelling flow produces Sdiff, ScS, and SKKS fast polarization directions and birefringence that vary gradually as a function of the back-azimuth from nearly parallel to inclined by up to 70° to CMB and from null to ∼5%. Change in the flow to shear parallel to the CMB results in dispersion of the CPO, weakening of the Anisotropy, and strong azimuthal variation of the S-wave splitting up to 250 km from the corner. Transition from horizontal shear to upwelling also produces weakening of the CPO and complex Seismic Anisotropy patterns, with dominantly inclined fast ScS and SKKS polarizations, over most of the upwelling path. Models that take into account twinning in PPv explain most observations of Seismic Anisotropy in D″, but heterogeneity of the flow at scales
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Transmission electron microscopy characterization of dislocations and slip systems in K-lingunite: Implications for the Seismic Anisotropy of subducted crust
Physics of the Earth and Planetary Interiors, 2010Co-Authors: A. Mussi, David Mainprice, P. Cordier, D. J. FrostAbstract:In order to estimate the Seismic Anisotropy of subducted crust, polycrystalline samples of KA1Si(3)O(8) K-lingunite (25% of the total subducted transformed sediments), have been synthesized and deformed under the temperature and pressure conditions of the subducted slabs. Transmission electron microscopy (TEM) characterizations of the recovered samples reveal that the microstructures are clearly dominated by [001] glide involving screw dislocations. For this reason, only {100} could be identified as glide planes, the question of [001] slip on {110} remains open. Few 1/2 < 111 > dislocations were observed gliding on {110} planes, which implies that 1/2 < 111 > {110} is a harder slip system than those involving [001] slip. The occurrence of sub-grain boundaries suggests that diffusion and climb might be active under these conditions.;To assess the texture of polycrystalline K-lingunite, the crystal preferred orientations (CP05) were calculated using visco-plastic self-consistent (VPSC) polycrystalline plasticity model in simple shear using the slip systems identified by TEM. Finally, the Seismic properties of K-lingunite aggregates were calculated from the CPO and single crystal elasticity tensor. K-lingunite is predicted to have a high Seismic Anisotropy, which could combine constructively with one of the stishovite (same proportion as K-lingunite at the transition zone depth ranges).
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fault induced Seismic Anisotropy by hydration in subducting oceanic plates
Nature, 2008Co-Authors: Manuele Faccenda, L Burlini, Taras Gerya, David MainpriceAbstract:In subduction zones, the anisotropic fast shear-wave component is generally observed to be orientated parallel to the strike of the trench. The interpretation of this shear-wave splitting above subduction zones has, however, been controversial and none of the inferred models appears to be explain the whole range of anisotropic patterns observed worldwide. Manuele Faccenda and colleagues now show that the amount and geometry of Seismic Anisotropy measured in the forearc regions of subduction zones strongly depend on the preferred orientation of hydrated faults in the subducting oceanic plate. The Anisotropy originates from the crystallographic preferred orientation of highly anisotropic hydrous minerals formed along steeply dipping faults and from the larger scale vertical layering consisting of dry and hydrated crust-mantle sections whose spacing is several times smaller than teleSeismic wavelengths. Faults orientations and estimated delay times are consistent with the observed shear-wave splitting patterns in most subduction zones. It is shown that the amount and geometry of Seismic Anisotropy measured in the forearc regions of subduction zones strongly depend on the preferred orientation of hydrated faults in the subducting oceanic plate. The Anisotropy originates from the crystallographic preferred orientation of highly anisotropic hydrous minerals formed along steeply dipping faults and from the larger-scale vertical layering consisting of dry and hydrated crust–mantle sections, the spacing of which is several times smaller than teleSeismic wavelengths. The variation of elastic-wave velocities as a function of the direction of propagation through the Earth’s interior is a widely documented phenomenon called Seismic Anisotropy. The geometry and amount of Seismic Anisotropy is generally estimated by measuring shear-wave splitting, which consists of determining the polarization direction of the fast shear-wave component and the time delay between the fast and slow, orthogonally polarized, waves. In subduction zones, the teleSeismic fast shear-wave component is oriented generally parallel to the strike of the trench1, although a few exceptions have been reported (Cascadia2 and restricted areas of South America3,4). The interpretation of shear-wave splitting above subduction zones has been controversial and none of the inferred models seems to be sufficiently complete to explain the entire range of anisotropic patterns registered worldwide1. Here we show that the amount and the geometry of Seismic anisotropies measured in the forearc regions of subduction zones strongly depend on the preferred orientation of hydrated faults in the subducting oceanic plate. The Anisotropy originates from the crystallographic preferred orientation of highly anisotropic hydrous minerals (serpentine and talc) formed along steeply dipping faults and from the larger-scale vertical layering consisting of dry and hydrated crust–mantle sections whose spacing is several times smaller than teleSeismic wavelengths. Fault orientations and estimated delay times are consistent with the observed shear-wave splitting patterns in most subduction zones.
