Structural and metamorphic evolution of the Schneeberg Complex

Transcription

1 Structural and metamorphic evolution of the Schneeberg Complex Gunnar Oeltzschner, 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

2 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 Ma old using Sm and Nd isotopes. Additionally, migmatites with an age of Ma 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 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 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 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

3 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

4 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 (SC). Fig. 3: Sketch of the fold geometries comprising initial, curvilinear and evolved folds. Taken from (Alsopa & Carreras, 2007) 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 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 Ötztal- Bundschuh nappe (ÖBN) and the Koralpe- Wölz 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 Schneeberg Complex and the Texel Complex. The deformation was accompanied by Fig. 4: Map view of the surrounding area of the Schneeberg Complex in the situation 115 Ma ago. The numbers 1 to 3 represent the relative age of the thrusting events. Modified 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).

5 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 is located to the WNW of the ÖBN and 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 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 above the Schneeberg Complex and the Silvretta-Seckau nappe system Fig. 5: Geological cross section of the Schneeberg Complex and related nappe systems in a situation 115 Ma ago. Modified after Krenn et al. (Swiss Journal of Geosciences, 2010). Fig. 6: Geological cross section of the SC area at Ma. Modified after Krenn et al. (Swiss Journal of Geosciences, 2010). (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 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).

6 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. References Alsopa, G. I., & Carreras, J. (2007). The structural evolution of sheats folds: A case study from Cap de Creus. Journal of Structural Geology Vol 29/12, Butler, R. W. (1989). The influence of pre-existing basin structure on thrust system evolution in the Western Alps. Geological Society Special Publications No. 44, Dobbs, H. T., Peruzzo, L., Seno, F., Spiess, R., & Prior, D. J. (2003). Unraveling the Schneeberg garnet puzzle: a numerical model of multiple nucleation and coalescence. Contributions to Mineralogy and Petrology, 1-9. Frisch, W. (1979). Tectonic progradation and plate tectonic evolution of the Alps. Tectonophysics 60, Frisch, W., Kuhlemann, J., Dunkl, I., & Brügel, A. (1998). Palinspastic reconstruction and topographic evolution of the Eastern Alps during late Tertiary tectonic extrusion. Tectonophysics 297, Frisch, W., Neubauer, F., & Satir, M. (1984). Concepts of the evolution of the Austroalpine basement complex (Eastern Alps) during the Caledonian-Variscan cycle. Geologische Rundschau, 73, Hoinkes, G., & Mogessie, A. (1986). Coexisting Cummingtonite and Calcic Amphibole in Amphibolites from the Schneeberg Complex, Tyrol, Austria. TMPM Tschermaks Mineralogische und Petrologische Mitteilungen, Konzett, J., & Hoinkes, G. (1996). Paragonite-hornblende assemblages and their petrological significance: an example from the Austroalpine Schneeberg Complex, Southern Tyrol, Italy. Journal of metamorphic Geology 14,

7 Krenn, K. (2010). Fluid inclusions in quartz related to subsequent stages of foliation development during a single metamorphic cycle (Schneeberg Fault Zone, Eastern Alps, Austria). Lithos 118, Krenn, K., Kaindl, R., & Hoinkes, G. (2004). Pumpellyite in metapelites of the Schneeberg Complex (Eastern Alps, Austria): a relict of the eo-alpine prograde P-T path? European Journal of Mineraology 16, Krenn, K., Kurz, W., Fritz, H., & Hoinkes, G. (2010). Eoalpine tectonics of the Eastern Alps: implications from the evolution of monometamorphic Austroalpine units (Schneeberg and Radenthein Complex). Swiss Journal of Geosciences, Lammerer, B. (1988). Thrust-regime and transpression-regime tectonics in the Tauern Window (Eastern Alps). Geologische Rundschau 77/1, Missoni, S., & Gawlick, H.-J. (2011). Evidence for Jurassic subduction from the Northern Calcareous Alps (Berchtesgaden; Austroalpine, Germany). International Journal of Earth Sciences, Sölva, H., Grasemann, B., Thöni, M., Thiede, R., & Habler, G. (2005). The Schneeberg Normal Fault Zone: Normal faulting associated with Cretaceous SE-directed extrusion in the Eastern Alps (Italy/Austria). Tectonophysics 401, Zanchetta, S. (2010). The Texel-Schneeberg boundary in the Pfossen valley (Merano, NE Italy): geological-structural map and explanatory notes. Italian Journal of Geosciences 129,