Comminution‐Induced Transient Frictional Behavior in Sheared Granular Halite.

Grain comminution is commonly observed in numerous geological settings. To elucidate the role of grain comminution in dry granular friction, we sheared breakable halite (NaCl) grains using a ring‐shear configuration at a constant slip rate under various normal stresses. We observed transient frictional behaviors: a constant regime exhibiting a high friction coefficient at small slip displacements, and a weakening regime showing a substantial decay in friction at large slip displacements. The characteristic slip lengths for both regimes decreased with normal stress and were characterized by similar exponents. Micro‐X‐ray tomography revealed the evolution of microstructure from distributed grain comminution to progressive shear localization for these two regimes. We propose that the filling processes of comminuted fine particles, during which fine particles saturate and then overflow the shear zone, define transient frictional behaviors. This study may hold significant implications for natural shear systems, given the ubiquity of comminution and localization phenomena. Plain Language Summary: Grain comminution and structural evolution are common phenomena in natural settings, including earthquake faults and landslides. However, their role in granular friction remains unclear. To investigate this, we experimentally sheared breakable NaCl grains to simulate the processes within growing fault zones and visualized microstructural evolution using micro‐X‐ray computed tomography (CT). We observed two distinct frictional behaviors: a constant regime exhibiting a high friction coefficient at small slip displacements, and a weakening regime showing a substantial decay in friction at large slip displacements. The characteristic slip lengths for both regimes decreased with normal stress. Micro‐observations revealed drastic grain comminution and segregation processes in the constant regime, while the weakening regime showed progressive shear localization evolving from multiple discontinuous shear planes to one extremely localized shear plane. The higher constant friction appeared to result from large grain contacts, while the substantially lower steady‐state friction arose from comminuted fine particle contacts. We propose that grain comminution generates fine particles, gradually filling the pores within the shear zone in the constant regime, ultimately leading to frictional weakening by effectively reducing large grain contacts. The characteristic lengths defining transient behavior may be influenced by geometrical complexities and boundary conditions in various geological settings. Key Points: Sheared granular halite exhibits constant friction at small slip displacement and substantial weakening at large displacementCharacteristic slip lengths for constant friction and weakening decrease with normal stress and are characterized by similar exponentsThe production, saturation, and overflow of comminuted fines in the shear zone are key factors determining transient frictional behavior [ABSTRACT FROM AUTHOR]