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Geophysical models at the contact of the Dinarides and Pannonian basin

Available gravity data, along with the new seismic data gathered from 2002, enabled developing of geophysical models of crust and upper mantle at the contact of the Dinarides and south-western part of the Pannonian basin. The study area is located in the boundary zone of the Adriatic microplate as part of the African plate and Pannonian segment as part of the European plate. Seismic data are available from two large international projects: wide-angle refraction and reflection experiment ALP 2002 – Seismic Exploration of the Alpine Lithosphere, and passive seismic experiment ALPASS DIPS (Alpine Lithosphere and Upper Mantel PASsive Seismic Monitoring-DInarides Pannonian Segment). Basic exploration was carried out on the profile Alp07 stretching from the edge of Adriatic microplate (Istra) through the northern part of the Dinarides, crossing wide ophiolite zone (Dinaridic ophiolite zone and Sava Vardar zone) and terminating in the Pannonian basin at the eastern part of the Tisia block. Direction of this 300 km long profile is almost perpendicular to the Dinarides. Velocity model was obtained from the active-source seismic data by inversion tomography and forward modelling using ray tracing method. Based on the velocity model, 2 D gravity modelling was performed on the profile, in order to determine lithosphere densities. The data gathered during passive seismic experiment were analysed by receiver function method and used to define velocity discontinuities in the crust and upper mantle. Since the profile Alp07 is located in the marginal part of the Dinarides, gravity modelling enabled extension of the study area to the central part of the Dinarides. Five gravity profiles were set up southeast from the Alp07 profile, covering the area of Croatia, Bosnia and Herzegovina and southern parts of Hungary. Structural units defined on the Alp07 profile on the basis of both models, velocity and density, can be followed in wider area. Calibrated densities, defined on the Alp07, enabled more precise gravity modelling on the other profiles. Density models show the greatest thickness of crust under the Dinarides, and thinning of the crust towards the Pannonian basin. Two-layered crust is observed under the Dinarides, as well as in the marginal part of the SW Pannonian basin, but under the Pannonian basin, crust can be considered as single-layered. Whereas the structure covered by the profiles is two-dimensional, the obtained results enabled the construction of structural map of the Moho and its three-dimensional image. It shows the greatest depth of the Mohorovičić discontinuity in the Dinarides root. 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 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. Structural Moho map of the area can be very helpful in planning future seismic experiments in the area. Density calibration was carried out on the profile Alp07, which is located at the edge of the contact, and structural map has been made assuming there are no lateral changes in densities. However, if stronger lateral changes are present, it can lead to significant changes in gravity model. The depth of interfaces, especially Mohorovičić discontinuity, as well as position of structural units can be modified. Therefore, it is necessary to set at least one profile across the central part of the contact of the Adriatic microplate and Pannonian segment. The future step will be setting a profile with dense deployment of broadband seismic stations, especially in the area of expected subduction. The data will be interpreted by high-resolution migration techniques to map interfaces and to obtain a physical evidence of the subduction. High-resolution migration techniques use scattered teleseismic waves. Waves are represented by a diffracted wavefield, backprojected to depth. Because the diffracted wavefield is caused by small scale-length perturbations, the position and depth of these smaller subsurface units can be easily estimated.

gravity modelling; seismic modelling; Dinarides; Pannonian Basin

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