89 Structural Geology (Posters)

QUARTZ C-AXIS TEXTURE DATA COLLECTED USING THE MELBOURNE FINE-GRAINED FABRIC ANALYZER (MIFA): A CASE STUDY ALONG THE MOINE THRUST ZONE, NW SCOTLAND

Paper number: 89-15
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

Using the fully automated c-axis texture data acquisition system (Melbourne Fine-Grained Fabric Analyzer, MiFA) we have determined quartz crystallographic fabrics for samples from the Moine thrust zone. In particular, data was collected from the footwall Cambrian quartzites and from quartz-rich layers of the hanging wall Moine schists around a well-studied salient-recess pair along the Moine thrust trace. This study confirms the results of previous investigations and demonstrates that the recess has a larger and more highly strained deformation zone. This deformation zone is characterized by the transition from an asymmetric quartz c-axis pattern to a non-plane strain flattening, small-circle girdle pattern farther from the thrust trace. Within the salient, the asymmetric c-axis pattern transitions into a fully symmetric cross-girdle pattern. Moreover, the transition from an asymmetric pattern occurs much farther from the thrust trace within the recess than the adjoining salient. This suggests that as the deformation zone grows in thickness the simple shear dominated portion of the deformation zone grows along with it, potentially maintaining the same proportion of the deforming zone.

We note a bias within the MiFA method against steeply plunging c-axis orientations. This bias may be inherent in the method or due to human-error during the subsequent data collection process. In either case, the method would benefit from a more objective and automated procedure for assembling the fabric data as well as from the collection of data from differently oriented sections (e.g. three mutually perpendicular sections). Furthermore, it is important to collect data from multiple fields of view because the samples show a highly domainal pattern of c-axis orientations in AVA (axial distribution analysis). Grains of similar crystallographic orientations will often be clustered together, and therefore, several of these clusters need to be sampled to determine the entire crystallographic fabric.