Successive episodes of normal faulting and fracturing resulting from progressive extension during the uplift of the Holy Cross Mountains, Poland

Conjugate normal faults, extension fractures and mesh-fracture structures were investigated in Devonian carbonate rocks from the southern part of the Holy Cross Mountains (HCM) (Central Poland). Strata folded during Variscan deformations were later subject to uplift, resulting in increasing extension in the upper part of the rock mass. At a relatively shallow depth, faults and fractures developed in an orderly vertical succession. First, mesh fracture structures and conjugate normal fault sets enclosing acute dihedral angles (2Θ) of over 45° were formed. Next, conjugate normal faults with 2Θ less than 45° and sub-vertical extension fractures developed. The occurrences of conjugate normal sets enclosing different dihedral acute angles and extension fractures with similar strikes, juxtaposed with each other at the same stratigraphic level, point to the fact that the uplifted rock mass underwent successive changes in a stress regime leading to the formation of these structures. The first sets of conjugate normal faults and fractures developed when the HCM were uplifted during a late stage of Variscan deformations. The next sets of extension fractures and conjugate normal faults developed during the following uplift events interrupted by periods of sedimentation of the Mesozoic and younger strata.