In January 2024, our team has launched an open, free, participative gravity-modelling initiative. The idea is to provide all the necessary, pre-processed input data to the community, so that modelling and inversion can be applied by anyone interested in constraining the shape of a geological-geophysical anomaly. This way we hope to step over hurdles related to personal biases. The full description can be found at https://zenodo.org/records/10390437 Beyond advertising the initiative, we would like to share some thoughts on the pathway to launching it: the motivation, the interest, the experienced past difficulties and further challenges on the horizon. By doing so, we hope to contribute to the goals of this session, and to foster further such (scientific and sharing) initiatives.

The Western Alps constitute a complex and heterogeneous orogenic system, generated by the continental collision between the European and Adriatic tectonic plates. Three main tectonic domains can be identified across local to regional scales, based on geophysical and geological observations: the European and Adriatic domains, and the high-density, high-velocity anomaly known as Ivrea geophysical body (IGB). Despite being one of the best-studied collisional systems in the world, the 3D Western Alpine lithosphere and its along-arc compositional and structural variations are still subjects of investigations. In this framework, we exploit the inherently-3D information provided by gravity data. In particular, we set up a 3D Bayesian joint inversion of Bouguer gravity anomaly and surface wave dispersion data, to obtain a new 3D ρ-vS model of the Western Alpine lithosphere. We benefit from the Bouguer anomaly map by Zahorec et al. (2021), obtained by homogeneous processing of gravity data across the Alpine domain, and from seismic data recorded by permanent and temporary seismic networks: e.g. IvreaArray, AlpArray (Hetényi et al. 2017, 2018), CIFALPS I and II (e.g. Paul et al. 2022). We perform 3D forward gravity modeling by discretizing the study area in unitary volumes of constant density (voxels), accounting for spherical Earth structure and surface topography. The gravity effect of each voxel is pre-computed, and then only needs to be scaled with density during the inversion. This significantly decreases the computational cost of the forward model, and thus allows us to explore the parameter space with Monte Carlo sampling. We use a Bayesian framework and implement a Markov chain Monte Carlo (McMC) algorithm. We test different types of parameterizations to reduce the non-uniqueness of gravity inversion. We plan to jointly invert gravity with surface wave dispersion data, providing complementary information on vS. Finally, existing receiver function studies (e.g. Monna et al. 2022, Paul et al. 2022, Michailos et al. 2023) provide prior information on crustal and lithospheric geometry.We expect to obtain a new 3D ρ-vS model for the Western Alpine crust and lithosphere. This will provide new information on the European-Adriatic collision boundary, together with the IGB structure, and their three-dimensional variation along the orogen. The new model will be also useful to constrain rock composition, upon comparison with the geological observations at the surface.

The Balmuccia peridotite exposes relatively fresh mantle rocks at the Earth’s surface, and as such it is of interest for geologists and geophysicists. The outcrop is a kilometre-scale feature, yet its extent at depth is insufficiently imaged. Our aim is to provide new constraints on the shape of the density anomaly this body represents, through 3D gravity modelling. In an effort to avoid personal or methodology bias, we hereby launch an invitation and call for participative modelling. We openly provide all the necessary input data: pre-processed gravity data, geological map, in situ rock densities, and digital elevation model. The expected inversion results will be compared and jointly analysed with all participants. This approach should allow us to conclude on the shape of the Balmuccia peridotite body and the associated uncertainty. This crowd effort will contribute to the site surveys preparing a scientific borehole in the area in frame of project DIVE.

