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

Area of Science:

• Solid Earth Geophysics
• Tectonophysics
• Experimental Rock Deformation

Background:

• The transition from frictional sliding to plastic or viscous flow in faults is poorly understood and poses a challenge for earthquake cycle modeling.
• Previous research established a frictional-to-viscous transition in simulated calcite fault gouge.
• The microphysics governing this transition, especially near the brittle-to-ductile zone, requires further investigation.

Purpose of the Study:

• To investigate the microphysical mechanisms controlling the transition from frictional sliding to plastic/viscous flow in simulated calcite fault gouge.
• To determine the critical slip rate at which this transition occurs under experimental conditions.
• To model and understand the observed deformation behaviors and transient responses during shear.

Main Methods:

• Conducted ring-shear deformation experiments on simulated calcite fault gouge at 550°C.
• Performed constant velocity and velocity-stepping tests across a wide range of slip rates (0.001–300 μm/s).
• Utilized mechanical data and post-mortem microstructural observations, analyzed with a micromechanical model.

Main Results:

• A critical slip rate (v_c) of approximately 0.1 μm/s was identified, marking the transition from flow-like (v > v_c) to frictional (v < v_c) behavior.
• Velocity-stepping tests below v_c exhibited 'semi-brittle' flow with high stress sensitivity and transient responses.
• Above v_c, deformation localized into a shear band; below v_c, the gouge became well-compacted and homogeneous with decreasing slip rate.

Conclusions:

• The flow-to-friction transition is strongly dependent on fault microstructural evolution, particularly the opening of transient microporosity at higher shear strain rates.
• A micromechanical model successfully reproduced both steady-state strength-velocity profiles and transient responses across deformation regimes.
• Findings provide new insights into the microphysics of earthquake rupture nucleation and propagation within the brittle-to-ductile transition zone.