INTEGRATED STRAIN, PETROFABRIC AND VORTICITY ANALYSIS OF MOINE THRUST FOOTWALL MYLONITES IN THE NORTHERN PART OF THE ASSYNT WINDOW, NW SCOTLAND

We present an integrated strain, petrofabric and vorticity analysis of greenschist facies mylonitic Cambrian quartzites lying in the footwall to the Moine thrust. The 12 samples analyzed were collected from a stream section in the Strath nan Aissinin area of northern Assynt and are structurally located at ~ 10 to 90 meters beneath the thrust plane. 3D strain analysis (Rf/phi technique) on plastically deformed relic detrital quartz grains indicates that mylonite formation was in the general flattening field. Estimated strain magnitude values, e_s, range from » 1.1 – 1.5 within the suite of footwall quartzites, with shortening estimates of 30-42% (assuming no volume loss) perpendicular to the gently dipping mylonitic foliation. Two techniques of vorticity analysis have locally proved applicable: a) the combined Rxz strain ratio and quartz c-axis fabric method (Wallis 1992) and, b) the rigid grain method (Wallis et al. 1993) in the more feldspar-rich quartzite units. Both methods indicate general shear histories. 3D strain and vorticity analyses from this suite of quartzites will be compared with previously published analyses of a thinner sequence (< 10 m) of mylonitic Cambrian quartzites preserved in the footwall to the Moine thrust at the Stack of Glencoul (4.6 km to the south) where strain magnitudes are higher (e_s ranges from » 1.7 – 2.1) but 3D strain type is similar. Implications of the integrated strain and vorticity data for stretching both along orogenic strike and parallel to the thrust transport direction will be highlighted, and tectonic significance of along strike variation in these parameters discussed.