title: Phyllosilicate-bearing fault zones: the mechanical role of fluids, lithology and temperature.
Creators: name: Ashman, Isabel
Publisher: name: University of Liverpool Repository
Year: 2023

description: Large earthquakes nucleate on crustal faults that have accumulated significant slip displacement and field observations show that these mature fault cores are comprised of extremely fine, low permeability, phyllosilicate-bearing gouges. Phyllosilicate minerals have layered anisotropic structures that are thought to impart an anomalously weak mechanical behaviour to the deformation of fault rocks and shear zones. An area of interest in rock mechanics research is the discrepancy between observed natural fault behaviour and the data collected from rock deformation experiments. Phyllosilicate-bearing materials in rock mechanics experiments consistently exhibit weak, stable sliding deformation behaviour, whereas clay-bearing, crustal-scale faults in nature can rupture via aseismic creep, slow slip or fast earthquakes. This thesis aims to investigate and quantify the controls on the deformation of phyllosilicate-bearing fault rocks including the dilatancy of granular fault materials, the effect of temperature on fault slip and the deformation of heterogeneous fault zones. Granular fault materials are known to dilate in response to increases in sliding velocity, resulting in significant pore pressure drops that can suppress instability, known as dilation hardening. Dilation hardening is a mechanism proposed to govern slow slip earthquakes. A suite of triaxial deformation experiments revealed that, upon a 10-fold velocity increase, gouges of systematically varied clay-quartz contents displayed measurable dilatancy. Peak dilation did not occur in the pure quartz gouges, but rather in gouges containing 10 to 20 wt% clay. A transition occurred at ~40 wt% clay content from strong, unstably sliding quartz-dominated gouges to weak but stably sliding clay-dominated gouges. These results indicate that in a low permeability, clay-rich fault zone, the increases in pore volume could transiently decrease pore-fluid pressure through dilatant hardening, thus contributing to the arrest of earthquake nucleation or the promotion of sustained slow slip. The vast majority of previous experiments that investigated the frictional stability of phyllosilicate-bearing gouges were performed at room temperatures (~20°C), but elevated temperatures are known to affect frictional behaviour. There has not, to date, been a study that systematically documents the effect of frictional stability as a function of clay content and temperature. A suite of triaxial experiments showed that gouges containing ≤ 50 wt% clay became increasingly unstable with increasing temperature so that the materials displayed conditionally unstable slip at temperatures between 100 and 180°C, whereas at room temperature the same materials hosted only stable slip. This reduction in stability with increasing temperature coincides with a greater degree of localization observed in the gouge ii microstructure. This indicates that a broad compositional range of clay-bearing fault rocks can nucleate unstable slip at conditions common to the shallow brittle crust. The Carboneras Fault Zone (CFZ) in southern Spain is a 1km wide left-lateral, strike slip fault zone that consists of high-strain fault strands of phyllosilicate-rich gouges anastomosing with lower-strain lenses of basement protolith. The mechanical strength contrasts and geometry of the fault rock domains affect the distribution of strain through a fault zone. Field work was conducted in the Rambla del Cajon site in the core of the CFZ to collect samples for microstructural, chemical and frictional analyses to study how strain transferred from localising to non-localising fault materials and to search for microstructural evidence of seismicity. The fault samples showed strain localisation across multiple scales, with fault gouge strands ~10 m wide in the field to 200 μm wide in thin section. Quantification of the mineral contents of the fault rocks by X-Ray Diffraction (XRD) techniques showed that phyllosilicate minerals (muscovite/illite, chlorite, paragonite, kaolinite and smectite) dominated the assemblages. Frictional experiments showed that all of the tested fault materials were velocity strengthening, with a relatively low frictional strength contrast between the protoliths and the fault gouges. The fault gouges included pulverised quartz clasts that contained dauphiné twins, which can form due to seismic shock. It was concluded that the phyllosilicate-rich fault materials caused deformation on the CFZ to be dominated by aseismic fault creep, and that the anastomosing structure of the CFZ arose because of the low strength contrast between the protolith and fault rocks. However, the identification of pulverised quartz clasts indicates that intermittent seismic slip on the localising materials in the CFZ should not be discounted.