Results of shallow scientific drillings in the Upper Nysa Kłodzka Graben and the Zieleniec area, Sudetes

A set of four new research boreholes, up to 200 m deep, were accomplished in the Nysa Kłodzka Graben and adjacent area, namely: Gniewoszów PIG-1, Międzygórze PIG-1, Krosnowice PIG-1 (plus Krosnowice PIG-1bis) and Zieleniec PIG-1. All boreholes were localized close to the important “frame dislocations” which determine the border between the graben filled with Upper Cretaceous sedimentary rocks and crystalline basement building the Orlica–Śnieżnik Dome. A purpose of the drillings was to confirm the presence of supposed thrusts or reverse faults, which was postulated for some of these dislocations in the past. Out of four boreholes, Upper Cretaceous rocks were encountered below metamorphic rocks only at one, Zieleniec PIG-1, in the strongly deformed footwall of the Zieleniec thrust. The deformed footwall and lack of fault-related folds in the region cast doubts on the thrust interpretation. In four other boreholes that drilled the footwalls of other supposedly reverse faults, Upper Cretaceous rocks invariably show signs of multiple tectonic deformation, which confirms multi-stage evolution of the Nysa Kłodzka Graben, first under extensional and then under compressional regime with duly changing kinematics.