Observations Of Seismic Anisotropy Research Articles

Advances in Mapping Lowermost Mantle Convective Flow With Seismic Anisotropy Observations

AbstractConvective flow in the deep mantle controls Earth's dynamic evolution, influences plate tectonics, and has shaped Earth's current surface features. Present and past convection‐induced deformation manifests itself in seismic anisotropy, which is particularly strong in the mantle's uppermost and lowermost portions. While the general patterns of seismic anisotropy have been mapped for the upper mantle, anisotropy in the lowermost mantle (called D′′) is at an earlier stage of exploration. Here we review recent progress in methods to measure and interpret D′′ anisotropy. Our understanding of the limitations of existing methods and the development of new measurement strategies have been aided enormously by the availability of high‐performance computing resources. We give an overview of how measurements of seismic anisotropy can help constrain the mineralogy and fabric of the deep mantle. Specifically, new and creative strategies that combine multiple types of observations provide much tighter constraints on the geometry of anisotropy than have previously been possible. We also discuss how deep mantle seismic anisotropy provides insights into lowermost mantle dynamics. We summarize what we have learned so far from measurements of D′′ anisotropy, how inferences of lowermost mantle flow from measurements of seismic anisotropy relate to geodynamic models of mantle flow, and what challenges we face going forward. Finally, we discuss some of the important unsolved problems related to the dynamics of the lowermost mantle that can be elucidated in the future by combining observations of seismic anisotropy with geodynamic predictions of lowermost mantle flow.

Hydrothermal Fluids and Where to Find Them: Using Seismic Attenuation and Anisotropy to Map Fluids Beneath Uturuncu Volcano, Bolivia

AbstractMapping fluid accumulation in the crust is pertinent for numerous applications including volcanic hazard assessment, geothermal energy generation, and mineral exploration. Here, we use seismic attenuation tomography to map the distribution of fluids in the crust below Uturuncu volcano, Bolivia. Seismic P wave and S wave attenuation, as well as their ratio (QP/QS), constrain where the crust is partially and fully fluid‐saturated. Seismic anisotropy observations further constrain the mechanism by which the fluids accumulate, predominantly along aligned faults and fractures in this case. Furthermore, subsurface pressure‐temperature profiles and conductivity data allow us to identify the most likely fluid composition. We identify shallow regions of both dry and H2O/brine‐saturated crust, as well as a deeper supercritical H2O/brine column directly beneath Uturuncu. Our observations provide a greater understanding of Uturuncu's transcrustal hydrothermal system, and act as an example of how such methods could be applied to map crustal fluid pathways and hydrothermal/geothermal systems elsewhere.

On the Implementation and Usability of Crystal Preferred Orientation Evolution in Geodynamic Modeling

AbstractFlow in the Earth's mantle causes the preferred orientation of crystals called lattice/crystal preferred orientation (LPO or CPO). This preferred orientation is one of the main reasons why seismic anisotropy is observed. Seismic anisotropy observations could therefore be used to constrain the mantle flow in geodynamic models through tracking CPO evolution, and computing the resulting elastic tensor and the anisotropy predicted at the surface. Even though there are many types of CPO models, only a few studies include CPO calculations due to the complexity and computational cost. Here, we implemented an extended version of the CPO model D‐Rex into the open‐source community geodynamics code ASPECT. We show that the implementation is correct, how to use it, and that it is feasible and important to use in large 3D models. We also show that it is important to calculate CPO, especially for models focusing on plate boundary of smaller scale flow because the resulting fast axis directions can greatly deviate from the flow direction. The added infrastructure will also allow for future enhancement, testing, and even replacement of the CPO model.