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Fault-induced Seismic Anisotropy by hydration in subducting oceanic plates
Nature, 2008Co-Authors: Manuele Faccenda, L Burlini, Taras Gerya, David MainpriceAbstract:It is shown that the amount and geometry of Seismic Anisotropy measured in the forearc regions of subduction zones strongly depend on the preferred orientation of hydrated faults in the subducting oceanic plate. The Anisotropy originates from the crystallographic preferred orientation of highly anisotropic hydrous minerals formed along steeply dipping faults and from the larger-scale vertical layering consisting of dry and hydrated crust–mantle sections, the spacing of which is several times smaller than teleSeismic wavelengths.
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Fault-induced Seismic Anisotropy by hydration in subducting oceanic plates
Nature, 2008Co-Authors: Manuele Faccenda, L Burlini, Taras Gerya, David MainpriceAbstract:The variation of elastic- wave velocities as a function of the direction of propagation through the Earth's interior is a widely documented phenomenon called Seismic Anisotropy. The geometry and amount of Seismic Anisotropy is generally estimated by measuring shearwave splitting, which consists of determining the polarization direction of the fast shear- wave component and the time delay between the fast and slow, orthogonally polarized, waves. In subduction zones, the teleSeismic fast shear- wave component is oriented generally parallel to the strike of the trench(1), although a few exceptions have been reported (Cascadia(2) and restricted areas of South America(3,4)). The interpretation of shear- wave splitting above subduction zones has been controversial and none of the inferred models seems to be sufficiently complete to explain the entire range of anisotropic patterns registered worldwide(1). Here we show that the amount and the geometry of Seismic anisotropies measured in the forearc regions of subduction zones strongly depend on the preferred orientation of hydrated faults in the subducting oceanic plate. The Anisotropy originates from the crystallographic preferred orientation of highly anisotropic hydrous minerals (serpentine and talc) formed along steeply dipping faults and from the larger- scale vertical layering consisting of dry and hydrated crust - mantle sections whose spacing is several times smaller than teleSeismic wavelengths. Fault orientations and estimated delay times are consistent with the observed shear- wave splitting patterns in most subduction zones.
Helene Couvy - One of the best experts on this subject based on the ideXlab platform.
