Stratigraphy and evolution of the northern Apenninic accretionary wedge with particular regard to the ophiolitic sequences: a review

The Northern Apennines are an east-migrating accretionary wedge in an ensialic stage, made up of several east-verging nappes. The most internal units (Ligurids) belong to the Ligund domain, the basement of which was the oceanic crust of the Western Tethys. The most external units (Tuscan and Umbria-Marches units) belong to the Adriatic continental domain. Only the Calcshists with Ophiolites and Vara Supergroup units present a Tethyan oceanic basement at the base of the Jurassic-Cretaceous cover. The other Ligurid units, cut below at the level of their Cretaceous 'basal complexes', do not show any record of their Jurassic oceanic basement. The Ligurid units more external than the Vara Sopergroup include ophiolites as olistolites and olistostromes in their Upper Cretaceous turbidites. Stratigraphic, sedimentologic, structural and palaeogeographic considerations led to the hypothesis that also these Ligurids have an oceanic basement. The opening of the Western Tethys occurred during the Middle-Upper Jurassic after a Triassic-Jurassic rifting stage. The oceanic crust of the Western Tethys is made up of a serpentiniticgabbroic basement, which was extensively exposed on the oceanic bottom and directly covered by ophiolitic breccias and pelagic sediments. The basaltas follow the latter and were discontinuous and scarce in thickness. During the Upper Cretaceous the Western Tethys began to dose and an embryonic (Ligurid) accretionary wedge of the Northern Apennine orogen formed. The progressive eastward migration. starting from Upper Cretaceous. of the turbiditic basins, and the western provenance of the siliciclastic material of the more internal (and older) turbidites, indicate the European margin (Corsica and Sardinia) as the active (overlying) one. Two main phases characterized the evolution of this tectonic wedge a) an Upper Cretaceous-Eocene Ligurid phase. during which the Tethyan oceanic crust was subducted and the Ligurid units (both of Corsica and the Northern Apennine) were accreted (and metamorphosed) by underplating processes; b) a post-Upper Eocene ensialic phase, which started after the Europa-Adria collision, occurred between Upper Eocene and Oligocene. during which the Tuscan and Umbria-Marches units were structured. After the Middle Eocene, probably during the collision, the metamorphic 'Alpine' Corsica units was exhumed and emplaced westward as backthrusts of the east-verging Northern Apennine accretionary wedge. The amount of shortening decreases from W to E, as does the age of the turbidites at the top of each unit. From the Middle Miocene, extensional tectonics, with related magmatism, active to the west of the compressive front, began to migrate east-wards. The magmatic activity seems to have ceased in Quaternary times. At present, the compressive front is active on the Po Valley- Adriatic Sea side; the distensive front remains 80-100 Km behind, near the Apenninic watershed divide. A new structural and rheologic model for the Northern Apennine lithosphere. based on old and new seismic reflectors in the Liguria-Parma Apennines, assume a pure shear behaviour for the upper crust, and a simple-shear model for the lower crust, and a lozenge geometric pattern for the crust-mantle boundary. A main listric fault dipping westwards separates (Mt. Orsaro) the extensional (Tyrrhenian) domain from the compressive domain. An east-dipping crustal shear zone (LANF) lies above relics of old deep west-dipping Moho compressional structures. A new west-dipping Moho plane, which may be a new compressional structure, is present eastwards below the compressional domain of the Northern Apennines.