ECOMAN: a new open source software for Exploring the Consequences of Mechanical Anisotropy in the maNtle

<p>Coupling large-scale geodynamic and seismological modelling appears to be a promising methodology for better understanding 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 investigation methodologies is still lacking.</p><p>In this contribution we introduce ECOMAN, a new open source software that allows modelling the strain-induced mantle fabrics and related elastic anisotropy, and for performing different seismological synthetics, such as SKS splitting measurements and P- and S-wave isotropic and anisotropic inversions (Faccenda et al., in preparation).  </p><p>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.</p><p>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). 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.</p><p>Extrinsic elastic anisotropy due to grain- or rock-scale fabrics or fluid-filled cracks can also be estimated with the Differential Effective Medium (DEM) (Ferreira et al., Nat. Geo; Sturgeon et al., Gcubed, 2019). Similarly, extrinsic viscous anisotropy can be modelled yielding viscous tensors to be used in large-scale mantle flow simulations (de Montserrat et al., in preparation).  </p><p>The crystal aggregates can then be interpolated in a tomographic grid for (i) 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; ), (ii) generating input files for large-scale synthetic waveform modelling (e.g., SPECFEM3D format; FSTRACK format to calculate SKS splitting (Becker et al., 2006, GJI)), or to perform teleseismic P- and S-wave isotropic and anisotropic inversions with the method developed recently by (VanderBeek and Faccenda, 2021, in review).</p>

Identifying Seismic Anisotropy Patterns in the Alps and Apennines with Splitting Intensity and Backazimuthal Dependencies

<p>The current tectonics of the Alps and Apennines are driven and influenced by current and<br>past subduction systems. Computational advances over the years made it possible to<br>identify remnant and active slabs until great depths and large seismic deployments<br>revealed mostly clockwise rotation SKS splitting measurements. But the effects of layered<br>anisotropy and regional upper mantle flow through possible tears in the slabs remain<br>unknown. A comparison of several seismological methods can be a very efficient tool to<br>separate lithospheric and asthenospheric anisotropy. This study tries to understand if<br>anisotropy patterns change with depth in some regions (e.g., possible subslab mantle flow<br>in the Western Alps) and if tears can be identified with shear wave splitting measurements<br>(e.g., Central Apennines). Furthermore, splitting intensities will be analyzed for<br>backazimuthal dependencies and used to correct velocities in a full-waveform tomography.<br>By mapping and comparing existing and new anisotropy measurements (e.g., SKS, Pn<br>anisotropy, azimuthal anisotropy from surface waves tomography, and splitting intensities)<br>we intend to identify anisotropic depth dependencies.</p>

Imaging azimuthal anisotropy in the alpine crust from ambient noise beamforming.

<p>Noise cross-correlations provide a good azimuthal coverage, limited only by the distribution of noise sources and the layout of the stations used. It is therefore a promising method to constrain azimuthal anisotropy. As noise cross-correlations consist mainly of surface waves, they are especially sensitive to the crust and provide good depth constraints, as opposed to SKS-splitting data that are more sensitive to the upper mantle. We use the AlpArray network as well as stations from permanent networks all across Europe to perform time-domain beamforming on noise cross-correlations. The extent and density of the AlpArray network allows us to obtain reliable measurements all across the Alps. We divide the area in smaller zones using all stations outside the zone as sources and all stations inside as a sub-array for beamforming. This allows us to estimate the quality of our measurements in a region where strong lateral heterogeneities make measurements challenging, by estimating the magnitude of bias due to heterogeneities using the cos(theta) amplitude and evaluating uncertainties with bootstrap. This way, we measure Rayleigh wave azimuthal anisotropy in several period bands between 15 s and 60 s period. Inversion of dispersion curves in specific areas allows us to constrain the depth of the observed anisotropy. The results are broadly similar to results from SKS-splitting as they are generally parallel to the mountain belt. However, we observe lower anisotropy at short periods (40 seconds and less) in the Alps themselves than in surrounding regions. We also observe several structures in the crust that are not observed with SKS-splitting data. The most striking is a strong and spatially coherent NE-oriented anisotropy to the NW of the Alps that is possibly related to Variscan inheritance (at 40 seconds and less, in the upper and lower crust).  In the Northern Apennines, we observe anisotropy perpendicular to the belt at 30 s period (middle crust) that correlates well with an area of strong radial anisotropy recently observed by Alder et al (in review) at 30 km depth. </p>

Supplemental Material: Seismic evidence for craton chiseling and displacement of lithospheric mantle by the Tintina fault in the northern Canadian Cordillera

Methodology for SKS splitting analysis and teleseismic body-wave tomography, calculation of thermal length scale, comparion of NCC with cratonic xenoliths, eight supplemental figures, and a table with compiled estimates.<br>

Supplemental Material: Seismic evidence for craton chiseling and displacement of lithospheric mantle by the Tintina fault in the northern Canadian Cordillera

Methodology for SKS splitting analysis and teleseismic body-wave tomography, calculation of thermal length scale, comparion of NCC with cratonic xenoliths, eight supplemental figures, and a table with compiled estimates.<br>