The Apennines mountain range develops all along Italy, presenting important variations in terms of both structural and tectonic environments, and seismogenic patterns as well. This is observed not only along the main NW-SE chain axis, but also by comparing multidisciplinary observations between the western Tyrrhenian and the eastern Adriatic domains (Di Luccio et al., 2022).We focus on the southern Apennines, where the Adriatic plate subducts westward under the thinner Tyrrhenian plate and the highest seismic release is documented.Recent studies showed that fluids play an important role in the seismic behavior of the area. The western domain is associated with heterogeneous and distributed patterns of CO2 gas emission at the surface; the latter ceasing in the east, where high-pressure fluids are trapped in crustal pockets and affect the seismogenic cycle (Chiodini et al., 2004; Improta et al., 2014; Di Luccio et al., 2022 and references therein).We perform regional-scale P- and S-body waveform analysis and forward numerical modeling, for a selected catalog of crustal events recorded by the broadband seismic stations of the italian network, as well as of temporary passive seismic experiments. We focus on a SW-NE transect, which cross-cuts the southern portion of the Apennines chain, and along which the recorded waveforms exhibit important differences in terms of frequency content and pulse shape. Along the same transect, the waveforms from two events (2013 Mw5 Sannio-Matese and 2014 Mw4.5 Gargano earthquakes) show significant differences in the propagation towards the east and west, respectively.Starting from two velocity models such as EPcrust (Molinari et al. 2011) and the adjoint tomographic model of Magnoni et al. (2022), we use the finite difference numerical modeling code nbpsv2d (Li et al. 2014) to produce synthetic waveforms to fit and explain the observations. By including information on the earthquake source mechanism and by improving the waveform fit in terms of both arrival time and body-wave coda, we provide new, preliminary information on the crustal structure of the southern Apennines, aimed at improving our understanding of the fluid-seismicity interaction in the area. Research performed in the framework of FURTHER project (https://progetti.ingv.it/en/further). References: Chiodini G., Cardellini, C., Amato, A., Boschi, E., Caliro, S., Frondini, F., and Ventura, G. (2004). Carbon dioxide Earth degassing and seismogenesis in central and southern Italy. Geophys. Res. Lett., 31, L07615, doi:10.1029/2004GL019480. Di Luccio et al., (2022). Geodynamics, geophysical and geochemical observations, and the role of CO2 degassing in the Apennines. Earth-Sci. Rev., https://doi.org/10.1016/j.earscirev.2022.104236 Improta L., P. De Gori, and C. Chiarabba (2014). New insights into crustal structure, Cenozoic magmatism, CO2 degassing, and seismogenesis in the southern Apennines and Irpinia region from local earthquake tomography, J. Geophys. Res. Solid Earth, 119, 8283–8311, doi:10.1002/ 2013JB010890. Li, D., Helmberger, D., Clayton, R. W., & Sun, D. (2014). Global synthetic seismograms using a 2-D finite-difference method. Geophysical Journal International, 197(2),1166-1183. Magnoni, F., Casarotti, E., Komatitsch, D., Di Stefano, R., Ciaccio, M. G., Tape, C., ... & Tromp, J. (2022). Adjoint tomography of the Italian lithosphere. Communications Earth & Environment, 3(1),1-12. Molinari, I., & Morelli, A. (2011). EPcrust: a reference crustal model for the European Plate. Geophysical Journal International, 185(1), 352-364.

SUMMARY We performed seismic ambient noise tomography to investigate the shallow crustal structure around the Ivrea geophysical body (IGB) in the Ivrea-Verbano Zone (IVZ). We achieved higher resolution with respect to previous tomographic works covering the Western Alps, by processing seismic data collected by both permanent and temporary seismic networks (61 broad-band seismic stations in total). This included IvreaArray, a temporary, passive seismic experiment designed to investigate the IVZ crustal structure. Starting from continuous seismic ambient noise recordings, we measured and inverted the dispersion of the group velocity of surface Rayleigh waves (fundamental mode) in the period range 4–25 s. We obtained a new, 3-D vS model of the IVZ crust via the stochastic neighbourhood algorithm (NA), with the highest resolution between 3 to 40 km depth. The fast and shallow shear wave velocity anomaly associated with the IGB presents velocities of 3.6 km s−1 directly at the surface, in remarkable agreement with the location of the exposed lower-to-middle crustal and mantle outcrops. This suggests a continuity between the surface geological observations and the subsurface geophysical anomalies. The fast IGB structure reaches vS of 4 km s−1 at 20–25 km depth, at the boundary between the European and Adriatic tectonic plates, and in correspondence with the earlier identified Moho jump in the same area. The interpretation of a very shallow reaching IGB is further supported by the comparison of our new results with recent geophysical investigations, based on receiver functions and gravity anomaly data. By combining the new geophysical constraints and the geological observations at the surface, we provide a new structural interpretation of the IGB, which features lower crustal and mantle rocks at upper crustal depths. The comparison of the obtained vS values with the physical properties from laboratory analysis of local rock samples suggests that the bulk of the IGB consists of a combination of mantle peridotite, ultramafic and lower crustal rocks, bound in a heterogeneous structure. These new findings, based on vS tomography, corroborate the recent interpretation for which the Balmuccia peridotite outcrops are continuously linked to the IGB structure beneath. The new outcomes contribute to a multidisciplinary framework for the interpretation of the forthcoming results of the scientific drilling project DIVE. DIVE aims at probing the lower continental crust and its transition to the mantle, with two ongoing and one future boreholes (down to 4 km depth) in the IVZ area, providing new, complementary information on rock structure and composition across scales. In this framework, we constrain the upper crustal IGB geometries and lithology based on new evidence for vS, connecting prior crustal knowledge to recent active seismic investigations.

