Exhumierungsmechanismen von Mittlerer und Unterer Kruste im Westlichen Tauern Fenster, Ostalpen

Declaration of Authorship ________________________________________________________ i Acknowledgements ____________________________________________________________ iii Kurzfassung __________________________________________________________________ v Abstract _____________________________________________________________________ vi Organization of the thesis and contributions of the authors and co-workers _________________ ix 1\. The western Tauern Window a natural laboratory, refining the geodynamics after more than a century of research ____________________________________________________________ 1 2\. Dating the longevity of ductile shear zones: Insight from 40Ar/39Ar in situ analyses _______ 3 2.1 Highlights _______________________________________________________________ 3 2.2 Keywords _______________________________________________________________ 3 2.3 Abstract ________________________________________________________________ 3 2.4 Introduction _____________________________________________________________ 4 2.5 Geological setting _________________________________________________________ 5 2.6 40Ar/39Ar methodology _____________________________________________________ 7 2.7 Results _________________________________________________________________ 9 2.7.1 Mineral reactions _____________________________________________________ 10 2.7.2 Isotopic data_________________________________________________________ 10 2.7.3 Isochron calculations __________________________________________________ 10 2.7.4 Extraneous argon _____________________________________________________ 13 2.7.5 Syn- and post-kinematic minerals ________________________________________ 14 2.7.5.1 Ahorn Shear Zone samples __________________________________________ 14 2.7.5.2 Tuxer Shear Zones samples _________________________________________ 16 2.7.5.3 Greiner Shear Zone samples _________________________________________ 18 2.8 Discussion _____________________________________________________________ 20 2.8.1 Errors ______________________________________________________________ 20 2.8.1.1. Analytical errors _________________________________________________ 20 2.8.1.2 Geological errors _________________________________________________ 21 2.8.2 Dating deformation ___________________________________________________ 21 2.8.3 Comparison with age results of previous studies ____________________________ 22 2.9 Conclusion _____________________________________________________________ 23 2.10 Acknowledgment _______________________________________________________ 24 2.11 References ____________________________________________________________ 24 2.12 Supplementary material __________________________________________________ 28 2.12.1 Extraneous argon ____________________________________________________ 28 2.12.2 Example ST0730 of the Tuxer Shear Zones _______________________________ 28 2.12.3 Pre- kinematic minerals _______________________________________________ 30 2.12.3.1 Muscovite ______________________________________________________ 30 2.12.3.2 Biotite _________________________________________________________ 31 2.12.3.3. K-feldspar _____________________________________________________ 31 2.12.4 Rb/Sr methodology __________________________________________________ 31 2.13 Supplement references ___________________________________________________ 32 3\. Translation of indentation into lateral extrusion across a restraining bend: The western Tauern Window, Eastern Alps ___________________________________________________ 33 3.1 Keywords ______________________________________________________________ 33 3.2 Abstract ________________________________________________________________ 33 3.3 Introduction _____________________________________________________________ 33 3.4 Geological setting of the western Tauern Window _______________________________ 34 3.4.1 Tectonometamorphic frame _____________________________________________ 34 3.4.2 Structural frame ______________________________________________________ 36 3.4.3 Timing of sinistral and extensional shear___________________________________ 37 3.4.4. Cooling history ______________________________________________________ 38 3.4.5. Surrounding faults ____________________________________________________ 38 3.4.5.1 Giudicarie Belt and Meran-Mauls basement _____________________________ 38 3.4.5.2. Salzach-Ennstal-Mariazell-Puchberg (SEMP) Fault ______________________ 39 3.4.5.3 Pustertal-Gailtal Fault ______________________________________________ 39 3.4.5.4 Defereggen-Antholz- Vals (DAV) and Speikboden faults __________________ 40 3.4.5.5 Brenner Fault ____________________________________________________ 40 3.5 Results _________________________________________________________________ 40 3.5.1 Deformation fabrics ___________________________________________________ 40 3.5.2. Shear zones of the western Tauern Window ________________________________ 42 