The Central Mediterranean region is an active plate margin characterized by the presence of both oceanic and continental lithosphere. The recent tectonic history is marked by intense seismic and volcanic activity triggered by episodes of continental collision and slab rollback leading to the formation of mountain ranges and extensional basins (Faccenna et al., 2014). Our understanding of the structural heterogeneity and tectonic complexity of this region requires accurate imaging of the subsurface. For this reason, since the late 1990s numerous seismological studies have been carried out to constrain upper mantle structure beneath the Mediterranean region (e.g.

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The Central-Western Mediterranean (CWM) is one of the most complex tectonic setting on Earth. Episodes of slab rollback, break-off and tearing, the opening of back-arc extensional basins (i.e., Liguro-Provencal, Alborean, Algerian and Tyrrhenian basins), the presence of large mountain ranges, active volcanoes and violent earthquakes have made the Mediterranean an ideal environment to study a wide range of geodynamic processes and an important target for seismological studies (e.g, seismic tomography). Here we build a geodynamic model which, although it does not reproduce its exact tectonic structure (e.g., due to the limits of the numerical method, approximations in the initial setup, etc), presents multiple and geometrically complex subduction systems analogous to those found in the CWM. The tectonic evolution of this model is estimated with petrological-thermo-mechanical 3D simulations, then, we dynamically compute the upper mantle fabrics and seismic anisotropy as a function of the strain history and local P-T conditions. After comparing the model with SKS splitting observations in order to quantify the discrepancies with the true Central-Western Mediterranean, we use the elastic tensors predicted for the modeled configuration to perform 3D P-wave anisotropic tomography by inverting synthetic P-wave delay times. Using the geodynamic model as reference, we evaluate the capabilities of a recently developed seismic tomography technique to recover the isotropic anomalies and anisotropy patterns related to a complex subduction environment in different conditions, such as poor data coverage and bad data quality. We observe that, although P-wave tomography still remains a powerful tool to investigate the upper mantle, the reliability of the retrieved structures strongly depends on data quality and data density. Furthermore, the recovered anisotropic patterns are consistent with those of the target model, but in general an underestimation of the anisotropy magnitude in the upper mantle is observed. In the light of future developments, our study suggests that by combining micro- and macro-scale geodynamic simulations and seismological modeling of seismic anisotropy it will be possible to reproduce, at least to a first order, the tectonic evolution of real study regions (e.g., the Mediterranean) thus providing fundamental constraints on the processes that have contributed in shaping their current geological scenario.

<p>Teleseismic travel-time tomography remains one of the most popular methods for obtaining images of Earth's upper mantle. While teleseismic shear phases, most notably SKS, are commonly used to infer the anisotropic properties of the upper mantle, anisotropic structure is often ignored in the construction of body wave shear velocity models. Numerous researchers have demonstrated that neglecting anisotropy in P-wave tomography can introduce significant imaging artefacts that could lead to spurious interpretations. Less attention has been given to the effect of anisotropy on S-wave tomography partly because, unlike P-waves, there is not a ray-based methodology for modelling S-wave travel-times through anisotropic media. Here we evaluate the effect that the isotropic approximation has on tomographic images of the subsurface when shear waves are affected by realistic mantle anisotropy patterns. We use SPECFEM to model the teleseismic shear wavefield through a geodynamic model of subduction that includes elastic anisotropy predicted from micromechanical models of polymineralic aggregates advected through the simulated flow field. We explore how the chosen coordinates system in which S-wave arrival times are measured (e.g., radial versus transverse) affects the imaging results. In all cases, the isotropic imaging assumption leads to numerous artefacts in the recovered velocity models that could result in misguided inferences regarding mantle dynamics. We find that when S-wave travel-times are measured in the direction of polarisation, the apparent anisotropic shear velocity can be approximated using sinusoidal functions of period pi and two-pi. This observation allows us to use ray-based methods to predict S-wave travel-times through anisotropic models. We show that this parameterisation can be used to invert S-wave travel-times for the orientation and strength of anisotropy in a manner similar to anisotropic P-wave travel-time tomography. In doing so, the magnitude of imaging artefacts in the shear velocity models is greatly reduced.</p>

<p>The Mediterranean region is an active plate margin characterized by the presence of both oceanic and continental lithosphere. Its tectonic history is marked by intense seismic and volcanic activity triggered by episodes of continental collision and slab rollback leading to the formation of mountain ranges and extensional basins. Our understanding of the structural heterogeneity and tectonic complexity of this region requires accurate imaging of the subsurface. Seismic anisotropy is a key parameter commonly used to study flow in the mantle and its relations with plate motions. In this study we present a three-dimensional anisotropic seismic tomography of the entire Mediterranean area performed using travel time from the new “Global Catalog of Calibrated Earthquake Locations” by Bergman et al. (2023). We present purely isotropic and anisotropic solutions. Compared to isotropic tomography, it is found that including the magnitude, azimuth, and, importantly, dip of seismic anisotropy in the inversions simplifies isotropic heterogeneity by reducing the magnitude of slow anomalies while yielding anisotropy patterns that are consistent with regional tectonics. The isotropic component of our preferred tomography model is dominated by numerous fast anomalies associated with retreating, stagnant, and detached slab segments. In contrast, relatively slower mantle structure is related to slab windows and the opening of back-arc basins. The anisotropic patterns reveal the deformation history of the area which has been characterized by intermittent phases of collision and tectonic relaxation. A diversity of dip angles is observed with near-horizontal and more steeply dipping fabrics found in different areas of the Entire Mediterranean, probably reflecting the entrainment effect of horizontal or vertical asthenospheric flows, respectively. We interpreted the high velocity zones of our best solution as subducting lithosphere and starting from this interpretation we built a 3D reconstruction of the main slabs found in the study region. To perform the tomography, we used the method proposed by Vanderbeek and Faccenda (2021) and already used by Rappisi et al. (2022) in a similar study on the Central Mediterranean area. This work returns the first anisotropic tomography of the entire Mediterranean and demonstrates the importance of seismic anisotropy to better constrain the upper mantle.</p> <p>Bergman, E. A., Benz, H. M., Yeck, W. L., Karasözen, E., Engdahl, E. R., Ghods, A., ... & Earle, P. S. (2023). A Global Catalog of Calibrated Earthquake Locations. Seismological Society of America, 94(1), 485-495.</p> <p>Rappisi, F., VanderBeek, B. P., Faccenda, M., Morelli, A., & Molinari, I. (2022). Slab Geometry and Upper Mantle Flow Patterns in the Central Mediterranean From 3D Anisotropic P‐Wave Tomography. Journal of Geophysical Research: Solid Earth, 127(5), e2021JB023488.</p> <p>VanderBeek, B. P., & Faccenda, M. (2021). Imaging upper mantle anisotropy with teleseismic P-wave delays: insights from tomographic reconstructions of subduction simulations. Geophysical Journal International, 225(3), 2097-2119.</p>