Phd THESIS Submitted to the Faculty of Geo-And Atmospheric Sciences of the University of Innsbruck

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

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.

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 reinvestigate 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.

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.

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., Schmid et al., 2004; Tollmann, 1976b). The thrust sheets of the NCA were defined at the beginning of the 20th century by Ampferer and Hammer (1911) and Ampferer (1912) in the western NCA, and by Hahn (1912; 1913) in the central and eastern NCA. Their classifications are in use ever since. In his review of the structure of the NCA, Tollmann (1970; 1976b) unified the classifications and defined a system of far travelled nappes. In the western NCA, these thrust sheets are from base to top the Allgäu-, the Lechtal- and the Inntal thrust sheets (Fig. 1). Ever since this subdivision was proposed, it was controversially debated, especially the boundary of the Inntal- against the Lechtal thrust sheet (Ampferer, 1912; 1914; 1931; 1942; Heißel, 1958; Loesch, 1915; Mylius, 1914; Richter, 1929; Rüffer and Bechstädt, 1995; Schlagintweit, 1912a; b) in the western NCA. Based on earlier tectonic sketches by Ampferer and Heißel (1950) and Heißel (1958), Tollmann defined a frontal slice of the Inntal thrust sheet (Karwendel- Stirnschuppe; Tollmann, 1970). Heißel (1977; 1978) and Donofrio et al. (1980) redefined the boundaries of the Inntal thrust sheet, and included this frontal slice in the Karwendel zone of slices (Karwendelschuppenzone) on which the Inntal thrust sheet was emplaced. Later studies, e.g., Eisbacher and Brandner (1995; 1996); Eisbacher et al. (1990); Tanner et al. (2003) used the nappe subdivision of Tollmann (1976b) even though new and clearer concepts had been developed to describe fold-and-thrust belts (Boyer and Elliott, 1982; Dahlstrom, 1969; Suppe, 1983).

Fig. 1: Tectonic subdivision of the western Northern Calcareous Alps following (Tollmann, 1976b). Tectonic boundaries were drawn according to the published and unpublished maps of the Austrian and Bavarian geological surveys. Synorogenic deposits according to Ortner (2003a) and Ortner and Gaupp (2007): youngest age of upper-footwall-deposits and oldest age of thrust-sheet-top deposits bracket the age of thrust sheet emplacement. KM = Karwendel mountains, MM = Mieming mountains, WM = Wetterstein mountains. Graticule: MGI Austria GK West (EPSG: 31254).

11 In the last years, serious doubts (re)emerged about the validity of the nappe subdivision of the western NCA as proposed by Tollmann (1976b). Part of the problems result from to the out-of- sequence nature of some principal thrusts of the NCA (Ortner, 2003a). This study discusses the thrust sheet boundaries in the Karwendel mountains of the western NCA, revaluates these boundaries and proposes a new subdivision.

1.2 Terminology For many years, nappes and thrust sheets were defined without clear definitions of the exact meaning of the terms used. An attempt has been done by Tollmann (1973), but unfortunately the terminology proposed was not applied systematically. Much of the abovementioned controversy resulted from the fact, that all authors tried to define nappes that are separated on all sides from the tectonically deeper unit by a basal thrust. This neglects the fact that, in an early stage, thrusts nucleate with a limited extent, and then continue to grow laterally, while offset increases. Therefore, an allochthon will be far-travelled near the centre of the basal thrust, while offset will diminish or disappear at the end of the thrust. It also neglects the possibility of out-of-sequence thrusting, i.e., thrusts cutting across existing structures like folds or older thrusts.

Here we use following definitions: A thrust is a low-angle fault that superimposes older on younger rocks. Out-of-sequence thrusts are thrusts that develop in the hanging wall or hinterland of an older thrust and cut across older thrust-related structures. They may emplace younger on older rocks. Thrusts and out-of-sequence thrusts may be high angle as a consequence of subsequent folding of the thrust.

1.3 Tectonic evolution of the Northern Calcareous Alps The Permian to Triassic sediments of the NCA were deposited on the passive continental margin of Eurasia, which was then part of Pangea, toward the Meliata branch of the Neotethys ocean (e.g., Haas et al., 1995; Lein, 1987; Schmid et al., 2004; Stampfli et al., 1998). Shallow to deep marine successions accumulated on this continental margin (Mandl, 2000). Early Jurassic rifting initiated opening of the Penninic ocean, and the Adriatic plate, including the future NCA, were separated from Eurasia (Froitzheim and Manatschal, 1996). The new continental margins drowned, ending shallow marine deposition, and pelagic sediments settled throughout the Jurassic and Early Cretaceous. Upper Albian basanitic dykes and sills locally intruded these sediments (Richter, 1928; Trommsdorff et al., 1990).

12 Alpine orogeny is related to the closure of the two neighbouring oceans (Froitzheim et al., 1996): (1) Cretaceous (Eoalpine) orogeny occurred after obduction of Meliata ophiolithes onto the southeastern Adriatic margin, with the Austroalpine, Adria-derived units being in lower plate position (Schmid et al., 2004; Stüwe and Schuster, 2010). Stacking of thrust sheets in the studied part of the NCA started at the end of the Albian based on the age of the youngest sediments (Eisbacher and Brandner, 1996; Ortner, 2003a). (2) Paleogene (Mesoalpine) orogeny was related to the extinction of the Penninic ocean in the Late Eocene, and this time the Austroalpine, including the NCA, was in upper plate position (Schmid et al., 2004; Stüwe and Schuster, 2010). Within the internal Austroalpine basement units, Eoalpine und Mesoalpine stacking is clearly separated by Upper Cretaceous extension (Froitzheim et al., 1994), but in the external thrust sheets, Cretaceous to Cenozoic shortening is continuous, as documented by growth strata in different synorogenic successions (Ortner, 2001; 2003a; Ortner and Gaupp, 2007; Ortner et al., 2016).

The age of movement of individual thrust sheets during Cretaceous nappe stacking can be deduced using the youngest sediments below a thrust and retransgression on top of the thrust sheet during and after emplacement (Ortner, 2003a; 2016; Ortner and Gaupp, 2007). Nappe stacking in the western NCA started in Albian by imbrication of the Lechtal thrust sheet onto the Allgäu thrust sheet and propagated progressively into the more external parts of the NCA fold-and-thrust belt, incorporating the Cenomanrandschuppe and the Penninic Arosa Zone in the Turonian to Coniacian, and the Rhenodanubian Flysch nappes in the Maastrichtian (Ortner, 2003a). However, post-Cenomanian stacking of the Inntal- onto the Lechtal thrust sheet postdated emplacement of the Lechtal thrust sheet, and was out-of-sequence (Ortner, 2003a). Eoalpine, mid-Cretaceous nappe stacking, transport and folding was NW- to NNW directed, while subsequent Mesoalpine Late Cretaceous to Paleogene shortening was N to NNE directed (Eisbacher and Brandner, 1996; Ortner, 2003b). Neoalpine late Paleogene to Neogene shortening was NW to NE-directed and repeated the Cretaceous-Paleogene history in terms of directions (Ortner, 2003b; Peresson and Decker, 1997)

2 Investigated area In the studied area (see Appendix for the geologic map), the Karwendel thrust between the Inntal- and the Lechtal thrust sheets is spectacularly exposed below the north-facing cliffs of the Laliderer Wände, and at the western and eastern side of the Dreizinkenspitze – Gumpenspitze – Gamsjoch ridge (Fig. 2). These outcrops were already described and drawn in cross section by Ampferer (1902), and the thrust termed the Karwendel thrust. All overturned 13 and/or faulted units below the main thrust sheet boundary are part of the Karwendel zone of slices (Fig. 2; Heißel, 1977; Heißel, 1978).

Fig. 2: Tectonic map of the eastern Karwendel mountains, using the subdivision of thrust sheets of Tollmann (1976b). Localities mentioned in text: DZ = Dreizinkenspitze, FS = Falkenstand, FT = Falzthurntal, GJ = Gamsjoch, GS = Gumpenspitze, GK = Grubenkarspitze, MK = Mahnkopf, SG = Sulzgraben, SJ = Sonnjoch, SS = Schaufelspitze. Extent of Karwendel zone of slices (Karwendelschuppenzone) taken from Heißel (1978). KS = frontal slice of Inntal thrust sheet (Karwendel-Stirnschuppe) of Tollmann (1970). Graticule: MGI Austria GK West (EPSG: 31254). 3 Lithology The sedimentary succession in the Karwendel mountains reaches from the Upper Permian Haselgebirge to the Upper Triassic Hauptdolomit in the tectonically higher Inntal thrust sheet, and from the Hauptdolomit to the Cretaceous Schrambach Fm. in the underlying Lechtal thrust sheet (c.f. Fig. 3).

3.1 Permo-Triassic sediments The oldest sediment in this part of the NCA is the Haselgebirge, which crops out in the direct vicinity of the study area (e.g., Mahnkopf, Halltal, Sulzgraben west, south and east of the study area, respectively; Heißel, 1978; see Fig. 2). Discontinuous grey shales, cellular dolomites and

14 reddish to greenish quartz-rich sandstones are found in surface outcrops. In the subsurface, large amounts of anhydrite and gypsum are known and salt has been mined (e.g., Schmidegg, 1950). The Haselgebirge is in contact to cellular dolomites and stromatolithic dolomites and limestones of the Reichenhall Fm. Above the last cellular dolomites, well-bedded micritic limestones prevail that are often strongly bioturbated (“Wurstelkalk” of the Virgloria Fm.), and have crinoids. Crinoidal arenites, bioclastic packstones with green algae and occasionally massive limestones build the carbonate ramp of the Steinalm Fm. (Nittel, 2006; Rüffer and Zamperelli, 1997). Break-up of this carbonate ramp initiates facies differentiation between basins filled by nodular cherty filament-bearing limestones intercalated with tuff layers of the Reifling Fm. (Brühwiler et al., 2007; Nittel, 2006), and the Wetterstein carbonate platform. Clinoforms in the lower Wetterstein limestone interfinger with the Reifling Fm. in the study area. The Wetterstein carbonate platform is more than 1700 m thick (Sarnthein, 1966; 1967), and the Alpine Muschelkalk Group (Virgloria-, Steinalm- and Reifling Fms.) 150 m in the study area (Nittel, 2006). These units are the “competent backbone” of the Inntal thrust sheet, whereas the décollement lies in the evaporitic units, i.e. cellular dolomites, anhydrite and salt of Reichenhall Fm. and Haselgebirge.

The Raibl beds, a succession of limestones, shales, dolomites and cellular dolomites (Jerz, 1966), that should follow on top of the Wetterstein limestone are not preserved in the study area. Another carbonate platform, the Norian Hauptdolomit, well bedded, partly stromatolithic dolomite (Fruth and Scherreiks, 1982; Müller-Jungbluth, 1971), is the oldest exposed unit of the Lechtal thrust sheet. Toward the top, the Hauptdolomit platform drowns, and the Plattenkalk was deposited in a subtidal environment (Müller-Jungbluth, 1971). Drowning culminates in establishment of the basins of the Kössen Fm. during the Rhaetian. The Kössen marls and marly limestones interfinger with a platform, the Upper Rhaetian limestone. Black, dm to ½ m bedded limestones characterize the Kössen Fm. of the Lechtal thrust sheet in the study area, and these are comparable to the appearance of these units in the Allgäu thrust sheet in the northwestern part of the NCA (facies of the type Kalkalpen-Nordrand of Fabricius, 1966).

Fig. 3: Sedimentary succession of the Inntal and the Lechtal thrust sheets, and thickness of the deposits.

3.2 Jurassic and Cretaceous sediments The turn from the Triassic to the Jurassic is a major transition in the stratigraphic and tectonic evolution of the NCA (“Adneter Wende”; Schlager and Schöllnberger, 1974). Onset of Penninic rifting caused drowning of the Triassic carbonate platforms and major subsidence, and rift- related normal faulting caused facies differentiation (e.g., Nagel et al., 1976). The Adnet Fm., condensed red nodular, micritic limestones, represents submarine highs whereas the Allgäu Fm. was deposited in basins and reaches more than 1 km thickness (Jacobshagen, 1965). Synrift deposition ends at the turn to the Upper Jurassic with the “Ruhpoldinger Wende” (Schlager and Schöllnberger, 1974) when the variegated cherts of the Ruhpolding radiolarite accumulated. The Upper Jurassic to Lower Cretaceous Ammergau Fm. consists of dense pelagic, sometimes marly, well bedded, micritic limestones intercalated with thin marl layers (Tollmann, 1976a). The Lower Cretaceous (Berriasian to Albian) Schrambach Fm. consists of marls intercalated with marls and, occasionally, sandstones (Nagel et al., 1976). 16 4 Results Here, we describe the structures observed in the Gumpenspitze-Gamsjoch ridge. The organization of the description follows the position of the described structures in the hanging wall and footwall related to the main Karwendel thrust, respectively, and according to the relative age of the structures. We use the cross-cutting relationships between the described structures to deduce a relative age. A relative chronology is also based on the comparison of the shortening/transport directions with other parts of the NCA, where young, syntectonic sediments are present.

4.1 Karwendel thrust (1) The best exposures of the Karwendel thrust are found at Halftergraben (Fig. 4) on the western side of the Gumpenspitze-Gamsjoch ridge. There, Triassic sediments (Reichenhall Fm.) lie on top of Cretaceous deposits (Schrambach Fm.; 1 of Fig. 4), and this old-on-young contact defines the Karwendel thrust and distinguishes it from other tectonic boundaries in the area. The thrust is parallel to bedding in the immediate footwall (above 3 of Fig. 4) and hanging wall, which is close to horizontal on the hectometric scale (see also Fig. 4c and d). In the footwall, the zone of deformation reaches approximately 70 m down into the Allgäu Fm., the most intense deformation being observed in the Schrambach Fm. right below the thrust. In contrast, the complete hanging wall is involved in the deformation.

The emplacement of the allochthon is associated with end-Early Cretaceous nappe stacking postdating the youngest sediments in the area, which is the Schrambach Fm. reaching into the Albian. It is folded in the core of the Gamsjoch anticline and is subvertical in the northern limb, thus disappearing into the subsurface toward the north (Figs. 7 and 10). The Karwendel thrust is offset by the Lamsenjoch fault system (see 4.5; Figs. 7 and 10), and both are cut by the out- of-sequence Eng thrust which causes a doubling within the footwall of the Karwendel thrust (Figs. 4 and 7).

Fig. 4: View of the Gamsjoch – Gumpenspitze – Grubenkarspitze ridge from the W. (a) photograph and (b) interpretation. (c) and (d) transport direction across the Karwendel thrust at Halftergraben based on the intersection lineation of the mean orientation of bedding of the Reichenhall Fm. (= Rh) in the hanging wall, which is parallel to the Karwendel thrust, and mean foliation in the Schrambach Fm. (= Sb) of the footwall in two domains. (e) Orientation of the main Karwendel thrust at Gumpenalm (red great circles). Foliation s and shear planes c of two s-c-pairs are given and the transport direction based on the intersection lineation is indicated by arrows on the c-planes. Small scale fold axes (black circles) are perpendicular to transport directions of (c), (d) and (e). (f) Poles to bedding and fold axis of the Einsiedel anticline. All diagrams in this and the subsequent figures are lower hemisphere equal area stereographic projections, and the TectonicVB software (Ortner et al., 2002) has been used to manipulate and plot the data.

The Karwendel thrust is accompanied by a discontinuous band of rock (Fig. 5) that consists of recrystallized micritic limestone, sparry calcite and stylolites (Fig. 6d). The orientation of the pressure solution seams is (N)NW and gives evidence of (E)NE-shortening (Z), parallel to the stylolite teeth (Fig. 6d, top). In a section perpendicular to the thrust and parallel to transport direction (Fig. 6d, bottom), the shortening direction (Z) from stylolithes is parallel to transport direction, and the stretching direction (X) perpendicular to the thrust. We interpret this band of 18 rock as a tectonite, that formed from slices of rock probably derived from the footwall, but that was strongly altered by fluid-rock interaction during progressive deformation. While the top surface of this unit is planar and parallel to bedding of the overlying Reichenhall Fm., the basal surface has a cuspate-lobate geometry on the meter-scale, with the Schrambach Fm. protruding into the cusps.

Fig. 5: The Karwendel thrust at Halftergraben (see Fig. 4 for location), and structural data collected. (a) axes of m-scale folds in the Reichenhall Fm., (b) of cm- to dm-scale folds in the Schrambach Fm. and (c) in the Radiolarite and Ammergau Fm. Orientation of the (d) basal surface of the tectonite, and (e) of faults crosscutting the tectonite. (f) Poles to bedding and fold axis of a fold in the Reichenhall Fm., also labelled in the photograph. Poles to bedding in the (g) Ammergau Fm. and in the (h) Allgäu Fm. in the footwall of the Karwendel thrust.

These cusps indicate that the Schrambach Fm. was the weaker unit during transport of thrust sheets. The axes of the cusps trend ESE (Fig. 5d), and are parallel to fold axes in the underlying Schrambach Fm. (Fig. 5b), Ammergau Fm. (Fig. 5g), and roughly parallel to fold axes in the overlying Reichenhall Fm. (Figs. 5a, 7c; see below). Locally, the long axis of the lobes of the tectonite is (sub)parallel to a system of normal faults (Figs. 5d, e, 6b). The stretching direction

19 from the faults is parallel to the (E)NE transport direction indicated by the mean bedding- cleavage intersection (Fig. 4c) and transport directions deduced from individual s-c-pairs in the Schrambach (Figs. 4e) and Ammergau Fms. (Fig. 7f), and open folding in the incompetent Allgäu Fm. (Fig. 5h) below the Karwendel thrust (see below). As the normal faults crosscut and offset the lobes of the tectonite, this stretching must be younger than NNE-directed transport that formed the mullion surface. It may be related to late transport-parallel stretching as observed in many melange zones (e.g., Biehler, 1990; Jeanbourquin, 1994; Kusky and Bradley, 1999; Needham, 1995).

Fig. 6: Outcrop-scale structures below the Karwendel thrust. (a) s-c-fabric in the Schrambach Fm. at the location of e in Fig. 4. (b) Second lobe of the tectonite from the south in Fig. 5 in contact to Schrambach Fm. Note the conjugate fault system, N-dipping faults offsetting the tectonite, S-dipping faults not. Arrow indicates hanging wall transport on fault surface. (c) Symmetric folds at the boundary of the Ruhpolding radiolarite to the Ammergau Fm., halfway between the locations of c and d in Fig. 4. (d) Oriented polished surfaces of a tectonite sample parallel (top) and perpendicular (bottom) to the Karwendel thrust. Sample location shown by c in Fig. 4. Note stylolitic foliation perpendicular to, and calcite veins parallel to the

20 direction of shortening (compare Fig. 4c). (e) Foliation and folds within the Schrambach Fm. at the location of c in Fig. 4. Stippled line indicates sandstone bed. Mean bedding parallel to hammer handle. (f) s-c-fabric in the Ammergau Fm. at the location of f in Fig. 7.