Seismic anisotropy of mid crustal orogenic nappes and their bounding structures: An example from the Middle Allochthon (Seve Nappe) of the Central Scandinavian Caledonides

We report compositional, microstructural and seismic properties from 24 samples collected from the Middle Allochthon (Seve Nappe) of the central Scandinavian Caledonides, and its bounding shear zones. The samples stem both from field outcrops and the continental drilling project COSC-1 and include quartzofeldspathic gneisses, hornblende gneisses, amphibolites, marbles, calc-silicates, quartzites and mica schists, of medium to high-strain. Seismic velocities and anisotropy of P (AVp) and S (AVs) waves of these samples were calculated using microstructural and crystal preferred orientation data obtained from Electron Backscatter Diffraction analysis (EBSD). Mica-schist exhibits the highest anisotropy (AVP ~ 31%; max AVs ~34%), followed by hornblende-dominated rocks (AVp ~5–13%; max AVs 5–10%) and quartzites (AVp ~6.5–10.5%; max AVs ~7.5–12%). Lowest anisotropy is found in calc-silicate rocks (AVp ~4%; max AVs 3–4%), where the symmetry of anisotropy is more complex due to the contribution to anisotropy from several phases. Anisotropy is attributed to: 1) modal mineral composition, in particular mica and amphibole content, 2) CPO intensity, 3) crystallization of anisotropic minerals from fluids circulating in the shear zone (calc-silicates and amphibolites), and to a lesser extent 4) compositional banding of minerals with contrasting elastic properties and density. Our results link observed anisotropy to the rock composition and strain in a representative section across the Central Scandinavian Caledonides and indicate that the entire Seve Nappe is seismically anisotropic. Strain has partitioned on the nappe scale, and likely on the microstructural scale. High- strain shear zones that develop at boundaries of the allochthon and internally within the allochthon show higher anisotropy than a more moderately strained interior of the nappe. The Seve Nappe may be considered as a template for deforming, ductile and flowing middle crust, which is in line with general observations of seismic anisotropy in mid-crustal settings.

Mantle Flow Patterns Beneath the Junction of Multiple Subduction Systems Between the Pacific and Tethys Domains, SE Asia: Constraints From SKS‐Wave Splitting Measurements

AbstractThe region around the Celebes Sea, SE Asia, is evolving within a convergent tectonic environment involving the Pacific plate to the east and the Indian‐Australian plate to the south. It is arguably one of the most tectonically complex regions in the world and serves as an ideal setting to study dynamic interactions between the Pacific and Tethys tectonic domains. The issue of which subducting plate plays a leading role in governing the regional mantle flow is not well understood. Mantle flow can be characterized by seismic anisotropy observations, providing clues for understanding regional tectonics. We conducted SKS‐wave splitting analysis by using data from seven seismic stations located around the Celebes Sea. Our results, when combined with previous observations, suggest the presence of various types of mantle flow in this area, including (a) corner flow in the mantle wedge above the westward‐subducting Molucca Sea (Sangihe) slab, (b) two‐layer anisotropy related to the eastward‐subducting Molucca Sea (Halmahera) slab, (c) deflected flow due to proximity to the Sangihe slab's edges, (d) trench‐normal mantle flow beneath southeastern Borneo due to the subduction of the Indian‐Australian plate, and (e) Northwest Borneo‐Palawan trough‐parallel mantle flow beneath northeastern Borneo. Various types of mantle flow indicate that the dynamic interactions of adjacent subduction zones played crucial roles in influencing the regional upper mantle dynamics between the Tethys and western Pacific domains since the breakup of the Gondwana supercontinent in the Mesozoic.

The elastic properties and anisotropic behavior of MgSiO3 akimotoite at transition zone pressures

