Mapping grain size distributions and surface density in experimentally faulted granitoids

In a suite of experimentally faulted granitoids we measured the grain size distribution and the grain boundary surface created during faulting and sliding on the fault. Coaxial deformation experiments were carried out on isotropic Verzasca gneiss using a Grigg's deformation apparatus at 300-500 °C, 500-1000 MPa, strain rates of 10-4s-1 to 10-7 s-1 and 0.2 %wt H2O added. On a log-log plot the grain size distribution for the freshly fractured fault rock (CRACKED) shows two slopes D: D=1.0 for the size range from the smallest grain size (approx. 15 nm) up to 1-3 μm. D=1.5 for the larger grain sizes up to 100 μm. After sliding on the fault, the cracked material turns into a mature gouge (GOUGE), and the grain size distribution changes: the slope for the larger grain size range is increases: D=2.0. Using SEM images and applying the D-mapping technique (Heilbronner and Keulen, in press) the slope of the grain size distribution of the cracked and gouge material in the sample is mapped. This procedure visualizes the development of the fault (fault propagation) and localization of the deformation (gouge formation) in the sample. From the grain size distributions, the grain boundary surface is calculated. For the cracked and gouge material, surface densities between approx. 10^{+6} (for cracked) to approx. 10^{+9} (for mature gouge) have been obtained. Both the map of the cracked and gouge material (the "damage zone") and the surface density values can be compared with the mechanical data of the experiement or the seismic data from earth quakes. The surface density found in our samples is very low and probably only accounts for a very small fraction of the energy expended in the experiment.