4.2 Structure of the footwall Below the Karwendel thrust, an upright sedimentary succession from the Hauptdolomit to the Schrambach Fm. is present. The latter is strongly deformed right below the thrust, but deformation features vary in different places. Where the Schrambach Fm. is rich in marly limestone and sandstone, a south to southeast-dipping axial planar foliation is developed (Fig. 6e). Marls have tight cm-scale NE-verging folds (diagram Fig. 5b), whereas sandstones are folded in metric folds. Separation of boudinaged sandstone fragments in overturned fold limbs is partly caused by removal of material during foliation development, but probably also by older boudinage, as the distance between fragments is large and cannot be caused by pressure solution alone. Foliation is defined by stylolite surfaces. Across stylolites, parts of cm-scale fold hinges in marls are removed by solution. Where the Schrambach Fm. is rich in marls, and sandstones are missing, s-c-fabrics are found, indicating transport toward the (E)NE (Fig. 6a and f, diagrams of Figs. 4c, e and 7f).

The underlying Ammergau Fm. and Ruhpolding radiolarite are more competent and intensely folded. The top surface of this unit is flat, while the lower boundary against the underlying less competent Allgäu Fm. has an irregular metric lobate geometry, and, superposed, the lower half of a decametric pinch-and-swell structure (Fig. 5). Within these boundaries these units are pervasively folded on the dm-scale (Fig. 6c). These folds are mostly symmetric, only a few verge to the N. Fold axes scatter between SW and WNW, and are distributed along a small circle with an ENE-trending axis (Fig. 5c), while poles to bedding plot along a poorly defined girdle about an ESE-trending axis (Fig. 5g).

The small circle distribution of fold axes indicates refolding about an ENE axis (e.g., Ramsay, 1960). A possible sequence of events is initial NW-directed transport, followed by NNW- transport. The poorly defined ESE-fold axis from the Ammergau Fm. bedding poles (Fig. 5g) may be related to still younger NNE-directed transport. On the outcrop scale, the wavy lower boundary of the Ammergau-Radiolarite packet is the only candidate for a superposed folded surface (Fig 5). The contact of this mullion-like surface against the less competent unit below is interpreted as an effect of buckling into the underlying Allgäu Fm. after the dm-scale folds were so tight that the whole Ammergau-Ruhpolding unit started to behave as a homogenous layer developing a new, larger wavelength of buckling.

The sedimentary succession below (Allgäu-, Adnet- and Kössen Fms.) is not affected by pervasive deformation. Isolated, very open, metric folds are present in the Adnet- and Allgäu Fms., and only in the latter, recumbent cm- to dm-scale slump folds. Poles to bedding in the Allgäu Fm. scatter about the mean orientation of the Karwendel thrust in the area (compare diagrams of Figs. 5h and 4c), but are also folded about a SE-trending axis (Fig. 5h).

East and southeast of the Halftergraben outcrop, the structure immediately below the main thrust is more complex. On the east side of the Gamsjoch-Gumpenspitze ridge, an additional slice repeating the Ammergau-Schrambach succession is present (Fig. 7). Further to the south, an isoclinal anticline-syncline pair in the Radiolarite-Ammergau-Schrambach succession are found in the hanging wall of the Eng out-of-sequence thrust (Fig. 10f). The small spread of the bedding poles in this diagram is a consequence of the isoclinal nature of the folds.

Fig. 7: View of the Gamsjoch – Gumpenspitze ridge from the NE. (a) photograph and (b) interpretation. Poles to bedding and fold axis of the (c) Teufelskopf syncline, (d) Gamsjoch anticline, and (e) the buttressed units below the Eng out-of-sequence thrust. (f) structural data at the reactivated Karwendel thrust: ss Rh. = mean bedding in the Reichenhall Fm. of the hanging wall, ss Kö. = mean bedding of the Kössen Fm. in the undeformed footwall. s, c = foliation and shear planes in s-c-fabrics in the Schrambach and Ammergau Fms, fa = fold axis. Arrows indicate hangingwall movement on shear planes below the Karwendel thrust, KS = frontal slice of the Inntal thrust sheet (Karwendel-Stirnschuppe) of Tollmann (1970), L.f.s. = Lamsenjoch fault system.

4.3 Structure of the hanging wall The sedimentary succession above the Karwendel thrust is not pervasively deformed. Generally, bedding is parallel to the main thrust (Figs. 4, 5). Metric to decametric folds are

23 localized above the cutoff of a bed right against the Karwendel thrust (Fig. 5). Above the cutoff, carbonate beds separated by cellular dolomites are disharmonically folded, the number of folds varies depending on the thickness of the beds. In the bed labelled (f), a fold train of three overturned folds is observed. Wavelength and amplitude of these folds decrease from N to S. Upsection, the wavelength of the largest of these folds increases to several tens of meters above a thick layer of cellular dolomites, while the smaller folds disappear. The syncline at Teufelskopf (Fig. 7c) and the middle fold of the fold train (diagram Fig. 5f) was measured, and verges to the N(NE).

4.4 Interpretation of mesoscale structures Three conclusions can be drawn at this point: (1) The mesoscale structures below the Karwendel thrust can be understood in terms of a shear zone displaying decreasing deformation intensity with increasing distance from the fault core marked by the tectonite. As the incompetent Schrambach and Allgäu Fms. lack structures that can be related to NW-transport, but the more competent Ammergau Fm. and Ruhpolding radiolarite do, we suggest that differences in the rheology of sedimentary units control the preservation of structures. While the more competent units preserve the complete record of polyphase deformation, incompetent units tend to preserve only the youngest events.

Fig. 8: Kinematic evolution of the cross section in the area of the Gamsjoch anticline. See text for discussion.

(2) In comparison to regional deformation of the Austroalpine and the NCA, Cretaceous, Eoalpine NW-directed shortening was therefore active in the Karwendel mountains but no large scale folds formed (Fig. 8a). N- to NNE-directed transport can be related to latest Cretaceous- Paleogene shortening (Eisbacher and Brandner, 1996; Froitzheim et al., 1994; Ortner, 2003b; Oswald et al., 2018), which is also seen within the Penninic nappes underlying the Northern Calcareous Alps (e.g., Biehler, 1990; Ring et al., 1988; Ring et al., 1990). However, a possible contribution of Miocene NE-directed transport can neither be excluded nor proven at this point (see Decker et al., 1994; Ortner, 2003b; Peresson and Decker, 1997).

24 (3) The hanging wall of the Karwendel thrust does not show evidence of Cretaceous NW- directed shortening, but only of Latest Cretaceous-Paleogene N- to NNE-directed transport. (4) Shortening was active from the latest Early Cretaceous into the Miocene.

4.5 Gamsjoch anticline (2) The km-scale Gamsjoch anticline folds the hanging wall and the footwall together and is therefore younger than the Karwendel thrust (Figs. 7, 8b). The stratigraphic succession in both fold limbs is comparable, including the old-on-young contact defining the Karwendel thrust (see above), and even the clinoforms in the Wetterstein limestone dip roughly to the south in both limbs (Fig. 7) in their restored geometry. The fold axis plunges to the WNW (Fig. 7d), parallel to the folds above the Karwendel thrust (see above). On larger scale geologic maps (e.g., Fig. 2), these folds are truncated by the northern branches of the Miocene Inntal shear zone (Ortner, 2003b; Ortner et al., 2006; Ortner and Stingl, 2001), thus a Paleogene age is more likely for these folds. Latest Cretaceous-Paleogene folding thus caused the Karwendel thrust to lock (Fig. 8b) and to disappear in the subsurface toward the north (see paragraph 4 in chapter 5.4).

The Gamsjoch anticline is part of a larger anticline that extends to the Eiskarspitze (Fig. 11). This fold formed during NNE-directed shortening, as most of the other km-scale folds of the Karwendel mountains (Heißel, 1978; Tollmann, 1976b). Folding the nappe stack lead to a regional uplift of anticlines, later out-of-sequence thrusting caused the todays structural situation (Fig.12).

4.6 Lamsenjoch fault system (3) The subvertical WNW-striking Lamsenjoch fault runs across the uppermost Falzthurn valley (label FT of Fig. 2) and the Eng valley, cuts the Gamsjoch anticline (Figs. 4, 7 and 14) and then continues further west. In the western flank of the ridge between Gumpenspitze and the Gamsjoch this strike-slip fault system cuts across the axial plane of the Gamsjoch anticline (Fig. 4). The upward diverging nature of the faults in Fig. 4 suggest the geometry of a flower structure. Across single fault branches, the southern block is systematically downthrown (e.g., fault south of Gamsjoch summit of Fig. 7). This is less obvious in other views. In Fig. 4, the steep, overturned limb of the Gamsjoch anticline is offset against the upright limb across the northernmost fault branch, and an upright limb is in contact with the overturned limb across the next fault branch. In Fig. 10, the overturned limb of the eastern continuation of the Gamsjoch anticline is in contact with the upright limb across the northernmost branch of the Lamsenjoch

25 fault. There, major normal offset is required, as the upright limb is south of the closure of the syncline, and this limb can only come from above.

The direct contact of the upright Kössen Fm. of the Lechtal thrust sheet and the overturned Reichenhall Fm. of the Inntal thrust sheet across the southern branch of the Lamsenjoch fault system (Fig. 11) is more difficult to explain. Obviously, the fault cuts out the core of the anticline by lateral movements. This branch of the fault reactivated the folded, pre-existing Karwendel thrust, where it is subvertical to north-dipping in the deeper part. The upper part is parallel to the axial plane of the Gamsjoch and was probably guided by bedding-perpendicular pre-existing fractures. In summary, a combination of lateral and south-block down normal offsets are required to explain the observed contacts in the area. Another indication of lateral offset is the different thickness of sedimentary units in the Muschelkalk Group, e.g., the Virgloria Fm. being 50 m thick S of the fault and 400 m N of it (Fig. 11). Unfortunately, no clear cutoffs are present to quantify offset.

Brittle fault data were collected in the Falzthurn valley (diagrams d and e of Fig. 10; for location see label FT of Fig. 2). The girdle distribution of the P- and B-axes and the shape factor of R = 0,75 of the incremental strain ellipsoid from inversion of the fault data using the NDA method (Spang, 1972) give evidence for sinistral transtension along WNW-striking faults. The fault data sets also contain dip-slip and oblique-slip normal faults, suggesting that faults with a relative normal offset and moderate dip (Figs. 4, 10) are also part of this fault system. Fault data collected at Falkenstand west of the Gamsjoch (label FS of Fig. 2), and around Gh. Eng of Fig. 10 show the same general fault pattern (Fig. 9).

Fig. 9: Brittle faults collected at (a) Falkenstand (see Fig. 2 for location) and near Gh. Eng (see Fig. 10 for location), and the respective kinematic axes. The fault data document sinistral transtension comparable to the data set of Fig. 10d and e.

The WNW striking sinistral transtensive Lamsenjoch fault system is younger than the Karwendel thrust and younger than the Gamsjoch anticline because it offsets the thrust and runs into bedding in the steep limb of the fold. We interpret the normal, south-block down faults of the study area as branches of the Lamsenjoch fault system. The structural relations (Figs. 4, 7, 11) showed that these faults were active in post- or latest Paleogene times (post-Gamsjoch anticline), probably Oligocene. Transtensive brittle faulting at sinistral WNW-trending master faults conjugate with ESE-trending faults (Figs. 10d, 9) has not been recorded previously. In older studies, dextral faulting across NW to WNW-trending tear faults has been associated with dextral tear faulting related to Cretaceous transport of thrust sheets (Eisbacher and Brandner, 1995; 1996). Sinistral reactivation of these has been attributed to Late Miocene E-W compression in other parts of the NCA (Ortner, 2003b; Peresson and Decker, 1997), and is not comparable to the observed Oligocene transtensive faulting.

Fig. 10: View of southern and eastern end of the Eng valley from the SW. (a) photograph and (b) interpretation. (c) Poles to bedding and fold axis in the overturned limb of a recumbent fold east of the Eng valley. This overturned limb reappears at Ruederkarspitze of Fig. 7. (d) Faults and (e) P-T-axes related to a transtensive strike slip fault zone. Data collected in the Falzthurn valley on the other side of the saddle marked by the arrow (location FT of Fig. 2). (e) Poles to bedding and fold axis of the fold system between the Karwendel thrust and the Eng thrust.

28 4.7 Out-of-sequence Eng thrust (4) The Eng thrust crosscuts the Karwendel thrust and doubles the Upper Triassic to Lower Cretaceous succession of the Lechtal thrust sheet. Toward the north, this Eng thrust merges with and reactivates the southern branch of the Lamsenjoch fault system in Figs. 7 and 11, which crosscuts the Gamsjoch anticline. Therefore the Eng thrust is youngest structure in the area, and out-of-sequence with respect to folding and stacking of thrust sheets. The Upper Triassic to Lower Cretaceous succession in the footwall of the Eng thrust is detached at the Kössen marls and buttressed against the Lamsenjoch fault, causing open to isoclinal folding within the Allgäu-, Ammergau and Schrambach Fms. with an axial planar foliation subparallel to the Lamsenjoch fault (inset of Fig. 11). Folding thickens and moves the buttressed units up the upper, south-dipping part of the Lamsenjoch fault, thus emplacing units belonging to the Lechtal thrust sheet onto the tectonically higher Inntal thrust sheet (Figs. 7 and 11; Fig. 8d).

This process is even more pronounced at the western side of the Gamsjoch-Gumpenspitze ridge, where the Eng thrust emplaces the Upper Triassic to Lower Cretaceous rocks in its hanging wall onto an upright folded slice of Reichenhall-Virgloria Fm. and truncates the crest of the fold (Fig. 4). There, S-C-fabrics in the Schrambach Fm. give a NE transport direction (Fig. 4e). Buttressing in the footwall of the Eng thrust caused stacking and tilting of several slices within the Kössen Fm. against a southern branch of the Lamsenjoch fault system (Fig. 4). (E)NE-directed transport related to activity of the Eng thrust also overprints the incompetent units and the tectonite in its hanging wall that are sandwiched between the Eng and the Karwendel thrusts (Figs. 4c, e, Fig. 7), and the folded units detached from the competent Hauptdolomit below the Eng thrust (Fig. 7e). It reactivates the Karwendel thrust, probably not causing large offsets.

The out-of-sequence Eng thrust is the youngest event in the study areas because it truncates all other structures. From structural data (see above), we suggest the transport across the out-of- sequence thrust was (E)NE-directed. The age of the out-of-sequence Eng thrust must be younger than the Gamsjoch anticline and the steep faults, and is probably Latest Oligocene to Miocene. Such an age is in accordance to observations in the Inn valley, where soft-sediment folding with NW-axes has been observed in Upper Oligocene deposits (Ortner, 2003b; Ortner and Stingl, 2001). However, also Miocene age NE-compression has been documented, which is mostly strike-slip, but also thrust-type (Decker et al., 1994; Ortner, 2003b; Peresson and Decker, 1997).

29 5 Discussion

5.1 Timing of the structures observed In summary, shortening from the late Early Cretaceous onward has been observed, roughly following the changing plate convergence direction changing from NW-directed to NE-directed (e.g., Dewey et al., 1989), while the Oligocene to Miocene/shortening direction is controlled by escape tectonics that creates secondary stress fields (Decker et al., 1994; Ortner, 2003b). On the local scale, the kinematic history as deduced from the observations starts with Eoalpine, end-Albian NW-directed stacking of thrust sheets. Transport and shortening continued into the Paleogene and changed to N- and NE-directed. In a late stage of this process, the first, km-scale, WNW-trending folds of the Karwendel mountains formed (Fig. 2), including the Gamsjoch anticline, that refolds the thrust boundary, which disappears into the subsurface. Folding locked the thrust boundary. Shortening was interrupted by Late Oligocene sinistral transtensive shearing at WNW-trending faults, which separates NE-directed Early Paleogene from (E)NE- directed Late Paleogene-Neogene transport, and initiated out-of-sequence thrusting. In the following we discuss some of the kinematic-mechanical boundary conditions of thrusting and out-of-sequence thrusting.

Fig. 11: Cross section parallel to the Gamsjoch-Gumpenspitze ridge. Geologic information from the ridge was projected into the section. See Fig. 2 for trace of section. Note the Karwendel thrust being parallel to bedding all along the cross section, as the complete Upper Triassic to Cretaceous succession reappears at the southern margin of the Karwendel mountains, except the Schrambach Fm. (e.g., Heißel, 1958; Moser, 2008).

31 5.2 Kinematic evolution, salt tectonics and fluid overpressure The difference in the kinematic histories recorded in the footwall and the hanging wall of the Karwendel thrust is conspicuous. While the full kinematic history is documented in the footwall, only (Paleogene) NNE-transport is seen in the hanging wall. Part of the explanation might be the presence of rock salt at the Karwendel thrust (see section 3.1), decoupling the hanging wall during a part of the kinematic history. However, in this case neither the footwall nor the hanging wall should show a kinematic record. The solution of the problem may be the fact, that the salt pillows of the Haselgebirge had a limited lateral extent (e.g., Leitner and Spötl, 2017; Spötl, 1988), and the salt is mobile and migrates laterally or upward (e.g., Hudec and Jackson, 2007; Jackson and Hudec, 2017). Therefore, the Karwendel thrust had to cut from the base to the top of the NCA sedimentary succession across a ramp, and to emplace the Inntal thrust sheet on the upper flat, until the salt reached the position of the present-day Karwendel mountains. During this part of the history the frontal parts of the Inntal thrust sheet would pass the footwall presently exposed in the Karwendel mountains, and record the kinematic history. A trailing part of the Inntal thrust sheet floored by salt would have been entirely decoupled from the footwall during transport, and not record this kinematic history. This is in line with the observation, that there was no large scale folding associated with early, Eoalpine transport. Only after flow of salt out of the thrust zone, the thrust sheet grounded, and mechanical coupling was possible, and both hanging- and footwall were affected by Early Paleogene deformation. Salt expulsion probably also contributed to strengthening of the thrust itself, and forced the décollement to move out of the Schrambach-Reichenhall contact into marls of the Kössen Fm. below.

Fluid overpressure at the thrust contact would have comparable effects, and the release of overpressure would leave only minor traces in the rock record (e.g., Dielforder, 2017; Dielforder et al., 2015). Finite strain in the tectonite immediately below the Karwendel thrust, with the long axis X perpendicular to and the short axis Z parallel to the thrust and ENE-trending (Fig. 6b, d) documents pure shear deformation, while simple shear is observed in the underlying foliated Schrambach marls (Fig. 6a, b, e). The absence of simple shear in the tectonite requires full decoupling and strain partitioning, which was probably achieved by near-lithostatic fluid pressures (compare, e.g., Tobin et al., 1994). This interpretation is in accordance with the outcrop-scale geometry of the tectonite. The cuspate-lobate interface between the tectonite and the Schrambach Fm. requires shortening parallel to the thrust, while the planar top surface against the Reichenhall Fm. requires mechanical decoupling to facilitate transport of the

32 hanging wall with low friction, thus not disturbing the tectonite. The orientation of the foliation in the tectonite and underlying Schrambach-Fm. points to Late Paleogene-Miocene formation of foliations and existence of overpressure, and therefore this event is distinct from the earlier, Cretaceous-Early Paleogene history (see above) in terms of orientations, and mechanics.

5.3 Tectonic subdivision of the NCA in the study area Here we discuss the thrust sheet boundaries as defined by previous authors in the light of our analysis. As outlined in the introduction, many problems arise from the fact that the simple criterion that thrusts emplace old on young rocks has not been applied systematically. In his discussion of the thrust sheet boundaries of the NCA, Heißel (1958, page 124) stated: “Es kann auch keine Bedingung für den Begriff einer Decke sein, daß sie überall mit Älterem auf Jüngerem liegt” (It cannot be a requirement for a nappe to emplace old on young rocks everywhere). This argument has been used to interpret all sorts of boundaries as thrust sheet boundaries.