We use seismic waveform data from the AlpArray Seismic Network and three other temporary seismic networks, to perform receiver function (RF) calculations and time-to-depth migration to update the knowledge of the Moho discontinuity beneath the broader European Alps. In particular, we set up a homogeneous processing scheme to compute RFs using the time-domain iterative deconvolution method and apply consistent quality control to yield 112 205 high-quality RFs. We then perform time-to-depth migration in a newly implemented 3D spherical coordinate system using a European-scale reference P and S wave velocity model. This approach, together with the dense data coverage, provide us with a 3D migrated volume, from which we present migrated profiles that reflect the first-order crustal thickness structure. We create a detailed Moho map by manually picking the discontinuity in a set of orthogonal profiles covering the entire area. We make the RF dataset, the software for the entire processing workflow, as well as the Moho map, openly available; these open-access datasets and results will allow other researchers to build on the current study.

The European Alps, formed by the interactions between the European and Adriatic plates, is a unique geological structure that has been extensively studied over the past decades. Despite numerous active and passive seismic investigations in the past, the crustal structure across the whole Alpine domain is somehow limited - mainly due to the limited number of seismometers available. The deployment of the AlpArray Seismic Network provides, which consisted of around 600 broadband seismometers and was operational from early 2016 till mid-2019, offers a unique opportunity to further update the current knowledge of the crustal structure beneath the European Alps by employing Receiver function (RF) analysis. RF method can provide an efficient way to image the structures and the discontinuities within the uppermost part of the Earth. We use teleseismic earthquakes with M≥5.5 and M<8.5 and epicentral distances ranging between 30 and 90 degrees that occurred during the operational time of the AlpArray Seismic Network. We compute RFs using a time-domain iterative deconvolution method. We apply quality control steps to both the original three-component waveforms and the calculated RFs to ensure that we only use high-quality signals. As of abstract submission, we are in the process of calculating the RFs. We also intend to perform a time to depth migration, in a 3D spherical coordinate system, to the RFs. This methodology, together with unprecedented data coverage, will provide us with migrated profiles that will image the structure of the crust and map the Moho depths at a great level of detail.

Modelling of geophysical data is often subject to choices made by the researcher undertaking the work. The level of structural complexity in the model, the bounds on parameters imposed by a priori knowledge, the thoroughness and efficiency in exploring the parameter space may all lead to bias in determining what the best fitting models can be. To avoid bias from our own ideas in constraining the subsurface shape of a given density anomaly, we hereby invite anyone interested to create their own models. This is planned by sharing the same gravity data measured in the field, the same digital elevation model, the main features of the local geological maps, and bounds on the encountered rock density values. These data will be shared openly, in the form of a modelling challenge: each participating researcher or group is expected to submit their solution(s). All these will be compared during a dedicated workshop, ultimately resulting in a joint publication. The target of this modelling challenge is the world-famous Balmuccia peridotite body (45.84°N, 8.16°E) in the Ivrea-Verbano Zone (IVZ). Here mantle rocks are naturally exposed at the surface, in the broader context of the IVZ, a middle- to lower crustal terrain along the Europe-Adria plate boundary’s eastern side. The surface exposure of the Balmuccia peridotite is ~ 4.4 km N-S by 0.6 km E-W, with outcrop elevation changes exceeding 1000 m. About 150 new gravity data points have been measured within a radius of 3 km from the centre of the peridotite body, along more or less accessible paths and slopes. The measurements have been carried out with a Scintrex CG-5 relative gravimeter, tied to a reference point, and all points located via differential GPS with typical vertical precision of a few cm. Farther away regional gravity data is available at few km spacing. Beyond the modelling challenge, the interest in constraining the subsurface shape of the Balmuccia peridotite body is its future target role in the ICDP DIVE continental drilling project (www.dive2ivrea.org).

