Clay smear in normal fault zones – The effect of multilayers and clay cementation in water-saturated model experiments

We studied the evolution of fault zones in water-saturated model experiments consisting of sand and clay layers above a normal fault dipping 70° in a stiff basal layer. The model is bounded below by a rigid metal basement with a pre-cut 70° fault and above by a metal plate, also with a 70° cut, aligned in the same plane as the basement fault. Quantitative analysis of particle displacements was undertaken with PIV (Particle Image Velocimetry) software. In these models, the structure of initial localized deformation evolves into a kinematically favorable fault zone. This evolution, which produces releasing or restraining relays across the clay layer, has a major role in controlling fault-zone structure. We show that a high competence contrast between sand and clay leads to a more complex fault zone due to the formation of secondary shear zones and segmentation-induced fault lenses. A high competence contrast also promotes a more complex temporal evolution of those shear zones. Weak clay layers are preferentially enriched in fault zones, whereas strong, brittle clay initially fractures and forms clay boudins that rotate in the deforming sand. With progressive deformation these boudins are abraded and transformed into a soft-clay gouge. Thin, weak clays deform continuously over large displacements, and the volume of clay-rich gouge increases as sand mixes into clay at the margins of the shear zone. Thus, we observe a wide range of fault zone and fault gouge evolution by adjusting the mechanical properties of the clay. Further physical insights into fault processes like those reached here may yield predictive models of fault-zone evolution that will transcend empirical methods (e.g., shale-gouge ratio, SGR).