Abstract
AbstractNanograins (≪1 μm) are common in the principal slip zones of natural and experimental faults, but their formation and influence on fault mechanical behavior are poorly understood. We performed transmission Kikuchi diffraction (spatial resolution 20–50 nm) on the principal slip zone of an experimental carbonate gouge (50 wt% calcite, 50 wt% dolomite) that was deformed at a maximum slip rate of 1.2 m/s for 0.4 m displacement. The principal slip zone (PSZ) consists of nanogranular aggregates of calcite, Mg‐calcite, dolomite and periclase, dominated by grain sizes in the range of 100–300 nm. Nanograins in the ultrafine (< 800 nm) PSZ matrix have negligible internal lattice distortion, while grains > 800 nm in size contain subgrains. A weak crystallographic preferred orientation is observed as a clustering of calcite c‐axes within the PSZ. The high‐resolution microstructural observations from transmission Kikuchi diffraction, in combination with published flow laws for calcite, are compatible with high‐velocity slip in the PSZ having been accommodated by a combination of grain size sensitive creep in the ultrafine matrix, and grain size insensitive creep in the larger grains, with the former process likely controlling the bulk rheology of the PSZ after dynamic weakening. If the activation energy for creep is lowered by the nanogranular nature of the aggregates, this could facilitate grain size sensitive creep at high (coseismic) strain rates and only moderate bulk temperatures of approximately 600 °C, although temperatures up to 1000 °C could be locally achieved due to processes such as flash heating.
Highlights
The high‐resolution microstructural observations from transmission Kikuchi diffraction, in combination with published flow laws for calcite, are compatible with high‐velocity slip in the principal slip zone (PSZ) having been accommodated by a combination of grain size sensitive creep in the ultrafine matrix, and grain size insensitive creep in the larger grains, with the former process likely controlling the bulk rheology of the PSZ after dynamic weakening
Calculations of the temperature rise suggest that the principal slip surface (PSS) reached a temperature of approximately 620 °C, which is consistent with thermocouple measurements of temperature increase in the adjacent PSZ
We suggest that grain growth in the calcite‐dolomite gouges was negligible, and that the microstructures and grain sizes preserved in our sample are similar to those that were active during the high‐velocity experiment
Summary
Nanograins (≪ 1 μm) are widely reported in both natural (e.g., Chester et al, 2005; Demurtas et al, 2016; Ma et al, 2006; Novellino et al, 2015; Pittarello et al, 2008; Siman‐Tov et al, 2013; Smeraglia et al, 2017; Wilson et al, 2005) and experimental fault slip zones (e.g., Yund et al, 1990; Han et al, 2007, 2010; Reches & Lockner, 2010; De Paola et al, 2011; Han et al, 2011; Tisato et al, 2012; Chen et al, 2013; Verberne et al, 2013, 2014; De Paola et al, 2015; Green et al, 2015; Spagnuolo et al, 2015; Yao et al, 2016; Aretusini et al, 2017; Smeraglia et al, 2017; Pozzi et al, 2018, 2019). Nanograins have been generated in rock deformation experiments simulating both aseismic and coseismic deformation conditions (e.g., Han et al, 2010; Tisato et al, 2012; Verberne et al, 2013; Spagnuolo et al, 2015; Smeraglia, Bettucci, et al, 2017, Pozzi et al, 2018, 2019). Journal of Geophysical Research: Solid Earth compaction due to active diffusive mass transfer In their case, the presence of a crystallographic preferred orientation (CPO) within shear bands cutting the gouge was interpreted to be the result of cataclastic flow aided by shear‐induced dislocation glide at the grain contacts (Verberne et al, 2013)
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