For illustration, the boundary of the Inntal against the Lechtal thrust sheet as shown in the maps of Tollmann (1976b) has been marked by a red stippled line in the hanging wall (= Inntal thrust sheet) of the respective boundaries in the panoramic views of the studied area (Figs. 4, 7, 10). While the uppermost thrust is straightforward and fits all definitions of a thrust, the other boundaries are problematic. In Fig. 4, the frontal slice of the Inntal thrust sheet is a fault block between two branches of the Lamsenjoch fault system, which is also truncated by the Eng out- of-sequence thrust at the top. In Fig. 7, the southern boundary of this slice is the main branch Lamsenjoch fault system, while the stratigraphic contact between the Virgloria and Reichenhall Fms. in the core of the Gamsjoch anticline forms the northern boundary. In Fig. 10, this frontal slice of the Inntal thrust sheet is bounded to the south by a segment of the Karwendel thrust, but the northern boundary is an out-of-sequence thrust that develops from bedding-parallel shear in the completely overturned limb of a recumbent syncline. Based on the ESE-trend of the fold axis (Fig. 10c), refolding of the overturned limb took place in the Early Paleogene. This thrust is not folded and therefore younger, and is related to Latest Paleogene to Miocene out- of-sequence thrusting, comparable to the Eng out-of-sequence thrust (see above).

The Gamsjoch - Gumpenspitze ridge has been regarded to be a key area for the understanding of the tectonic subdivision of NCA in the past. The folding of the Karwendel thrust in the core of the Gamsjoch anticline has not been recognized previously (see cross sections in Ampferer, 1902; 1928; Richter, 1929), except Brandner (2014), probably because of the overprinting by

33 the Lamsenjoch fault system and by the Eng out-of-sequence thrust. Ampferer (1928) tried to explain the observed multiphase cross section geometry by a single tectonic event and claimed that a thrust onto a existing surface relief (his “Reliefüberschiebung”) could explain the observations. At the same time, Richter (1929) connected the Karwendel thrust and the southern branch of the Lamsenjoch fault system in a sketch of the western side of the Gamsjoch - Gumpenspitze ridge (the view of Fig. 4) and concluded correctly that the two summits should be in the same tectonic unit, based on faulty arguments. Looking at the cross sections with today’s background, these are not admissible (e.g., Dahlstrom, 1969). Based on his schematic sketches the “Reliefüberschiebung” of Ampferer (1928) would correspond to a break-through or out-of-sequence thrust cutting across predeformed units. A thrust emplacing its hanging wall onto a land surface like, e.g., the present-day eastern frontal thrust of the Andes (e.g., Costa et al., 2000; Schmidt et al., 2011; Vergés et al., 2007) is not at all comparable to the thrusts of the Karwendel mountains, mainly because of the absence of syntectonic continental deposits in the footwall.

The geometry of the Karwendel thrust in Figs. 7 and 11, disappearing into the subsurface in the northern limb of the Gamsjoch anticline, shows that the thrust must continue to the north below the Karwendel syncline (Fig. 1), and the Inntal and Lechtal thrust sheets are connected. This observation is not isolated, and the connection has also been documented in the Wetterstein and Mieming mountains further west (see Fig. 1; Ortner and Bitterlich, 2016; Schlagintweit, 1912a). The thrust reappears at the northern margin of the NCA (Figs. 1 and 12), where the youngest sediments on top of the Allgäu thrust sheet are uppermost Albian in age W of Oberammergau (Gaupp, 1982; Höfle et al., 1969) and on the trace of the cross section of Fig. 12 (Doben et al., 1991). This increases the amount of transport of the upper onto the lower thrust sheet to a minimum of 38 km which is the horizontal N-S distance between the southern and northern margin of the upper thrust sheet. The estimate disregards folds, minor thrusts, and the changing transport direction through time (see above). However, using any other transport direction would increase the estimate.

Fig. 12: Conceptual N-S cross sections of the NCA illustrating (a) the tectonic subdivision of Tollmann (1976b), and (b) the tectonic subdivision suggested in this paper. White vertical hatch in (a) shows the extent of the Karwendel zone of slices as proposed by Heißel (1978). Dashed black line indicates the Raibl beds. Deep structure based on the cross section of Bachmann and Müller (1981) across the well Vorderriß 1, projected 7 km from the W. The segment of the cross section between the Vorderriß well and the Karwendel syncline based on enclosure 6 in Freudenberger and Schwerd (1996). See Fig. 1 for trace of section and color code of (a), and Fig. 13 for color code of (b). Note Albian age synorogenic sediments both below the Lechtal- and Inntal thrust sheets of (a), justifying the merge of the Inntal- and Lechtal thrust sheets into the Karwendel thrust sheet of (b). In this context, the Karwendel zone of slices appears to be a zone of out-of- sequence duplexing uplifting the southern part of the Karwendel thrust sheet. See Reiter et al. (2018) for a more elaborated cross section nearby.

How to define a thrust sheet? An important argument are geometric relationships, as discussed in chapter 5.1. A second argument is the time of emplacement, which is given by the age of the youngest sediments below the thrust (Figs. 1 and 12; Mandl et al., 2017; Ortner, 2003a). Using this argument, the eastern Inntal- and the Lechtal thrust sheet were emplaced at the same time. In combination with geometry (see above), only two large thrust sheets are left on the NCA, the deeper one corresponding to the Allgäu thrust sheet in the north and the Lechtal thrust sheet in the eastern Karwendel mountains, the upper including the main body of the Lechtal- and the Inntal thrust sheets (Figs. 2 and 12a). To avoid confusion of different nomenclatures, we name the tectonically deeper thrust sheet the Tannheim thrust sheet, as the largest area exposing this thrust sheet is the Tannheim valley ENE of Oberstdorf (Fig. 1), and the upper thrust sheet the Karwendel thrust sheet, which has the best exposures of its lower boundary in the Karwendel mountains (Figs. 4, 5). We show this new subdivision in Fig. 13, for a preview of this new subdivision on the scale of the western NCA see Fig. 24 in Ortner (2016).

Fig. 13: Tectonic map of the studied area distinguishing in-sequence thrusts (Karwendel thrust) and out-of- sequence thrusts (e.g., Eng thrust). The latter locally superimpose younger on older rocks, and are frequently not parallel to bedding. For explanation of abbreviations, see Fig. 2.

It is not useful to separate thrust sheets along local out-of-sequence thrusts such as the Eng thrust (Figs. 4 and 7), as this obscures the older, in-sequence history of the Karwendel thrust. However, these thrusts are still tectonic boundaries, and can have considerable offset. In Fig. 13, transport of klippen of the Tannheim thrust sheet onto the Karwendel thrust sheet is at least 3 km, but still only a tenth of the transport of the Karwendel thrust sheet. Separating a thrust sheet in the hanging wall of the Eng thrust would mean to include the Inntal thrust sheet of Tollmann (1976b) and the upper part of his Lechtal thrust sheet in a tectonic unit detached during the Late Paleogene-Miocene. This is in contrast to the concept of Eoalpine, Cretaceous nappe stacking in the NCA.

The thrust sheets of the NCA have traditionally been arranged in groups or nappe systems, based on their paleogeographic position on the Triassic Eurasian continental margin (e.g., Haas et al., 1995; Lein, 1987). As transport of thrust sheets was directed toward the continent, the tectonic position correlates with their paleogeographic origin. Therefore, the most ocean-near

36 parts of the continental margin are represented by the tectonically highest Juvavic group of thrust sheets, the units near the shelf break by the Tirolic thrust sheets occupy an intermediate tectonic position, and the continent-near Bajuvaric thrust sheets are the tectonically deepest units (Mandl, 2000; Pestal et al., 2009; Tollmann, 1976b). Within this system, the Allgäu and Lechtal thrust sheets were regarded to be part of the Bajuvaric units, whereas the Inntal thrust sheet belongs to the Tirolic units (Mandl, 2000; Schmid et al., 2004; Tollmann, 1976b). Merging the Inntal and Lechtal thrust sheets into the Karwendel thrust sheet, creates the problem to which larger unit it belongs. Within the sedimentary succession of the Tirolic thrust sheets, two sedimentary units are characteristic: the Upper Jurassic Oberalm Fm. that contains allodapic limestones (Flügel and Fenninger, 1966; Steiger, 1981; Tollmann, 1976a), and the Lower Cretaceous Roßfeld Fm. (Decker et al., 1987; Faupl and Tollmann, 1979) that are both absent in the Bajuvaric thrust sheets. The Jurassic and Cretaceous sedimentary succession in the Karwendel and the easterly adjacent Thiersee synclines (Figs. 1) in the Karwendel thrust sheet includes both the Oberalm Fm. (Nagel et al., 1976; Ortner and Kilian, 2016), and up to 150 m thick Lower Cretaceous, Aptian-Albian sandstones in the eastern Thiersee syncline (Wilmers, 1971) comparable to the Roßfeld Fm. Therefore the Karwendel thrust sheet belongs to the Tirolic thrust sheets (compare Mandl et al., 2017).

6 Conclusion The structural analysis of the Karwendel thrust in the Gamsjoch-Gumpenspitze ridge of the eastern Karwendel mountains, that is a key outcrop in the discussion of the tectonic subdivision of the NCA, resulted in the proposal of a new tectonic subdivision. As the Karwendel thrust is seen to be folded in the core of the anticline, it continues to the N below the Karwendel syncline, and the Inntal- and Lechtal thrust sheets must be part of a larger tectonic unit that has been termed the Karwendel thrust sheet here (Fig. 12). For the same reason, the former Allgäu thrust sheet increases in size as well and now includes the former Lechtal thrust sheet of the eastern Karwendel mountains in the new Tannheim thrust sheet. The Karwendel thrust sheet was emplaced at the end of the Albian, based on the age of the youngest synorogenic sediments on top of the Tannheim thrust sheet (Fig. 12).

The failure to recognize the folded thrust has its reason in the multiphase overprinting of the initial thrust boundary (Fig. 8). Post-emplacement strike-slip faults offset the thrust, and forced it to climb across a vertical step, causing initial buttressing against the fault in the footwall of an out-of-sequence thrust, and then emplacement of young on old rocks (Fig. 8). The recognition and differentiation of in-sequence versus out-of-sequence thrusts is of fundamental 37 importance when resolving the structural history of an area, and depends on the strict application of the rule that initial stacking emplaces old on young rocks. Here we demonstrate that out-of-sequence thrusts may emplace klippen and halfklippen as well, but the structural context is different from in-sequence thrusts, as these locally emplace young on old rocks, and crosscut pre-existing structures.

The analysis demonstrated that shearing across the thrust sheet boundary continued from the Cretaceous into the Miocene. Regarding the structural record, Early Paleogene N to NNE- directed transport produced the most pervasive structures, even if transport was mostly accomplished during the Cretaceous. However, also Latest Paleogene-Miocene transport left a structural imprint on the thrust, even if transport was small.

7 Acknowledgments We thank Midland Valley who provided their Move software that was used to construct the cross sections in figures 8, 11 and 12 in the frame of their academic software initiative. The work of Sinah Kilian was supported by the Geologic Survey of Austria and a scholarship of the University of Innsbruck. We thank Alfred Gruber for help during fieldwork and many lengthy discussions. Some of the structural data and the photograph of Figure 6c were collected by Kajetan Heigert in the course of preparation of his bachelor thesis in the Halftergraben. Some of the fault data were collected by Leonhard Rauch and Stefan Griesser in the frame of their bachelor thesis in the Falzthurn valley. We thank the journal referees Gerhard Bryda and Oscar Fernandez, who significantly contributed to the improvement of the manuscript.

8 References Ampferer, O., 1902. Bericht über die Neuaufnahme des Karwendelgebirges. Verhandlungen der Geologischen Bundesanstalt, 1902, 274–276. Ampferer, O., 1912. Gedanken über die Tektonik des Wettersteingebirges. Verhandlungen der k.k. Geologischen Reichsanstalt, 1912, 197–212. Ampferer, O., 1914. Besprechung mit O. Schlagintweit, K. Ch. v. Loesch und H. Mylius über das Wettersteingebirge. Verhandlungen der k.k. Geologischen Reichsanstalt, 1914, 338–352. Ampferer, O., 1928. Die Reliefüberschiebung des Karwendelgebirges. Jahrbuch der Geologischen Bundesanstalt, 78, 241–256. Ampferer, O., 1931. Zur neuen Umgrenzung der Inntaldecke. Jahrbuch der Geologischen Bundesanstalt, 81, 25–48. Ampferer, O., 1942. Geologische Formenwelt und Baugeschichte des östlichen Karwendelgebirges. Denkschriften der Akademie der Wissenschaft, 106/1, 1–95. Ampferer, O., Hammer, W., 1911. Geologischer Querschnitt durch die Alpen vom Allgäu zum Gardasee. Jahrbuch der k.k. Geologischen Reichsanstalt, 61, 531–710.

38 Ampferer, O., Heißel, W., 1950. Das östliche Karwendel: Erläuterungen zur geologischen Karte des östlichen Karwendel und des Achensee-Gebietes. Wagner, Innsbruck, 55 pp. Bachmann, G.H., Müller, M., 1981. Geologie der Tiefbohrung Vorderriß 1 (Kalkalpen, Bayern). Geologica Bavarica, 81, 17–53. Biehler, D., 1990. Strukturelle Entwicklung der penninisch-ostalpinen Grenzzone am Beispiel der Arosa-Zone im Ost-Rätikon. Eclogae Geologicae Helvetiae, 83, 221–402. https://doi.org/10.5169/seals-166585 Boyer, S.E., Elliott, D., 1982. Thrust systems. AAPG Bulletin, 66, 1196–1230. Brandner, R., 2014. Die Karwendel-Plattform - ein Schlüssel zum Verständnis des Deckenbaues. In: Sonntag, H., Straubinger, F. (Eds.), Großer Ahornboden - eine Landschaft erzählt ihre Geschichte. Berenkamp, Wattens, 40–45. Brühwiler, T., Hochuli, P.A., Mundil, R., Schatz, W., Brack, P., 2007. Bio- and chronostratigraphy of the Middle Triassic Reifling Formation of the westernmost Northern Calcareous Alps. Swiss Journal of Geosciences, 100, 443–455. https://doi.org/10.1007/s00015-007-1240-2 Costa, C.H., Gardini, C.E., Diederix, H., Cortés, J.M., 2000. The Andean orogenic front at Sierra de Las Peñas-Las Higueras, Mendoza, Argentina. Journal of South American Earth Sciences, 13, 287–292. https://doi.org/10.1016/S0895-9811(00)00010-9 Dahlstrom, C.D.A., 1969. Balanced cross sections. Canadian Journal of Earth Sciences, 6, 743– 757. https://doi.org/10.1139/e69-069 Decker, K., Faupl, P., Müller, A., 1987. Synorogenic sedimentation in the Northern Calcareous Alps during the Early Cretaceous. In: Faupl, P., Flügel, H.W. (Eds.), Geodynamics of the Eastern Alps. Deuticke, Wien, 126–141. Decker, K., Peresson, H., Faupl, P., 1994. Die miozäne Tektonik der östlichen Kalkalpen: Kinematik, Paläospannungen und Deformationsaufteilung während der "lateralen Extrusion" der Zentralalpen. Jahrbuch der Geologischen Bundesanstalt, 137, 5–18. Dewey, J.F., Helman, M.L., Turco, E., Hutton, D.H.W., Knott, S.D., 1989. Kinematics of the Western Mediterranean. In: Coward, M.P., Dietrich, D., Park, R.G. (Eds.), Alpine Tectonics. Special Publications. Geological Society, London, 45, pp. 265–283. Dielforder, A., 2017. Constraining the strength of megathrusts from fault geometries and application to the Alpine collision zone. Earth and Planetary Science Letters, 474, 49– 58. https://doi.org/10.1016/j.epsl.2017.06.021 Dielforder, A., Vollstaedt, H., Vennemann, T., Berger, A., Herwegh, M., 2015. Linking megathrust earthquakes to brittle deformation in a fossil accretionary complex. Nature Communications, 6, 7504. https://doi.org/10.1038/ncomms8504 Doben, K. et al., 1991. Erläuterungen zum Blatt Nr. 8335 Lenggries. Geologische Karte von Bayern 1:25000. Bayerisches Geologisches Landesamt, München, 120 pp. Donofrio, D.A., Heissel, G., Mostler, H., 1980. Beiträge zur Kenntnis der Partnachschichten (Trias) des Tor- und Rontales und zum Problem der Abgrenzung der Lechtaldecke im Nordkarwendel. Mitteilungen der Österreichischen Geologischen Gesellschaft, 73, 55– 94. Eisbacher, G.H., Brandner, R., 1995. Role of high-angle faults during heteroaxial contraction, Inntal thrust sheet, Northern Calcareous Alps, western Austria. Geologisch- Paläontologische Mitteilungen Innsbruck, 20, 389–406. Eisbacher, G.H., Brandner, R., 1996. Superposed fold thrust structures and high angle faults, northwestern Calcareous Alps, Austria. Eclogae Geologicae Helvetiae, 89, 553–571. https://doi.org/10.5169/seals-167913 Eisbacher, G.H., Linzer, G.-H., Meier, L., 1990. A depth extrapolated structural transect across the Northern Calcareous Alps of Western Tirol. Eclogae Geologicae Helvetiae, 83/3, 711–725. https://doi.org/10.5169/seals-166610

