Geological features of Larderello-Travale and Mt. Amiata geothermal areas (southern Tuscany, Italy)

This paper summarises the geological features of the Larderello-Travale and Monte Amiata areas, where the world's most ancient exploited geothermal fields are located. In both geothermal areas, three regional tectonostratigraphic elements are distinguished, from the top: (a) Late Miocene-Pliocene and Quaternary, continental to marine sediments; (b) the Ligurian and Sub-Ligurian complexes, which include remnants of the Jurassic oceanic realm and of the transitional area to the Adriatic margin, respectively; (c) the Tuscan Unit (Tuscan Nappe), composed of sedimentary rocks ranging in age from Late Triassic to Early Miocene. The sub-stratum of the Larderello and Monte Amiata areas is referred to as the Tuscan Metamorphic Complex. This is mainly known through drilling of geothermal wells. This complex is composed of two metamorphic units: the upper Monticiano-Roccastrada Unit and the lower Gneiss Complex. The Monticiano-Roccastrada Unit consists of (from top to bottom): the Verrucano Group, the Phyllite-Quartzite Group and the Micaschist Group. The Gneiss Complex consists only of pre-Alpine polymetamorphic gneiss. The Tuscan Metamorphic Complex is affected by contact metamorphism by Plio-Quaternary granitoids and their dyke swarms. Hydrothermal phenomena still occur in both geothermal fields. The Larderello-Travale and Mt. Amiata geothermal fields are located in the inner Northern Apennines, in an area that has been subject to extension since the ? Early-Middle Miocene. Two main extensional events are well expressed in the structures of the geothermal areas. The first extensional event (?Early-Middle Miocene) determined the tectonic delamination of the Ligurian Units and Tuscan Nappe. The second extensional event (Late Miocene-Present) is characterized by high-angle normal faults bounding the Neogene tectonic depressions of southern Tuscany.