We have applied transmission electron microscopy (TEM) analyses coupled with viscoplastic self-consistent (VPSC) numerical modelling to identify the active slip systems and to better understand the crystal preferred orientation (CPO) development of the Torridon quartz mylonite (NW Scotland). TEM analyses showed evidence of activation of 1/3kal{p′}, 1/3kal{z} and possible kal(c) slip systems, as well as dislocation climb and dynamic recrystallization. All the CPOs generated by VPSC models share common characteristics with the Torridon quartz mylonite, but only Models 2 and 3 reproduce the [c]-axes maxima at low angle (,208) to the foliation pole along the YZ plane, as observed in the mylonite. In Model 2, this concentration only occurs at g ≥ 2.6, whereas in Model 3 this maxima occurs at lower shear strains. The models that start with a previous preferred orientation acquire very strong CPOs after small-imposed strains, followed by the rapid rotation of the fabric in relation to the new imposed finite strain axes. The combined activation of kal{p′}, kal{z} and possibly kal(c) slip systems, as demonstrated by TEM analyses, suggests that the VPSC model that best predicts CPO development in the Torridon quartz mylonite is Model 2, where the critical resolved shear stress (CRSS) of kal{p/p′} is assumed to be slightly stronger than kal(c). Quartz mylonites are typical rocks of high-strain zones and result from localized deformation in quartzites and quartz veins and by the intense deformation of granitoids in the presence of fluids under upper to middle-crustal conditions (e.g. Dixon & Williams 1983; Law et al. 1986, 1990; Lloyd et al. 1992; Fitz Gerald & Stunitz 1993; Goodwin & Wenk 1995; Hippertt 1998; Wibberley 1999; Lloyd 2004; Jefferies et al. 2006; Pennacchioni et al. 2010). Less common but no less important is the occurrence of quartz mylonites in the lower crust, suggesting high differential stresses for their generation and implying a contrasting geological behaviour for the lower crust under certain conditions (e.g. Fitz Gerald et al. 2006). In all of these cases, quartz mylonites play an important role in controlling lithospheric strength as they may allow deformation localization (e.g. Carter & Tsenn 1987). Independently of the processes in which the quartz mylonites are generated, a common feature observed in these rocks is the development of strong crystal preferred orientation (CPO) of quartz. Quartz CPOs can be used to infer deformation temperatures, magnitude and symmetry of strain and kinematic framework and the mechanisms for deformation (e.g. Schmid & Casey 1986; Law 1990; Stipp et al. 2002; Law et al. 2004). The development of CPO usually implies the activation of dislocation creep. CPO patterns are usually interpreted in terms of the relative activation of different slip systems, essentially controlled by temperature and variation in the strain and kinematic conditions (e.g. Schmid & Casey 1986; Okudaira et al. 1995; Takeshita 1996; Kurz et al. 2002; Heilbronner & Tullis 2006). Nevertheless, quartz CPOs depart from these expected patterns in many cases given the deformation conditions of certain rocks, and their interpretation by purely intracrystalline plasticity mechanisms is not straightforward (e.g. Hippertt 1998). Mechanisms such as dynamic recrystallization, dissolution/precipitation, grainboundary sliding and diffusion creep have also been used to explain the variety of quartz fabrics observed in both nature and experiments (e.g. From: Prior, D. J., Rutter, E. H. & Tatham, D. J. (eds) Deformation Mechanisms, Rheology and Tectonics: Microstructures, Mechanics and Anisotropy. Geological Society, London, Special Publications, 360, 151–174. DOI: 10.1144/SP360.9 # The Geological Society of London 2011. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Wenk & Christie 1991; Gleason et al. 1993; Hippertt & Egydio-Silva 1996; van Daalen et al. 1999; Heilbronner & Tullis 2002, 2006; Stipp et al. 2002; Halfpenny et al. 2006; Vernooij et al. 2006). In addition, CPO patterns are typically a ‘post-mortem’ feature for which we usually do not know the deformation path that produced the preferred orientation. In general, CPO patterns are interpreted assuming initially randomly distributed crystal orientations in a volume of rock prior to deformation. This deformation, and the mechanisms responsible for its accommodation, is responsible for the development of a given CPO. Indeed, in many cases this is what happens (e.g. Pennacchioni et al. 2010). In other cases, for example, the assumption has to be made simply because of the lack of exposure from