39 Fabricius, F., 1966. Beckensedimentation und Riffbildung an der Wende Trias/Jura in den Bayerisch-Tiroler Kalkalpen. International Sedimentary Petrological Series, 9. Brill, Leiden, 143 pp. Faupl, P., Tollmann, A., 1979. Die Roßfeldschichten: Ein Beispiel für Sedimentation im Bereich einer tektonisch aktiven Tiefseerinne aus der kalkalpinen Unterkreide. Geologische Rundschau, 68/1, 93–120. https://doi.org/10.1007/bf01821124 Flügel, H., Fenninger, A., 1966. Die Lithogenese der Oberalmer Schichten und der mikritischen Plassenkalke (Tithonium, Nördliche Kalkalpen). Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen, 123/3, 249–280. Freudenberger, W., Schwerd, K., 1996. Erläuterungen zur geologischen Karte von Bayern 1:500 000. Bayerisches Geologisches Landesamt, München, 329 pp. Froitzheim, N., Manatschal, G., 1996. Kinematics of Jurassic rifting, mantle exhumation, and passive-margin formation in the Austroalpine and Penninic nappes (eastern Switzerland). Geological Society of America Bulletin, 108, 1120 –1133. https://doi.org/10.1130/0016-7606(1996)108<1120:KOJRME>2.3.CO;2 Froitzheim, N., Schmid, S., Conti, P., 1994. Repeated change from crustal shortening to orogen parallel extension in the Austroalpine units of Graubünden. Eclogae Geologicae Helvetiae, 87, 559–612. https://doi.org/10.5169/seals-167471 Froitzheim, N., Schmid, S.M., Frey, M., 1996. Mesozoic paleogeography and the timing of eclogite-facies metamorphism in the Alps: A working hypothesis. Eclogae Geologicae Helvetiae, 89, 81–110. https://doi.org/10.5169/seals-167895 Fruth, I., Scherreiks, R., 1982. Hauptdolomit (Norian), Stratigraphy, Paleogeography and Diagenesis. Sedimentary Geology, 32, 195–231. https://doi.org/10.1016/0037- 0738(82)90050-1 Gaupp, R., 1982. Sedimentationsgeschichte der kalkalpinen Mittelkreide (Allgäu, Tirol, Vorarlberg). Zitteliana, 8, 33–72. Haas, J., Kovács, S., Krystyn, L., Lein, R., 1995. Significance of Triassic facies zones in terrane reconstructions in the Alpine-North Pannonian domain. Tectonophysics, 242, 19–40. https://doi.org/10.1016/0040-1951(94)00157-5 Hahn, F.F., 1912. Versuch einer Gliederung der austroalpinen Masse westlich der österreichischen Traun. Verhandlungen der k. k. Geologischen Reichsanstalt, 1912, 337–344. Hahn, F.F., 1913. Grundzüge des Baues der nördlichen Kalkalpen zwischen Inn und Enns. Mitteilungen der österreichischen Geologischen Gesellschaft, 9, 374–500. Heißel, G., 1977. Die geologische Neuaufnahme des Karwendelgebirges und seine tektonische Ausdeutung. Doctoral Thesis, Universität Innsbruck, Innsbruck, 371 pp. Heißel, G., 1978. Karwendel - geologischer Bau und Versuch einer tektonischen Rückformung. Geologisch-Paläontologische Mitteilungen Innsbruck, 8, 227–288. Heißel, W., 1958. Zur Tektonik der Nordtiroler Kalkalpen. Mitteilungen der Österreichischen Geologischen Gesellschaft, 50, 95–132. Höfle, H.-C. et al., 1969. Erläuterungen zum Blatt Nr. 8331 Baiersoien. Geologische Karte von Bayern 1:25000. Bayerisches Geologisches Landesamt, München, 122 pp. Hudec, M.R., Jackson, M.P.A., 2007. Terra infirma: Understanding salt tectonics. Earth- Science Reviews, 82/1, 1–28. https://doi.org/10.1016/j.earscirev.2007.01.001 Jackson, M.P.A., Hudec, M.R., 2017. Salt Tectonics - Principles and Practice. Cambridge University Press, Cambridge, 510 pp. https://doi.org/10.1017/9781139003988 Jacobshagen, V., 1965. Die Allgäuschichten (Jura-Fleckenmergel) zwischen Wettersteingebirge und Rhein. Jahrbuch der Geologischen Bundesanstalt, 108, 1–114. Jeanbourquin, P., 1994. Early deformation of Ultrahelvetic mélanges in the Helvetic nappes (Western Swiss Alps). Journal of Structural Geology, 16, 1367–1383. https://doi.org/10.1016/0191-8141(94)90003-5

40 Jerz, H., 1966. Untersuchungen über Stoffbestand, Bildungsbedingungen und Paläogeographie der Raibler Schichten zwischen Lech und Inn (Nördl. Kalkalpen). Geologica Bavarica, 56, 3–100. Kusky, T.M., Bradley, D.C., 1999. Kinematic analysis of mélange fabrics: examples and applications from the McHugh Complex, Kenai Peninsula, Alaska. Journal of Structural Geology, 21, 1773–1796. https://doi.org/10.1016/S0191-8141(99)00105-4 Lein, R., 1987. Evolution of the Northern Calcareous Alps during Triassic times. In: Flügel, H.W., Faupl, P. (Eds.), Geodynamics of the Eastern Alps. Deuticke, Wien, 85–102. Leitner, C., Spötl, C., 2017. Chapter 21 - The Eastern Alps: Multistage Development of Extremely Deformed Evaporites. In: Soto, J.I., Flinch, J.F., Tari, G. (Eds.), Permo- Triassic Salt Provinces of Europe, North Africa and the Atlantic Margins. Elsevier, 467– 482. Loesch, K.C.v., 1915. Der Schollenbau im Wetterstein- und Mieminger- Gebirge. Jahrbuch der k. k. Geologischen Reichsanstalt, 64 (1914), 1–98. Mandl, G., 2000. The Alpine sector of the Tethyan shelf - examples for Triassic to Jurassic sedimentation and deformation from the Northern Calcareous Alps. Mitteilungen der Österreichischen Geologischen Gesellschaft, 92, 61–77. Mandl, G.W., Brandner, R., Gruber, A., 2017. Zur Abgrenzung und Definition der Kalkalpinen Deckensysteme (Ostalpen, Österreich). In: Wimmer-Frey, I., Römer, A., Janda, C. (Eds.), Arbeitstagung 2017 – Angewandte Geowissenschaften an der GBA. Geologische Bundesanstalt, Wien, 254–255. Moser, M., 2008. Provisorische Geologische Karte von Österreich, Blatt 118 - Innsbruck, GeoFAST 1:50.000. Geologische Bundesanstalt, Wien. Müller-Jungbluth, W.-U., 1971. Sedimentologische Untersuchungen des Hauptdolomits der östlichen Lechtaler Alpen, Tirol. In: Mostler, H. (Ed.), Beiträge zur Mikrofazies und Stratigraphie von Tirol und Vorarlberg, Festband des Geol. Inst., 300-Jahr-Feier Univ. Innsbruck, Innsbruck, 255–308. Mylius, H., 1914. Berge von scheinbar ortsfremder Herkunft in den bayrischen Alpen. Landeskundliche Forschungen, 22, 1–44. Nagel, K.H., Schütz, K.I., Schütz, S., Wilmers, W., Zeil, W., 1976. Die geodynamische Entwicklung der Thiersee- und Karwendelmulde (Nördliche Kalkalpen). Geologische Rundschau, 65, 536–557. https://doi.org/10.1007/BF01808480 Needham, D.T., 1995. Mechanisms of mélange formation: examples from SW Japan and southern Scotland. Journal of Structural Geology, 17, 371–388. https://doi.org/10.1016/0191-8141(94)00132-J Nittel, P., 2006. Beiträge zur Stratigraphie und Mikropaläontologie der Mitteltrias der Innsbrucker Nordkette (Nördliche Kalkalpen, Austria). Geo.Alp, 3, 93–145. Ortner, H., 2001. Growing folds and sedimentation of the Gosau Group, Muttekopf, Northern Calcareous Alps, Austria. Int. J. Earth Sci. (Geol. Rundsch.), 90, 727–739. https://doi.org/10.1007/s005310000182 Ortner, H., 2003a. Cretaceous thrusting in the western part of the Northern Calcareous Alps (Austria) - evidences from synorogenic sedimentation and structural data. Mitteilungen der Österreichischen Geologischen Gesellschaft, 94, 63–77. Ortner, H., 2003b. Local and far field stress – analysis of brittle deformation in the western part of the Northern Calcareous Alps, Austria. Geologisch-Paläontologische Mitteilungen Innsbruck, 26, 109–131. Ortner, H., 2016. Field Trip 4: Deep water sedimentation on top of a growing orogenic wedge - interaction of thrusting, erosion and deposition in the Cretaceous Northern Calcareous Alps. Geo.Alp, 13, 141–182. Ortner, H., Bitterlich, L., 2016. The Zugsitze cross section and the structure of the Northern Calcareous Alps. In: Ortner, H. (Ed.), Abstract Volume of GeoTirol2016 - Annual

41 Meeting of DGGV and PANGEO Austria. Institute of Geology, University of Innsbruck, Innsbruck, 248. Ortner, H., Gaupp, R., 2007. Synorogenic sediments of the western Northern Calcareous Alps. Geo.Alp, 4, 133–148. Ortner, H., Kilian, S., 2016. Sediment creep on slopes in pelagic limestones: Upper Jurassic of Northern Calcareous Alps, Austria. Sedimentary Geology, 350–363. https://doi.org/10.1016/j.sedgeo.2016.03.013 Ortner, H., Kositz, A., Willingshofer, E., Sokoutis, D., 2016. Geometry of growth strata in a transpressive fold belt in field and analogue model: Gosau Group at Muttekopf, Northern Calcareous Alps, Austria. Basin Research, 28/6, 731–751. https://doi.org/10.1111/bre.12129 Ortner, H., Reiter, F., Acs, P., 2002. Easy handling of tectonic data: the programs TectonicVB for Mac and TectonicsFP for Windows(TM). Computers & Geosciences, 28, 1193– 1200. https://doi.org/10.1016/S0098-3004(02)00038-9 Ortner, H., Reiter, F., Brandner, R., 2006. Kinematics of the Inntal shear zone–sub-Tauern ramp fault system and the interpretation of the TRANSALP seismic section, Eastern Alps, Austria. Tectonophysics, 414, 241–258. https://doi.org/10.1016/j.tecto.2005.10.017 Ortner, H., Stingl, V., 2001. Facies and Basin Development of the Oligocene in the Lower Inn Valley, Tyrol/Bavaria. In: Piller, W., Rasser, M. (Eds.), Paleogene in Austria. Schriftenreihe der Erdwissenschaftlichen Kommissionen, 14. Österreichische Akademie der Wissenschaften, Wien, 153–196. Oswald, P., Ortner, H., Gruber, A., 2018. Deformation around a detached half-graben shoulder during nappe stacking (Northern Calcareous Alps, Austria). Swiss Journal of Geosciences. https://doi.org/10.1007/s00015-018-0333-4 Peresson, H., Decker, K., 1997. The Tertiary dynamics of the northern Eastern Alps (Austria): changing palaeostresses in a collisional plate boundary. Tectonophysics, 272, 125–157. https://doi.org/10.1016/S0040-1951(96)00255-7 Pestal, G., Hejl, E., Braunstingl, R., Schuster, R., 2009. Geologische Karte von Salzburg 1:200 000: Erläuterungen. Geologische Bundesanstalt, Wien, 162 pp. Ramsay, J.G., 1960. The deformation of early linear structures in areas of repeated folding. The Journal of Geology, 68/1, 75–93. Reiter, F. et al., 2018. Active seismotectonic deformation in front of the Dolomites indenter, Eastern Alps. Tectonics, 37/12, 4625–4654. https://doi.org/10.1029/2017TC004867 Richter, M., 1928. Ein neues Vorkommen von Diabasen im Karwendel. Verhandlungen der Geologischen Bundesanstalt, 1928, 117–120. Richter, M., 1929. Die Struktur der nördlichen Kalkalpen zwischen Rhein und Inn. Neues Jahrbuch für Mineralogie, Geologie und Paläontologie, Beilage-Band, 63, Abteilung B, 1–62. Ring, U., Ratschbacher, L., Frisch, W., 1988. Plate Boundary Kinematics in the Alps: Motion in the Arosa Suture Zone. Geology, 16, 696–698. https://doi.org/10.1130/0091- 7613(1988)016<0696:PBKITA>2.3.CO;2 Ring, U., Ratschbacher, L., Frisch, W., Dürr, S., Borchert, S., 1990. The internal structure of the Arosa Zone. Geologische Rundschau, 79, 725–739. https://doi.org/10.1007/BF01879211 Rüffer, T., Bechstädt, T., 1995. Interpretation des Deckenbaus in den westlichen nördlichen Kalkalpen: Widerspruch zwischen tektonischen und sedimentologischen Daten. Jahrbuch der Geologischen Bundesanstalt, 138, 701–713. Rüffer, T., Zamperelli, V., 1997. Facies and Biota of Anisian to Carnian Carbonate Platforms in the Northern Calcareous Alps (Tyrol and Bavaria). Facies, 37, 115–136. https://doi.org/10.1007/BF02537374

42 Sarnthein, M., 1966. Sedimentologische Profilreihen aus den mitteltriadischen Karbonatgesteinen der Kalkalpen nördlich und südlich von Innsbruck. 1. Fortsetzung. Berichte des naturwissenschaftlich-medizinischen Vereins Innsbruck, 54, 33–59. Sarnthein, M., 1967. Versuch einer Rekonstruktion der mitteltriadischen Paläogeographie um Innsbruck. Österreich. Geologische Rundschau, 56/1, 116–127. https://doi.org/10.1007/bf01848710 Schlager, W., Schöllnberger, W., 1974. Das Prinzip stratigraphischer Wenden in der Schichtfolge der Nördlichen Kalkalpen. Mitteilungen der Österreichischen Geologischen Gesellschaft, 66/67, 165–193. Schlagintweit, O., 1912a. Die Mieminger-Wetterstein Überschiebung. Geologische Rundschau, 3/2, 73–92. Schlagintweit, O., 1912b. Zum Problem des Wettersteingebirges. Verhandlungen der k. k. Geologischen Reichsanstalt, 1912, 313–327. Schmid, S.M., Fügenschuh, B., Kissling, E., Schuster, R., 2004. Tectonic map and overall architecture of the Alpine orogen. Eclogae Geologicae Helvetiae, 97, 93–117. https://doi.org/10.1007/s00015-004-1113-x Schmidegg, O., 1950. Die Stellung der Haller Salzlagerstätte im Bau des Karwendelgebirges. Jahrbuch der Geologischen Bundesanstalt, 94, 159–205. Schmidt, S., Hetzel, R., Mingorance, F., Ramos, V.A., 2011. Coseismic displacements and Holocene slip rates for two active thrust faults at the mountain front of the Andean Precordillera (∼33°S). Tectonics, 30/5. https://doi.org/10.1029/2011TC002932 Spang, J.H., 1972. Numerical method for dynamic analysis of calcite twin lamellae. Geological Society of America Bulletin, 83/1, 467–472. https://doi.org/10.1130/0016- 7606(1972)83[467:NMFDAO]2.0.CO;2 Spötl, C., 1988. Sedimentologisch-fazielle Analyse tektonisierter Evaporitserien – eine Fallstudie des Alpinen Haselgebirges (Permoskyth, Nördliche Kalkalpen). Geologisch- Paläontologische Mitteilungen Innsbruck, 15, 59–69. Stampfli, G.M. et al., 1998. Subduction and obduction processes in the Swiss Alps. Tectonophysics, 296, 159–204. https://doi.org/10.1016/S0040-1951(98)00142-5 Steiger, T., 1981. Kalkturbidite im Oberjura der Nördlichen Kalkalpen (Barmsteinkalke, Salzburg, Österreich). Facies, 4, 215–348. Stüwe, K., Schuster, R., 2010. Initiation of subduction in the Alps: Continent or ocean? Geology, 38, 175–178. https://doi.org/10.1130/G30528.1 Suppe, J., 1983. Geometry and kinematics of fault-bend folding. American Journal of Science, 283, 684–721. https://doi.org/10.2475/ajs.283.7.684 Tanner, D.C., Behrmann, J.H., Dresmann, H., 2003. Three-dimensional retrodeformation of the Lechtal nappe, Northern Calcareous Alps. Journal of Structural Geology, 25, 737–748. https://doi.org/10.1016/S0191-8141(02)00057-3 Tobin, H.J., Moore, J.C., Moore, G.F., 1994. Fluid pressure in the frontal thrust of the Oregon accretionary prism: Experimental constraints. Geology, 22/11, 979–982. https://doi.org/10.1130/0091-7613(1994)022<0979:FPITFT>2.3.CO;2 Tollmann, A., 1970. Tektonische Karte der Nördlichen Kalkalpen, 3. Teil: Der Westabschnitt. Mitteilungen der Österreichischen Geologischen Gesellschaft, 62 (1969), 78–170. Tollmann, A., 1973. Grundprinzipien der alpinen Deckentektonik. Monographie der Nördlichen Kalkalpen, Teil I. Deuticke, Wien, 404 pp. Tollmann, A., 1976a. Analyse des klassischen nordalpinen Mesozoikums. Monographie der Nördlichen Kalkalpen, Teil II. Deuticke, Wien, 580 pp. Tollmann, A., 1976b. Der Bau der Nördlichen Kalkalpen. Monographie der Nördlichen Kalkalpen, Teil III. Deuticke, Wien, 449 pp. Trommsdorff, V., Dietrich, V., Flisch, M., Stille, P., Ulmer, P., 1990. Mid - Cretaceous, primitive alkaline magmatism in the Northern Calcareous Alps: significance for

43 Austroalpine geodynamics. Geologische Rundschau, 79/1, 85–97. https://doi.org/10.1007/BF01830448 Vergés, J. et al., 2007. Crustal wedging triggering recent deformation in the Andean thrust front between 31°S and 33°S: Sierras Pampeanas-Precordillera interaction. Journal of Geophysical Research, 112(B03), B03S15. https://doi.org/10.1029/2006JB004287 Wilmers, W., 1971. Geologie der Mulde von Thiersee zwischen Landl und Kufstein in Tirol (Nördliche Kalkalpen). Doctoral Thesis, TU Berlin, Berlin, 75 pp.

9 Appendix

Fig. 14: Geologic map of the study area. Graticule: MGI Austria GK West.

44 2.2 Study 2

A new tectonic subdivision in the Northern Calcareous Alps of western Austria and southern Germany resolves a 100 year old controversy Ortner Hugo and Sinah Kilian Submitted to International Journal of Earth Sciences, May 2020

46 A new tectonic subdivision in the Northern Calcareous Alps of western Austria and southern Germany resolves a 100 year old controversy

Hugo Ortner*, Sinah Kilian Institut für Geologie, Universität Innsbruck, Innrain 52, 6020 Innsbruck * corresponding author, [email protected] ORCID Hugo Ortner 0000-0001-6909-6627

Abstract We reinvestigate tectonic subdivision of the western Northern Calcareous Alps of the Eastern European Alps in the Wetterstein and Mieming mountains, where this subdivision was controversial since it was introduced. Using key outcrops, we demonstrate, that the Triassic rocks of the Wetterstein and Mieming mountains were initially emplaced onto Lower Cretaceous rocks toward the NW. Latest Cretaceous to Paleogene NNE-directed transport took place after the decollement shifted to a deeper position within the footwall of the original thrust, possibly after salt moved out of the initial thrust boundary. This shortening created out-of- sequence foreland-directed and backthrusts cutting across the original thrust that end laterally and were localized by major Albian strike-slip faults, that offset the original thrust boundary. The out-of-sequence thrusts locally exhume the original old-on-young thrust and emplace young-on-old rocks.

The revised tectonic subdivision is strictly based on old-on-young stacking, while the existing subdivision delimits thrust sheets along out-of-sequence thrusts. We need only two thrust sheets in this part of the Calcareous Alps: (1) the tectonically deeper Tannheim thrust sheet, that includes the former Allgäu and part of the former Lechtal thrust sheet, and (2) the tectonically higher Karwendel thrust sheet that merges the former Inntal and Lechtal sheets, that sit on the same decollement. West of the study area, the Imst half klippe is stacked by an out-of-sequence thrust onto the Karwendel thrust sheet, which is, in its southeastern part, in lateral contact with the latter across a tear fault.

Keywords Tectonic subdivision, Thrust kinematics, Out-of-sequence thrusts, Eastern Alps

Funding (information that explains whether and by whom the research was supported) Part of the work done by SK for this study was supported by Land Tirol (Tiroler Wissenschaftsfond) and the Doctoral programme of the University of Innsbruck.

47 1. Introduction Tectonic subdivisions of larger geologic units reflect the geologic knowledge at the time of creation. The tectonic subdivision of the Northern Calcareous Alps (NCA) thrust belt of the Eastern Alps was originally defined at the end of the 19th century and was based on the first geologic mapping campaign in the Austro-Hungarian empire, and of the Bavarian geologic survey (Ampferer and Hammer 1911; Ampferer 1912; Hahn 1912, 1913). One of the key problems that these early mappers encountered was the ambiguity of the structural situation in the area of the Wetterstein and Mieming mountains in Bavaria and the Tyrol. At that time, the first large-scale thrusts had already been recognized in the Alps, based on spatial distribution of rock types, and old-on-young relationships (Richthofen 1859, plate III, section XII; Ampferer 1902; Rothpletz 1902; Termier 1904), however, textbooks were not yet available, and an understanding of thrust dynamics was not yet developed. As a consequence, a long lasting controversy developed (see below and Schneider 1962).

