Stratigraphy and deformation of Pleistocene talus in relation to a normal fault zone (central Apennines, Italy)

Abstract

In studies of neotectonism, alluvial-fan and talus deposits commonly are used as deformation markers, but rarely are studied themselves and in relation to adjacent faults. Herein we report on the facies, diagenesis and deformation of a talus succession and fault cataclasites in the central Apennines of Italy. The study site is located near the western end of the Assergi normal fault zone that accommodates >2000 m of vertical throw, but was dormant since a longer interval of time.The preserved talus succession is confined to the fault hangingwall. Deposition and deformation of the talus overlapped with the terminal phase of fault activity. The talus accumulated mainly from grain flows, cohesive debris flows and ephemeral fluid flows; it comprises (i) two superposed units of scree breccias, partly cemented before deformation, and intercalated with (ii) an interval of unlithified scree and soils. The exposed succession accumulated between ≥33–30 cal ka BP to less than ~22 cal ka BP. Talus breccias record complex diagenetic successions including eluviation/dissolution of primary matrix, growth of interstitial cements interrupted by episodes of dissolution and/or fracturation, and late-stage dissolution porosity development. Downthrow by faulting produced two types of folds, (i) a syncline-anticline pair with inclined fold axes roughly normal to the fault plane, and (ii) a recumbent fold with an axis subparallel to depositional strike of the scree slope; the complicated stratigraphic architecture of the deformed succession is truncated along and hidden beneath the topsoil of the present, uniform slope surface. Further deformation structures include conjugate fractures locally coated by speleothem flowstones, deformation bands, sediment fabrics characterized by planar clast contacts, and lithoclasts crushed in situ while embedded in the sediment. Cementation overlapped with and post-dated in-situ crushing of clasts.The core of the normal fault is an ultracataclasite that (i) shows different degrees of diagenetic recrystallization into micro- to pseudospar, and (ii) that is riddled with solution pores fringed or filled with successive ‘generations’ of calcite cement. These cements are locally sharply capped along discrete levels, and overlain by ultracataclasite with floating chunks of calcite cement crystals ripped off during increments of faulting. Stable isotopes of oxygen and carbon indicate that the ultracataclasites lithified under influence of meteoric waters, but at variable degrees of rock buffering; the calcite cements within the talus breccias and the flowstones along fractures, in turn, precipitated from meteoric-derived waters with low to negligible rock buffering. The structural juxtaposition of ultracataclasites (probably formed in 1–2 km depth) with talus breccias indicates that the preserved fault/talus ensemble records only the terminal phase of total faulting. The interstitial cements in talus, the flowstones on fracture walls, and the cements in ultracataclasites should be datable with the 234U/230Th errorchron method. This promises to be a new approach to derive age constraints on talus cementation and fracturation, and on palaeoactivity and final dormance of normal faults.