Seismic wave velocities in the Earth's transition zone, between 410 and 660 km depth, are poorly matched by mineralogical models. Mainly due to the presence of majoritic garnet, wave velocities calculated for a peridotite composition lithology are slower than seismic reference models, particularly towards the base of the transition zone. One possible resolution for this discrepancy is if the MgSiO3 polymorph akimotoite replaces majoritic garnet in some regions of the transition zone. This is possible in peridotitic material if temperatures are slightly lower than a typical geotherm or in harzburgitic composition material. The presence of akimotoite might serve as an explanation not only for the discrepancy of seismic velocities at the base of the transition zone but also for observations of transition zone seismic anisotropy in regions of current subduction. In order to provide the data required to test this possibility, two high-quality single-crystals of MgSiO3 akimotoite were studied using combined Brillouin spectroscopy and X-ray diffraction up to 24.86(3) GPa, i.e. to the limit of the akimotoite stability field. The resulting equation of state yields an adiabatic bulk modulus and first pressure derivative of KS0 = 209(2) GPa and K′ = 4.4(1), respectively, and a shear modulus and first pressure derivative of G0 = 131(1) GPa and G′ = 1.7(1). This is in overall agreement with several experimental and computational studies, however, the resulting aggregate velocities are slower than previously reported, especially at high pressures. The elastic anisotropy of akimotoite is found to decrease only slightly with pressure; akimotoite hence remains the most elastically anisotropic mineral in the transition zone and may therefore play a major role in explaining seismic anisotropy observations in the proximity of subducting slabs.

Seismic anisotropy and mantle deformation beneath Eastern Ghats Mobile Belt using direct-S waves

Seismic anisotropy observations provide a potential tool to constrain the signatures of various continental-scale deformations that have shaped the lithospheric structure of various mobile belts and ancient cratons across the world. They shed light on various aspects of evolutionary tectonics in any region. The current study primarily aims at examining the direct S-wave derived seismic anisotropy beneath the Eastern Ghats Mobile Belt (EGMB) and the adjacent Archean Singhbhum and Bastar Cratons. The granulite Terrane of EGMB represents the complex deformational history and is a crucial link in the reconstruction of Rodinia and Gondwana supercontinents. The Terrane underwent collision and rifting in Mesoproterozoic, followed by thermal overprint of the mid-Neoproterozoic to early Phanerozoic orogeny modifying its lithospheric structure. We have implemented the Reference Station Technique and obtained 854 well-constrained individual splitting measurements. The observations are based on 185 earthquake events (with Mw⩾5.5, and epicentral distance ranging 30° to 90°) recorded at 27 seismic stations deployed in the study area. The large splitting delay times (1.2–2.3 s) suggest that the lithospheric mantle is highly anisotropic. The NNE-SSW oriented Fast Polarization Directions (FPDs) observed at the Singhbhum Craton can be devoted to the evolutionary tectonic regime of the craton. The strained minerals at the deeper depths, result in FPD patterns sub-parallel to the Kerajung Fault Zone (~70°) in the Angul Domain. The FPDs are predominantly oriented along the Absolute Plate Motion (APM) direction of the Indian plate (~39°) for the Bastar Craton. However, the varying FPD patterns in the Eastern Ghats Boundary Shear Zone (Khariar Domain) signify the influence of the lithospheric deformation along with APM. The imprints of the continental building orogenies, that have modified the lithospheric structure of the study area over the Mesoproterozoic, mid-Neoproterozoic, and early Phanerozoic periods, are preserved as frozen anisotropic signatures. Our observations highlight these signatures. The study also investigates seismic anisotropic patterns in the hitherto uncharted regions close to Chilka Lake using the core-mantle refracted (SKS, SKKS, and PKS) and direct S phases. The markedly distinct E-W (average ~95°) oriented fast wave azimuths, against the ~47° average FPD of the adjacent western Phulbani Domain, distinguishes it as a separate block amongst the collage of domains forming the Eastern Ghats Province.

SKS Splitting and Upper Mantle Anisotropy Beneath the Southern New England Appalachians: Constraints From the Dense SEISConn Array

