A 3-D geological model of the Western Alps and Ligurian basin from joint interpretation of geological data and seismic imaging models

The Western Alps exhibit a non-cylindrical lithospheric structure that cannot be understood using 2-D sections. To improve our understanding of the dynamics of the W-Alps and their peripheral basins, we have synthesised the latest geological and geophysical knowledge into a lithospheric-scale 3-D numerical geomodel. This work has been conducted by a team of geologists and geophysicists in the framework of the French Geological Reference Platform (RGF, http://rgf.brgm.fr), worksite “Alps and peripheral basins” (RGF-Abp). Our 3-D crustal-scale geological model covers the area [41.5°N - 48°N; 4°E - 10°E], from the Jura and Subalpine chains to the Ligurian-Provence basin and Corsica, and from the South-East basin of France to the western Po basin. We have modelled the crust-mantle boundary and the 2 boundaries of the highly metamorphosed subduction complex of the internal Alps, i.e. the Penninic front and the Insubric line.

The high-quality 3-D S-wave velocity models computed from ambient-noise tomography using data of the AlpArray, Cifalps and Cifalps-2 temporary seismic networks were key elements in our geomodelling (Nouibat et al. 2022, 2023). We have also used the receiver-function profiles along the Cifalps and Cifalps-2 transects (Paul et al. 2022), and the recent active seismic reflection-refraction and wide-angle profiles SEFASILS (Dessa et al. 2020) and LOBSTER-P02 (Dannowski et al.,  2020) in the Ligurian-Provence basin. All previous models or data have been taken into account, including 3-D P-wave velocity models (Diehl et al. 2009; Solarino et al. 2018), the ECORS-CROP deep seismic reflection profile, Moho depth models from active and passive seismic imaging (e.g. Spada et al. 2013), a Moho depth model of the Ligurian basin from gravity inversion (Chamot-Rooke et al. 1999) and results of active seismic profiles in the Ligurian basin (e.g. Rollet et al. 2002). The digital terrain model, geological map, and geophysical models have been input in the GeoModeller software to share data in the same reference frame, and model the geological boundaries in 3-D (Calcagno et al. 2008). All available models have been carefully cross-checked along reference 2-D cross-sections to determine which Moho proxy should be picked in the 3-D Vs model. The Penninic Front and the Insubric line were picked by geologists based on their surface trace and extended to depth based on velocity heterogeneities. The geological boundaries, Moho and boundaries of the subduction complex, have then been interpolated from interpreted 2-D cross-sections to 3-D surfaces by GeoModeller.

Unlike previously published Moho models in the W-Alps, our 3-D geomodel highlights the downthrusting of the European Moho in the subduction of Europe beneath Adria, and it emphasizes its strong shape changes along the arc. The 3-D shape of the Adriatic Moho on top of the Ivrea geophysical body is also clearly highlighted. This first version of the crustal-scale geomodel of the W-Alps will be later augmented by the addition of earthquake hypocentres and focal mechanisms. It will also be used as initial model in the inversion of gravity data. Within the RGF-Abp programme, the geomodel will help to integrate results of local studies in a crustal reference frame.