High-strain deformation tests on natural gypsum aggregates in torsion

Evaporitic minerals, such as gypsum, within sedimentary sequences play an important role in localizing deformation, especially in thrust tectonics, implying that their strength is generally lower than that of other rocks. To study the rheological and microstructural evolution of gypsum with strain, a set of experiments was performed on natural gypsum samples from Volterra (Italy). To reach high shear strain (up to γ = 5), deformation tests were performed in torsion at 300 MPa confining pressure, at temperatures up to 127 °C, and at shear strain rates between 10-3 and 10-5 s-1. All samples were studied by optical microscopy, to investigate the evolution of the microstructure with strain, and by X-ray diffraction (XRD) analyses, to determine whether and to what extent gypsum dehydrated during deformation. The shear stress increased with shear strain rate and decreased with temperature. A peak stress was usually reached at γ between 0.5 and 1.5. After the peak, 30-40% of weakening occurred but mechanical steady-state conditions were never reached. The microstructure evolved from a plastic deformation microstructure, where grains changed shape according to the bulk strain imposed, into a recrystallization-dominated microstructure, where grains were more equant. The shear stress sensitivity to strain rate increases with progressive strain, thus a meaningful constitutive flow law can only be determined from experiments in which steady-state flow is eventually reached. These results imply that gypsum in nature will flow plastically at shear stress levels lower than those expected from previous experimental studies due to the strain weakening associated with dynamic recrystallization, which can occur at temperatures even lower than gypsum dehydration. © The Geological Society of London 2005.