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Experimental deformation of forsterite, wadsleyite and ringwoodite: Implications for Seismic Anisotropy of the Earth's mantle
2005Co-Authors: Helene CouvyAbstract:The rheological properties of the major minerals of the Earth's mantle are still not well constrained. However, these properties are crucial for the understanding of a wide range of processes in the Earth's interior such as mantle convection. The purpose of this work is to address the issue of the rheology of the lowermost upper mantle and of the transition zone through the mechanical properties at high pressure of olivine (with forsterite composition Mg2SiO4) and of its high-pressure polymorphs wadsleyite and ringwoodite. Indeed, the properties of the Earth's mantle can be inferred as a first approximation from the mechanical properties of those polymorphs which volumetrically dominate the mineralogy of the region of concern. Deformation experiments have been performed on hot-pressed forsterite samples and on pre-synthesized wadsleyite and ringwoodite samples under pressure conditions of the Earth's mantle and at 1300-1400°C. The possible influence of the phase transformation from forsterite to wadsleyite on rheology has been also investigated. Deformation has been achieved by shear using the Kawai-type multianvil apparatus. Complementary experiments on forsterite have been performed in the newly developed Deformation-DIA. Some of them have been carried out on a synchrotron beam line to perform in-situ stress and strain measurements. In order to gain a maximum of information on the deformation mechanisms and on the Crystallographic Preferred Orientation (CPO), a special attention has been devoted to the microstructural characterisation of the samples. Electron BackScattering Diffraction (ESBD) and Transmission Electron Microscope (TEM) have been mainly used. An important pressure-induced change in deformation mechanism is shown in forsterite. The deformation of forsterite at high pressure and temperature is dominated by the [001](hk0) slip system rather than the [100](010) glide which is extensively observed at low pressure and high temperature.. Concerning the high-pressure polymorphs, their plastic behaviour has been studied with a strong emphasis on the formation of CPO. ViscoPlastic Self Consistent (VPSC) modelling is used to link the CPO with known elementary deformation mechanisms of these phases. The main features of the CPO of wadsleyite are characterized by the alignment of the [100] axes parallel to the shear direction and the alignment of the [001] axes toward the normal to the shear plane. Too many uncertainties remain on the ringwoodite CPO for them being used to interpret Seismic Anisotropy. Finally, we suggest that strain-induced CPO might be responsible for the Seismic Anisotropy observed in the lowermost upper mantle and in the upper part of the transition zone. The low Seismic Anisotropy of the lowermost upper mantle can be explained from the slip system change in forsterite and the CPO of wadsleyite point toward a dominant tangential flow in the upper part of the transition zone.
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pressure sensitivity of olivine slip systems and Seismic Anisotropy of earth s upper mantle
Nature, 2005Co-Authors: David Mainprice, Helene Couvy, Andrea Tommasi, Patrick Cordier, Daniel J FrostAbstract:The mineral olivine dominates the composition of the Earth's upper mantle and hence controls its mechanical behaviour and Seismic Anisotropy. Experiments at high temperature and moderate pressure, and extensive data on naturally deformed mantle rocks, have led to the conclusion that olivine at upper-mantle conditions deforms essentially by dislocation creep with dominant [100] slip. The resulting crystal preferred orientation has been used extensively to explain the strong Seismic Anisotropy observed down to 250 km depth1,2,3,4. The rapid decrease of Anisotropy below this depth has been interpreted as marking the transition from dislocation to diffusion creep in the upper mantle5. But new high-pressure experiments suggest that dislocation creep also dominates in the lower part of the upper mantle, but with a different slip direction. Here we show that this high-pressure dislocation creep produces crystal preferred orientations resulting in extremely low Seismic Anisotropy, consistent with seismological observations below 250 km depth. These results raise new questions about the mechanical state of the lower part of the upper mantle and its coupling with layers both above and below.
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strain induced Seismic Anisotropy of wadsleyite polycrystals and flow patterns in the mantle transition zone
Journal of Geophysical Research, 2004Co-Authors: Andrea Tommasi, Helene Couvy, David Mainprice, Patrick Cordier, Catherine ThoravalAbstract:[1] We use forward models based on recent high-pressure experimental data on mantle minerals to predict the Seismic Anisotropy produced by plastic strain of orthorhombic wadsleyite, the dominant mineral in the upper transition zone. These models predict a weak Seismic Anisotropy for a polycrystal of pyrolitic composition (60% wadsleyite, 40% garnet) at transition zone conditions: ∼2% for P and ∼1% for S waves for a shear strain of 1. Both P and S wave Anisotropy patterns show an orthorhombic symmetry. P waves propagate faster at low angle to the shear direction and slower at high angle to the shear plane. S wave Anisotropy is characterized by faster propagation of waves polarized at low angle to the shear direction. Horizontal shearing results therefore in higher velocities for horizontally propagating P waves (PH) and horizontally polarized S waves (SH), as well as in weak azimuthal variation of SV and SH velocities. On the other hand, vertical flow leads to higher velocities for vertically propagating P waves (PV) and vertically polarized S waves (SV) and to a weak azimuthal variation of SV velocity but to a roughly constant SH velocity. Analysis of global observations of Seismic Anisotropy in the transition zone in the light of these models supports dominant horizontal flow in the uppermost transition zone, in agreement with predictions of geodynamical models that explicitly introduce phase transitions.
