Articles | Volume 8, issue 6
Solid Earth, 8, 1193–1209, 2017
Solid Earth, 8, 1193–1209, 2017

Research article 19 Dec 2017

Research article | 19 Dec 2017

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 James Gilgannon et al.
  • 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. Establishing models for the formation of well-mixed polyphase domains in ultramylonites is difficult because the effects of large strains and thermo-hydro-chemo-mechanical feedbacks can obscure the transient phenomena that may be responsible for domain production. We use scanning electron microscopy and nanotomography to offer critical insights into how the microstructure of a highly deformed quartzo-feldspathic ultramylonite evolved. The dispersal of monomineralic quartz domains in the ultramylonite is interpreted to be the result of the emergence of synkinematic 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.

Short summary
We examine rocks from the middle crust to explore how fluids circulate and influence a rock’s response to larger-scale tectonic movements. A model is developed in which 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.