Deformation temperatures, vorticity of flow and strain symmetry in the Loch Eriboll mylonites, NW Scotland: implications for the kinematic and structural evolution of the northernmost Moine Thrust zone J. RYAN THIGPEN 1 *, RICHARD D. LAW 1 , GEOFFREY E. LLOYD 2 , SUMMER J. BROWN 1 & BRIAN COOK 3 1 Department of Geosciences, Virginia Tech, Blacksburg, Virginia 24061, USA 2 School of Earth and Environment, The University, Leeds LS2 9JT, UK 3 Department of Earth and Environmental Sciences, University of Kentucky, Lexington, Kentucky 40506, USA *Corresponding author (e-mail: thigpe05@vt.edu) Abstract: The Moine Thrust zone (MTZ) marks the Caledonian foreland-to-hinterland transition zone at the base of the Scandian (c. 430 Ma) orogenic wedge. In the Loch Eriboll region, the upper ductile part of the MTZ is composed in ascending order of two regionally extensive thrust sheets (Upper Arnaboll-Creag na Faoilin and Creagan) and is overlain by the Moine Nappe. Quartz crystal fabrics, kinematic vorticity (W m ), and strain estimates from the ductile thrust sheets in this region are used to determine how pure and simple shear components of deformation are partitioned, and indicate that these processes may be thermally, structurally, and lithologically dependent. At the lowest structural levels, quartzite and gneiss in the Upper Arnaboll-Creag na Faoilin (UA-CNF) thrust sheet yield rigid grain-based arithmetic mean minimum (W m min) and mean maximum (W m max) vorticity estimates of 0.57 and 0.67, respectively (60–53% pure shear). Creagan thrust sheet mylonites yield W m min and W m max estimates of 0.59 and 0.72 (59–48% pure shear). At the highest structural levels, Moine Nappe mylonites yield W m min and W m max esti- mates of 0.59 and 0.71 (59–49% pure shear). Quartz c- and a-axis fabrics qualitatively indicate an increase in non-coaxial deformation (top-to-the-west) traced towards structurally higher levels, which is accompanied by increases in deformation temperature (c. 370 8C to c. 550 8C). Integrated strain and vorticity estimates indicate that significant sub-vertical foliation normal short- ening has occurred as nappe stacking progressed. Recent quantitative vorticity studies have indicated that ductile deformation in the hinterlands of colli- sional orogenic systems (e.g. High Himalaya, Scot- tish Caledonides, Greek Hellenides) involves a significant component of pure shear deformation (Grasemann et al. 1999; Xypolias & Doutsos 2000; Xypolias & Koukouvelas 2001; Law et al. 2004; Jessup et al. 2006; Law et al. 2010; Xypolias et al. 2010) and ductile nappes in these orogens are often dominated by a pervasive subhorizontal foli- ation, which is unlikely to result solely from simple shear deformation (Ring & Kassem 2007). Traditional models of ductile thrust sheet emplace- ment that assume strictly simple shear as the domi- nant deformation symmetry fail to explain the observed variation of grain shape fabrics (L-, L-S, and S-tectonites; Fig. 1a – c) observed in high strain zones of most orogens (Strand 1945; Kvale 1945; Flinn 1956, 1961; Wood 1973; Mendum 1976; Law et al. 1984, 2004, 2010; Fossen 1993a, b; Strine & Mitra 2004; Strine & Wojtal 2004; Xypolias et al. 2010). If these data reflect actual deformation con- ditions, then a number of implications must be con- sidered regarding orogenic wedge evolution (Platt 1986). Firstly, pure shear strain applied orthogonal to the pervasive subhorizontal foliation should, in the absence of significant volume loss, lead to verti- cal ductile thinning of the nappe pile (Ring & Kassem 2007). In simple models that assume approximately plane strain isochoric conditions, vertical ductile thinning should result in transport- parallel lengthening of thrust sheets and subsequent extrusion of material towards the synorogenic topo- graphic surface. Also, along-strike variation in extrusion magnitude could produce strike-parallel gradients in ductile displacement and may even be responsible for observed variations of foreland From:LAW, R. D., BUTLER, R. W. H., HOLDSWORTH, R. E., KRABBENDAM, M. & STRACHAN, R. A. (eds) Continental Tectonics and Mountain Building: The Legacy of Peach and Horne. Geological Society, London, Special Publications, 335, 623–662. DOI: 10.1144/SP335.26 0305-8719/10/$15.00 # The Geological Society of London 2010.