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Experimental deformation of forsterite, wadsleyite and ringwoodite: Implications for Seismic Anisotropy of the Earth’s mantle
2004Co-Authors: Helene CouvyAbstract:The rheological properties of the major minerals of the Earth's mantle are still not well constrained. However, these properties are crucial for the understanding of a wide range of processes in the Earth's interior such as mantle convection. The purpose of this work is to address the issue of the rheology of the lowermost upper mantle and of the transition zone through the mechanical properties at high pressure of olivine (with forsterite composition Mg2SiO4) and of its high-pressure polymorphs wadsleyite and ringwoodite. Indeed, the properties of the Earth's mantle can be inferred as a first approximation from the mechanical properties of those polymorphs which volumetrically dominate the mineralogy of the region of concern. Deformation experiments have been performed on hot-pressed forsterite samples and on pre-synthesized wadsleyite and ringwoodite samples under pressure conditions of the Earth's mantle and at 1300-1400°C. The possible influence of the phase transformation from forsterite to wadsleyite on rheology has been also investigated. Deformation has been achieved by shear using the Kawai-type multianvil apparatus. Complementary experiments on forsterite have been performed in the newly developed Deformation-DIA. Some of them have been carried out on a synchrotron beam line to perform in-situ stress and strain measurements. In order to gain a maximum of information on the deformation mechanisms and on the Crystallographic Preferred Orientation (CPO), a special attention has been devoted to the microstructural characterisation of the samples. Electron BackScattering Diffraction (ESBD) and Transmission Electron Microscope (TEM) have been mainly used. An important pressure-induced change in deformation mechanism is shown in forsterite. The deformation of forsterite at high pressure and temperature is dominated by the [001](hk0) slip system rather than the [100](010) glide which is extensively observed at low pressure and high temperature.. Concerning the high-pressure polymorphs, their plastic behaviour has been studied with a strong emphasis on the formation of CPO. ViscoPlastic Self Consistent (VPSC) modelling is used to link the CPO with known elementary deformation mechanisms of these phases. The main features of the CPO of wadsleyite are characterized by the alignment of the [100] axes parallel to the shear direction and the alignment of the [001] axes toward the normal to the shear plane. Too many uncertainties remain on the ringwoodite CPO for them being used to interpret Seismic Anisotropy. Finally, we suggest that strain-induced CPO might be responsible for the Seismic Anisotropy observed in the lowermost upper mantle and in the upper part of the transition zone. The low Seismic Anisotropy of the lowermost upper mantle can be explained from the slip system change in forsterite and the CPO of wadsleyite point toward a dominant tangential flow in the upper part of the transition zone.
Andrea Tommasi - One of the best experts on this subject based on the ideXlab platform.
