Observations Of Seismic Anisotropy Research Articles

Flow in the western Mediterranean shallow mantle: Insights from xenoliths in Pliocene alkali basalts from SE Iberia (eastern Betics, Spain)

AbstractMantle xenoliths in Pliocene alkali basalts of the eastern Betics (SE Iberia, Spain) are spinel ± plagioclase lherzolite, with minor harzburgite and wehrlite, displaying porphyroclastic or equigranular textures. Equigranular peridotites have olivine crystal preferred orientation (CPO) patterns similar to those of porphyroclastic xenoliths but slightly more dispersed. Olivine CPO shows [100]‐fiber patterns characterized by strong alignment of [100]‐axes subparallel to the stretching lineation and a girdle distribution of [010]‐axes normal to it. This pattern is consistent with simple shear or transtensional deformation accommodated by dislocation creep. One xenolith provides evidence for synkinematic reactive percolation of subduction‐related Si‐rich melts/fluids that resulted in oriented crystallization of orthopyroxene. Despite a seemingly undeformed microstructure, the CPO in orthopyroxenite veins in composite xenoliths is identical to those of pyroxenes in the host peridotite, suggesting late‐kinematic crystallization. Based on these observations, we propose that the annealing producing the equigranular microstructures was triggered by melt percolation in the shallow subcontinental lithospheric mantle coeval to the late Neogene formation of veins in composite xenoliths. Calculated seismic properties are characterized by fast propagation of P waves and polarization of fast S waves parallel to olivine [100]‐axis (stretching lineation). These data are compatible with present‐day seismic anisotropy observations in SE Iberia if the foliations in the lithospheric mantle are steeply dipping and lineations are subhorizontal with ENE strike, implying dominantly horizontal mantle flow in the ENE‐WSW direction within vertical planes, that is, subparallel to the paleo‐Iberian margin. The measured anisotropy could thus reflect a lithospheric fabric due to strike‐slip deformation in the late Miocene in the context of WSW tearing of the subducted south Iberian margin lithosphere.

Overriding plate, mantle wedge, slab, and subslab contributions to seismic anisotropy beneath the northern Central Andean Plateau

AbstractThe Central Andean Plateau, the second‐highest plateau on Earth, overlies the subduction of the Nazca Plate beneath the central portion of South America. The origin of the high topography remains poorly understood, and this puzzle is intimately tied to unanswered questions about processes in the upper mantle, including possible removal of the overriding plate lithosphere and interaction with the flow field that results from the driving forces associated with subduction. Observations of seismic anisotropy can provide important constraints on mantle flow geometry in subduction systems. The interpretation of seismic anisotropy measurements in subduction settings can be challenging, however, because different parts of the subduction system may contribute, including the overriding plate, the mantle wedge above the slab, the slab itself, and the deep upper mantle beneath the slab. Here we present measurements of shear wave splitting for core phases (SKS, SKKS, PKS, and sSKS), local S, and source‐side teleseismic S phases that sample the upper mantle beneath southern Peru and northern Bolivia, relying mostly on data from the CAUGHT experiment. We find evidence for seismic anisotropy within most portions of the subduction system, although the overriding plate itself likely makes only a small contribution to the observed delay times. Average fast orientations generally trend roughly trench‐parallel to trench‐oblique, contradicting predictions from the simplest two‐dimensional flow models and olivine fabric scenarios. Our measurements suggest complex, layered anisotropy beneath the northern portion of the Central Andean Plateau, with significant departures from a two‐dimensional mantle flow regime.

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. 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 spreading1,2. 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.

Structural insights and elasticity of single-crystal antigorite from high-pressure Raman and Brillouin spectroscopy measured in the (010) plane

We report high-pressure Raman and Brillouin spectroscopy results measured in the (010) plane of a natural antigorite single crystal. We find that structural changes at >6 GPa lead to (1) an intensity crossover between Raman modes of the Si-O-Si bending vibrations, (2) changes of the compression behavior of Raman modes related to the SiO4 tetrahedra, (3) changes of the pressure derivative of the Raman shifts associated with OH stretching vibrations, (4) the emergence of a new Raman band in the OH spectral region, (5) a softening of the elastic constants c33 and c11, and (6) a directional change of the slowest compressional wave velocity in the a-c plane. In addition to the structural insights at high-pressure, the unique characteristics of our single-crystal sample allows for first direct measurements of the acoustic velocity anisotropy in a plane perpendicular to the basal a-b plane. Comparison to previously published data indicates that the elastic anisotropy of antigorite strongly depends on the FeO and/or Al2O3 content. In contrast, it seems not to be affected by increasing temperature as inferred from an additional high-temperature experiment performed in our study. These constraints are important for the interpretation of seismic anisotropy observations in subduction zone environments.

