Comparison of joint sets orientation in the Lower Paleozoic shales exposed in Scania (SW Sweden) and concealed in N Poland : a multi-methodological approach

The Lower Paleozoic shales of SW Sweden and Eastern Pomerania (Poland) have a common history related to the depositional and tectonic evolution of the Baltic Basin. The major tectonic events are recorded, among others, as joints, which are either exposed in outcrops in SW Sweden or recognised in deep boreholes located in Pomerania. We present a comparison of the regional joint systems recognized by multiple methods in the studied region. In effect of a statistical compilation of measurements, five joint sets (named JS 1 to JS 5) were identified, traceable between Eastern Pomerania and SW Sweden. Our analysis showed a general consistency of joint set orientations, independent of their distance to the Tornquist tectonic zone passing through the study region. Three of the joint sets, JS 1 striking NNE, JS 2 striking WNW, and JS 3 striking NNW, are found to be the most frequent sets, occurring in most sites. Having more constant orientation, the JS 1 and JS 3 served as indicators of possible rotation of the tectonic block or stress field in the region. JS 1 and JS 2 are interpreted as an effect of late Carboniferous stress relaxation after Variscan collision, while JS 3 and the E–W striking JS 4 might be related to the Early Devonian Caledonian compressive stage.