This paper deals with the results of new geological field-mapping and a structural study, carried out in the western area of the Monte Amiata volcano, in southern Tuscany. The study area is located in the northern part of the Mt. Aquilaia-Mt. Buceto-Mt. Labbro ridge, representing a structural culmination situated in the western side of the Bagnore geothermal field (Calamai et alii, 1970). In the Mt. Aquilaia area tectono-stratigraphic units belonging to the Tuscan Nappe and Argille e calcari Unit, crop-out. The Tuscan Nappe mainly consists of the Scaglia toscana stratigraphic units and Macigno Fm. with minor outcrops of Marne a Posidonomya, Diaspri and Maiolica Fms. The Argille e calcari Unit is represented by the Argilliti e calcari di Canetolo-Cirone and of the Calcareniti di Groppo del Vescovo (Boccaletti et alii, 1987). Structural analysis and geological mapping have allowed the tectonic setting of this area to be defined. Three main tectonic phases have been recognised. The oldest (D <sub>1</sub> ) is characterised by brittle and ductile structures. It is related to the thrusting of the Argille e calcari Unit onto the Tuscan Nappe and to a thrust recognised within the Tuscan Nappe, defining two tectonic sub-units, here named SU1 and SU2. The SU1 consists of the stratigraphic units comprised between the Marne a Posidonomya and the Macigno Fm, whereas the SU2 consists of the Maiolica and Diaspri Fms. together with the stratigraphic units belonging to the Scaglia toscana. S-c structures, folds and tectonic foliation associated with the D <sub>1</sub> tectonic phase are very well developed close to the thrust surface. The axes of the folds, the direction of the reverse fault planes and the intersections between the s and c planes are oriented N150 degrees -N200 degrees . All these structures indicate an eastward tectonic vergence of the D <sub>1</sub> tectonic phase. The D <sub>2</sub> deformational phase produced low-angle normal faults and east-verging folds, metres to decametres in size, with axes ranging from N20 degrees to N70 degrees . The D <sub>3</sub> tectonic phase gave rise to high-angle normal faults dissecting all the previous structures. The fault directions ranging from N150 degrees to N180 degrees are coherent with the main normal faults bordering the Neogene-Quaternary basins of Southern Tuscany. The Mt. Aquilaia thrust, previously unidentified, is a D <sub>1</sub> structure developed during the Northern Apennine syn-collisional stage, but unfortunately its complete geometry is unlikely to be detectable owing to the scarcity of continuous outcrops. The Mt. Aquilaia thrust probably could be a part of a major thrust-duplex system affecting the Tuscan Nappe. This supported by the occurrence of other thrusts recognised in the neighbouring areas: Monte Labbro area (Crescenti & Giussani, 1969) and Monti dell'Uccellina (Campetti et alii, 1995, 1999). The tectonic setting detected in this area represents an important contribution to the knowledge of the structural setting of Tuscan Nappe and, more generally, of the tectonic history of this sector of the northern Apennine Chain.

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