Flow‐to‐Friction Transition in Simulated Calcite Gouge: Experiments and Microphysical Modeling

Abstract

A (micro)physical understanding of the transition from frictional sliding to plastic or viscous flow has long been a challenge for earthquake cycle modeling. We have conducted ring-shear deformation experiments on layers of simulated calcite fault gouge under conditions close to the frictionalto-viscous transition previously established in this material. Constant velocity (v) and v-stepping tests were performed, at 550°C, employing slip rates covering almost 6 orders of magnitude (0.001-300 μm/s). Steady-state sliding transitioned from (strong) v-strengthening, flow-like behavior to v-weakening, frictional behavior, at an apparent "critical" velocity (v cr) of~0.1 μm/s. Velocity-stepping tests using v < v cr showed "semi-brittle" flow behavior, characterized by high stress sensitivity ("n-value") and a transient response resembling classical frictional deformation. For v ≥ v cr , gouge deformation is localized in a boundary shear band, while for v < v cr , the gouge is well-compacted, displaying a progressively homogeneous structure as the slip rate decreases. Using mechanical data and post-mortem microstructural observations as a basis, we deduced the controlling shear deformation mechanisms and quantitatively reproduced the steady-state shear strength-velocity profile using an existing micromechanical model. The same model also reproduces the observed transient responses to v-steps within both the flow-like and frictional deformation regimes. We suggest that the flow-to-friction transition strongly relies on fault (micro)structure and constitutes a net opening of transient microporosity with increasing shear strain rate at v < v cr , under normal stress-dependent or "semi-brittle" flow conditions. Our findings shed new insights into the microphysics of earthquake rupture nucleation and dynamic propagation in the brittle-to-ductile transition zone.

Hydrothermal Friction Experiments on Simulated Basaltic Fault Gouge and Implications for Megathrust Earthquakes

“…Results from previous studies on simulated gouges prepared from halite-phyllosilicate mixtures, calcite, illite-rich shale, granite, quartz, and gabbro, under hydrothermal conditions, showed similar transitions in frictional behavior from V strengthening, V weakening, to V strengthening with respect to V or T conditions (Blanpied et al, 1995;Chen et al, 2020;Chester & Higgs, 1992;Den Hartog & Spiers, 2013;He et al, 2007;Niemeijer & Spiers, 2007;Verberne et al, 2015). This three-regime behavior can be explained by a microphysical model based on a competition between a rate/time-dependent, thermally activated deformation/compaction mechanism, such as pressure solution, and slip-dependent dilatation resulting from intergranular sliding (granular flow) characterized by mild V-strengthening, namely the Chen-Niemeijer-Spiers (CNS) model (Chen & Spiers, 2016;Niemeijer & Spiers, 2007).…”

“… Schematic views of (a) the hydrothermal ring shear apparatus and (b) the close‐up view of the sample assembly (Chen et al., 2020; Den Hartog, Niemeijer, et al., 2012). After the experiment, the sample was retrieved and microstructure was observed using the cross section shown in (c).…”

“…Therefore, we used 0.004 mm −1 as a universal slip-hardening trend (gray line in Figure 4a) to obtain the first estimation of μ values with slip-hardening trend removed (Figure 3d, see also Figure 4b). The detrended (Chen et al, 2020;Den Hartog, Niemeijer, et al, 2012). After the experiment, the sample was retrieved and microstructure was observed using the cross section shown in (c).…”

The physics of fault friction: insights from experiments on simulated gouges at low shearing velocities

“…The aim was to elucidate the combined role of pressure solution creep and foliation development in controlling the strength of faults in the upper crust, viewing the halitekaolinite mixtures as a mid-crustal rock analogue (see e.g. Shimamoto, 1986;Hiraga and Shimamoto, 1987;Chester and Logan, 1990). Velocity (v) and normal stress (σ n ) stepping experiments (Fig.…”

A Microphysical Model of Rock Friction and the Brittle‐Ductile Transition Controlled by Dislocation Glide and Backstress Evolution

“…Decades of work to describe the microphysics of RSF for intact rocks and gouge, typically using a thermally activated rheology, have led to an improved understanding (e.g., Aharonov & Scholz, 2018, 2019; Barbot, 2019; Bar‐Sinai et al., 2014; Bos & Spiers, 2002; Brechet & Estrin, 1994; Chen et al., 2017, 2020; Estrin & Brechet, 1996; Hatano, 2015; Ikari et al., 2016; Molinari & Perfettini, 2017; Niemeijer & Spiers, 2007; Noda & Takahashi, 2016; Perfettini & Molinari, 2017; Putelat et al., 2010; Rice et al., 2001; Tian et al., 2018; van den Ende et al., 2018; Verberne et al., 2020), but significant uncertainties still remain when extrapolating beyond the relatively limited set of conditions tested in the laboratory. In particular, frictional behavior near the brittle‐ductile transition has remained poorly constrained, despite some high‐temperature laboratory experiments (Blanpied et al., 1991, 1995, 1998; Boettcher et al., 2007; Chester & Higgs, 1992; Chester, 1994; King & Marone, 2012; Stesky, 1978).…”