Shear zones in porous sand: Insights from ring-shear experiments and naturally deformed sandstones

We have used thin section and particle size analyses to relate stress–strain relationships in ring-shear experiments with burial depth at the time of faulting in naturally deformed samples. We show that the burial depth (level of stress) and the amount of shear displacement at the time of deformation are important factors influencing the type of grain breakage and also the type of shear zone that forms. Further, petrographic image analyses with porosity estimations show systematic change related to progressive development of the shear zones and the development of two end-member types of shear zones: (a) Shear zones with diffuse boundaries formed at low levels of stress, and (b) Shear zones with sharp boundaries formed at higher levels of stress. We consider the mechanism of deformation at shallow depth/low level of stress to be dominated by reorganization, rolling and flaking of grains. This mechanism causes rough surfaces of the grains at the margins and within the shear zones. At greater depths or at higher levels of stress, the predominant mode of fracturing transgresses from flaking to grain splitting, resulting in lower porosity values and greater particle size reduction. Furthermore, this transition results in a slow increase in the power law dimension from 1.4 to 1.6 with respect to the increased displacement.