The western NCA have been subdivided into three major tectonic units: These are, from base to top, the (1) Allgäu thrust sheet, forming a narrow band at the northern margin of the NCA, except in the westernmost Tyrol and the Allgäu, the (2) Lechtal thrust, representing the main body of the western NCA, and the (3) Inntal thrust sheet in the south-central part of the NCA of the Tyrol (Heißel 1958; Tollmann 1970b, 1976). In terms of structure, the Allgäu and Inntal thrust sheets have been interpreted as a frontal and trailing imbricate of the Lechtal thrust sheet, respectively (Tollmann 1976).

1.1 Sedimentary succession The Permo-Triassic sedimentary succession building the NCA thrust sheets was deposited on the SE passive margin of Pangea, facing the Neotethys ocean (e.g., Lein 1987; Haas et al. 1995). The succession starts with Permian continental clastics (Stingl 1982, 1984), interfingering with salt-bearing evaporites (Spötl 1988, 1989; Leitner and Spötl 2017), followed by Lower Triassic evaporites and dolomites of the Reichenhall Formation (e.g., Schenk 1967). Haselgebirge and Reichenhall Fm. form the basal decollemént of the NCA (Linzer et al. 1995; Eisbacher and Brandner 1996). Well bedded limestones of the Virgloria and Steinalm formations are the first basinal and carbonate ramp deposits of the area (Bechstädt and Mostler 1976), and are grouped together with the Reifling Formation into the Alpine Muschelkalk Group (Bechstädt and Mostler 1974). Breakup of the Steinalm ramp caused facies heteropy in the Middle Triassic, when the basinal Reifling Formation interfingered with the the Wetterstein limestone (Bechstädt and Mostler 1976), the latter representing the first major carbonate platform of the 48 NCA, more than 1400 m thick at Zugspitze (Ortner 2015) and in the Mieming mountains (Miller 1965). In the Carnian, the younger part of the Wetterstein platform interfingers with the marly Partnach beds. Later in the Carnian, the mixed carbonatic-siliziclastic succession of the Raibl beds ends growth of the Wetterstein platform, and cover the platforms and interplatform basins (Brandner 1984). A new carbonate platform develops in the Norian, and the more than 1000 m thick Hauptdolomit is deposited (Fruth and Scherreiks 1982, 1984). Toward the end of the Norian, this platform starts to break apart, and the basins of the Kössen Formation developed in the Rhätian, that interfinger with another platform, the Upper Rhätian limestone (Golebiowski 1991).

The Jurassic sedimentary succession of the NCA is controlled by rifting and opening of the Penninic ocean, that separated the African continent and the Adriatic microplate from Pangea (Froitzheim and Manatschal 1996; Manatschal 2004). The syn-rift succession in this part of the NCA includes red nodular limestones of the Adnet formation, that accumulated on deep, submarine highs, and are meters to tens of meters thick, and the basinal Allgäu-formation with spiculitic, siliceous limestones, and occasional breccias and olistolithes (Jacobshagen 1965; Bischof et al. 2010), with thicknesses of hundreds of meters. After breakup the Late Jurassic, the Ruhpolding radiolarite, and the pelagic limestones of the Ammergau formation accumulated, the latter reaching into the lower Cretaceous (Miller 1963c).

Synorogenic sedimentation started diachronously in the Early Cretaceous, with deposition of the marly Schrambach Formation, that contains quartz-bearing calcarenites in its youngest part. According to Vidal (1953) and Miller (1963c) it reaches at least into the Hauterivian. No younger deposits are known in the study area, but basanitic volcanic dykes and sills are found within the Jurassic-Cretaceous units between the Wetterstein and Mieming mountains, that have been given the local name Ehrwaldites (Pichler 1866). These volcanites have been radiometrically dated to 100 Ma (Albian-Cenomanian boundary; Trommsdorff et al. 1990).

1.2 Structural evolution of the NCA Nappe stacking in the NCA is closely related to syntectonic sedimenation, therefore these synorogenic deposits will be discussed together with the relevant tectonic events. The initial inversion of this continental margin took place during Eoalpine Cretaceous intracontinental subduction within the Adriatic plate (Janák et al. 2004; Stüwe and Schuster 2010). Eoalpine shortening caused NW-directed stacking of thrust sheets in the NCA, that travelled tens of kilometers onto the units in their footwall (Eisbacher et al. 1990; Linzer et al. 1995; Eisbacher 49 and Brandner 1996). The age of syntectonic sediments below a thrust sheet allow to define the maximum age of emplacement, while synorogenic clastics unconformably overlying the exhumed thrust sheet give a minimum age (upper-footwall deposits and thrust-sheet-top deposits of Ortner 2003a; Ortner and Gaupp 2007; Ortner 2016; see Fig. 1). The ages of these deposits suggest thrust sheet emplacement between the Hauterivian and Turonian: Based on the ages of upper footwall deposits, the northern Lechtal thrust sheet was thrust onto the Allgäu thrust sheet at the end of the Albian. The western Inntal thrust sheet did override the Lechtal thrust sheet out-of-sequence after the Cenomanian (Ortner 2003a) while the eastern part of the Inntal thrust sheet rests on Hauterivian sediments (see above) on top of an in-sequence thrust. The contact between the Allgäu nappe und the underlying marginal slice of the Cenoman- Randschuppe is Turonian or younger and documents foreland-propagation of thrust activity.

Thrust sheet emplacement was generally not accompanied by major folding, however during the Late Cretaceous, when the NCA travelled across the tectonically deeper Penninic units, the major thrusts were folded (the "Deckenfaltung" = refolding of nappes of Kockel 1926), which is recorded in the growth strata of the thrust-sheet-top deposits (Branderfleck Formation and Gosau Group; Ortner 2001, 2007; Ortner and Gaupp 2007; Ortner et al. 2016; Sieberer 2020). In the Late Cretaceous, subduction of the Penninic ocean had begun (Frisch 1979; Winkler 1988; Froitzheim et al. 1996; Eynatten and Gaupp 1999). This shortening persisted into the Paleogene and progressively changed toward a N- and NNE-direction in the Paleogene (Mesoalpine shortening; Eisbacher and Brandner 1996). During the Miocene, renewed NNE- to NE-directed Neoalpine shortening affected the NCA, that was largely related to activity of strike-slip faults (Decker et al. 1994; Ortner 2003b), and contemporanous to transport of the Alpine wedge into the Cenozoic foreland basin on the European margin (Ortner et al. 2015; Schuller et al. 2015).

Both the Wetterstein and Mieming mountains are built by the km-thick limestones of the Wetterstein limestone and the underlying Muschelkalk Group, often floored by the evaporitic Reichenhall Formation. These Triassic rocks are emplaced onto Cretaceous marls of the Schrambach Formation (Fig. 2). The abovementioned controversy centered on the question, whether the Wetterstein mountains are part of the Lechtal or Inntal thrust sheet. One group argued, that there is a continuous sedimentary succession connecting the northeastern foothills of the Wetterstein mountains, which is clearly part of the Lechtal thrust sheet, with the Wetterstein mountains (Ampferer 1912, 1914; Leuchs 1924; Ampferer 1931; Leuchs 1935;

50 Ampferer 1942; Beurlen 1944; Vidal 1953; Bögel 1960; Ferreiro Mählmann and Morlok 1992). If such an interpretation is adopted, major south-directed backthrusting is required (Zugspitze thrust of Mylius 1916; Miller 1963a, b; Eisbacher and Brandner 1995). Another group connected the thrusts of the Mieming and Wetterstein mountains but then faced the problem of separating the thrusted units from the Lechtal thrust sheet (Schlagintweit 1912b, a; Richter 1929; Haber 1934; Kraus 1957; Rüffer and Bechstädt 1995).

Fig. 1: Tectonic map of the Northern Calcareous Alps following Heißel (1958); Tollmann (1970a, 1976). Synorogenic deposits are distinguished into upper-footwall- and thrust sheet-top deposits (Ortner 2003a; Ortner and Gaupp 2007; Ortner 2016) that allow to reconstruct the timing of thrust sheet emplacement (see text). KM = Karwendel mountains, MM = Mieming mountains and WM = Wetterstein mountains.

52 1.3 Kinematics of thrusts At the time when the tectonic structure of the NCA was controversially discussed, thrust kinematics were not yet properly understood. It was only from the 1970ies onward that some key papers on the geometry and kinematics of thrusting appeared (e.g., Boyer and Elliott 1982; Suppe 1983; Suppe and Medwedeff 1990) and applied to the major thrusts of the NCA (Eisbacher et al. 1990). From the very beginning, it was known that thrusts have lateral terminations and grow laterally during foreland propagation (Elliott 1976). In the NCA, however, all authors tried to define thrust sheets that are surrounded by a throughgoing thrust on all sides, a state that a thrust sheet can only aquire in an advanced state of its development, after significant transport and erosion that isolates klippen of a formerly continuous thrust sheet. Geometry might be complex as thrust boundaries get folded as new thrusts form in the footwall of an existing thrust during in-sequence thrust propagation. Folded thrust are likely places where out-of-sequence thrusts form when shortening persists (e.g., Gilluly 1960; Boyer 1992).

Based on a structural analysis of macro- and mesostructures in the Mieming- and Wetterstein mountains, we demonstrate here that the problems separating the thrust sheets of the NCA properly originate from failure to recognize late out-of-sequence thrusts, that terminate laterally. We show that the western NCA structure needs only two thrust sheets, and show how out-of-sequence thrusts separate parts of thrust sheets, that were emplaced at a late stage of thrust transport (cf., Kilian and Ortner 2019). Finally, we present and discuss this new tectonic subdivision of the western NCA.

2. Results Here we present the results of our structural investigation in the Mieming and Wetterstein mountains, based on the relative succession of structures based on cross-cutting relationships.

2.1 Western and southern flank of the Wetterstein and Mieming mountains The most prominent structure in the investigated area is a thrust that emplaces Triassic carbonates onto marls of the Cretaceous synorogenic Schrambach Formation. On outcrop scale, this thrust is parallel to bedding in the footwall, but oblique to bedding in the hanging wall. The thrust is associated with a shear zone with a thickness of several tens of meters, reaching downsection to the Kössen Formation, but not affecting the underlying competent Hauptdolomit. The main thrust is not continuously exposed, but well visible along the complete length of the western face of the Zugspitze (Zugspitze thrust; Fig. 2). Toward the E, numerous outcrops below the southern cliffs of the Wetterstein mountains (Leuchs 1930) indicate the 53 regional nature of this contact. It is well exposed at Marienbergjoch (9 of Fig. 2; van Kooten 2016) at the SW edge of the Mieming mountains. Isolated outcrops of the Jurassic-Cretaceous succession directly below the cliffs of the Seebenwände (10 of Fig. 2) at the northwestern side of the Mieming mountains, and in mining galleries reaching from Schachtkopf under the western Mieming mountains (Schachtkopf mine of Fig. 2; Ampferer and Ohnesorge 1924; Mutschlechner 1955) demonstrate the continuity of the Mieming thrust at the base of the Mieming mountains. The contact is seen to cut upsection continuously from the Reichenhall Formation in the South (above 9 of Fig. 2) to the Raibl beds in the North (IV of Fig. 2) at a rather constant angle of ca. 10° in the Mieming mountains. In the Wetterstein mountains, the thrust cuts from the Wetterstein limestone downsection into the Muschelkalk Group toward the North, and also to the East, giving the impression of a syncline truncated at the base with the maximum stratigraphic reduction above 3 of Fig. 2.

The sedimentary succession in the footwall of the thrust below the western wall of the Zugspitze and the Mieming mountains is exposed from the Schrambach Formation down to the Kössen Formation (Fig. 2). At the SW-corner of the Zugspitze, the succession is deformed into the hectometric, overturned, SSW-facing Holzerwies anticline (3 of Fig. 2). Adjacent to the south, the symmetric, tight to isoclinal Koppenwald syncline with a vertical axial plane (4 of Fig. 2) follows.

55 Fig. 2: (a) View of the Wetterstein and Mieming mountains from the west, and (b) geologic interpretation. Stereograms 1 – 8: analysis of bedding orientations in lower hemisphere stereographic projections and calculated fold axes. The respective white numbers in (b) indicate, where the data were collected. Stereograms I – V: mesofaults measured at major fault contacts in lower hemisphere stereographic projections, and mean transport direction ( 1) calculated from the fault data set using the NDA method (Spang 1972) as implemented by Ortner et al. (2002). Note that data of V were collected at the eastern side of the mountain, behind the ridge shown. Data in diagram III from Kreidl (2015). All stereograms in this and the following figures were created using the TectonicVB software (Ortner et al. 2002). Number 9-11 indicate locations mentioned in the text. VB = Vorbergzug.

S-C structures, dm-scale vergent folds and brittle faults within the shear zone below the main thrust consistently show NW- to NE-directed transport of the hanging wall (Fig. 3). In an outcrop below the Zugspitze south face (Fig. 4; see Fig. 2 for location), dm-scale folds (Fig. 4c, g, f), s-c-fabrics (Fig. 4, 2 of f) and the relationship between the main thrust and foliation in the underlying incompetent units (Fig. 4 g, e) indicate NW-directed transport at the Zugspitze thrust.

The thrust between the Triassic carbonates and the Schrambach Formation is lined by a discontinuous band of rock, showing a thin or thinned, intensly deformed, completely overturned Jurassic-Cretaceous succession (Figs. 4, 6; compare Ferreiro Mählmann and Morlok 1992). This succession is folded on the dm-scale, but the competent band of the Radiolarite and Ammergau Formation does also show round symmetric folds on the dekametric scale, with small cusps of the incompetent Schrambach Formation (Fig. 6b). The axial planes of these folds are perpendicular to the thrust contact (compare Kilian and Ortner 2019).

56 ss

Fig. 3: Tectonic map of the Mieming, Wetterstein and eastern Karwendel mountains. See Fig. 1 for location. (a) Tectonic subdivision of the study area following Heißel (1958); Tollmann (1970a, 1976). Trace of axial planes of major fold indicated, and numbered 1 – 15 from S to N: 1 = Höllkopf syncline, 2 = Gleierschtal syncline, 3 = Mieming - Bettelwurf anticline, 4 = Gaistal – Hinterautal syncline, 5 = Lermoos syncline, 6 = Ehrwald anticline, 7 = Reintal syncline, 8 = Zugspitze anticline, 9 = Wetterstein – Karwendel syncline, 10 = Heiterwangersee anticline, 11 = Wamberg anticline, 12 = Katzenstein syncline, 13 = Loisachtal anticline, 14 = Plansee syncline, 15 = Krottenkopf syncline. Names largely following Tollmann (1976). (b) New tectonic subdivision. OOS = out-of-sequence. Tectonic transport directions of hanging wall of thrusts documented in Figs. 2 and 4, additional data south of Ehrwald taken from van Kooten (2016), east of Ehrwald from Zambanini (2014). Major faults: H = Höll shear zone, O = Obermoos thrusts, T = Telfs fault, Z = Zugspitze thrust. KM = Karwendel mountains, MM = Mieming mountains and WM = Wetterstein mountains. 57

Fig. 4: Key outcrop of the thrust emplacing the Triassic onto the Lower Cretaceous Schrambach Formation at the base of the cliffs at the south face of the Wetterstein mountains at Angerbach. (a) Overview of the outcrop giving the location of subfigures b, c and d. (b) Detail of the contact. An overturned slice with a stratigraphic succession with red limestones, Ruhpolding radiolarite and Ammergau Formation is observed below the Wetterstein limestone on top of the Schrambach Formation. Thickness of the succession is reduced as a result of pervasive shear band deformation. (c) Detail of the contact with isoclinal folding of the Schrambach Formation into the Wetterstein limestone. (d) Detail of the contact. Between the Wetterstein limestone and Schrambach Formation, a upright slice with red limestones of the Ammergau Formation and Schrambach Formation is intercalated, that is isoclinally folded and sheared into the Wetterstein limestone. The overturned contact is refolded about fold axes subparallel to the outcrop surface, 58 causing the complex boundary (label 1 of d). (e) Detail of d. A foliation within the Schrambach Formation is oblique to the contact against the Wetterstein limestone. Limbs of dm-scale isoclinal folds are parallel to the foliation. The intersection of bedding and foliation is subparallel to the majority of fold axes (see diagram g). (f) Detail of d. The Ammergau and Schrambach Formations are pervasively deformed into s-c-fabrics that are refolded (label 2 of f). Fold axes of c and f trend NE-SW, and verge to the NW (see diagram g). Note flame-like structures where the Schrambach Formation protrudes into the Wetterstein limestone in b, d and e.

2.2 The zone of Upper Triassic to Lower Cretaceous rocks S of the Wetterstein mountains (Puitental zone) The roughly E-W trending strip of Late Triassic to Cretaceous rocks S of the cliffs of the Wetterstein mountains has been named the Puitental zone (Fig. 3a) by Tollmann (1976). These cliffs are fault-controlled (Ampferer 1912; Leuchs 1930). Three sets of strike-slip faults can be observed: (1) a set of sinistral, E-trending faults, (2), a set of dextral, WNW-trending faults and (3) a set of sinistral, NE-trending faults. All of them offset the Zugspitze thrust in the area, and each other in the sequence described. Figs. 5 and 6c give an example: The thrust is vertically uplifted on the southern side of a sinistral strike-slip fault in (a), the sinistral kinematics indicated by s-c fabrics (c) within Lower Cretaceous marls (Schrambach Fm.). The shear zone is intruded by Ehrwaldite dykes (b), that are found both along the s- and c-planes of the shear zone. Most probably the shear zone existed already, and provided the anisotropy used by the dykes, but the dykes are also deformed by the shear zone. Otherwise we would expect to find Ehrwaldite dykes also in a oblique orientation with respect to the shear zone, being sheared only locally.

Fig. 5: (a) View to the East of the key outcrop of the Zugspitze thrust at Angerbach. The Zugspitze thrust is offset by a E-striking sinistral fault, that deforms the Schrambach Formation pervasively (b, c). (b) Ehrwaldite dyke (label 1) within the Schrambach Formation. The dyke intruded both the s- and the c-planes of the shear zone, suggesting that the shear zone already existed. (c) S-C-fabrics having shearband geometry within the Schrambach Formation seen from above. Sinistral offset is observed at the c-planes (diagram d),

59 and the distribution of B- and T-axes along a girdle indicate transpressive sinistral shearing (diagram e; compare Meschede 1994).

The structure of the Puitental zone is relatively cylindric. A broad zone of Lower Cretaceous marls is found in contact with the Middle Triassic rocks of the Wetterstein mountains across a steep fault (Fig. 5a) or below the Zugspitze thrust (Figs. 2 and 4). To the south, the Upper Jurassic to Upper Triassic (Ammergau Fm. to Kössen Fm.) sedimentary units are tightly folded in isoclinal folds with vertical axial plane, tens of meters wavelength, but hundrets of meters of height (Fig. 6a). The contact to Triassic rocks in the south is again subvertical to N-dipping, and parallel to the fold axial planes. As the oldest sediment in this fold train are Upper Triassic marls of the Kössen Fm., it was most probably detached within this unit, and separated from the underlying Upper Triassic Hauptdolomit-Plattenkalk (Fig. 7a). During shortening, the incompetent Puitental zone rocks were backthrusted and buttressed against the more competent Triassic rocks in the south.

