Variscan tectonics in the Holy Cross Mountains (Poland) and the role of structural inheritance during Alpine tectonics

Abstract

The present study was carried out in the Holy Cross Mountains (HCM) of south-central Poland and includes computation of palaeostresses following Angelier's method and field structural analysis. The Palaeozoic basement of the HCM comprises two tectonic units separated by the major WNW–ESE-striking Holy Cross Fault (HCF). Fold analysis indicates a N–S to NNE–SSW direction of Variscan shortening. Micro-structures and fold analysis from Upper Devonian rocks further reveal: (1) a brittle tectonic event due to a NW–SE compression preceding folding that could be related to pre-Late Carboniferous tectonics, due to block transport within the Tornquist–Teisseyre Zone (TTZ), and (2) polyphase Variscan folding comprising (a) an early stage of N–S shortening marked by north-verging ramps, (b) a main folding event and axial cleavage formation involving N–S to NNE–SSW shortening, and (c) a late stage of shortening deforming older folds and cleavage. A mainly extensional tectonic regime dominated from the Permian until the Cretaceous, during which time the HCF was reactivated as a normal fault. Large NW–SE faults bordering the Mid-Polish Trough (MPT) developed. Subsequent tectonic inversion of the MPT resulted in basin uplift (`Mid-Polish Swell', MPS). Palaeostress computations from Mesozoic strata suggest a NE–SW direction for the main Maastrichtian–Paleocene shortening phase, in addition to two minor brittle events resulting from N–S and E–W compression. Analysis of local folds in the Mesozoic cover indicates a causal relationship with the Maastrichtian–Paleocene reactivation of older faults. In particular, en-échelon folds in the Radomsko Elevation suggest a sinistral reactivation of the Palaeozoic HCF. Folds in the southwestern part of the HCM argue for reactivation in the reverse mode of a NW–SE-trending fault bordering the MPS that originated in the Mesozoic. In Palaeozoic strata, post-Variscan brittle deformation and micro-fault reactivation are attributed to the tectonic events of Maastrichtian–Paleocene age. Reactivation of N–S fractures is demonstrated in the Rzepka Quarry, where fractures in Devonian dolomites are filled by calcite, probably formed during a late Variscan event, and by younger Late Permian–Early Triassic sediments. They were subsequently reactivated as dextral strike-slip faults during the NE–SW Maastrichtian–Paleocene compression.