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

Jianye Chen, B. A. Verberne, A. R. Niemeijer

Research output: Contribution to journalArticleScientificpeer-review

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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 frictional-to-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 (vcr) of ~0.1 μm/s. Velocity-stepping tests using v < vcr showed “semi-brittle” flow behavior, characterized by high stress sensitivity (“n-value”) and a transient response resembling classical frictional deformation. For v ≥ vcr, gouge deformation is localized in a boundary shear band, while for v < vcr, 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 < vcr, 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.

Original languageEnglish
Article numbere2020JB019970
Pages (from-to)1-25
Number of pages25
JournalJournal of Geophysical Research: Solid Earth
Volume125
Issue number11
DOIs
Publication statusPublished - 2020

Keywords

  • brittle-to-ductile transition
  • calcite friction
  • earthquake nucleation
  • flow-to-friction transition
  • microphysical model
  • rock deformation mechanisms

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