Modern tectonic activity of Alexandrovsky graben (Lower Volga) on the re-levelling data

The article presents the results of the current tectonic mobility analysis of the Alexandrovsky graben - widely known tectonic structure in the Lower Volga region. This graben is one of the strongest occurrence of fault tectonics on the Russian Plain. Like other dislocations, it is laid on the contact of two major tectonic elements of the East European platform - the Pachelmsko-Saratov aulacogen and the Caspian syneclise. When the Volgograd Hydroelectric Power Station (1950s) was designed, the faults systems of the Lower Volga region were studied in details to identify the presence of modern movements. During that period, passivity of faults was ascertained. In the middle of the 2000s, the authors identified geomorphological features of active modern tectonic movements in the Alexandrovsky graben area. In 2009, a reference geodetic network was laid to perform long-term monitoring and to determine the rates of tectonic movements and a primary levelling and elevation alignment of reference points were carried out. The depth and method of referent points laying exclude the influence of seasonal soil processes on the high-altitude position of the geodetic base points. Re-levelling was conducted in 2016-2017. The results of the double levelling match the III (third) class. The results of levelling allow us to estimate the average velocity of vertical displacement in 0,6-1,1 mm / year. Such speeds are almost limitless for platform sections of the earth’s crust. In different parts of the graben, both upward and downward movements were detected. The obtained results indicate the presence of complex and multidirectional modern movements in a limited area of the Aleksandrovsky graben.