Contemporary stress field distortion in the Polish part of the Western Outer Carpathians and their basement

Abstract

The Western Carpathians seem to be the crucial place to understand the nature of the regional turn of the present-day maximum horizontal stress ( S Hmax) direction from the northwest, which is characteristic for the West European stress province, towards the north, specific for the western edge of the East European Platform. S Hmax orientations have been investigated in fourteen deep wells located in the western part of the Polish Outer Carpathians by borehole breakout analyses from six-arm dipmeter data. This tool proved to be useful to determine breakout orientation and to discriminate stress-induced breakouts from mechanical failures of the borehole wall. The analysed dipmeter data come from wells (most of them deeper than 3000 m) which penetrate Cretaceous–Tertiary sequences of the Carpathian flysch nappes (4 wells), the Palaeozoic–Tertiary autochthonous sedimentary cover of the Upper Silesian and the Małopolska Massifs (13 wells), and their Precambrian metamorphic basement (4 wells). In the investigated part of the Upper Silesian Massif S Hmax for separate depth sections shows a general, counterclockwise rotation with increasing depth. For the Carpathian nappes, breakout data of moderate quality indicate NNE-oriented S Hmax while for the autochthonous basement sequence good quality data point to a NNW mean direction of S Hmax. In the deepest well sections which penetrate the crystalline basement of the Upper Silesian Massif the tendency to further stress rotation towards the northwest was observed. The range of the angular S Hmax rotation with depth for the utmost structural levels exceeds 60° in the central part, and decreases to 40° for the eastern edge of the Upper Silesian Massif. No stress rotation is evident in the westernmost part of this massif. Numerous second- and third-order stress deviations give evidence for recent fault reactivation and rotational stresses in horizontal planes. For the autochthonous sedimentary cover of the Małopolska Massif, N- to NNE-oriented compression was determined. Hitherto acquired data suggest insignificant stress rotation from the north in unfolded sequences of the Miocene molasse beneath the nappes, to the north-northeast in deeper Palaeozoic–Mesozoic complexes. In one well, NE-trending S Hmax was determined in folded Miocene molasse beneath the main decollement surface. This stress orientation is similar to stress directions in the nappes covering the Upper Silesian Massif. S Hmax directions in the autochthonous complex beneath the nappes of the Western Outer Carpathians generally are subperpendicular to the overall trend of the orogen creating a fan-like pattern of stress trajectories. NNE-oriented compression within the nappes covering the Upper Silesian Massive, within the autochthonous complex of the Małopolska Massif and in the Carpathian foreland probably is exerted by the Carpatho-Pannonian plate advancing towards the north-northeast relative to the European plate. Active movement occurs along the Mur–Muerz–Źilina fault zone on the western boundary of this plate and probably along the Eastern Carpathians suture on its eastern margin. NW-directed compression in the basement of the Upper Silesian Massif is in agreement with S Hmax orientation within the Bohemian Massif and the West European stress province, thus plate forces are thought to contribute to its origin.