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

The Western Carpathians seem to be the crucial place to understand the natu re of the regional turn of the present-day maximum horizontal stress (S-Hma x) direction from the northwest, which is characteristic for the West Europ ean stress province, towards the north, specific for the western edge of th e East European Platform. S-Hmax orientations have been investigated in fou rteen deep wells located in the western part of the Polish Outer Carpathian s by borehole breakout analyses from six-arm dipmeter data. This tool prove d to be useful to determine breakout orientation and to discriminate stress -induced breakouts from mechanical failures of the borehole wall. The analy sed dipmeter data come from wells (most of them deeper than 3000 m) which p enetrate Cretaceous-Tertiary sequences of the Carpathian flysch nappes (4 w ells), the Palaeozoic-Tertiary autochthonous sedimentary cover of the Upper Silesian and the Malopolska Massifs (13 wells), and their Precambrian meta morphic 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 o f moderate quality indicate NNE-oriented SHmax while for the autochthonous basement sequence good quality data point to a NNW mean direction of SHmax. In the deepest well sections which penetrate the crystalline basement of t he Upper Silesian Massif the tendency to further stress rotation towards th e northwest was observed. The range of the angular S-Hmax rotation with dep th for the utmost structural levels exceeds 60 degrees in the central part, and decreases to 40 degrees for the eastern edge of the Upper Silesian Mas sif. 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 au tochthonous sedimentary cover of the Malopolska Massif, N- to NNE-oriented compression was determined. Hitherto acquired data suggest insignificant st ress rotation from the north in unfolded sequences of the Miocene molasse b eneath the nappes, to the north northeast in deeper Palaeozoic-Mesozoic com plexes. In one well, NE-trending S-Hmax was determined in folded Miocene mo lasse beneath the main decollement surface. This stress orientation is simi lar to stress directions in the nappes covering the Upper Silesian Massif. S-Hmax directions in the autochthonous complex beneath the nappes of the We stern Outer Carpathians generally are subperpendicular to the overall trend of the orogen creating a fan-like pattern of stress trajectories. NNE-orie nted compression within the nappes covering the Upper Silesian Massive, wit hin the autochthonous complex of the Malopolska Massif and in the Carpathia n foreland probably is exerted by the Carpatho-Pannonian plate advancing to wards the north-northeast relative to the European plate. Active movement o ccurs along the Mur-Muerz-Zilina fault zone on the western boundary of this plate and probably along the Eastern Carpathians suture on its eastern mar gin, NW-directed compression in the basement of the Upper Silesian Massif i s in agreement with S-Hmax orientation within the Bohemian Massif and the W est European stress province, thus plate forces are thought to contribute t o its origin. (C) 1998 Elsevier Science B.V. All rights reserved.