Bernd Kolenprat* 1, Manfred Rockenschaub 2, Wolfgang Frank 1
|Institut für Geologie, Universität Wien, Austria|
|Geologische Bundesanstalt, Austria|
|Correspondence: Althanstr. 14, A-1090 Wien, Austria (email@example.com)|
The Austroalpine nappe pile in the area of investigation shows a complex, polymetamorphic Pre-Alpine to Alpine evolution, with evidence of Hercynian, Permian, several Alpine-Cretaceous (Eoalpine) and several Alpine-Tertiary tectonometamorphic events. Depending on their palaeogeographic and tectonic position during this deformation history, the different units show a variable structural development. However, the different rheological behaviour of the various rock types and their spatial occurrence within a unit led to a variable overprinting of the older structures, as well as the formation of new structural elements. Furthermore, strain partitioning during thrusting, normal and strike-slip faulting led to different deformation mechanisms in the adjacent tectonic blocks.
Some parts of the Brenner Area, the Brenner Normal Fault Zone and the Brenner Mesozoic Unit, were already well studied (e.g. Fügenschuh, 1995 and Selverstone et al., 1995) . The aim of this FWF-project (P 11771-Geo: The structural and thermal evolution of the Brenner Area – Innsbruck Quartzphyllite Zone, Stubaikristallin, Brenner Mesozoic Unit and Steinach Nappe) is to improve and complete the understanding of the tectonometamorphic evolution of the Austroalpine nappe pile by detailed structural investigations and different geochronological methods (see Part II.).
Pre-Alpine evolution of Austroalpine basement (IQP-Zone, StN)
The Innsbruck Quartzphyllite Zone (IQP-Zone), part of the Lower Austroalpine Unit (LAA), shows, in comparison to the other Austroalpine units, the most complete deformation history, with evidence for most of the tectonometamophic events. In the case of the Pre-Alpine evolution of the IQP-Zone, distinction between a Permian and an earlier event, probably Hercynian in age, can be made. The formation of a relict S2-foliation (= axial plane foliation of relict isoclinal folds, IsoF1) is probably of Hercynian age, whereas geochronological data, mainly Ar/Ar-ages, indicate a Permian event. Another indication for such an event is the post-S2/pre-S3 garnet growth in mica schists.
Geochronological dating of the different mica generations of the Steinach Nappe (StN), even those from the basal part, gave Ar/Ar-plateau ages which indicate Hercynian cooling (320 to 311 Ma). In contrast to the Lower Austroalpine IQP-Zone, there are no evidence for a Permian event. Furthermore, these data indicate that no significant recrystallisation of phengites, and no rejuvenation of the older mica generations occurred during the different Cretaceous deformation events.
Alpine-Cretaceous (Eoalpine) evolution of the Austroalpine Units
During the Trupchun Phase, (W to NW-directed nappe stacking of the Austroalpine units at c.100-80 Ma), the formation of the penetrative mylonitic S3-structure with top to W stretching lineation, W-E-striking isoclinal folds IsoF2 and top to W shear bands occurred in both the IQP-Zone and the Patscherkofelkristallin (PK). Pre-Alpine structures, S2-foliation and S1-compositional layering, were crenulated to S3, thus lithological layering is in the northern and the central part of the IQP-Zone parallel to the NW dipping moderately inclined mylonitic S3-foliation.
However, despite all the geometric similarities in these units, the Trupchun W-directed nappe stacking did not lead to the present tectonic position of the Patscherkofelkristallin (PK) on top of the IQP-Zone. The PK shows post-S3 growth of garnet, whereas there was no Alpine garnet growth in the IQP; thus this "inverted metamorphism" has to be transported. Field evidence for a post-Trupchun Phase nappe stacking was found in a distinct chlorite-schist-layer at the base of the PK, with top to NW shear band structures and top to NW shear bands in the footwall (the IQP-Zone). Furthermore, this NW-directed nappe stacking shows cooler condition than the W-directed S3-mylonitic thrusting. The age of the this deformation phase is still in doubt and should be resolved by Ar/Ar-laser-dating of the shear bands.