Fig. 6: (a) Geologic map of the Puitental zone near Hoher Kamm, illustrating the deformation style at the southern margin of the Wetterstein massif (partly from Hildebrandt 2016). See Fig. 3a for location. Background hillshade © Land Tirol. (b) View to the west of the boundary between the Wetterstein limestone and the Schrambach Formation at Gatterl across a steep fault plane. Right below the contact, a thin, overturned slice with a succession of the Allgäu Formation, Ruhpolding radiolarite and Ammergau Formation is present (compare Fig. 4). Note the cuspate-lobate geometry of the contact with the Schrambach Formation. (c) View to the east to Kleinwanner. The overturned slice is seen right at the contact between Wetterstein limestone and Schrambach Formation in this perspective. Bedding within the Wetterstein limestone is folded in an anticline, that is offset across vertical faults with sinistral offset together with the thrust contact. Abbreviations: am = Ammergau Fm., rr = Ruhpolding radiolarite, al = Allgäu Fm. 61 In map view, the rocks of the Puitental zone were also involved in the sinistral shearing across NE-trending faults, at their southern margin, but this margin does not seem to be involved in the dextral faulting (see above). The northern tips of 600-800 m wide fault blocks separated by sinistral strike-slip faults reach into the zone of tight folds, but most sinistral faults disappear within the folded zone, except the Gatterl fault that has 1 km offset (Ferreiro Mählmann and Morlok 1992). Thus, sinistral strike-slip faulting, backthrusting and folding were contemporaneous.

Fig. 7: Cross sections of the Wetterstein mountains. See Fig. 3 for location. (a) Section across the Zugspitze summit to Ehrwalder Alm. Note zone of Upper Triassic to Lower Cretaceous rocks below the Zugspitze (compare section 6/7 on plate 6 of Tollmann 1976) terminating in an overturned position against upright Upper Triassic Plattenkalk, precluding a continuous sedimentary succession. The footwall cutoff related to this hanging wall cutoff of the thrust on top of this succession does not crop out, and has therefore been drawn schematically in the subsurface below the Plattspitzen. (b) Section across the Waxenstein ridge to Hoher Kamm in the Puitental zone. The space needed for downthrowing the Wetterstein platform against the basinal succession in the north has been interpreted to be related to salt tectonics in the Haselgebirge- Reichenhall succession, that also facilitates tight recumbent folding of the Wamberg anticline, and folding of the Zugspitz anticline. Note that the Puitental backthrust was probably localized by the vertical step across the strike-slip fault.

2.3 Mieming mountains and Vorbergzug The backbone of the Mieming mountains is the Mieming anticline, that is a simple, 40° E- plunging anticline N of Telfs (3 of Fig. 3a; Mair 2020). Between Telfs and Ehrwald, the Mieming anticline has an axial plane dipping to the north (Becke 1983). Toward the West, the 62 plunge diminishes to 25° (Leo 2020), and then disappears in the western Mieming mountains (diagram 6 of Fig. 2). The anticline is dissected by the NW-trending Telfs fault (T of Fig. 3b) that has a total offset of 5 km across its two branches. The western branch is connected to a steep oblique thrust, that stacks the core of the anticline onto its northern limb (Miller 1963b; Becke 1983; Ferreiro Mählmann and Morlok 1992) and continues into its northern limb (11 of Fig. 2), but offset diminishes. Adjacent to the North is the very open Gaistal-Hinterautal syncline (Fig. 2, 4 of Fig. 3a), and then the open anticline at Issentalköpfl (Figs. 2 and 7a). This anticline disappears East of the Gatterl fault, only the northern limb of this anticline continues further to the east in the Vorbergzug (Figs. 2 and 3). Parts of the folds are cut out across E- trending faults that most probably belong to the same fault set as the one decribed in chapter 2.2. As the Puitental zone, the Mieming mountains are also dissected by 800 m spaced NE- trending sinistral faults, that delimit structurally different units. In the panoramic view of Fig. 2, such a fault is seen in the west face of Wamperter Schrofen, that separates the N-dipping Wetterstein platform interfingering with Partnach marls from Wetterstein limestone with a subvertical bedding attitude.

To the south, the Wetterstein limestone of the Mieming mountains is bounded by a major strike slip fault that follows the shales of the Raibl beds (Höll shear zone; H of Fig. 3b; diagram V of Fig. 2). This is a major break in the structural architecture in this part of the Northern Calcareous Alps: Fold axes N of this zone are parallel to the fault (diagrams 1 – 4 of Fig. 2), and this applies for the Karwendel mountains as well (traces of fold axial planes 2, 3, 4 of Fig. 3a; see also Heißel 1978; Kilian and Ortner 2019). North of Telfs, the Höll shear zone is truncated by the Telfs fault (T of Fig. 3b), that is linked to the western part of the Mieming anticline. There, folds more variably WNW-ESE to WSW-ENE oriented (diagrams 5 – 7 of Fig. 2). South of the Höll shear zone fold axes are uniformely SW-NE oriented (e.g., diagram 8 of Fig. 2) and terminate against the fault.

2.4 Northern flank of the Wetterstein mountains At the northern side of the Wetterstein mountains, stratigraphy and structure change. The Wetterstein carbonate platform interfingers to the NE with basinal sediments of the Reifling and Partnach basin with clinoforms seen in the north flank of Zugspitze and Waxenstein (Petschick 1983; Hornung and Haas 2017) and is entirely replaced by these basinal deposits in the Mittenwald area (Ampferer 1912; Mylius 1916; Jerz and Ulrich 1966). A tightly folded continuous sedimentary succession is present from the Partnach beds into the Hauptdolomit,

63 and therefore the Wetterstein mountains is in stratigraphic contact with its northern foreland (see chapter 1.2; Fig. 7b).

Further to west, the units of the Alpine Muschelkalk are exposed below the Wetterstein cliffs in the western part of the north wall of the Zugspitze. At Ehrwalder Köpfe at the NW edge of the Wetterstein mountains, the Alpine Muschelkalk is stacked in a 500 m thick duplex with six slices (Obermoos thrusts; Fig. 8). Slice 2 is 220 m long between the synclinal hinge in the South (Fig. 8a) to the anticlinal hinge in the North (Fig. 8b) measured parallel to transport direction (diagram I of Fig. 2). Assuming a comparable length of all slices, a minimum offset of 1320 m is deduced. The floor and roof thrusts of this duplex emplaces strongly deformed „Jungschichten“ between the Triassic units (Miller 1963a), and sigificantly larger transport has to be assumed. Slice 3 (Fig. 8a) is at least 800 m long, and the floor thrust in Fig. 7a has 7 km offset. Summing up, shortening in the duplex is about 9 km. Toward the east, the thickness and the number of slices of this duplex diminishes (Fig. 8b).

Fig. 8: Panoramic views of the NW edge of the Wetterstein mountains showing the Obermoos thrusts. (a) Photograph and (b) interpretation of the Gamskar as seen from the West. Three slices of the Virgloria- Steinalm succession are stacked. In each slice an anticline is developed at the frontal end (see slice 1), and a syncline at the trailing end (seen in slices 2 and 3). The „Jungschichten“, i.e., uppermost Triassic to Lower Cretaceous rocks are emplaced young-on-old onto slice 3. Therefore this thrust on top of slice three is out- of-sequence. (c) photograph and (d) interpretation of the Zugspitze north face as seen from the North. Slice 1 and 2 cannot be distinguished in this view as the thrust is parallel to bedding. At least six slices are seen, and the thrust at the base terminates to the East. The Zugspitze anticline („Hauptsattel“ of Miller 1963a) is 64 therefore partly a consequence of stacking of slices within this duplex. However, the anticline also coincides with the transition from lagoon to fore-reef within the Wetterstein carbonate platform. The dip of the clinoforms is therefore also contained in the Zugspitze anticline.

The Jurassic succession associated with the floor thrust of the duplex is exposed at the SW shore of Eibsee. An overturned succession of Schrambach to Kössen Fm., the latter in the core of a tight, recumbent fold, overlies the upright Hauptdolomit (Fig. 7a). A NNW-trending normal fault downthrows this succession to the Eibsee on its eastern side, but it is hidden below scree in the West. The Jurassic succession associated with the roof thrust is exposed in the northern Gamskar. A thin veneer of intensly sheared Jurassic rock (mostly Allgäu Formation) is found atop a thrust truncating the Virgloria and Steinalm Fms. of the Alpine Muschelkalk (Fig. 8a, b). The shear plane transports the hanging wall to the NNE (Fig. 2, diagrams I and II).

In summary, shortening increases significanly from E to W between in the northern foreland of the Wetterstein mountains, from a zone of tight folding NW of Mittenwald (Fig. 3), to a zone of duplexing at the NW corner of the Zugspitze. Vertical uplift related to the W-ward increase of shortening causes the 30° eastward plunge of the folds on top (e.g., Reintal syncline, Fig. 2, diagram 2) (Ortner 2015). This plunge is also associated with an eastward tightening of the Reintal syncline and Zugspitze anticline from very open (Fig. 7a) at Zugspitze to tight north of Gatterl (Fig. 7b). Shortening is increasingly transferred from duplexing to folding to toward the east, as the Wetterstein carbonate platform looses thickness.

The western boundary of this duplex is a branch of the Loisach faults, which is a bundle of NE trending, sinistral faults (Fig. 3b; Loisach fault set; Kockel et al. 1931; Schmidt-Thomé 1954). These NE-trending faults have been related to Mesoalpine shortening and have been interpreted to be tear faults (Eisbacher and Brandner 1996). West of the Zugspitze, the Loisach fault separates the Lermoos syncline with a km-thick Jurassic succession (Jacobshagen 1965; Köhler 1986) in the W from the easterly adjacent Ehrwald anticline where the Jurassic succession is condensed. Duplexing during activity of the Obermoos thrusts also ends against this Ehrwald branch of the Loisach fault (Fig. 3b).

65 3. Discussion

3.1 Structural synthesis of the study area

3.1.1 The Obermoos thrusts solving the controversy on the tectonic position of the Wetterstein mountains The structural observations of chapter 2 illustrate the contradictory structural position of the Wetterstein mountains. On one hand, the Wetterstein mountains are part of a continuous sedimentary succession connecting it to its NE foreland and should therefore be part of the Lechtal thrust sheet, on the other hand, the structural geometry of the Mieming thrust and Zugspitze thrust at the base of the Wetterstein mountains is comparable, and transport across the Zugpitze thrust has been shown to be NW-directed. Therefore the Mieming chain and Wetterstein mountains should be part of the same structural unit.

The solution to solve these contradictions are the Obermoos thrusts at the northern flank of the Wetterstein mountains, that die out to the east as shortening is transferred from thrusting to folding (see Fig. 9). These thrusts are clearly out-of-sequence, as they emplace Jurassic to Cretaceous onto Triassic rocks. These rocks were originally in the footwall of the Zugspitze thrust, thus the thrust is repeated. In the cross sections of Fig. 7a and in the inset of Fig. 9, only the hanging wall cutoff of the Zugspitze thrust and the Jurassic-Cretaceous succession is exposed at the surface, the footwall cutoff needs to be drawn in the subsurface and the Zugspitze thrust must continue to the North.

Fig. 9: 3D view of the Wetterstein mountains looking to the SE. This oblique view illustrates the Obermoos thrusts loosing offset to the East, where they end within an anticlinorium related to the large scale Wamberg anticline. Inset shows a further simplified version showing the hanging wall cutoff of the Zugspitze thrust at the Obermoos thrust, that is cut out by the normal fault east of Eibsee in the large version.

67 At the Zugspitze and Mieming thrust the hanging wall was transported to the NW where it was not overprinted (see transport directions of Figs. 3b, and Fig. 4). In an outcrop at Marienbergjoch south of Ehrwald (Fig. 3b) top NW movements were recorded by NW-facing mesofolds in pelagic limestones of the Ammergau Formation, whereas the less competent, marly Allgäu and Schrambach Formations are deformed by NNE-facing mesofolds and s-c- fabrics (van Kooten 2016). Following the interpretation of thrust-related fabrics of Kilian and Ortner (2019) in the Karwendel mountains, we suggest an older, Eoalpine, pervasive fabric related to NW-directed transport, overprinted by younger, Mesoalpine N- to NNE-directed movements, that did affect the incompetent units. Based on the age of the youngest sediments below the thrust (see chapter 1.1), Eoalpine thrusting has a maximum age of Hauterivian in the study area.

3.1.2 Ehrwaldites and E-trending sinistral faults Offset of the Zugspitze and Mieming thrusts by sinistral E-W strike-slip faults is dated by the intrusion of the Albian Ehrwaldite dykes into the shear zone (Fig. 5). This is in accordance with the observation of Ehrwaldites in the Reichenhall Formation at the base of the overlying thrust sheet in the Karwendel mountains (Mutschlechner 1954; Jerz and Ulrich 1966). Unlike previous studies, that argued that the absence of the Ehrwaldite dykes and sills in the hanging wall of the thrusts means that thrusting had not yet taken place (e.g., Eisbacher and Brandner 1996), we conclude that the Ehrwaldites postdate major thrust motion. Otherwise the dykes and sills in the hanging wall would have been transported out of the area of observation.

In their study on the Ehrwaldites, Trommsdorff et al. (1990) argue that these were sourced from a subcontinental mantle, and exclude a subduction setting, i.e., the Penninic subduction underlying the NCA thrust sheets. It is, however, compatible with a foreland setting in the external zone of NCA nappe stacking (Ortner et al. 2016) related to Cretaceous intracontinental subduction(Stüwe and Schuster 2010). In such a context, sinistral shearing at E-trending faults can probably be related to intracontinental transform faults in the context of opening of the Penninic ocean. In paleogeographic reconstructions, such faults have been interpreted to crosscut the Austroalpine domain (e.g., Weissert and Bernoulli 1985; Froitzheim et al. 1996; Handy et al. 2010), but have not yet been observed in the field.

3.1.3 Meso- to Neoalpine out-of-sequence thrusting The vertical N-down km-scale offset across these sinistral faults localized backthrusting at the Puitental backthrust by the connection of the Kössen marl decollement north and south of the

68 sinistral faults with the decollement in the Raibl evaporites and shales (Fig. 7b). At least 2 km offset are seen in Fig. 7b, however this number may be underestimated as the Puitental backthrust is kinematically linked with the sinistral, NE-trending Gatterl fault (chapter 2.2), and the southern block moves out of the section.

The out-of-sequence Obermoos thrusts and the Puitental backthrust emplace young on old rocks. The N- to NNE-direction of transport suggest a younger, Mesoalpine to Neoalpine age as compared to initial NW-directed, Eoalpine stacking. The footwall cutoff at the Obermoos thrusts must be below the rocks in the foreland of the Wetterstein mountains, and should be at the base of the sedimentary succession of the Lechtal thrust sheet north of Zugspitze (Fig. 7). Therefore, the Wetterstein mountains must have been in continuation with the Lechtal thrust sheet, as previously claimed. However, connecting the footwall and hanging wall cutoffs across the Puitental backthrust implies also continuity of the Zugspitze and Mieming thrusts.

3.1.4 Salt tectonics in the western NCA? The geometry of this Mieming-Zugspitze thrust remains enigmatic. While the geometry of the Mieming thrust in Fig. 2, that cuts upsection could be interpreted in terms of a hanging wall ramp in a fault-bend fold, the Zugspitze thrust cutting downsection in transport direction is much more difficult to explain. The geometry suggests that the thrust formed out-of-sequence and postdated folding of the hanging wall. However, the low angle of the cutoffs in the hanging wall is mechanically difficult to explain. Known examples of out-of-sequence thrusts show that thrusts maintain their orientation with respect to gravity if they are folded during activity, and create imbricate stacks in the forelimb of anticlines (Gilluly 1960; Boyer 1992). This is supported by evidence from numerical experiments (Fig. 10): in an already folded succession new faults form crosscutting a competent layer. Newly formed faults do not follow the anisotropy between layers 1 and 2 having a large rheological contrast at relatively low angle. Therefore an origin of the Mieming-Zugspitze thrust as an out-of-sequence thrust is improbable. (a)

(b) Layer 1 (Bottom) Layer 2 (Middle) Layer 3 (Top) Thickness (m) 600 1500 1600

69 Stratigraphy Haselgebirge and Alp. Muschelkalk Gr. Gosau Group Reichenhall Fm. to Hauptdolomit E-Module (kN/m²) 2 000 000 80 000 000 10 000 000 Poisson ratio 0.25 0.2 0.25 Friction angle (φ) 42 41 42 Cohesion (c) 1 1 1 Fig. 10: Finite-element numerical experiment performed in ABAQUS using the Mohr Coulomb model (linear elastic, ideal plastic model with the failure surface according to Mohr-Coulomb). (a) Plastic strain in a new increment of shortening in x-direction postdating folding. Three-layer model based on the sedimentary succession of the western NCA. The middle layer with a higher E-Module and a lower friction angle fails where plastic strain is high, and the orientation of these zones is dependent on the coefficient of internal friction. (b) Input data for the numeric model. The input data base on tests on modelling folding using a linear elastic model and literature: von Soos and Engel (2008) use φ =35-51° for dense limestone (layer 2); Czech and Huber (1990) give φ =45° for the Haselgebirge (layer 1) and φ =41° for Cretaceous marls (equivalent to Gosau Group of layer 3).

An alternative, more speculative explanation would be, that the Triassic succession, that sits now on top of the Mieming-Zugspitze thrust, originally onlapped a salt anticline, i.e., an anticline formed by uplifting salt (e.g., Hudec and Jackson 2007). During Cretaceous inversion the Triassic succession would have been detached along the unconformity, separated from the underlying salt and thrust onto the Schrambach shales. Salt has not been observed in Wetterstein and Mieming mountains, but further east in the Karwendel mountains (KW of Fig. 1) at the base of the Inntal thust sheet (Schmidegg 1950; Heißel 1978; Spötl 1989). Around the salt occurrence and in its wider vicinity, the Wetterstein limestone is frequently in direct contact with cellular dolomites (Ampferer 1928; Ampferer and Heißel 1950; Krauter 1968; Moser 2008a, b), that are also often found in direct contact with Haselgebirge salt and shales (Spötl 1988), and may represent zones related to salt evacuation toward the surface and related salt tectonics. Such zones are also found in the Mieming mountains close to the core of the Mieming anticline (3 of Fig. 3a; see Moser 2010), and may suggest the presence of salt in the study area.

The jump of the decollement from the Haselgebirge-Reichenhall succession during initial Eoalpine separation of the allochthon from its basement to the Kössen marls during Mesoalpine out-of-sequence thrusting may support this interpretation: The original decollement must have gained strength to force it into another position. If the Zugspitze-Mieming thrust was originally related to salt, evacuation of this salt and welding could have caused strengthening of this contact, and subsequently the activation of the Kössen marl decollement during Mesoalpine shortening (compare Kilian and Ortner 2019). Backthrusting across this decollement also creates the Holzerwies anticline (3 of Fig. 2) that was one of the main arguments for Miller (1963a) to propose S-directed transport of the Wetterstein mountains, however, this is superimposed after the switch of decollements into the Kössen marls. It is also conspicouos that fold axes in the Karwendel and Wetterstein mountains are consistently ENE-trending and seem 70 to be entirely related to Mesoalpine shortening, while major Eoalpine transport of thrust sheets is only documented in the strongly deformed Jurassic to Cretaceous succession in the footwall of the Mieming-Zugspitze thrust (see transport directions of Fig. 3b), and there is no evidence in the hanging wall north of the Höll shear zone.

