Water Availability and Deformation Processes in Smectite‐Rich Gouges During Seismic Slip

Abstract

Smectite clays occur in subduction zone fault cores at shallow depth (approximately 1 km; e.g., Japan Trench) and landslide décollements (e.g., Vajont, Italy, 1963). The availability of pore fluids affects the likelihood that seismic slip propagates from deeper to shallow fault depths or that a landslide accelerates to its final collapse. To investigate the deformation processes active during seismic faulting we performed friction experiments with a rotary machine on 2-mm-thick smectite-rich gouge layers (70/30 wt % Ca-montmorillonite/opal) sheared at 5-MPa normal stress, at slip rates of 0.001, 0.01, 0.1, and 1.3 m/s, and total displacement of 3 m. Experiments were performed on predried gouges under vacuum, under room humidity and under partly saturated conditions. The fault shear strength measured in the experiments was included in a one-dimensional numerical model incorporating frictional heating, thermal, and thermochemical pressurization. Quantitative X-ray powder diffraction and scanning electron microscopy investigations were performed on pristine and deformed smectite-rich gouges. Under dry conditions, cataclasis and amorphization dominated at slip rates of 0.001-0.1 m/s, whereas grain size sensitive flow and, under vacuum, frictional melting occurred at fast slip rates (1.3 m/s). Under partly saturated conditions, frictional slip in a smectite foliation occurred in combination with pressurization of water by shear-enhanced compaction and, for V = 0.01-1.3 m/s, with thermal pressurization. Pseudotachylytes, the only reliable microstructural markers for seismic slip, formed only with large frictional power (>2 MW/m 2 ), which could be achieved at shallow depth with high slip rates, or, at depth, with high shear stress in dehydrated smectites.

Mechanical Amorphization of Synthetic Fault Gouges During Rotary‐Shear Friction Experiments at Subseismic to Seismic Slip Velocities

“…About 7 wt% of the quartz and kaolinite gouges was amorphized at frictional work of 30.33 and 0.77 MJ kg −1 , respectively (Tables 3 and 4), clearly showing that less work is required to amorphize kaolinite. This result is consistent with the results of previous studies that demonstrated preferential mechanical amorphization of clay minerals over quartz in clay‐bearing fault gouges during friction experiments (Aretusini et al, 2017, 2019). For instance, rotary‐shear friction experiments by Aretusini et al (2019) on samples composed of a mixture of smectite and opal‐CT showed that the amorphous material produced was mechanically amorphized smectite.…”

“…This result is consistent with the results of previous studies that demonstrated preferential mechanical amorphization of clay minerals over quartz in clay‐bearing fault gouges during friction experiments (Aretusini et al, 2017, 2019). For instance, rotary‐shear friction experiments by Aretusini et al (2019) on samples composed of a mixture of smectite and opal‐CT showed that the amorphous material produced was mechanically amorphized smectite. Our results show that less frictional work is needed to distort the crystal structure of kaolinite than is the case for quartz, thus providing a clear explanation for the preferential amorphization of clay minerals during fault slip.…”

Faulting Processes Unveiled by Magnetic Properties of Fault Rocks

“…The temperature within a slip zone is locally elevated by coseismic frictional heating to quite variable maximum temperatures, even at shallow crustal levels. This is surmised from widely reported thermochemical reactions: clay mineral reactions (e.g., Brantut et al, 2008;L.-W. Kuo et al, 2011), decarbonation reactions (e.g., Han et al, 2007;Rowe, Fagereng, et al, 2012), amorphization (Aretusini et al, 2019;Rowe et al, 2019), trace element partitioning features (e.g., Ishikawa et al, 2008;Tanikawa et al, 2015), graphitization of carbonaceous material (e.g., Kuo et al, 2014;Oohashi et al, 2011), thermal maturation of Open and solid circles represent projection of the vector on the vertical and horizontal planes, respectively.…”

Fluid pressurisation and earthquake propagation in the Hikurangi subduction zone

“…d ranged from 0.0012 to 0.0017 m). The gouge subdomain had a thermal conductivity of 1.5 W/(m·K), a density of 2400 kg/m 3 and a heat capacity of 1000 J/(kg·K) 51 . Two subdomains, representing the steel porous plates, were set in the intervals –d − 0.02 ≤ x ≤ − d and d ≤ x ≤ d + 0.02, with a thermal conductivity of 8.6 W/(m·K), density of 6162 kg/m 3 and heat capacity of 475 J/(kg·K) (Mottcorp datasheet).…”