Displays

Anisotropy beneath the Kalahari Craton in southern Africa has been a subject of a long-standing controversy: does the shear-wave splitting measured on it—small in amplitude but following a smoothly varying, regional fast-azimuth pattern—indicate dominant anisotropy within the lithosphere or, instead, within the underlying asthenosphere? Here we show that the thick lithosphere of cratons can contain multiple anisotropic layers with different rock fabric within each, recording different episodes of deformation at different times in the ancient past. We invert very broadband measurements of surface-wave phase velocities for the layering of anisotropy from the upper crust down to the asthenosphere (up to 350 km depth) beneath different cratonic blocks within the Kalahari Craton. Our Bayesian inversion yields both the most likely values and the uncertainties of S-velocity isotropic averages and the azimuthal and radial anisotropy at different depths.

We detect four main layers of azimuthal anisotropy. In the upper crust, fast-propagation directions across the region are aligned N-S, perpendicular to the direction of extension, as evidenced by the earthquake source mechanisms. The upper-crustal anisotropy can thus be accounted by aligned micro-cracks, opened by the regional tectonic stress. In the asthenosphere (350 km depth), fast-propagation directions are also uniform across the region and aligned NNE-WSW, parallel to the absolute plate motion of Africa. This indicates that athenospheric anisotropy reflects the shear associated with the plate motion. In the lower lithosphere, anisotropic fabric is oriented differently in every cratonic sub-block. This anisotropy is likely to pre-date the assembly of the Kalahari Craton. Finally, in the lower crust and upper mantle down to ~80 km, the fabric is oriented uniformly E-W.

The regionally uniform anisotropic fabric in the upper lithosphere and the contrast of this uniformity with the lateral variability shown by the lower lithosphere suggest a previously unknown style of tectonics, likely to be unique to the Archean-Paleoproterozoic times. Following the formation of the cratons’ thick continental crust, the high radiogenic heat production within it resulted in peculiar geotherms (as modelled previously), with particularly hot lower crust and uppermost mantle. Ductile flow within this mechanically weak layer, driven by regional stresses, could account for the observed anisotropy; the geological record confirms the occurrence of significant, late-Archean, E-W extension. The mechanically stronger deep lithosphere, by contrast, appears to have remained largely undeformed, preserving pre-existing fabric. 

References

Ravenna, M., S. Lebedev, J. Fullea, J. M.-C. Adam. Shear-wave velocity structure of southern Africa's lithosphere: Variations in the thickness and composition of cratons and their effect on topography. Geochem. Geophys. Geosyst., 19, 1499–1518, https://doi.org/10.1029/2017GC007399, 2018.

Ravenna, M., S. Lebedev. Bayesian inversion of surface-wave data for radial and azimuthal shear-wave anisotropy, with applications to central Mongolia and west-central Italy. Geophys. J. Int., 213, 278-300, DOI:10.1093/gji/ggx497, 2018.

Adam, J. M.-C., S. Lebedev. Azimuthal anisotropy beneath southern Africa, from very-broadband, surface-wave dispersion measurements. Geophys. J. Int., 191, 155–174, 2012.

How to cite: Lebedev, S., Ravenna, M., and Adam, J. M.-C.: The peculiar style of Archean continental tectonics: Insights from multi-layered seismic anisotropy beneath southern Africa, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11620, https://doi.org/10.5194/egusphere-egu2020-11620, 2020.

Azimuthal anisotropy in the NW U.S. crust is derived using 3-17 s Rayleigh waves derived using ambient noise from about 300 broadband stations. Velocity is resolved between all station pairs in close proximity, and velocity as a function of azimuth is determined for each station. Azimuthal anisotropy orientations point strongly toward tomographically-imaged high-velocity structures in the underlying mantle, but show no relation to the underlying mantle anisotropy field. We suggest that the crustal anisotropy is decoupled from lateral tectonic forces and is created by upper mantle vertical loading, which in turn generates lateral pressure gradients that drive channelized flow in the ductile mid and lower crust. This idea is tested with geodynamic modeling. Using reasonable values for crustal viscosity and mantle buoyancy structure, we find that the local buoyancy sources within the upper mantle will drive the viscous crustal flow in a manner that reproduces well the imaged crustal anisotropy. We conclude that mantle vertical loading, acting independently from mantle flow, can actively control crustal deformation on a scale of several hundred kilometers.

How to cite: Humphreys, E., Castellanos, J., Clayton, R., Perry-Houts, J., Kim, Y., Niday, B., and Stanciu, C.: Northwest U.S. crustal seismic anisotropy suggests crustal flow driven by vertical loads in the underlying mantle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12438, https://doi.org/10.5194/egusphere-egu2020-12438, 2020.

A proper understanding of crustal seismic anisotropy beneath the tectonically complex northwestern part of the North Anatolian Fault Zone (NAFZ) will shed light into the depth extent of deformation zones. To investigate the seismic anisotropy in the crustal part of the NAFZ, we applied the harmonic decomposition technique on receiver functions from teleseismic earthquakes (with epicentral distances between 30° and 90°) recorded at the Dense Array for North Anatolia (DANA) seismic network. Harmonic coefficients, k=0, k=1, and k=2 were obtained by applying the harmonic decomposition method to the depth migrated receiver functions. Results from k=0 harmonics suggest south to north (e.g. from Sakarya Zone to Istanbul Zone) increase in crustal thickness. The depth variations of energy associated with k=1 and k=2 harmonic components imply significant lateral variation. For instance, the energy calculated for k=1 harmonics in the north (Istanbul Zone) indicates that seismic anisotropy is likely concentrated in the upper crust (within the first 15 km). However, further south, the signature of anisotropy in Armutlu-Almacik and Sakarya Zones becomes more significant in close proximity to the fault zone and dominates at greater (15-30 km and 30-60 km). Furthermore, k=2 harmonic energy maps exhibit relatively high intensities nearby the fault for all depth ranges.