The arc of the Western Alps is characterized by a complex crustal structure. Lower-to-middle crustal composition outcrops are exposed in the Ivrea-Verbano Zone (IVZ) and a major crustal anomaly, known as Ivrea Geophysical Body (IGB), presents dense and seismically fast rocks right below the surface. Understanding better their relation provides a key to refine our understanding of orogeny formation mechanisms. We performed seismic ambient noise tomography using data from the IvreaArray and the AlpArray Seismic Network, selected within a radius of ca. 100 km around the study area. Previous seismic investigations provided knowledge on the crustal structure in the Western Alps, by means of active refraction seismics and of more recent local earthquake and ambient noise tomography at regional scales (e.g. Solarino et al. 2018 Lithos, Lu et al. 2018 GJI). Recently, gravity data and receiver function analysis imaged the IGB as a dense and fast seismic anomaly, related to upper mantle material, reaching up to few km depth below sea level (Scarponi et al. 2021 Frontiers). However, local high-resolution constraints on the absolute vS distribution remain unknown. We used raw summer seismic data (June to September) across 3 years of recording, and computed daily ambient noise cross-correlation traces, for all the available station pairs (61 stations in total) in the 2-20s period range. Daily cross-correlations were stacked and processed to extract Green’s functions. Subsequently, we performed frequency-time analysis to get group velocity dispersions for the fundamental mode of surface Rayleigh waves. We computed 2D surface group velocity maps at each period, which clearly show the slow sediment area of the Po Plain, and the fast IGB structure within the crust. We are going to use the 2D group velocity maps to derive local dispersions curves and invert for 1D vS-depth profiles with the use of the Neighborhood Algorithm, to produce a 3D vS velocity model for the IVZ at high-resolution. This will also provide new geophysical constraints in the target area of the scientific drilling project DIVE (www.dive2ivrea.org) and reliable information for crustal corrections, which are necessary for upper mantle studies in such a complex area.

The Gotthard Base Tunnel (GBT) is a 57 km long railway tunnel, constructed in the Central Alps in Switzerland and extending mainly North–South across numerous geological units. We acquired 80 new gravity data points at the surface along the GBT profile and used 77 gravity measurements in the tunnel to test and constrain the shallow crustal, km-scale geological model established during the tunnel construction. To this end, we developed a novel processing scheme, which computes a fully 3D, density-dependent gravity terrain-adaptation correction (TAC), to consistently compare the gravity observations with the 2D geological model structure; the latter converted into a density model. This approach allowed to explore and quantify candidate rock density distributions along the GBT modelled profile in a computationally-efficient manner, and to test whether a reasonable fit can be found without structural modification of the geological model. The tested density data for the various lithologies were compiled from the SAPHYR rock physical property database. The tested models were evaluated both in terms of misfit between observed and synthetic gravity data, and also in terms of correlation between misfit trend and topography of the target profile. The results indicate that the locally sampled densities provide a better fit to the data for the considered lithologies, rather than density data averaged over a wider set of Alpine rock samples for the same lithology. Furthermore, using one homogeneous and constant density value for all the topographic corrections does not provide an optimal fit to the data, which instead confirms density variations along the profile. Structurally, a satisfactory fit could be found without modifying the 2D geological model, which thus can be considered gravimetry-proof. From a more general perspective, the gravity data processing routines and the density-dependent corrections developed in this case study represent a remarkable potential for further high-resolution gravity investigations of geological structures.

