The continental crust in the Mt. Amiata geothermal area was involved, in the Early-Middle Miocene, in widespread megaboudinage which affected all the cover tectonic units of the Northern Apennines and, in particular, the Tuscan Nappe. Field work, deep boreholes and interpretation of seismic reflection profiles has allowed us to recognise three Tuscan Nappe megaboudins occurring in the subsurface of the geothermal area. The westernmost and central megaboudins partially crop out in the Castiglioncello Bandini-Mt. Labbro and in the Poggio Zoccolino-Campiglia d'Orcia areas, respectively. The megaboudins show elliptical shape and were produced by east-dipping normal faults showing flat-ramp-flat geometry. The megaboudins are delimited by two flats which are located at the base of, or within, the Ligurian l.s. Units (the uppermost flat) and within the Triassic evaporites of the Tuscan Nappe (the lower-most flat). Secondary flats also occur within, or at the base of the terrigenous succession ("Scaglia Toscana" and "Macigno" Fms.). These flats are connected to ramps located in the Mesozoic carbonates and in the Tertiary terrigenous Fms of the Tuscan Nappe. On the whole, these megaboudins can be considered as elements of an extensional duplex system. An asymmetrical profile typifies the megaboudins which show a steeply-dipping western margin, coinciding with the rotated westernmost hangingwall and uppermost flat, and a gently-dipping eastern margin, coinciding with the easternmost ramp. The "Scaglia Toscana" and "Macigno" Fms. occur in the westernmost side of each megaboudin, showing west-dipping or sub-vertical bedding. In the eastern side, the Triassic-Early Miocene succession of the Tuscan Nappe can be observed showing sub-horizontal bedding. The areas interposed between the Tuscan Nappe megaboudins are typified by the direct superimposition of the Ligurian l.s. Units on the Triassic evaporites representing the basal formation of the Tuscan Nappe. This geometrical setting is known as "serie ridotta" (reduced sequence). Late Miocene deposits infilling the Cinigiano-Baccinello, the Valdorcia and the Radicofani Basins occur where such a reduced sequence has been identified. This suggests that the space accommodation for the Late Miocene continental deposits was mainly related to the Tuscan Nappe megaboudinage. In fact, it can be envisaged that megaboudin development caused morphotectonic depressions offering suitable conditions for Late Miocene continental sedimentation and subsequently, for the development of Pliocene extensional structures.

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