Mid-mantle seismic anisotropy beneath southwestern Pacific subduction systems and implications for mid-mantle deformation

Observations of seismic anisotropy can offer relatively direct constraints on patterns of mantle deformation, but most studies have focused on the upper mantle. While much of the lower mantle is thought to be isotropic, several recent studies have found evidence for anisotropy in the transition zone and uppermost lower mantle (the mid-mantle), particularly in the vicinity of subducting slabs. Here we investigate anisotropy at mid-mantle depths in the Tonga-Kermadec, Sumatra, New Britain, New Hebrides, and Philippines subduction zones using the source-side shear wave splitting technique. We measure splitting of direct teleseismic S phases originating from deep events (>300km) that have been corrected for the effect of upper mantle anisotropy beneath the seismic stations. We find evidence for considerable anisotropy at mid-mantle depths in all subduction systems studied, with delay times averaging ∼1.0–1.5s. Several measurements originating from depths greater than 600km exhibit delay times greater than 1s, suggesting a significant contribution from anisotropy in the uppermost lower mantle. We combine our results with those documented in previous studies into a quasi-global set of source-side shear wave splitting measurements that reflect mid-mantle anisotropy. We document significant variability in the dominant fast directions both within and among individual subduction systems, suggesting different deformation geometries among different subduction systems. As further constraints on the elasticity and deformation of mid-mantle minerals become available, our dataset can be used to constrain patterns of mid-mantle flow associated with subduction.

Lowermost mantle flow at the eastern edge of the African Large Low Shear Velocity Province

Observations of seismic anisotropy in the lowermost mantle are plentiful, but their interpretation in terms of mantle flow remains challenging. Here we interrogate the anisotropic structure of the lowermost mantle beneath the Afar region, just outside the edge of the African Large Low Shear Velocity Province, using a combination of shear wave splitting techniques applied to phases propagating at five distinct raypath orientations. We then model the resulting data set by testing various candidate mechanisms for anisotropy. The observations are best fit by a model that invokes the lattice preferred orientation (LPO) of post-perovskite, with the [100] crystallographic axis oriented either nearly vertically or highly obliquely to the horizontal plane. Plausible corresponding mantle flow scenarios involve a significant vertical flow component, which suggests that the African Large Low Shear Velocity Province edge may deflect ambient mantle flow upwards or may be associated with a sheet-like upwelling.

Is inner core seismic anisotropy a marker for plastic flow of cubic iron?

AbstractThis paper investigates whether observations of seismic anisotropy are compatible with a cubic structure of the inner core Fe alloy. We assume that anisotropy is the result of plastic deformation within a large‐scale flow induced by preferred growth at the inner core equator. Based on elastic moduli from the literature, body‐centered cubic or face‐centered cubic Fe produces seismic anisotropy well below seismic observations (<0.4%). A Monte Carlo approach allows us to generalize this result to any form of elastic anisotropy in a cubic system. Within our model, inner core global anisotropy is not compatible with a cubic structure of Fe alloy. Hence, if the inner core material is indeed cubic, large‐scale coherent anisotropic structures, incompatible with plastic deformation induced by large‐scale flow, must be present.

Looking for layered anisotropic structures in the mantle beneath the northern Apennines

Competing geodynamic scenarios proposed for northern Apennines (Italy) make very different predictions for the orientation of strain in the upper mantle. Constraints on the pattern are offered by observations of seismic anisotropy. Previous study of the anisotropy beneath the northern Apennines used birefringence of core-refracted shear waves (SKS phases), and demonstrated the presence of two domains: Tuscan and Adria. In the transition between the two domains, across the Apennines orogen, anisotropy measurements reflect a complex deep structure. To define better the upper-mantle structure beneath this area we analyze seismological data recorded by a set of seismic stations that operated for 3 years, between 2003 and 2006, located in the outer part of the Apennines belt, in the Adria terrane, collected by the RETREAT Project. Directionally distributed sets of SKS records were inverted for layered anisotropic structures with a well-tested method, adding new results to previous hypotheses for this area. New data analysis argues for two-layer anisotropy for sites located on the Apennines wedge and also one site in the Tuscan terrane. Beneath the wedge an upper layer with nearly north-south fast polarization pervades the lithospheric mantle, while at depth a nearly NW–SE Apennines-parallel direction is present in the lower layer. Beneath Tuscany a shallower NW–SE direction and a deeper E–W one suggest the deeper strain from active slab retreat, with a mantle-wedge circulation (i.e. an east–west corner flow), overlain by an Apennines-parallel fast polarization that could be a remnant of lower-crust deformation.

