Tablet boudinage of an anhydrite layer in rock-salt matrix: Results from thermomechanical experiments

A single competent layer of anhydrite, embedded in a matrix of incompetent rock salt, was deformed in a flattening type of bulk deformation at temperature, T = 345 °C, strain rate, ė = 10−7 s−1, to a maximum finite strain, eZ = −30%. As the anhydrite layer was oriented perpendicular to the main shortening direction, Z, the layer underwent equal layer-parallel extension in all directions resulting in tablet-shaped boudins. Boudinage results from tensile fracture of anhydrite. The rock-salt matrix, on the other hand, behaved viscously. Crystal plastic deformation of halite was accommodated by slip on {110}<110> which led to formation and rotation of subgrains and a 001-maximum parallel to the principal shortening axis, Z. An axisymmetric texture of halite, however, is present only in high-strain domains at the contact to the anhydrite layer where the differential stress, obtained from subgrain size of halite, reaches maximum values of ca. 6 MPa. Subgrains remote from the anhydrite layer yielded 2–3 MPa, which is consistent with the flow stress recorded by the load cell of the machine. n-isometric shape of the tablet boudins in plan-view is indicated by the ratio between long and short axis (R = 1.2–1.9), and can be explained by the interaction of concentric and radial tensile fractures. Both the mean diameter of the tablet boudins in plan-view, Wa, and the number of boudins, N, show a linear relation to the layer thickness. Progressive finite strain results in a higher number and a smaller mean diameter of the boudins. The thickness of the boudins, Hf, is almost the same like the initial layer thickness, Hi, while the aspect ratio (Wd = Wa/Hf) decreases with finite strain. The mean Wd values obtained from all runs are ranging from ca. 0.8 to ca. 1.5. This range is much lower than the aspect ratio of boudins in viscous or brittle/viscous layers, but is similar to the aspect ratio expected from fracture-saturation models. The aspect ratio of the tablet-shaped anhydrite boudins is further consistent with aspect ratios published so far for tensile-fracture boudins which developed in a plane or constrictional type of bulk deformation.