The sub-Ligurian and Ligurian units of the Mt. Amiata geothermal Region (south-eastern Tuscany): new stratigraphic and tectonic data and insight into their relationships with the Tuscan Nappe

A new geological survey and micropaleontological and petrographical studies were performed on the Ligurids l.s. in the Montalcino-Mt. Amiata area. The relationships between the Ligurids l.s., the Tuscan Nappe and the underlying Triassic-Paleozoic succession of the crystalline basement were also defined in the outcrops and at depth using data from ENEL deep geothermal wells. The results improve the stratigraphic and structural framework of southeastern Tuscany crossed by the southern transect of the CROP-18 profile. In particular, three main units have been distinguished within the Ligurids l.s. (from the geometric bottom to top): (a) the Canetolo Unit (or Sub-Ligurian Unit, ?Paleocene-Eocene), represented by dark grey to green shales with grey calcilutite and calcarenite beds, locally with K-feldspar-free, more or less calcareous sandstone intercalations; (b) the Santa Fiora Unit, consisting of a complex pile of tectonic elements made up of Cretaceous successions and, in particular, a dominant marly-calcareous flysch with local Pietraforte-like intercalations (Santa Fiora Formation) and of the siliciclastic turbiditic Pietraforte Fm. with its basal shaly-silty lithofacies (Manganesiferous Varicoloured Shales Fm.); (c) Ophiolitiferous Unit, mostly represented by often chaotic shales with siliceous limestone intercalations and quartz-arenites of Lower Cretaceous age (Palombini Shales Fm), which locally include ophiolitic olistoliths (mostly serpentinites locally associated with ophicalcites and ophiolitic breccias), Lower Cretaceous Murlo-like marls and Albian-Turonian olivine-basalt dykes, sills and pillow-lavas. The tectonic setting of the Ligurian, Sub-Ligurian and Tuscan units in the Mt. Amiata area is similar to that in southern Tuscany. In particular, the performed studies indicate a complex tectonic evolution which mostly occurred in the Lower Miocene to Pliocene time interval and including the following events: (1) syn-collisional and <<serrage>> folds and stacking of the units (Early Miocene-Middle Miocene); (2) tectonic lamination of the Tuscan Nappe (the so-called <<Reduced Tuscan Sequence>>) and of the overlying Ligurian l.s. stack in the Serravallian-Lower Tortonian; (3) development of the intramontane continental to marine basins (Upper Miocene-Pliocene) and of high-angle faulting. A tectonic doubling of the Tuscan Nappe (recently defined also in other parts of the Mt. Amiata region and, further to the west, in the Larderello-Travale geothermal field: see text) is suggested for the Tuscan inlier of Poggio Zoccolino, where an upper tectonic element of Triassic carbonates and evaporites overlies the Upper Triassic to Tertiary Tuscan succession. At least some of these doublings (e.g. the Poggio Zoccolino doubling), generally related to the main Late Oligocene-Early Miocene shortening, could have taken place during the Burdigalian-Lower Tortonian time interval during which the regional Tuscan Nappe Front (Mt. Orsaro-Lima Valley-Mts. Chianti-Mt. Cetona structural alignment) was originated and syn-tectonic extension took place in front of the main crustal thrusts (onset of the <<Reduced Tuscan Sequence>>). This event also produced doublings in the underlying crystalline Triassic-Paleozoic successions which probably belong to the Umbrian Domain. Since ?Upper Miocene-Pliocene times, this complex tectonic pile was dismembered by high-angle normal fault systems (which mainly strike NW-SE, NE-SW and N-S) and produced the growth of the Siena-Radicofani and the Baccinello morphological-structural depressions and of the Montalcino-Mt. Amiata Ridge