How to cite: Keleş, D., Eken, T., Licciardi, A., and Taymaz, T.: Crustal and Upper Mantle Deformation Beneath Northwestern part of North Anatolian Fault Zone from Harmonic Decomposition of Receiver Functions , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-527, https://doi.org/10.5194/egusphere-egu2020-527, 2020.

Gas chimneys are locations where natural gas leaks from the subsurface causing seabed pockmarks and potentially creating leakage pathways from CO₂ storage or other subsurface reservoirs. The CHIMNEY project seeks evidence of changes in anisotropy between a gas chimney and the surrounding sediments, which would corroborate theories on the chimney permeability being caused by fractures.

Twenty-five ocean bottom seismometers (OBSs) were placed in an asterisk-shaped array over the Scanner pockmark in the UK License Block 15/25 in the North Sea and at a reference location ~1.5 km away. The OBSs recorded for several days while an active source survey was undertaken. Rayleigh wave data were also available from ambient seismic noise observed by using a low pass filter to remove active sources from the data.

We use 2D beamforming to observe the azimuthal dependence of the Rayleigh wave phase velocity. 2D beamforming uses radon transforms summed over time for a range of different azimuths which gives the distribution of wave energy passing across an array as a function of group velocity.

Using narrowly band-passed data for the beamforming, we observe phase velocities of 250 - 650 m/s in the 0.8 - 1.2 Hz range. Initial results show θ, 2θ and 4θ anisotropy components in the measured phase velocities at the frequencies with the best ambient sources. We observe different fast orientations at the reference site than the chimney site. Varying anisotropy between the two sites supports the hypothesis that there is different fracturing in the chimney than in the surrounding geology.

With lower frequency surface waves penetrating deeper into the subsurface, dispersion of surface waves provides information about velocity variations with depth. Despite the array aperture imposing a lower limit on observable frequencies at around 0.7 Hz and noise source availability imposing a higher limit of about 1.2 Hz, strong dispersion was evident at both sites within this frequency window. The orientation and degree of anisotropy also appears to vary with frequency, indicating a variation in velocity and anisotropy with depth.

This work was undertaken with funding from NERC through the E3 Doctoral Training Partnership (E3 DTP; NE/L002558/1). The data was acquired with funding from the NERC (CHIMNEY; NE/N016130/1) and EU Horizon 2020 programme (STEMM-CCS; No.654462).

How to cite: Parkes, L., Chapman, M., Curtis, A., Minschull, T. A., Bull, J. M., Henstock, T., Bayrakci, G., and MacDonald, C.: Azimuthal anisotropy of Rayleigh waves across a gas chimney structure, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20436, https://doi.org/10.5194/egusphere-egu2020-20436, 2020.

To use observations of seismic anisotropy to constrain mantle flow patterns, we need a model for how progressive deformation of a rock leads to preferred orientation (CPO) of its constituent crystals. An important class of such models comprises so-called `self-consistent' (SC) models such as VPSC (viscoplastic SC) and SOSC (second-order SC). However, calculations based on SC models are far too costly for use in 3-D time-dependent convection simulations. To overcome this difficulty, we have developed two new analytical models that combine the accuracy of SC models with a greatly enhanced (by orders of magnitude) computational efficiency. The basis of our new models is the discovery that the crystallographic spin predicted by SC models as a function of crystal orientation is always a generalized spherical harmonic of degree 2, regardless of the CPO of the aggregate. This fact allows us to find an analytical expression for the spin to within an arbitrary amplitude, which we then determine by fitting to the predictions of the SOSC model.  Our first new model, ANPAR, uses the analytical expression for the spin to calculate evolving CPO in an aggregate comprising many (typically 2000) individual grains. The resulting CPO is visually indistinguishable from the SOSC predictions, but is ~ 50000 times faster to compute. Our second model, SBFTEX, is based on a more economical representation of CPO as a weighted sum of a small number of  analytical `structured basis functions' (SBFs), each of which represents the virtual CPO that would be produced by one intracrystalline slip system acting alone. The model consists of analytical expressions for the weighting coefficients of the SBFs as functions of the finite strain experienced by the aggregate. While somewhat less accurate than ANPAR, SBFTEX is ~ 2000 times faster, or ~ 10⁸ times faster than SOSC. We will illustrate the predictions of ANPAR and SBFTEX for pure olivine polycrystals, a simple model for the upper 400 km of the mantle.

How to cite: Ribe, N., Castelnau, O., Goulding, N., Hielscher, R., Walker, A., and Wookey, J.: Fast analytical models for texture evolution in anisotropic polycrystals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-608, https://doi.org/10.5194/egusphere-egu2020-608, 2020.

Over the last two decades, there has been significant progress in the development of tomographic schemes to infer the anisotropic properties of the upper mantle from teleseismic XKS phases. The methods are based on linking the anisotropic material properties and their changes with surface observables and waveform proxies, such as splitting parameters and intensities, through finite-frequency sensitivity kernels. These approaches are supported by increasingly dense seismic networks that allow for a more precise characterization of short-scale waveform variations due to lateral variations of anisotropy.

