Analysis of the post-metamorphic fault network of the Ligurian Alps and Ligurian Sea as a key for the understanding the Alps-Apennine transition (CARG project)

The new detailed geological mapping at 1:10.000 scale of >1800 km² collected in the frame of the Italian Geological Survey Project (CARG project), resulted in a large amount of structural data throughout the Ligurian Alps and the adjacent Ligurian Sea, representing the connection area between the Alpine and Apennine orogenic arches. Field- and remote-sensing-based data highlight the presence of a dense fault-fracture network developed throughout the entire orogenic sector. Detailed structural analysis has been performed in selected sites, through the building of Digital Outcrop Models (DOM) derived from digital photogrammetry and the use of Unmanned Aircraft Vehicles (UAV). The onshore survey is based on the integration of a new set of seismic reflection lines and high-resolution bathymetric data (MBES) with pre-existing available geological and geophysical datasets. The integrated geological mapping and structural analysis can be summarized as follows: i) the entire orogenic sector encompasses a very dense network of relatively small fractures and faults showing complex intersection relationships; ii) from this network, several, previously unreported, km-scale transtensive fault zones emerge; iii) these faults postdate the metamorphic foliation associated with the main thrust-and-fold architecture of the Ligurian Alps; iv) the recognition of earthquake-related soft-sediments deformation structures in Miocene-Pliocene deposits highlights continuous seismicity related to the fault development. The offshore area consists of a continental shelf and the upper slope both intersected by well carved submarine canyons and characterized by a complex fault network. The fault network is characterized by dominant E-W left-lateral and NE-SW right-lateral faults. The entire fault network is interpreted as a complex network of subsidiary, en-échelon Riedel fractures encompassed within a regional sinistral transtensional/transpressional shear zone. The fault network primarily developed in the Oligo-Miocene times, thus during the pre-to-syn Corsica drifting phase. Moreover, both the Ligurian orogen and Ligurian Basin fault systems experienced Miocene to present-day seismicity marked by i) the cyclic occurrence of earthquake-induced soft sediment deformation structures found in the Miocene to Pleistocene sediments and ii) the present-day instrumental recording. In light of the new dataset, we reconsider the role of the local-scale fault network as a direct surface expression of the ongoing bending of the Ligurian Alps, driven by the combination of Adria rotation and the pull of the Apennine subduction.