Mylonity strefy Niemczy

Stanisław Mazur, Jacek Puziewicz


Mylonites of the Niemcza fault zone

The Niemcza Fault Zone, at the NE foreland of the Sudetes (SW Poland), consists of mylonitized Góry Sowie gneisses with intercalations of amphibolites, quartzo-graphitic schists and plutonic rocks. The mylonites are composed of fine-grained, in places laminated, matrix with porphyroclasts of plagioclase (20 - 30% an), garnet and, locally, cordierite. The matrix is formed of quartz and plagioclase (20% an), accompanied by syndeformational biotite and fibrolite or muscovite and chlorite. Biotite and fibrolite occur in mylonites deformed under the conditions of the amphibolite facies, whereas chlorite and muscovite occur in mylonites deformed under greenschist facies conditions.

There is abundant unambiguous evidence for non-coaxial deformation of the mylonites. Various kinematic indicators consistently indicate a sinistral sense of shear. The most frequent shear sense indicators are: S-C fabric, extensional crenulation cleavage, asymmetric tails around porphyroclasts, asymmetric pressure shadows and displaced broken crystals.

The gradual transition from the Góry Sowie gneisses to the mylonites demonstrates the progressive increase of strain. This is seen as a greater intensity of foliation, a reduction of grain-size and a decreasing number of porphyroclasts.

The mylonites were formed in a strike-slip, sinistral shear zone located in the gneisses along the eastern margin of the Góry Sowie gneissic block. The same deformation produced the dominant fabric in the mica schists of the Niemcza-Kamieniec Metamorphic Complex east of the Niemcza Zone.

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