Manuele Faccenda’s research while affiliated with University of Padua and other places

Seismic anisotropy is key to constrain mantle flow, but it is challenging to image and interpret it. Existing large-scale tomography models of seismic anisotropy typically show large discrepancies, which can lead to completely distinct geodynamical interpretations. To better quantify the robustness of anisotropy tomography, we create a 2-D ridge-to-slab geodynamic model and compute the associated fabrics. Using the resulting 21 elastic constants we compute seismic full waveforms, which are inverted for isotropic and radially anisotropic structure. We test the effects of different data coverage and levels of regularisation on the resulting images and on their geodynamical interpretation. Within the context of our specific imposed conditions and source-receiver configuration, the retrieved isotropic images exhibit substantial artificial slab thickening and loss of the slab’s high velocity signature below ∼100 km depth. Our results also show that the first order features of radial anisotropy are well retrieved despite strong azimuthal anisotropy (up to 2.7 %) in the input model. On the other hand, regularisation and data coverage strongly control the detailed characteristics of the retrieved anisotropy, notably the depth-age dependency of anisotropy, leading to an artificial flat depth-age trend shown in existing anisotropy tomography models. Greater data coverage and additional complementary data types are needed to improve the resolution of (an)isotropic tomography models.

The distribution of water within the Earth's mantle has significant implications for the Earth's dynamics and evolution. Recent mineral physics experiments indicate that dense hydrous magnesium silicates can contain large amounts of water stable up to 60 GPa or even beyond along slab geotherms. Here we perform petrological‐thermomechanical numerical simulations of water transportation by deep slab subduction and related magmatism in the mid‐mantle. Key parameters including those defining the slab thermal parameter and the water storage capacity in the oceanic lithosphere and surrounding mantle are explored. The results show two major dehydration events of ultramafic rocks at around 150 and 750 km by dehydration of serpentine at 600°C and superhydrous phase B in the entrained wet upper mantle, respectively. Large amounts of water, ∼1.5 wt% at least locally, are carried down to the mantle transition zone and lower mantle. We estimate an upper limit of slab water flux into the mid‐mantle of 0.1–0.28 × 10¹² kg/yr, which is ∼13%–37% of the input water from the serpentinized mantle. Moreover, a substantial fraction of the water released by the slab is absorbed by the entrained mantle and overlying mid‐mantle portions, such that ∼30%–70% of the water injected at the trench could be delivered to the lower mantle. The deepest magmatism is observed at ∼1,500 km in case of phase H breakdown (MgO‐SiO2‐H2O system), coinciding with the depth of strong seismic attenuation. Overall, these simulations suggest that up to 0.2 ocean mass per billion years could be transported down to the mid‐mantle and beyond.

Seismic radial anisotropy (the squared ratio between the speeds of horizontally and vertically polarized shear waves, ξ=VSH2VSV2 $\xi =\frac{{{V}_{SH}}^{2}}{{{V}_{SV}}^{2}}$) is a powerful tool to probe the direction of mantle flow and accumulated strain. While previous studies have confirmed the dependence of azimuthal anisotropy on plate speed, the first order control on radial anisotropy is unclear. In this study, we develop 2D ridge flow models combined with mantle fabric calculations to report that faster plates generate higher tectonics stresses and strain rates which lower the dislocation creep viscosity and lead to deeper anisotropy than beneath slower plates. We apply the SGLOBE‐rani tomographic filter, resulting in a flat depth‐age trend and stronger anisotropy beneath faster plates, which correlates well with 3D global anisotropic mantle models. Our predictions and observations suggest that as plate speed increases from 2 to 8 cm/yr, radial anisotropy increases by ∼0.01–0.025 in the upper 100–200 km of the mantle between 10 and 60 Ma.

We present the first three‐dimensional (3D) anisotropic teleseismic P‐wave tomography model of the upper mantle covering the entire Central Mediterranean. Compared to isotropic tomography, it is found that including the magnitude, azimuth, and, importantly, dip of seismic anisotropy in our 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. To better understand the complexities in slab geometry and their relationship to surface geological phenomenon, we present a 3D reconstruction of the main Central Mediterranean slabs down to 700 km based on our anisotropic model. P‐wave seismic anisotropy is widespread in the Central Mediterranean upper mantle and is strongest at 200–300 km depth. The anisotropy patterns are interpreted as the result of asthenospheric material flowing primarily horizontally around the main slabs in response to pressure exerted by their mid‐to‐late Cenezoic horizontal motion, while sub‐vertical anisotropy possibly reflects asthenospheric entrainment by descending lithosphere. Our results highlight the importance of anisotropic P‐wave imaging for better constraining regional upper mantle geodynamics.

The origin of intermediate-depth subduction seismicity is a topic of research since long time. While plate unbending is considered as one of the main stress loading mechanisms, the processes responsible for earthquake nucleation are still unclear and depend on the question of whether failure occurs in the wet dehydrating portion of the slab or in the predominantly dry portion. Recently, the seismogenic portions of subducting oceanic slabs have been proposed to consist of dominantly dry metaperidotite that deforms by seismic brittle failure in absence of fluid-mediated embrittlement. In this work, we quantify by numerical modelling the differential stress achievable during unbending of a subducting slab. We show that the presence of discrete hydrated domains in a dry, strong slab amplifies the differential stress to high seismogenic values (ca. 3-4 GPa in the 100-200 km depth range) at intermediate depths. We also consider the effects of low temperature plasticity in olivine that can hinder the build-up of high differential stress to the first 100 km of depth.

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