MINERALIZED VEINS RESULTING FROM FLUID-FLOW IN DECOLLEMENT ZONES OF THE SICILIAN PRISM - EVIDENCE FROM FLUID INCLUSIONS

Imbrications took place at the frontal part of the Sicilian accretionary prism during the Plio-Quaternary. The structural setting shows a superposition of southward directed thrust-sheets bounded by two decollement levels. Syntectonic veins were sampled in thrust fault zones located at the Frontal part of the prism. In the lower thrust fault zone, hydraulic breccias include quartz and calcite veins. The initial crystallisation fluids have been entrapped in these veins and preserved as different generations of fluid inclusions. Primary intracrystalline aqueous fluid inclusions are composed of low salinity water (near 0.8 wt % equ.NaCl) depleted in gas and display homogenisation temperatures increasing from quartz to calcite to values up to 200 degrees C. In calcite veins, hydrocarbon fluid inclusions containing light aliphatic oil indicate the occurrence of organic matter in the tectonic wedge with a thermal evolution comprised within the oil window. The pressure required is the lithostatic pressure, in agreement with the conditions implied by the tectonic setting accounting for the formation of hydraulic breccias. It ranges from 140 degrees C-0.8kbar for quartz up to 235 degrees C-1kbar for calcite deposition. Intersection of oil and aqueous solution isochores lead to comparable values. In addition the occurrence of pyrobitumen particles in the veins indicates that the fluids reached temperatures higher than 150 degrees C. These high temperature fluids are localized in the lower fault zone. In the upper fault zone, syntectonic calcite veins only display low temperature fluid inclusions. The results of X-ray studies on the foliated host-rock of the veins provide evidence for a highly localized incipient thermal transformation of the clay minerals (I/S of R(3) type) at the contact with the hydraulic breccia, while the overall mass of the surrounding shales remained unaffected (I/S of R(0) type). The strict localization of the veins in the fault zones suggest that dewatering of the Sicilian wedge occurred along high-permeability decollements and faults. In the lower fault zone, warm fluid circulation of fresh water was responsible for the hydraulic fracturing and the subsequent vein formation restricted to the basal decollement, where a thermal desequilibrium occurred between the fluids and the shale matrix. This may be interpreted as the result of large-scale advection of fluids arising from the deeper internal zone of the prism and flowing along the basal decollement.