Strain, influence of dynamic recrystallization on quartz crystal fabric development, and estimates of flow vorticity in mylonites at the Stack of Glencoul, Northwest Scotland R.D. Law 1 , B. Cook 1&2 , M. Casey 3 , G.E. Lloyd 3 , R.J. Knipe 3 , D.H. Mainprice 4 and R. Thigpen 1 1 Department of Geosciences, Virginia Tech., Blacksburg, Virginia 24061, U.S.A. 2 Earth & Environmental Sciences, University of Kentucky, Lexington, Kentucky 40506, U.S.A. 3 Earth Sciences, School of Earth and Environment, The University, Leeds LS2 9JT, U.K. 4 Université Montpellier II, Lab. de Tectonophysique, 34095 Montpellier Cedex 05, France Since the early descriptions published by Callaway in1884, 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 (hangingwall), have figured prominently in: a) early research on the influence of crystal plastic deformation and recrystallization on microstructural and crystal fabric evolution (e.g. Christie 1960, 1963) and, b) in debates on kinematic interpretation of macro- and micro-structures and crystal fabrics (e.g. Johnson 1965 vs. Christie 1965; Johnson 1967; McLeish 1971; Wilkinson et al. 1975; Law et al. 1986; Law 1987; Strine and Wojtal 2004; Halfpenny et al. 2006). The classic outcrops of mylonite at the Stack of Glencoul will form the primary focus for one of the fieldtrips associated with the Peach and Horne conference. In this poster the microstructural and crystal fabric data (both published and unpublished) that we have collected over the last 25 years from these mylonites is summarize and integrated with previously unpublished strain and vorticity analyses. Strain analysis of the mylonitic Cambrian quartzites, using deformed detrital grain shapes (Rf/φ and harmonic mean) exposed on three mutually perpendicular sections in each sample, indicate that the plastically deformed footwall quartzites fall in the apparent flattening field (Lodes Unit υ in the 0.25 - 0.50 range) as previously suggested by small circle quartz a-axis fabrics (X-ray texture goniometry) in these samples (Law et al. 1986, Schmid & Casey 1986). Stretches parallel to the X (transport direction parallel lineation) and Y (orogenic strike) principal strain directions are estimated at 170-260% and 26-49% respectively, while shortening perpendicular to foliation is estimated at 75-90%, assuming constant volume deformation. A volume loss of 50- 70% would be required to explain these data in terms of a plane strain deformation with no elongation parallel to orogenic strike - for which, at least optically, there is no supporting microstructural evidence. We have studied quartz crystal fabric development in the Stack of Glencoul mylonites using optical microscopy/universal stage and SEM/EBSD single grain techniques as well as X-ray texture goniometry and neutron diffraction multigrain techniques. In this poster we concentrate on c-axis fabric development in plastically deformed detrital grains from the mylonitic Cambrian quartzite and on comparing these fabrics with those developing in the surrounding dynamically recrystallized matrix grains (10-15 micron grain size, recrystallization dominated by subgrain rotation). We also describe c-axis fabric development from deformed quartz layers (veins?) within the overlying Moine mylonites. Quartz c-axis fabrics from the detrital grains define Type One cross-girdle fabrics, which in XZ sections exhibit a high degree of both internal and external skeletal symmetry, and have previously been interpreted as at least qualitatively indicating a dominantly coaxial deformation history (cf. Christie 1963; Law et al. 1986). In contrast, surrounding matrix recrystallized grains define Type One cross-girdle fabrics which are subtly more asymmetric in terms of both internal and external skeletal outline and have larger opening angles measured in the XZ plane, the sense of asymmetry being consistent with top to the WNW thrusting. A close correlation has been established between the various angular fabric parameters used to define the degree of internal and external skeletal symmetry, with an increase in external fabric asymmetry always being accompanied by an increase in internal fabric asymmetry. In addition to changes in skeletal outline, our analyses also indicate that recrystallization leads to significant changes in density distribution on the fabric diagram. This has been quantified for individual samples by counting and comparing the number of data points from detrital and recrystallized grains that plot within individual cells of the numerical counting grid used for fabric contouring. The results are displayed as fabric density difference plots and indicate that, relative to the detrital grains which have deformed by dislocation creep, recrystallization produces a large number of grains whose c-axes define point maxima located on a small circle girdle centered about the foliation pole. The degree of dynamic recrystallization increases in the Cambrian quartzites traced upwards to the overlying Moine thrust plane and a progressive transition can also be tracked upwards within the recrystallized grains from subtly asymmetric cross-girdle c-axis fabrics to obviously asymmetric cross-girdle and single girdle fabrics. Similar asymmetric fabrics are also recorded within the deformed and dynamically recrystallized quartz veins in the overlying Moine mylonites.