Refined Imaging and Modelling of the Ivrea Geophysical Body Through New Gravity Data, Anomaly Products and Seismic Constraints

The Ivrea Verbano Zone (IVZ) is a unique portion of the Western Alps, lying along the European-African plate boundary and presenting evidences of continental collision processes. At the surface, widespread outcrops of lower to middle-crustal composition rocks serve as a unique catalog of Earth lithosphere samples and properties. Below the surface, a sliver of Adriatic lithosphere is at shallow depth, producing the so-called Ivrea Geophysical Body (IGB), mainly characterised by gravity and seismic velocity anomaly.

The denser structure of the IGB has been documented by significant positive anomalies on several gravity maps, which, however, differ from each other because of heterogeneous and sparse data coverage and interpolation artefacts. Recent tomographies present mantle-like seismic velocities at shallow depths. In order to investigate the IVZ crustal roots at a high and unprecedented resolution, we gathered new multi-disciplinary geophysical data: 180 new gravity points, planned to best approach the homogeneous coverage of 1 point every 4 to 9 km² and a 10-broadband seismic profile, operated for 2 years and crossing the IGB in the same area. Finally, rock sample analyses and geological field observations have been used to produce a surface density map of the whole area.

Surface densities exhibit significant deviations from 2.67 g/cm³ - the standard density value for Bouguer anomaly computation. Therefore, we defined a new gravity anomaly product: the Niggli anomaly, by incorporating into the computation the observed in-situ densities. This approach allows consistency between the observed gravity anomalies and the geological observations. Together with this data product, we computed receiver functions out of the seismic data, to constrain the location of seismic velocity discontinuities.

First, we present a 3D density model of the IGB, that has been derived by using the gravity data products. Subsequently, we focus on the 2D structure below our seismic station profile, moving towards a joint inversion of seismic and gravity data products. The joint use of these two datasets, together with the geological observations incorporated into the computations, offers stronger constraints on the IGB structure and brings the geophysical investigations closer to the geological observation scales.