Strain hardening and viscosity anisotropy in foliated rock salt
Final Report Abstract
The present project was dealing with the impact of strain hardening and planar anisotropies on the viscous flow of rock salt. The main aims were (1) to reveal the reasons for various stages of strain hardening, and (2) to quantify the microstructural processes responsible for mechanical anisotropy in rock salt. To achieve these aims, halite single crystals and foliated rock salt were deformed incrementally to high finite viscous strain under different bulk strain fields (flattening, plane strain, constriction) using a thermomechanical apparatus, which allowed deformation at elevated temperature (345°C), slow strain rates (10^-7 s^-1), and low differential stress (<5 MPa). The microfabrics of the deformed samples, as revealed by optical microscopy, electron backscattered diffraction (EBSD), were used to constrain the deformation mechanisms. Our studies can be subdivided into four parts. In a first study high strain constriction experiments have been conducted on natural halite cuboids from the Morsleben mine. The experiments were stopped at different final strains (ey=z of ~10, 20, 30 and 40%). The halite is deformed by dislocation creep, and the size of developed subgrains corresponds to the applied stress. Slip was accommodated by the {110}<110> slip systems, with minor contribution by slip on the {100}<110> slip systems. Subgrains with small misorientations lead to large cumulative misorientations within a single grain (>40°). However, no subgrain rotation recrystallization was observed. All the experiments show strain hardening, suggesting that recrystallization by grain boundary migration was not extensive. The results of the deformation experiments show that very high strain (170%) can be achieved in coarse-grained relatively dry rock salt by dislocation creep, without extensive dynamic recrystallization. In a second study natural rock salt was deformed under bulk plane and constrictional strain, until a maximum shortening strain (εZ) of -36%. The dominant deformation mechanism was dislocation creep. Two types of microstructures form during deformation: (1) equiaxed subgrains with small misorientations (1-10°), and (2) deformation bands, which result from (sub) grain boundaries with higher misorientations (some >30°) aligned throughout the grain. No traditional subgrain rotation recrystallization is observed (i.e. an increase of subgrain boundary misorientation with strain). The analyses show that within one grain and one sample, a large variation occurs in subgrain size, misorientation angles, variations in orientations, and formation of high angle grain boundaries. The variations have consequences for the use of the subgrain paleopiezometer in natural samples and for our understanding of dynamic recrystallization in rock salt. In a third study the effect of a shape-preferred orientation (SPO) on deformation of rock salt was studied by ductile deformation experiments using natural halite cubes with and without an SPO from the Asse mine. The SPO was oriented at different angles (ß = 0°, 45° and 90°) to the main shortening direction, Z. There is a clear influence of the SPO on the mechanical data. At a shortening strain of ca. -10%, the samples with ß = 0° and 45° are weaker than the samples without an SPO. In contrast, the sample with ß = 90° is stronger than the samples without an SPO. A new SPO is formed in all samples with the long axis of the grain shape fabric being subparallel to the principal stretching axis, X. At these larger shortening strains, the samples show similar strength, with an exception of the 90° oriented sample that is still stronger than the samples without an SPO. The new results show that the SPO of rock salt likely has to be taken into account when building a repository for nuclear waste and could have an effect on the stability of caverns. A fourths study was focusing on the natural deformation processes in Permian Knäuel- and Streifensalz formations of the Gorleben salt dome. The reconstruction of 3D halite grain shape ellipsoids reveals small grain size (3.4 ±0.6 mm) and heterogeneous grain shapes in both formations. The grain shape ellipsoids of halite are ranging from oblate to prolate and show high average axial ratios (ARs) compared to published data from other salt structures. Strain was accommodated by strain-induced grain boundary migration, subgrain formation and minor subgrain rotation at very low differential stress (1.1–1.3 MPa) as deduced from subgrain-size piezometry. No continuous anhydrite layers are preserved, and halite has acted as a sealing matrix embedding the disrupted anhydrite fragments prohibiting any potential migration pathways for fluids. Thus, anhydrite should not have a negative effect on the barrier properties of the Gorleben rock salts investigated in this study.
Publications
- 2016. Experimental deformation of coarse-grained rock salt to high strain. J. Geophys. Res. Solid Earth, 121, 6150–6171
Linckens, J., Zulauf, G. and Hammer, J.
(See online at https://doi.org/10.1002/2016JB012890) - 2016. Microfabrics and 3D grain shape of Gorleben rock salt: constraints on deformation mechanisms and paleodifferential stress. Tectonophysics, 676, 1-19
Thiemeyer, N., Zulauf, G., Mertineit, M., Linckens, J., Pusch, M., Hammer, J.
(See online at https://doi.org/10.1016/j.tecto.2016.02.046) - 2018. Subgrain development in experimentally deformed rock salt. Mechanical Behavior of Salt IX. Proceedings Conf. on mechanical behaviour of salt, SaltMech IX, pp. 15-24; Hannover; Taylor & Francis Group, London
Linckens, J., Heeb, J., Zulauf, G., Mertineit, M.
- (2019) The influence of a grain-shape fabric on the mechanical behaviour of rock salt: Results from deformation experiments. Tectonophysics 751 73-82
Linckens, Jolien; Zulauf, Gernold; Mertineit, Michael
(See online at https://doi.org/10.1016/j.tecto.2018.12.009)