Here we focus on the general capability of the tomographic schemes to uniquely resolve the anisotropic structure of the upper mantle from surface observations. For this purpose, we perform full-waveform calculations for relatively simple, canonical models of upper-mantle anisotropy. Our approach involves checkerboard-style tests similar to those typically used to assess the resolving power of tomographic schemes. The models are characterized by four zones of different anisotropic properties. Specifically, we assume orthorhombic symmetry with arbitrarily chosen strength of the anisotropy and orientation of the horizontal a-axis. XKS waveforms, generated from plane-wave initial conditions, traverse through anisotropic models and are recorded at the surface by a dense station profile. In addition to waveforms, we also consider the effects of different anisotropic models on splitting parameters and splitting intensities.

The results show that it is, generally, not possible to uniquely resolve the eight anisotropic parameters (a-axis orientation and strength of anisotropy in four zones) of a given model, even if complete waveforms (under noise-free conditions) are considered. This is related to the fact that waveforms for significantly different anisotropic models, often, are indistinguishable. We conclude that finite-frequency XKS-splitting tomography, alone, is not suited to resolve the anisotropic structures of the upper mantle and that combinations with alternative methods, based on e.g. receiver-function splitting or surface waves, are required.

How to cite: Rümpker, G. and Kaviani, A.: On the limitations of finite-frequency XKS-splitting tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8077, https://doi.org/10.5194/egusphere-egu2020-8077, 2020.

Observations of P-wave (Vp) and S-wave (Vs) velocities in Antarctic and Greenland ice sheets show a strong decrease of 25% of Vs in their deep parts, while Vp remains approximately constant. The drastic Vs decrease corresponds to the basal “echo free zone”, where large-scale disturbances and strong preferred ice crystal orientation are found. According to Wittlinger and Farra (2014), the low Vs may be due to the presence of unfrozen liquids resulting from pre-melting at grain joints and/or melting of chemical solutions buried in ice. In this contribution we investigate the evolution of seismic velocity anisotropy during deformation of temperate ice by means of microdynamic numerical simulations. Temperate ice is modelled as a two-phase non-linear viscous aggregate constituted by a solid phase (ice polycrystal) and a liquid phase (melt). The viscoplastic full-field numerical approach (VPFFT-ELLE) (Lebensohn and Rollet, 2020) is used to calculate the mechanical response of the two-phase aggregate, which deforms purely by dislocation glide. Viscoplastic deformation is coupled with dynamic recrystallisation processes, such as grain boundary migration, intracrystalline recovery and polygonisation (Llorens et al., 2017), all driven by the reduction of surface and strain energies. The changes in P- and S-wave velocities are calculated with the AEH-EBSD software (Vel et al., 2016) from single crystal stiffness and microstructural measurements of crystal preferred orientations (CPO) during deformation. Regardless the amount of melt and intensity of recrystallisation, all simulations evolve from a fabric defined by randomly oriented c-axes to a c-axis preferred orientation (CPO) distribution approximately perpendicular to the shear plane.  For purely solid aggregates, the results show that the highest Vp and lowest Vs velocities are rapidly aligned with the CPO (at a shear strain of 1), and then evolve to a strong single maximum with progressive deformation. This alignment has been previously predicted in models, experiments and measured in ice core samples. When melt is present, the maximum and minimum seismic velocities are not aligned with the CPO and both Vp and Vs are considerably lower than in cases without melt.  However, if the bulk modulus of ice is assumed for the melt phase, the presence of melt produces a remarkable decrease in S-wave velocity while Vp is maintained constant. These results suggest that the decrease in S-wave velocity observed at the base of ice sheets could be explained by the presence of overpressured melt, which would be unconnected at triple grain junctions in the ice polycrystal.

References:

Wittlinger and Farra. 2014. Polar Science 9, 66-79.

Lebensohn and Rollet. 2020. Computational Mat. Sci. 173, 109336.

Llorens, et al. 2017. Philosophical Transactions of the Royal Society A, 375, 20150346.

Vel, et al. 2016. Computer Methods in Applied Mechanics and Engineering 310, 749-779.

How to cite: Llorens, M.-G., Griera, A., Bons, P. D., Gomez-Rivas, E., Weikusta, I., Prior, D., and Lebensohn, R.: The effect of melt on seismic anisotropy of ice polycrystalline aggregates , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19134, https://doi.org/10.5194/egusphere-egu2020-19134, 2020.

To characterize crustal anisotropy beneath the North China Craton (NCC), we apply a recently developed deconvolution approach to effectively remove near-surface reverberations in the receiver functions recorded at 200 broadband seismic stations and subsequently determine the fast orientation and the magnitude of crustal azimuthal anisotropy by fitting the sinusoidal moveout of the P to S converted phases from the Moho and intracrustal discontinuities. The magnitude of crustal anisotropy is found to range from 0.06 s to 0.54 s, with an average of 0.25 ± 0.08 s. Fault-parallel anisotropy in the seismically active Zhangjiakou-Penglai Fault Zone is significant and could be related to fluid-filled fractures. Historical strong earthquakes mainly occurred in the fault zone segments with significant crustal anisotropy, suggesting that the measured crustal anisotropy is closely related to the degree of crustal deformation. The observed spatial distribution of crustal anisotropy suggests that the northwestern terminus of the fault zone probably ends at about 114°E. Also observed is a sharp contrast in the fast orientations between the western and eastern Yanshan Uplifts separated by the North-South Gravity Lineament. The NW-SE trending anisotropy in the western Yanshan Uplift is attributable to “fossil” crustal anisotropy due to lithospheric extension of the NCC, while extensional fluid-saturated microcracks induced by regional compressive stress are responsible for the observed ENE-WSW trending anisotropy in the eastern Yanshan Uplift. Comparison of crustal anisotropy measurements and previously determined upper mantle anisotropy implies that the degree of crust-mantle coupling in the NCC varies spatially.

