New insights into the late Miocene extent of the Ligurian nappe on the Adriatic side of the Northern Apennines, SE of the Sillaro Line, Italy

On the Adriatic side of the Northern Apennines, SE of the Sillaro Line, both the original extent/areal distribution and the stratigraphic composition of the Ligurian thrust-nappe are still unknown and represent a highly debated topic. Few contraints have been proposed in recent papers. Based on the presence of “Ligurian olistostromes” within the Tortonian Marnoso arenacea Fm and their absence, instead, within younger deposits, some workers have been suggested that to SE of the Sillaro Line (i.e., in the Romagna and Umbria Apennines) the Ligurian nappe was located in a more internal position. However, other workers claimed for a Ligurian thrust-sheet up to 5 km thick, partially or completely covering the internal Marnoso arenacea Fm of the Romagna Apennines. The latter hypothesis was based either on vitrinite reflectance data and apatite fission tracks data or on the composition of the Quaternary fluvial terraced deposits cropping out along the Romagna foothills. Following this latter hypothesis, the complete disappearance of the Ligurian nappe between the Sillaro Line and the Marecchia area, was the result of a differential widespread event of tectonic uplift since the Pliocene times enhancing subaerial erosion. Here we show that a re-examination and reinterpretation of the existing knowledge about the regional geology of the Northern Apennines is essential to solve this problem. In fact, in the External Ligurian Units cropping out in the south-eastern sector of the Emilia Apennines and in the Romagna Apennines (between the Enza and the Marecchia valleys) a particular distinct unit, variously named as Val Samoggia, Val Sillaro or Val Marecchia Varicoloured Shales - AVS - have been recently distinguished from the rest of the Ligurian Units. The AVS typically consist of green, black, red and grey shales with interbedded layers of predominantly limestones, marlstones, sandstones and manganese-rich thin layers ranging in age from the Early Cretaceous and the early Eocene. The AVS are stratigraphically inchoerent and pervasively deformed at all scales of observation, showing the typical characters of dismembered and broken formations. The AVS form the stratigraphic base of different early-middle Eocene flysches of the External Ligurian Units (i.e., Sporno, Savigno and M. Morello formations) and they share the same lithological, stratigraphical and structural character independent of the overlying formation. Interestingly, the AVS crop out also either in few scattered outcrops (klippens) in eastern Tuscany (Casentino and Pieve Santo Stefano areas) and Umbria (between Umbertide and Nocera Umbra) where they have been interpreted not as klippens but as “olistostromes” within the Marnoso arenacea Fm. In both latter areas the AVS are unconformably overlain by few scattered remnants of the late Oligocene to middle Miocene epi-Ligurian Antognola, Monte Fumaiolo or S. Marino formations. These data clearly imply that in the whole Romagna and Umbria Apennines on the internal Marnoso arenacea Fm a thin thrust sheet of Ligurian rocks exclusively formed of AVS and an uncomplete overlying epi-Ligurian succession, similar to that cropping out in the Val Marecchia area, was present at least up to the middle Miocene. Therefore, it follows that the thickness of the Ligurian nappe was not the same across the Sillaro Line, and that the thrust-front of the nappe to NW and SE of this tectonic lineament clearly had a quite different evolution. These findings again show that in the Northern Apennines the Sillaro Line played also an important role inside the evolution of the Ligurian nappe. KEY WORDS: Ligurian Units, Ligurian nappe, Northern Apennines, Romagna Apennines, Sillaro Line, Umbria Apennines, Varicoloured shales.