TSK 11 Göttingen 2006 Laukamp & Angerer
 Eo-Alpine imbrication of
 Middle Austroalpine units in
 the Gurktal Alps, Carinthia
 — questioning the existence
 of the Upper Austroalpine
 Murau Nappe Poster
 Carsten Laukamp1 Thomas Angerer1
 Structural analysis of Paleozoic Middle
 and Upper Austroalpine (MAA, UAA)
 units of the Gurktal Nappe Complex
 (GNC) in combination with electron mi-
 croprobe analyses of multiphase gar-
 nets reveal a complex Variscan and Al-
 pidic tectonometamorphic evolution of
 the GNC. Our study is focussed on the
 UAA ‘Murau Nappe’ and its tectonic
 transition to the MAA respectively to
 the overlying UAA Stolzalpen Nappe.
 The ‘Murau Nappe’ may rather be in-
 terpreted as a major zone of imbricate
 thrust slices of the underlying and over-
 lying units than a distinct nappe.
 The tectonometamorphic evolution of
 the MAA and UAA units in the GNC
 comprises the following events:
 1. D1: pre-Alpine (Variscan), anchi-
 zonal to greenschist-facies meta-
 morphism of the MAA (40Ar/39Ar-
 ages on micas of the western GNC
 about 315–310Ma; Neubauer et al.
 1999),
 2. D2: after subduction of the North-
 Penninic ocean thrusting of the
 Austroalpine units during Creta-
 ceous times west- to northwest-
 wards over the North-Penninic and
 Helvetic units,
 3. D3: Paleogene movement of thrusts
 to the north/northwest (v. Gosen
 1 Geologisch-Paläontologisches Institut, Uni-
 versität Heidelberg, Im Neuenheimer Feld 234
 1989), and
 4. syn- to post-D3: transition of
 the N–S-compressional regime to
 E–W-extension, causing sliding of
 the extruding wedge of the Mur-
 alps from the Tauern Window
 eastwards towards the Pannon-
 ian Basin (Ratschbacher et al.
 1991a,b) from Eocene/Oligocene to
 Middle Miocene times.
 The ‘Murau Nappe’ (Stohwasser
 1947/1956) was postulated because of
 two Mesozoic units (Stangalm respec-
 tively Pfannock unit) separating the
 nappe from the underlying MAA and
 overlying Stolzalpen Nappe. The oc-
 currence of Mesozoic units, however, is
 locally restricted; therefore the ‘Murau
 Nappe’ as a distinct thrust unit is not
 confirmed yet.
 The analyses on spessartine-rich gar-
 nets indicate upper greenschist-facies
 to epidote-amphibolite-facies in the
 phyllitic micaschists and epidote-
 amphibolite-facies in the phyllites,
 which are presumably related to the
 pre-Alpine peak-metamorphism. The
 carbonate dominated suite (Murau
 Limestones) displays conditions of
 higher greenschist-facies and has been
 tectonically imbricated between the
 overlying phyllitic micaschists and the
 overlying phyllites during D2.
 The Stolzalpen Nappe lacks Variscan
 ductile structures (v. Gosen 1989).
 During the Eo-Alpine orogeny the
 Stolzalpen Nappe overthrusted the
 MAA and the direction of movement
 changes from top-NW (syn-D2) to top-
 E/NE (syn-D3). A NW- to SW-dipping
 penetrative cleavage (S3) is attributed
 to the late phase of the Eo-Alpine
 orogeny (D3) and is a ubiquitous feature
 of all MAA- and UAA-units in the cen-
 1
Laukamp & Angerer TSK 11 Göttingen 2006
 tral Gurktal Alps. Linked to D3 is a ret-
 rogressive overprint (diaphthoresis) of
 the MAA and ‘Murau Nappe’ under low
 to high greenschist-facies conditions.
 The Stolzalpen Nappe is not affected by
 this retrogression. The boundary ‘Mu-
 rau Nappe’ and Stolzalpen Nappe ap-
 pears to be transitional with respect to
 lithology and metamorphic grade. Lo-
 cally a tectonic boundary is defined by
 chloritoid-bearing quartzitic mylonites
 (Angerer et al. 2000), indicating a
 thrust zone, with lower greenschist fa-
 cies metamorphosis.
 Except for the Murau Limestones a
 continuous metamorphic gradient from
 MAA to the Stolzalpen Nappe was es-
 tablished during the Eo-Alpine orogeny.
 The Murau Nappe appears as a con-
 tinuous shear zone, incorporating less
 deformed imbricate slices of the Mu-
 rau Limestones. No discrete tectonic
 boundary between MAA and ‘Murau
 Nappe’ has been observed in the central
 GNC.
 The results of our study lead to a rein-
 terpretation of the ‘Murau Nappe’ as a
 major shear zone generated during the
 Eo-Alpine thrusting of the Stolzalpen
 Nappe over the MAA. Concerning the
 central GNC we conclude, that the ge-
 netic term ‘Murau Nappe’ for the lower
 UAA unit between MAA and Stolzalpen
 Nappe is not valid. Instead, its reinter-
 pretation as a major shear zone (‘Murau
 shear zone’), genetically belonging to
 the MAA, generated during Eo-Alpine
 overthrusting of the Stolzalpen Nappe,
 is favoured. The Mesozoic Stangalm
 unit is interpreted as a local sequence
 imbricated during the northward move-
 ment of the MAA units.
 References
 Angerer T, Laukamp C & Leonhard A (2000)
 Strukturgeologische Untersuchungen in den
 Teileinheiten des Gurktaler Deckensystems
 (Ostalpen) und seines Kontaktes zum Mit-
 telostalpin. Terra Nostra 2000/5: 4–5
 v. Gosen W (1989) Gefügeentwicklungen,
 Metamorphosen und Bewegungen der os-
 talpinen Baueinheiten zwischen Nockgebiet
 und Karawanken (Österreich). Geotekt
 Forsch 72, pp 247
 Neubauer F, Hoinkes G, Sassi FP, Handler R,
 Höck V, Koller F & Frank W (1999) Pre-
 Alpine metamorphism of the Eastern Alps.
 Schweiz Min Petr Mitt 79, 41–62
 Ratschbacher L, Merle O, Davy Ph & Cobbold
 P (1991a) Lateral extrusions in the Eastern
 Alps, Part 1: Boundary conditions and ex-
 periments scaled for gravity. Tectonics 10,
 245–256
 Ratschbacher L, Frisch W, Linzer H-G & Merle
 O (1991b) Lateral extrusions in the Eastern
 Alps, Part 2: Structural analysis. Tectonics
 10, 257–271
 Stowasser H (1947/1956) Zur Schichtfolge,
 Verbreitung und Tektonik des Stangalm-
 Mesozoikums (Gurktaler Decke). Jb Geol B-
 A 99, 75–199
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