Large grain-size-dependent rheology contrasts of halite at lowdifferential stress: evidence from microstructural study of naturallydeformed gneissic Zechstein 2 rock salt (Kristallbrockensalz) from thenorthern Netherlands

Constitutive laws to predict long-term deformation ofsolution-mined caverns and radioactive-waste repositories in rock salt playan important role in the energy transition. Much of this deformation is atdifferential stresses of a few megapascals, while the vast majority of laboratorymeasurements are at much higher differential stress and requireextrapolation. This can be much improved by including microstructural dataof samples deformed in natural laboratories. Deformation of rock salt canoccur by dislocation creep and grain-size-dependent dissolution-precipitationcreep processes (pressure solution); this mechanism is not commonly includedin current engineering predictions. Here we show evidence for large grain-size-dependent differences in rocksalt rheology based on microstructural observations from Zechstein rock saltcores of the northern Netherlands that experienced different degrees oftectonic deformation. We studied the relatively undeformedhorizontally layered Zechstein 2 (Z2) salt (Stassfurt Formation) from Barradeel andcompared it with a much more strongly deformed equivalent in diapiric salt fromWinschoten, Zuidwending, and Pieterburen. We used optical microscopy ofthin gamma-irradiated sections for microtectonic analysis, recrystallizedgrain-size measurements and subgrain-size piezometry, electron microscopywith energy-dispersive X-ray spectroscopy, and X-ray diffraction analysis forsecond-phase mineralogy. Subgrain-size piezometry shows that thisdeformation took place at differential stresses between 0.5 and 2 MPa. In the undeformed, layered salt from Barradeel we find centimetre-thick layers ofsingle crystalline halite (Kristalllagen or megacrystals) alternating withfine-grained halite and thin anhydrite layers. The domal salt samples aretypical of the well-known "Kristallbrockensalz" and consist of centimetre-sizetectonically disrupted megacrystals surrounded by fine-grained halite with agrain size of a few millimetres. We infer high strains in the fine-grained halite asshown by folding and boudinage of thin anhydrite layers, as compared to themegacrystals, which are internally much less deformed and develop subgrainsduring dislocation creep. Subgrain size shows comparable differentialstresses in Kristallbrocken as in matrix salt. The fine-grained matrix saltis dynamically recrystallized to some extent and has few subgrains andmicrostructures, indicating deformation by solution-precipitation processes.We infer that the finer-grained halite deformed dominantly via pressuresolution and the megacrystals dominantly by dislocation creep. The samples show that the fine-grained matrix salt is much weaker thanKristallbrocken because of different dominant deformation mechanisms. Thisis in agreement with microphysical models of pressure solution creep in which grain size has asignificant effect on strain rate at low differential stress. Our resultspoint to the importance of pressure solution creep in rock salt at lowdifferential stresses around engineered structures but also in most salttectonic settings. We suggest that including results of microstructuralanalysis can strongly improve engineering models of rock salt deformation. We recommend that this mechanism of grain-size-dependent rheology isincluded more consistently in the constitutive laws describing thedeformation of rock salt.