Integrated gravity modelling of the onshore-offshore transect Ligurian Sea – Po Basin: Revisiting the lithosphere architecture of the Northern Apennines (Northern Italy)

Orogenic system, which may often have onshore and offshore portions, requires different methodologies of investigation that, on their hand, provide results not always easy to compare. This is a drawback that typically hampers the reconstruction of unifying geological models of the orogenic belts and associated sedimentary basins. These limitations are even more evident in orogenic systems that were affected by late synorogenic extension such as the Apennines of Italy. In the attempt to create a unifying geological model of the Northern Apennine orogen, we integrated onshore geology, marine geology with geophysical data (magnetic and gravimetric anomalies, heat flow, refraction seismology, surface wave tomography, particular noteworthy seismic reflection profiles) and previous lithospheric studies to constrain a new gravity model along a transect that crosses the whole orogenic system from the Ligurian Sea, through the northwestern portion of the Northern Apennines adjacent to the Alps (Ligurian knot), to the Po Basin, up to the Southern Alps of Italy. The new gravity model fits with densities of a lithosphere made of different crustal types: from extended continental crust, 18 km thick in the offshore oceanic domain of Ligurian Sea, to the continental crust ≤40 km-thick including up to ∼20 km sedimentary cover beneath the Po Basin in the onshore continental domain. In between, the offshore-onshore transition domain, ∼150-km wide and up to 25 km thick, includes most of the inner Northern Apennines' thrust stacks which are thinned and dismembered by extensional faulting. Within this transition domain, around ∼20 km depth, an eastward-dipping High-Velocity/high-Density Body (HVDB) is highlighted. It abruptly ends within the Ligurian Sea, where it is ∼7 km thick, progressively thins out toward the NE and disappears at ∼36 km depth beneath the watershed zone of the Northern Apennines. The new onshore-offshore lithospheric transect provides a unifying and updated geological model of the Northern Apennines and sheds new lights on their complexity in relationship with the colliding Europe and Africa continental plates that shape the Alpine-Apenninic orogenic system in the Mediterranean Region.