Il termalismo di Val Calaona in relazione con l'assetto geologico-strutturale del settore occidentale dei Colli Euganei

The Val Calaona is located in the southwestern area of Euganean Hills and characterized by the presence of the most important hydrothermal occurrence (T = 35°C) of the western margin of the hills, which was exploited for balneotheraphy in the nineteenth century. The Val Calaona is bordered by the Cinto and Gemola Mountains to the north and by the Cero Mountain to the south. The top of these reliefs is made of trachytic and rhyolitic magmatic bodies, while the slopes are carved into Mesozoic sedimentary layers. A geological-structural study of the area was performed to understand the origins of the warm waters of Val Calaona. During field-work, the attitude of sedimentary beds and fault and fracture planes has been measured. The measures have been included into a GIS system and then integrated with data extrapolated by CARG sheet Padova Sud and by the geological map of Euganean Hills (1:25000). The analysis of these data has allowed the recognizing of several blocks separated by faults. These are normal and strike-slip faults with mainly NNW-SSE e ENE-WSW orientations. The field work at Bomba quarry (base of southern side of Cinto Mountain) has led to the identification of a fold, that is evident by the downwards increasing of the bed dip (from 25° to 35°) with a mean dip direction of N189°. The origin of the Val Calaona valley has been attributed to a pull-apart basin developed at the right step of two overlapping faults oriented NNW-SSE, with a right strike-slip movement. This could explain the steep dip of the sedimentary layers observed at the base of Cinto Mountain. Finally, the Val Calaona hydrothermal springs could be related to the permeability increase due to fracturing, which occurs in fault zone, combined with a local tensional regime of faults bordering a pull-apart basin.