Faults linkage, damage rocks and hydrothermal fluid circulation: Tectonic interpretation of the Rapolano Terme travertines (southern Tuscany, Italy) in the context of Northern Apennines Neogene-Quaternary extension

Abstract

The tectonic history of Neogene-Quaternary extensional structures has been documented in the inner part of the Northern Apennines (Rapolano Terme area) from Quaternary travertines and faults relationships. In particular, structural studies, field mapping and tectono-stratigraphical considerations on the pre-Neogene, Pliocene and Quaternary units indicate two fault populations of different ages. The travertines, broadly exposed in the Rapolano Terme area, were related to hydrothermal fluid circulation and upwelling along the damage zones of Pleistocene normal faults. These structures were superimposed upon normal (mainly NNW-SSE and SW-NE oriented) and transtensional (mainly SW-NE oriented) faults, Early-Middle Pliocene in age, related to development of the Siena Basin. The age of such Pliocene normal and transtensional faults has been inferred on the basis of the relationships between tectonic elements and Pliocene deposits. In fact, Middle Pliocene marine sandy and gravelly sediments suture the faults. The Pleistocene normal faults (mainly SW-NE oriented) reactivated the oldest SW-NE transtensional faults, cut the NNW-SSE normal faults, and displaced Middle Pliocene marine deposits. Pleistocene-Holocene travertines and present-day thermal springs mainly occur at the intersection points between the Pleistocene SW-NE faults and the Pliocene NNW-SSE normal faults, or along the SW-NE Pleistocene normal faults.