Mesostructural and kinematic characteristics of core gneisses in western part of the Orlica-Śnieżnik Dome, West Sudetes

Abstract

The Orlica-Śnieżnik Dome in the West Sudetes consists of an orthogneissic core and a schist-phyllite mantle. It is subdivided into two parts tectonically separated by the Upper Cretaceous Nysa Graben. In this paper structures developed during polyphase deformation of the c. 500 Ma core gneisses in the western part (Orlické hory, Góry Orlickie, Góry Bystrzyckie) are compared with those of the eastern part (Śnieżnik massif). The effects of deformations D₁ to D₄ structures are, in general, mutually comparable in the gneissic series of the Góry Orlickie (Mountains) and the Śnieżnik complex. D₅ structures, manifested in the Góry Orlickie as S-C structures which show a top-to-the-south or top-to-the-southwest sense of displacement, are, however, not known in this shape from the eastern part. Structures of such geometry in the gneisses of the latter region were ascribed to deformation D₃. The C element of the S-C structures is represented in the Góry Orlickie by a rough S₅ domainal cleavage, spatially related to the NW-SE fault zone of Olešnice-Uhřínov, which separated the gneissic core from the Nové Město series and the Zabřeh series of the mantle. This relationship suggests that both the S₅ cleavage and the fault zone are associated with the same deformation event D₅ and originated under similar rheological and kinematic conditions. In both cases cataclastic deformation was accompanied by only local and limited recrystallization of white micas. Kinematic analysis of the D₅ tectonic structures shows that these structures were formed in a sinistral transtension regime. In the Góry Bystrzyckie the tabular crystals of microcline, usually preserving their original shapes, are, in general, arranged along the S₃ gneissosity planes in which they show a considerable scatter. They are more or less lenticularly deformed, in concordance with the SSW-trending L₃ mineral recrystallization lineation, and point to a top-to-the-north sense of shearing during D₃ event.