‘See-through’ deformation experiments on brittle–viscous norcamphor at controlled temperature, strain rate and applied confining pressure

An integrated experimental–analytical deformational system allows the observation of evolving microstructures in rock analogues under an optical macroscope at controlled confining pressure (Pc=0.001–0.3 MPa), temperature (T=−10–150°C) and strain rate (10−2–10−6 s−1). This system is ideally suited for studying grain-scale processes across the complete spectrum of deformational behaviour from brittle (cataclastic) to fully viscous (mylonitic) flow. The experimental system comprises two main elements: (1) a modified Urai–Means deformation rig, and (2) a steel pressure vessel that accommodates the deformation rig, yet is small enough to fit under an optical microscope. First results of controlled confining pressure experiments on fluid-bearing polycrystalline norcamphor, an organic quartz analogue, indicate that deformation involves a combination of dislocation creep and intergranular hydrofracturing leading to the formation of discrete shear surfaces comparable to C′ planes in mylonite. At applied confining pressure, the relative activity of brittle and viscous deformation mechanisms varies across the sample. This in turn affects the distribution of strain on the sample scale. The pressure-dependence of strain localization in these experiments is interpreted to reflect a combination of local variations in the effective stresses and strain incompatibilities associated with the experimental, simple-shear configuration.