Non-steady-state subduction and trench-parallel flow induced by overriding plate structure

The direction of plate tectonic motion and the direction of mantle flow, as inferred from observations of seismic anisotropy measurements, show a good global correlation far from subduction zones. However, this correlation is poor near subduction zones, where below the slab seismic anisotropy is aligned parallel to the trench and above the slab has a complex pattern, which has not been fully explained. Here we present time-dependent three-dimensional (3D) fully-dynamic simulations of subduction to study the effect of overriding plate structure on the evolution of slab geometry and induced mantle flow. We find that along-strike variation in thermal thickness of the overriding plate causes increased hydrodynamic suction and shallower slab dip beneath the colder portion of the overriding plate; the variation in slab geometry drives strong trench-parallel flow beneath the slab and a complex flow pattern above the slab. This new mechanism for driving trench-parallel flow provides a good explanation for seismic anisotropy observations from the Middle and South America subduction zones, where both slab dip and overriding plate thermal state are strongly variable and correlated, and thus may be an important mechanism in other subduction zones. The location and strength of trench-parallel flow vary with the time-dependent evolution of the slab, suggesting that the global variability in seismic anisotropy observations in subduction zones is in part due to the non-steady-state behavior of these systems.

Sub‐slab anisotropy beneath the Sumatra and circum‐Pacific subduction zones from source‐side shear wave splitting observations

AbstractUnderstanding the dynamics of subduction is critical to our overall understanding of plate tectonics and the solid Earth system. Observations of seismic anisotropy can yield constraints on deformation patterns in the mantle surrounding subducting slabs, providing a tool for studying subduction dynamics. While many observations of seismic anisotropy have been made in subduction systems, our understanding of the mantle beneath subducting slabs remains tenuous due to the difficulty of constraining anisotropy in the sub‐slab region. Recently, the source‐side shear wave splitting technique has been refined and applied to several subduction systems worldwide, making accurate and direct measurements of sub‐slab anisotropy feasible and offering unprecedented spatial and depth coverage in the sub‐slab mantle. Here we present source‐side shear wave splitting measurements for the Central America, Alaska‐Aleutians, Sumatra, Ryukyu, and Izu‐Bonin‐Japan‐Kurile subduction systems. We find that measured fast splitting directions in these regions generally fall into two broad categories, aligning either with the strike of the trench or with the motion of the subducting slab relative to the overriding plate. Trench parallel fast splitting directions dominate beneath the Izu‐Bonin, Japan, and southern Kurile slabs and part of the Sumatra system, while fast directions that parallel the motion of the downgoing plate dominate in the Ryukyu, Central America, northern Kurile, western Sumatra, and Alaska‐Aleutian regions. We find that plate motion parallel fast splitting directions in the sub‐slab mantle are more common than previously thought. We observe a correlation between fast direction and age of the subducting lithosphere; older lithosphere (>95 Ma) is associated with trench parallel splitting while younger lithosphere (<95 Ma) is associated with plate motion parallel fast splitting directions. Finally, we observe source‐side splitting for deep earthquakes (transition zone depths) beneath Japan and Sumatra, suggesting the presence of anisotropy at midmantle depths beneath these regions.

