TSK 11 Göttingen 2006 Schneider & Rosenberg
 Kinematics of the SEMP-
 fault in the western Tauern
 Window (Stillupp Valley)
 Poster
 Susanne Schneider1
 Claudio Rosenberg1
 Introduction
 The working area is located in the
 Stillupp Valley (Tirol, Austria), where
 the western termination of the SEMP-
 fault (Salzach, Ennstal, Mariazell,
 Puchberg), overprints the northernmost
 margin of the Zentralgneiss. This sinis-
 tral shearzone, which has a length of
 about 300km, and causes a lateral dis-
 placement of 60 km (Linzer et al. 2002),
 marks part of the northern border of the
 Tauern Window.
 Macroscopic Structures
 The northern part of the study area
 shows a penetrative, mylonitic folia-
 tion, which strikes ENE–WSW, dips
 subvertically, and contain flat-lying, but
 generally WSW-dipping stretching lin-
 eations. This mylonitic zone, which has
 a width of approximately 2 km is sys-
 tematically associated with shear bands
 and σ-clasts, indicating a sinistral sense
 of shear. Occasionally, shear bands
 are also contained in the YZ plane of
 deformation, where they indicate a S-
 side-up sense of shear. Besides mostly
 in this part the deformation of the
 rocks is overprinted by brittle tecton-
 ics. South of the mylonitic zone, the
 foliation is folded by isoclinal folds with
 axial planes subparallel to the foliation
 described above. These folds can be cor-
 related to the upright, large-amplitude
 1 Freie Universität Berlin, Malteserstr 74–100,
 12249 Berlin Germany
 folds, which form the three gneiss cores
 of the western Tauern Window. Further
 south, folds become more open, with ax-
 ial planes dipping gently southward. We
 attribute these folds to the early Alpine
 phase of shortening, which affected the
 Zentral Gneiss (e.g. Lammerer 1988),
 before doming of the Tauern Window.
 This area is also affected by ductile
 shear zones of several meters width
 and a periodic distance of about 100m.
 These shear zones strike WSW–ENE,
 dip steeply southwards and show a S-
 side-up sense of shear, indicated by the
 foliation drag and shear bands.
 Microscopic Structures
 On the basis of the quartz microstruc-
 tures a gradient in the deformation tem-
 perature across the sinistral shear zone
 can be inferred. Quartz grains in the
 northernmost samples were deformed by
 dislocation glide associated with incipi-
 ent bulging recrystallization. Towards
 the South, the deformation and recrys-
 tallization mechanisms of quartz show a
 transition to dislocation creep and sub-
 grain rotation recrystallization, followed
 by dislocation creep and grain bound-
 ary migration at the southernmost end
 of the sinistral shear zone. This change
 in the deformation and recrystallization
 mechanisms goes together with a grain
 size increase of the new grains from
 ca. 0.04mm in the North to ca. 0.12mm
 in the South. The folded foliation
 south of the sinistral shear zone shows
 a high-temperature fabric, characterized
 by dynamic recrystallization of quartz.
 Hence, there is no evidence for a static
 overprint separating the first and the
 second phase of folding.
 1
Schneider & Rosenberg TSK 11 Göttingen 2006
 Interpretation
 In summary, the structures and mi-
 crostructures described above suggest
 that the upright folds of the western
 Tauern Window formed during shear-
 ing along the SEMP Fault, which be-
 comes ductile as it enters the northern
 margin of the Tauern Window. In addi-
 tion, a pronounced, southward increase
 in structural level, as exposed across
 the strike of the shear zone, points
 to a transpressive type of deformation.
 Therefore, in addition to a left-lateral
 displacement, the SEMP Fault accom-
 modates a vertical S-side-up component
 of displacement, which contributes to
 the exhumation of the Tauern Window.
 References
 Lammerer B (1988) Thrust-regime and
 transpression-regime tectonics in the Tauern
 Window (Eastern Alps). Geologische
 Rundschau 77, 143–156.
 Linzer H-G, Decker K, Peresson H, Dell’Mour
 R, & Frisch W (2002) Balancing orogenic
 float of the Eastern Alps. Tectonophysics
 354, 211–237.
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