How to cite: Zheng, T., Gao, S. S., Ding, Z., and Fan, X.: Distribution of crustal azimuthalanisotropy beneath the North China Craton: Insights from analysis of receiver functions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1455, https://doi.org/10.5194/egusphere-egu2020-1455, 2020.

Most of existing rocks are typically fractured that effectively result in anisotropic model of different complexity (Tsvankin and Grechka, 2011). The anisotropy signatures can be defined by traveltime and reflection&transmission amplitudes at plane interface between the rocks of different properties. Here, we focus on reflection amplitude by introducing the uniform approach to embed the fracture sets of different orientation. We utilize the linear slip theory (Schoenberg and Helbig, 1997) to add the fractures of arbitrary weakness parameters. The anisotropic model sequence consists of isotropic model (later used as a background), transversely isotropic model with a vertical symmetry axis (due to horizontal fracture set), orthorhombic model (due to horizontal and vertical fracture sets), monoclinic model with a horizontal symmetry plane (due to horizontal and two non-orthogonal vertical fracture sets) and triclinic model (due to horizontal, two non-orthogonal vertical and one inclined fracture sets). The general equation for the matrix of stiffness coefficients is given by the inverse sum of the fracture weakness matrices multiplied with density. The isotropic background stiffness coefficient matrix is defined by inverse of background weakness matrix. Each fracture weakness matrix generally has three independent parameters that are normal, tangential and horizontal weaknesses. In addition to these parameters, the fracture orientation angles in 3D space are also taken into account, and the rotation matrix is defined for each set of fractures. The uniform non-rotated weakness matrix can be chosen for brevity’s sake, however, all fracture sets might have their own weaknesses. We analyze the plane P wave reflection coefficient computed at plane interface between isotropic background and fractured background half-spaces. It is convenient to show reflection coefficient versus horizontal slowness projections. To compute reflection coefficients, we use the method developed by Jin and Stovas (2020).

The fracture sets of different orientation affect azimuthally dependent amplitude signatures. By using proposed method, the fracture set parameters and orientation can be estimated from seismic data.

References

Tsvankin, I., and V. Grechka, 2011, Seismology of azimuthally anisotropic media and seismic fracture characterization. SEG.

Schoenberg, M., and K. Helbig, 1997, Orthorhombic media: Modeling elastic wave behavior in avertically fractured earth. Geophysics, 62(2). 1954-1974.

Jin, S., and A. Stovas, 2020, Reflection and transmission approximations for monoclinic media with a horizontal symmetry plane. Geophysics (early view).

How to cite: Stovas, A. and Jin, S.: Amplitude signatures in fractured media, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2011, https://doi.org/10.5194/egusphere-egu2020-2011, 2020.

Telesesimic earthquake data recorded at eight seismograph stations across the northeast India are analysed for shear-wave splitting from core-refracted XKS phases (collectively PKS, SKS and SKKS). Shear-wave splitting parameters, derived from the analysis provide information about seismic anisotropy and deformation of the crust and upper mantle beneath each seismograph stations site. The results point towards the presence of complex and highly anisotropic crust and upper mantle beneath northeast India. Being surrounded by two seismically active plate boundaries, to the north by India-Eurasia collision plate boundary and to the east by Indo-Burman subduction plate boundary, the crust and upper mantle beneath the northeast India has been assumed to have complex deformation pattern. This present study provides an evidence for this assumption. According to station locations, we have one station BONG situated near the Main boundary thrust (at India-Eurasia collision zone), one station NAMS and eastern syntexis Himalaya, five station AZWL, SILS, DIPH and NKCR at Indo-Burman subduction plate boundary, one station SHLS and Shillong plateau bounded by Oldham Fault, Dauki Fault and Kopli fault, and one station AGAR at the boundary of Bengal basin. The direction of anisotropy is nearly E-W at BONG, NE-SW in the Indo-Burman subduction zone, nearly N-S on Shillong plateau and NW-SE at eastern syntexis of Himalaya. Source of anisotropy in the Himalaya collision boundary is result of lithospheric deformation due to finite strain induced by collision. In Shillong plateau and Indo-burman subduction boundary, source of anisotropy seems to be the asthenospheric flow-related strain which is also in harmony with the absolute plate motion (APM) of the Indian plate in a no net reference frame.

How to cite: Kanaujia, J., Surve, G., and Hazarika, N.: Upper Mantle deformation patterns beneath norteast India from Shear-wave splitting analysis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4694, https://doi.org/10.5194/egusphere-egu2020-4694, 2020.

The surface wave dispersion data with azimuthal anisotropy can be used to invert for the wavespeed azimuthal anisotropy, which provides essential dynamic information about depth-varying deformation of the Earth’s interior. In this study, we adopt an rj-MCMC (reversible jump Markov Chain Monte Carlo) technique to invert for crustal and upper mantle shear velocity and azimuthal anisotropy beneath the Japan Sea using Rayleigh wave azimuthally anisotropic phase velocity measurements from Fan et al. (2019). The rj-MCMC implements trans-dimensional sampling in the whole model space and derives the distribution for each model parameter (shear wave velocity and anisotropy parameters) directly from data. Without the prejudiced parameterization for model, the result can be more credible, from which some more reliable estimates for shear wave velocity and azimuthal anisotropy as well as their uncertainties can be acquired. Our preliminary results, together with shear wave splitting observations, show a layered anisotropy beneath the Japan Sea and NE China, suggesting the complicated mantle flow that is controlled by the subduction of the Pacific plate and the large-scale upwelling beneath the Changbaishan volcano.

How to cite: Zhao, Y., Guo, Z., Fan, X., and Wang, Y.: Layered anisotropy beneath the Japan Sea and NE China from inversion of surface wave dispersion using rj-MCMC method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6381, https://doi.org/10.5194/egusphere-egu2020-6381, 2020.