AbstractThe geology of southern New England reflects subduction and terrane accretion during the Appalachian Orogeny and rifting during the breakup of Pangea. The presence of a low‐velocity seismic anomaly in the upper mantle beneath New England suggests the possible presence of vertical mantle flow (upwelling). It remains poorly understood how the lithosphere beneath southern New England was deformed by past tectonic events; furthermore, the details of the present‐day mantle flow field remain elusive. Observations of seismic anisotropy have the potential to constrain both past and present upper mantle deformation. Here, we present SK(K)S splitting observations at stations of the Seismic Experiment for Imaging Structure Beneath Connecticut array, a deployment of 15 broadband seismic stations across northern Connecticut. This linear array crosses a number of major terrane boundaries and traverses the Mesozoic Hartford Rift Basin in its central portion; its dense station spacing affords an opportunity to probe anisotropic structure on length scales that are relevant for the complex geology of southern New England. We find evidence for average fast splitting directions that are generally parallel to the absolute motion of the North American Plate, but in a few specific regions they are aligned with local tectonic boundaries. We document a striking decrease in splitting delay times (measured at low frequencies) from 0.9 s at the western end of the array to 0.2 s at the eastern end. We discuss several scenarios that might explain this observation, and explore the implications of our measurements for both present‐day mantle flow and past lithospheric deformation beneath southern New England.

Seismic anisotropy in southern Costa Rica confirms upper mantle flow from the Pacific to the Caribbean

AbstractSurrounded by subducting slabs and continental keels, the upper mantle of the Pacific is largely prevented from mixing with surrounding areas. One possible outlet is beneath the southern part of the Central American isthmus, where regional observations of seismic anisotropy, temporal changes in isotopic composition of volcanic eruptions, and considerations of dynamic topography all suggest upper mantle flow from the Pacific to the Caribbean. We derive new constraints on the nature of seismic anisotropy in the upper mantle of southern Costa Rica from observations of birefringence in teleseismic shear waves. Fast and slow components separate by ∼1 s, with faster waves polarized along the 40°–50° (northeast) direction, near-orthogonally to the Central American convergent margin. Our results are consistent with upper mantle flow from the Pacific to the Caribbean and require an opening in the lithosphere subducting under the region.

Geneses of Two Contrasting Antigorite Crystal Preferred Orientations and Their Implications for Seismic Anisotropy in the Forearc Mantle

AbstractCrystal preferred orientation (CPO) of antigorite is an important parameter for the interpretation of seismic anisotropies and related geodynamic processes in subduction zones. However, mechanisms for the development of various antigorite CPOs and their effects on seismic anisotropies in the forearc mantle still remain to be constrained. In this study, we investigated olivine and antigorite CPOs and calculated seismic anisotropies of the serpentinized peridotites from Val Malenco, Central Alps. The results show that antigorite in the olivine‐rich (weakly serpentinized) layer displays an L‐type CPO (i.e., (h0l) plane//foliation and [010] axis//lineation), whereas antigorite in the antigorite‐rich (highly serpentinized) layer develops an LS‐b‐type CPO (i.e., (001) plane//foliation and [010] axis//lineation). The antigorite L‐type CPO is most likely formed by diffusion and dislocation creep accommodated phase boundary sliding and substantial grain rotation at a higher temperature and smaller strain regime. In contrast, the development of an antigorite LS‐b‐type CPO requires diffusion and dislocation creep accommodated grain boundary sliding, smaller contribution of grain rotation, and significant role of dissolution‐precipitation creep at a lower temperature and larger strain regime. In this context, we propose an antigorite CPO distribution model in the forearc mantle. Based on this model and under vertically incident teleseismic waves, if significant serpentinization and asymmetric shearing do indeed occur within serpentinized layers, then strong and weak trench‐parallel seismic anisotropies can be expected for cold and warm moderate‐angle subduction zones, respectively. This model may provide an alternative interpretation on the seismic anisotropy observations in some modern subduction systems.

Serpentinization, Deformation, and Seismic Anisotropy in the Subduction Mantle Wedge