The AlpArray Gravity Research Group (AAGRG), as part of the European AlpArray program, focuses on the compilation of a homogeneous surface-based gravity data set across the Alpine area. In 2017 10 European countries in the Alpine realm agreed to contribute with gravity data for a new compilation of the Alpine gravity field in an area spanning from 2 to 23∘ E and from 41 to 51∘ N. This compilation relies on existing national gravity databases and, for the Ligurian and the Adriatic seas, on shipborne data of the Service Hydrographique et Océanographique de la Marine and of the Bureau Gravimétrique International. Furthermore, for the Ivrea zone in the Western Alps, recently acquired data were added to the database. This first pan-Alpine gravity data map is homogeneous regarding input data sets, applied methods and all corrections, as well as reference frames. Here, the AAGRG presents the data set of the recalculated gravity fields on a 4 km × 4 km grid for public release and a 2 km × 2 km grid for special request. The final products also include calculated values for mass and bathymetry corrections of the measured gravity at each grid point, as well as height. This allows users to use later customized densities for their own calculations of mass corrections. Correction densities used are 2670 kg m−3 for landmasses, 1030 kg m−3 for water masses above the ellipsoid and −1640 kg m−3 for those below the ellipsoid and 1000 kg m−3 for lake water masses. The correction radius was set to the Hayford zone O2 (167 km). The new Bouguer anomaly is station completed (CBA) and compiled according to the most modern criteria and reference frames (both positioning and gravity), including atmospheric corrections. Special emphasis was put on the gravity effect of the numerous lakes in the study area, which can have an effect of up to 5 mGal for gravity stations located at shorelines with steep slopes, e.g., for the rather deep reservoirs in the Alps. The results of an error statistic based on cross validations and/or “interpolation residuals” are provided for the entire database. As an example, the interpolation residuals of the Austrian data set range between about −8 and +8 mGal and the cross-validation residuals between −14 and +10 mGal; standard deviations are well below 1 mGal. The accuracy of the newly compiled gravity database is close to ±5 mGal for most areas. A first interpretation of the new map shows that the resolution of the gravity anomalies is suited for applications ranging from intra-crustal- to crustal-scale modeling to interdisciplinary studies on the regional and continental scales, as well as applications as joint inversion with other data sets. The data are published with the DOI https://doi.org/10.5880/fidgeo.2020.045 (Zahorec et al., 2021) via GFZ Data Services.

We present results from a joint inversion study of new seismic and gravity data to constrain a 2D high-resolution image of one of the most prominent geophysical anomalies of the European Alps: the Ivrea geophysical body (IGB). Our work exploits both new data and multidisciplinary a priori constraints, to better resolve the shallow crustal structure in the Ivrea-Verbano zone (IVZ), where the IGB is known to reach anomalously shallow depths and partially outcrop at the surface. A variety of previous studies, ranging from gravity surveys to vintage refraction seismics and recent local earthquake tomographies (Solarino et al. 2018, Diehl et al. 2009), provide comprehensive but spatially sparse information on the IGB structure, which we aim at investigating at higher resolution, along a linear profile crossing the IVZ. To this purpose, we deployed 10 broadband seismic stations (MOBNET pool, IG CAS Prague), 5 km spaced along a linear West-East profile, along Val Sesia and crossing Lago Maggiore. This network operated for 27 months and allowed us to produce a new database of ca. 1000 seismic high-quality receiver functions (RFs). In addition, we collected new gravity data in the IVZ, with a data coverage of 1 gravity point every 1-2 km along the seismic profile. The newly collected data was used to set up an inversion scheme, in which RFs and gravity anomalies are jointly used to constrain the shape and the physical property contrasts across the IGB interface. We model the IGB as a single interface between far-field constraints, whose geometry is defined by the coordinates of four nodes which may vary in space, and density and VS shear-wave velocity contrasts associated with the interface itself, varying independently. A Markov chain Monte Carlo (MCMC) sampling method with Metropolis-Hastings selection rule was implemented to efficiently explore the model space, directing the search towards better fitting areas. For each model, we perform ray-tracing and RFs migration using the actual velocity structure both for migration and computation of synthetic RFs, to be compared with the observations via cross-correlation of the migration images. Similarly, forward gravity modelling for a 2D density distribution is implemented and the synthetic gravity anomaly is compared with the observations along the profile. The joint inversion performance is the product of these two misfits. The inversion results show that the IGB reaches the shallowest depths in the western part of the profile, preferentially locating the IGB interface between 3 and 7 km depth over a horizontal distance of ca. 20 km (between Boccioleto and Civiasco, longitudes 8.1 and 8.3). Within this segment, the shallowest point reaches up to 1 km below sea level. The found density and velocity contrasts are in agreement with rock physics properties of various units observed in the field and characterized in earlier studies.

