Phd THESIS Submitted to the Faculty of Geo-And Atmospheric Sciences of the University of Innsbruck
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THRUST SYSTEMS IN THE WESTERN NORTHERN CALCAREOUS ALPS - A FIELD BASED NUMERIC MODELLING APPROACH ___________________________________________________________________________ PhD THESIS Submitted to the Faculty of Geo-and Atmospheric Sciences of the University of Innsbruck Sinah Kilian Innsbruck, August 2020 Supervisor: A.o. Prof. Dr. Hugo Ortner Insitute of Geology, University of Innsbruck Co-Supervisor: Priv.Doz. Dr. Barbara Schneider-Muntau Unit of Geotechnical and Tunnel Engineering, University of Innsbruck 1. Reviewer: Prof. Dr. Herwegh Marco Universität Bern, Insitute of Geological Sciences 2. Reviewer: Prof. Dr. Jonas Kley Georg-August-Universität Göttingen, Departement of Structural Geology and Geodynamics Acknowledgements The Geological Survey of Austria, the Tiroler Wissenschaftsfonds and a scholarship from the Doctoral program of the University of Innsbruck, supported this research. At this point I would like to thank everyone who supported this PhD Thesis. First, I would like to thank my supervisors Hugo Ortner and Barbara Schneider-Muntau. Hugo supported me with his excellent knowledge about the western Northern Calcareous Alps and Barbara introduced me very patient into numeric modelling. I also want to thank the Unit of geotechnical and tunnel engineering, the whole Team welcomed me warmly, especially Gertraud Medicus, Iman Bathaeian and Sarah-Jane Lorenz-Theodorine. Furthermore, I want to thank my colleges, Hannah Pomella who helped me with all GIS-affairs, Martin Reiser (in the first year) and Kathrin Faßmer (in the last years) for the coffee-breaks. A big thank goes to my family, Andi for numerous corrections and encouragements and Karin and Inge for babysitting. Their support made this thesis possible. CONTENTS Abstract .................................................................................................................. 3 1 Introduction ................................................................................................. 5 2 Structural geology ....................................................................................... 5 2.1 Study 1 ...................................................................................................... 7 2.2 Study 2 .................................................................................................... 45 3 Numeric Modelling ................................................................................... 87 3.1 Initial situation ........................................................................................ 88 3.2 Aims of the study .................................................................................... 89 3.3 Software and model set up ...................................................................... 93 3.4 Study 3 .................................................................................................... 97 3.5 Analysing folds ..................................................................................... 127 3.6 From folding to faulting........................................................................ 130 4 Discussion ............................................................................................... 131 5 Conclusion .............................................................................................. 134 6 References ............................................................................................... 135 7 Appendix ................................................................................................. 139 1 2 Abstract In this study the tectonic subdivision of the western Northern Calcareous Alps (NCA), which was a matter of controversies since it was introduced, is reinvestigated. We concentrated on the Karwendel, Mieming and Wetterstein Mountains and showed that only two main thrust sheets, instead of three, are necessary in the western NCA. We emphasise that the tectonic subdivision must be based on old-on-young relationships, while the previously used subdivision included out-of-sequence thrusts to delimit thrust sheets. For the western NCA we renamed the thrust sheets in a tectonically deeper Tannheim thrust sheet and a tectonically higher Karwendel thrust sheet. The two thrust sheets are separated by the Karwendel thrust. In a numeric model, performed with the finite element program Abaqus, we investigated some folding in the study area. Mainly the behaviour of the Karwendel thrust and the structures above the latter were of interest. We suggested that the large-scale folds of the Karwendel thrust sheet are salt-cored buckle folds on top of a salt-bearing décollement horizon. We summarise the results of the structural investigations