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.

Abstract The classical Alpine folded belt of the Northern Calcareous Alps (NCA) of Austria is correlated with the Transdanubian Central Range (TCR) of Hungary using structural and stratigraphic relationships to restore the system. The semiquantitative map-view restoration of several consecutive Alpine deformational periods reveals unexpected similarities between the NCA and TCR. In fact, some west–northwest-trending right lateral strike-slip faults in the TCR (e.g., Telegdi-Roth, Padrag, and Vargesztes faults) are interpreted here for the first time to be analogous to those described from the NCA (e.g., Lammertal, Wolfgangsee-Windischgarsten, and Hochwart faults). These middle to late Miocene transpressional faults are reactivated in the Late Cretaceous tear faults, as can be documented by reflection seismic data in the subsurface of the southeastern Danube Basin. The structural correlation between the NCA and TCR provides further evidence for the much debated interpretation of the TCR in terms of a large Eo-Alpine (Cretaceous) nappe-system in an Uppermost Austroalpine structural position. Furthermore, recognition of a once continuous, regional-scale, right lateral strike-slip fault system in the NCA-TCR areas has a significant impact on the pre-Tertiary kinematic reconstructions of the broader Eastern Alps and Pannonian Basin region.

Abstract In this study, we focus on the transition from the host rock to the damage zone within brittle shear zones in order to document the structures forming during the initial phases of deformation, i.e., the fractures that formed prior to the formation of fault breccias and cataclasites. Structural analyses of rock samples from sites of the Talhof– and Palten–Liesing faults in the eastern Alps show that in these cases well-known R- and P-fracture patterns do not play a dominant role in the early stages of the generation of brittle fracture zones. In the studied layered marble and foliated impure quartzite samples, the boundary between the host rock and the damage zone is characterized by the formation of closely spaced fractures at high angles (70–90°) to the shear zone boundaries, being parallel to pre-existing layering/foliation planes. These fractures bound and define slender slab-like or columnar rock elements, here being termed lamellae. It is assumed that subsequent rotation of these lamellae in the bookshelf and domino modes associated with impeded dilation across the actual shear zone boundaries leads to kinking, splitting and final granulation of the lamellae to generate breccias of later fault core zones parallel to the shear zone boundary. In some cases, the observed bending and buckling of lamellae indicate additional ductile deformation subsequent to the development of the dominant lamellar structures formed by brittle fracturing.

Abstract The Periadriatic fault system (PFS) is an array of late orogenic faults (35-15 Ma) in the retro-wedge of the Alpine orogen that accommodated dextral transpression during oblique indentation by the southern Alpine crust. Decoupling along the leading edges of the southern Alpine indenter occurred where inherited lithological and rheological contrasts were accentuated by lateral thermal gradients during emplacement of the warm orogenic retro-wedge next to the cold indenter. In contrast, decoupling within the core and retro-wedge of the orogen occurred in a network of folds and mylonitic faults. In the Eastern Alps, this network comprises conjugate sets of upright, constrictional folds, strike-slip faults and low-angle normal faults that accommodated nearly coaxial NNE-SSW shortening and E-W extensional exhumation of the Tauern thermal dome. The dextral shear component of oblique convergence was taken up by a discrete, brittle fault parallel to the indenter surface. In the Central and Western Alps, a steep mylonitic backthrust, upright folds, and low-angle normal faults effected transpressional exhumation of the Lepontine thermal dome. Mylonitic thrusting and dextral strike-slip shearing along the steep indenter surface are transitional along strike to low-angle normal faults that accommodated extension at the western termination of the PFS. The areal distribution of poles to mylonitic foliation and stretching lineation of these networked structures is related to the local shape and orientation of the southern Alpine indenter surface, supporting the interpretation of this surface as the macroscopic shearing plane for all mylonitic segments of the PFS. We propose that mylonitic faults nucleate as viscous instabilities induced by cooling, or more often, by folding and progressive rotation of pre-existing foliations into orientations that are optimal for simple shearing parallel to the eigenvectors of flow. The mechanical anisotropy of the viscous continental crust makes it a preferred site of decoupling and weakening. Networking of folds and mylonitic fault zones allow the viscous crust to maintain strain compatibility between the stronger brittle crust and upper mantle, while transmitting plate forces through the lithosphere. Decoupling within the continental lithosphere is therefore governed by the symmetry and kinematics of strain partitioning at, and below, the brittle-to-viscous transition.