Estimating Strain Due to Comminution in Simulated Gouge

We attempt to estimate local strain due to comminution in large-displacement simulated gouge.The local strain is an estimate of the intensity of comminution over a small area of the sample.The values thus measured could be used to construct profiles for the desired sections of the deformed gouge. Comminution intensity Ψ , is approximated as [1-A/(A₀-Φ A₀)], where A₀ is the image area, A is the total sectional area of deformed particles >= critical particle size S_c, and Φ is the initial porosity. Particle size distribution (PSD) plots (size S vs. cumulative number < S) merge for the least deformed and most deformed samples below a certain particle size. This supports the assumption that since PSD evolves mainly due to comminution, S_c is the particle size below which the typical PSD is generally insensitive to strain. The effect was observed in this study as well as in experimental cataclastic deformation of porous quartz sandstone (Φ =11%, 12-53% strain at 15-200 MPa confining pressures) where the PSD data suggested S_c ~ 7μm. For the sandstone samples the range of axial strain was predictable by e₁₁ = Ψ ⁿ for 4 >= n >= 2. Microstructures from two rotary shear experiments on simulated Westerly granite gouge taken to 499, and 928mm total displacements were studied using the equation described above and in comparison with the results of the sandstone study. The cataclastic textures include gradational zones of different particle size, sharp particle size gradients and multiple slip surfaces. Particle size, and particle area data were obtained from 500x digital images in optical reflected light along several traverses perpendicular to the shearing direction (32 overlapping images/traverse). The gouge PSD data suggested S_c ~ 3μm (useful measurement resolution ~ 0.7 μm); Φ = 0.25 was used. The results are presented as strain profiles, on which the slip surfaces (highly localized deformation) appear as distinct discontinuities because of impossible Ψ values. Distinct slip surfaces in the gouge are typically associated with Ψ > 0.7. Using 4 >= n >= 2, the measured Ψ values give local strain values in the simulated gouge that correspond to axial strain between 34-50% in the sandstone. However, the actual engineering shear strains Γ in the gouge are often much larger than this, ranging from 10 to several hundred or several thousand. Thus the results imply that 1. The slip surfaces developed early in the strain history, and have accommodated most of the displacement. 2. Particle comminution may not be the dominant deformation mechanism in the relatively small volume surrounding slip surfaces. Particle rolling and sliding are possible mechanisms. In addition, TEM of the fine gouge fragments in previous studies has revealed the presence of many particles much smaller than 3μm as well as amorphous material within the highly localized slip zones. This suggests that at high strain, comminution and additional strain-related processes may go on below the apparent S_c

for these samples.