Ongoing tectonic reactivation of the Outer Carpathians and its impact on the foreland: Results of borehole breakout measurements in Poland

Abstract

The direction of present-day maximum horizontal compression (= S Hmax = tectonic stress) was interpreted for 62 wells in Poland, using the method of borehole breakout analysis of 4-arm and 6-arm dipmeter logs. The study area covers complex tectonic junction of the Carpathian orogen and its foreland, which comprises the East European Craton (EEC) divided by the Teisseyre–Tornquist zone (TTZ) from the Palaeozoic platform of western Poland. For this area, frequent deviation of the S Hmax direction from NW–SE characteristic for the Atlantic ridge push has been interpreted in terms of the ALCAPA tectonic push. In the Upper Silesian segment of the Polish Outer Carpathians (POC), NNE–SSW-oriented S Hmax in the accretionary wedge differs significantly from NNW–SSE S Hmax in the autochthonous basement. The above discrepancy points to uncoupled type of the suture in this segment of the POC. In this scenario the ALCAPA push involves the nappes and is compensated in the top of the basement, which is expressed by systematic S Hmax rotations. In the accretionary wedge of the eastern Małopolska segment, S Hmax follows the trend perpendicular to the strike of nappes. It is in general agreement with NNE–SSW-oriented S Hmax in the autochthonous basement that also parallels the ALCAPA push direction. Similarity in stress orientation between these structural levels implies coupled suture zone in this segment of the POC. Further to the north, ALCAPA push is transmitted into the foreland plate where it causes N–S orientation of S Hmax, as determined for sedimentary cover of the EEC. Within the Baltic portion of the EEC, further S Hmax rotation towards the intermediate NNW–SSE position suggests balance between the ALCAPA push and the ridge push components. Within the TTZ, common S Hmax rotations from N–S to NW–SE indicate structurally controlled accommodation of the ALCAPA push. In the Palaeozoic platform of western Poland, Mesozoic complex of the Fore-Sudetic Monocline reveals NNE–SSW-directed S Hmax that differs from NW–SE-oriented S Hmax in the Variscan accretionary wedge. Here, mechanical decoupling along Zechstein evaporates is likely. Presented set of breakout data from Poland shows that recent compressive reactivation of the Carpathians exerts strong impact on the stress field of the foreland plate at a distance of 700 km from the suture. Due to this effect, the Fore-Carpathian stress domain within the North European stress province can be discriminated.