AbstractAntigorite is a hydrous sheet silicate with strongly anisotropic seismic and rheological properties. Hydrous minerals such as antigorite have been invoked to explain numerous geologic observations within subduction zones including intermediate‐depth earthquakes, arc volcanism, the persistent weakness of the subduction interface, trench‐parallel S wave splitting, and episodic tremor and slip. To understand how the presence of antigorite‐bearing rocks affects observations of seismic anisotropy, three mylonites from the Kohistan palaeo‐island arc in Pakistan were analysed using electron backscatter diffraction. A fourth sample, which displayed optical evidence for crystallographically controlled replacements of olivine, was also investigated using electron backscatter diffraction to identify potential topotactic relationships. The resulting data were used to model the bulk seismic properties of antigorite‐rich rocks. The mylonitic samples exhibit incredibly strong bulk anisotropy (10–20% for the antigorite + olivine). Within the nominally undeformed protomylonite, two topotactic relationships were observed: (1) (010)ant//(100)ol with [100]ant//[001]ol and (2) (010)ant//(100)ol with [100]ant//[010]ol. However, the strength of a texture formed by topotactic replacement is markedly weaker than the strength of the textures observed in mylonitic samples. Since antigorite is thought to be rheologically weak, we hypothesise that microstructures formed from topotactic reactions will be progressively overprinted as deformation is localised in regions with greater percentages of serpentine. Regions of highly sheared serpentine, therefore, have the potential to strongly influence seismic wave speeds in subduction settings. The presence of deformed antigorite in a dipping structure is one explanation for observations of both the magnitude and splitting pattern of seismic waves in subduction zones.

SKS Splitting Beneath Mount St. Helens: Constraints on Subslab Mantle Entrainment

AbstractObservations of seismic anisotropy can provide direct constraints on the character of mantle flow in subduction zones, critical for our broader understanding of subduction dynamics. Here we present over 750 new SKS splitting measurements in the vicinity of Mount St. Helens in the Cascadia subduction zone using a combination of stations from the iMUSH broadband array and Cascades Volcano Observatory network. This provides the highest density of splitting measurements yet available in Cascadia, acting as a focused “telescope” for seismic anisotropy in the subduction zone. We retrieve spatially consistent splitting parameters (mean fast direction Φ: 74°, mean delay time ∂t: 1.0 s) with the azimuthal occurrence of nulls in agreement with the fast direction of splitting. When averaged across the array, a 90° periodicity in splitting parameters as a function of back azimuth is revealed, which has not been recovered previously with single‐station observations. The periodicity is characterized by a sawtooth pattern in Φ with a clearly defined 45° trend. We present new equations that reproduce this behavior based upon known systematic errors when calculating shear wave splitting from data with realistic seismic noise. The corrected results suggest a single layer of anisotropy with an ENE‐WSW fast axis parallel to the motion of the subducting Juan de Fuca plate; in agreement with predictions for entrained subslab mantle flow. The splitting pattern is consistent with that seen throughout Cascadia, suggesting that entrainment of the underlying asthenosphere with the subducting slab is coherent and widespread.

An investigation of seismic anisotropy in the lowermost mantle beneath Iceland

SUMMARYIceland represents one of the most well-known examples of hotspot volcanism, but the details of how surface volcanism connects to geodynamic processes in the deep mantle remain poorly understood. Recent work has identified evidence for an ultra-low velocity zone in the lowermost mantle beneath Iceland and argued for a cylindrically symmetric upwelling at the base of a deep mantle plume. This scenario makes a specific prediction about flow and deformation in the lowermost mantle, which can potentially be tested with observations of seismic anisotropy. Here we present an investigation of seismic anisotropy in the lowermost mantle beneath Iceland, using differential shear wave splitting measurements of S–ScS and SKS–SKKS phases. We apply our techniques to waves propagating at multiple azimuths, with the goal of gaining good geographical and azimuthal coverage of the region. Practical limitations imposed by the suboptimal distribution of global seismicity at the relevant distance ranges resulted in a relatively small data set, particularly for S–ScS. Despite this, however, our measurements of ScS splitting due to lowermost mantle anisotropy clearly show a rotation of the fast splitting direction from nearly horizontal for two sets of paths that sample away from the low velocity region (implying VSH > VSV) to nearly vertical for a set of paths that sample directly beneath Iceland (implying VSV > VSH). We also find evidence for sporadic SKS–SKKS discrepancies beneath our study region; while the geographic distribution of discrepant pairs is scattered, those pairs that sample closest to the base of the Iceland plume tend to be discrepant. Our measurements do not uniquely constrain the pattern of mantle flow. However, we carried out simple ray-theoretical forward modelling for a suite of plausible anisotropy mechanisms, including those based on single-crystal elastic tensors, those obtained via effective medium modelling for partial melt scenarios, and those derived from global or regional models of flow and texture development in the deep mantle. These simplified models do not take into account details such as possible transitions in anisotropy mechanism or deformation regime, and test a simplified flow field (vertical flow beneath the plume and horizontal flow outside it) rather than more detailed flow scenarios. Nevertheless, our modelling results demonstrate that our ScS splitting observations are generally consistent with a flow scenario that invokes nearly vertical flow directly beneath the Iceland hotspot, with horizontal flow just outside this region.

