Structural and metamorphic evolution of the Schneeberg Complex

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Structural and metamorphic evolution of the Schneeberg Complex
Structural and metamorphic evolution of the Schneeberg Complex
Gunnar Oeltzschner, 316111
Introduction
The Schneeberg Complex is a part of the austroalpine nappe stack system and is located west of
the Tauern Window (Fig. 1). While the surrounding nappe units experienced a polymetamorphic
history, the Schneeberg Complex is basically a monometamorphic sedimentary sequence which
experienced its main deformation phase during the late cretaceous (Frisch, Kuhlemann, Dunkl, &
Brügel, 1998). The Schneeberg Complex (SC) is overlain by the Ötztal nappe (ÖN) and underlain by
the Texel Complex (TC) (Krenn, Kurz, Fritz, & Hoinkes, 2010). Furthermore, it is part of an eoalpine
high-pressure wedge that forms an intracontinental suture. During the eoalpine orogeny, the
formations of the SC were south-dipping and experienced a tectono-metamorphic history from 115
Ma ago until the unroofing of the Tauern Window in Miocene times (Frisch, Kuhlemann, Dunkl, &
Brügel, 1998).
Fig. 1: Geological sketch of the area west of the Tauern Window including the most important nappe systems and the
SAM-line. Modified after (Krenn et al., Lithos 118, 2010).
Geological setting
The Schneeberg Complex is part of a high-pressure nappe system that is juxtaposed at its
southern margin by fault zones, which were active in paleogene times. These faults are related to the
early evolution of the periadriatic fault system and they delineate the so called southern limit of
alpine metamorphism (SAM-line). The SAM-line separates areas of eoalpine metamorphic imprint in
the north and formations with a weakly to non-metamorphic history in the south. Staurolites have
been found in the Paragneisses of the Texel Complex, south of the Schneeberg Complex. The Ötztal
nappe also contains Gabbro´s of pre-variscian age which were dated to be 530-520 Ma old using Sm
and Nd isotopes. Additionally, migmatites with an age of 490 +- 9Ma have also been found. In the
center of the Ötztal nappe, acidic orthogneisses with an age of 485 to 420 Ma (dated using Rb and Sr
isotopes) have been located. Within the upper australoalpine nappe system, the maximum
metamorphic grade reached the eclogite facies in the southern part of the Koralpe-Wölz highpressure nappe. Variscian eclogites in the northwestern and central parts and amphibolite facies
metamorphic overprint indicate pressure-temperature conditions of up to 27kbar and 730°C. The
central parts of the Schneeberg Complex are of eoalpine age and display pressure-temperature
conditions of 550-600°C and 8-10kbar, indicated by paragonite bearing amphibolites (Krenn, Kurz,
Fritz, & Hoinkes, 2010).
Deformation stages
According to Krenn et al. (Swiss Journal of Geosciences, 2010), the history of the Schneeberg
Complex can be divided into four deformation stages:
D1:
The first deformation stage is defined by a WNW-directed shear movement (resulting
folds are referred to as F1) under conditions of 550 - 600 °C. This phase is related to
the initial exhumation of the area within the high-pressure wedge.
D2:
The second phase is linked to a coaxial movement at conditions of 450 - 550 °C as a
result of the advanced exhumation. This phase is also associated to the folding (F2
folds) of the high-pressure wedge including the Ötztal nappe on top as well as the
Texel Complex below.
D3:
Stage D3 is associated with refolding of pre-existing structures of the stage D2
resulting in folds (F3 folds) with axial planes perpendicular to the older ones. These
structures mainly occur in the southern part of the SC and in the marble units of the
southern Lodner synform (“Laaser Serie”). The interference of F2 and F3 folding is
assumed to be the cause of the large-scale synforms.
D4:
The phase D4 is supposed to be a result of the tilting of individual basement block
along a large-scale strike-slip fault zone which originated in Oligo- to Miocene age.
According to Sölva et al. (2005), the area experienced even five different deformation stages. The
last stage is assumed to be a brittle deformation occurring in several cataclastic zones, slickensides
and pseudotachylites. Indicators like secondary foliation and stylolites indicate a top-to-NW shear
sense.
In the area west of Tauern, fold interference resulted in the formation of large-scale sheath-folds
in the frontal part of the nappe stack. Earlier thrusts have been reactivated during late cretaceous
normal faulting at the base of the Ötztal-Bundschuh nappe system and its cover. The structural
evolution of the austroalpine nappes is largely controlled by the interplay of compressional and
extensional phases (Krenn et al., Swiss Journal of Geosciences, 2010). The nappe stack developed
during cretaceous times by NWN to N directed thrusting and was followed by extension in ESE
direction resulting in normal faulting during late cretaceous and paleogene (90-60Ma).
Macro- scale structures
The Ötztal nappe on top, the Schneeberg Complex and the Texel Complex at the base build a
northwest dipping nappe stack. Furthermore, the Schneeberg Complex contains the following four
major synforms:




Schneeberg main synform (1)
Seewerspitz synform (2)
Schrottner synform (3)
(Krenn et al., Lithos 118, 2010)Lodner synform (4)
Fig. 2: Lithologies of the SC, TC and ÖN. The markers A-F represent the field research areas, where measurement were
taken to identify the deformation stages. PJF = Passeier-Jaufen Fault. Modified after (Krenn et al., Swiss Journal of
Geosciences, 2010)
Intensive field research in six different locations within the area of the SC and TC allowed for a
precise reconstruction of the tectonic phases by the use of deformation markers (Fig. 2). The general
structure is a result of thrusting in a NW to WNW direction. The Ötztal nappe is showing a foliation
with a general direction striking W to E which is bent to the south, when approaching the vicinity of
the eoalpine metamorphosed Schneeberg Complex. This bending of the foliation is suggested to be a
result of the deformation of the SC, thus
providing evidence of the eoalpine age of
the sheath folds (formerly called
“Schlingentektonik”) of the Schneeberg
Complex (Fig. 3). It is suggested that this
bending rotated the pre-existing W to E
foliation to a N to S direction. The macroscale fold interference in the Pfossen valley
is a result of the structural overprint
dominant in the southern Lodner synform
(TC) and the southern Schrottner synform Fig. 3: Sketch of the fold geometries comprising initial, curvilinear
and evolved folds. Taken from (Alsopa & Carreras, 2007)
(SC).
