'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 obser vation of evolving microstructures in rock analogues under an optical macro scope at controlled confining pressure (P-c = 0.001-0.3 MPa), temperature ( T = -10-150 degrees C) and strain rate (10(-2)-10(-6) s(-1)). This system i s ideally suited for studying grain-scale processes across the complete spe ctrum of deformational behaviour from brittle (cataclastic) to fully viscou s (mylonitic) flow. The experimental system comprises two main elements: (1 ) a modified Urai-Means deformation rig, and (2) a steel pressure vessel th at accommodates the deformation rig, yet is small enough to fit under an op tical microscope. First results of controlled confining pressure experiment s on fluid-bearing polycrystalline norcamphor, an organic quartz analogue, indicate that deformation involves a combination of dislocation creep and i ntergranular hydrofracturing leading to the formation of discrete shear sur faces comparable to C' planes in mylonite. At applied confining pressure, t he relative activity of brittle and viscous deformation mechanisms varies a cross the sample. This in turn affects the distribution of strain on the sa mple scale. The pressure-dependence of strain localization in these experim ents is interpreted to reflect a combination of local variations in the eff ective stresses and strain incompatibilities associated with the experiment al, simple-shear configuration. (C) 2000 Elsevier Science Ltd. All rights r eserved.