3.1.5 General relevance of out-of-sequence thrusting at the Obermoos thrusts Out-of-sequence thrusting is rarely documented, because it is difficult to balance, and often leads to inadmissible structures in cross sections, such as truncated anticlines and synclines (e.g., Butler 1987; Morley 1988). However, in recent years increasing evidence accumulates on the existence of out-of-sequence thrusting (e.g., Kley 1996 - Bolivian Andes; McDowell 1997 - Rocky mountains of Montana; Molinaro et al. 2005 - Zagros mountains; Olivetti et al. 2010 - Peloritani mountains of Sicily; Sieniawska et al. 2010 - Polish Carpathians; Ortner et al. 2015 - Subalpine Molasse, Alps). So far, only few structural concepts for out-of-sequence thrusts exist. In Fig. 11 we illustrate the case of two parallel decollements, the upper one abandoned after an increase of strength related to salt expulsion or release of overpressure. Young-on-old relationships are observed across some parts of the out-of-sequence thrust. In more detail, an old-on-young contact across the out-of-sequence thrust is observed (1) close to the tip and (3) above the footwall cutoff of the out-of-sequence thrust (orange segments in Fig. 11), and a (2) young-on-old contact between the hanging wall and footall cutoff of the in-sequence thrust (yellow segments in Fig. 11). This latter segment increases in length as the hanging wall travels across the upper footwall flat of the out-of-sequence thrust. The fault tip and segments (1) and (2) are visible in the inset of Fig. 9. The right side of Fig. 11 gives the local names used in this study.

Fig. 11: Geometry of out-of-sequence thrusting in cases when the decollement shifts to a deeper position. Model cut parallel to upper end of ramp. Hanging wall transport on out-of-sequence thrust increases from zero at left to right, where the entire hanging wall has climbed the ramp. Old-on-young contacts and young- on-contacts across out-of-sequence thrust are color coded, and alternate depending on position on the thrust (left). After the entire hanging wall has climbed the ramp, the length of the young-on-old contact on the upper footwall increases. The coloring of stratigraphic units is similar to the Fig. 9, but is used here in a more general way to distinguish old and young deposits.

3.2 Tectonic subdivision of the NCA

3.2.1 Revised tectonic subdivision of the western NCA Joining the Zugspitze and Mieming thrusts by connecting the hanging- and footwall cutoffs across the out-of-sequence thrusts (Fig. 7) leads to one, continuous thrust, that disappears in the subsurface north of the Wetterstein mountains. The next old-on-young contact to the north is the contact of the Lechtal on top of Allgäu thrust sheet close to the northern margin of the NCA (Fig. 12a). In our interpretation, this contact must be the continuation of the Zugspitze-Mieming thrust. This interpretation renders the accepted tectonic subdivision of the western NCA of Ampferer (1912); Heißel (1958) and Tollmann (1976) obsolete. Here we apply the new subdivision suggested by Kilian and Ortner (2019) and discuss it on the scale of the western Northern Calcareous Alps (Fig. 14). To avoid confusion between different teconic concepts, Kilian and Ortner (2019) introduced new terms and renamed the unit including most of the former Lechtal and Inntal thrust sheets as the Karwendel thrust sheet, and the underlying unit the Tannheim thrust sheet. The latter includes the former Allgäu thrust sheet, and the tectonic windows of the Puitental zone, the half window of the eastern Karwendel mountains, and the Lechtal thrust sheet E of Innsbruck (Fig. 1).

72 However, W of the study area, the half-klippe and klippen of the former Inntal thrust sheet belong to the same tectonic unit as the underlying unit in the new subdivision. It has been known for a long time, that the thrust at the base of this part of the Inntal thrust sheet is out-of-sequence, as it has a Cenomanian age (Ortner 2003a). It is therefore younger than the more external Lechtal thrust of Albian age (Figs. 1, 12c; Ortner 2003a). This is in line with the observations reported here. The difference between the Karwendel- (Kilian and Ortner 2019) and Wetterstein/Mieming mountains (this study), and the units further west is the amount of transport across the out-of-sequence thrusts. In the Karwendel mountains, offset across the out- of-sequence thrust(s) is between one and a few kilometers. At the north side of the Wetterstein mountains, offset increases to 9 km (see above), but is also redistributed to folding, and backthrusts, and the out-of-sequence thrusts are not continuous. West of the study area, out-of- sequence offset increases to 20 km at the base of the Inntal thrust sheet (12 km thrust transport, 8 km shortening by folding; Eisbacher et al. 1990).

This increase occurs across the Höll shear zone (see above), that separates a domain folded with NE-axes in the South and West from a domain with ENE-axes parallel to the fault in the North and East (Fig. 3). To avoid a stack of units having the same name, we propose to rename the half-klippe and klippen of the former Inntal thrust sheet west of the Höll shear zone as the Imst half-klippe and klippen, which are part of the larger Karwendel thrust sheet. Fig. 12b shows that the Karwendel thrust sheet and the Imst half klippe sit on the same thrust, and have consequently not been separated in previous tectonic subdivisions, and are still part of the same thrust sheet.

Fig. 12: Conceptual cross sections of the NCA. Cretaceous synorogenic sediments color-coded for type (UFD = upper footwall deposits; TSTD = thrust-sheet-top deposits) and age. (a) Zugspitze section using the existing tectonic subdivision. The „Puitental slices“ are needed to account for thickening in the southern part of the cross section. (b) Zugspitze section using the new tectonic subdivision. Thickening of the NCA wedge below the Wetterstein and Mieming mountains is accomplished by the out-of-sequence Obermoos thrusts. (c) Section west of the study area simplified from Eisbacher et al. (1990). (d) Reinterpreted section west of the study area using the revised tectonic subdivision.

3.2.2 Multiple events of out-of-sequence thrusting West of the Höll shear zone, the Tannheim thrust sheet tapers to the South below the Imst half klippe (Figs. 12d, 13; compare Eisbacher et al. 1990). East of the Höll shear zone, it continues further to the southeast, and reappears at the surface near Innsbruck (Fig. 14). Therefore, a lateral ramp (seen in Fig. 13b) or a tear fault sitting on the basal decollement is required, that separates the Tannheim thrust sheet from the Karwendel thrust sheet and localizes the Höll shear zone at surface (Fig. 12d). The Imst half klippe was emplaced during the Cenomanian (see above), therefore the Höll shear zone must have been active during that time to delimit relative NW-directed movement. In the N-S section of Fig. 12b, the Imst half klippe moves out of the section. Only during Latest Cretaceous to Paleogene, Mesoalpine, N to NNE-directed shortening, the Wetterstein mountains and Puitental zone were uplifted relative to the units further west.

Fig. 13: Block diagrams illustrating the lateral relationship between the Zugspitze and Lechtal cross sections of Fig. 12. (a) Geometry of the nappe stack at the end of the Cenomanian. The Karwendel thrust sheet had already been stacked between the Hauterivian and Albian, and the Imst half klippe had been emplaced during the Cenomanian. East of the Höll shear zone, the Tannheim thrust sheet reaches further to the south, and a lateral ramp or tear fault is required, that localizes the Höll shear zone at surface (see also Fig. 12b). (b) During the Latest Cretaceous to Paleogene, the Obermoos out-of-sequence thrust stacks the alredy existing nappe pile. In this view, the Imst half klippe south of the Höll shear zone moved toward the observer. Transport during out-of-sequence thrusting was almost perpendicular to preceding stacking.

Fig. 14: New tectonic subdivision of the western NCA. This subdivision avoids the contradictions of older tectonic subdivisions and is in accordance with the distribution of all synorogenic sediments of the western NCA. All thrusts that and related tear faults, that are out-of-sequence with repect to the general foreland- propagating thrust sequence are classified as out-of-sequence thrusts/faults, and faults that are not related to thrusting are classified separately. 76 3.2.3 Thrust distance and rates Combining the former Lechtal and Inntal thrust sheets increases the thrust distance to at least 38 km in the Karwendel mountains (Kilian and Ortner 2019), and to 35 km in the study area, which is the surface N-S length of the Karwendel thrust sheet (Fig. 12b). This value excludes folding and minor thrusting within the unit. Considering NW-directed transport (chapters 1.2 and 2.1) would increase this estimate significantly. The Karwendel thrust sheet is floored by upper-footwall deposits that are exposed close to the northern margin of the NCA near Oberammergau (Fig. 14), where these reach into the Albian. S of Zugspitze, upper-footwall deposits are found within the Puitental zone and at Eibsee (Figs. 3b, 12), where they reach into the Hauterivian (Vidal 1953; Miller 1963c). This allows to calculate thrust rates; in the cross section studied here, the rate of thrust propagation using the thrust distance given would amount to 0.04 cm/year in N-S direction, which is rather slow as compared to active present-day orogenic wedges (e.g., 1.2 cm/ year, Himalayan thrust front - Wesnousky et al. 1999; 0.45 cm/year, thrust front of the central Andes - Schmidt et al. 2011; 3 cm/year, Taiwan orogenic front - Malavieille et al. 2020 in press).

The tectonic units of the NCA haven been grouped into nappe complexes based on the original position of the rocks on the passive continental margin bordering the Meliata ocean (Tollmann 1976; Schuster 2015). The nappe complexes are the Juvavic, Tirolic and Bajuvaric nappe groups that had their paleogeographic position at the continental slope, at the shelf break, and continentward behind the shelf break, respectively. This arrangement is also the vertical succession from top to base in the nappe stack. The Juvavic units are absent west of the Inn valley. In contrast to the former Lechtal thrust sheet, the new Karwendel thrust sheet is part of the Tirolic nappe complex (see Kilian and Ortner 2019 for a discussion). In the new tectonic map, the Tirolic Karwendel thrust sheet now borders the Tirolic Staufen-Höllengebirgs nappe across the Inntal shear zone. The Inntal shear zone is a major, subvertical strike slip fault zone with a sinistral offset of up to 40 km (Ortner 2003b; Ortner et al. 2006); It does not stack thrust sheets, and two units belonging to the same group of nappes is more appropriate.

4 Conclusions Based on our structural analysis, we propose a new tectonic subdivision of the western NCA. This subdivision has only two large thrust sheets, the tectonically deeper unit being the Tannheim thrust sheet, the tectonically higher Karwendel thrust sheet. The observation of out- of-sequence thrusts, that crosscut and locally repeat the original thrust and end laterally, allows to keep the Wetterstein mountains in stratigraphic contact with its northeastern foreland and, at 77 the same time, sitting on the thrust at the base of the Karwendel thrust sheet. This solves a 100 year old controversy, in which all arguments used by the opposing parties were correct (see chapter 1.2).

The following steps are evident in the kinematic evolution of the study area: (1) Between the Hauterivian and Albian, the Karwendel thrust sheet travelled at least 35 km to the NW over the Tannheim thrust sheet, possibly detached on salt. (2) Activity of E-trending sinistral faults separates Eoalpine NW-directed emplacement of thrust sheets from Mesoalpine to Neoalpine N- to NNE-directed out-of-sequence thrusting. These faults did crosscut the entire nappe stack and reached into the underlying crust and mantle, facilitating the ascent of mantle-derived melts leading to intrusion of Ehrwaldite dykes into the shear zones. Near the surface, these faults did create km-scale vertical, N-down offset. (3) The steps across these faults localized Mesoalpine out-of-sequence thrusting, both foreland- directed (Obermoos thrusts), and hinterland-directed (Puitental backthrust) in the study area. Almost all large fold axes are perpendicular to the Mesoalpine shortening direction and therefore related to Mesolpine shortening. The western part of the Karwendel thrust sheet, the Imst half klippe, was thrust onto its footwall out-of-sequence during the Cenomanian.

5 Acknowledgements High resolution DEMs and orthophotographic images were used to analyze stratal and fault geometries. These were provided by the state of Tyrol and the Bavarian Agency for Digitisation, High-Speed Internet and Surveying, which is greatly acknowledged. We thank Petroleum Experts Ltd. who provided their Move software package that was used to create Figures 7, 9 and 11. The work of Sinah Kilian was supported by the state of Tyrol (Tiroler Wissenschaftsfond) and the Doctoral programme of the University of Innsbruck. We thank Gabor Heja (Budapest) for help during field work.

6 References Ampferer O (1902): Bericht über die Neuaufnahme des Karwendelgebirges. Verh Geol Bundesanst 1902:274-276 Ampferer O (1912): Gedanken über die Tektonik des Wettersteingebirges. Verhandlungen der kk Geologischen Reichsanstalt 1912:197-212 Ampferer O (1914): Besprechung mit O. Schlagintweit, K. Ch. v. Loesch und H. Mylius über das Wettersteingebirge. Verhandlungen der kk Geologischen Reichsanstalt 1914:338- 352 Ampferer O (1928): Die Reliefüberschiebung des Karwendelgebirges. Jb Geol Bundesanst 78:241-256 78 Ampferer O (1931): Zur neuen Umgrenzung der Inntaldecke. Jb Geol Bundesanst 81:25-48 Ampferer O (1942): Geologische Formenwelt und Baugeschichte des östlichen Karwendelgebirges. Denkschriften d Akad d Wiss 106 (1):1-95 Ampferer O, Hammer W (1911): Geologischer Querschnitt durch die Alpen vom Allgäu zum Gardasee. Jahrbuch der kk Geologischen Reichsanstalt 61:531 - 710 Ampferer O, Heißel W (1950): Das östliche Karwendel: Erläuterungen zur geologischen Karte des östlichen Karwendel und des Achensee-Gebietes. Wagner, Innsbruck Ampferer O, Ohnesorge T (1924): Erläuterungen zur Geologischen Spezialkarte der im Reichsrate vertretenen Königreiche und Länder der österreichisch-ungarischen Monarchie 1:75.000, 5046 Zirl und Nassereith. k. k. Geologische Reichsanstalt, Wien Bechstädt T, Mostler H (1974): Mikrofazies und Mikrofauna mitteltriadischer Beckensedimente der nördlichen Kalkalpen Tirols. Geol-Paläontol Mitt Innsbruck 4:1- 74 Bechstädt T, Mostler H (1976): Riff-Becken-Entwicklung in der Mitteltrias der westlichen nördlichen Kalkalpen. Zeitschr Dt Geol Ges 127:271-289 Becke M (1983): Zur Geologie des Mieminger Gebirges. Geol-Paläontol Mitt Innsbruck 12:317-340 Beurlen (1944): Zum Problem der Inntaldecke. Sitzungsberichte der mathematisch- naturwissenschaftlichen Abteilung der Bayerischen Akademie der Wissenschaften zu München 1943:239-264 Bischof M, Garber C, Mackowitz J, Postl M, Ortner H (2010): Jurassische Beckenbildung in den westlichen nördlichen Kalkalpen. PANGEO 2010, Abstracts, Journal of Alpine Geology 52:93-94 Bögel H (1960): Der geologische Bau des Wettersteingebirges und seiner Umgebung. Jahrbuch des Deutschen Alpenvereins 85:20-27 Boyer SE (1992): Geometric evidence for synchronous thrusting in the southern Alberta and northwest Montana thrust belts. In: McClay KR (ed) Thrust Tectonics. Chapman & Hall, London, pp 377-390. https://doi.org/10.1007/978-94-011-3066-0_34 Boyer SE, Elliott D (1982): Thrust systems. AAPG Bull 66:1196 - 1230 Brandner R (1984): Meeresspiegelschwankungen und Tektonik der NW - Tethys. Jb Geol Bundesanst 126 (4):435 - 475 Butler RWH (1987): Thrust sequences. Jour Geol Soc London 144:619-634 Czech J, Huber H (1990): Gesteinskennwerte aus Laborversuchen. Felsbau 8 (3):129-133 Decker K, Peresson H, Faupl P (1994): Die miozäne Tektonik der östlichen Kalkalpen: Kinematik, Paläospannungen und Deformationsaufteilung während der "lateralen Extrusion" der Zentralalpen. Jb Geol Bundesanst 137:5-18 Eisbacher GH, Brandner R (1995): Role of high-angle faults during heteroaxial contraction, Inntal thrust sheet, Northern Calcareous Alps, western Austria. Geol-Paläontol Mitt Innsbruck 20:389 - 406 Eisbacher GH, Brandner R (1996): Superposed fold thrust structures and high angle faults, northwestern Calcareous Alps, Austria. Eclogae Geol Helv 89:553 - 571. https://doi.org/10.5169/seals-167913 Eisbacher GH, Linzer G-H, Meier L (1990): A depth extrapolated structural transect across the Northern Calcareous Alps of Western Tirol. Eclogae Geol Helv 83 (3):711 - 725. https://doi.org/10.5169/seals-166610 79 Elliott D (1976): The energy balance and deformation mechanisms of thrust sheets. Phil Trans Roy Soc Lond, part A 283:289 - 312 Eynatten Hv, Gaupp R (1999): Provenance of Cretaceous synorogenic sandstones in the Eastern Alps: constraints from framework petrography, heavy mineral analysis and mineral chemistry. Sediment Geol 124:81 - 111 Ferreiro Mählmann R, Morlok J (1992): Das Wettersteingebirge, Widerlager der allochtonen Inntaldecke, und die Ötztalmasse, Motor tertiärer posthumer NW - Bewegungen im Mieminger Gebirge. Geol-Paläontol Mitt Innsbruck 18:1 - 34 Frisch W (1979): Tectonic Progradation and Plate Tectonic Evolution of the Alps. Tectonophysics 60:121 - 139 Froitzheim N, Manatschal G (1996): Kinematics of Jurassic rifting, mantle exhumation, and passive-margin formation in the Austroalpine and Penninic nappes (eastern Switzerland):. Geol Soc Am Bull 108:1120 - 1133. https://doi.org/10.1130/0016- 7606(1996)108<1120:KOJRME>2.3.CO;2 Froitzheim N, Schmid SM, Frey M (1996): Mesozoic paleogeography and the timing of eclogite-facies metamorphism in the Alps: A working hypothesis. Eclogae Geol Helv 89:81-110. https://doi.org/10.5169/seals-167895 Fruth I, Scherreiks R (1982): Hauptdolomit (Norian), Stratigraphy, Paleogeography and Diagenesis. Sediment Geol 32:195 - 231. https://doi.org/10.1016/0037-0738(82)90050- 1 Fruth I, Scherreiks R (1984): Hauptdolomit, sedimentary and paleogeographic models (Norian, Northern Calcareous Alps). Geol Rundsch 73 (1):305 - 319 Gilluly J (1960): A folded thrust in Nevada-inferences as to time relations between folding and faulting. American Journal of Science 258A:68-79 Golebiowski R (1991): Becken und Riffe der alpinen Obertrias - Lithostratigraphie und Biofazies der Kössener Formation. In: Nagel D, Rabeder G (eds) Exkursionen im Jungpaläozoikum und Mesozoikum Österreichs. Österreichische Paläontologische Gesellschaft, Wien, pp 79-119 Haas J, Kovács S, Krystyn L, Lein R (1995): Significance of Triassic facies zones in terrane reconstructions in the Alpine-North Pannonian domain. Tectonophysics 242:19-40. https://doi.org/10.1016/0040-1951(94)00157-5 Haber G (1934): Bau und Entstehung der bayerischen Alpen, vol 3. Deutsche Landschaftskunde. C.H. Beck’sche Verlagsbuchhandlung, München Hahn FF (1912) Versuch einer Gliederung der austroalpinen Masse westlich der österreichischen Traun. Verhandlungen der k k Geologischen Reichsanstalt 1912:337- 344 Hahn FF (1913): Grundzüge des Baues der nördlichen Kalkalpen zwischen Inn und Enns. Mitt Österr Geol Ges 9:374-500 Handy MR, M. Schmid S, Bousquet R, Kissling E, Bernoulli D (2010): Reconciling plate- tectonic reconstructions of Alpine Tethys with the geological–geophysical record of spreading and subduction in the Alps. Earth-Science Reviews 102 (3):121-158. https://doi.org/10.1016/j.earscirev.2010.06.002 Heißel G (1978): Karwendel - geologischer Bau und Versuch einer tektonischen Rückformung. Geol-Paläontol Mitt Innsbruck 8:227 - 288

