Recent tectonic stress field investigations in Poland: a state of the art

Abstract

The paper summarizes up-to-date knowledge of the contemporary tectonic stress field in Poland and compares the results of geophysical measurements with mathematical models. The extensive set of data provided by borehole breakout analyses is supplemented by hydraulic fracturing tests, earthquake focal mechanism solutions and preliminary resolution of regional intraplate motions from GPS measurements. Frequent breakout presence shows that tectonically driven anisotropy of horizontal stress is a common feature in the study area. Roughly N-S direction of maximum horizontal stress (SHmax) in Eastern Poland differs significantly from Western European stress domain. This difference is produced by tectonic push of Alcapa, which is successively compensated within the Teisseyre-Tornquist Zone (TTZ) and in the Upper Silesian segment of the Outer Carpathians. In the western part of Poland stress directions are ambiguous due to interplay of several additional tectonic factors. Most of hydraulic fracturing data and earthquake focal mechanism solutions indicate strike-slip stress regime in Eastern Poland where stresses are in equilibrium with preferentially oriented faults of low friction (0.16). Limited data from Western Poland suggest normal fault stress regime. Good conformity between directions of SHmax and intraplate motions occurs from comparison of breakout and GPS data. Finite element modelling shows that the most important factor shaping the stress field in Eastern and Central Poland is the Adria push transmitted through the Pannonian region. Secondary, but still notable factors are differentiation of loads along the Mediterranean collision zone and changes in magnitude of the ridge push force along the NW continental passive margin of Europe. Results of rheological modelling indicate that the crust is entirely decoupled from the mantle in the Fore-Sudetic Platform, partial uncoupling in the base of the upper crust is possible in the TTZ while in the East European Craton (EEC) the whole lithosphere is coupled. The comparison of different set of data and models presented here provides a comprehensive geodynamic scenario for Poland, however, a number of unresolved questions still remains to be addressed.