Hierarchical creep cavity formation in an ultramylonite and implications for phase mixing

Abstract

Abstract. Establishing models for the formation of wellmixed polyphase domains in ultramylonites is difficult because the effects of large strains and thermo-hydro-chemomechanical 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 Y Z 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 grainboundary 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.

Multiscale porosity changes along the pro- and retrograde deformation path: an example from Alpine slates

“…In addition, Kaufhold et al (2016) acknowledge the resolution issue by a comparison of MIP with FIB-SEM data from shales and show that only 20 vol% of the total porosity can be resolved with FIB-SEM; the remaining porosity content should be measured with a bulk rock measurement technique such as He pycnometry. Additionally, large microfractures are not incorporated into the representative domain of the BIB-SEM but do play an important role in the MIP data (Hemes et al, 2013). This correlates with our observations: larger cracks are measured in the bulk rock methods, whereas the BIB-SEM data only provide matrix porosity as we avoid imaging the large fractures.…”

“…Therefore, for image analysis procedures on slate samples, sample preparation like ion polishing techniques, either on 2-D thin sections or on cross sections of sub-samples, are necessary (e.g. Hemes et al, 2013;Keller et al, 2013a;Klaver et al, 2012Klaver et al, , 2015. The bulk rock measurements are strongly influenced by the presence of natural and possible artificial fractures and sample heterogeneity.…”

“…Differences in porosity estimates from bulk rock measurements and image analysis techniques are related to the different principles underlying the techniques, which are the sample volumes (Ougier-Simonin et al, 2016) and the limits in spatial resolution (e.g. Hemes et al, 2013). In addition, Kaufhold et al (2016) acknowledge the resolution issue by a comparison of MIP with FIB-SEM data from shales and show that only 20 vol% of the total porosity can be resolved with FIB-SEM; the remaining porosity content should be measured with a bulk rock measurement technique such as He pycnometry.…”

“…Boom clay has a high pore connectivity, and it is shown that in samples with a higher clay content the pore connectivity is controlled by pores in the clay matrix, whereas in samples with a smaller clay content, large inter-particle pores have the largest contribution to the total connectivity (e.g. Hemes et al, 2013). A second clay stone type, the Opalinus clay is more compacted and cemented than the Boom clay (about 10 % porosity from MIP: Houben and Urai, 2013 and references therein).…”

“…A possible evolution starts with early compaction and diagenesis on the prograde path, evolving to maximum burial, followed by metamorphism and possibly tectonic deformation on the retrograde path, eventually resulting in surface exposure (Horseman et al, 1996). Porosity is produced by processes such as grain dilation, grain boundary sliding, cavity formation and dissolution (Herwegh and Jenni, 2001;Fusseis et al, 2009;Gilgannon et al, 2017), and porosity is sealed by grain size reduction, compaction and precipitation, as observed for example in clay fault gauges (Holland and Urai, 2006;Laurich et al, 2014).…”