Tectonically-related fluid circulation in the San Casciano dei Bagni - Sarteano area (M. Cetona Ridge - Southern Tuscany): a coupled structural and geochemical investigation.

The aim of this investigation is to examine the relationship between springs and structural setting of Mt. Cetona Ridge. This area represents an important regional morpho-tectonic feature oriented NNW-SSE, separated from the geothermal area of Monte Amiata to the West by the Radicofani Neogene Basin, while bound to the East by Valdichiana Neogene-Quaternary Basin. This Ridge is constituted by the Tuscan Nappe and the overthrusted S.Fiora Unit. A new detailed field mapping and structural study allowed recognising a polyphase structural setting defined by four plicative followed by three extensional and transtensional phases. Coupled to the structural investigation, 13 spring water samples were collected in the area in May and in August 2005. Groundwater ranges from cold, low mineralized calcium-bicarbonate to warm, highly mineralised, calcium sulphate. Results are in general agreement with previous studies indicating the recharge area in the Cetona relief and a circulation in a highly fractured, quasicontinuous reservoir constituted by the Mesozoic limestone and the underlying Burano anhydrite formation. Nevertheless, geochemical modelling indicates that the high variability in the hydrochemical and isotopic composition cannot be explained by simple binary mixing between two groundwater types nor by a geochemical evolution involving gypsum dissolution and calcite precipitation. Outflows are closely related to the structural setting of the area and hydrochemistry allows discriminating different fluids associated with specific hydrogeological circuits. The interpretation of the structural setting for Mt. Cetona Ridge was obtained by fieldwork studies integrating geochemical data of both thermal and cold springs. Results of this investigation allow to propose for this area a new tectonic evolution and fluid circulation model.