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Fine grain bands in faults are normally interpreted as indicative of slip localization. However, grains can migrate systematically during shear resulting in nonintuitive effects on the final structure. Here we report the results of shear experiments performed on natural granular materials and discuss their possible implications for natural shear systems. We sheared dry grains in a rheometer over a range of velocities, followed the structural evolution with video during shear, and characterized the final structures. Fine particles comminuted during high-velocity shear migrated from the highest shear rate regions to a layer where shear rate is near zero. This migration is an example of shear segregation that is driven by grain-inertia effects. The segregation forms an apparent shear band displaced from the principal slip surface and has a stabilizing effect on subsequent flow. This shear segregation may result in fine-particle layers and slip localization in natural shear zones.

Plain Language Summary Geologists commonly observe layers in fault zones, yet the formation of these layers remains a poorly understood process. Here we use laboratory experiments to show that layers can form by small particles being kicked out of the fast-moving regions of a fault zone. The small particles pond where the grains are moving more slowly and form a distinct shear layer. The accumulation of fine particles in between larger grains makes this layer harder. This hardness change could be critical for the slip behavior during subsequent earthquakes.