Lower Oligocene thrust-system in the epi-Ligurian succession: evidence from the Enza Valley (northern Apennines, Italy)

Within the lower part (Upper Eocene-Oligocene) of the epi-Ligurian successi on, outcropping in the Emilian side of the northern Apennines (Enza Valley) , duplications by thrust tectonics were recognized through the systematic i ntegration of field geology with calcareous nannofossil biostratigraphy. Th is thrust system, derived from the overthrusting of two thrust-sheets over a footwall, is unconformably overlain by a Rupelian succession. The thrust structure of the Enza Valley, affected by a subsequent wide overturned sync line together with the unconformable succession, shows a remarkable Lower O ligocene contractional tectonics, previously not recognized in the northern Apennines. The comparison of this thrust system with other outcropping areas of the ep i-ligurian succession makes probable the widespread occurrence of the Lower Oligocene tectonics in the uppermost structural levels of the chain (epi-L igurian domain). In a regional tectonic framework, the Rupelian thrust tectonics affecting t he epi-Ligurian succession can be related to the Lower Oligocene closure of the innermost portion of the Subligurian basin (Aveto-Petrignacola Formati on) due to the NE-verging overthrusting of the External Ligurian Units. In this context the unconformable succession of the Enza Valley seems correlab le with the basal portion of the Subligurian Bratica Sandstone (Rupelian-Ch attian) which unconformably overlies a deformed substratum (Mesoalpine Phas e). (C) Elsevier, Paris.