Grain scale processes in fault rock : a comparison between experimental and natural deformation

The aim of this study is to identify the processes that play a role in and after the formation of fault rock by investigating their microstructures. We apply these observations to interpret the processes related to faulting in natural rocks. This thesis focusses on (1) the development of analytical methods to quantify the grain size and grain shape of fault rock, (2) the investigation of the microstructures that formed during deformation of the rock with emphasis on the grain size distribution of the fault rock, (3) the effect of time, temperature and strain rate on the microstructures of fine grained gouge in the presence of a fluid, and (4) to compare gouge formed under controlled laboratory conditions with natural gouge. For these purposes a series of deformation experiments with and without subsequent heat treatment on intact granitoid and basalt rock samples were performed at T = 300 – 500 ◦ C, Pc = 500 MPa, at ˙ = 3.5 x 10−4 – 1.3 x 10−7 s−1 and with 0.2 wt% H2 O. The heat treatment was performed at T = 200 – 500 ◦ C, Pc = 500 MPa forhours to 14 days under hydrostatic and non-hydrostatic conditions. Thin sections of the samples were investigated with scanning electron and optical polarization microscopy. The results of the laboratory experiments were compared to three natural granitoid fault systems, (1) the Nojima Fault Zone (Japan), (2) fault zones in the Black Forest (Germany), and (3) the Orobic Thrust (Italian Southern Alps). The grain shape analysis provides a clear distinction between cracked grains and gouge of quartz and feldspar grains with the following shape descriptors (1): aspect ratio (longest / shortest diameter); cracked quartz (range: 1.0–8.0, average 2.9) has a higher ratio than K-feldspar (range: 1.0–4.0, average 2.1), gouge has a low aspect ratio (range: 1.0–3.0, average 1.5), (2) paris factor and the deltA (difference between a shape, perimeter and area, respectively, and its convex envelope); cracked grains yield higher values (range: 0–100%, average: 15% for feldspar and 5% for quartz) than gouge (range: 0–20%, average: 2.5%), and (3) Ω-value (fraction of angles < 0◦ in a histogram of vertex angles); cracked material reaches higher values (30–40%) than gouge (10–20%). The grain size distributions (GSD) of the deformed samples are quantified using the D-value (slope of log(frequency)-log(radius) of the grain size distribution) for quartz and feldspar gouge. Cracked grains and gouges can be distinguished on the basis of their D-value. For both types of fault rocks two slopes are observed: for grain sizes, r, of ∼ 30 nm < r <µm, D< ≈ 0.9–1.1 for all fault rock; for r >µm, cracked material shows D> ≈ 1.5–1.6, while gouge has D> ≥ 2.0 for quartz and feldspar, and D> = 1.8 for pyroxene gouge. D> of quartz and feldspar gouge is dependent on the deformation conditions; an increase in the confining pressure or temperature,