Present‐Day Stress Decoupling Across the Zechstein Salt: Borehole Constraints From the Eastern Southern Permian Basin (Poland)

ABSTRACT

We investigate present‐day maximum horizontal stress ( S _Hmax ) orientations in the Fore‐Sudetic Homocline (FSH) and their changes across the Zechstein salt strata deposited in the eastern Southern Permian Basin, using borehole image and calliper logs from 39 boreholes. Stress direction indicators represented by compressive borehole breakouts (BB), tensile drilling‐induced fractures (DIF) and breakout‐like structures (BBL) of uncertain origin are grouped into five geomechanical units, which show internally consistent S _Hmax orientations but exhibit significant inter‐unit variability. In the sub‐salt units: (1) DIF data from the Carboniferous indicate a NWN–SES S _Hmax in the western FSH segment consistent with the regional tectonic stress; (2) BB data from the Rotliegend show a N–S S _Hmax in the eastern segment of the FSH, consistent with stress rotation across fault‐bounded crustal blocks. In the intra‐salt unit, where the dolomite–anhydrite complex is welded to the basement: (3) DIF indicate a distinctive W–E S _Hmax that we interpret to be the result of tension generated by downslope gravity‐driven gliding of the dense dolomite–anhydrite ‘plate’ within light and ductile salt; this mechanism, locally enhanced by gravitational collapse, produces similarly oriented tension in the underlying basement highs. (4) BBL in Zechstein salt suggest an ENE–WSW S _Hmax which we relate to lithostatic pressure gradients arising from variations in salt overburden. In a supra‐salt unit: (4) BB in the Mesozoic succession reveal a dominant NNE–SSW S _Hmax subparallel to the homocline dip, interpreted as downslope compression in the northern FSH that transitions to N–S tension in its southern FSH. We provide first‐order analytical estimates of gravity‐driven and tectonic stresses magnitudes at the intra‐salt and supra‐salt levels and discuss the structural control on a layered stress pattern in the FSH.