The effect of water on strain localization in calcite fault gouge sheared at seismic slip rates

Abstract

Strain localization during coseismic slip in fault gouges is a critical mechanical process that has implications for understanding frictional heating, the earthquake energy budget and the evolution of fault rock microstructure. To assess the nature of strain localization during shearing of calcite fault gouges, high-velocity (vmax=1m/s) rotary-shear experiments at normal stresses of 3–20 MPa were conducted under room-dry and wet conditions on synthetic calcite gouges containing dolomite gouge strain markers. When sheared at 1 m/s, the room-dry gouges showed a prolonged strengthening phase prior to dynamic weakening, whereas the wet gouges weakened nearly instantaneously. Microstructural analysis revealed that a thin (<600 μm) high-strain layer and through-going principal slip surface (PSS) developed after several centimeters of slip in both dry and wet gouges, and that strain localization at 1 m/s occurred progressively and rapidly. The strain accommodated in the bulk gouge layer did not change significantly with increasing displacement indicating that, once formed, the high-strain layer and PSS accommodated most of the displacement. Thus, a substantial strain gradient is present in the gouge layer. In water-dampened gouges, localization likely occurs during and after dynamic weakening. Our results suggest that natural fault zones in limestone are more prone to rapid dynamic weakening if water is present in the granular slipping zones

Localized Slip and Associated Fluidized Structures Record Seismic Slip in Clay‐Rich Fault Gouge

“…The character of microfoliated zones that develop during experiments on carbonate Boulton et al, 2017;Rempe et al, 2014;Smith et al, 2017), exhumed clay-bearing (Kitajima et al, 2010;Mizoguchi et al, 2007) and granitic (Stunitz et al, 2010) gouges are not as sensitive to pore fluid content as the CDZ gouge and other clay-rich gouges, and the fabric of most of these zones resemble that described for our Unit 4-D. Similar to our Unit 4-D, the localized slip zones in these other lithologies are distinguished by a fine grain size and laminae that have been interpreted to show internal variations in porosity (Rempe et al, 2017;Stunitz et al, 2010). Laminated slip zones in the ultracataclasite of the Punchbowl fault have also been interpreted to develop by progressive wear and accumulation (Chester et al, 2004), which is consistent with our observations that the inferred older laminae show increasing density and decreasing porosity.…”

“…The mechanisms that result in localization at seismic rates are thought to be controlled by shear heating and associated thermal weakening processes Platt et al, 2014Platt et al, , 2015, and these processes can produce unique microstructures (Brantut & Sulem, 2012). Recent work provides quantitative measurements of foliation orientations and grain size distributions that develop during seismic slip in carbonates (e.g., Rempe et al, 2017;Smith et al, 2017) and a wealth of qualitative observations on grain size and foliation orientations for siliciclastic and clay-rich lithologies (Aretusini et al, 2017;Kitajima et al, 2010;Mizoguchi et al, 2009;Ujiie & Tsutsumi, 2010). Our goals are (1) to provide quantitative measures of the microstructures that develop during seismic slip of clay-rich gouge at low normal stress, (2) to determine the processes that lead to the development of these microstructures, and (3) to evaluate if structures in exhumed localized clay-rich slip zones can be interpreted as having developed at seismic rates.…”

Microscale cavitation as a mechanism for nucleating earthquakes at the base of the seismogenic zone

“…3b, c ). The shear band marks an abrupt reduction in grain size, which is typically interpreted to indicate that most of the imposed shear displacement was accommodated within it 39 – 41 , suggesting an internal shear rate of ≈ 2–3 s −1 . The shear band further shows strong, uniform birefringence and optical extinction (Fig.…”

The importance of Thermo-Hydro-Mechanical couplings and microstructure to strain localization in 3D continua with application to seismic faults. Part II: Numerical implementation and post-bifurcation analysis

“…Field observations of fault zones show that, during seismic slip, shear deformation is localized in a very thin zone of finite thickness, which is called Principal Slip Zone (PSZ) (see Sibson (2003), Rempe et al (2017), Rattez et al (2017a) for a detailed discussion). This phenomenon is favored by weakening 5 of the gouge material and it is enhanced by multi-physical couplings .…”