Receiver function analysis reveals layered anisotropy in the crust and upper mantle beneath southern Peru and northern Bolivia

Subduction systems play a key role in plate tectonics, but the deformation of the crust and uppermost mantle during continental subduction remains poorly understood. Observations of seismic anisotropy can provide constraints on dynamic processes in the crust and uppermost mantle in subduction systems. The subduction zone beneath Peru and Bolivia, where the Nazca plate subducts beneath South America, represents a particularly interesting location to study subduction-related deformation, given the along-strike transition from flat to normally dipping subduction. In this study we constrain seismic anisotropy within and above the subducting slab (including the overriding plate) beneath Peru and Bolivia by examining azimuthal variations in radial and transverse component receiver functions. Because anisotropy-aware receiver function analysis has good lateral resolution and depth constraints, it is complementary to previous studies of anisotropy in this region using shear wave splitting or surface wave tomography. We examine data from long-running permanent stations NNA (near Lima, Peru) and LPAZ (near La Paz, Bolivia), and two dense lines of seismometers from the PULSE and CAUGHT deployments in Peru and Bolivia, respectively. The northern line overlies the Peru flat slab, while the southern line overlies the normally dipping slab beneath Bolivia. We applied harmonic decomposition modeling to constrain the presence, depth, and characteristics of dipping and/or anisotropic interfaces within the crust and upper mantle. We found evidence for varying multi-layer anisotropy, in some cases with dipping symmetry axes, underneath both regions. The presence of multiple layers of anisotropy with distinct geometries that change with depth suggests a highly complex deformation regime associated with subduction beneath the Andes. In particular, our identification of depth-dependent seismic anisotropy within the overlying plate crust implies a change in deformation geometry, dominant mineralogy, and/or rheology with depth, shedding light on the nature of deep crustal deformation during orogenesis.

Crustal anisotropy and state of stress at Uturuncu Volcano, Bolivia, from shear-wave splitting measurements and magnitude–frequency distributions in seismicity

The physical signatures of unrest in large silicic magma systems are commonly observed in geophysical surveys, yet the interactions between magmatic processes and crustal stresses are often left unconstrained. Stresses in the mid and upper crust exert a strong control on the propagation and stalling of magma, and magma ascent can in turn change the magnitude and orientation of these stresses, including those associated with hydrothermal systems. This study assesses the state of stress at the restless Uturuncu Volcano in the Bolivian Andes with space, depth and time using observations of seismic anisotropy and the magnitude–frequency distributions of local earthquakes. Shear-wave splitting measurements are made for 677 events in the upper crust (1–25 km below sea level) between June 1, 2009 and March 10, 2012, and b-values are calculated using the Aki maximum likelihood method for a range of catalog subsets in the entire crust (−5 to 65 km below sea level). The b-value of the crustal events is unusually low (b=0.66±0.09), indicating that the seismogenic region features strong material with high stresses that are released with limited influence from hydrothermal fluids. The 410 good quality shear-wave splitting results have an average delay time of 0.06 ± 0.002 s and an average percent anisotropy ranging from 0.25 ± 0.04% to 6.2 ± 0.94% with a mean of 1.70 ± 0.32%. Fast shear-wave polarization directions are highly variable and appear to reflect a combination of tectonic and magmatic stresses that overprint the regional E–W compressive stress associated with the convergence of the Nazca and South American Plates. The shear-wave splitting results and b-values suggest that the upper crust beneath Uturuncu (∼0–7 km below the summit) is characterized by a weak and localized hydrothermal system in a poorly developed fracture network. We conclude that stresses imposed by crustal flexure due to magmatic unrest above the Altiplano-Puna Magma Body activate crack opening on a pre-existing fault beneath the volcano, generating seismicity and a spatially variable 1–10% anisotropy above the brittle–ductile transition zone. These results suggest that strong stresses in relatively unfractured upper crustal rocks may locally inhibit fluid migration in large silicic magma systems, leading to pluton emplacement and effusive volcanism rather than explosive eruptions.