Subduction‐induced mantle flow, finite strain, and seismic anisotropy: Numerical modeling

AbstractSurface measurements of shear wave splitting patterns are widely used to infer the mantle circulation around subducting slabs; however, the relation between mantle flow and seismic anisotropy is still elusive. Finite strain is a direct measurement of time‐dependent deformation and has been proposed as a proxy for the crystal‐preferred orientation (CPO) of mantle minerals. We have conducted a series of numerical models to systematically investigate the mantle flow, finite strain, olivine CPO, and SKS wave splitting in oceanic subduction zones with variable slab width. They demonstrate that the preferred orientations of olivine a axes generally agree with the long (extensional) axes of the finite strain ellipsoid (FSE), even in these very complex mantle flow fields; however, neither the a axis nor the FSE axes necessarily aligns with the instantaneous mantle velocity vector. We identify two domains with distinct deformation mechanisms in the central subplate mantle, where simple shear induced by plate advance dominates at shallow depths and produces trench‐normal fast splitting, while pure shear induced by slab rollback dominates the deeper mantle and results in trench‐parallel fast splitting. The SKS splitting patterns are thus dependent on the competing effects of these two mechanisms and also on the subduction partition ratio γ = Xp/Xt: trench parallel when γ< 1 and trench normal when γ>1. In addition, different mantle deformation mechanisms and SKS splitting patterns are observed in the mantle wedge and around the slab edges, which may aid in the general interpretation of seismic anisotropy observations in natural subduction zones.

Seismic anisotropy and large-scale deformation of the Eastern Alps

Mountain chains at the Earthʼs surface result from deformation processes within the Earth. Such deformation processes can be observed by seismic anisotropy, via the preferred alignment of elastically anisotropic minerals. The Alps show complex deformation at the Earthʼs surface. In contrast, we show here that observations of seismic anisotropy suggest a relatively simple pattern of internal deformation. Together with earlier observations from the Western Alps, the SKS shear-wave splitting observations presented here show one of the clearest examples yet of mountain chain-parallel fast orientations worldwide, with a simple pattern nearly parallel to the trend of the mountain chain. In the Eastern Alps, the fast orientations do not connect with neighboring mountain chains, neither the present-day Carpathians, nor the present-day Dinarides. In that region, the lithosphere is thin and the observed anisotropy thus resides within the asthenosphere. The deformation is consistent with the eastward extrusion toward the Pannonian basin that was previously suggested based on seismicity and surface geology.

Strong inheritance of texture between perovskite and post-perovskite in the D′′ layer

About 200 km above the core–mantle boundary, the D′′ seismic discontinuity marks the depth where magnesium silicate perovskite—the main mantle mineral—is transformed into its high-pressure phase of post-perovskite1,2. Observations of seismic anisotropy within the D′′ region are inferred to arise from textures within post-perovskite3,4,5 that are created by flow in the deep mantle. Specifically, mantle flow is thought to cause post-perovskite to deform, creating a lattice-preferred orientation within the post-perovskite6,7,8,9,10. However, it is difficult to explain all of the observed patterns of seismic anisotropy in the D′′ region from this deformation mechanism alone. Here we use a low-pressure fluoride analogue system11 to study the transformation from perovskite to post-perovskite in laboratory experiments. We find that post-perovskite can inherit texture from the perovskite phase. If a similar transformation mechanism operates in the Earth, post-perovskite will inherit textures from deformed perovskite and vice versa, as lower-mantle material passes into and out of the D′′ region. We find that this textural inheritance, combined with lattice-preferred orientation in post-perovskite, can explain the observed patterns of anisotropy in the lowermost mantle.

Mantle flow and deformation of subducting slab at a plate junction

At junctions between plate boundaries there are several general characteristic features. These include trench-normal fast polarization direction of S-wave splitting in the mantle wedge, along-arc variation of the angle of subduction, and the different behavior of slab stagnation along the strike of the trench. In this paper, we focus on the junction of the Tohoku–Kurile arc. We show the dynamical effects of the junction using a numerical model of mantle convection with a realistic curved shape of the trench. We obtain 3D flow in the mantle wedge which is consistent with the observation of seismic anisotropy even with the oblique subduction. We also find the along-arc variation of subduction angle which agrees with the observations, although the match becomes worse when we include the effect of dislocation creep, that is, non-linear viscosity. Along-arc variation of the angle of subduction partly contributes to the different behavior of slab stagnation in the Tohoku–Kurile arc. Our results show that the shape of the trench is an important factor which considerably affects mantle flow and deformation of subducting slabs. Thus, 3D modeling is necessary to constrain the dynamics of subduction zones near the junction.

