Abstract
Optical microscopy and transmission electron microscopy (TEM) have been used to investigate deformation-induced microstructures in Alpine pseudotachylytes, mylonites and ultramylonites from the Upper Austro-Alpine Silvretta nappe at the southwestern margin of the Lower Engadine Window in the Eastern Alps of Switzerland and Austria. The earliest Alpine stage of deformation (D1) produced mylonitic gneisses, with dislocation glide being the dominant deformation mechanism in quartz. In the second stage (D2a), mylonites were formed as a result of strain softening in quartz (manifest by dynamic recrystallization and a lattice preferred orientation). TEM observations are used to characterize the initial stages of migration and rotation recrystallization. Pseudotachylytes were generated during D 2b when strain hardening due to dislocation tangling was not outweighed by strain softening. Several generations of mylonites and pseudotachylytes are observed. A model for the propagation of seismic fractures, initiated in comparatively rigid amphibolites, into an otherwise ductile environment is proposed for the pseudotachylytes. As such they are formed as transient discontinuities in the mylonites. D 1 and D 2a/b are related to the Eo-Alpine detachment of the Silvretta nappe. Subsequent Eo-Alpine metamorphism in the investigated area reached the stilpnomelane zone, and this coincided with the development of ultramylonites from both the mylonites and pseudotachylytes during D 3 and peak metamorphism. The increased temperature combined with fluid infiltration from the overridden Penninic strata favoured grain-boundary sliding (superplasticity) as the dominant deformation mechanism during D 3.
Published Version
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