It is widely accepted that the ongoing India-Asia collision since approximately 50 Ma ago has resulted in the uplift and eastward expansion of the Tibetan Plateau. Yet the interpretations of its dynamic process and deformation mechanism still remain controversial. Distinct models that emphasize particular aspects of the tectonic features have been proposed, including fault-controlled rigid blocks, continuous deformation of lithosphere and lower crust flow.

One possible way to reconcile these models is to investigate crustal deformation at multiple depths simultaneously, as well as crust-mantle interaction. Seismic anisotropy is considered as an effective tool to study the geometry and distribution of subsurface deformation, due to its direct connection to the stress state and strain history of anisotropic structures and fabrics. In the eastern margin of Tibetan plateau, previous studies of seismic anisotropy have already provided useful insights into the bulk anisotropic properties of the entire crust or upper mantle, based on shear wave splitting analyses of Moho Ps and XKS phases.

In this study, we went further to extract anisotropic parameters of multiple crustal layers by waveform inversion of teleseismic receiver function (RF) data from the western-Sichuan temporal seismic array using particle swarm optimization. Instead of directly fitting the backazimuthal stacking of RFs from each station, we translated the RF data into backazimuthal harmonic coefficients using harmonic decomposition technique, which separates the signals (of planar isotropic structure and anisotropy) from the scattering noise generated by non-planar lateral heterogeneity. The constant (k=0) and k=1, 2 terms of backazimuthal harmonic coefficients were used in our inversion. We also fixed the anisotropic model to slow-axis symmetry to avoid ambiguous interpretations.

Our results show that:

(1) Anisotropy with a titled anisotropy axis of symmetry is more commonly observed than pure azimuthal anisotropy in our data, which has been also reported by other RF studies across the surrounding areas of Tibetan plateau.

(2) The trends of slow symmetry axis vary from the upper to lower part of the crust in both Chuandian and Songpan units, indicating the deformation of the upper crust is decoupled from that of the lower crust in these two regions, while the trends are more consistent throughout the crust in the Sichuan basin.

(3) In the upper crust, the trends show a degree of tendency to lie parallel to the major geological features such as the Xianshuihe and Longmenshan faults, exhibiting a fault-controlled deformation or movement. In the middle and lower crust, the trends are NS or NW-SE in Chuandian unit and NE-SW in Songpan unit, which are coincident with the apparent extension directions of the ductile crustal flow.

How to cite: Qi, S., Liu, Q., Chen, J., and Guo, B.: Multilayered Crustal Anisotropy in Eastern Tibet Revealed by Receiver Function Data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7623, https://doi.org/10.5194/egusphere-egu2020-7623, 2020.

Earth's mantle rocks are poly-aggregates where different mineral phases coexist.  These rocks may often be approximated as two-phase aggregates with a dominant phase and less abundant one (e.g. bridgmanite-ferropericlase aggregates in the lower mantle). Severe shearing of these rocks leads to a non-homogeneous partitioning of the strain between the different phases. The resulting bulk rock is mechanically not isotropic, and the elastic and the viscous tensor depend on the volume fraction and viscosity contrast between the mineral phases and the fabric.

Here we employ three-dimensional mechanical models to reproduce and parametrise fabrics typical of mantle rocks and quantify the evolution of the viscous tensor. These fabrics are produced by shearing a mechanically heterogeneous medium comprised by randomly distributed isotropic inclusions embedded in: i) a weak inclusion-strong matrix aggregate where strain is mainly accommodated by the weak phase, that flattens and yields a penetrative foliation; and, ii) a strong inclusion-weak matrix where strain is mainly accommodated by the matrix, in this case, the strong phase deforms primarily parallel to the direction of the flow, producing cigar-shaped inclusions.

Finally, we combine the fabric parametrisation of a two-phase aggregate with the Differential Effective Medium (DEM) theory to study the evolution of the viscous tensor and its effects in mantle dynamics. The results of two-dimensional models of thermal convection show that a viscosity contrast of one order of magnitude between the two mineral phases is enough to deflect mantle plumes and produce convection patterns that differ considerably from the ideal isotropic media.

How to cite: de Montserrat Navarro, A. and Faccenda, M.: Extrinsic viscous anisotropy in two-phase aggregates, fabric parametrisation and application to mantle convection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11325, https://doi.org/10.5194/egusphere-egu2020-11325, 2020.

Coupling large-scale geodynamic and seismological modelling appears a promising methodology for the understanding of the Earth’s recent dynamics and present-day structure. So far, the two types of modelling have been mainly conducted separately, and a code capable of linking these two methodologies of investigation is still lacking.

In this contribution I present MAVEPROS, a new open source software that allows both for the modelling of strain-induced mantle fabrics and seismic anisotropy, and for the generation of realistic synthetic tomographic models.

As an input, the software requires the velocity, pressure, temperature (and additionally the fraction of deformation accommodated by dislocation creep) fields (averaged each 100 kyr for typical mantle strain rates) outputted by the large-scale mantle flow models.

The strain-induced mantle fabrics are then modelled with D-Rex (Kaminski et al., 2004, GJI), an open source code that has been parallelized and modified to account for fast computation, combined diffusion-dislocation creep (Faccenda and Capitanio, 2012a, GRL; 2013, Gcubed), LPO of transition zone and lower mantle polycrystalline aggregates, P-T dependence of single crystal elastic tensors (Faccenda, 2014, PEPI), advection and non-steady-state deformation of crystal aggregates in 2D/3D cartesian/spherical grids with basic/staggered velocity nodes (Hu et al., 2017, EPSL), homogeneous sampling of the mantle by implementation of the Deformable PIC method (Samuel, 2018, GJI), apparent anisotropy in layered or crack-bearing rocks estimated with the Differential Effective Medium (DEM) (Sturgeon et al., Gcubed, 2019). The new version of D-Rex can solve for the LPO evolution of 100.000s polycrystalline aggregates of the whole mantle in a few hours, outputting the full elastic tensor of poly-crystalline aggregates as a function of each single crystal orientation, volume fraction and P-T scaled elastic moduli.

