3-D imaging of the Balmuccia peridotite body (Ivrea-Verbano zone, NW-Italy) using controlled source seismic data

We provide new results from a controlled-source seismic experiment on the deepest part of the Val Sesia crust-mantle section of the Ivrea-Verbano zone (IVZ) in the Italian Alps. The IVZ is a tilted, almost complete section through the continental crust and exposes gabbros and peridotites in the structurally deepest level, coinciding with high-resolution gravity anomalies imaging the Ivrea geophysical body. The seismic experiment SEIZE (SEismic imaging of the Ivrea ZonE) was conducted along two crossing profiles: an NNE-SSW profile of similar to 11 km length and an E-W profile of similar to 16 km length. 432 vibration points were recorded with 110 receivers resulting in 24 392 traveltime picks. Inversion methods using Markov chain Monte Carlo techniques have been used to derive an isotropic 3-D P-wave velocity model based on first break traveltimes (refracted phases) from controlled source seismic data. Resulting seismic P-wave velocities (V-p ) range from 4.5 to 7.5 km s(-1), with an expected general trend of increasing velocities with depth. A sharp velocity change from low V-p in the West to high V-p in the East marks the Insubric Zone (ISZ), the Europe-Adria plate boundary. The most prominent feature of the 3-D tomography model is a high-velocity body (V-p increases from 6 to 7.5 km s(-1)) that broadens downwards. Its pointy shape peaks the surface East of Balmuccia at a location coincident with the exposed Balmuccia peridotite. Considering rock physics, high-resolution gravity and other geophysical data, we interpret this high-velocity body as dominantly composed of peridotite. The dimension of this seismically imaged peridotite material is far bigger than interpreted from geological cross-sections and requires a revision of previous models. The interpretation of ultramafic bodies in the IVZ as fragments of mantle peridotites interfingered in the crust during pre-Permian accretion is not supported by the new data. Instead, we revive a model that the contact between the Balmuccia peridotite and the Permian mafic magmas might represent a fossil continental crust-mantle transition zone.