The western termination of the SEMP Fault (eastern Alps) and its bearing on the exhumation of the Tauern Window
Claudio L. Rosenberg, Susanne Schneider, 2008. "The western termination of the SEMP Fault (eastern Alps) and its bearing on the exhumation of the Tauern Window", Tectonic Aspects of the Alpine-Dinaride-Carpathian System, S. Siegesmund, B. Fügenschuh, N. Froitzheim
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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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The Alps, Carpathians and Dinarides form a complex, highly curved and strongly coupled orogenic system. Motions of the European and Adriatic plates gave birth to a number of ‘oceans’ and microplates that led to several distinct stages of collision. Although the Alps serve as a classical example of collisional orogens, it becomes clearer that substantial questions on their evolution can only be answered in the Carpathians and Dinarides. Our understanding of the geodynamic evolution of the Alpine-Dinaride-Carpathian System has substantially improved and will continue to develop; this is thanks to collaboration between eastern and western Europe, but also due to the application of new methods and the launch of research initiatives. The largely field-based contributions investigate the following subjects: pre-Alpine heritage and Alpine reactivation; Mesozoic palaeogeography and Alpine subduction and collision processes; extrusion tectonics from the Eastern Alps to the Carpathians and the Pannonian Basin; orogen-parallel and orogen-perpendicular extension; record of orogeny in foreland basins; tectonometamorphic evolution; and relations between the Alps, Apennines and Corsica.