Are the Carpathians tectonically active?: Geomechanical study in deep boreholes in the outer Carpathians (Poland)

Abstract

Present-day tectonic stress state was investigated in three deep boreholes located in the eastern segment of the Polish Outer Carpathians (POC). Significant rotations of the maximum horizontal stress (S_H) were observed in these boreholes, located at the hinge of the anticlines in the upper part of the nappes. For the deepest borehole, D-1 (5.5 km depth), 1D geomechanical modelling was performed to determine the stress gradient profiles. An optimal solution of the model, validated by numerous compressional and extensional failures (breakouts and drilling-induced fractures, respectively) of the borehole wall, was obtained for variable elastic horizontal strain. The strain varies stepwise across the Main Thrust Fault (MTF) and linearly within its walls. The dominance of a strike-slip faulting stress regime was determined for the Carpathian nappes, with contributions from thrust faulting above the MTF and normal faulting below the MTF. A critical stress state for reactivation of preferentially oriented pre-existing faults and fractures was inferred for the competent strata. A consistent interpretation of the variations in stress orientation and magnitude, suggests a contemporary refolding of the anticline at a shallower structural level, enhanced by the reactivation of the MTF and a lack of reactivation of the Carpathian Bottom Thrust. Integration of these results with measurements from previous studies in the eastern segment of the POC indicates a different regional orientation of S_H in the autochthonous basement (N-S) and in the nappes (NE-SW). These results indicate a thin-skinned compressive reactivation of the upper part of the accretionary wedge, with the lower part of the nappes remaining passive, or locally prone to minor strike-slip or normal faulting. These results contradict the hypothesis of a contemporaneous extensional collapse of the POC.