Grain size distributions of fault rocks: A comparison between experimentally and naturally deformed granitoids

We have investigated the grain size distribution (GSD) of granitoid fault rock by comparing experimentally produced gouge with fault rock from the Nojima Fault Zone. Triaxial experiments were carried out on wet and dry intact samples of Verzasca Gneiss at T = 300 and 500 °C, Pc = 500 and 1030 MPa, ε˙=0.013–1.6×10−4s−1. The GSD has been determined from SEM-micrographs and is characterized by the slope, D, of its log(frequency)–log(radius) histogram. The GSD is not fractal; we observe two slopes for all GSDs. The larger grains in experimentally deformed samples have a D-value, D>, of 2.04 and 2.26 for feldspar and quartz gouge. Cracked grains yield values of D = 1.5–1.6. Increasing the confining pressure or temperature decreases the D-value. For grains smaller than ∼2 μm another D-value, D<, of 0.9–1.1 is observed. The grain radius at the slope change, rK, corresponds to the grinding limit of quartz, so that rK probably represents a change in the dominant comminution mechanism from grinding to attrition processes. The GSD obtained from deformation experiments agrees well with results for the Nojima Fault Zone: D> = 2.02 for gouge and 1.64 for cracked grains; D< = 0.97. Grain size reduction in fault zones develops by a two-stage process: rupturing creates cracked grains; further displacement of fragments causes further comminution by wear and attrition. Cracked grains have been used to calculate the surface energy associated with faulting; it follows that this energy forms a small fraction in the total energy-budget of earthquakes.