GNGTS 2013 - Atti del 32° Convegno Nazionale

of the Adria microplate, well documented by GPS data (i.e. Weber et al. , 2010). In the Gulf of Trieste and its immediate surroundings, no historical records of local seismic events are known, and only low magnitude events (< M 4) with both shallow and deep hypocenters have been recorded by the Sismometric Network of the OGS, operating since 1977. However, the very modest seismic record may be influenced by the low deformation rate and/or long recurrence times of earthquakes. Both the MCS and Chirp data show seismic evidence of fluid occurrences and migration in the Quaternary sediment covering the main fault zones (Fig. 2). These fluids can be related to either water or gas occurence, as the presence of both is well documented in the area. Low enthalpy waters occur within the carbonates and constitute the geothermal reservoir of the region (Calore et al. , 1995). They are well known in the coastal Friuli Plain, were they are already used as geothermal resources. To further exploit the geothermal potential, the Grado-1 well was drilled in 2008. The well encountered thermal water in fractured carbonates at 42-45° C at depths between 736 and 740 meters, and from 1040 to downhole at 1108 meters (Della Vedova et al. , 2008; Cimolino et al. , 2010). Low-entalphy (about 40°C) natural springs of sulphurous-salty-alkaline waters (Doro, 1939) are present on land close to the northeastern coast of the gulf at the Roman Baths of Monfalcone, which are exploited since ancient times. The springs are located few meters below the plain, and are coming out along faults and karstic conduits. On the southeastern coast of the Gulf, close to the town of Izola, eight submarine thermal springs occur with temperature from 22 to 30°C (Žumer, 2004). The low-frequency horizons in the carbonate sequence, seen in our multichannel seismic profiles, are usually considered as an indication of the presence of fluids. They may indicate the occurrences of salty low-enthalpy waters, belonging to the same reservoir system that was drilled in the Grado-1 well. The Izola hot springs are located close to the crest of the Izola anticline. This geological setting provides suitable conditions for the rising of the thermal water from the reservoir to the sea floor. Fluid seepings from the sea floor, mainly related to gas, are also quite frequent in the Gulf. They are well known to the local population in the Northern Adriatic, as they favour the development of carbonatic concretions that form up to a few meters of relief on the seabed and are called trezze or grebeni by local population (Gordini et al. , 2004, 2010). High-resolution seismic data show the presence of biogenic gas pockets in surface sediments, which chemical analyses revealed that the gas consists primarily of methane (81-84%), nitrogen (15-18%) and oxygen (0.7-1.3%) (Gordini et al. , 2004, 2010). In multichannel seismic profiles, up to several hundred metres wide areas with very low amplitude signals occur in the Quaternary sedimentary sequence. These low-amplitude areas may indicate the presence of fluids within the sediment. In Chirp profiles, fluid accumulation zones in the late Pleistocene sediments are characterized by transparent acoustic facies. Accumulations of fluids are usually associated with major fault zones occurring in the underlying pre-Quaternary carbonate and turbidite sequence, especially with the faults that indicate Quaternary activity. We therefore speculate that the origin of accumulations is related to fluid migration along the main fracture/fault zones. Fluids can also rise up to the sea floor as plumes in the water column that were recorded in the Chirp profiles. Fluid seepings produce small relief forms on the seabed, such as the abovementioned trezze or tegnue , but also small mud volcanoes and small pockmarks a few meters in diameter. As already documented by Gordini et al. (2004, 2010) these resiliences are constituted mainly of methane of biogenic origin. The fracture systems may therefore facilitate the migration of both deep and shallow fluids, that constitute of low enthalpy thermal waters in carbonates, as well as of gas, primarily methane, occurring in the Quaternary and Neogene sediments. Conclusions. Interpretation of seismic data demonstrated a polyphase tectonic evolution of the area, which started by Mesozoic rifting and deposition of a several km thick carbonate 33 GNGTS 2013 S essione 3.1