The crystal aggregates can then be interpolated in a tomographic grid for either visual inspection of the mantle elastic properties  (such as Vp and Vs isotropic anomalies; radial, azimuthal, Vp and Vs anisotropies; reflected/refracted energy at discontinuities for different incidence angles as imaged by receiver function studies; ), or to generate input files for large-scale synthetic waveform modelling (e.g., SPECFEM3D format; FSTRACK format to calculate SKS splitting (Becker et al., 2006, GJI)).

How to cite: Faccenda, M.: MAVEPROS: a new open source software to predict mantle elastic properties and build realistic tomographic models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18884, https://doi.org/10.5194/egusphere-egu2020-18884, 2020.

Despite the well-established anisotropic nature of Earth’s upper mantle, the influence of elastic anisotropy on teleseismic tomographic images remains largely ignored. In subduction zones, unmodeled anisotropic heterogeneity can lead to substantial isotropic velocity artefacts that may be misinterpreted as compositional heterogeneities (e.g. Bezada et al., 2016). Recent studies have demonstrated the possibility of inverting P-wave delay times for the strength and orientation of seismic anisotropy assuming a hexagonal symmetry system (e.g. Huang et al., 2015; Munzarová et al., 2018). However, the ability of P-wave delay times to constrain complex anisotropic patterns, such as those expected in subduction settings, remains unclear as the aforementioned methods are tested using ideal self-consistent data (i.e. data produced using the assumptions built into the tomography algorithm) generated from simplified synthetic models. Here, we test anisotropic P-wave imaging methods on data generated from geodynamic simulations of subduction. Micromechanical models of polymineralic aggregates advected through the simulated flow field are used to create an elastic model with up to 21 independent coefficients. We then model the teleseismic wavefield through this fully anisotropic model using SPECFEM3D coupled with AxiSEM. P-wave delay times across a synthetic seismic array are measured using conventional cross-correlation techniques and inverted for isotropic velocity and the strength and orientation of anisotropy using travel-time tomography methods. We propose and validate approximate analytic finite-frequency sensitivity kernels for the simplified anisotropic parameters. Our results demonstrate that P-wave delays can reliably recover horizontal and vertical changes in the azimuth of anisotropy. However, substantial isotropic artefacts remain in the solution when only inverting for azimuthal anisotropy parameters. These isotropic artefacts are largely removed when inverting for the dip as well as the azimuth of the anisotropic symmetry axis. Due to the relative nature of P-wave delay times, these data generally fail to reconstruct anisotropic structure that is spatially uniform over large scales. To overcome this limitation, we propose a joint inversion of SKS splitting intensity with P-wave delay times. Preliminary results demonstrate that this approach improves the recovery of the magnitude and azimuth of anisotropy. We conclude that teleseismic P-wave travel-times are a useful observable for probing the 3D distribution of upper mantle anisotropy and that anisotropic inversions should be explored to better understand the nature of isotropic velocity anomalies in subduction settings.

How to cite: VanderBeek, B. and Faccenda, M.: Can Teleseismic Travel-Times Constrain 3D Anisotropic Structure in Subduction Zones? Insights from Realistic Synthetic Experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14886, https://doi.org/10.5194/egusphere-egu2020-14886, 2020.

Upper mantle dynamics (e.g. subduction processes, slab roll-back, slab tearing and mantle upwelling) impact eastern Mediterranean region tectonics but a detailed understanding of the acting forces has remained elusive. Further progress requires more accurate measurements not just of the surface kinematics (from GPS) but also of indirect indicators of kinematics throughout the lithosphere and convecting upper mantle from seismology. A robust quantification of the magnitude, location and orientation of seismic anisotropy is a primary source of information to provide constraints on tectonic processes of the formation and evolution of the Anatolian Peninsula and the surrounding regions. Direct shear-wave splitting measurements in the Aegean to revealed mostly NNE-SSW oriented fast polarization directions, perpendicular to the trench and parallel to the mantle flow induced by the roll-back and large time delays (1.15-1.62 s) in the upper mantle. In southwestern Turkey the FPDs are more confusing and probably related to the tearing of the slab in the upper mantle underneath this region. With complex non-steady state 3D geodynamic modelling, the plate movement, mantle flow, anisotropy and SKS splitting parameters for the last 20-30 Ma in the regional subduction system of the eastern Mediterranean and Anatolia were calculated. The model shows that tearing underneath southwestern Turkey, a break-off in the collitional regime of eastern Anatolia as well as the retreat of the slab in the Aegean influence on the strength and direction of the mantle flow and anisotropy. At last a P-wave tomography study of the Eastern Mediterranean region, focusing on the upper mantle with a large data set was done. Since anisotropy is present in the region especially due to the active subduction system, travel times were corrected by including anisotropy as an aprori constraint, from the numerical model and SKS splitting parameters. In isotropic inversions as well as the ones corrected for anisotropy, tears in the northern Hellenic slab, underneath southwestern Turkey and in the Cyprian slab can be seen. Spatially large first order velocity perturbations are stable and similar in isotropic and anisotropy corrected models. But differences up to 2% and small geometrical discrepancies beween the models show the importance of including anisotropy to P-wave tomographies.

