Crystal‐Plastic Deformation in Seismically Active Carbonate Fault Rocks

Abstract

The spatial separation of macroscopic rheological behaviors has led to independent conceptual treatments of frictional failure, often referred to as brittle, and viscous deformation. Detailed microstructural investigations of naturally deformed carbonate rocks indicate that both frictional failure and viscous mechanisms might operate during seismic deformation of carbonates. Here, we investigate the deformation mechanisms that were active in two carbonate fault zones in Greece by performing detailed slip‐system analyses on data from automated crystal‐orientation mapping transmission electron microscopy and electron‐backscatter diffraction. We combine the slip‐system analyses with interpretations of nanostructures and predictions from deformation mechanism maps for calcite. The nanometric grains at the principal slip surface should deform by diffusion creep but the activation of the (0001)<1¯21¯0> slip system is evidence for a contribution of crystal plasticity. A similar crystallographic preferred orientation appears in the cataclastic parts of the fault rocks despite exhibiting a larger grain size and a different fractal dimension, compared to the principal slip surface. The cataclastic region exhibits microstructures consistent with activation of the (0001)<1¯21¯0> and {101¯4}<2¯021> slip systems. Postdeformational, static recrystallization, and annealing produce an equilibrium microstructure with triple junctions and equant grain size. We propose that repeated introduction of plastic strain and recrystallization reduces the grain size and offers a mechanism to form a cohesive nanogranular material. This formation mechanism leads to a grain‐boundary strengthening effect resulting in slip delocalization which is observed over 6 orders of magnitude (μm‐m) and is expressed by multiple faults planes, suggesting cyclic repetition of deformation and annealing.

Microscale Spatial Variations in Coseismic Temperature Rise on Hematite Fault Mirrors in the Wasatch Fault Damage Zone

“…The highest temperatures (>1000°C) are achieved where the shear zone is both narrow and underlies geometric asperities (Figures 5a and 10a), as shown in Figure 10c (panels 1–5). Regions of the slipping surface dominated by crystal plasticity experience viscous flow and low average dynamic shear strength (Figure 10a; e.g., Ohl et al., 2021; Pozzi et al., 2019; Siman‐Tov et al., 2015). Geometric asperities are loci of shear stress, resulting in extreme heating and weakening (Rice, 2006).…”

“…Rotary shear experiments and field studies of exhumed fault rocks document a myriad of dynamic weakening mechanisms, including flash heating and failure at geometric asperities (Goldsby & Tullis, 2011; Kohli et al., 2011; Rice, 2006), melting of rock and lubrication (Di Toro et al., 2006; Spray, 2005; Ujiie et al., 2007), thermal pressurization and fluidization (Ujiie et al., 2011; Wibberley & Shimamoto, 2005), gel formation (Faber et al., 2014; Kirkpatrick et al., 2013; Rowe et al., 2019), and nanoparticle lubrication by comminution and phase changes (Han et al., 2011; Verberne et al., 2014). Coseismic temperature rise, despite its short duration (on the scale of seconds), may be sufficient to trigger local crystal plasticity, even in the brittle crust (De Paola et al., 2015; Ohl et al., 2021; Pozzi et al., 2019, 2021). Different dynamic weakening mechanisms are active at variable threshold temperatures, and thus the distribution of heat across a fault surface directly impacts earthquake mechanics (e.g., Di Toro et al., 2011; Kirkpatrick & Shipton, 2009; Spagnuolo et al., 2016).…”

“…sufficient to trigger local crystal plasticity, even in the brittle crust (De Paola et al, 2015;Ohl et al, 2021;Pozzi et al, 2019Pozzi et al, , 2021. Different dynamic weakening mechanisms are active at variable threshold temperatures, and thus the distribution of heat across a fault surface directly impacts earthquake mechanics (e.g., Di Toro et al, 2011;Kirkpatrick & Shipton, 2009;Spagnuolo et al, 2016).…”

Decoding the nanoscale porosity in serpentinites from multidimensional electron microscopy and discrete element modelling

“…Fractal geometry concepts have been extensively employed to characterize and quantify irregularities (Mandelbrot 1982;Turcotte 1989), specifically in the size distribution of grains and pores observed in natural phenomena (e.g., Houben et al 2013;Ohl et al 2021). We utilize the slope of the log(frequency)-log(diameter) histogram to assess the fractal properties of serpentinite porosity.…”