Present-day tectonic stress from borehole breakouts in the North-Sudetic Basin (northern Bohemian Massif, SW Poland) and its regional context

We analyze borehole breakouts in 18 boreholes in the North-Sudetic Basin (NSB), located on the Lower Silesian tectonic block, to provide the first present-day stress analyses performed for the NSB. Dipmeter logs from the Upper Permian sedimentary succession showed short breakout profiles in 15 boreholes, from 458 to 1368 m depth interval. Integration of log and core interpretation revealed a close correlation between breakout occurrence and fault zones. The breakouts, although scarce, indicate consistent changes of maximum horizontal stress (SHmax) direction among three spatial clusters of boreholes: a prominent cluster of NNW–SSE SHmax direction and two subordinate ones of NW–SE and NE–SW SHmax directions. These stress patterns in the NSB may be placed in the tectonic stress context of the broader Sudetic region comprising the northern part of the Bohemian Massif and the Fore-Sudetic Homocline. For this area, statistics of compiled stress indicators exhibit a tri-modal SHmax direction distribution, quite similar to that obtained for the NSB. We proposed that stress directions and regime changes were triggered by minor fault reactivation in a transtensional stress regime, and this might be applicable also to the NSB.