The use of syntectonic fibres to determine strain estimates and deformation paths: an appraisal

Abstract

Fibrous pressure shadows developed adjacent to rigid brittle inclusions are familiar features in many tectonites. Belief that these structures reflect the deformation path, as once adjacent grains move apart, has led to the derivation of a variety of techniques to determine both incremental and finite strain estimates. These methods, which can be divided into rigid fibre models (e.g. Durney and Ramsay) and deformable fibre models (e.g. Ellis), display potentially serious problems when applied to field examples. Recent studies of microstructures within veins and pressure shadows support the concept that syntectonic fibre complexes can reflect deformation paths and are therefore useful markers of incremental and finite strain. The problems mentioned therefore stem directly from the application of the techniques. Worked examples using these techniques give a variety of strain values dependent upon the extent to which a particular technique allows for volume change or fibre deformation. This is useful locally. However, if the results are to be meaningful on a regional scale, then it is imperative that the same technique be applied in the analysis of all fibres regardless of the local deformation path. In such studies the merits of a particular technique in determining a “correct” strain value come second to allowing the spatial distribution of strain to be determined semi-quantitatively, and thus allowing comparison of similarly derived results with one another.