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Deformation, crystal preferred orientations, and Seismic Anisotropy in the Earth's D '' layer
Earth and Planetary Science Letters, 2018Co-Authors: Andrea Tommasi, Patrick Cordier, Alexandra Goryaeva, Philippe Carrez, David MainpriceAbstract:We use a forward multiscale model that couples atomistic modeling of intracrystalline plasticity mechanisms (dislocation glide ± twinning) in MgSiO3 post-perovskite (PPv) and periclase (MgO) at lower mantle pressures and temperatures to polycrystal plasticity simulations to predict crystal preferred orientations (CPO) development and Seismic Anisotropy in D″. We model the CPO evolution in aggregates of 70% PPv and 30% MgO submitted to simple shear, axial shortening, and along corner-flow streamlines, which simulate changes in flow orientation similar to those expected at the transition between a downwelling and flow parallel to the core–mantle boundary (CMB) within D″ or between CMB-parallel flow and upwelling at the borders of the large low shear wave velocity provinces (LLSVP) in the lowermost mantle. Axial shortening results in alignment of PPv [010] axes with the shortening direction. Simple shear produces PPv CPO with a monoclinic symmetry that rapidly rotates towards parallelism between the dominant [100](010) slip system and the macroscopic shear. These predictions differ from MgSiO3 post-perovskite textures formed in diamond-anvil cell experiments, but agree with those obtained in simple shear and compression experiments using CaIrO3 post-perovskite. Development of CPO in PPv and MgO results in Seismic Anisotropy in D″. For shear parallel to the CMB, at low strain, the inclination of ScS, Sdiff, and SKKS fast polarizations and delay times vary depending on the propagation direction. At moderate and high shear strains, all S-waves are polarized nearly horizontally. Downwelling flow produces Sdiff, ScS, and SKKS fast polarization directions and birefringence that vary gradually as a function of the back-azimuth from nearly parallel to inclined by up to 70° to CMB and from null to ∼5%. Change in the flow to shear parallel to the CMB results in dispersion of the CPO, weakening of the Anisotropy, and strong azimuthal variation of the S-wave splitting up to 250 km from the corner. Transition from horizontal shear to upwelling also produces weakening of the CPO and complex Seismic Anisotropy patterns, with dominantly inclined fast ScS and SKKS polarizations, over most of the upwelling path. Models that take into account twinning in PPv explain most observations of Seismic Anisotropy in D″, but heterogeneity of the flow at scales
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Deformation and Seismic Anisotropy of the lithospheric mantle in the southeastern Carpathians inferred from the study of mantle xenoliths
Earth and Planetary Science Letters, 2008Co-Authors: György Falus, Andrea Tommasi, Jannick Ingrin, Csaba SzabóAbstract:Peridotite xenoliths with a broad range of textures provides evidence for consistent microstructural evolution in a vertical transect of the shallow lithospheric mantle (35?55 km depth) beneath the Persani Mountains, SE Carpathians, Romania, due to ongoing plate convergence in the Carpathian Arc nearby. The recrystallized grain size, crystal preferred orientations strength, and resulting Seismic Anisotropy vary continuously and display a strong correlation to equilibrium temperatures, suggesting a continuous change in deformation conditions with depth. The shallowmost xenoliths have microstructures typical of high stress deformation, marked by strong recrystallization to fine grain sizes, which results in weak crystal preferred orientations and Anisotropy. The deepest xenoliths have coarse-grained porphyroclastic microstructures and strong crystal preferred orientations. Replacive orthopyroxene structures, consuming olivine, and high H2O concentrations in the pyroxenes are observed in some xenoliths indicating limited percolation of fluids or volatile-rich melts. Despite the high stress deformation and high H2O contents in some of the studied xenoliths, analysis of olivine crystallographic orientations indicates that [100] slip systems, rather than ?wet? [001] accommodate most of the deformation in all samples. Seismic Anisotropy estimated from the measured olivine and pyroxene crystal preferred orientations suggests that the strike-parallel fast SKS polarization directions and ~ 1 s delay times measured in the SE Carpathians are likely the consequence of convergence-driven belt-parallel flow in the lithospheric mantle.
