Large grain-size-dependent rheology contrasts of halite at low differential stress: evidence from microstructural study of naturally deformed gneissic Zechstein 2 rock salt (Kristallbrockensalz) from the northern Netherlands

Abstract

Abstract. Constitutive laws to predict long-term deformation of solution-mined caverns and radioactive-waste repositories in rock salt play an important role in the energy transition. Much of this deformation is at differential stresses of a few megapascals, while the vast majority of laboratory measurements are at much higher differential stress and require extrapolation. This can be much improved by including microstructural data of samples deformed in natural laboratories. Deformation of rock salt can occur by dislocation creep and grain-size-dependent dissolution–precipitation creep processes (pressure solution); this mechanism is not commonly included in current engineering predictions. Here we show evidence for large grain-size-dependent differences in rock salt rheology based on microstructural observations from Zechstein rock salt cores of the northern Netherlands that experienced different degrees of tectonic deformation. We studied the relatively undeformed horizontally layered Zechstein 2 (Z2) salt (Stassfurt Formation) from Barradeel and compared it with a much more strongly deformed equivalent in diapiric salt from Winschoten, Zuidwending, and Pieterburen. We used optical microscopy of thin gamma-irradiated sections for microtectonic analysis, recrystallized grain-size measurements and subgrain-size piezometry, electron microscopy with energy-dispersive X-ray spectroscopy, and X-ray diffraction analysis for second-phase mineralogy. Subgrain-size piezometry shows that this deformation took place at differential stresses between 0.5 and 2 MPa. In the undeformed, layered salt from Barradeel we find centimetre-thick layers of single crystalline halite (Kristalllagen or megacrystals) alternating with fine-grained halite and thin anhydrite layers. The domal salt samples are typical of the well-known “Kristallbrockensalz” and consist of centimetre-size tectonically disrupted megacrystals surrounded by fine-grained halite with a grain size of a few millimetres. We infer high strains in the fine-grained halite as shown by folding and boudinage of thin anhydrite layers, as compared to the megacrystals, which are internally much less deformed and develop subgrains during dislocation creep. Subgrain size shows comparable differential stresses in Kristallbrocken as in matrix salt. The fine-grained matrix salt is dynamically recrystallized to some extent and has few subgrains and microstructures, indicating deformation by solution–precipitation processes. We infer that the finer-grained halite deformed dominantly via pressure solution and the megacrystals dominantly by dislocation creep. The samples show that the fine-grained matrix salt is much weaker than Kristallbrocken because of different dominant deformation mechanisms. This is in agreement with microphysical models of pressure solution creep in which grain size has a significant effect on strain rate at low differential stress. Our results point to the importance of pressure solution creep in rock salt at low differential stresses around engineered structures but also in most salt tectonic settings. We suggest that including results of microstructural analysis can strongly improve engineering models of rock salt deformation. We recommend that this mechanism of grain-size-dependent rheology is included more consistently in the constitutive laws describing the deformation of rock salt.