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

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 underex-plored. 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 part of the central-southern Apennines accretionary wedge, Italy. This melange developed by shear deformation at the base of a elastic succession. 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 syntectonic calcite veins suggest calcite precipitation in a closed system from warm (108 degrees-147 degrees C) paleofluids, with delta O-18 vlaues between +9%o and 14 parts per thousand, such as trapped pore waters after extensive O-18 exchange with the local limestone host rock and/or derived by clay dehydration processes at T > 120 degrees C. The He-3/He-4 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 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.