How to cite: Confal, J., Eken, T., Bezada, M., Faccenda, M., Saygin, E., and Taymaz, T.: Anisotropy and Mantle Kinematics in the Eastern Mediterranean Region based on Shear Wave Splitting Measurements, Numerical Models and P-wave Tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8096, https://doi.org/10.5194/egusphere-egu2020-8096, 2020.

The present tectonics of Iran has resulted from the continental convergence of the Arabian and Eurasian plates. Our study area, in NW Iran comprises a part of this collision zone and consists of an assemblage of distinct lithospheric blocks including the central Iranian Plateau, the South Caspian Basin, and the Talesh western Alborz Mountains. A proper knowledge of mantle flow field is required to bettwer constrain mantle kinematics in relation to the dynamics of continental deformation in NW Iran. To achieve this aim, we examined splitting of teleseismic shear waves (e.g. SKS and S) arriving with steep arrival angles beneath the receiver, which provide excellent lateral resolution in the upper mantle. We used data from 68 temporary broadband stations with varying operation periods (4 to 31 months) along 3 linear profiles. We perfomed splitting analyses on SK(K)S and direct S waves. Resultant splitting parameters obtained from both shear phases exhibit broad similarities. Relatively large time delays observed for direct S-waves, however, are anticipated since these waves travel longer than SKS along a non-vertical propagation path in an anisotropic layer. Overall, the fast polarization directions (FPDs) in the Alborz, Talesh, Tarom Mountain and in NW Iran indicate a strong consistency with NE-SW anisotropic orientations. Besides, we observe a good accordance between S and SKS results. A comparison of splitting parameters with the absolute plate motion (APM) vector and structural trends in Iran and eastern Turkey suggests asthenospheric flow field as the dominant source for observed seismic anisotropy. The lithospheric layer beneath these regions is relatively thin (compared to the adjacent Zagros region), explaining why it appears to only make a partial contribution to the observed anisotropy. The stations located in central Iran just southwest of the Alborz yield angular deviations from the general NE-SW trend as this may be explained by changing style of deformation across the different tectonic blocks. These stations indicate significant misfit between SK(K)S and direct S-waves that could be caused by local heterogeneities developed due to a diffuse boundary from the flow organization in the upper mantle of central Iran. Another possibility for large differences between two types of waves might be reflect the anisotropic structure of a remnant slab segment or a foundered lithospheric root beneath central Iran with a volume small enough to be detected by SKS phases, but not by the direct S waves.

How to cite: Arvin, S., Sobouti, F., Priestley, K., Ghods, A., Motaghi, S. K., Tilmann, F., and Eken, T.: Seismic anisotropy and mantle deformation in NW Iran through splitting measurements of SKS and direct S phases, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11368, https://doi.org/10.5194/egusphere-egu2020-11368, 2020.

A general review on measurements of upper mantle seismic anisotropy in the Alpine and Apennines region is now encouraged by the large amount of data produced by several projects (i.e AlpArray, Cifalps1). Geodynamic studies need to have a sketch of mantle flows that drives the evolution of a
tectonically active region. This is particularly important for the Italian peninsula, where several slabs have been involved in the Alps and Apennines building and where they are still interacts with the Adriatic plate. Draw mantle flows starting from seismic anisotropy requires to locate the source of what SKS phases detect. The answer, often undetermined, it is frequently hypothesized cross-checking different seismological observation. Overlapping SKS data with tomographic models in this region gives little help, because of the large differences in the shape, depth and dimension of fast bodies identified by different tomographic studies. Mapping and comparing SKSs data with other types of anisotropy measurements (Pn anisotropy, azimuthal anisotropy from surface waves tomography, crustal anisotropy) allow to discretise where fast anisotropy direction is much more probably astenospheric or where it pervades also regions at shallower depths.

How to cite: Pondrelli, S., Salimbeni, S., and Faccenda, M.: Where is seismic anisotropy located beneath the Alps and the Apennines?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10624, https://doi.org/10.5194/egusphere-egu2020-10624, 2020.

This study concerns crustal anisotropy at 16 permanent seismic stations to investigate preferentially aligned cracks or structures and their relation to the stress-state in the South Central Alborz (northern Iran). We consider plunging anisotropy and dipping interfaces of multiple layers using harmonic functions to correct the arrival time variations of Ps phases from different back-azimuths.

The dominant fast orientation of integrated crustal anisotropy strikes NE, almost parallel to the stress direction in the upper crust. The magnitude of crustal anisotropy is found to be in range of 0.1 s to 0.5 s. In some stations, intracrustal interface is observed, for which we analyzed harmonic decomposition of receiver functions to consider anisotropy in the upper crust. Upper crustal anisotropy strikes NE, close to the principal stress direction, indicating that stress in the upper crust plays a major role in producing anisotropy and deformation. In a few stations, crustal anisotropy display different directions rather than NE, which maybe controlled by cracks and fractures of dominant faults.

Keywords: Anisotropy, Receiver function, harmonic decomposition, Northern Iran.

How to cite: Azqandi, M., Abbassi, M. R., Mahmoodabadi, M., and Sadidkhouy, A.: Crustal Anisotropy beneath Northern Iran calculated by harmonic decomposition of Receiver Functions., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-962, https://doi.org/10.5194/egusphere-egu2020-962, 2020.

