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Lithosphere structure in the area of the Dinarides and SW part of the Pannonian Basin based on 2-D seismic and gravity modelling

Deep seismic exploration has been carried out on the profile ALP07 in the framework of the international scientific project ALP 2002-Seismic Exploration of the Alpine Lithospere, whose scientific objective is to acquire fundamental data about deep structures in crust and upper mantle, including Mohorovičić discontinuity. These data are very useful in the seismology and other deep explorations. The ALP07 profile is set up approximately perpendicular to extension of geological structures, respectively to the dinaridic direction. The profile length is 270 km and covers areas of the Dinarides and edge part of the Pannonian basin in Croatia. The profile extends from Istra, across the Krk island, city of Karlovac, beside the Zagreb and ends at the Đurđevac by border with Hungary. Four shots were set on the profile: Koromačno in Istra, Tribalj at Crikvenice, Ivanić Grad and the shot in Hungary, and 72 stations were deployed. Data quality is much higher from the shots placed in the Pannonian Basin, than from the shots placed in the area of the Dinarides. High quality data can be seen on the measured seismograms of the shots Ivanić Grad (32070) and Hungary (38070), where first and even later arrivals are very clear. The shot in Hungary provides us with the most integral data about deep crustal structures and Mohorovičić discontinuity. Two-dimensional interpretation has been carried out by the forward modelling software based on the Ray Tracing Method, which consists of two modules: ZPLOT (author Colin Zelt), and SEIS 83 (authors Vlastislav Červeny and Ivan Pšenčik). Automatic inversion by the Seismic Tomography Method has been carried out by the HOLE software (author J. A. Hole). Two-dimensional gravity modelling on the profile ALP07 has been performed by the software GM-SYS in order to reducing of the interpretation ambiguity. The aim was to define better attributes of the layers determined by the seismic modelling. However, new significance has been associated to some events on the seismic profile, so gravity modelling has been more useful than expected. Depths of the Mohorovičić discontinuity on the seismic model obtained by the forward modelling are the greatest in the area of the Dinarides (40 km). Depths in the area of the Pannonian Basin are in the range 20-30 km with the smallest value at the end of the profile. Depth changes are not smooth and uneven relief of the Mohorovičić discontinuity can be noticed. Seismic tomographic model indicates this event, as well. In the upper crust seismic velocities are generally very low, around 6 km/s, but with lateral variations. Two anomalies of higher velocities can be noticed: shallow anomaly in the beginning and deeper anomaly in the center of the profile. These anomalies can be also seen on the tomographic model. Relatively high seismic velocities characterise the lower crust in the area of the Dinarides (6.6-7.1 km/s), but in the area of the Pannonian Basin velocities are very low, around 6.0 km/s. According to the velocity distribution there is no need to divide the crust on the upper and the lower, but the crust can be considered as unique layer. This conclusion is also confirmed by the gravity modelling. Therefore two types of crust are defined: Pannonian Crust and Dinaridic Crust, which are separated by wide Transition Zone. This zone is characterised by sharp changes of seismic velocities and densities that point to tectonic anomalies and fracturing.

deep seismic refraction; lithosphere structure; Dinarides; Pannonian Basin; 2D modelling

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