Kinematic analysis of the Permian and Triassic Formations of the Hronic Unit (Western Carpathians, Slovakia)

Our investigation focused on the kinematic analysis and paleostress reconstruction of the brittle structures of the Hronic Unit in the Malé Karpaty Mountains and Strážovské vrchy Mountains. The Hronic nappe system represents the structurally highest tectonic unit in the Late Cretaceous thrust stack of the Central Western Carpathians. Structurally, the Hronic Unit is an internally complex nappe system with numerous partial nappes. It consists mainly of a Permian volcano-sedimentary sequence (Malé Karpaty Mts.) and Triassic carbonate sediments (Strážovské vrchy Mts.).

 Based on kinematic analyses of mesoscale faults (slickensides), several brittle deformation stages characterized by distinct properties of the reconstructed stress field were identified. We employed the Win Tensor program to calculate stresses and to separate the faults into homogeneous groups. The relative superposition of individual palaeostress states was derived from field structural relationships. The observed chronology of deformation phases can be divided into five different palaeostress fields. The evolution is characterised by a continuous change in the orientation of the maximum principal stress axis σ1, from W–E through N–S to NE–SW. The kinematic analysis of fault-slip data confirmed predominant strike-slip nature of the fault during the entire deformation history.

The oldest deformation event is characterized by a transpressional tectonic regime, in which the maximum compressional stress axis was oriented W–E. This stage includes dextral strike-slip faults trending NE–SW and sinistral strike-slip faults trending NW–SE at both localities. At the Strážovské vrchy Mts., reverse faults with an approximately N–S orientation were also documented. The second deformation stage, associated with NW–SE-directed compression, represents a direct continuation of the transpressional tectonic regime. Acting compression resulted in the development of W–E-trending dextral strike-slip faults and N–S-trending sinistral strike-slip faults. Reverse faults with NE–SW orientation were documented at the Malé Karpaty Mts. The subsequent third deformation stage includes NW–SE-trending dextral strike-slip faults and NE–SW-trending sinistral strike-slip faults at both localities. At the Malé Karpaty Mts., an additional group of WNW–ESE oriented reverse faults were identified. A transpressional tectonic regime persists during this stage, however, the orientation of the compression differs slightly between the two localities. At the Strážovské vrchy Mts., compression is oriented approximately N–S, whereas at the Malé Karpaty Mts. it is shifted eastward to an NNE–SSW orientation. The fourth deformation event represents the final stage associated with a transpressional tectonic regime. The acting compression was oriented NE–SW. Dextral strike-slip faults are N–S-trending and sinistral strike-slip faults are W–E-trending. NW–SE-oriented reverse faults were documented at both localities. However, at the Strážovské vrchy Mts., two groups of reverse faults with a paired relationship were identified. The youngest deformation event represents a change of tectonic regime from a transpressional to an extensional one. At both localities, extension oriented NNW–SSE and NE–SW was documented. In addition, WNW–ESE-oriented extension was identified at the Strážovské vrchy Mts.

Figure 1: Chronological summary of deformation events in the study areas.

Acknowledgements

This work was supported by the Slovak Research and Development Agency under the contracts APVV-22-0092.