Brittle tectonic events in the western boundary of the East Serbian Carpatho-Balkanides: preliminary results based on structural and paleostress analyses in the Gornjak area

The Carpatho-Balkanides in the East Serbia are composed of multiple longitudinal tectonic zones characterised by various stratigraphic/lithofacial differences and very complex structural or tectonic compositions. In this paper we analyse and discuss a relatively small amount of collected data and present determination of paleostress tensors in the Gornjak area.These are preliminary results that are going to be further documented and reinterpreted by data collected in three more cross-sections in the areas of Ravanica, Kučevo and Despotovac.
   The Gornjak area represents Serbian part of the larger Saska-Gornjak unit that is considered a part of the Getic nappe. It is mostly composed of Triassic and Jurassic limestone which provide abundant evidence of Alpine ductile and brittle deformation stages. Kilometerscale folds have uniform geometry trending from north to south, which provide general trends of tectonics shortening during the oldest deformation stage. Ductile tectonic event in the Gornjak unit predominantly produced gentle to mostly open cylindrical and planar folds. Upright linear folds have almost gently plunged axis towards the N and NW.
   Well developed fault planes, often with multiple striations were observed and later statistically analysed by direct inversion method, and to a lesser extent by NDA method (where applicable). The paleostress analysis is based on high quality data of 175 faults and striation datasets and they were processed in specialized  software - Tectronics FP. Relative ages of these events were mainly indicated by superimposing fault surface kinematics indicators.
   According to preliminary analysis, four main brittle deformation phases, composed of six unique kinematics events were determined. The oldest kinematic event (D₁) indicates predominant NW-SE compression. The shape of stress ellipsoid, orientation of σ₁, σ₂, σ₃ axes (sub-vertical σ₃ axes) and stress ratio R=0.9 suggest a stress regime close to radial compression. This stress regime resulted with formation of ductile structures with axes dipping gently to N-NE. The continuation of this tectonic phase in brittle deformation conditions caused mostly reverse movements along NE-SW to ENE-WSW fault systems. During the same tectonic phase, this compressional kinematics was followed by strike-slip kinematic events. The stress ellipsoid ratio indicates increasing intensity difference acting along maximal and medium stress directions. This change caused a transition from almost radial compression to strike-slip tension. During this tectonic phase NNW-SSE to NNE-SSW sinistral faults were activated. During D₂ phase an E-W compression was exerted. A stress ratio of R=0,3 implies a pure strike-slip regime. It probably resulted in activation of dextral movements along ENE-WSW to ESE-WNW striking fault systems. The D₃ tectonic phase started with pure strike-slip events having a NE-SW oriented maximal compression axis. This kinematic act gave rise to a NE-SW striking fault system. Initially sinistral-normal obliqueslip movements were slightly changed resulting in pure sinistral strike-slip movements along the same fault system. Changes of magnitude in minimal (σ₃) and medium (σ₂) main stress
directions while retaining the maximal (σ₁) compression direction caused a change in stress regime to pure compression. The D₄ tectonic phase comprises of single kinematic event with maximal compression in N-S direction. Stress ratio and main stress axes orientations indicate a pure strike-slip regime resulting with activation of predominantly dextral movement along NNW-SSE to ENE-WSW striking fault systems.