Dynamic recrystallisation can produce porosity in shear zones

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.