Key travertine tectofacies for neotectonics and palaeoseismicity reconstruction: effects of hydrothermal overpressured fluid injection

Abstract: Travertine deposits have long been considered as powerful tools for investigating neotectonics and reconstructing palaeoseismic events. We document, for the first time, the effects of overpressured hydrothermal fluids injected within travertine deposits. We also describe tectofacies interpreted as a consequence of coseismic events. Calcite veins, banded or massive, associated with hydrofracture and fluid-escape features, promoted hydrothermal eruptions and lithoclast formation in latest Quaternary travertine exposed in two quarries near Rapolano Terme (Northern Apennines, Italy). The isotopic composition of the calcite veins confirms the hydrothermal origin of the parent fluids and their rapid ascent, as indicated by the estimated palaeo-temperatures (43 – 50°C). Integration of U–Th ages obtained for the calcite veins with palaeoseismic evidence from a local archaeological site built at the top of one of the analysed travertine deposits suggests that hydrofracture and fluid- escape structures were associated with five main seismic events that occurred from the latest Pleistocene to the fourth century AD. In sum, the travertine tectofacies have a key role in better constraining the seismotectonic setting of a region and thus offer a powerful tool for the evaluation of seismic hazard for areas characterized by low seismicity and travertine deposition.