Deformation, crystal preferred orientations, and seismic anisotropy in the Earth's D″ layer

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 <1000 km is needed to comply with the seismological evidence for low apparent birefringence in D″.

Quantifying seismic anisotropy induced by small-scale chemical heterogeneities

Observations of seismic anisotropy are usually used as a proxy for lattice-preferred orientation (LPO) of anisotropic minerals in the Earth's mantle. In this way, seismic anisotropy observed in tomographic models provides important constraints on the geometry of mantle deformation associated with thermal convection and plate tectonics. However, in addition to LPO, smallscale heterogeneities that cannot be resolved by long-period seismic waves may also produce anisotropy. The observed (i.e. apparent) anisotropy is then a combination of an intrinsic and an extrinsic component. Assuming the Earth's mantle exhibits petrological inhomogeneities at all scales, tomographic models built from long-period seismic waves may thus display extrinsic anisotropy. In this paper, we investigate the relation between the amplitude of seismic heterogeneities and the level of induced S-wave radial anisotropy as seen by long-period seismic waves. We generate some simple 1-D and 2-D isotropic models that exhibit a power spectrum of heterogeneities as what is expected for the Earth's mantle, that is, varying as 1/k, with k the wavenumber of these heterogeneities. The 1-D toymodels correspond to simple layeredmedia. In the 2-D case, our models depict marble-cake patterns in which an anomaly in shear wave velocity has been advected within convective cells. The long-wavelength equivalents of these models are computed using upscaling relations that link properties of a rapidly varying elastic medium to properties of the effective, that is, apparent, medium as seen by long-period waves. The resulting homogenized media exhibit extrinsic anisotropy and represent what would be observed in tomography. In the 1-D case, we analytically show that the level of anisotropy increases with the square of the amplitude of heterogeneities. This relation is numerically verified for both 1-D and 2-D media. In addition, we predict that 10 per cent of chemical heterogeneities in 2-D marble-cake models can induce more than 3.9 per cent of extrinsic radial S-wave anisotropy.We thus predict that a non-negligible part of the observed anisotropy in tomographic models may be the result of unmapped small-scale heterogeneities in the mantle, mainly in the form of fine layering, and that caution should be taken when interpreting observed anisotropy in terms of LPO and mantle deformation. This effect may be particularly strong in the lithosphere where chemical heterogeneities are assumed to be the strongest. © The Authors 2017.

Mantle flow through a tear in the Nazca slab inferred from shear wave splitting

AbstractA tear in the subducting Nazca slab is located between the end of the Pampean flat slab and normally subducting oceanic lithosphere. Tomographic studies suggest mantle material flows through this opening. The best way to probe this hypothesis is through observations of seismic anisotropy, such as shear wave splitting. We examine patterns of shear wave splitting using data from two seismic deployments in Argentina that lay updip of the slab tear. We observe a simple pattern of plate‐motion‐parallel fast splitting directions, indicative of plate‐motion‐parallel mantle flow, beneath the majority of the stations. Our observed splitting contrasts previous observations to the north and south of the flat slab region. Since plate‐motion‐parallel splitting occurs only coincidentally with the slab tear, we propose mantle material flows through the opening resulting in Nazca plate‐motion‐parallel flow in both the subslab mantle and mantle wedge.