Tectonic and metamorphic evolution
Although the exact source of the Schneeberg Complex is still unclear, it is suggested that it is
derived either from palaeozoic or Permian to Triassic carbonatic and clastic sequences. These units
were deposited on older units, which have been intruded by magmatites during Permian age
resulting in the Texel complex. All these units are part of the eoalpine high-pressure wedge (Fig. 4)
and have been exhumed between 90 and 70-60 Ma. The reconstruction of the temperature history
of the relevant units also supports the model of a retrograde meta-morphosis. The reconstruction of
the prograde metamorphic path is almost impossible, because of the retrograde overprint. Though,
in some areas Pumpellyite remnants have been found encased in rigid host minerals like garnets (e.g.
Almandines) where the prograde break-down stopped (Krenn et al., 2004).
The Texel Complex has reached
temperatures below 300°C 70 Ma ago and
the Ötztal nappe has been cooled down to
temperatures below 100°C at an age of 60
Ma. During the situation 115 Ma ago, the first
units to develop thrust faults were the ones
on top of the sequence. Because the ÖtztalBundschuh nappe (ÖBN) and the KoralpeWölz high-pressure wedge share the same
metamorphic evolution, it can be assumed
that both units originated from a position
close to each other. During exhumation, the
ÖBN has been folded together with the Fig. 4: Map view of the surrounding area of the Schneeberg
Complex in the situation 115 Ma ago. The numbers 1 to 3
Schneeberg Complex and the Texel Complex. represent the relative age of the thrusting events. Modified
The deformation was accompanied by after Krenn et al. (Swiss Journal of Geosciences, 2010).
eoalpine mineral growth at the base of the ÖBN. Within the SC, staurolite has been formed and the
pre-eoalpine staurolite contained in the Ötztal-Bundschuh nappe has seen retrogression into finegrained white mica (Krenn et al., Swiss Journal of Geosciences, 2010).
Starting in the position shown in
figures 4 and 5, the Ötztal-Bundschuh
nappe has been thrusted to the WNW
in several steps. In the situation 115
Ma ago, the first thrust fold to develop
was located between the future ÖBN
and the Drauzug-Gurktal nappe (DGN)
system to the ESE. The Texel Complex Fig. 5: Geological cross section of the Schneeberg Complex and related
is located to the WNW of the ÖBN and nappe systems in a situation 115 Ma ago. Modified after Krenn et al.
(Swiss Journal of Geosciences, 2010).
is overlain by the Schneeberg Complex,
which is a part of the Caledonianformed Permo-Mesozoic Cover (PMC)
(Frisch et al., 1984). As the thrusting
continued to the NW in 90-80 Ma (Fig.
6),
the
Schneeberg
Complex
experienced its first deformation
phase (D1). The PMC is partially
sheared off the older units and the
ÖBN has now been over-thrusted Fig. 6: Geological cross section of the SC area at 90-80 Ma. Modified
after Krenn et al. (Swiss Journal of Geosciences, 2010).
above the Schneeberg Complex and
the Silvretta-Seckau nappe system
(SSN). Additionally, in this phase the Texel complex has reached its maximum burial depth which is
indicated by eclogites preserved within garnet amphibolites in several areas (Zanchetta, 2010).
The situation 80 to 60 Ma ago (Fig. 7a) induced the coaxial deformation phases two and three,
causing the stage D1 to be refolded. With the unroofing of the Tauern window in the situation at 30
Ma (Fig. 7b), the Schneeberg Complex has now been exposed to the surface due to erosion. As a
result of the continuous convergence, the SC experienced the fourth deformation stage under
semiductile to brittle conditions (Krenn et al., Swiss Journal of Geosciences, 2010).
(a)
(b)
Fig. 7: Geological cross section at 80-60 Ma (a) and <30 Ma (b). BM = Brenner Mesozioc, PFS = Periadratic Fault System, PJF =
Passeier-Jaufen Fault, SN = Steinach nappe, TW = Tauern Window. Modified after Krenn et al. (Swiss Journal of Geosciences,
2010).
The WNW shearing in the area west of the Tauern Window is generally compatible with the
overall kinematics during eoalpine nappe stacking. The Schneeberg Complex reached peak
metamorphic condition of 600°C (Hoinkes & Mogessie, 1986) and about 10 kbar (Konzett & Hoinkes,
1996), which are linked to static growth of poikiloblastic garnets followed by syn-kinematic growth
during WNW-directed shearing under early retrograde conditions. Following this events, the area has
been affected by the second stacking phase which is a stage of NW-directed thrusting. Finally, the
third phase resulted in a rapid exhumation of the high-pressure wedge.
Conclusion
Unlike the surrounding nappe system, the Schneeberg complex has only experienced a monometamorphic history and the recent tectonic situation is a result of several stacking phases which are
preserved in deformation indicators. At least four distinct deformation phases can be observed
within the area of the Schneeberg Complex (at least five, according to Sölva et al. (2005)), which are
responsible for the complex structures resulting from fold interferences.
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