• Rutter, Ernest

Abstract

mectite 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 porefluidsaffects the likelihood that seismic slip propagates from deeper to shallow fault depths or that a landslideaccelerates to itsfinal collapse. To investigate the deformation processes active during seismic faultingwe 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, and1.3 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 theexperiments was included in a one‐dimensional numerical model incorporating frictional heating,thermal, and thermochemical pressurization. Quantitative X‐ray powder diffraction and scanning electronmicroscopy investigations were performed on pristine and deformed smectite‐rich gouges. Under dryconditions, cataclasis and amorphization dominated at slip rates of 0.001–0.1 m/s, whereas grain sizesensitiveflow and, under vacuum, frictional melting occurred at fast slip rates (1.3 m/s). Under partlysaturated conditions, frictional slip in a smectite foliation occurred in combination with pressurization ofwater by shear‐enhanced compaction and, forV= 0.01–1.3 m/s, with thermal pressurization.Pseudotachylytes, the only reliable microstructural markers for seismic slip, formed only with largefrictional power (>2 MW/m2), which could be achieved at shallow depth with high slip rates, or, at depth,with high shear stress in dehydrated smectites.

Topics

• grain
• experiment
• strength