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pressure sensitivity of olivine slip systems and Seismic Anisotropy of earth s upper mantle
Nature, 2005Co-Authors: David Mainprice, Helene Couvy, Andrea Tommasi, Patrick Cordier, Daniel J FrostAbstract:The mineral olivine dominates the composition of the Earth's upper mantle and hence controls its mechanical behaviour and Seismic Anisotropy. Experiments at high temperature and moderate pressure, and extensive data on naturally deformed mantle rocks, have led to the conclusion that olivine at upper-mantle conditions deforms essentially by dislocation creep with dominant [100] slip. The resulting crystal preferred orientation has been used extensively to explain the strong Seismic Anisotropy observed down to 250 km depth1,2,3,4. The rapid decrease of Anisotropy below this depth has been interpreted as marking the transition from dislocation to diffusion creep in the upper mantle5. But new high-pressure experiments suggest that dislocation creep also dominates in the lower part of the upper mantle, but with a different slip direction. Here we show that this high-pressure dislocation creep produces crystal preferred orientations resulting in extremely low Seismic Anisotropy, consistent with seismological observations below 250 km depth. These results raise new questions about the mechanical state of the lower part of the upper mantle and its coupling with layers both above and below.
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strain induced Seismic Anisotropy of wadsleyite polycrystals and flow patterns in the mantle transition zone
Journal of Geophysical Research, 2004Co-Authors: Andrea Tommasi, Helene Couvy, David Mainprice, Patrick Cordier, Catherine ThoravalAbstract:[1] We use forward models based on recent high-pressure experimental data on mantle minerals to predict the Seismic Anisotropy produced by plastic strain of orthorhombic wadsleyite, the dominant mineral in the upper transition zone. These models predict a weak Seismic Anisotropy for a polycrystal of pyrolitic composition (60% wadsleyite, 40% garnet) at transition zone conditions: ∼2% for P and ∼1% for S waves for a shear strain of 1. Both P and S wave Anisotropy patterns show an orthorhombic symmetry. P waves propagate faster at low angle to the shear direction and slower at high angle to the shear plane. S wave Anisotropy is characterized by faster propagation of waves polarized at low angle to the shear direction. Horizontal shearing results therefore in higher velocities for horizontally propagating P waves (PH) and horizontally polarized S waves (SH), as well as in weak azimuthal variation of SV and SH velocities. On the other hand, vertical flow leads to higher velocities for vertically propagating P waves (PV) and vertically polarized S waves (SV) and to a weak azimuthal variation of SV velocity but to a roughly constant SH velocity. Analysis of global observations of Seismic Anisotropy in the transition zone in the light of these models supports dominant horizontal flow in the uppermost transition zone, in agreement with predictions of geodynamical models that explicitly introduce phase transitions.
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viscoplastic self consistent and equilibrium based modeling of olivine lattice preferred orientations implications for the upper mantle Seismic Anisotropy
Journal of Geophysical Research, 2000Co-Authors: Andrea Tommasi, David Mainprice, G R Canova, Yvan ChastelAbstract:Anisotropy of upper mantle physical properties results from lattice preferred orientation (LPO) of upper mantle minerals, in particular olivine. We use an anisotropic viscoplastic self-consistent (VPSC) and an equilibrium-based model to simulate the development of olivine LPO and, hence, of Seismic Anisotropy during deformation. Comparison of model predictions with olivine LPO of naturally and experimentally deformed peridotites shows that the best fit is obtained for VPSC models with relaxed strain compatibility. Slight differences between modeled and measured LPO may be ascribed to activation of dynamic recrystallization during experimental and natural deformation. In simple shear, for instance, experimental results suggest that dynamic re-crystallization results in further reorientation of the LPO leading to parallelism between the main (010)[100] slip system and the macroscopic shear. Thus modeled simple shear LPOs are slightly misoriented relative to LPOs measured in natural and experimentally sheared peridotites. This misorientation is higher for equilibrium-based models. Yet Seismic properties calculated using LPO simulated using either anisotropic VPSC or equilibrium-based models are similar to those of naturally deformed peridotites; errors in the prediction of the polarization direction of the fast S wave and of the fast propagation direction for P waves are usually <15°. Moreover, overestimation of LPO intensities in equilibrium-based and VPSC simulations at high strains does not affect Seismic Anisotropy estimates, because these latter are weakly dependent on the LPO intensity once a distinct LPO pattern has been developed. Thus both methods yield good predictions of development of upper mantle Seismic Anisotropy in response to plastic flow. Two notes of caution have nevertheless to be observed in using these results: (1) the dilution effect of other upper mantle mineral phases, in particular enstatite, has to be taken into account in quantitative predictions of upper mantle Seismic Anisotropy, and (2) LPO patterns from a few naturally deformed peridotites cannot be reproduced in simulations. These abnormal LPOs represent a small percent of the measured natural LPOs, but the present sampling may not be representative of their abundance in the Earth's upper mantle.
