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

Abstract

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, e Z = −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. The non-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, W a , 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, H f , is almost the same like the initial layer thickness, H i , while the aspect ratio ( W d = W a / H f ) decreases with finite strain. The mean W d 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.