The role of trapped fluids during the development and deformation of a carbonate/shale intra-wedge tectonic mélange (Mt. Massico, Southern Apennines, Italy)

Abstract: Numerous studies exist on exhumed tectonic melanges along subduction channels whereas, in accretionary wedge interiors, deformation mechanisms and related fluid circulation in tectonic melanges are still underexplored. We combine structural and microstructural observations with geochemical (stable and clumped isotopes and isotope composition of noble gases in fluid inclusions of calcite veins) and U-Pb geochronological data to define deformation mechanisms and syn-tectonic fluid circulation within the Mt. Massico intra-wedge tectonic melange, located in the inner (western) part of the central-southern Apennines accretionary wedge, Italy. This melange developed at the base of a clastic succession, and shear deformation was characterized by disruption of the primary bedding, mixing, and deformation of relicts of competent olistoliths and strata within a weak matrix of deformed clayey and marly interbeds. Recurrent cycles of mutually overprinting fracturing/veining and pressure-solution processes generated a block-in-matrix texture. The geochemical signatures of syn-tectonic calcite veins suggest calcite precipitation in a closed system from warm (108°-147 °C) paleofluids, with δ18O vlaues between +9‰ and 14‰, such as trapped pore waters from diagenesis after extensive 18O exchange with the local limestone host rock and/or derived by clay dehydration processes at T > 120 °C. The 3He/4He ratios in fluid inclusions are lower than 0.1 Ra, indicating that He was exclusively sourced from the crust. We conclude that: (1) intraformational rheological contrasts, inherited trapped fluids, and low-permeability barriers such as clay-rich stylolites and marly-shaly matrix, can promote the generation of intra-wedge tectonic melanges and the development of transient fluid overpressure; (2) clay-rich tectonic melanges, developed along intra-wedge decollement layers, may generate low-permeability barriers hindering the fluid redistribution within accretionary wedges.