Evolution of c axis pole figures and grain size during dynamic recrystallization: Results from experimentally sheared quartzite

Abstract

[1] Quartzite samples experimentally sheared at conditions where subgrain rotation and grain boundary migration recrystallization are active show a striking change in the c axis crystallographic preferred orientation (CPO) with increasing shear strain (up to = 8) and degree of recrystallization (up to 100%). We used optical methods to determine the c axis CPO for the bulk samples and for porphyroclasts and recrystallized grains separately and to track the strengths of the CPOs in five different orientation domains with increasing . The c axis pole figure evolves from a broad peripheral maximum indicative of basal hai slip, to an inclined single girdle with two maxima indicative of rhomb hai slip, and finally an elongate single maximum at the girdle center indicative of prism hai slip; throughout this sequence the fabric skeleton rotates with the sense of shear. The bulk preferred c axis orientation results from growth and shrinkage of differently oriented crystallographic domains. Grain boundary density analysis indicates that the size of prism hai recrystallized grains is 1.5 times that of average and 2 times that of rhomb grains, indicating that prism hai slip is significantly easier at our experimental conditions. This inference is supported by autocorrelation function analysis of the various orientation domains and how they evolve with strain. Similar progressive changes in c axis CPO are observed in quartzites naturally sheared at conditions where grain boundary migration recrystallization operates. These results indicate that achievement of microstructural and mechanical steady state requires high strain and complete recrystallization.

Strain within the ultrahigh-pressure Western Gneiss region of Norway recorded by quartz CPOs

“…Progressive strain of a polycrystal leads to progressive rotation of the crystal lattices and, therefore, to a relationship between strain magnitude and CPO strength (Lister et al 1978); this relationship has been demonstrated in both experimentally and naturally deformed rocks (Marjoribanks 1976;Bouchez 1977;Carreras et al 1977;Miller & Christie 1981;Law 1986;Dell'Angelo & Tullis 1989;Heilbronner & Tullis 2006). Other factors that influence CPO strength include the particular slip systems that are active, the activity of other deformation mechanisms, the rate of recrystallization, the presence of other phases and variations in strain path (e.g.…”

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

“…This perpendicularity seems to be reproduced by some numerical simulations (Etchecopar and Vasseur, 1987;Jessell and Lister, 1990). It has been found in many natural rocks (Carreras and Garcia-Celma, 1982;Mainprice et al, 1986), but this is not always the case at all (Law et al, 1984;Tullis, 2002;Heilbronner and Tullis, 2006;Law et al, 2010;Kilian et al, 2011).…”

“…In simple-shearing experiments made by Heilbronner and Tullis (2006, Fig. 6), quartz c-axis fabric evolves from an asymmetrical crossed girdle with a broad peripheral maximum to an inclined single girdle with single girdle with two maxima and finally a fabric with one maximum at the girdle center, indicative of a change of the dominant slip system from basal <a> slip to prism <a> slip.…”

“…Both crossed girdle and single girdle of quartz c-axes are generally considered as a representation of active slip systems, mainly including basal <a>, prism <a> and rhomb <a> slip systems (Passchier and Trouw, 2005;Heilbronner and Tullis, 2006). They can be reproduced using numerical software that is based upon crystal plasticity theory (Nemat-Nasser and Okinaka, 1996;De Souza Neto et al, 2008).…”

“…For example, in the presence of basal <a> slip as the dominant slip system, quartz c-axes appear in a cleft girdle under axial extension, in a small circle girdle under axial shortening, and in a crossed girdle with symmetrical or asymmetrical legs under plane coaxial or non-coaxial strain (Tullis, 1977;Tullis et al, 1973;Lister and Hobbs, 1980;Dell'Angelo and Tullis, 1989;Gleason et al, 1993). Crossed girdles may form initially and then switch into a single girdle or even a single c-axis maximum, when dynamic recrystallization sets in and gradually increases with increasing strain (Schmid and Casey, 1986;Law, 1990;Schmid, 1994;Stipp et al, 2002;Mancktelow and Pennacchioni, 2004;Heilbronner and Tullis, 2006). Despite these approaches, there remain two unresolved issues regarding the development of quartz c-axis fabric in non-coaxial deformation.…”