Abstract

The Narni-Amelia chain (southern Umbria) is a 45 km long, NW-SE-trending calcareous mountain ridge. It is bounded westward by the Paglia-Tevere basin and eastward by the Middle Tevere-Terni basin; both basins are referred Co the Plio-Pleistocene interval. Previous works have exhaustively described the stratigraphic sequence of the calcareous Meso-Cenozoic multilayer, from the Raethavicula contorta limestone (Late Triassic) to the Bisciaro marly limestone and volcanoclastic beds (Early Miocene). However in this work, for the first time, we have recognised that even in this inner part of the Umbria domain, a distinction between complete and reduced Jurassic successions is possible (see for instance the Fornole reduced succ). The Narni mountains are a good sample-area for the geometrical reconstruction of the chain because in the "Nera window" the deepest part of the structure outcrops and because a number of subsurface data are available. The compressional structure is characterised by three stacked tectonic units which have developed from the progressive deformation of a recumbent east-verging anticline by thrusting. The upper unit corresponds to the western limb and to the wide flat hinge zone of the Narni anticline. The intermediate unit is the overturned eastern limb of the anticline. The lower M. Ippolito unit corresponds to the footwall of the Narni thrust, folded and thrusted with adriatic vergence. The above mentioned units are separated by two main thrust planes displaced by following strike slip and extensional faults. The observed tectonic features can be interpreted hypothesising a complex deformational history: - buckling-induced parallel folding of the calcareous multilayer and origin of small-scale blind thrusts in the core of the Narni anticline; - nucleation of the Narni thrust system and emplacement of the three above described main tectonic units; - activation of a conjugate strike slip fault system among which the most important structures are the right-handed Narni-Poggio Macchie fault and the left-handed S. Casciano-Nera Montoro fault; - break up of the compressional architecture by means of two main normal fault systems bordering the Narni-Amelia chain towards W (SW-dipping normal faults) and towards E (NE-dipping normal faults) and origin of the present horst-graben setting. Correlating the Narni thrust with the northernmost Umbria thrusts (Massicci Perugini and M. Peglia), the compressional tectonic phase might be referred to the Middle-Late Miocene (Late Serravallian-Early Tortonian). The divergence between the axial direction of the Narni anticline and of the M. Ippolito unit, suggests an out of sequence re-activation of the sole thrust in more recent times. Palynological and magneto-stratigraphical data from the "Fosso Bianco" lacustrine clays, which are the oldest deposits outcropping in the Middle Tevere basin, suggest an Upper Pliocene age for the beginning of the extension along the eastern border of the Narni-Amelia chain. The onset of extension is probably older along the western border of the chain, as suggested by the age of the "Argille di Fabbro" unit (Lower Pliocene), which is the oldest unit of the marine-transitional succession outcropping W of the chain (Paglia-Tevere basin).

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