The offshore geology of the Gulf of Trieste, in the border area between Italy and Slovenia, has been investigated in several geophysical campaigns in the years 2005, 2009, and 2013, during which nearly 700 km of multichannel seismic profiles and chirp data were acquired. 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 succession, dissected by normal faults. During Cenozoic orogeny, the Gulf area represented the frontal part of the SW-ward advancing Dinaric thrust system, and was initially buried by foredeep flysch sediments of Eocene age, then dissected by thrusts in the final stage of Dinaric deformation. In Neogene, continuing convergence of the Adriatic microplate towards Eurasia produced transpressional reactivation of the faults, which might be continuing into the present time. Additionally, field investigations in Slovenian Istria suggest that a part of the ongoing Adria-Eurasia convergence may be absorbed by renewed thrusting in the coastal thrustbelt. High-resolution sub-bottom profiling has revealed that tectonic deformation is affecting the Quaternary marine and continental sediments in the Gulf of Trieste. Some of the observed faults and thrusts may therefore be active and seismogenic, although major earthquakes are not known from the historical record, and only low magnitude and shallow events have been recorded, perhaps because of the low deformation rate and long recurrence times of earthquakes. We found seismic evidence for the presence of fluids in the Mesozoic carbonates as well as in the Quaternary sediments. The fault zones are the preferential way of fluid migration upwards to the sea floor. Chirp data show fluid accumulation zones in the late Pleistocene sediments, with seepings providing positive sea floor morphologies, like little mud volcanoes and/or carbonate concretions, pockmarks, and plumes in the water column. Seepings of gas, mainly biogenic methane from Pleistocene sediment, and of low enthalpy water occurring from the carbonates, are well-known by the local population and were also recorded during the drilling of the Grado-1 well.
COBISS.SI-ID: 1113694
In the SLOMARTEC 2013 campaign with research vessel R/V OGS Explora we acquired 132 km of multichannel seismic reflection profiles and 150 km of Chirp subbottom profiles. The survey covers the Slovenian part of the Gulf of Trieste and thus complements earlier multichannel surveys in the Italian part of the Gulf as well as the onshore structural investigations in Slovenian Istria. Three principal stratigraphic units can be observed in seismic sections: subhorizontal sediments of Quaternary age, which onlap an erosional surface on top of Eocene turbiditic sequence, followed by weakly-bedded Mesozoic carbonates. Subsurface structure clearly indicates polyphase tectonic evolution. Dinaric thrusting produced a series of SW-verging thrusts and folds. These structures are offset by later suvertical faults, which may partly be reactivated mesozoic normal faults, and partly neoformed neotectonic faults. Some of the faults can be traced into Quaternary strata, which could suggests their recent activity and therefore a potential seismic hazard for the densely populated and industrialized coast of the Gulf of Trieste.
COBISS.SI-ID: 1112670
The Istria Peninsula in the northern Adriatic Sea (Croatia and Slovenia) is considered to be a nearly aseismic part of the Adriatic microplate, as opposed to the seismically active frontal ranges of the highly active Dinaric orogen. New archaeoseismological data from the Medieval Eufrasius cathedral in Poreč on the west coast of Istria demonstrate two, previously unknown major earthquakes, which occured approx. a millennium apart. In our interpretation of the seismic cycle, Istria is slowly raised, about 1-2 m in a millennium, during stress accumulation caused by a locked fault plane. Stress release produces sudden subsidence of the coast, which is recorded by subsided coastal features (marine notch, terrestrial sediments, cathedral of Poreč). We speculate that the thrust fault responsible for the earthquakes lies below the 2-5 km thick Triassic-Cretaceous carbonate platform sequence. These observations challenge the current notion of Istria as the region of low seismic hazard and invite further paleoseismological research.
COBISS.SI-ID: 1113950