New constraints on the Ivrea Geophysical Body at intra-crustal scales: a combination of gravimetry with passive seismology and rock’s physical properties

We present a high-resolution investigation of the Ivrea Geophysical Body (IGB) at intra-crustal scales in the Western Alps. The IGB is a sliver of Adriatic lower lithosphere, located at anomalously shallow depths, and presenting positive gravity and fast seismic anomalies. Despite comprehensive information from previous studies, structural questions persist on the IGB and on its structural relation with the Ivrea-Verbano zone (IVZ), which exposes lower-to-middle crustal composition outcrops at the surface. Therefore, we measured 207 new gravity data points, obtaining a coverage of ca. 1 point every 4-to-9 km2 across the IVZ, and we installed 10 broadband seismic stations (IvreaArray) along the linear West-East profile of Val Sesia, operated for 27 months. We compiled a surface rock-density map and used it to define the density-dependent terrain-corrected “Niggli” gravity anomaly to properly model the IGB density structure at depth. We modelled the IGB as a 3D, single density-contrast interface, obtaining 400 ± kg·m-3 as optimal density contrast and a 20-km wide protruding structure, as shallow as 1 ± 1 km below sea level. The seismic data was then used to constrain the IGB shape along the 2D Val Sesia cross-section by means of a joint inversion of seismic receiver functions and gravity anomaly data. This has confirmed the marked density contrast and shallow segments reaching 1 to 3 km depth below sea level, and provide agreement with the rock’s physical properties (ρ, vS) and the geological structures observed at the surface. These results are now published (doi:10.1093/gji/ggaa263 and doi:10.3389/feart.2021.671412).