We present results from a joint inversion of new seismic and recently compiled gravity data to constrain the structure of a prominent geophysical anomaly in the European Alps: the Ivrea Geophysical Body (IGB). We investigate the IGB structure along the West-East oriented Val Sesia profile at higher resolution than previous studies. We deployed 10 broadband seismic stations at 5 km spacing for 27 months, producing a new database of ∼1000 high-quality seismic receiver functions (RFs). The compiled gravity data yields 1 gravity point every 1–2 km along the profile. We set up an inversion scheme, in which RFs and gravity anomalies jointly constrain the shape and the physical properties of the IGB. We model the IGB’s top surface as a single density and shear-wave velocity discontinuity, whose geometry is defined by four, spatially variable nodes between far-field constraints. An iterative algorithm was implemented to efficiently explore the model space, directing the search toward better fitting areas. For each new candidate model, we use the velocity-model structures for both ray-tracing and observed-RFs migration, and for computation and migration of synthetic RFs: the two migrated images are then compared via cross-correlation. Similarly, forward gravity modeling for a 2D density distribution is implemented. The joint inversion performance is the product of the seismic and gravity misfits. The inversion results show the IGB protruding at shallow depths with a horizontal width of ∼30 km in the western part of the profile. Its shallowest segment reaches either 3–7 or 1–3 km depth below sea-level. The latter location fits better the outcropping lower crustal rocks at the western edge of the Ivrea-Verbano Zone. A prominent, steep eastward-deepening feature near the middle of the profile, coincident with the Pogallo Fault Zone, is interpreted as inherited crustal thickness variation. The found density and velocity contrasts of the IGB agree with physical properties of the main rock units observed in the field. Finally, by frequency-dependent analysis of RFs, we constrain the sharpness of the shallowest portion of the IGB velocity discontinuity as a vertical gradient of thickness between 0.8 km and 0.4 km.

Abstract One of the few near‐complete continental crustal sections exposed on Earth's surface is the Ivrea‐Verbano Zone (Western Alps, Italy), which is considered as a petro‐geophysical reference of the continental lithosphere. Exposed peridotite slivers embedded in lower crustal rocks at the surface and large density, seismic velocity anomalies of the Ivrea Geophysical Body in the subsurface suggest that mantle‐like rocks are located as shallow as a few kilometers depth, but the actual composition of the rocks producing these anomalies is unknown. Here we investigate how the published seismological and new gravimetric data in the location of Valsesia could be reconciled with petrologic data and models of the Ivrea‐Verbano Zone. We use the Perple_X software to calculate densities and compressional wave velocities for a range of possible deep crustal rock types. We argue that amphibole gabbros (<18 km depth) and pyroxene hornblendites (>18 km depth) provide the best fit to the joint geophysical and petrologic constraints, whereas residual ultramafic rocks and anhydrous gabbros are inconsistent with the existing data. This indicates that the Ivrea Geophysical Body beneath the Valsesia area in the Ivrea‐Verbano Zone preserves the structure of an igneous complex formed during magmatic underplating from the crystallization of hydrous mafic magmas. This would imply melting of a damp mantle source that produced a continental crust of an original thickness of up to ∼48 km in the Permian, of which ∼30 km are exposed at Earth's surface today.

We collected new seismological and gravity data in the Val Sesia and Lago Maggiore regions in NW Italy to constrain the geometry and properties of the Ivrea Geophysical Body. This piece of lower Adriatic lithosphere is known to be at anomalously shallow depth along the inner arc of the Western Alps, yet existing seismological constraints (vintage seismic refraction data, local earthquake tomography) are spatially sparse. With the aim to reach higher spatial resolution in imaging the structure of the IGB, we analyze the seismological data with various receiver function approaches to map the main velocity discontinuities, followed by joint inversion with gravity data to fill the bulk properties of bodies with densities. The new data acquisition consisted of two type of campaigns. For seismology, we deployed 10 broadband seismic stations (MOBNET pool, IG CAS Prague) along a linear West-East profile at 5 km spacing along Val Sesia and across the Lago Maggiore. This network continuously recorded seismic data for 27 months at 100 Hz sampling rate. For gravimetry, we compiled existing datasets and then completed the spatial gaps by relative gravity surveys, tied to absolute reference points, to achieve 1 gravity point every 1-2 km along the profile. The receiver function (RF) analyses aim at detecting velocity increases with depth: primarily the Moho and the shallow IGB interfaces and their crustal reverberations (multiples), together with their potential dip by analyzing the transverse component RFs. Furthermore, we aim at investigating the sharpness of the velocity gradient across the discontinuities by analyzing the frequency dependence of the corresponding RF peaks. We aim at reproducing the observations by simple synthetic models. The 2D joint inversion combines S wave velocity VS and bulk density as physical parameters to match both the seismological and gravimetry data. The relationship between the two parameters is initially chosen from the literature, but depending on the first results the relation itself may be inverted for, considering the various high-grade metamorphic rocks observed at the surface in the area, whose properties may not align with classical VS–density equations. In conclusion, we propose new constraints on the IGB, demonstrating the advantage of using multi-disciplinary geophysical observations and improved data coverage across the study area.

