Estimates of stress and strain rate in mylonites based on the boundary between the fields of grain‐size sensitive and insensitive creep

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<jats:p>Microstructural analyses of mylonites next to the Median Tectonic Line (MTL), SW Japan, reveal a transition in the dominant deformation mechanism of quartz from grain‐size‐insensitive dislocation creep to grain‐size‐sensitive grain‐boundary sliding (GBS). The transition occurred under greenschist‐facies conditions (∼300–400°C) during grain‐size reduction by dynamic recrystallization. The stereologically corrected grain size for the transition is approximately 4.3 <jats:italic>μ</jats:italic>m. At the boundary between the fields of dislocation creep and GBS, as calculated from creep constitutive relations, the differential stress and strain rate for this corrected grain size are estimated to be ∼280 MPa and 1.2 × 10<jats:sup>−11</jats:sup> s<jats:sup>−1</jats:sup> for 300°C, and ∼110 MPa and 1.0 × 10<jats:sup>−10</jats:sup> s<jats:sup>−1</jats:sup> for 400°C. The strain rates estimated for the mylonites next to the MTL are much higher than those estimated for the surrounding metamorphic rocks (∼10<jats:sup>−14</jats:sup> s<jats:sup>−1</jats:sup>), and the displacement rates calculated based on the thickness of high‐strain mylonites and their strain rates are comparable with the average slip rates of the most active intraplate faults in Japan. These inferences suggest that the high‐strain mylonite zones next to the MTL are the exhumed downward extension of a seismogenic fault in the ductile region. The zones were highly localized (<10 m) and experienced very high strain rates (10<jats:sup>−11</jats:sup> to 10<jats:sup>−10</jats:sup> s<jats:sup>−1</jats:sup>).</jats:p>

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