3.5.2.1 Ahorn Shear Zone _________________________________________________ 42 3.5.2.2. Tuxer Shear Zones ________________________________________________ 44 3.5.2.3 Greiner Shear Zone ________________________________________________ 44 3.5.2.4 Olperer Shear Zone ________________________________________________ 44 3.5.2.5. Ahrntal Shear Zone _______________________________________________ 44 3.5.2.6 Meran-Mauls basement and Jaufen Fault _______________________________ 45 3.5.3 Structures of the western Tauern Window __________________________________ 46 3.5.3.1 S1 foliation_______________________________________________________ 46 3.5.3.2 F2 fold axes and axial planes _________________________________________ 46 3.5.3.3 S2 foliations and L2 stretching lineations _______________________________ 46 3.5.3.4 Shear zones ______________________________________________________ 48 3.5.4 Periphery of the western sub-dome _______________________________________ 50 3.5.4.1 Domain D _______________________________________________________ 50 3.5.4.2 Domain E _______________________________________________________ 50 3.5.5 Shear bands _________________________________________________________ 53 3.5.6 Structural summary ___________________________________________________ 53 3.5.7 Shortening accommodated by upright folding ______________________________ 53 3.6 Discussion _____________________________________________________________ 54 3.6.1 Comparison with earlier studies _________________________________________ 54 3.6.2 Corner effect of the Dolomites Indenter ___________________________________ 54 3.6.3 Deformation fabrics and crustal level _____________________________________ 54 3.6.4 From upright folding to localized shearing _________________________________ 55 3.6.5 The western Tauern Window: a restraining bend ____________________________ 56 3.6.6. Age of doming in the Tauern Window ____________________________________ 56 3.6.7 Decoupling along the western margin _____________________________________ 57 3.6.8 Decoupling to the east, transition to lateral extrusion _________________________ 58 3.6.9 Estimates of shortening, displacement and extension _________________________ 59 3.7 Conclusion _____________________________________________________________ 62 3.8 Acknowledgement _______________________________________________________ 63 3.9 References _____________________________________________________________ 63 3.10 Supplementary material __________________________________________________ 71 4\. U-Pb ages of apatite in the western Tauern Window (Eastern Alps): Tracing the onset of collision- related exhumation in the European plate __________________________________ 104 4.1 Keywords _____________________________________________________________ 104 4.2 Abstract ______________________________________________________________ 104 4.3 Introduction ___________________________________________________________ 104 4.4 Geological setting _______________________________________________________ 106 4.5 Sample description ______________________________________________________ 110 4.6 Methods ______________________________________________________________ 113 4.7 Theory and Calculations __________________________________________________ 113 4.8 Results and Discussion ___________________________________________________ 114 4.8.1 Analytical and geological errors ________________________________________ 119 4.8.2 Formation or cooling ages _____________________________________________ 121 4.8.3 Comparison to other geochronometers ___________________________________ 122 4.8.4 Tectono- metamorphic implications ______________________________________ 123 4.9 Conclusion ____________________________________________________________ 124 4.10 Acknowledgement _____________________________________________________ 125 4.11 References ___________________________________________________________ 125 4.12 Supplementary material _______________________________________________ 132 5\. The dynamic evolution of the Tauern Window: a crustal scale expression of upper mantle dynamics ___________________________________________________________________ 145 5.1 Methodological conclusions _______________________________________________ 145 5.2 Geological conclusions ___________________________________________________ 145 5.3 Geodynamic implications _________________________________________________ 146 6\. What will be next? _______________________________________________________ 149 6.1 Methodological focus ____________________________________________________ 149 6.1.1 How can we improve the 40Ar/39Ar geochronometer? ________________________ 149 6.1.2 Can we use other noble gases to understand orogenic processes? _______________ 149 6.2 Microstructural focus ____________________________________________________ 150 6.2.1 How can we better understand major-element mass-transfer?