SUMMARY We provide a high-resolution image of the Ivrea Geophysical Body (IGB) in the Western Alps with new gravity data and 3-D density modelling, integrated with surface geological observations and laboratory analyses of rock properties. The IGB is a sliver of Adriatic lower lithosphere that is located at shallow depths along the inner arc of the Western Alps, and associated with dense rocks that are exposed in the Ivrea-Verbano Zone (IVZ). The IGB is known for its high seismic velocity anomaly at shallow crustal depths and a pronounced positive gravity anomaly. Here, we investigate the IGB at a finer spatial scale, merging geophysical and geological observations. We compile existing gravity data and we add 207 new relative gravity measurements, approaching an optimal spatial coverage of 1 data point per 4–9 km2 across the IVZ. A compilation of tectonic maps and rock laboratory analyses together with a mineral properties database is used to produce a novel surface rock-density map of the IVZ. The density map is incorporated into the gravity anomaly computation routine, from which we defined the Niggli gravity anomaly. This accounts for Bouguer Plate and terrain correction, both considering the in situ surface rock densities, deviating from the 2670 kg m–3 value commonly used in such computations. We then develop a 3-D single-interface crustal density model, which represents the density distribution of the IGB, including the above Niggli-correction. We retrieve an optimal fit to the observations by using a 400 kg m–3 density contrast across the model interface, which reaches as shallow as 1 km depth below sea level. The model sensitivity tests suggest that the ∼300–500 kg m–3 density contrast range is still plausible, and consequently locates the shallowest parts of the interface at 0 km and at 2 km depth below sea level, for the lowest and the highest density contrast, respectively. The former model requires a sharp density discontinuity, the latter may feature a vertical transition of densities on the order of few kilometres. Compared with previous studies, the model geometry reaches shallower depths and suggests that the width of the anomaly is larger, ∼20 km in west–east direction and steeply E–SE dipping. Regarding the possible rock types composing the IGB, both regional geology and standard background crustal structure considerations are taken into account. These exclude both felsic rocks and high-pressure metamorphic rocks as suitable candidates, and point towards ultramafic or mantle peridotite type rocks composing the bulk of the IGB.

TABLE CAPTIONS Table 1: Average and specific bulk compositions of the representative exposed primitive rocks of the deep lower crust and upper mantle of the IVZ (Balmuccia and Lower Mafic Complex in Valsesia, Finero in Val Cannobina, and Sessera river in Val Sessera) used for the Perple_X calculations using the two thermodynamic models STX11 and HP02. Mn, Ni, P, and Cr were not used for the Perple_X calculations. Mg# is calculated as Mg / (Mg + Fe) (mole fractions). Symbols in the last row indicate the rock compositions that are reported (✓) and not reported (✗) in Figure 5. Average compositions are normalized to 100 wt.% on a volatile-free basis; specific compositions report original totals and H2O contents estimated based on the volumetric proportions hydrous minerals (amphibole and phlogopite) at the scale of the bulk rock. References: 1) Voshage et al. (1990); 2) Voshage et al. (1990), Shervais and Mukasa (1991), Hartmann and Wedepohl (1993), Rivalenti et al. (1995), and Mukasa and Shervais (1999); 3) Pin and Sills (1986); 4) Pin and Sills (1986), Shervais and Mukasa (1991), Hartmann and Wedepohl (1993), Rivalenti et al. (1995), and Mukasa and Shervais (1999); 5) Shervais and Mukasa (1991) and Mazzucchelli et al. (2009); 6) Sinigoi et al. (1991; 2011); 7) Hartmann and Wedepohl (1993); 8) Coltorti and Siena (1984); 9) Siena and Coltorti (1989), Lu et al. (1997a; b), and Stähle et al. (2001). Table 2: Summary of VP and r from the thermodynamic calculations based on representative rock compositions listed in Table 1. Mineral abbreviations: Pl = plagioclase, Spl = spinel, Grt = garnet, Ky = kyanite, Px = pyroxene, Cr-Di = Cr-diopisde, Am = amphibole, Phl = phlogopite.