Abstract. The ice microstructure in the lower part of the North Greenland Eemian Ice Drilling (NEEM) ice core consists of relatively fine grained glacial ice with a single maximum crystallographic preferred orientation (CPO) alternated by much coarser grained Eemian ice with a partial girdle type of CPO. In this study, the grain size sensitive (GSS) composite flow law of Goldsby and Kohlstedt (2001) was used to study the effects of grain size and premelting on strain rate in the lower part of the NEEM ice core. The results show that the strain rates predicted in the fine grained glacial layers are about an order of magnitude higher than in the much coarser grained Eemian layers. The dominant deformation mechanisms between the layers is also different with basal slip accommodated by grain boundary sliding (GBS-limited creep) being the dominant deformation mechanism in the glacial layers, while GBS-limited creep and dislocation creep (basal slip accommodated by non-basal slip) contribute both roughly equally to bulk strain in the coarse grained layers. Due to the large difference in microstructure between the impurity-rich glacial ice and the impurity-depleted Eemian ice at premelting temperatures (T>262 K), it is expected that the fine grained layers deform mainly by simple shear at high strain rates, while the coarse grained layers are relatively stagnant. The difference in microstructure, and consequently in viscosity, between glacial and interglacial ice at temperatures just below the melting point can have important consequences for ice dynamics close to the bedrock.
Summary The Dutch Westland area, situated in the onshore part of the West Netherlands Basin, is of interest for geothermal energy exploration because good geothermal reservoirs are present in the Late Jurassic to Early Cretaceous deposits, and a high demand for hot water exists from the numerous greenhouses in the area. This study aimed to provide a better regional understanding of the Late Jurassic to Early Cretaceous tectonic history of the West Netherlands Basin, in particular regarding reservoir distribution and the possible influence of early tectonic structures on the deposition of reservoir sands. In addition, detailed knowledge of the fault pattern is important for the placement of geothermal wells. Detailed seismic mapping combined with stratigraphic cross-sections enabled a study of the distribution and development of geothermal reservoirs within the Alblasserdam, Delft Sandstone and the Berkel Sandstone Members. By flattening seismic cross-sections at several mapped horizons, the pre-inversion geometry of the basin was reconstructed. In addition, seismic attribute analyses have significantly improved the mapping of the faults and may illustrate depositional patterns within the Berkel Sandstone Member. The revealed tectonic history and reservoir distribution aid to accurately plan and execute geothermal wells.
Abstract. Constitutive laws of rock salt are required for the prediction of long-term deformation of radioactive waste repositories and solution mined caverns, which are used for energy storage and play an important role in the energy transition. Much of this deformation is at differential stresses of a few MPa. The vast majority of laboratory measurements of salt creep are at much higher differential stress and require extrapolation over many orders of magnitude. This extrapolation can be made more reliable by including microphysical information on the deformation mechanisms in the laboratory samples, integrated with microstructural analysis of samples deformed in natural laboratories at low differential stress. Rock salt can deform at widely different rates at the same temperature and deviatoric stress, depending on state variables such as grain size, solid solution- and second phase- impurities, crystallographic preferred orientation, water content and grain boundary structure. Both dislocation creep and dissolution-precipitation creep processes are common, but dissolution-precipitation creep (pressure solution) is not commonly included in current engineering predictions. Here we show evidence for large grain size-dependent differences in halite 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 Z2 (Stassfurt Formation)‚ horizontal-layered salt from Barradeel, and compare it with much stronger deformed equivalent in diapiric salt form Winschoten, Zuidwending, and Pieterburen. We used optical microscopy of Gamma-irradiated thin sections for microtectonic analysis, recrystallized grain size measurements and subgrain size piezometry, SEM-EDX and XRD for second phase mineralogy. Subgrain size piezometry shows that this deformation took place at differential stress between 0.5 and 2 MPa, providing a natural laboratory. In the undeformed, layered salt from Barradeel we find cm-thick layers of single crystalline halite (Kristalllagen) alternating with fine-grained halite and thin anhydrite layers. The domal salt samples are typical of the well-known "Kristallbrocken" salt, and consist of cm-size tectonically disrupted megacrystals surrounded by fine-grained halite with grain size of a few mm. 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 than in matrix salt. The fine-grained matrix salt is dynamically recrystallized, 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. This provides evidence 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 these low differential stress. Our results on the operation of pressure solution creep in rock salt at differential stress of a few MPa point to the importance of this mechanism 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 deformation of engineered structures in rock salt.
Abstract Field studies of calcite mylonites often document microstructures produced by dislocation creep. In contrast, flow laws derived from experiments predict that calcite rocks should deform mostly by diffusion creep during tectonic processes. To investigate this apparent discrepancy, we compare stresses estimated by microstructural piezometers to those obtained by extrapolation of experimentally derived flow laws. Considering shear zones from different geological settings, a clear trend is observed of increasing recrystallized grain size with increasing temperature. However, there is a large spread in grain size and associated stress. Because separate flow laws have been defined for various different marbles and limestones, the strengths predicted for a given set of conditions differ significantly. The stress estimates based on the piezometers and strength extrapolated from the various experimentally derived dislocation creep flow laws agree qualitatively, but no single flow law predicts all the palaeostress estimates. Even if experimental data are disregarded, the field observations are not consistent with a hypothetical law for Coble creep; they are consistent with a power law for dislocation creep, but only if the material constants are different from those currently determined in laboratory experiments.
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