Buckle folding in the Kielce Unit, Holy Cross Mountains, central Poland

Abstract

Gradual changes of shape profiles of map-scale as well as mesoscopic folds, recognised as symmetric and asymmetric buckle folds, were investigated. These folds developed in the Holy Cross Mountains in Palaeozoic unmetamorphosed sedimentary rocks during Variscan deformation. During folding, layerparallel shortening prevailed, although a layer-parallel shear component, resulting from deformation of multilayers by compression at low angles to the layering, has also been recognized. Different contraction and extension fault sets, as well as cleavage, small-scale duplexes, boudinage and stylolites, developed progressively on both gradually steepening limbs of the symmetric folds, as well as mainly on the short limbs of the asymmetric folds. Based on the comparison of  laboratory data for different types of rocks, the range of values of horizontal compressive stress levels occurring during fold growth has been estimated. The stress levels for the high pore fluid pressures probably did not exceed 150 MPa. These conditions and temperatures below 150°C favoured buckle folding. In the late phase of the folding or in the post-folding stage of Variscan deformation, the separation of the Kielce Unit into fault-bounded block domains was initiated along longitudinal and transverse faults. This resulted in changes in trend of parts of the map-scale folds.