__________________ 150 6.2.2 What tell us trace element distributions? __________________________________ 150 6.2.3 Do structural elements act the same or are there fundamental differences? _______ 150 6.3 Focus on Regional Geology _______________________________________________ 151The Tauern Window, a tectonic window, is subject to research in earth sciences for more than a century. European lower crustal rocks of the western Tauern Window, once buried below 35 km, were uplifted and exhumed to surface level during Cenozoic times. Although it has been studied exhaustively there is a vital discussion which geological processes contributed in which amount to uplift and exhumation and also when and in which temporal succession these processes occurred. The Tauern Window plays a key role and is significantly involved into Eastern Alps tectonics. This work supplies contributions in structural geology and geochronology to this discussion. Comprehensive structural field measurements were performed to determine the dominant structural features. A systematic sampling strategy was performed where highly and almost not deformed samples were collected for microstructural, petrological and geochronological investigations. The highly deformed samples were analyzed with the 40Ar/39Ar in situ technique to obtain deformation ages. The almost not deformed samples were analyzed with the U-Pb apatite geochronometer to contrast the cooling history of the rocks with the timing of localized deformation. The results of this study show that the western Tauern Window was deformed by a transpressive belt consisting of upright folds and sinistral shear zones that has the scale of an orogen. It forms a restraining bend by crustal buckling between two major Alpine fault systems. The obtained cooling pattern from the U-Pb apatite ages indicates a dome structure in agreement with earlier studies. The novel aspect is that an early stage of this cooling event in the Lower Oligocene epoch was dated. Hence, the duration of cooling and the driving processes causing it were substantially extended. The longevity of sinistral shear zones comprises for all dated structures several million years. The termination of those structures was precisely figured out in some cases. Localized deformation initiated within the entire western sub-dome contemporaneously to the cooling mentioned above. It continued until the Upper Miocene and followed the cooling of the dome from the margins to the center. The sinistral shear zones and for this reason the entire transpressive belt dominated the uplift and exhumation of the western Tauern Window.Das Tauernfenster, ein tektonisches Fensters, ist seit über einem Jahrhundert Gegenstand geowissenschaftlicher Forschung. Im westlichen Tauernfenster wurden während des Känozoikums Gesteine der europäischen Unterkruste, die einst bis in eine Tiefe von >35 km versenkt wurden, durch tektonische Prozesse an die Erdoberfläche gebracht. Trotz eingehender Studien existiert eine lebendige Debatte darüber welche geologischen Prozesse welchen Beitrag zur Hebung und Exhumierung geleistet haben auch darüber wann und in welcher Reihenfolge die geologischen Ereignisse stattfanden. Das Tauernfenster nimmt eine Schlüsselrolle ein, von der die Tektonik der Ostalpen maßgeblich abhängt. Diese Arbeit leistet einen strukturgeologischen und einen geochronologischen Beitrag zu dieser Debatte. Umfangreiche strukturgeologische Messungen wurden durchgeführt um die dominanten tektonischen Strukturen zu erfassen. Systematisch wurden stark und möglichst wenig verformte Proben genommen um mikrostrukturelle, petrologische und geochronologische Untersuchungen durchzuführen. Die stark verformten Proben wurden mit der 40Ar/39Ar in situ Methode datiert um das Alter der Verformung zu erfahren. Die wenig verformten Proben wurden mit Hilfe des U-Pb Apatit Geochronometers datiert um die Abkühlung der Gesteine der lokalisierten Verformung zeitlich gegenüberzustellen. Die Ergebnisse dieser Studie zeigen, dass das westliche Tauernfenster durch einen transpressiven Gürtel aus aufrechten Falten und sinistralen Scherzonen im orogenen Maßstab verformt würde. Es bildet eine blockierende Krümmung bestehend aus einem Krustenstapel zwischen zwei alpinen Hauptstörungssystemen. Das Abkühlmuster der U-Pb Apatit Alter wiederspiegelt eine Domstrutur im Einklang mit früheren Studien. Neu ist, dass ein frühes Stadium der Abkühlung im Unteren Oligozän datiert wurde. Damit wird die Dauer der Abkühlung, folglich auch die der verursachenden Prozesse, erheblich verlängern. Die Langlebigkeit der sinistralen Scherzonen umfasst für alle datierten Strukturen mehrere Millionen Jahre. Ihr Ende konnte in einigen Fällen genau erfasst werden. Lokalisierte Verformung entlang der sinistralen Scherzonen beginnt im gesamten westlichen Sub-dom zeitgleich mit der oben beschriebenen Abkühlung. Sie dauert bis ins Obere Miozän an und folgt dabei zeitlich der Abkühlung des Doms vom Rand zum Zentrum. Die sinistralen Scherzonen und damit der gesamte transpressive Gürtel des westlichen Tauernfensters dominieren dessen Heraushebung