Clastic injectites and seismic-induced liquefaction in latest Quaternary travertine deposits (Serre di Rapolano, Italy)

Abstract

ABSTRACT The liquefaction and consequent injection of clastic dykes (injectites) is often associated with the occurrence of large earthquakes. Injectites are most documented in thick unconsolidated sedimentary sequences and less commonly in rocks as a result of localized clastic mud overpressure due to earthquake-induced shaking. This paper documents near-surface clastic dykes injected into Middle-Late Pleistocene travertine deposits following repeated seismic events. The novelty is that there is a clear relationship between the development of the injectites and the activity of seismogenic faults that cut the entire body of the travertine, deposited at least at 45.5 ± 8.5 ka, and the topographic surface. The clastic slurry, derived from the fluidization of silty-sandy continental deposits at the base of, or intercalated with the travertine body, was in geometric connection with the faults. When this occurred, the over-pressured slurry was injected into the fault zone, filling the fractures and assisting their development by over-pressure hydrofracturing and counteracting the frictional forces on the fault. This evidence, in addition to documenting another case of liquefaction and associated emplacement of clastic dykes, has a twofold significance: i) it records the effects of clastic dykes intruded at very shallow depths, up to the surface, within a capable fault zone; ii) it documents a polyphase injection of clastic material into travertine deposits, never before described, induced by repeated liquefaction phenomena following major seismic events, thus highlighting and reinforcing the importance of travertine for the study of active tectonics and earthquake studies.