Complex, multilayered azimuthal anisotropy beneath Tibet: evidence for co-existing channel flow and pure-shear crustal thickening

Of the two debated, end-member models for the late-Cenozoic thickening of Tibetan crust, one invokes “channel flow” (rapid viscous flow of the mid-lower crust, driven by topography-induced pressure gradients and transporting crustal rocks eastward) and the other—“pure shear” (faulting and folding in the upper crust, with viscous shortening in the mid-lower crust). Deep-crustal deformation implied by each model is different and would produce different anisotropic rock fabric. Observations of seismic anisotropy can thus offer a discriminant. We use broadband phase-velocity curves—each a robust average of tens to hundreds of measurements—to determine azimuthal anisotropy in the entire lithosphere-asthenosphere depth range and constrain its amplitude. Inversions of the differential dispersion from path pairs, region-average inversions and phase-velocity tomography yield mutually consistent results, defining two highly anisotropic layers with different fast-propagation directions within each: the middle crust and the asthenosphere. In the asthenosphere beneath central and eastern Tibet, anisotropy is 2–4 per cent and has a NNE–SSW fast-propagation azimuth, indicating flow probably driven by the NNE-ward, shallow-angle subduction of India. The distribution and complexity of published shear-wave splitting measurements can be accounted for by the different anisotropy in the mid-lower crust and asthenosphere. The estimated splitting times that would be accumulated in the crust alone are 0.25–0.8 s; in the upper mantle—0.5–1.2 s, depending on location. In the middle crust (20–45 km depth) beneath southern and central Tibet, azimuthal anisotropy is 3–5 and 4–6 per cent, respectively, and its E–W fast-propagation directions are parallel to the current extension at the surface. The rate of the extension is relatively low, however, whereas the large radial anisotropy observed in the middle crust requires strong alignment of mica crystals, implying large finite strain and consistent with high-rate horizontal flow. Together, radial and azimuthal anisotropy suggest eastward mid-crustal channel flow in central Tibet, along the regional topography gradient. In NE high Tibet, mid-crustal azimuthal anisotropy is 4–8 per cent and has WNW–ESE and NW–SE fast-propagation directions, parallel to the net extension at the surface. These fast directions are inconsistent with channel flow following the SW–NE regional topography gradient. Instead, they suggest similar net deformation in the (decoupled) shallow and deep crust. In the brittle upper crust, it is accommodated by strike-slip faulting; in the ductile mid-lower crust—by shear oriented at ∼45° to the faults. Although mid-crustal flow beneath NE Tibet may transport some material towards the plateau periphery at a low region-average rate, the dominant mid-crust deformation pattern is shear parallel to the plateau boundary. This implies that channel flow from central Tibet is not the main cause of the on-going crustal thickening farther northeast.

Superweak asthenosphere in light of upper mantle seismic anisotropy

AbstractEarth's upper mantle includes a km thick asthenosphere underneath the plates where viscosity and seismic velocities are reduced compared to the background. This zone of weakness matters for plate dynamics and may be required for the generation of plate tectonics itself. However, recent seismological and electromagnetic studies indicate strong heterogeneity in thinner layers underneath the plates which, if related to more extreme, global viscosity reductions, may require a revision of our understanding of mantle convection. Here, I use dynamically consistent mantle flow modeling and the constraints provided by azimuthal seismic anisotropy as well as plate motions to explore the effect of a range of global and local viscosity reductions. The fit between mantle flow model predictions and observations of seismic anisotropy is highly sensitive to radial and lateral viscosity variations. I show that moderate suboceanic viscosity reductions, to –0.1 times the upper mantle viscosity, are preferred by the fit to anisotropy and global plate motions, depending on layer thickness. Lower viscosities degrade the fit to azimuthal anisotropy. Localized patches of viscosity reduction, or layers of subducted asthenosphere, however, have only limited additional effects on anisotropy or plate velocities. This indicates that it is unlikely that regional observations of subplate anomalies are both continuous and indicative of dramatic viscosity reduction. Locally, such weak patches may exist and would be detectable by regional anisotropy analysis, for example. However, large‐scale plate dynamics are most likely governed by broad continent‐ocean asthenospheric viscosity contrasts rather than a thin, possibly high melt fraction layer.