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Abstract

The SEMP (Salzach–Ennstal–Mariazell–Puchberg) Fault strikes along more than 300 km from the southern margin of the Vienna Basin to the northern Tauern Window accommodating a sinistral displacement of 60 km during Tertiary time. We present new structural data, showing that the SEMP Fault continues into the Tauern Window within a 50 km long mylonitic belt of approximately 2 km width, which we term the Ahorn shear zone. This sinistral shear zone, which marks the northern boundary of the Zentral Gneiss, strikes E to ENE, dips subvertically, and is characterized by gently W-dipping to subhorizontal stretching lineations. S-side-up kinematic indicators in the Y–Z fabric plane and a pronounced southward increase in the inferred temperature of sinistral shearing are observed within the shear zone. Microstructural observations indicate that deformation of quartz at the northernmost boundary of the Zentral Gneiss occurred by dislocation glide with only incipient dynamic recrystallization, suggesting a temperature of approximately 300 °C. Further south, temperatures greater than 300 °C are inferred because all samples are affected by dynamic recrystallization of quartz, and dynamic recrystallization of feldspars also occurred in the southernmost part of the shear zone. These findings point to transpressive deformation accommodating a significant component of south-side-up displacement in addition to sinistral shearing. The sinistral mylonitic foliation forms the axial-plane foliation of the large-scale, ENE-striking upright folds of the western Tauern Window. From east to west, deformation becomes increasingly distributed, passing from an area of interconnected shear zones in the east to a homogeneously deformed mylonitic belt in the west, which terminates into a belt of WNW-striking, upright folds. From the above, we suggest the following: (1) the SEMP Fault extended beyond the brittle-ductile transition to a depth where temperatures exceeded 500 °C (>20 km depth?). These mylonites should be included in the seismic interpretation profiles as a major crustal discontinuity; (2) the large-amplitude, upright folds of the Tauern Window formed at the same time as the sinistral mylonites, and hence during south-side-up differential displacement; and (3) part of the 60 km lateral displacement of the SEMP fault is transferred into a vertical displacement at the western end of the Ahorn shear zone and into a fold belt accommodating NNE-oriented shortening, west of the Ahorn shear zone.

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