Water-induced shear localization and slip mode partitioning in montmorillonite gouge

Abstract

Montmorillonite is a clay mineral that commonly occurs in shallow subduction zones and crustal faults and may exhibit a lower frictional strength than other clay minerals when wet. Water in the interlayers can modify the frictional strength of montmorillonite by forming thin water films that lubricate grain contacts and, by reducing effective normal stress under undrained compression where fluid cannot escape. However, little is known about how water affects the development of deformation structures within montmorillonite gouge and slip behavior. We conducted low-velocity shear experiments to explore such aspects using dry powders and wet pastes of Ca-montmorillonite and Na-montmorillonite. The dry powder experiments exhibited high frictional strength, high apparent shear stiffness (or the stiffness reflecting the combined effects of loading frame, forcing blocks, and gouge layers), and stable slip behavior with predominant foliated structures. In contrast, the wet paste experiments showed a lower frictional strength, lower apparent shear stiffness, and unstable stick-slip behavior, with the development of both foliated zones and shear localization zones. The stable slip in the dry powder was possible because the rheological stiffness, or the rate of fault weakening during slip, of the material with foliated zones was negative and thus lower than the apparent shear stiffness. In the wet paste, unstable stick-slip was possible because the shear localization zone had a rheological stiffness higher than the apparent shear stiffness, and stable slip is inferred to have occurred in the foliated zones. Our findings reveal that water in montmorillonite gouges not only significantly lowers the frictional strength but also promotes the formation of shear localization zones as well as foliated zones, eventually leading to the partitioning of slip into different structural zones of montmorillonite-bearing faults. While the velocity-strengthening behavior of a slip zone is generally interpreted as precluding unstable slip, our study provides evidence that this is not necessarily the case, as velocity-weakening and stick-slip events can still occur within slip localization zones, which are parts of the slip zone. This structure-related slip mode partitioning offers a new perspective on conventional fault stability assessments. © 2025 Elsevier Ltd.