The geological research focused on the southern Tuscany continental crust and finalised on the Cro.P.18 Project (Lazzarotto & Liotta, 1994; Morelli, 2000), allowed reconsideration of the geological setting for some areas located along the Cro.P.18 crustal seismic profile, which crossed the two geothermal areas of southern Tuscany (Larderello-Travale and Monte Amiata). For this purpose the Tuscan Nappe, emerging from neighbouring Ligurian l.s. Units and cropping out in a nucleus located in the western side of the Monte Amiata geothermal area, has been studied. New geological field work (1:5.000 scale) and structural analysis have been carried out. The study area is located along the Castiglioncello Bandini-Monte Labbro ridge and, in particular, in the Mt. Aquilaia and Mt. Buceto area. There, the Tuscan Nappe formations consists of broad exposures; in particular the Jurassic-Oligocene succession, represented by a sequence from "Marne a Posidonomya" to "Macigno" formations, crop out. The new detailed field mapping and structural analysis allowed both the stratigraphical succession and the structural setting to be refined. Stratigraphical features of some Tuscan Nappe formations must be underlined. In fact distinctive characters of the Maiolica Fm. have been observed: the upper part of this formation is characterised by red or pink limestones with interbedded marls and shaly-marls. Also, a thick succession (up to 50 m) represents the transitional lithofacies between the "Maiolica" and "Argilliti di Brolio" Fms. This succession consists of alternations between centimetre-thick strata of grey calcilutites and decimetre-thick red shale and shaly-marl levels. The "Scaglia Toscana" stratigraphical succession shows similar characters as documented in the Monti del Chianti area (Canuti et alii, 1965; Fazzuoli et alii, 1996). Concerning the structural setting, pre-, syn- and post-collisional structures affected the Tuscan Nappe. The oldest tectonic event (FT <sub>0</sub> ) produced syn-sedimentary normal faults, with centimetre-scale displacement, affecting the "Maiolica" and "Marne del Sugame" Fms. These structures are associated with pre-collisional extensional tectonics. Thrust and related parasitic structures (deformational event FT <sub>1</sub> ) followed the syn-sedimentary normal faults. These compressive structures caused the repetition of the stratigraphical succession within the Tuscan Nappe. The thrust is well exposed in the Monte Aquilaia (Brogi & Lazzarotto, 2002) and in the Monte Buceto areas. Minor east-verging structures, oriented at N150 degrees -N190 degrees , are associated with the thrust; they are: folds and related tectonic foliation, reverse faults and s-c structures. These structures are mainly located along the shear zones corresponding to the thrusting zone. Very low-grade metamorphism, consisting of clay-matrix alteration (illite-smectite), is locally associated with the tectonic foliation, mainly affecting pelitic rocks. The FT <sub>1</sub> event is linked to the syn-collisional tectonics building the Northern Apennines tectonic pile. Southeast verging folds, oriented N25 degrees -N30 degrees (deformational event FT <sub>2</sub> ), deformed the preceding structures. Low angle normal faults with flat-ramp-flat geometry (deformational event FT <sub>3</sub> ) produced tectonic delamination within the Tuscan Nappe and the Ligurian l.s. Units. In particular, the relationships between the Tuscan Nappe and the overlying Ligurian Units are driven by these structures. Surface geological data as well as boreholes and seismic data attest the existence of a widespread megaboudinage of the Tuscan Nappe in the Mt. Amiata area. In particular, the study area consists of a main Tuscan Nappe megaboudin partially exposed in the Castiglioncello Bandini-Mt. Labbro ridge. This geological body is composed of the Late Triassic-Early Miocene stratigraphical succession and it is associated with a minor megaboudin occurring in the eastern side, well controlled by boreholes penetrating the subsurface of the Bagnore geothermal field. Such megaboudins are delimited at the top and bottom by low-angle extensional detachments (flats) and laterally by east-dipping normal faults (ramps) flattening at the bottom in the Triassic evaporites and at the top in the Ligurian l.s. Units. These structures are related to the Early-Middle Miocene extensional tectonics affecting the inner part of the Northern Apennines (Carmignani et alii, 1994; Liotta et alii, 1998). All these structures were displaced by high angle normal faults (deformational event FT <sub>4</sub> ) Late Miocene-Quaternary in age. In summary, the structures in the study area related to the tectonic evolution of the Northern Apennines have been documented. In particular, relicts of pre- and syn-collisional structures have been recognised as relicts preserved during the Miocene extensional tectonics.

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