Updated gravity and geophysical model for the crust and upper mantle transectfrom the Ligurian Sea to the Po Basin

A new gravity model was constructed for the composite line, 465 km long, crossing from the Ligurian Sea through the Northern Apennines to the Po Basin as far as Verona province and derived from published cross-sections. Gravity field along the transect varies strongly from high positive values of Bouguer anomalies (160 mGal) offshore to the low-amplitude gravity minima (-160 mGal) above the Po Basin. The structure of the sedimentary succession, basement, and the crystalline crust of the density model were constrained by offshore-onshore WARR (wide-angle reflection and refraction) and reflection seismic profiles. In addition, the European Moho compilation was used as well. We also constrained the upper mantle structure by the S-wave tomography model of Italy. Using the known velocity-density conversion functions for different velocity values and rock types, the velocity model of the crust was transferred into density one, from which the gravity effect was calculated by the GRAV3D software. A stable solution of the modelling was obtained for an oceanic crustal segment, a continental crust, and a transition zone from a thin (18 km) oceanic crust of the Ligurian Sea to ≤ 40 km-thick continental crust with a deep (up to 18 km) meta-sedimentary succession of the Po Basin, which causes the mentioned gravity minimum.

The ocean-continent transition zone, ~100-km wide, including much of the accretionary wedge, is a thinned crust (up to 25 km) with a thick basement (Tuscan metamorphic unit) overlain by Mz carbonate rocks, Oligocene-Miocene foredeep siliciclastic sediments and Ligurian units. A spectacular feature of the transition zone is a underplated sub-Moho high-velocity/density body, which is ~7 km thick and deepens northeastwards, below the Po Basin.  This transition zone is separated from the oceanic crust by a block, ~40 km wide, with subvertical flanks, marked by local magnetic anomaly which we associate to exhumed HP/LT metamorphic rocks. This could be indicative of the complex nature of the transition zone, which was affected by various geodynamic processes during the long-lived history of the Europe and Africa plates and the closure of the Tethys ocean between them since the Late Cretaceous. These processes included the compressional deformation event of the crust (in Eocene times) during the closure of the Piedmont-Ligurian Ocean, between Europe and Adria/Apulia paleocontinent, and the Apenninic subduction. Later (since middle Miocene, ~20-15 Ma), rifting occurred in the area of the modern Ligurian Sea, and it led to formation of the modern Western Mediterranean Basin and southwards opening of the Tyrrhenian Sea which began under the influence of asthenospheric flows. The latters, in the offshore part of our transect, are recorded as low-velocity layers (from S-wave tomography) in the subcrustal region, at a depth of about 30 km, and in the upper mantle. Corresponding zones of low density (up to 3.20-3.25 g/cm³) are present in the upper mantle of the Liguria-Verona density model. The distribution of the high heat flow zones strictly corresponds to the subcrustal astenospheric heterogeneities confirming that these heterogeneities are formed recently in the evolution of the Northern Apennines.