Dynamic Recrystallization Can Produce Porosity in Shear Zones

Abstract

Creep cavities are increasingly recognized as an important syn-kinematic feature of shear zones, but much about this porosity needs investigation. Largely, observations of creep cavities are restricted to very fine grained mature ultramylonites, and it is unclear when they developed during deformation. Specifically, a question that needs testing is should grain size reduction during deformation produce creep cavities? To this end, we have reanalyzed the microstructure of a large shear strain laboratory experiment that captures grain size change by dynamic recrystallization during mylonitization. We find that the experiment does contain creep cavities. Using a combination of scanning electron microscopy and spatial point statistics, we show that creep cavities emerge with, and because of, subgrain rotation recrystallization during ultramylonite formation. As dynamic recrystallization is ubiquitous in natural shear zones, this observation has important implications for the interpretation of concepts such as the Goetze criterion, paleopiezometery, and phase mixing. Plain Language SummaryAt great depths inside the Earth, rocks called mylonites slowly deform and accommodate tectonic forces. Generally, these rocks are considered to have no porosity because the pressure they experience is very large. However, it is frequently documented that these mylonites focus the transport of mass, both fluid and solid, through the crust. This implies that mylonites host a permeable porosity. To better understand this paradox, we reanalyzed an old laboratory experiment that documented the formation of a mylonite. We showed that a porosity, known as creep cavities, forms synchronously with the mylonite. This is an important experimental finding because it suggests that creep cavities are a fundamental feature of mylonites. Our results showcase a rare snapshot into the dynamics of rocks important for tectonics and advance larger questions about their transport properties.

Seismic Anisotropy and Its Impact on Imaging the Shallow Alpine Fault: An Experimental and Modeling Perspective

“…Journal of Geophysical Research: Solid Earth cavities (Fusseis et al, 2009;Gilgannon et al, 2020;Menegon et al, 2015), grain boundaries (Kruhl et al, 2013;Mancktelow et al, 1998), or well-developed cleavage (Babuska & Cara, 1991;Kern & Wenk, 1990;. With the FEM approach shown here, future studies could focus on characterizing the effect of real fracture networks, mineral phase, CPO, and microstructural arrangement data mapped from EBSD on the elastic anisotropic of shear zone rocks (Zhong et al, 2014).…”

“…Kern et al (2008) suggest that low aspect ratio intercrystalline and intracrystalline microfractures related to biotite minerals should close at high effective pressures (>100 MPa), but Kern and Wenk (1990) conclude that aligned microfractures with foliation in mylonites remain open for effective pressures <50 MPa. Moreover, porosity in shear zone rocks can be as high as 8% (Géraud et al, 1995), and in addition to microfractures, porosity can be present in creep Journal of Geophysical Research: Solid Earth cavities (Fusseis et al, 2009;Gilgannon et al, 2020;Menegon et al, 2015), grain boundaries (Kruhl et al, 2013;Mancktelow et al, 1998), or well-developed cleavage (Babuska & Cara, 1991;Kern & Wenk, 1990;. With the FEM approach shown here, future studies could focus on characterizing the effect of real fracture networks, mineral phase, CPO, and microstructural arrangement data mapped from EBSD on the elastic anisotropic of shear zone rocks (Zhong et al, 2014).…”

How Does Viscosity Contrast Influence Phase Mixing and Strain Localization?

“…Mechanical phase mixing can occur by dynamic recrystallization coupled with grain switching during grain boundary sliding (Farla et al, 2013; Linckens et al, 2014), the sequential formation, attenuation (stretching), and disaggregation of compositional layering (Cross & Skemer, 2017) and/or the nucleation of well‐mixed grains at interphase triple junctions (Bercovici & Skemer, 2017). Chemical phase mixing, on the other hand, may involve the formation of a well‐mixed metamorphic reaction product (Dijkstra et al, 2002; Kenkmann & Dresen, 2002; Kruse & Stünitz, 1999; Marti et al, 2018; Newman et al, 1999), the exchange of chemical species between phases (Tasaka et al, 2017), and/or the precipitation of phases into dilational sites (Kenkmann & Dresen, 2002; Kilian et al, 2011; Platt, 2015) including creep cavities formed via grain boundary sliding (Czertowicz et al, 2016; Lopez‐Sanchez & Llana‐Fúnez, 2018; Menegon et al, 2015; Précigout et al, 2017; Viegas et al, 2016), Zener‐Stroh cracking (Gilgannon et al, 2017), or dynamic recrystallization (Gilgannon et al, 2020). Chemical phase mixing mechanisms may be extremely efficient, since well‐mixed reaction products can form even in the absence of deformation.…”

Porosity induced by dislocation dynamics in quartz-rich shear bands of granitic rocks

“…Alternatively, Gilgannon et al 14 recently used a statistical approach to highlight micro-cavities emerging during dynamic recrystallization of experimentally deformed Carrara marble. They proposed that micro-pores may arise from sub-grain rotation with many implications, but without expanding much on the mechanism itself.…”

“…This corroborates previous observations that documented the presence of micro-pores in polymineralic shear zones dominated by dislocation creep 5 . Moreover, statistical characterizations of experimentally deformed Carrara marble have lately shown that a significant porosity may emerge with dynamic recrystallization accommodated by sub-grain rotation 14 . It seems therefore that, like diffusion creep via GBS, dislocation creep might be involved to produce micro-pores via crystal plasticity.…”