The Oligocene ophiolite-derived breccias and sandstones of the Val Marecchia Nappe: Insights for paleogeography and evolution of Northern Apennines (Italy)

Abstract

Ophiolite-derived debrites, microbreccias and olistoliths are interbedded in the Monte Morello and Argille Varicolori Formations of the Val Marecchia Nappe, constituting the highest tectonic unit of the north-eastern Apennines and usually considered as having originated from the External Ligurian Domain.The ophiolite-derived clastic rocks were supplied exclusively by an oceanic sequence, consisting of peridotite, gabbro, basalt, radiolarite, pelagic limestone and shale. They are interbedded within a succession made up of fine-grained carbonate and siliciclastic turbidites, and of pelagic claystones.Petrographic and sedimentological features of the ophiolite-derived breccia and sandstone units testify to an intrabasinal source area for these clastic rocks rather than an extrabasinal origin from subaerially exposed oceanic crust, forming the inner flank of the basin, as previously suggested. Due to their Oligocene–earliest Miocene age, the deposits in which debrites and olistoliths occur, are related to Early Neo-Alpine tectonic events that caused reactivation and/or inversion of old normal/transform faults. Unstable fault escarpments provided a preferential path for submarine landslide and turbidite emplacement. Debrites, microbreccias and olistoliths, therefore, were added to the basin fill of fine-grained turbidite and pelagic deposits.This study suggests that the Val Marecchia Nappe succession was deposited on an oceanic substratum. Stratigraphy and age of the succession of this nappe can be well framed only in the evolution of the Sub-ligurian Domain, whereas they conflict with the interpretation of the Val Marecchia Nappe as a nappe originated from the External Ligurian Domain, as suggested previously by most authors. This interpretation also requires an oceanic substratum for the Sub-ligurian Domain, i.e. the existence of an oceanic belt external to the Ligurian Domain, which was deformed only in the Early Miocene. The Sub-ligurian Domain, therefore, would be an eastern branch of the Central Tethys and would represent the extension in the Northern Apennines of the Maghrebian–Lucanian oceanic realm, as recognized in the Betic Cordillera, Maghrebian Chain and Southern Apennines.