A geodynamical perspective on the subduction of Cocos and Rivera plates beneath Mexico and Central America

Abstract The Middle America subduction zone (MASZ) is one of the world’ most complex convergent margins as it involves the subduction of the Rivera and Cocos young oceanic plates beneath the North American and Caribbean plates and is bounded by the Gulf of California rift and the Panama slab window. Characterized by contorted and unusual slab geometry, irregularly distributed seismicity and volcanism, exceptionally large slow slip events (SSE) and non-volcanic tremors (NVT), this subduction system represents a great natural laboratory for better understanding geodynamic processes at a fundamental level. Based on a solid observational foundation, and incorporating the latest experimental results into a coherent geodynamical framework, we shed light on the main processes controlling the subduction system evolution in this region. The tectonics, volcanism, slab geometry and segmentation along the margin are reviewed from a geodynamical perspective. We proposed and discussed a series of evolutionary scenarios for the Mexican and Central American subduction zones, providing a coherent starting base for future geodynamical modeling studies tailored to this active margin. We discuss comparatively the recently discovered SSEs and NVTs along the MASZ, and try to differentiate among the proposed mechanisms responsible for these observations. Finally we discuss the recent seismic anisotropy observations in a geodynamic context, offering an integrated view of mantle flow pattern along the entire active margin. Although the MASZ as a whole may be considered a fairly complicated region with many unusual features and sometimes controversial interpretations, its complexity and unusual characteristics can improve our knowledge about the linkage between deep and surface processes associated with subduction zone dynamics.

CONSTRAINTS ON SUBDUCTION GEODYNAMICS FROM SEISMIC ANISOTROPY

AbstractMuch 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.

Sub-slab seismic anisotropy and mantle flow beneath the Caribbean and Scotia subduction zones: Effects of slab morphology and kinematics

Subduction systems are vitally important to plate tectonics and mantle convection, but questions remain about many aspects of subduction dynamics, particularly the nature of sub-slab mantle flow. Observations of seismic anisotropy can shed light on the pattern of mantle flow in subduction systems, but major uncertainties remain regarding the interpretation of sub-slab anisotropy. Here, we present measurements of shear wave splitting due to anisotropy in the sub-slab mantle beneath the Caribbean and Scotia subduction zones. These subduction systems are morphologically similar, with high arc curvature and short arc length, but there are differences between them in terms of their kinematic and tectonic settings. We apply the source-side shear wave splitting technique to direct teleseismic S phases originating from slab earthquakes. We restrict our analysis to seismic stations at which we have examined the receiver-side splitting in detail to ensure accurate corrections for anisotropy in the upper mantle beneath the station. We observe a well-defined pattern of trench-parallel fast directions (ϕ) in the northern half of the Caribbean subduction zone, with a transition to dominantly trench-perpendicular ϕ at the southern end. There is more scatter in the measurements for Scotia, but we observe generally trench-parallel ϕ in the northern portion of the subduction system, with a mix of trench-parallel, -perpendicular, and -oblique fast directions to the south. Our preferred interpretation of these splitting patterns is that they reflect 3-D return flow of the sub-slab mantle due to trench rollback. In both systems, we infer that sub-slab flow is being driven from south to north. Beneath Scotia, this is likely driven by differential migration of the Scotia trench along strike. Beneath the Caribbean, we hypothesize that mantle flow around the southern edge of the slab is inhibited by the presence of the South American continental keel, enabling trench-perpendicular stretching in the sub-slab mantle and forcing mantle flow to escape to the north. The similarities and differences we observe between the two systems yield insight into the relative contributions of slab morphology and plate kinematics in controlling mantle flow beneath subducting slabs.

Pressure‐ and stress‐induced fabric transition in olivine from peridotites in the Western Gneiss Region (Norway): implications for mantle seismic anisotropy

