3D velocity depth model in the Gulf of Trieste by means of tomographic analysis from multichannel seismic reflection data

The Gulf of Trieste (GT) is a shallow epicontinental basin that is part of the northern Adriatic Sea (Central Mediterranean). It is settled at the proximal southern edge of an active plate boundary, where the convergence between the Adriatic block and the Eurasian plate gave rise to two Meso-Cenozoic Alpine-Himalayan orogens: the Dinarides (NW-SE trending) and the Eastern Alps (E-W oriented). The Gulf of Trieste is the foredeep of the northern External Dinarides and the eastern Alps. Its evolution started from a Mesozoic rifting that generated NE-SW normal faults allowing the aggradation of the Mesozoic-Paleogene Friuli-Dinaric Carbonate Platform. From the Upper Cretaceous to the Oligocene, the Dinaric compression generated NW-SE oriented thrust faults, whose most external elements are located in the GT. Since the Oligocene, the Alpine compression, related to the ongoing N-ward motion of the Adriatic plate, reactivated the tectonic structures with a transpressive kinematics. The GT represents the foot-wall of the Dinaric Karst Thrust, extending along the coast of the Karst Plateau and displacing the carbonates with an important vertical throw. The carbonates lying in the gulf are tilted E and N-wards, due to the weight of the neighbouring chains. They are covered by Eocene turbiditic Flysch, whose top is depicted by an unconformity related to the Messinian erosional phase, that in the eastern part of the gulf was active also during the Pliocene, as proved by the thin package of Quaternary marine and continental deposits covering the surface. While the geological structure of the onshore areas bounding the gulf have been studied since the 19th Century, the geological characteristics of the offshore remained widely undiscovered until recent time. During the last decade, the National Institute of Oceanography and Experimental Geophysics - OGS, carried out three geophysical surveys in the Gulf of Trieste, collecting 630 km of Multichannel Seismic (MCS) reflection profiles. These data are acquired in the time domain and provide geological information about the investigated subsurface. A focused velocity analysis, encompassing tomographic inversion and depth seismic imaging, is crucial for the reconstruction of detailed and reliable geometries in the depth domain. In the present work, two main reflectors are considered: the top surface of the flysch and that of the carbonates. Their reflected and refracted events are picked on the prestack seismic data and used as input for the traveltime tomographic inversion. The adopted software detects the three-dimensional seismic velocity field and surface geometries, through an iterative algorithm that minimizes the differences between the modelled and measured traveltimes. The code inverts in sequence the velocity field and updates the reflector structure, until the variations with respect to the previous step become sufficiently small. The obtained model is then improved using an iterative imaging technique involving pre-stack depth migration, residual move-out analysis and grid tomography. The comparison between the results of reflection tomography and refraction analysis allows to gain information on the flysch anisotropy, which can be related to its petrophysical parameters. The results provide an adequate 3D elastic velocity model in depth of the flysch and carbonates units and of their top surfaces. The top of carbonate platform lies at a maximum depth of around 1.6 km below the seabed, at about 2 km offshore the city of Trieste. This result provide that the vertical displacement of the Karst Thrust is about 1600-1800 m. The obtained information constitutes a valuable basis for the reliable construction of a 3D geological and structural depth model of the GT. This would, in turn, permit the correlation with the onshore setting and enhance the understanding of the neotectonic evolution of the area.