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Subduction of the Adriatic microplate under the Pannonian segment based on new seismic and gravity explorations

The transition area from Dinarides to the Pannonian basin was, in the last decade, covered by several experiments in order to obtain the structure of the lithosphere and relationship between the Adriatic microplate and the Pannonian segment. The beginning of a new research was within the international wide-angle refraction and reflection project ALP 2002. This active-source seismic experiment was followed by the ALPASS-DIPS project (Alpine Lithosphere and Upper Mantel PASsive Seismic Monitoring-DInarides Pannonian Segment). The project is passive seismic survey which uses natural earthquakes in order to determine structure of the Earth's crust and upper mantle. Basic exploration had been carried out on the profile Alp07 stretching from Koromačno in Istra over the Dinarides to the Hungarian Croatian border in a WSW–ENE direction. It is oriented almost perpendicularly to the Dinarides and to main faults in this region. Tectonically, the profile stretches from the edge of Adriatic microplate through the northern part of the Dinarides, crosses wide ophiolite zone (Dinaridic ophiolite zone and Sava Vardar zone) and terminates in the Pannonian basin at the eastern part of the Tisia block. The data within ALPASS-DIPS project were gathered on 15 temporary stations in Croatia and 2 stations in southern Hungary. Most of the stations were deployed along the profile Alp07 where the velocity model has already been defined. Recorded data have been processed using the receiver function method, based on the converted P to S phase. Gravity modelling on the Alp07 profile enabled a definition of calibrated rock densities for different structural units. The study area has been extended, covering the area of Croatia, Bosnia and Herzegovina and southern parts of Hungary, in order to obtain structural relations in wider area. Thereafter five more profiles were set up, where 2-D gravity modelling was carried out (ŠUMANOVAC, 2010). Seismic modelling, both inverse and forward, was performed on the Alp07 profile (ŠUMANOVAC et al, 2009). Following seismic modelling, 2-D gravity modelling was used to determine densities in the lithosphere. On the basis of both data sets, three types of crust were defined along the profile: the Dinaridic and the Pannonian crusts that are separated by a Transition zone. The Dinaridic upper crust is characterized by low seismic velocities and low densities, while the lower crust shows high seismic velocities and densities. The Pannonian crust can be seen as single layered, characterized by low seismic velocities and low densities. The strongest lateral changes in seismic velocities and densities were found on the margin of the Pannonian basin and in the Sava depression area, where a wide Transition zone was defined. Calibrated densities were used in gravity modelling of the wider area. Structural units defined on the Alp07 profile can be followed on other profiles. Since the structure covered by the profiles is two-dimensional, the obtained results enabled the construction of 3-D Mohorovičić discontinuity map. The Mohorovičić discontinuity depth is the greatest in the Dinarides root. On the Alp07 profile, located in the NW part of the study area, the depth is about 40 km and increases to the SE where it reaches about 46 km. The subsidence of the Moho is particularly marked on the north side of the Dinarides, at the contact with the Pannonian basin, where, based only on structure geometry, subduction is assumed. The shallowest Mohorovičić discontinuity is located in the NE part of the study area (the Pannonian basin) at depth less than 20 km. Based on receiver functions analysis, there are three types of seismograms that can be derived: Dinaridic, Transitional and Pannonian. Pannonian type can be observed at station CBP4M which belongs to the Pannonian crust segment. Transitional type can be seen at station Cro_07 because it is located in a transitional zone, while Dinaridic type can be observed at station Cro_03, and belongs to Dinaridic crust. Three major lithospheric discontinuities can be defined at the Dinaridic type and the Transitional type, while the Pannonian type reveals only two discontinuities. The shallowest discontinuity observed at station CBP4M, can be defined as a sedimentary basement, and because of strong acoustic impedance contrast, the amplitude is very high. Deeper discontinuity appears at about 4 s, and is also characterized with high amplitude. However, it can be interpreted as the Mohorovičić discontinuity, because high amplitude is most likely the result of velocity contrast between crust and upper mantle. Analysis shows that there are no more discontinuities between them in the Pannonian type. Therefore, based on active seismic data analysis and gravity modelling, this corresponds very well with the unique-layer interpretation of Pannonian crust (ŠUMANOVAC et al., 2009). Receiver functions that were calculated for stations under the Dinarides and in the Transition zone showed two-layered crust, which was also a confirmation of earlier active-source seismic experiment and gravity modelling. From receiver functions analysis Moho depth along Alp07 profile is the greatest under the Dinarides, and is shallowest in the Pannonian basin.

Dinarides; Pannonian basin; gravity modelling; receiver functions

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