AbstractThe effects of pressure, water and differential stress on the crystallographic preferred orientation (CPO) of olivine were investigated by an integrated study on 13 peridotite samples from four areas in the Western Gneiss Region (WGR) of the Norwegian Caledonides. The highly depleted dunites are mantle residues after Archean melt extraction, while the garnet peridotites were formed by refertilization of the dunites via melt percolation in the Proterozoic. These orogenic peridotites were tectonically emplaced into Proterozoic gneisses and metasedimentary rocks of the WGR and were recrystallized under high‐pressure (HP) to ultrahigh‐pressure (UHP) conditions when the Baltic Shield was subducted beneath Laurentia during the 420–400 Ma Scandian orogeny. The CPOs of olivine, enstatite and diopside were measured using the electron backscatter diffraction technique. The weakly deformed garnet wehrlites and dunites from the HP central WGR (the Gurskøy, Almklovdalen and Tafjord areas) show the A‐type olivine fabric defined by the [100] axis subparallel to the stretching lineation and the (010) plane subparallel to the foliation, which may be a relict fabric formed at high temperature and low strain rate in the mantle wedge prior to subduction. The [001](010) fabric (B‐type) is observed in strongly sheared HP garnet peridotites and dunites from the Almklovdalen and Tafjord areas, suggesting the dominant activation of [001](010) slip system at high stress and high strain rate. Recrystallized fine olivine grains may develop a fabric different from porphyroclasts and record a gradual fabric transition. The olivine CPO in garnet lherzolites from Otrøy in the northernmost WGR is characterized by the concentration of [001] axes parallel to the stretching lineation and [100] axes normal to the foliation, indicating a prevailing [001](100) slip (C‐type fabric) at P–T conditions of >6 GPa and 850–950 °C. Despite very different deformation microstructures, Fourier transform infrared analysis reveals very low water contents (<13 ppm H2O by the Paterson calibration) in olivine for both coarse porphyroclasts and recrystallized small grains, which is consistent with the high‐degree partial melting of the peridotites during the Archean. Therefore, a combination of UHP and low temperature plays a more important role than water in promoting the C‐type olivine fabric in a dry, cold and deeply subducted continental slab. With increasing subduction depths, the C‐type olivine fabric will predominate over the A‐type fabric and make the maximum P‐wave velocity and shear wave splitting of peridotites normal to the foliation. However, the stress‐induced fabric transition in olivine from the A‐type to the B‐type may be localized in shear zones and not distinguishable in seismic anisotropy observations.

Crust–mantle coupling at the northern edge of the Tibetan plateau: Evidence from focal mechanisms and observations of seismic anisotropy

The Tarim basin is distinct from the Tibetan plateau by the apparently low degree of internal deformation. Relative motion between the lithospheres of Tibet and Tarim likely results in coherent mantle deformation, with a diagnostic signature of anisotropic seismic wave speed. Birefringence (splitting) in core-refracted shear phases (SKS, PKS) observed along the Tibet–Tarim border is indicative of seismic anisotropy along their path. The mantle deformation direction inferred from shear wave splitting under the assumption of a single source of anisotropy is approximately E–W. The size of the splitting delays inferred in our analysis suggests that most of the anisotropic signature is coming from the mantle. Furthermore, our data suggest vertical stratification of rock fabric.Focal mechanisms of regional earthquakes, both published and derived from new data, provide evidence for ~NNE–SSW compression within the Tarim basin north of the border with Tibet. This type of internal deformation in the lithosphere is consistent with its underthrusting (subduction?) beneath Tibet. Within the crust of the Tibetan plateau we find a more complex pattern suggestive of the ~E–W extension, and generally consistent with the sense of motion on the Altyn Tagh fault that extends along the Tibet–Tarim boundary. Therefore, at the Tibet–Tarim border we find close similarity between deformation directions within the crust of the plateau and within the upper mantle on both sides of the boundary. A plausible explanation of such similarity would be the coupling of the crust and the upper mantle, with no weak zone being present in the lower crust. One scenario for such coupling would involve an extension of a deformation zone associated with the Altyn Tagh fault into the uppermost mantle, making this fault zone very similar to a plate boundary.

High resolution three‐dimensional X‐ray diffraction study of dislocations in grains of MgGeO3 post‐perovskite at 90 GPa

MgGeO3 post‐perovskite (pPv) is a lower pressure analogue of MgSiO3‐pPv, which is believed to be the main constituent of the Earth's D″ layer. Understanding the physical properties of this phase is critical to explain seismological observations as seismic anisotropy is likely linked to lattice preferred orientation in post‐perovskite, which is governed by the motion of defects such as dislocations. Here, we apply in‐situ three‐dimensional X‐ray diffraction to a polycrystalline sample of MgGeO3‐pPv at 90 GPa. We demonstrate how the method can be used to follow individual grains within the material, including their individual orientations, positions, and strain tensors. We then use X‐ray line profile analysis to characterize dislocations in the grains. The most probable slip systems are 〈110〉 gliding on or (001) and [110](001), with a smaller contribution of [100](001) and [001] dislocations. These slip systems are shown to be consistent with observations of seismic anisotropy in D″.