Ocean-continent transition zone on the updated lithospheric transect from the Ligurian Sea to the Po Basin (Italy)

In order to investigate the deep structure along the transect Ligurian Sea - Northern Apennines - Po Basin, a new gravity model was constructed on the composite cross-section (465 km long) derived from published seismic profiles, geophysical data and surface (marine and onland) geology.

Along the transect, the gravity field shows great changes: 160 mGal offshore in the Ligurian Sea; a wide minimum of -160 mGal onshore in the Po Basin. The corresponding density model was constrained by: the structure of the sedimentary successions and basement-crystalline crust, the offshore-onshore WARR (wide-angle reflection and refraction), reflection seismic profiles and Moho depth derived from European Moho compilations. A shear-wave seismic tomography model was used to constrain the upper mantle; the derived seismic velocity model was converted into density one. At the end, the calculated gravity effect was compared with the observed gravity.

The modelled density transect shows an oceanic crust, a continental crust, and an Ocean Continent Transition (OCT) zone; the crustal thickness varying from ~15.5 km in the Ligurian Sea to ≤40-km (including 18 km of meta-sedimentary and sedimentary successions) in the Po Basin. The latter causes the mentioned gravity minimum. The OCT is abruptly separated from the oceanic crust by a block, ~40 km wide and with steep flanks marked by local magnetic anomaly, which we relate to exhumed HP/LT alpine metamorphic ocean-derived rocks. Specifically, the OCT zone is ~ 120 km wide and it affects the Northern Apennines orogenic wedge made of basement (Tuscan metamorphic unit) overlain by Mesozoic carbonate rocks, Oligocene-Miocene foredeep siliciclastic sediments and Ligurian ophiolite-bearing units which, as a whole, form a transitional crust up to 25 km thick. A peculiar feature of this OCT is a wedge-shaped sub-Moho body which is ~ 7 km thick maximum, deepens and thins northeastwards and has velocity/density value higher than lower crust and lower than upper mantle.

All these features are indicative of the complex nature of this OCT, which was affected by different geodynamic processes during the long-lived history of the Europe and Adria plates convergence since the Late Cretaceous. These processes include the Cretaceous-Eocene subduction of European plate underneath Adria with the closure of the ancient Piedmont-Ligurian Ocean and formation of the northernmost segment of the “Mediterranean Alps”. The exhumation of the inner portion of the wedge between West-Liguria and Corsica and the post-Eocene rifting associated to asthenospheric flow resulted in the dismemberment of the Alpine orogenic wedge during the early Apennines deformation history which enhanced the formation of the modern Ligurian Sea at the northeastern tip of the Liguro-Provencal Basin (Western Mediterranean Sea) and at the northern tip of the Tyrrhenian Sea. In the offshore-onshore part of the OCT, the astenopheric zones are recorded as low-velocity layers (from S-wave tomography) in the subcrustal region and in the upper mantle where they correspond to zones of low density. The distribution of the high heat flow zones strictly corresponds to the low-velocity upper mantle heterogeneities confirming their recent origin associated with magmatic activity.