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3D Velocity Model of the Crust and Uppermost Mantle in the Area of the Dinarides and Southwestern Pannonian Basin

The study area represents the boundary zone between the African and European plate i.e., the contact between the Adriatic microplate as part of the African plate and the Pannonian basin as part of the European plate. Recent geophysical efforts greatly contributed to the clarification of the crustal and lithospheric structure in this region. ŠUMANOVAC et al. (2017) imaged a fast velocity anomaly extending underneath the entire Dinaridic mountain belt, whereas KAPURALIĆ et al. (2019) reported first crustal high-resolution 3D velocity model in the northern Dinarides. This investigation is a continuation of geophysical studies focused on the Dinarides and its adjacent areas. In this study, Local Earthquake Tomography method (LET) was used in order to advance our understanding of the crustal structure and its relationship to the upper mantle in the contact area between the Adriatic microplate and European plate. P-wave travel-times are calculated from earthquakes, which were recorded by temporary and permanent seismic stations placed in the survey area. Phase arrivals (Pg and Pn phases) were manually picked. The used method employs the multi-stage fast marching method (FMM) as the grid-based forward modelling eikonal solver to predict the travel-times (SETHIAN & POPOVICI, 1999). The non-linear inversion problem is solved by iteratively adjusting model parameters in order to reconcile the objective function (e.g., velocity, interface depth, and/or source location) and travel-time perturbations (i.e., achieved with a subspace inversion scheme ; see KENNETT et al., 1988). The study resulted in a new crustal and upper mantle three-dimensional (3D) P-wave velocity model, which provides new insights on the deep geologic features. The inverted velocity model shows that the crust under the Dinarides is characterized by relatively stronger lateral and vertical velocity changes when compared to the crust in the Pannonian basin area. The velocity model reveals crustal thickening beneath the Dinarides and significant crustal thinning beneath the Pannonian basin. The most reliable feature in the model concerns the structure of the Mohorovičić discontinuity. The Moho shape can be determined in vertical cross sections based on the highest vertical velocity gradient in the lower crust. Tomography model indicates that the Moho flanks could be steep and with a sudden increase in depth on both sides of the Dinarides. There is a deep low- velocity zone beneath the Dinarides (Figure 1), which extends to a depth of more than 50 km and has characteristic NW–SE trending. The pronounced low-velocity anomaly (Vp≈ 7 km/s) is surrounded by higher velocity of about 8 km/s that is a typical velocity in the uppermost mantle. This zone could be interpreted as the fragmentation in the uppermost mantle. The low- velocity anomaly in the upper mantle correlates well with the Moho fragmentation determined by the geometrical relationships (Figure 1). In the map of the Mohorovičić discontinuity obtained by gravity modelling, the fragmentation is interpreted based on the asymmetry of the flanks at the Moho below the Dinarides (ŠUMANOVAC, 2010). High-velocity and low-velocity alteration in the narrow area below the Dinarides could be the first geophysical evidence of the contact of two upper mantles in the survey area. According to the velocity pattern, the contact zone can be located on the NE flank of the Dinarides.

Dinarides ; SW Pannonian Basin ; Travel-time tomography ; 3D velocity model ; Upper mantle structure

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