Mapping the Ivrea Geophysical Body and its anisotropic properties beneath the Western Alps using receiver functions analysis

We constrain the extent and anisotropic properties of the Ivrea Geophysical Body (IGB) beneath the Western Alps using receiver function (RF) analysis of 66 teleseismic datasets. The IGB represents one of the most prominent examples of shallow mantle material within continental crust, yet its geometry, composition, and tectonic significance remain debated beyond its well-known positive gravity anomaly. Using teleseismic waves recorded from temporary seismic deployments and permanent seismic networks across the western Alps, we perform a comprehensive receiver function (RF) analysis that indicates the presence of anisotropic mantle materials at shallow depth, associated with the occurrence of the IGB, in terms of P-to-s converted energy out of the radial plane. We characterise the anisotropic rock volumes solving an inverse problem using a Neighbourhood Algorithm. The results indicate that 35 out of 66 RF data-sets from this study, together with five additional stations from a previous study, display coherent anisotropic characteristics directly above the high-gravity anomaly and can be associated with the IGB. These stations exhibit strong anisotropy (~15%) and a coherent fast-axis pattern that systematically rotates from south to north, following the arcuate geometry of the Alpine trench. The depth distribution of the anisotropic interfaces constrains the IGB as a continuous lithospheric-scale body approximately 170 km long, 30-50 km wide, and 20-45 km thick, with its upper boundary as shallow as 1 km depth. The whole body is slightly dipping towards the East. Seismic velocities and anisotropy magnitudes indicate a dominantly mantle-derived composition, consistent with a peridotitic protolith variably overprinted by serpentinite-rich shear zones. Our results refine the three-dimensional extent of the IGB and demonstrate that its anisotropic fabric records the deformation associated with Alpine subduction, slab rollback, and subsequent exhumation, providing new constraints on the tectonic evolution of the western Alpine lithosphere.