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

The present study was carried out in the Holy Cross Mountains (HCM) of sout h-central Poland and includes computation of palaeostresses following Angel ier's method and field structural analysis. The Palaeozoic basement of the HCM comprises two tectonic units separated by the major WNW-ESE-striking Ho ly Cross Fault (HCF), Fold analysis indicates a N-S to NNE-SSW direction of Variscan shortening. Micro-structures and fold analysis from Upper Devonia n rocks further reveal: (1) a brittle tectonic event due to a NW-SE compres sion preceding folding that could be related to pre-late Carboniferous tect onics, due to block transport within the Tornquist-Teisseyre Zone (TTZ), an d (2) polyphase Variscan folding comprising (a) an early stage of N-S short ening marked by north-verging ramps, (b) a main folding event and axial cle avage formation involving N-S to NNE-SSW shortening, and (c) a late stage o f shortening deforming older folds and cleavage. A mainly extensional tecto nic regime dominated from the Permian until the Cretaceous, during which ti me 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 compu tations from Mesozoic strata suggest a NE-SW direction for the main Maastri chtian-Paleocene shortening phase, in addition to two minor brittle events resulting from N-S and E-W compression. Analysis of local folds in the Meso zoic cover indicates a causal relationship with the Maastrichtian-Paleocene reactivation of older faults. In particular, en-echelon folds in the Radom sko 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 Mes ozoic. In Palaeozoic strata, post-Variscan brittle deformation and micro-fa ult reactivation are attributed to the tectonic events of Maastrichtian-Pal eocene age. Reactivation of N-S fractures is demonstrated in the Rzepka Qua rry, where fractures in Devonian dolomites are filled by calcite, probably formed during a late Variscan event, and by younger Late Permian-Early Tria ssic sediments. They were subsequently reactivated as dextral strike-slip f aults during the NE-SW Maastrichtian-Paleocene compression. (C) 1999 Elsevi er Science B.V. All lights reserved.