Exhumation mechanisms of middle to lower crust in the Tertiary Eastern Alps (Tauern Window)
Final Report Abstract
The most significant results of this project concern the new ages of deformation, the new ages of cooling and the new structures mapped in and around the Tauern Window. Our structural results suggest that the western Tauern Window represents a zone of localized, sinistral transpressive deformation on the scale of the orogen, consisting of tight, upright folds associated to a network of sinistral mylonites striking from the southwestern termination to the northern central area of the Tauern Window (Krimml, Mittersill). This zone, finds its continuation in the sinistral NE-striking Giudicarie belt to the SW, and in the ENE-striking SEMP Fault to the NE, forming a large-scale, restraining bend between them. Our structural investigations show that sinistral displacements of the SEMP Fault do not terminate at the northern termination of the Brenner Fault, as previously postulated, but strike along its southern termination before passing into the Giudicarie Belt. The inferred amounts of displacement (~70 km), related to NNE-directed indentation along the Giudicarie Belt, coincides with the amount of sinistral displacements inferred for the SEMP fault. Shortening accommodated by the upright ENE-striking folds inside- and south of the western Tauern Window amounts to ~50 km. In due consideration that upright folding and sinistral shearing are coeval and kinematically linked in the western Tauern Window the displacement accommodated by the sinistral shear zone network can be estimated to be ~20 km. As a consequence, sinistral displacement of the SEMP Fault largely (completely?) continues into the Giudicarie Fault, linking indentation to lateral extrusion of the Eastern Alps. Therefore, extension along the Brenner Fault, which is inferred to be moderate, does not accommodate lateral displacement of the SEMP. A series of 40Ar/39Ar analyses of pre-kinematic muscovite, biotite and K-feldspars, of syn-kinematic phengite and K-feldspar, and of post-kinematic phengite within the same samples of sinistral mylonites from the western Tauern Window (EasternAlps) confirms microstructural observations of cross-cutting relationships showing that syn-kinematic minerals are, in general, younger than pre-kinematic minerals and older or of equal age than the post-kinematic minerals of the same sample. Absolute ages of syn-kinematic phengite and K-feldspar yield a time range of 33 to 15 Ma for the northernmost shear zone (Ahorn Shear Zone), of 24 to 12 Ma for the central shear zone network (Tuxer Shear Zones), and of 20 to 7 Ma for the southern, Greiner Shear Zone. The age variation of syn-kinematic phengite and K-feldspar analyses is larger than the analytical error of each age obtained. In addition, isochron calculations of the syn-kinematic minerals reveal atmospheric-like 40Ar/36Ar intercepts. Therefore, the obtained age values of the syn-kinematic minerals are interpreted as formation ages, which date increments of a long lasting deformation period. The time range of deformation of each shear zone system is bracketed by the oldest and youngest formation ages of syn-kinematic phengite and K-feldspar. Post-kinematic phengite laths show the youngest formation ages and overlap with the youngest syn-kinematic formation ages. This relationship indicates that post-kinematic growth occurred immediately after syn-kinematic mineral formation at the end of ductile sinistral shear. Hence, the termination of deformation is dated by the ages of these post-kinematic phengite blasts.
Publications
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2008, The western termination of the SEMP fault (Eastern Alps) and its bearing on the exhumation of the Tauern Window. In: S. Siegesmund, B. Fügenschuh, and N. Froitzheim (eds.), Tectonic aspects of the Alpine-Carpathian-Dinaride System, Geological Society of London, spec. publ., 298, 197-218
Rosenberg, C.L., and Schneider, S.
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2009, On the causes and modes of exhumation and lateral growth of the Alps, Tectonics, 28, TC6001
Rosenberg, C.L. and Berger, A.
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2013, Dating the longevity of ductile shear zones: insight from 40Ar/39Ar in situ analyses. Earth and Planetary Science Letters
Schneider S, Hammerschmidt K., and Rosenberg C.L.
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2013, The Tauern Window (Eastern Alps. Austria): a new tectonic map, with cross-sections and a tectonometamorphic synthesis. Swiss Journal of Geosciences, Swiss Journal of Geosciences, 106:1-32
Schmid SM, Scharf A, Handy MR, Rosenberg CL
