Mohorovičić Discontinuity at the Contact of the Adriatic Microplate and Pannonian Segment

Two-dimensional seismic and gravity modellings were applied to the contact of the Dinarides and the Pannonian basin as a relation between the Adriatic microplate and Pannonian segment, as a contribution to solving structural and tectonic relations. The basic exploration was carried out on the profile Alp07, which was a part of the ALP 2002 experiment (Brückl et al., 2003). This profile stretches in Croatia from Istra to the Drava River at Hungarian-Croatian border in a WSW-ENE direction. Two-dimensional seismic modelling was carried out using tomographic inversion and ray tracing technique. The Moho depth is the greatest in the area of the Dinarides, reaching about 40 km, and is shallowest (30 to 20 km) in the Pannonian basin. In order to obtain additional constraints on the crustal structure, 2 D gravity modelling was also performed. The layer boundaries were retained from the seismic model and varying of densities in the model produced a good fit to the data. A geological model was constructed based on both geophysical models (Šumanovac et al., 2009). Three types of the crust were found along the profile: the Dinaridic and the Pannonian crusts that are separated by a relatively wide Transition zone. The Dinaridic upper crust is characterised by low seismic velocities and densities, but its lower crust has high velocities and densities. The Pannonian crust can be seen as unique layer characterised by both low seismic velocities and densities. Large lateral and vertical changes in densities and seismic velocities can be found in the Transition zone. Gravity modelling on the Alp07 profile enabled a definition of calibrated rock densities that were applied on other profiles set up in the study area, generally covering the area of Croatia and Bosnia-Herzegovina, for the purpose of determining the structural relations. This significantly improved the resolution of gravity modelling, as well. The structural units determined on the Alp07 profile (Pannonian crust, Transition zone and Dinaridic crust) can be easily followed on all other profiles. Transition zone is very wide and covers the major part of the SW part of the Pannonian basin. It can be concluded that it corresponds with the ophiolite zones in more recent tectonic maps. The Tisia block can be compared with the Pannonian crust, whereas the Dinaridic crust would relate to the structural units of the Dinarides. However, the NE boundary of the Transition zone is located much further north than the boundary of the Tisia block in published tectonic maps. Due to a marked 2-D structure, the obtained results enabled the development of a 3-D structural map of the Mohorovičić discontinuity. The main fault in the upper mantle, which was defined on the basis of geometrical relations and a 3-D presentation of the interface, indicates a subduction of the Adriatic microplate beneath the Pannonian segment in the NE wing of the Dinarides. Brückl, E., et al., 2003. ALP 2002 Seismic Experiment. Studia Geophysica et Geodaetica, 47, 671-679. Šumanovac, F., et al., 2009. Crustal structure at the contact of the Dinarides and Pannonian basin based on 2-D seismic and gravity interpretation of the Alp07 profile in the ALP2002 experiment, Geophys. J. Int., 179, doi:10.1111/j.l365-246X.2009.04288.x.