Rheological behaviour of rocks in highly ductile shear zones: a review of indicators of shear direction

The research is devoted to the problem of kinematics of ductile shearing under high grade metamorphic conditions. Particularly, it focuses on the orientation of stretching lineation with respect to direction of shear. We study an area in the Eastern Baltic shield composed of Precambrian highly metamorphosed rocks that differs in kinematical features from the shear zones that have been usually described. Specifically, stretching lineation in this area represents the axis of bulk rotation rather than the vector of maximum displacement. We accept that in many regions stretching lineation is parallel to shear direction, but here we are dealing with a field example where they are perpendicular. The suggested research provokes the review of some reconstructions of the tectonic movement picture, especially in areas where evidence for parallelism of the lineation and shear direction are poor. The aim of the research is to describe in detail the observed phenomenon of b-stretching lineation. The task- maximum is to understand its genesis, i.e. to find factors causing different orientations of stretching lineation with respect to shear direction. In order to prove the main thesis we suggest the following independent lines of evidence: mapping and structural analysis, analysis of deformed dykes and veins (Talbot method), strain analysis of deformed conglomerates and amigdals, microfabric analysis, and X-ray texture analysis of quartz. Penetrative lineations are common if not ubiquitous in ductilly deformed crystalline rocks. Therefore, understanding the kinematic significance of this fabric is important for both theoretical studies and regional applications. The main consequence of this study is that where stretching lineation represents rotational axes, the vectors of tectonic transport may be perpendicular to those previously interpreted. Whole orogenic belts and large ductile shear zones that were previously assumed to involve strike flow may instead have been due to dip flow and vice versa. Archean high grade polymetamorphic rocks are poorly exposed in Europe except of the Baltic shield. Therefore, the data obtained from ancient high grade ductile shear zones of the Kola Peninsula are of interest for Austrian geologists. They give a key to understanding deformation processes at deep levels of the lithosphere which took place in the basement of Alps.