Brittle plus plastic deformation of gypsum aggregates experimentally deformed in torsion to high strains: quantitative microstructural and texture analysis from optical and diffraction data

This contribution presents a quantitative microstructural analysis of a polycrystalline aggregate of gypsum, deformed in torsion (T 1⁄4 70–90 8C) at g (shear strain) ranging from 0 to 4.82. Quantitative microstructural analysis is used to compare the evolution of microstructures observed by optical microscope with those obtained from analysis of X-ray and neutron diffraction data. This analysis shows that during experimental deformation, gypsum accommodated strain by brittle and plastic deformation mechanisms, developing Riedel-like microfaults with plastic foliations and crystallographic preferred orientation (CPO). The relations of microstructures show that with increasing strain, the Riedel systems start from R planes with an angle of 308 to the Imposed Shear Plane. This angle decreases (58–158) when strain increases, and Y planes develop. Quantitative texture analysis (QTA) shows that S-foliations start developing at low g and maintain their orientation up to high g, and that the most active slip system is the (010) along normal to (100) and the [001]-axis. Shape preferred orientation (SPO) of gypsum does not coincide with the theoretical orientation as it does not decrease with increasing strain. This discrepancy is explained by the role of the brittle shear planes that impose a back rotation to gypsum. No brittle to plastic transition occurs. But both plastic and brittle structures contemporaneously accommodate and localize strain. Many rocks (metamorphic, igneous and sedimentary) show non-random orientation distributions of their crystallites that results in anisotropies of macroscopic physical properties (Turner & Weiss 1963; Wenk 1985; Randle & Engler 2000; Karato 2008). The crystallographic/lattice or shape preferred orientation (SPO) of crystals with respect to macroscopic fabric axes may be attained in response to deformation processes. The interpretation of textures (i.e. crystallographic preferred orientation) in materials (e.g. rocks or rock analogues) relies on a quantitative description of the orientation features. Together with several other texture analysis methods, quantitative texture analysis (QTA) using neutron and X-ray diffraction has been successfully used in recent years to completely describe the crystallographic preferred orientation (CPO) of naturally deformed rocks (e.g. Kocks et al. 1998; Leiss et al. 2000; Zucali et al. 2002; Zucali & Chateigner 2006; Wenk 2006 and references therein). In this work we present, together with a detailed and complete microstructural analysis, the results of QTA measurements from neutron and X-ray experiments carried out on samples of natural gypsum that were experimentally deformed in torsion at high shear strains and confining pressure, and at various temperatures and strain rates (Barberini et al. 2005). The QTA measurements were performed at the ILL neutron facility (Grenoble, From: SPALLA, M. I., MAROTTA, A. M. & GOSSO, G. (eds) Advances in Interpretation of Geological Processes: Refinement of Multi-scale Data and Integration in Numerical Modelling. Geological Society, London, Special Publications, 332, 79–98. DOI: 10.1144/SP332.6 0305-8719/10/$15.00 # The Geological Society of London 2010. France) and at the Laboratoire de Cristallographie et Sciences des Materiaux (CRISMAT), Ecole Nationale Superieure d’Ingenieurs de Caen (ENSICAEN, France). The study of the deformation behaviour of minerals that form evaporitic rocks is of great importance because these rocks can easily localize deformation. Several studies have described the structures and behaviour of gypsum deformed under various conditions (Craker & Schiller 1962; Baumann 1984; Panozzo Heilbronner & Olgaard 1987; Harland et al. 1988; Ko et al. 1995; Stretton 1996; Barberini et al. 2005) and the tectonic implications of gypsum dehydration (Heard & Rubey 1966), but only a few have described the microstructural and textural evolution (Levykin & Parfenov 1983; Kern & Richter 1985; Panozzo Heilbronner & Dell’Angelo 1990; Panozzo Heilbronner 1993). Such studies describe the evolution of a preferred orientation of poles to planes (010), perfect cleavage planes, which is parallel to s1 (main axial stress). (010) , 001. is considered the most common slip plane but others have also been recognized (Muegge 1898; De Meer 1995). The deformation of gypsum has also been studied under various conditions in order to investigate the behaviour under transient drained conditions (Olgaard et al. 1995). Moreover, the instantaneous and long-term behaviour of natural gypsum has been studied to understand the traces of dissolution observed in pillars of underground gypsum quarries (Hoxha et al. 2006; Castellanza et al. 2008) and to model its long-term behaviour (Hoxha et al. 2005). Finally, some authors have studied the creep of wet gypsum aggregates, and the relationships with the mechanics of thrust faults and other large-scale structures, associated with oil and gas accumulations (De Meer & Spiers 1995).