Seismic anisotropy studies can provide important constraints on geodynamic processes and deformation styles in the upper mantle of tectonically active regions. Seismic anisotropy parameters (e.g. delay time and fast polarization direction) can give hints at the past and recent deformations and can be most conventionally obtained through core-mantle refracted SKS phase splitting measurements. In order to explore the complexity of anisotropic structures in the upper mantle of a large part of the Aegean region, in this study, we estimate splitting parameters beneath 25 broad-band seismic stations located at NW Anatolia, North Aegean Sea and Greece mainland. To achieve this we employ both transverse energy minimization and eigenvalue methods. Waveform data of selected earthquakes (with M_w ≥ 5.5; 2008-2018 and with epicentral distances between 85°–120°) were retrieved from Earthquake Data Center System of Turkey (AFAD; http://tdvm.afad.gov.tr/) and European Integrated Data Archive (EIDA; http://orfeus-eu.org/webdc3/). A quite large data set, the majority of which have not been studied before, were evaluated in order to estimate reliable non-null and null results. In general, station-averaged splitting parameters mainly exhibit the NE-SW directed fast polarization directions throughout the study area. These directions can be explained by the lattice-preferred orientation of olivine minerals in the upper mantle induced by the mantle flow related to the roll-back process of the Hellenic slab. We further observe that station-averaged splitting time delays are prone to decrease from north to south of the Aegean region probably changing geometry of mantle wedge with a strong effect on  the nature of mantle flow along this direction. The uniform distribution of splitting parameters as a function of back-azimuths of earthquakes refers to a single-layer horizontal anisotropy for the most part of the study area. However, back azimuthal variations of splitting parameters beneath most of northerly located seismic stations (e.g., GELI, SMTH etc.) imply the presence of a double-layer anisotropy. To evaluate this, we performed various synthetic tests especially beneath the northern part of study region. Yet, it still remains controversial issue due to the large azimuthal gap and thus requires further modelling which may involve the use of joint data sets.

How to cite: Erman, C., Yolsal-Çevikbilen, S., Eken, T., and Taymaz, T.: Detailed Investigation of Seismic Anisotropy in the Upper Mantle of the Northern Aegean Region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1088, https://doi.org/10.5194/egusphere-egu2020-1088, 2020.

An azimuth-dependent dispersion curve inversion (ADDCI) method is applied to Rayleigh waves to extract 3D velocity and azimuthal anisotropy. The synthetic tests show that the ADDCI method is able to extract azimuthal anisotropy at different depths. The errors of the fast propagation direction (FPD) and the magnitude of the anisotropy (MOA) are less than 10° and 1-2%, respectively. The 3D anisotropic model shows large variations in the FPDs and MOAs with depth and blocks; strong contrasts are observed across major faults, and the average MOA in the crust is approximately 3%. The FPDs are positively correlated with the GPS velocity direction and the strikes of regional faults in most of the blocks. The low-velocity zones (LVZs) in the middle to lower crust are widely observed in the Songpan Ganze Terrence, the north Chuan-Dian block, and surprisingly in the Huayingshan thrust and fold belt. The LVZs in the middle crust are also positively correlated with the low-velocity belt in the uppermost mantle. These observations may suggest that large-scale deformation is coupled vertically from the surface to the uppermost mantle. Crust shortening by the pure shearing process, which involves the thrusting and folding of the upper crust and the lateral extrusion of blocks, may be the major mechanism causing the growth of the eastern Tibetan Plateau.

How to cite: Liang, C.: The azimuth-dependent dispersion curve inversion method to extract 3D anisotropic structure and its application to the eastern Tibetan Plateau , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6768, https://doi.org/10.5194/egusphere-egu2020-6768, 2020.

Seismic anisotropy plays a key role in understanding deformation patterns of Earth’s material.  Surface wave dispersion data have been widely used to invert for azimuthal and radial anisotropy of shear wave speeds in the crust and upper mantle typically based on a 1-D pointwise inversion scheme. Here we present new methods of inverting for 3-D shear wave speed azimuthal and radial anisotropy directly from surface-wave traveltime data with the consideration of period-dependent surface wave raytracing. For the inversion of 3-D azimuthal anisotropy, our new method includes two steps: (1) inversion for the 3-D isotropic Vsv model directly from Rayleigh wave traveltime data (DSurfTomo; Fang et al., 2015, GJI); (2) joint inversion for both 3-D Vsv azimuthal anisotropy and additional 3-D isotropic Vsv perturbation. The joint inversion can significantly mitigatethe trade-off between the strong heterogeneity and azimuthal anisotropy. We apply the new method (DAzimSurfTomo) (Liu et al., 2019, JGR)to a regional array in Yunnan, southwestern China using the Rayleigh-wave phase velocity dispersion data in the period band of 5-40 s extracted from ambient noise interferometry. The obtained 3-D model of shear wave speed and azimuthal anisotropy indicates differentdeformation styles between the crust and upper mantle insouthern Yunnan. For the inversion of 3-D radial anisotropy, we presented a new inversion matrix that directly inverts Rayleigh and Love wave traveltime data jointly for 3-D Vsv and radial anisotropy parameters (Vsh/Vsv) simultaneously without intermediate steps (Hu et al., submitted to JGR).  The new approach allows for adding the smoothing or model regularization terms directly on the radial anisotropy parameters, which helps to obtain more reliable radial anisotropy structures compared to the previous division approach (Vsh/Vsv) from separate inversion of Vsv and Vsh structures. We apply this new approach (DRadiSurfTomo) to the region around the eastern Himalayan syntaxis using ambient noise dispersion data (5-40s). The obtained 3-D Vs and radial anisotropy models reveals complex distribution of crustal low velocity zones and spatial variation of deformation patterns around the eastern syntaxis region.

How to cite: Huajian, Y., Chuanming, L., and Shaoqian, H.: Direct inversion of 3-D shear wave speed azimuthal and radial anisotropy from surface-wave traveltime data: methodology and applications, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22100, https://doi.org/10.5194/egusphere-egu2020-22100, 2020.