Travertine deposits constraining transfer zone neotectonics in geothermal areas: An example from the inner Northern Apennines (Bagno Vignoni-Val d’Orcia area, Italy)

Abstract

Studying travertine deposits and the network of banded calcite veins that form their roots in the substratum can place important constraints on neotectonic activity and the seismotectonic settings of geothermal areas. In this paper, we present the results of integrated studies of a geothermal area located in the inner Northern Apennines (Bagno Vignoni area, Italy), where low magnitude (M < 4) seismicity suggests the occurrence of active faults, but information on their location, time-span of activity and seismotectonic setting is lacking. The study area is characterised by thermal springs (T<50 °C) and travertine deposits well exposed in saw-cut walls of abandoned quarries. We investigate the relationships between faults and the travertine deposits, in order to reconstruct the syn-sedimentary tectonic activity and the age of faulting, by combining analyses of: i) geological setting and structural/kinematic analyses of faults; ii) travertine morpho-structural and architectural setting; iii) travertine facies; iv) U-Th radiometric, stable- and clumped isotopes of travertine and banded calcite veins. The results highlight the occurrence of a wide brittle shear zone (>3 km) formed by two orthogonal faults systems, NE- and NW-striking, characterised by oblique-slip to normal kinematics, respectively; these faults belong to a tract of the so-named “Grosseto-Pienza” transfer zone, crossing the southern Tuscany from the sea-cost to the outer Apennines belt. The Grosseto-Pienza transfer zone formed with extensional tectonics that have been affecting the inner Northern Apennines since the middle Miocene. U-Th dating of travertine and banded calcite veins indicates that faulting enhanced the hydrothermal fluid circulation since the middle Pleistocene, in an unvaried tectonic setting, as indicated by the δ18O signature and temperature of the hydrothermal fluids, which remained stable through time. The activity of the faults continued until the Holocene and still produces seismicity. Finally, our findings permit to define the seismo-tectonic setting of this sector of the inner Northern Apennines, demonstrating more broadly the utility of travertine deposits in reconstructing the neotectonics in geothermal areas.