Maureen D. Long - One of the best experts on this subject based on the ideXlab platform.
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Internal deformation of the subducted Nazca slab inferred from Seismic Anisotropy
Nature Geoscience, 2016Co-Authors: Caroline M. Eakin, Susan L Beck, George Zandt, Maureen D. Long, A. C. Scire, Lara S. Wagner, Hernando TaveraAbstract:Within oceanic lithosphere a fossilized fabric is often preserved originating from the time of plate formation. Such fabric is thought to form at the mid-ocean ridge when olivine crystals align with the direction of plate spreading^ 1 , 2 . It is unclear, however, whether this fossil fabric is preserved within slabs during subduction or overprinted by subduction-induced deformation. The alignment of olivine crystals, such as within fossil fabrics, can generate Anisotropy that is sensed by passing Seismic waves. Seismic Anisotropy is therefore a useful tool for investigating the dynamics of subduction zones, but it has so far proved difficult to observe the anisotropic properties of the subducted slab itself. Here we analyse Seismic Anisotropy in the subducted Nazca slab beneath Peru and find that the fast direction of Seismic wave propagation aligns with the contours of the slab. We use numerical modelling to simulate the olivine fabric created at the mid-ocean ridge, but find it is inconsistent with our observations of Seismic Anisotropy in the subducted Nazca slab. Instead we find that an orientation of the olivine crystal fast axes aligned parallel to the strike of the slab provides the best fit, consistent with along-strike extension induced by flattening of the slab during subduction (A. Kumar et al. , manuscript in preparation). We conclude that the fossil fabric has been overprinted during subduction and that the Nazca slab must therefore be sufficiently weak to undergo internal deformation. 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.
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Internal deformation of the subducted Nazca slab inferred from Seismic Anisotropy
Nature Geoscience, 2015Co-Authors: Caroline M. Eakin, Susan L Beck, George Zandt, Maureen D. Long, A. C. Scire, Lara S. Wagner, Hernando TaveraAbstract: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.
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constraints on subduction geodynamics from Seismic Anisotropy
Reviews of Geophysics, 2013Co-Authors: Maureen D. LongAbstract:[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.
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Mantle dynamics and Seismic Anisotropy
Earth and Planetary Science Letters, 2010Co-Authors: Maureen D. Long, Thorsten W BeckerAbstract: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.
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the subduction zone flow field from Seismic Anisotropy a global view
Science, 2008Co-Authors: Maureen D. Long, Paul G. SilverAbstract:Although the morphologies of subducting slabs have been relatively well characterized, the character of the mantle flow field that accompanies subduction remains poorly understood. To analyze this pattern of flow, we compiled observations of Seismic Anisotropy, as manifested by shear wave splitting. Data from 13 subduction zones reveal systematic variations in both mantle-wedge and subslab Anisotropy with the magnitude of trench migration velocity |Vt|. These variations can be explained by flow along the strike of the trench induced by trench motion. This flow dominates beneath the slab, where its magnitude scales with |Vt|. In the mantle wedge, this flow interacts with classical corner flow produced by the convergence velocity Vc; their relative influence is governed by the relative magnitude of |Vt| and Vc.
