Dynamic Recrystallization Can Produce Porosity in Shear Zones
James Gilgannon (University of Bern)
Thomas Poulet (University of New South Wales, CSIRO Mineral Resources)
Alfons Berger (University of Bern)
A Barnhoorn (TU Delft - Applied Geophysics and Petrophysics)
Marco Herwegh (University of Bern)
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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.