Bayesian Joint Inversion of Bouguer Gravity and Surface Wave data: application to the Western Alps

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-v_S 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 v_S. 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-v_S 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.