Breaking Up: Comminution Mechanisms in Sheared Simulated Fault Gouge

Abstract

Abstract-The microstructural state and evolution of fault gouge has important implications for the mechanical behaviour, and hence the seismic slip potential of faults. We use 3D discrete element (DEM) simulations to investigate the fragmentation processes operating in fault gouge during shear. Our granular fault gouge models consist of aggregate grains, each composed of several thousand spherical particles stuck together with breakable elastic bonds. The aggregate grains are confined between two blocks of solid material and sheared under a given normal stress. During shear, the grains can fragment in a somewhat realistic way leading to an evolution of grain size, grain shape and overall texture. The 'breaking up' of the fault gouge is driven by two distinct comminution mechanisms: grain abrasion and grain splitting. The relative importance of the two mechanisms depends on applied normal stress, boundary wall roughness and accumulated shear strain. If normal stress is sufficiently high, grain splitting contributes significantly to comminution, particularly in the initial stages of the simulations. In contrast, grain abrasion is the dominant mechanism operating in simulations carried out at lower normal stress and is also the main fragmentation mechanism during the later stages of all simulations. Rough boundaries promote relatively more grain splitting whereas smooth boundaries favor grain abrasion. Grain splitting (plus accompanying abrasion) appears to be an efficient mechanism for reducing the mean grain size of the gouge debris and leads rapidly to a power law size distribution with an exponent that increases with strain. Grain abrasion (acting alone) is an effective way to generate excess fine grains and leads to a bimodal distribution of grain sizes. We suggest that these two distinct mechanisms would operate at different stages of a fault's history. The resulting distributions in grain size and grain shape may significantly affect frictional strength and stability. Our results therefore have implications for the earthquake potential of seismically active faults with accumulations of gouge. They may also be relevant to the susceptibility of rockslides since non-cohesive basal shear zones will evolve in a similar way and potentially control the dynamics of the slide.

Shear localization, velocity weakening behavior, and development of cataclastic foliation in experimental granite gouge

“…Model simulations of Mair and Abe (2008) showed a strong correlation between regions of enhanced grain size reduction and localized strain. Particle shape and roughness as well as size were shown to be significant contributing factors in strength and behavior of fault gouge (Guo and Morgan, 2006;Anthony and Marone, 2005;Mair and Abe, 2011). We showed that D > 1.6 in the experimental gouge not only indicates a fining of the particles, it also indicates a narrowing of the particle size range (decrease in sorting ratio Q), and a tendency to develop a non-fractal PSD.…”

Application of open‐source photogrammetric software MicMac for monitoring surface deformation in laboratory models

“…Although commonly used, these models have serious limitations: They model simplistic intrusion and fault shapes, such as point source [ Mogi , ; Masterlark , ], tensile [e.g., Okada , ; Amelung et al ., ; Wright et al ., ; Chang et al ., ; Sigmundsson et al ., ], or shear dislocation rectangle [ Okada , ; Shen et al ., ; Gusman et al ., ; Yue et al ., ]. These deformation sources are not representative of the complex shapes of magmatic intrusions or fault planes in nature [e.g., Burchardt , ; Lohr et al ., ; Burchardt et al ., ]. They model static intrusions/fault planes, such that they do not account for the complex magma propagation mechanisms [ Mathieu et al ., ; Abdelmalak et al ., ] or fault mechanics [ Mair and Abe , , ; Brodsky and Lay , ]; It is impossible to quantify the uncertainties of the model results and so to test their robustness. The main reason is that active geological processes occur in the subsurface, so that the results of the modeling cannot be validated by direct observations. …”

Deformation mechanisms and evolution of the microstructure of gouge in the Main Fault in Opalinus Clay in the Mont Terri rock laboratory (CH)

“…In experiments, cataclastically generated gouges yield power-law GSDs with exponents (D) increasing with strain until a static value of D ∼ 2 is reached. Henceforward, abrasion and wear dominate over grain splitting (e.g Sammis and King, 2007;Mair and Abe, 2011). For the OPA, undeformed material shows D = 1.97 already in existence, in parity with the gouge (D = 1.98).…”