The evolution of high-temperature mylonitic microfabrics: evidence from simple shearing of a quartz analogue (norcamphor)

Abstract

Abstract-Plane strain simple shearing of norcamphor (C,HiuO) in a see-through deformation rig to a shear strain of y = 10.5 at a homologous temperature of Ts = 0.81 yields a microfabric similar to that of quartz in amphibolite facies mylonite. Synkinematic analysis of the norcamphor microfabric reveals that the development of a steady-state texture is linked to changes in the relative activities of several grain-scale mechanisms. Three stages of textural and microstructural evolution are distinguished: (1) rotation and shearing of the intracrystalline glide planes are accommodated by localized deformation along three sets of anastomozing microshears. A symmetrical c-axis girdle reflects localized pure shear extension along the main microshear set (S,) oblique to the bulk shear zone boundary (abbreviated as SZB); (2) progressive rotation of the microshears into parallelism with the SZB increases the component of simple shear on the S, microshears. Grain-boundary migration recrystallization favours the survival of grains with slip systems oriented for easy glide. This is associated with a textural transition towards two stable c-axis point maxima whose skeletal outline is oblique with respect to the S, microshears and the SZB; and (3) at high shear strains (y > 8), the microstructure, texture and mechanism assemblage are strain invariant, but strain continues to partition into rotating sets of microshears. Steady state is therefore a dynamic, heterogeneous condition involving the cyclic nucleation, growth and consumption of grains.

The effect of different second-phase particle regimes on grain growth in two-phase aggregates: insights from in situ rock analogue experiments

“…Sample preparation was performed following Herwegh and Handy (1996). Depending on the required volume fraction of second phases, glass beads with the corresponding weight fraction were mixed with the correlating amount of norcamphor powder.…”

“…The sample assembly was placed in a heated Urai-Means rockanalogue rig (see Fig. 1 in Herwegh and Handy 1996), which was mounted underneath a modified Wild Macroscope. The Wild Macroscope was equipped with a SONY DW-F-X 700 camera and the automatic rotation polarizing system of Fueten (1997).…”

Development of quartz c-axis crossed/single girdles under simple-pure shear deformation: Results of visco-plastic self-consistent modeling

“…Such a scenario is partly or wholly observed in other experiments (e.g. Tullis, 1977;Dell'Angelo and Tullis, 1989;Herwegh and Handy, 1996). Our models with an invariable dominant slip system are only able to reproduce the early part of the scenario, a fabric transition from crossed girdle to single girdle.…”

Evolution of mylonitic microfabrics (EMM), a computer application for educational purposes

“…It is to this end that we have created 'Evolution of Mylonitic Microfabrics' (EMM). It is a publicly available computer package that contains movies, illustrations and explanations of the see-through experiments published in Herwegh and Handy (1996), Herwegh et al (1997) and Herwegh and Handy (1998). Below, we briefly specify the computer requirements for EMM and outline the essential contents and advantages of the application.…”

“…Following on a series of simple shear experiments on norcamphor as an analogue material for mylonitic quartz (Herwegh and Handy, 1996), we have recently applied the new method of computer-integrated polarization microscopy (CIP, see Panozzo Heilbronner and Pauli, 1993) to document all aspects of microstructure development in seethrough experiments . The CIP method allowed us to calculate the position of norcamphor c-axes for each individual image pixel and to attribute a specific color to it.…”