lowto high-strain conditions along a shear zone. The role of pre-existing CPO on fabric development in high-strain zones is only rarely considered (e.g. Lister & Williams 1979; Ralser et al. 1991; Lloyd et al. 1992; Toy et al. 2008; Pennacchioni et al. 2010). Such ‘initial’ CPOs may have a strong influence on the ‘final’ CPO patterns observed in shear-zone mylonites. The quartz mylonite studied here originated from intense simple-shear deformation of a quartzfeldspar vein infilling a joint in Lewisian gneiss of the Upper Loch Torridon area (NW Scotland). This vein is perhaps one of the most-studied quartzbearing mylonites in terms of microstructures and preferred orientations (Law et al. 1990; Lloyd et al. 1992; Trimby et al. 1998; Lloyd 2004; Lloyd & Kendall 2005). Nevertheless, specific points regarding the nature and evolution of the CPO of the mature mylonite (centre of the vein/shear zone) and its relation to the CPO of the original quartz vein (now only preserved at the margins of the quartz vein/shear-zone walls) remain poorly constrained. Specifically, it is not well understood whether the crystallographic orientation observed in the mature mylonite is the result of a single episode of deformation dominated by single or multiple slip systems (Law et al. 1990) or if the observed preferred orientation reflects an inherited initial fabric from the margin of the sheared quartz vein (Lloyd et al. 1992). The preferred orientation observed in the quartz mylonite from Torridon cannot be solely explained by intra-crystalline plasticity via mutual activation of basal, rhomb and prismatic slip in the kal direction, as commonly observed in quartz-rich tectonites deformed at relatively low temperatures (e.g. samples R-405, P-248 and C-156 of Schmid & Casey 1986); other mechanisms have to be active, such as Dauphine twinning and dynamic recrystallization (Law et al. 1990; Lloyd et al. 1992; Trimby et al. 1998; Lloyd 2004). The Torridon quartz mylonite is therefore a good case study for testing, through systematic modelling of crystallographic fabric development via the viscoplastic self-consistent (VPSC) approach (Lebensohn & Tome 1993; Tome & Lebensohn 2004), whether CPO results from a single deformation episode (starting from a random orientation) or if the observed CPO is inherited from the margin of the sheared vein. To better constrain the CPO predictions by numerical modelling, transmission electron microscopy (TEM) images from the mature mylonite are also presented, providing new information regarding the microstructural evolution of this shear zone and complementing previous work carried out by Law et al. (1990), Lloyd et al. (1992), Trimby et al. (1998), Lloyd (2004) and Lloyd & Kendall (2005). Sample description and quartz CPO The quartz feldspar vein from the head of Upper Loch Torridon (Fig. 1a, b) was sheared during relative motion of joint blocks of Lewisian gneiss on either side of the vein. Foliation (XY) associated with shearing increases in intensity traced in to the centre of the vein and simultaneously curves towards parallelism with the vein/shear-zone margins (sensu Ramsay & Graham 1970) but is still inclined at 98 to the shear-zone margins in the ‘mature’ high-strain centre of the vein (Law et al. 1990). Original vein textures are only locally preserved at the low-strain margins of the vein. The high-strain centre of the vein is characterized by a typical Type-2 S–C′ microstructure (Law et al. 1990). The vein microstructure is arranged in two planar domains aligned parallel to the macroscopic foliation. One domain consists of elongate dynamically recrystallized quartz grains (,100 mm) whose long axes are oblique to the main foliation, while the second domain is characterized by an equigranular texture of quartz and feldspar with grain size of less than 5 mm. The microstructure suggests that deformation occurred under constant-volume plane strain flow conditions very close to strict simple shear (Law et al. 1990; Wallis 1995, p. 1082). A band-contrast electron backscatter diffraction (EBSD) map (Fig. 1a) and cathodoluminescence (CL) image (Fig. 1b) illustrate development of the shear zone with a spatially rapid development of foliation, grain-size reduction and fracturing of feldspar crystals within the quartz matrix. The mature mylonite (MM) in the centre of the vein consists of dynamically recrystallized grains originating from a few larger grains preserved at the vein margins (Lloyd et al. 1992; Trimby et al. 1998). Close inspection of the boundary of the quartz vein material at the margins of the vein (starting L. F. G. MORALES ET AL. 152