Texture and fabric studies across the Kishorn Nappe, near Kyle of Lochalsh, Western Scotland

Abstract

The progressive development of mylonitic fabrics in a series of Torridonian sandstones and shales has been studied along traverses across the Kishorn Nappe. The fabrics developed have been investigated using the following techniques. 1. 1. Optical examination of thin sections. 2. 2. Measurements of the anisotropy of magnetic susceptibility. 3. 3. X-ray texture goniometry. The results are used in support of a proposed deformation history of the area and the relative advantages of the techniques used are discussed. The early deformation was well lubricated with layer-parallel sliding and little internal deformation of the rocks, except for development, in the east, of a layer-parallel penetrative fabric with an extension direction to the ESE. This deformation produced a westward facing isoclinal anticline and a recumbent syncline in the Torridonian rocks which became at least partly decoupled from the basement. The important phases of fabric development post date this folding. In the west the sandstones developed a spaced, pressure solution cleavage, but in the east the grain shape fabric has been produced by both dislocation and diffusion processes. The shales reveal more details of the deformation episodes than do the sandstones and thus show different fabric intensities and orientations when measured by magnetic and X-ray techniques. The magnetic anisotropy technique of fabric analysis gives a rapid method of mapping the deformation domains formed by different deformation mechanisms and intensities. However, the rocks carry several magnetic components and these have different anisotropy tensors and different responses to deformation, also, measurements made at high fields (5 kOe) give magnitudes and orientations of the magnetic anisotropy tensor which are different from those made at low fields. It is concluded that it is not possible to relate variations in the magnitude and shape of the magnetic anisotropy ellipsoid quantitatively to the deformation. Chlorite and muscovite fabrics measured by X-ray techniques show variations in intensity and orientation similar to those of the magnetic anisotropy ellipsoid due to paramagnetic minerals. However, the data demonstrate the difficulty of correlating this fabric intensity with deformation intensity where there has been a change in deformation mechanisms with time and space.