Abstract The Sicilian accretionary prism shows a large, southward imbricated system of thrust sheets. The frontal part of the belt has been tectonically emplaced in Plio-Pleistocene times. Major décollement surfaces and sole thrusts at the base of and inside the prism consist of mélanges with a “blocks in matrix” fabric. In the frontal part, two superposed décollement levels are recognized, the upper one located at the base of the Paleogene-Neogene sedimentary series, and the basal one at the base of duplexes (Triassic-Miocene sedimentary series). Syntectonic dewatering of the sedimentary sections occurred along décollements and thrust faults, involving hydrofracture and mineralized vein development. Inside the deep sole thrusts (rooted in the basal décollement); hydrofractured blocks show different generations of syntectonic quartz and calcite extensional veins, while in the upper décollement only calcite veins have been observed. Both quartz and calcite veins show aqueous primary fluid inclusions of low salinity, with low trapping temperature for fluids in the upper décollement (< 60°C) and high trapping temperature for fluids in the deep sole thrusts (up to 235°C in calcite, for a pressure near 0.8–1.0 kbar). If we assume that, at the time of deformation, the main heat source was the regional conductive geothermal gradient, the trapping temperatures of the fluids inside veins of the deep sole thrusts are higher than those predicted by the Pilo-Pleistocene gradient for this subduction zone and correspond to a thermal anomaly. We suggest that the frontal Sicilian prism supported localized transient fluid flow of deep freshwater sourced from two different origins: a shallow one for fluids now trapped in the upper décollement, and a deep internal origin (6–10 km minimum depth) for the hot fluids channelized along the basal décollement and now trapped inside the sole thrust of the Mt. Scalpello duplex area.