Dynamic recrystallisation can produce porosity in shear zones

Abstract

Creep cavities are increasingly recognised 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 reanalysed the microstructure of a large shear strain laboratory experiment that captures grain-size change by dynamic recrystallisation during mylonitisation. 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, sub-grain rotation recrystallisation during ultramylonite formation. As dynamic recrystallisation is ubiquitous in natural shear zones, this observation has important implications for the interpretation of concepts such as the Goetze criterion, palaeopiezometery and phase mixing.

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

“…(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 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; Naus‐Thijssen et al., 2011). 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).…”