and the numeric model in a kinematic model and explain the importance of salt tectonics for the study area. 3 4 1 Introduction This thesis is a combination of field studies in the Karwendel, Mieming and Wetterstein Mountains of the western Northern Calcareous Alps (NCA) and numeric modelling. During the field campaign the focus was on structural analysis to clarify the nappe structure of the western NCA that was controversially discussed since the nappes were defined in the early 20th century (Ampferer, 1912; 1914; 1931; 1942; Heißel, 1958; Loesch, 1915; Mylius, 1914; Richter, 1929; Rüffer and Bechstädt, 1995; Schlagintweit, 1912a;b). With detailed structural work in the Karwendel Mountains, we solved some of the problems associated with the definition of nappes (Ortner and Kilian, 2013). The results were incorporated into a new tectonic subdivision of the western NCA (Kilian and Ortner, 2019; Ortner, 2016a; Ortner and Bitterlich, 2016). The numeric modelling bases on the structural results in the Karwendel, Mieming and Wetterstein Mountains. With the numeric model, we investigated some key features of field observed structures to better understand the structural style of the western NCA. 2 Structural geology Two field-based studies in the western NCA are presented here. Both studies aimed to re- investigate the tectonic subdivision of the western Northern Calcareous Alps on the base of today’s knowledge of thrust kinematics. In the western NCA three major tectonic units were distinguished from base to top: The Allgäu thrust sheet, the Lechtal thrust sheets and the Inntal thrust sheet (Ampferer and Hammer, 1911; Ampferer, 1912; Heißel, 1958; Tollmann, 1970; 1976b). Both study areas deal with the border between the Lechtal and the Inntal thrust sheets. We show that the separation of the Inntal and the Lechtal thrust sheet is not necessary for the western NCA. In addition, we point out how out-of-sequence thrusts separate parts of thrust sheets that were emplaced in a later stage. To avoid confusion, we renamed the main thrust sheets as the Upper (Karwendel) thrust sheets and the lower (Tannheim) thrust sheet. The two thrust sheets are separated by the main (Karwendel) thrust. 5 6 2.1 Study 1 Structural evidence of in-sequence and out-of-sequence thrusting in the Karwendel mountains and the tectonic subdivision of the western Northern Calcareous Alps Sinah Kilian and Hugo Ortner Published in the Austrian Journal of Earth Sciences, 2019, v.112/1, p.62-83. 7 8 Structural evidence of in-sequence and out-of-sequence thrusting in the Karwendel mountains and the tectonic subdivision of the western Northern Calcareous Alps Sinah Kilian, Institute of Geology, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria; corresponding author, [email protected] Hugo Ortner, Institute of Geology, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria Abstract We present the results of a field study in the Karwendel mountains in the western Northern Calcareous Alps, where we analysed the boundary between two major thrust sheets in detail in a key outcrop where nappe tectonics had been recognized already at the beginning of the 20th century. We use the macroscopic structural record of thrust sheet transport in the footwall and hanging wall of this boundary, such as folds, foliation and faults. In the footwall, competent stratigraphic units tend to preserve a full record of deformation, while incompetent units get pervasively overprinted and only document the youngest deformation. Transport across the thrust persisted throughout the deformation history of the Northern Calcareous Alps from the late Early Cretaceous to the Miocene. As a consequence of transtensive, S-block down strike-slip tectonics, postdating folding of the major thrust, new out- of-sequence thrusts formed that climbed across the step, and ultimately placed units belonging to the footwall of the initial thrust onto its hanging wall. One of these out-of-sequence thrusts had been used to delimit the uppermost large thrust sheet (Inntal thrust sheet) of the western Northern Calcareous against the next, tectonically deeper, (Lechtal) thrust sheet. Based on the structural geometry of the folded thrust, and the age of the youngest sediments below the thrust, we redefine the thrust sheets, and name the combined former Inntal- and part of the Lechtal thrust sheet as the new Karwendel thrust sheet and the former Allgäu- and part of the Lechtal thust sheet as the new Tannheim thrust sheet. Keywords: Northern Calcareous Alps, Karwendel, out-of-sequence thrusting, thrust boundary, Austria, Tirol 9 1 Introduction 1.1 Background and aim of the study The Northern Calcareous Alps (NCA) represent the sedimentary cover of the topmost tectonic unit of the Austroalpine nappe system, that represents the upper plate in the Alpine orogen (e.g.,