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Solid Earth An interactive open-access journal of the European Geosciences Union

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https://doi.org/10.5194/se-2017-96
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
28 Aug 2017
Review status
This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Solid Earth (SE) and is expected to appear here in due course.
Hierarchical creep cavity formation in an ultramylonite and implications for phase mixing
James Gilgannon1,2, Florian Fusseis2, Luca Menegon3, Klaus Regenauer-Lieb4, and Jim Buckman5 1Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, 3012 Bern, Switzerland
2School of Geosciences, The University of Edinburgh, Grant Institute, Edinburgh EH9 3JW, UK
3School of Geography, Earth and Environmental Sciences, Plymouth University, Plymouth PL4 8AA, UK
4School of Petroleum Engineering, The University of New South Wales, Kensington NSW 2033, Australia
5Institute of Petroleum Engineering, Heriot-Watt University, Edinburgh EH14 4AS, UK
Abstract. The dispersal of monomineralic quartz domains in a quartzofeldspathic ultramylonite is interpreted to be the result of the emergence of syn-kinematic pores, called creep cavities. The cavities can be considered the product of two distinct mechanisms that formed hierarchically: Zener-Stroh cracking and viscous grain boundary sliding. In initially thick and coherent quartz ribbons deforming by grain size-insensitive creep, cavities were generated by the Zener-Stroh mechanism on grain-boundaries aligned with the YZ plane of finite strain. The opening of creep cavities promoted the ingress of fluids to sites of low stress. The local addition of a fluid lowered the adhesion and cohesion of grain-boundaries and promoted viscous grain boundary sliding. With the increased contribution of viscous grain boundary sliding, a second population of cavities formed to accommodate strain incompatibilities. Ultimately, the emergence of creep cavities is interpreted to be responsible for the transition of quartz domains from a grain size-insensitive, to a grain size-sensitive rheology.

Citation: Gilgannon, J., Fusseis, F., Menegon, L., Regenauer-Lieb, K., and Buckman, J.: Hierarchical creep cavity formation in an ultramylonite and implications for phase mixing, Solid Earth Discuss., https://doi.org/10.5194/se-2017-96, in review, 2017.
James Gilgannon et al.
James Gilgannon et al.

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Short summary
We examine rocks from the middle crust to explore how fluids circulate and influence a rock’s response to the larger scale tectonic movements. A model is developed where fluids deep in the Earth migrate to clusters of pores generated during those movements. We document how distinct pores form in a specific order in association with local changes in how quartz deforms. The porosity evolves out of the deformation, changing the rate the rock moved under tectonic forces.
We examine rocks from the middle crust to explore how fluids circulate and influence a rock’s...
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