Tectonics of the Chęciny Anticline (Holy Cross Mts., Central Poland) in the light of new cartographic data and calcite vein analysis

Abstract

The Variscan orogeny of NE–SW compression has folded the Paleozoic core of the Holy Cross Mountains (Central Poland). The Chęciny Anticline, formed during this tectonic event, is located in the southwestern part of the Kielce Unit. This paper presents structural data from two newly found outcrops of Cambrian rocks that modify the geometric reconstruction of the Chęciny Anticline, especially within its southern limb that is subdivided into two second-order structures: the Rzepka Syncline and the Wrzosy Anticline. The eastern part of the Chęciny Anticline has been reconstructed, pinpointing its fragmentation into a series of blocks (horsts and grabens) separated by faults semi-perpendicular to the anticline axis. New mapping data reveals deep, pre-Triassic erosion of folded Variscan basement, uncovering Cambrian rocks outcropping in the hinge of the Wrzosy Anticline. In the Chęciny Anticline, folding-related shortening has been accompanied by along-strike extension and the formation of syn-tectonic calcite veins, which filled fractures oriented perpendicular to the fold axis. The magnitude of extension has been estimated along a 215 m long main quarry wall of Rzepka Hill, approximately parallel to the Variscan structures and located within the southern limb of the Chęciny Anticline. The total thicknesses of veins filling extension fractures and spaces between clasts in tectonic breccia were summed and indicate 8.4% of strike-parallel extension (~120o) in the Chęciny area. This value was compared to ca. 30% of folding-induced shortening, related to NE–SW late Variscan compression. We hence obtain a strain ratio of 30:8.4 = 3.5. This high value of strain ratio indicates that longitudinal extension was a significant component contributing to the late Variscan deformation in the Kielce Unit.