Geological setting of the Bagno Vignoni area (northern side of the Mt. Amiata geothermal area, Italy): collisional structures recorded in the Tuscan Nappe

Geological studies carried out in the Bagno Vignoni area (southern Tuscany) for the realisation of the new Geological Map of Tuscany (1:10,000 scale), and the interpretation of the Cro.P.18 crustal seismic line, highlighted new structural data for the Tuscan Nappe occurring on the northern side of the Mt. Amiata geothermal area. The Tuscan Nappe was affected by polyphase tectonics, as documented by structural data mainly detected from map-scale and microstructural analyses. The Tuscan Nappe in the Bagno Vignoni area consists of small outcrops emerging from Ligurian l.s. Units, mainly composed of the <> Unit. The Tuscan Nappe is typified by repetitions of its stratigraphical succession due to the presence of imbricate thrusts. They are associated with a collisional, southeast-verging thrust system which produced three Tuscan Nappe subunits, highlighted by field mapping and borehole stratigraphy. The lowermost outcropping subunit (SU3) is composed of the stratigraphical succession ranging from the <>. Fm. to the <> Fm.; the overlying SU2 is characterised only by the <> and <> Fms., whereas the uppermost SU1 is composed of the <> Fm. and the <> Fm. The Tuscan Nappe subunits stack was later deformed by Early-Middle Miocene low-angle normal faults which produced widespread tectonic elision between the thickened Tuscan Nappe and Ligurian Units. In fact, in the study area, the <> Unit, mainly composed of <> and <> Fms., lies on different Tuscan Nappe formations, suggesting the tectonic elision of the uppermost Tuscan Nappe formations, of the Subligurian Unit and of the lowermost formations of the <> Unit. On the whole, Tuscan Nappe collisional structures occur-ring in the Bagno Vignoni area represent relict structures preserved during extensional tectonics. The evidence also demonstrates the occurrence of collisional structures in the northern part of the Mt. Amiata geothermal area, as well as in the western and eastern sides (Mt. Aquilaia-Mt. Buceto and Poggio Zoccolino-Campiglia d'Orcia).