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

Abstract

[1] 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 mm. 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 Â 10 À11 s À1 for 300 C, and $110 MPa and 1.0 Â 10 À10 s À1 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 À14 s À1), 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 À11 to 10 À10 s À1 ).Citation: Okudaira, T., and N. Shigematsu (2012), Estimates of stress and strain rate in mylonites based on the boundary between the fields of grain-size sensitive and insensitive creep,

Theoretical derivation of flow laws for quartz dislocation creep: Comparisons with experimental creep data and extrapolation to natural conditions using water fugacity corrections

“…We suggest therefore that the flow laws used in these three papers, as well as Holyoke and Kronenberg [2010], who corrected the stresses given by Gleason and Tullis [1995], are the most reliable in terms of stress accuracy and the smallest effects from secondary phases. These flow laws are therefore often applied to natural quartz samples deformed by dislocation creep [Wightman et al, 2006;Menegon et al, 2011;Okudaira and Shigematsu, 2012;Boutonnet et al, 2013]. We use the flow laws reported in these four experimental papers.…”

“…However, there are large variations in the proposed values of n (2–4) and Q dis (120–300 kJ/mol). As a result, when the flow laws are extrapolated to natural temperature conditions, the targeted parameters such as strain rates and flow stresses show variations of more than a few orders of magnitude [e.g., Wightman et al , ; Menegon et al , ; Okudaira and Shigematsu , ; Boutonnet et al , ].…”

“…However, there are large variations in the proposed values of n (2-4) and Q dis (120-300 kJ/mol). As a result, when the flow laws are extrapolated to natural temperature conditions, the targeted parameters such as strain rates and flow stresses show variations of more than a few orders of magnitude [e.g., Wightman et al, 2006;Menegon et al, 2011;Okudaira and Shigematsu, 2012;Boutonnet et al, 2013]. Hirth et al, 2001;Brodie, 2004a, 2004b;Holyoke and Kronenberg, 2013] and other minerals (e.g., Kohlstedt [2000a, 2000b] and Karato and Jung [2003] for olivine, Hier-Majumder et al [2005] for pyroxene, and Rybacki et al [2006] for anorthite).…”

Brittle grain-size reduction of feldspar, phase mixing and strain localization in granitoids at mid-crustal conditions (Pernambuco shear zone, NE Brazil)

“…Tullis, 2002;Okudaira and Shigematsu, 2012), so that shear zones in granitoids have often been used as natural laboratories to investigate strain localization in the middle crust and the associated microstructural modifications experienced by quartz and feldspars (Behrmann and Mainprice, 1987;Fitz Gerald and Stünitz, 1993;Fliervoet et al, 1997;Ingles et al, 1999;Tullis, 2002;Pennacchioni and Mancktelow, 2007;Menegon and Pennacchioni, 2010;Oliot et al, 2010;Kilian et al, 2011;Sullivan et al, 2013;Czaplinska et al, 2015). At pressure-temperature conditions typical of the middle crust, quartz is expected to be mechanically weaker than feldspars when deformation is accommodated by crystalline plasticity (Tullis, 2002, and references therein).…”

The Rheological Evolution of Brittle‐Ductile Transition Rocks During the Earthquake Cycle: Evidence for a Ductile Precursor to Pseudotachylyte in an Extensional Fault System, South Mountains, Arizona

“…Though there are no “wet” diffusion creep flow laws for quartz, wet feldspar aggregates show an expanded field of diffusion creep in experimental deformation mechanism maps (Rybacki & Dresen, ), indicating that water facilitates the operation of diffusion creep under a greater range of deformation conditions. Further, studies of naturally deformed rocks note a discrepancy between the wider range of conditions under which they interpret diffusion creep in quartz and those indicated by experimental quartz data (e.g., Kilian et al, ; Okudaira & Shigematsu, ; Rahl & Skemer, ). In the South Mountains, molecular water was present in fluid inclusions during crystallization of the composite pluton (Smith et al, ), but we suspect that this water was mobilized and reincorporated into structurally bound OH defects that enabled hydrolytic weakening and the onset of diffusion creep.…”