Structural style, deformation mechanisms and paleodifferential stress along an exposed crustal section: constraints on the rheology of quartzofeldspathic rocks at supra- and infrastructural levels (Bohemian Massif)

Abstract

Investigations of Variscan shortening of quartzofeldspathic rocks have been carried out along an exposed crustal section located in the western part of the Teplá Barrandian unit (Bohemian Massif). The study focuses on determining differential stress, deformation mechanisms and geometry of Cadomian metagreywackes and Cambrian granitoids which show different grade of Variscan Barrovian-type metamorphism ranging from the greenschist-/subgreenschist-facies boundary (semibrittle deformation regime) to the high-grade amphibolite facies.The semibrittle deformation regime (T=ca. 300°C) is characterized by large-scale thrusting within a folding environment and by similar intensities of cataclasis, diffusive mass transfer, and crystal plasticity of quartz, the latter including low-temperature migration recrystallization. Within Cadomian quartz veins of the greenschist-facies part, dislocation creep of quartz operated at differential stresses ranging from ca. 70MPa in the lowermost greenschist-facies to ca. 20MPa in the uppermost greenschist-facies. These values have been derived by paleopiezometry using the grain size of recrystallized quartz. They are interpreted as upper bounds for the bulk stress because of intense diffusive mass transfer within the adjacent metagreywackes. A marking increase in recrystallized grain size of quartz occurs within the lower greenschist-facies, where low-temperature grain boundary migration is replaced by subgrain rotation. Thus, causion is needed when applying the recrystallized grain size paleopiezometer to this crustal level. Pressure solution of quartz and development of discrete crenulation cleavage was common in fine-grained metagreywackes of the lower greenschist-facies, whereas the upper greenschist-facies (T=ca. 500–570°C) is characterized by increasing intensity of metamorphic reactions, grain coarsening and development of a schistosity. The entire greenschist-facies level forms a suprastructure where viscosity contrasts between mica- and quartz-rich layers were sufficiently high for buckle folding. Strain-protected intercalated metagranitoids, on the other hand, do not show pervasive deformation fabrics.The amphibolite-facies part forms an infrastructure characterized by pervasive mylonitic shearing of both Cambrian granitoids and metagreywackes. It is suggested that the combination of the following processes has contributed to weaken the amphibolite-facies level making nonlocalized flow possible: (1) increased recrystallization of feldspar; (2) crystal-plastic softening of quartz due to high-temperature grain boundary migration and activity of prism slip; and (3) enhanced diffusive mass transfer in the form of metamorphic reactions.The structural breaks at the semibrittle crustal level and at the greenschist-/amphibolite-facies boundary supports the view that rheological boundaries are prone to form major detachment horizons in the continental crust.