PARTICLE SIZE-DRIVEN SHEAR LOCALIZATION AND CATACLASTIC FOLIATION IN SIMULATED GRANITE GOUGE

Frictional sliding tests were performed in rotary-shear at 25 MPa normal stress on simulated Westerly granite gouge (particle size≤85μm, 35% porosity). Sliding velocity was stepped to 1 and 10µm/s during each test. Tests were terminated after: (1) 49 mm of sliding, after a persistent decrease in friction, (2) 79 mm of sliding involving that decrease followed by a persistent increase in friction, and (3) 387mm of sliding involving several variable amplitude fluctuations in friction. Microstructural attributes, including slip surfaces, particle-size, and porosity was measured and studied using SEM imaging, image processing, and EDX analyses.

The velocity dependence of friction was negative during most of the sliding, and fluctuations in friction, velocity dependence, and gouge thickness changes were reduced with increased sliding. A master Y-shear formed in all experiments at sliding distances <50mm. The most notable microstructures in (2) and (3) were stacks of disrupted bands on either sides of the main shear. The term ‘asymmetric shear band' is used to describe the bands since the average particle size across individual band underwent an exponential reduction ranging from ≈2μm on one side to ≤15nm on the opposite side (the side undergoing slip). Porosity across the bands showed similar changes. Commonly, the bands contained extremely dense gouge at the slip side with no discernible particle outlines or pore spaces.

The observed features of the asymmetric shear bands including stacking, consistency of thickness and fabric, absence of ductile deformation, and association with a master Y-shear make them comparable to natural cataclastic foliation. It is proposed that locally non-linear shear strain (dγ/dx) becomes asymptotic to lines of Y-shear orientation leading to repeated shear localizations. A band may continue to localize strain until particles that line the slip-side achieve a critical average size, or a critical packing density that harden the slip side. Microstructural relations suggest that there might be two distinct strength thresholds, one resulting from the band slip-hardening, and another that requires the wholesale breakup of the stacks. It is possible that the interaction or net outcome of these two thresholds produce the observed mechanical effects.