Porosity and particle shape changes leading to shear localization in small-displacement faults

A microstructural study of shear localization in fault gouge was carried out in small-displacement faults so there would be minimum masking effects from a complex deformation history. We studied particle size, shape, and porosity changes in gouge adjacent to zones of shear localization in natural and synthetic gouges subjected to shear displacements δ, of up to 1.2 m. Scanning electron microscope images were used for estimating image porosity ΦI, and measuring particle size of the deformed and undeformed gouges. The particle size data were used for calculating simulated porosity ΦS from computer-generated simple fractal gouge model of each sample. Modeled microstructures contained round grains and a fractal distribution matched to that of the measured natural samples. Changes in ΦI, ΦS, and ΦI/ΦS with increasing δ were used for tracking changes in particle shape and porosity of the gouges precursory to shear localization. The ΦI and ΦS values for the natural and synthetic gouges converge at δ ∼ 0.1 m, suggesting that gouge particles adjacent to shear localization sites tend to become rounded. Porosity for such densified regions of the gouge adjacent to Y-shear zones was determined to be <1% at large displacements. In the same regions, the porosity reductions were also associated with decreased sorting coefficient and fractal dimensions D > 2.6. The study suggests that brittle shear localization may involve favorably-oriented micro porous pockets of gouge that result from competing changes in particle shape and particle size, which tend to affect gouge porosity in different ways.