IN SITU OBSERVATION OF THE DEVELOPMENT OF SHEAR-ZONE MICROSTRUCTURES AND STRAIN LOCALIZATION USING A TRANSPARENT ROTARY SHEAR APPARATUS

Using a transparent rotary shear apparatus and rock-analog granular materials, we investigate the micromechanics of slip in shear zones. We evaluate the effects of grain size distribution on grain kinematics, such as prevalence of rolling and sliding. We track the emergence and evolution of shearing-related microstructures and investigate conditions that lead to strain localization. Our experimental apparatus consists of a ring-shaped transparent sample chamber, with a width of 1.5 cm and a thickness of 2 cm. The upper wall of the chamber functions as a piston and also transmits the shearing force to the sample. An immersion fluid, with an index of refraction similar to that of the transparent sample material, facilitates the observation of individual grains throughout the width of the sample chamber. Normal stresses of up to 344 KPa and slip rates of up to 60 μm/s can be attained. Granulated sugar and potassium alum powder are used as rock analog materials. Initial experiments have been conducted at P = 0.3 MPa, a steady slip rate of 50 μm/s and at room temperature, on sugar assemblages with uniform grain sizes of approximately 0.5 mm, immersed in a saturated sugar solution that acts as immersion fluid. During our initial tests we have observed limited grain sliding and distributed deformation throughout the thickness of the samples.