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

Abstract

Although the effects of mechanical amorphization by fault motion on fault rocks have been investigated both in nature and experiments, the relationship between slip processes and the amount of amorphous materials produced remains unclear. We performed rotary-shear friction experiments on synthetic quartz and kaolinite gouges at room temperature, normal stresses of 1 or 3 MPa, slip velocities of 0.001 or 1 m s −1 , and displacements of 1-101 m. X-ray diffraction and microscopic observation data revealed that mechanical amorphization in both materials was accompanied by grain-size reduction, particle rounding, and the formation of ultrafine particles. The amount of amorphous materials produced was strongly dependent on mineralogy and total frictional work regardless of slip velocity. Therefore, mechanical amorphization can occur during both coseismic and subseismic slip. Amorphization of~7 wt% of quartz gouge required 30.33 MJ kg −1 of frictional work, whereas for kaolinite gouge, only 0.77 MJ kg −1 was required, which is consistent with observed preferential amorphization of clay minerals in faults. For kaolinite, up to 6% of frictional work can be used for mechanical amorphization, indicating its potential importance for faulting energetics. Because some kinds of amorphous phyllosilicates release water at lower temperature than crystalline, mechanical amorphization of fault rocks may influence thermochemical pressurization. Gradual weakening of quartz gouge at 0.001 m s −1 slip velocity suggested that <4 wt% of amorphous silica can weaken a fault by amorphous silica-related lubrication. Mechanical amorphization may accelerate pressure solution healing of faults if dissolution can occur under high porous conditions.

Formation of Nanocrystalline and Amorphous Materials Causes Parallel Brittle‐Viscous Flow of Crustal Rocks: Experiments on Quartz‐Feldspar Aggregates

“…Nanocrystalline to amorphous materials have been increasingly frequently identified within zones of strain localization in experiments on felsic, feldspar‐rich rocks such as granitoids (Hadizadeh et al., 2015; Pec, Stünitz, & Heilbronner, 2012; Pec, Stünitz, Heilbronner, Drury, et al., 2012; Pec et al., 2016; Yund et al., 1990) and mafic, feldspar‐rich rocks such as diabase and gabbro (Marti et al., 2017, 2020; Weiss & Wenk, 1983). While feldspars seem to be especially prone to amorphization, nanocrystalline to amorphous material has also been documented in experiments on quartzites (Di Toro et al., 2004; Goldsby & Tullis, 2002; Hayward et al., 2016; Rowe et al., 2019; Toy et al., 2015), in clay minerals (Aretusini et al., 2017; Kaneki et al., 2020), and even carbonates (Delle Piane et al., 2018; Verberne et al., 2013, 2014) suggesting that comminution to nanometric grain sizes and concomitant amorphization is prevalent during experimental rock deformation.…”

“…While feldspars seem to be especially prone to amorphization, nanocrystalline to amorphous material has also been documented in experiments on quartzites (Di Toro et al, 2004;Goldsby & Tullis, 2002;Hayward et al, 2016;Rowe et al, 2019;Toy et al, 2015), in clay minerals (Aretusini et al, 2017;Kaneki et al, 2020), and even carbonates (Delle Piane et al, 2018;Verberne et al, 2013Verberne et al, , 2014 suggesting that comminution to nanometric grain sizes and concomitant amorphization is prevalent during experimental rock deformation.…”

Seismic fault weakening via CO2 pressurization enhanced by mechanical deformation of dolomite fault gouges

“…2B). However, mechanical decarbonation in a fault depends on the magnitude of the mechanical energy applied to the fault zone during slip (Aretusini et al, 2017;Kaneki et al, 2020). Thus, the occurrence of mechanical decarbonization depends on the fault.…”

Nanoscale Textural and Chemical Evolution of Silica Fault Mirrors in the Wasatch Fault Damage Zone, Utah, USA

“…In this model, coseismic fault slip and associated comminution facilitates amorphization in three ways including (1) generation of frictional heat (i.e., shear heating; Ben‐Zion & Sammis, 2013), (2) creation of more reactive surfaces (Di Toro et al., 2011); and (3) liberation of any water in fluid inclusions, along grain boundaries, and within hydrous mineral phases in the Xfc (Goldsby & Tullis, 2002). Amorphous fault material can be generated via mechanical amorphization in the absence of elevated temperatures at sub‐seismic slip rates (Kanecki et al., 2020; Marti et al., 2020; Pec et al., 2012, 2016; Yund et al., 1990). However, our observations, including N substitution in anatase and titanite, indicate elevated temperatures accompany amorphization of what is now the FM volume.…”

“…Fault weakening mechanisms identified in natural rocks and deformation experiments include mineral decomposition and dehydration (Collettini et al., 2013; Han et al., 2007), dynamic recrystallization (Smith et al., 2013), nanoparticle lubrication (De Paola et al., 2011), amorphization (Rowe et al., 2019), asperity flash heating (Beeler et al., 2008; Goldsby & Tullis, 2011; Kohli et al., 2011; O'Hara, 2005; Rice, 2006), and frictional melt and gel lubrication (Di Toro et al., 2006; Goldsby & Tullis, 2011; Kirkpatrick et al., 2013). Some dynamic weakening mechanisms require coseismic temperature rise; but others, such as mechanical amorphization, do not (Kaneki et al., 2020; Marti et al., 2020; Pec et al., 2012; Yund et al., 1990). In addition, dynamic weakening may cause rheological changes that ultimately restrengthen faults (Ault et al., 2019; Di Toro et al., 2011; Mitchell et al., 2016; Proctor & Lockner, 2016; Rowe et al., 2019; Yasuhara et al., 2005).…”