Abstract We present a selective overview of current issues and outstanding problems in the field of deformation mechanisms, rheology and tectonics. A large part of present-day research activities can be grouped into four broad themes. First, the effect of fluids on deformation is the subject of many field and laboratory studies. Fundamental aspects of grain boundary structure and the diffusive properties of fluid-filled grain contacts are currently being investigated, applying modern techniques of light photomicrography, electrical conductivity measurement and Fourier Transform Infrared (FTIR) microanalysis. Second, the interpretation of microstructures and textures is a topic of continuous attention. An improved understanding of the evolution of recrystallization microstructures, boundary misorientations and crystallographic preferred orientations has resulted from the systematic application of new, quantitative analysis and modelling techniques. Third, investigation of the rheology of crust and mantle minerals remains an essential scientific goal. There is a focus on improving the accuracy of flow laws, in order to extrapolate these to nature. Aspects of strain and phase changes are now being taken into account. Fourth, crust and lithosphere tectonics form a subject of research focused on large-scale problems, where the use of analogue models has been particularly successful. However, there still exists a major lack of understanding regarding the microphysical basis of crust- and lithosphere-scale localization of deformation.
Abstract Optical measurements of microstructural features in experimentally deformed Carrara marble help define their dependence on stress. These features include dynamically recrystallized grain size (Dr), subgrain size (Sg), minimum bulge size (Lρ), and the maximum scale length for surface-energy driven grain-boundary migration (Lγ). Taken together with previously published data Dr defines a paleopiezometer over the range 15–291 MPa and temperature over the range 500–1000 °C, with a stress exponent of −1.09 (CI −1.27 to −0.95), showing no detectable dependence on temperature. Sg and Dr measured in the same samples are closely similar in size, suggesting that the new grains did not grow significantly after nucleation. Lρ and Lγ measured on each sample define a relationship to stress with an exponent of approximately −1.6, which helps define the boundary between a region of dominant strain-energy-driven grain-boundary migration at high stress, from a region of dominant surface-energy-driven grain-boundary migration at low stress.
This paper reports uniaxial compaction creep experiments performed on porous calcite aggregates in the presence of CO 2 at controlled conditions similar to those relevant for geological storage of CO 2 in carbonate reservoirs. The experiments were conducted on pre‐compacted calcite aggregates of various mean grain sizes in the range 1 to 250 μ m, under dry and wet conditions, at temperatures of 28–100°C and applied effective stresses of 4–40 MPa. Carbon dioxide was added to wet samples at pressures up to 10 MPa. The results demonstrate that dry granular calcite shows virtually no creep, but that significant creep occurs when saturated aqueous solution is added. In wet samples, the strain rate increases with increasing grain size and applied stress. When CO 2 is added from the outset, the strain rate decreases with increasing grain size up to 106 μ m, and increases with grain size above 106 μ m. Below 106 μ m, the strain rate also increases with applied stress and strongly with CO 2 (partial) pressure, but decreases with increasing temperature. The mechanical data together with microstructural evidence indicate that combined grain scale microcracking and diffusion controlled pressure solution best explain the behavior observed. Notably, in experiments where CO 2 was added before loading, pressure solution dominated creep at fine grain size, giving way to subcritical cracking control at grain sizes above 106 μ m. Our results point to pressure solution accelerating by up to 50 times at CO 2 pressures increased from 6 to 10 MPa. Integrating our findings, we suggest that if a depleted carbonate reservoir exhibits measurable compaction creep due to diffusion‐controlled pressure solution, then injection of CO 2 has the potential to speed this up by amounts up to 50 times or more.
Abstract. The effect of grain size on strain rate of ice in the upper 2207 m in the North Greenland Eemian Ice Drilling (NEEM) deep ice core was investigated using a rheological model based on the composite flow law of Goldsby and Kohlstedt (1997, 2001). The grain size was described by both a mean grain size and a grain size distribution, which allowed the strain rate to be calculated using two different model end-members: (i) the microscale constant stress model where each grain deforms by the same stress and (ii) the microscale constant strain rate model where each grain deforms by the same strain rate. The model results predict that grain-size-sensitive flow produces almost all of the deformation in the upper 2207 m of the NEEM ice core, while dislocation creep hardly contributes to deformation. The difference in calculated strain rate between the two model end-members is relatively small. The predicted strain rate in the fine-grained Glacial ice (that is, ice deposited during the last Glacial maximum at depths of 1419 to 2207 m) varies strongly within this depth range and, furthermore, is about 4–5 times higher than in the coarser-grained Holocene ice (0–1419 m). Two peaks in strain rate are predicted at about 1980 and 2100 m depth. The prediction that grain-size-sensitive creep is the fastest process is inconsistent with the microstructures in the Holocene age ice, indicating that the rate of dislocation creep is underestimated in the model. The occurrence of recrystallization processes in the polar ice that did not occur in the experiments may account for this discrepancy. The prediction of the composite flow law model is consistent with microstructures in the Glacial ice, suggesting that fine-grained layers in the Glacial ice may act as internal preferential sliding zones in the Greenland ice sheet.
An overview of the deformation and recrystallization mechanisms that are active in the North Greenland Eemian Ice Drilling (NEEM) ice core is given, based on microscale models, light microscopy and cryogenic electron backscatter diffraction (cryo-EBSD). The Holocene ice (0-1419 m depth) deforms by dislocation creep with basal slip accommodated by non-basal slip. The amount of non-basal slip is controlled by the extent of strain induced boundary migration (SIBM). The most important recrystallization mechanisms and processes in the Holocene ice are grain dissection, strain induced boundary migration (SIBM), and bulging nucleation. In the glacial ice (1419-2207 m of depth) basal slip is accommodated by both non-basal slip and grain boundary sliding (GBS). Rotation recrystallization is more important, while SIBM is less important in the glacial ice compared to the Holocene ice. In the Eemian ice (2207-2540 m depth), which is at high temperature, different microstructures occur depending on the impurity content of the ice. The difference in microstructure and deformation mechanisms, between interglacial and glacial ice can have important consequences for ice rheology and ice sheet dynamics.
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