Moine Thrust zone mylonites at the Stack of Glencoul: I – microstructures, strain and influence of recrystallization on quartz crystal fabric development

Abstract

Since the early descriptions published by Callaway in 1884, the gently dipping mylonites exposed along the Moine Thrust at the Stack of Glencoul have drawn generations of geologists to the northern part of the Assynt district. These mylonites, derived from Cambrian quartzites (footwall) and Moine pelites and psammites (hanging wall), have figured prominently in: a) early research into the influence of crystal plastic deformation and recrystallization on microstructural and crystal fabric evolution; b) debates on the kinematic interpretation of macro-and microstructures and crystal fabrics; and c) debates on the tectonic significance of such kinematic data. In this paper first we briefly review the historical aspects of this research and then, using both previously published and unpublished data, document the finite strain and quartz fabric development at this classic mylonite locality. A tectonic interpretation of these data, together with quantitative estimates of flow vorticities associated with mylonite formation at the Stack of Glencoul, are presented in a companion paper by Law (2010).

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).…”

Water loss during dynamic recrystallization of Moine thrust quartzites, northwest Scotland

“…In this study, we revisit the classic mylonitic Cambrian quartzites in the footwall to the Moine thrust (defined by the Cambrian quartzite-Neoproterozoic Moine schist contact; Peach et al, 1907;Christie, 1963;Law et al, 2010) exposed at the Stack of Glencoul in the Assynt region of northwest Scotland (Fig. 1) in order to measure molecular water contents as a function of structural level, from 70 m below the principal thrust surface to within 5 mm of the thrust contact.…”

Microstructures and quartz lattice‐preferred orientations in the eclogite‐bearing migmatitic gneisses of the D'Entrecasteaux Islands, Papua New Guinea

“…Kruhl [] suggested that the opening angles in plane strain may be used as a deformation thermometer, whereas Law et al . [] provided the caveat that the degree of dynamic recrystallization and non‐plane strain deformation probably also affect the opening angle. Opening angles for our samples that featured [c]‐axis crossed‐girdles (and their estimated uncertainty) are presented in Table B and Figure , and are also depicted on the complete set of [c]‐axis pole figures (Appendix C).…”

“…[33] The crossed-girdle opening angle of quartz c-axis LPOs may be a function of deformation temperature, hydrolytic weakening, or strain rate [Tullis et al, 1973;Lister & Hobbs, 1980;Kruhl, 1998;Law et al, 2004]. Kruhl [1998] suggested that the opening angles in plane strain may be used as a deformation thermometer, whereas Law et al [2010] provided the caveat that the degree of dynamic recrystallization and non-plane strain deformation probably also affect the opening angle. Opening angles for our samples that estimated uncertainty) are presented in Table B and Figure 10, and are also depicted on the complete set of [c]-axis pole figures (Appendix C).…”