Evidence for non-plane strain flattening along the Moine thrust, Loch Srath nan Aisinnin, North-West Scotland Matthew Strine * , Steven F. Wojtal Department of Geology, Oberlin College, Oberlin, OH 44074-1044, USA Received 23 May 2003; received in revised form 15 January 2004; accepted 24 February 2004 Available online 13 May 2004 Abstract We report quartz c-axis patterns, grain-shape fabrics, and microstructures for 11 mylonitic quartzites and quartz-phyllosilicate schists from a transect across the Moine thrust at Loch Srath nan Aisinnin, North-West Scotland. In the footwall samples collected more than 42 m normal distance from the thrust surface, quartz c-axis textures indicate a general flattening strain (i.e. 0 , k , 1). Samples within 19 m normal distance of the thrust are completely recrystallized and exhibit asymmetric c-axis patterns. Recrystallized hanging wall fault rocks exhibit random c-axis patterns on the scale of a standard thin section. Relict footwall grains provide the closest approximation of finite strain; they have octahedral shear strains (e s ) between 1.10 and 1.47 and exhibit general flattening k-values (0.0524 – 0.659). The long axis of the mean relict grain shape trends parallel to the regional transport direction and plunges gently to the ESE. In contrast, recrystallized footwall grains have a mean grain shape with the longest axis oriented nearly perpendicular to the transport direction. Furthermore, these samples have grain shape k-value ranges from 0.157 to 0.295. Recrystallized hanging wall grain shapes exhibit the lowest octahedral shear ‘strains’ (e s ¼ 0.532 – 0.733) and largest mean k-values (0.351 – 0.961) of this sample set. The long axes of the mean recrystallized hanging wall grain shapes are parallel to transport, similar to that of relict footwall grains. Unrecrystallized quartz overgrowths about opaque mineral grains suggest concurrent elongation in all directions within the mylonitic foliation and support the inference of general flattening deformation. The mylonitic foliation and penetrative lineation are consistent with a WNW shearing direction; however, both were folded during later deformation increments. Recrystallized grains in footwall quartzites suggest a 305 – 3208 azimuth for the shearing direction. The best-fit p- axis of the poles to the foliation is 18,145, close to the mean trend/plunge of mesoscopic fold hinges at this site (18,151). Both suggest a NNW azimuth for the shearing direction. Thus, structural, microstructural and fabric indicators suggest a change in the azimuth of the shearing direction from , 286 to , 3318. q 2004 Elsevier Ltd. All rights reserved. Keywords: c-Axes; Fold–thrust belts; Grain shape analysis; Kinematics; Moine; Mylonites; Non-plane strain; North-West Scotland; Preferred orientation; X-ray goniometry 1. Introduction Most fold-and-thrust belts are arcuate in nature, giving rise to a pattern of large-scale salients and recesses (Marshak, 1988; Lawton et al., 1997; Mitra, 1997). Similarly, individual thrust faults have arcuate traces in map view (above and beyond the effect of interaction with an irregular surface topography) suggesting that thrust faults, in general, have non-planar geometries (Dahlstrom, 1970; Elliott and Johnson, 1980; Boyer and Elliott, 1982). Three-dimensional complexities in the shapes of individual thrust faults and in the overall configuration of fold-and- thrust belts lead to three-dimensional variations in the velocity and displacement fields within thrust sheets and, thus, in the distribution of incremental and finite strains within those sheets. Lateral changes in macroscopic geometry of thrust-related structures and documented non- plane strains at the mesoscopic and microscopic scales (e.g. Durney and Ramsay, 1973; Ramsay and Huber, 1983; Geiser, 1988; Mukul and Mitra, 1998; Twiss and Unruh, 1998) attest to the three-dimensional nature of displacement fields within thrust sheets. Moreover, variations in material behavior over space and time also contribute to the 0191-8141/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2004.02.011 Journal of Structural Geology 26 (2004) 1755–1772 www.elsevier.com/locate/jsg * Corresponding author. Present address: Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USA. Tel.: þ 1-585-275-5713; fax: þ1-585-244-5689. E-mail address: matty@earth.rochester.edu (M. Strine).