ADVECTION OF FLUIDS AT THE FRONT OF THE SICILIAN NEOGENE SUBDUCTION COMPLEX

The Sicilian accretionary prism shows a large, southward imbricated sy stem of thrust sheets, The frontal part of the belt has been tectonica lly emplaced in Plio-Pleistocene times, Major decollement surfaces and sole thrusts at the base of and inside the prism consist of melanges with a ''blocks in matrix'' fabric. In the frontal part, two superpose d decollement 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 d ewatering of the sedimentary sections occurred along decollements and thrust faults, involving hydrofracture and mineralized vein developmen t.Inside the deep sole thrusts (rooted in the basal decollement); hydr ofractured blocks show different generations of syntectonic quartz and calcite extensional veins, while in the upper decollement only calcit e veins have been observed. Both quartz and calcite veins show aqueous primary fluid inclusions of low salinity, with low trapping temperatu re for fluids in the upper decollement (< 60 degrees C) and high trapp ing temperature for fluids in the deep sole thrusts (up to 235 degrees C in calcite, for a pressure near 0.8-1.0 kbar). If we assume that, a t the time of deformation, the main heat source was the regional condu ctive geothermal gradient, the trapping temperatures of the fluids ins ide veins of the deep sole thrusts are higher than those predicted by the Plio-Pleistocene gradient for this subduction zone and correspond to a thermal anomaly. We suggest that the frontal Sicilian prism suppo rted localized transient fluid flow of deep freshwater sourced from tw o different origins: a shallow one for fluids now trapped in the upper decollement, and a deep internal origin (6-10 km minimum depth) for t he hot fluids channelized along the basal decollement and now trapped inside the sole thrust of the Mt. Scalpello duplex area.