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Characterization of the contact between Adriatic and Pannonian lithosphere based on the 3D velocity model from local earthquake tomography

The study area represents the contact between the Adriatic microplate as part of the African plate and the Pannonian basin as part of the Eurasian plate. This investigation is a continuation of the geophysical studies focused on the Dinarides and its adjacent areas. So far, several structural models based on 3D geophysical models of the upper mantle have been proposed, while there was no complete high-resolution geophysical model of the crust and the transition zone to the upper mantle. Our goal was to provide the first high-resolution 3D velocity model of the crust and the uppermost mantle in the broader Dinarides region. We applied the method of Local Earthquake Tomography (LET). The data sets of P-wave propagation times, calculated from earthquakes recorded by temporary and permanent seismic stations in the study area, were inverted to obtain a three-dimensional P-wave velocity model. The new geophysical model provided the best resolution of the uppermost mantle yet achieved in this study area and in a model of this scale. We assumed that the model would better reveal the structures of the crust. However, our model revealed features at the level of the lowermost crust and upper mantle that had previously been considered only on the basis of geologic relationships at the surface or structures within the crust projected to the level of the mantle. To date, there's no structural model of the crust- mantle transition based on 3D geophysical data that directly samples the lower crust and upper mantle. The resolution documented by the resolution tests is less than a hundred kilometers in the horizontal direction and a few kilometers in the vertical direction in the area with good ray coverage. The most reliable feature of the model concerns the structure of the lower crust and uppermost mantle. The model confirms previous findings of crustal thickening beneath the Dinarides and crustal thinning beneath the Pannonian Basin. The Moho shape is based on the highest vertical velocity gradient in the lower crust, and the depth of the boundary corresponds to the velocity surface of about 7 km/s. Under the Dinarides, there is a deep low velocity zone that extends to depths greater than 55 km and has a characteristic NW–SE trend. The pronounced low velocity anomaly (Vp less than 7.5 km/s) is surrounded by higher velocities typical of the uppermost mantle. The deep, low velocity anomaly occurring in the uppermost mantle and traced along the Dinarides can be explained by two models: a broad shear zone at the contact between two mantles or a subsided part of the lower Adriatic crust beneath the Pannonian lithospheric mantle. Therefore, we performed two- dimensional forward gravimetric modeling to reduce the ambiguity of interpretation. The anomaly is interpreted as fragmentation in the uppermost mantle. The high and low velocity alteration in the narrow region below the Dinarides may be the first geophysical evidence of contact between the Adriatic and Pannonian mantles. According to the velocity pattern, the contact zone can be located at the NE flank of the Dinarides.

Adriatic and Pannonian lithosphere ; 3D velocity model ; local earthquake tomography

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