GNGTS 2024 - Atti del 42° Convegno Nazionale

Session 1.1 GNGTS 2024 useful in actve tectonics studies due to (a) the close genetc relatonships between actve faultng and travertne depositon, (b) the optmal recording and preservaton of any britle deformaton events, (c) the quick and permanent sealing of such episodes due to the circulaton of pressurized fuids (e.g., Hancock et al., 1999). We describe in detail and for the frst tme an actve, capable and seismogenic fault system that we identfed in the saw-cut walls of Cava Capanni, an actve travertne quarry near Serre di Rapolano, few km south-east of the city of Siena. As a mater of fact, the Late Pleistocene-Holocene travertne deposits of the eastern Siena Basin preserved the record of several past earthquakes, in the form of earthquake-induced sof-sediment deformaton structures and co-seismic injecton of overpressured hydrothermal fuids, in diferent quarries and archeological sites (e.g. the Campo Muri site near the Serre di Rapolano fssure ridge; Brogi et al., 2017). To document the geometrical and kinematc characteristcs of the actve faults, we conducted a detailed geological and structural feld survey of the quarry outcrop, collected samples for U-Th datng, and made a virtual outcrop model (VOM) of Cava Capanni using a set of photos taken using an airborne drone. The actve fault system we mapped consists of at least three main WNW-ESE-striking fault segments, rupturing up to the ground surface a sequence of travertne younger than 45 ka. The system has a total vertcal displacement of 111 cm, and can be followed in the quarry over a distance of 200+ m. However, its total mapped length is ~700 m, as it contnues eastwards for at least 500 m, cutng the Jurassic Tuscan succession and the overlying Zanclean deposits, before disappearing due to the heavy vegetaton cover. To the west of the quarry, the fault zone disappears below the Holocene colluvial deposits of the Siena Basin. In additon to the main splays, the fault system exhibits many fractures, collectvely defning a several tens of meters-wide damage zone. The fractures are near-parallel to parallel to the main fault segments and locally exhibit anastomosed traces associated with linkage zones (releasing step-overs) and relay ramps, testfying for the transtensive kinematcs of the system. The walls of the faults are covered by cm-thick, well-cemented carbonate siltstone, which ofen also flls the fractures of the damage zone. Such material derives from sediment liquefacton at the base of the travertne succession, as suggested by the occurrence of fossil remains, forming three generatons of clastc dykes that were presumably injected in the fault zone during earthquake- induced shaking. Following this observaton, we suggest that the measured displacement is the result of at least three earthquakes. The Cava Capanni fault system is evidence of a poorly understood but regionally-extensive and potentally seismogenic tectonic mechanism. Based on total displacement and using standard empirical relatonships, we estmate that it may generate earthquakes in the Mw range 6.0-6.3 recurring every ~10 4 years. Its orientaton and kinematcs are compatble with the actvity of faults orthogonal to the main chain axis (“ant-Apennines striking”), in contrast with the current tectonic regime of the axial and outer zone of the Northern Apennines, where extension and compression are accommodated by Apennines-parallel faults.