Experimental deformation of a single-layer anhydrite in halite matrix under bulk constriction. Part 1: Geometric and kinematic aspects

A new deformation apparatus has been used to model the internal kinematics of salt diapirs. Composite natural samples consisting of a single layer of anhydrite, embedded in halite matrix, were constrictionally deformed at temperature, T = 345 °C, strain rate, ė = 10−7 s−1, maximum viscosity, η = 2 × 1013 Pa s, and maximum finite strain, ex = 122%. The anhydrite layer, oriented parallel to the major stretching axis, X, was deformed by fracturing, whereas halite behaved viscously. At advanced state of constriction (ex > 90%) a strong increase in strain hardening of halite led to a transient tension fracture that healed up and was shortened by folding during the final phase of viscous deformation.Tiny prismatic anhydrite inclusions disseminated inside the halite matrix were reoriented during constriction resulting in a linear grain-shape fabric. 3D-images of the anhydrite layer, based on computer tomography, revealed rare kink folds with axes subparallel to X, and boudins which result from tension fracture. With increasing layer thickness, Hi, the width of boudins, Wa, increases linearly (Wa = −0.3 + 1.3 × Hi). The normalized width of boudins (Wd = Wa/Hi) is almost constant at 1.5 ± 1.0. These geometrical parameters can be used to reveal fracture boudinage under bulk constriction. The oblique orientation of most of the boudins, with respect to the principal strain axes, results from folding of the boudins by a second generation of folds, the latter with axes subperpendicular to the layer.