Microstructural Deformation in Quartzites from the Moine Thrust Zone near Loch Eriboll, NW Scotland: An Optical-SEM/EBSD Analysis

Daniel J. Tatham

School of Earth Science, University of Leeds, Leeds, LS2 9JT, U.K.

d.tatham@earth.leeds.ac.uk

This study describes a re-analysis of a suite of quartzite samples collected from the Moine Thrust Zone of Loch Eriboll, northwest Scotland, by Nadir (1980). The original assessment involved optical universal stage microscopy alone to determine the effects of thrusting on grain shape fabric and crystal c-axis preferred orientation. In addition to optical analysis, the present study employed modern automated SEM electron backscattered diffraction (EBSD) techniques, which provide detailed and accurate data on the complete crystal fabric of quartz together with a variety of other information (eg. grain size and aspect ratio distributions, crystallographic orientation images, etc.).

Nadir (1980) concluded from his study that deformation by dislocation slip was prevalent throughout all his samples, from relatively undeformed to intensely deformed quartzite. To test this conclusion, three quartzite samples were chosen to represent a strain gradient (i.e. low, medium and high) through the thrust zone. Both optical and SEM/EBSD analyses illustrate that strain is manifested through a grain shape fabric, which, as Nadir (1980) envisaged, could have developed through crystal plastic processes involving dynamic recrystallisation. However SEM/EBSD analysis revealed only a minor LPO development. In addition, there is an obvious discrepancy between the principle strain axis ratio (X/Z) and the expected associated angles that elongate grains make with the shear plane. This problem was encountered also by Nadir (1980), who noted a discrepancy between different strain markers, in his case grain shape and the boudinage of acicular rutile inclusions, in resolving shear strain magnitudes.

SEM/EBSD analyses clearly show that initial bulk crystal plastic deformation generates an immature grain shape fabric and develops a weak LPO. With increased strain, localised intragranular dynamic recrystallisation and cataclastic fracture zones develop. Subsequent shear preferentially occurs along these by grain rotation and/or grain boundary sliding, including diffusion mass transfer, which in part destroys the early LPO. Tabular grain fragments shear past each other along these recrystallisation/fracture planes to varying length-scales so that the original detrital grain may be impossible to reconstruct, leading to contradictory grain shape axial ratios and angles between 'grain' long axes and macroscopic shear/foliation plane. These observations support the model proposed by Bowler (1989) for quartz deformation in other parts of the northern Moine Thrust Zone (e.g. Inverhope and Strath Beag, Upper Loch Eriboll).

Bowler S. 1989. Shape fabric formation by cataclasis in a quartzite from the Moine Thrust Zone, northwest Scotland, Geology, 17, 353-356.

Nadir P.Y. 1980 unpub. The structure and deformation history of the Cambro-Ordovician sediments of the Moine Thrust Zone near Loch Eriboll, northwest Scotland, PhD thesis, Dept. Earth Sciences, University of Leeds.