The Ducan Ela Phase (Late-Cretaceous ESE-directed asymmetric extensional collapse with associated sedimentation of the Gosau Formation at c. 80-67 Ma) led to the formation of a ESE-directed normal fault zone which emplaced low metamorphic grade to non-metamorphic units (the StN and the Blaser Nappe, Upper Austroalpine) on top of both the Alpine greenschist facies SK and the BMU (Fügenschuh 1995). This normal fault zone contained the uppermost part of the SK, the whole BMU and the basal parts of the StN and led to the formation of a mylonitic foliation with top to ESE stretching lineations and ESE plunging isoclinal folds. Temperature conditions during deformation reached a lower temperature limit of 450° C. Ar/Ar-plateau-ages of phengites of the BMU show a range between 86-96 Ma, thus reflecting regional cooling during exhumation. FT-dating by Fügenschuh (1995) determined the final cooling of the SK with zircon ages of about 65 Ma and apatite ages of about 60 Ma.
In the IQP-Zone, no evidence of structures related to the Ducan Ela Phase have been found. The following deformation phases, which were superimposed on the Trupchun-S3-structures and on the top to NW shear bands, could clearly be related to the subduction of Penninic Units and European basement and followed deformation phases during the Alpine-Tertiary evolution.
This event can be subdivided in several different phases. In this overview, we focus on the Blaisun Phase (N-directed thrusting of Austroalpine nappes over the subducting Penninic Units, at c. 50-35 Ma, or even earlier) and the Turba Phase (exhumation of the TW during N-S-shortening and E-W-extension, at c. 35-30 Ma).
As the SK and the other hanging-wall units (BMU and StN) had already cooled during the Ducan Ela Phase to below 200° C, these units reacted as more or less rigid blocks during these Alpine-Tertiary events. The formation of ENE-WSW-orientated microfolds are related to the Blaisun Phase; the Turba Phase led to the formation of brittle structures (normal faults antithetic to the Brenner Normal Fault).
In contrast to the other Austroalpine Units, the LAA-IQP-Zone shows a strong influence of these Alpine-Tertiary phases. In particular, studies in the southern part of the IQP-Zone and the neighbouring tectonic units, the Tarntaler Mesozoic Unit (TMU) and the Penninic Nordrahmenzone, has allowed a multitude of structural elements related to these Tertiary phases to be differentiated.
In the IQP-Zone and the underlying Penninic Nordrahmenzone, this event is characterised by:
a) Imbrication of slices of the Lower Austroalpine within the Penninic Nordrahmenzone and internal imbrications of the Lower Austroalpine;
b) Formation of a complex transpressional S4-structure during exhumation of the TW; open F3-NE-SW trending chevron folds in the northern IQP-Zone, tight to close F3-W-E trending folds in the southern IQP-Zone, as well as crenulation of Eoalpine-S3-foliation and the formation of a penetrative S4-foliation (dipping steeply inclined to the N), with a W-E stretching lineation (and a subordinate SE-NW stretching lineation); in the TMU and the Nordrahmenzone superposition of this S4´-structure on pre-existing foliations, in the TMU occurrence of a SE-NW stretching lineation on a S3´; in the southern parts of the IQP-Zone and the Penninic Nordrahmenzone lithological layering is subparallel to the N dipping steeply inclined S4- and S4´-foliation.
c) Ongoing exhumation of the TW and formation of a mylonitic shear band foliation S5´ in the TW and associated brittle structures in the IQP-Zone.
As this Tertiary evolution is fundamental for the full understanding of the evolution of the IQP-Zone, as well as of the TMU (which shows a HP-metamorphism; Dingeldey et al. 1997) and the Penninic Nordrahmenzone, we now focus our investigations on this critical border zone.
Dingeldey, Ch. , Dallmeyer, D.R. , Koller, F. , Massonne, H.-J. , 1997, P-T-t history of the Lower Austroalpine Nappe Complex in the "Tarntaler Berge" NW of the Tauern Window: implications for the geotectonic evolution of the central Eastern Alps. Contrib. Mineral. Petrol., 129:1-19.
Fügenschuh, B. , 1995, Thermal and kinematic history of the Brenner Area (Eastern Alps, Tyrol)., Ph.D. thesis ETH Zürich, Diss. ETH Nr. 11196:226 p.
Selverstone, J. , Axen, G.J. , Bartley, J.M. , 1995, Fluid inclusion constraints on the kinematics of footwall uplift beneath the Brenner Line normal fault, eastern Alps. Tectonics, 14/2:264-278.