Estimating local strain due to comminution in experimental cataclastic textures

Abstract

Cataclastic textures were produced by deforming porous quartz sandstone samples at room temperature, 15–200 MPa confining pressure, and 13–57% strain ( γ∼2–20). Particle size distribution (PSD) and fractal dimension ( D) values were measured on different digital images of each sample. Over the range of applied axial strain the detectable effects of comminution (particle-size reduction through fracture) on PSD were limited to the population of particles ≥7 μm. Textural variable ψ, named comminution intensity, is defined in terms of total sectional area of particle population greater than a critical size S c. For the Massillon sandstone the S c is close to four times the suggested value for quartz grinding limit. The comminution intensity vs. axial strain data due to the entire range of applied pressure and strain fits a power function of the form ϵ 11= ψ m , with 0.25≤ m≤0.5. The local strain due to constrained comminution is then predictable by model function ϵ L= ψ n , where n=1/ m, with empirical n=2.97 for the studied sandstone. The model predicts relatively monotonous textures for very small and very large strains, while the highest variety of particle size occurs at between 2 and 13% strain. Applications of the method include millimeter to meter scale mapping of the relative degree of comminution in natural fault gouge and the local strain values in experimental samples.