Preliminary geochemical and isotopic data on springs along the Fella-Sava fault zone (NE Italy)

During 2006, a geochemical survey was carried out on three perennial springs discharging along a segment of the Fella-Sava line, a transpressive lineament running in the eastern Southern Alps (Friuli, NE Italy). Waters have both a Ca-bicarbonate and a CaMg(Na)-HCO3SO4 sulfureous character. Oxygen and hydrogen isotopic data indicate a meteoric origin from catchments with different mean topographic elevation. The Sr isotopic composition of the Ca-bicarbonate water is consistent with circulation in Middle Triassic limestones; sulfureous waters have 87Sr/86Sr isotopic ratios which indicate interactions with the Upper Permian dolomite-evaporite layers, the latter representing the preferential decollement horizons during the compressive Alpine cycle. The isotopic data, the negative redox potential and the tritium activity for sulfureous springs suggest a long-lasting flow through different fracture systems. Limited effects of mixing between the deeper and superficial, low-salinity waters are observed. Speciation calculations indicate that sulfureous waters are oversaturated in calcite and silicate minerals, suggesting that cement deposition is an active process along the flowpaths. The formations of diagenetic seals in a compressive regime would contribute to fluid overpressuring in pores at a relatively shallow depth, where part of the seismic activity is concentrated. The SO4, Ca and Mg concentrations, in one of the sulfureous springs, show a marked increase which seems to be related to the seismic events in the area, and is attributed to the expulsion of pore fluids with high concentration of sulphate due to strain changes.