80 Heißel W (1958): Zur Tektonik der Nordtiroler Kalkalpen. Mitt Österr Geol Ges 50 (1957):95- 132 Hildebrandt E (2016): Strukturgeologischen Untersuchung in der Puitentalzone am Hohen Kamm östlich der Ehrwalder Alm. Unpublished Bachelor Thesis, Innsbruck Hornung T, Haas U (2017): Geologische Karte von Bayern 1:25.000, 8532/8632 Garmisch- Partenkirchen. Geologische Karte von Bayern 1:25.000. Bayerisches Geologisches Landesamt, München Hudec MR, Jackson MPA (2007): Terra infirma: Understanding salt tectonics. Earth-Science Reviews 82 (1):1-28. https://doi.org/10.1016/j.earscirev.2007.01.001 Jacobshagen V (1965): Die Allgäuschichten (Jura-Fleckenmergel) zwischen Wettersteingebirge und Rhein. Jb Geol Bundesanst 108:1-114 Janák M, Froitzheim N, Lupták B, Vrabec M, Krogh Ravna EJ (2004): First evidence for ultrahigh-pressure metamorphism of eclogites in Pohorje, Slovenia: Tracing deep continental subduction in the Eastern Alps. Tectonics 23:TC5014 Jerz H, Ulrich R (1966): Geologische Karte von Bayern 1:25.000, 8533/8633 Mittenwald. Geologische Karte von Bayern 1:25.000. Bayerisches Geologisches Landesamt, München Kilian S, Ortner H (2019): Structural evidence of in-sequence and out-of-sequence thrusting in the Karwendel mountains and the tectonic subdivision of the western Northern Calcareous Alps. Austrian Journal of Earth Sciences 112 (1):62-83. https://doi.org/10.17738/ajes.2019.0005 Kley J (1996): Transition from basement-involved to thin-skinned thrusting in the Cordillera Oriental of Southern Bolivia. Tectonics 15:763-775 Kockel CW (1926): Die Deckenfalten der Hohenschwangauer Berge. Geol Rundsch 17 (2):159- 160. https://doi.org/10.1007/BF01801862 Kockel CW, Richter M, Steinmann HG (1931): Geologie der Bayerischen Berge zwischen Lech und Loisach. Wissenschaftliche Veröffentlichungen des DÖAV 10:231 p. Köhler M (1986): Lermooser Tunnel (Ausserfern, Tirol): Baugeologische Verhältnisse, Prognose und tektonische Schlussfolgerungen. Geol-Paläontol Mitt Innsbruck 13:363- 379 Kraus E (1957): Zum Verankerungs-Problem der kalkalpinen Decken im Bereich des Wetterstein-Gebirges. Zeitschr Dt Geol Ges 108 (1956):141-155 Krauter E (1968): Zur Frage der Reliefüberschiebung an Staner-Joch (Östliches Karwendel, Tirol). Mitt Österr Geol Ges 60:23-64 Kreidl S (2015): Geologie und Tektonik am Westfuß der Zugspitze. unpubl. Master-Thesis, Innsbruck Lein R (1987): Evolution of the Northern Calcareous Alps during Triassic times. In: Flügel HW, Faupl P (eds) Geodynamics of the Eastern Alps. Deuticke, Wien, pp 85-102 Leitner C, Spötl C (2017): Chapter 21 - The Eastern Alps: Multistage Development of Extremely Deformed Evaporites. In: Soto JI, Flinch JF, Tari G (eds) Permo-Triassic Salt Provinces of Europe, North Africa and the Atlantic Margins. Elsevier, Amsterdam, pp 467-482. https://doi.org/10.1016/B978-0-12-809417-4.00022-7 Leo D (2020): Die Geologie der Hochwand im Mieminger Gebirge. Unpublished Bachelor Thesis, Innsbruck

81 Leuchs K (1924): Der geologische Bau des Wettersteingebirges und seine Bedeutung für die Entwicklungsgeschichte der deutschen Kalkalpen. Zeitschr Dt Geol Ges 75 (1923):100- 113 Leuchs K (1930): Der Bau der Südrandstörung des Wettersteingebirges. Geol Rundsch 21 (2):81-96. https://doi.org/10.1007/BF01802266 Leuchs K (1935): Tektonische Untersuchungen im Wettersteingebirge. Zeitschr Dt Geol Ges 87:703-719 Linzer H-G, Ratschbacher L, Frisch W (1995): Transpressional collision structures in the upper crust: the fold thrust belt of the Northern Calcareous Alps. Tectonophysics 242:41 - 61. https://doi.org/10.1016/0040-1951(94)00152-Y Mair G (2020): Die Geologie der Hohen Munde bei Telfs. Unpublished Bachelor Thesis, Innsbruck Malavieille J, Dominguez S, Lu C-Y, Chen C-T, Konstantinovskaya E (2020 in press) Deformation partitioning in mountain belts: insights from analogue modelling experiments and the Taiwan collisional orogen. Geological Magazine:1-20. https://doi.org/10.1017/S0016756819000645 Manatschal G (2004): New models for evolution of magma-poor rifted margins based on a review of data and concepts from West Iberia and the Alps. Int Jour Earth Sci 93:432- 466 McDowell RJ (1997): Evidence for synchronous thin-skinned and basement deformation in the Cordilleran fold-thrust belt: the Tendoy Mountains, southwestern Montana. J Struct Geol 19 (1):77-87. https://doi.org/10.1016/S0191-8141(96)00044-2 Meschede M (1994): Methoden der Strukturgeologie. Enke, Stuttgart Miller H (1963a): Der Bau des westlichen Wettersteingebirges. Zeitschr Dt Geol Ges 113:409- 425 Miller H (1963b): Die tektonischen Beziehungen zwischen Wetterstein- und Mieminger Gebirge. N Jb Geol Paläont, Abh 118:291-320 Miller H (1963c): Gliederung und Altersstellung der jurassischen und unterkretazischen Gesteine am Südrand des Wetterstein-Gebirges. Mitt Bayer Staatsslg Paläont Hist Geol 3:51-72 Miller H (1965): Die Mitteltrias der Mieminger Berge mit Vergleichen zum westlichen Wettersteingebirge. Verh Geol Bundesanst 1965:187-212 Molinaro M, Leturmy P, Guezou J-C, Frizon de Lamotte D, Eshraghi SA (2005): The structure and kinematics of the southeastern Zagros foldthrust belt, Iran: From thin-skinned to thick-skinned tectonics. Tectonics 24:TC3007 Morley CK (1988): Out-of-sequence thrusts. Tectonics 7:539-561 Moser M (2008a): Zusammenstellung ausgewählter Archivunterlagen der Geologischen Bundesanstalt, Blatt 118 - Innsbruck, GeoFAST 1:50.000 (Ausgabe 2008/09). Geologische Bundesanstalt, Wien Moser M (2008b): Zusammenstellung ausgewählter Archivunterlagen der Geologischen Bundesanstalt, Blatt 119 - Schwaz, GeoFAST 1:50.000 (Ausgabe 2008/11). Geologische Bundesanstalt, Wien Moser M (2010): Zusammenstellung ausgewählter Archivunterlagen der Geologischen Bundesanstalt, Blatt 116 - Telfs, GeoFAST 1:50.000 (Ausgabe 2011/04) Geologische Bundesanstalt, Wien 82 Mutschlechner G (1954): Die Massengesteine der Nordtiroler und Vorarlberger Kalkalpen. Tschermaks mineralogische und petrographische Mitteilungen 4 (1):386-395. https://doi.org/10.1007/bf01140410 Mutschlechner G (1955): Der Erzbergbau in Außerfern. Schlern-Schriften 111:25-52 Mylius H (1916): Ein Beitrag zum geologischen Bau des Wettersteingebirges. N Jb Min Geol Paläont 1916 (1):10-40 Olivetti V, Balestrieri ML, Faccenna C, Stuart FM, Vignaroli G (2010): Middle Miocene out- of-sequence thrusting and successive exhumation in the Peloritani Mountains, Sicily: Late stage evolution of an orogen unraveled by apatite fission track and (U-Th)/He thermochronometry. Tectonics 29 (5):TC5005. https://doi.org/10.1029/2009TC002659 Ortner H (2001): Growing folds and sedimentation of the Gosau Group, Muttekopf, Northern Calcareous Alps, Austria. Int J Earth Sci (Geol Rundsch) 90:727-739. https://doi.org/10.1007/s005310000182 Ortner H (2003a): Cretaceous thrusting in the western part of the Northern Calcareous Alps (Austria) - evidences from synorogenic sedimentation and structural data. Mitt Österr Geol Ges 94:63-77 Ortner H (2003b): Local and far field stress – analysis of brittle deformation in the western part of the Northern Calcareous Alps, Austria. Geol-Paläontol Mitt Innsbruck 26:109-131 Ortner H (2007) Styles of soft-sediment deformation on top of a growing fold system in the Gosau Group at Muttekopf, Northern Calcareous Alps, Austria: Slumping versus tectonic deformation. Sediment Geol 196:99-118. https://doi.org/10.1016/j.sedgeo.2006.05.028 Ortner H (2015): Fernerkundung mit Hilfe von Orthofotos und Geländemodellen in der Geologie – Beispiele aus den Nördlichen Kalkalpen. GeoAlp 11 (2014):5-27 Ortner H (2016): Field Trip 4: Deep water sedimentation on top of a growing orogenic wedge - interaction of thrusting, erosion and deposition in the Cretaceous Northern Calcareous Alps. GeoAlp 13:141-182 Ortner H, Aichholzer S, Zerlauth M, Pilser R, Fügenschuh B (2015): Geometry, amount and sequence of thrusting in the Subalpine Molasse of Western Austria and Southern Germany, European Alps. Tectonics 34 (1):1-30. https://doi.org/10.1002/2014TC003550 Ortner H, Gaupp R (2007): Synorogenic sediments of the western Northern Calcareous Alps. GeoAlp 4:133-148 Ortner H, Kositz A, Willingshofer E, Sokoutis D (2016) Geometry of growth strata in a transpressive fold belt in field and analogue model: Gosau Group at Muttekopf, Northern Calcareous Alps, Austria. Basin Res 28 (6):731–751. https://doi.org/10.1111/bre.12129 Ortner H, Reiter F, Acs P (2002): Easy handling of tectonic data: the programs TectonicVB for Mac and TectonicsFP for Windows(TM). Computers & Geosciences 28:1193-1200. https://doi.org/10.1016/S0098-3004(02)00038-9 Ortner H, Reiter F, Brandner R (2006): Kinematics of the Inntal shear zone–sub-Tauern ramp fault system and the interpretation of the TRANSALP seismic section, Eastern Alps, Austria. Tectonophysics 414:241-258. https://doi.org/10.1016/j.tecto.2005.10.017 Petschick R (1983): Sedimentpetrographie und sehr schwache Metamorphose mitteltriadischer Beckengesteine der zentralen Westlichen Kalkalnen (Bayern und Tirol). Mit 83 geologischer Kartierung des Nordwestlichen Wettersteingebirges 1:10000. unpubl. Diploma thesis, Frankfurt Pichler A (1866): Beiträge zur Geognosie Tirols. Jahrbuch der k k geologischen Reichsanstalt 16:503-504 Richter M (1929): Die Struktur der nördlichen Kalkalpen zwischen Rhein und Inn. Neues Jahrbuch für Mineralogie, Geologie und Paläontologie, Beilage-Band 63, Abteilung B:1-62 Richthofen FFv (1859): Die Kalkalpen von Vorarlberg und Nord-Tirol: Erste Abtheilung. Jb kk Geol Reichsanst 10:72-137 Rothpletz A (1902): Geologischer Führer durch die Alpen I. Das Gebiet der zwei großen rätischen Überschiebungen etc. Sammlung geologischer Führer, 10. Bornträger, Berlin Rüffer T, Bechstädt T (1995): Interpretation des Deckenbaus in den westlichen nördlichen Kalkalpen: Widerspruch zwischen tektonischen und sedimentologischen Daten. Jb Geol Bundesanst 138:701 - 713 Schenk V (1967): Die Faziesentwicklung der Reichenhaller Schichten und die Tektonik im Süden des Achensees, Tirol. Geol Rundsch 56 (1):464-473. https://doi.org/10.1007/BF01848736 Schlagintweit O (1912a): Die Mieminger-Wetterstein Überschiebung. Geol Rundsch 3 (2):73- 92 Schlagintweit O (1912b): Zum Problem des Wettersteingebirges. Verhandlungen der k k Geologischen Reichsanstalt 1912:313-327 Schmidegg O (1950): Die Stellung der Haller Salzlagerstätte im Bau des Karwendelgebirges. Jb Geol Bundesanst 94:159 - 205 Schmidt S, Hetzel R, Mingorance F, Ramos VA (2011): Coseismic displacements and Holocene slip rates for two active thrust faults at the mountain front of the Andean Precordillera (∼33°S). Tectonics 30 (5). https://doi.org/10.1029/2011TC002932 Schmidt-Thomé P (1954): Klufttektonik und Grosstrukturen in den nördlichen Kalkalpen. Geol Rundsch 42 (2):172-187. https://doi.org/10.1007/bf01773956 Schneider H-J (1962): Bau des Wetterstein-und Mieminger Gebirges im Lichte von 100 Jahren geologischer Forschungsgeschichte. Jahrbuch des Deutschen Alpenvereins 87:77-94 Schuller V, Frisch W, Herzog U (2015) Critical taper behaviour and out-of-sequence thrusting on orogenic wedges – an example of the Eastern Alpine Molasse Basin. Terra Nova 27 (3):231-237. https://doi.org/10.1111/ter.12152 Schuster R (2015): Zur Geologie der Ostalpen. Abh Geol Bundesanst 64:143-165 Sieberer A (2020): Structural evolution of the northern Austroalpine margin, western Ammergau Alps, Bavaria. unpubl. Diploma thesis, Innsbruck Sieniawska I, Aleksandrwski P, Rauch M, Koyi H (2010): Control of synorogenic sedimentation on back and out of sequence thrusting: Insights from analog modeling of an orogenic front (Outer Carpathians, southern Poland). Tectonics 29:TC6012 Spang JH (1972): Numerical method for dynamic analysis of calcite twin lamellae. Geol Soc Am Bull 83 (1):467 - 472. https://doi.org/10.1130/0016- 7606(1972)83[467:NMFDAO]2.0.CO;2 Spötl C (1988): Zur Altersstellung permoskythischer Gipse im Raum des ostlichen Karwendelgebirges. Geol-Paläontol Mitt Innsbruck 14 (9):192-212

84 Spötl C (1989): The Alpine Haselgebirge Formation, Northern Calcareous Alps (Austria): Permo-Scythian evaporites in an alpine thrust system. Sediment Geol 65 (1):113-125. https://doi.org/10.1016/0037-0738(89)90009-2 Stingl V (1982): Sedimentologie und Vererzung des alpinen Verrucano im Stanzertal (Tirol). Geol-Paläontol Mitt Innsbruck 12:71 - 80 Stingl V (1984): Lagerungsverhältnisse des Permoskyths im Stanzertal, West-Tirol, Österreich. Mitt Geol Ges Bergbaustud 30/31:117 - 131 Stüwe K, Schuster R (2010): Initiation of subduction in the Alps: Continent or ocean? Geology 38:175-178. https://doi.org/10.1130/G30528.1 Suppe J (1983): Geometry and kinematics of fault-bend folding. American Journal of Science 283:684 - 721. https://doi.org/10.2475/ajs.283.7.684 Suppe J, Medwedeff DA (1990): Geometry and kinematics of fault-propagation folding. Eclogae Geol Helv 83:409-454 Termier P (1904): Les nappes des Alpes orientales et la synthése des Alpes. Bull Soc geol France 4 (3 (1903)):711-766 Tollmann A (1970a): Der Deckenbau der westlichen Nord-Kalkalpen. N Jb Geol Paläont, Abh 136:80 - 133 Tollmann A (1970b): Tektonische Karte der Nördlichen Kalkalpen, 3. Teil: Der Westabschnitt. Mitt Österr Geol Ges 62 (1969):78-170 Tollmann A (1976): Der Bau der Nördlichen Kalkalpen. Monographie der Nördlichen Kalkalpen, Teil III. Deuticke, Wien Trommsdorff V, Dietrich V, Flisch M, Stille P, Ulmer P (1990): Mid - Cretaceous, primitive alkaline magmatism in the Northern Calcareous Alps: significance for Austroalpine geodynamics. Geol Rundsch 79 (1):85 - 97. https://doi.org/10.1007/BF01830448 van Kooten WSMT (2016): The thrust between the Inntal and Lechtal nappe near the Nassreither Alm. Unpublished Bachelor Thesis, Innsbruck Vidal H (1953): Neue Ergebnisse zur Stratigraphie und Tektonik des nordwestlichen Wettersteingebirges und seines nördlichen Vorlandes. Geol Bav 17:56-88 von Soos P, Engel J (2008): Eigenschaften Von Boden und Fels–Ihre Ermittlung im Labor. In: Witt KJ (ed) Grundbau‐Taschenbuch. Ernst & Sohn, Berlin, pp 123-218. https://doi.org/10.1002/9783433600221.ch3 Weissert HJ, Bernoulli D (1985): A transform margin in Mesozoic Tethys: evidence from the Swiss Alps. Geol Rundsch 74:665-679 Wesnousky SG, Kumar S, Mohindra R, Thakur VC (1999): Uplift and convergence along the Himalayan Frontal Thrust of India. Tectonics 18 (6):967-976. https://doi.org/10.1029/1999tc900026 Winkler W (1988): Mid- to Early Late Cretaceous Flysch and Melange Formations. Paleotectonic Implications. Jb Geol Bundesanst 131:341-390 Zambanini J (2014): Geologie der Zugspitze und Umgebung. unpubl. Dipl.-Arb., Univ. Innsbruck, Innsbruck

86 3 Numeric Modelling The results of the structural investigations provide the starting point for the numeric modelling - the second part of this thesis. The numeric model focuses on the deformation behaviour of the Karwendel thrust and –thrust sheet, which is different in the Karwendel Mountains on one hand and the Mieming and Wetterstein Mountains on the other hand (see Figure 3 for location). In the Karwendel Mountains, the Karwendel thrust runs over kilometres parallel to bedding of the upper and lower thrust sheets, which is close to horizontal (Figure 4). In the Mieming and Wetterstein Mountains the same thrust is parallel to the bedding of the footwall but oblique to bedding of the hanging wall. In the Mieming and Wetterstein Mountains the Karwendel thrust truncates synclines of the hanging wall (Figure 5). Understanding the different appearances of the Karwendel thrust in the Mieming and Wetterstein Mountains and the Karwendel Mountains was the aim of the numeric modelling. A detailed description of the numeric model is given below. The numeric model was developed in cooperation with the Unit of Geotechnical and Tunnel Engineering from the University of Innsbruck. The wording of the geotechnical engineers and geologists partly differs. Some of the used terms are described here. The rheology of rocks is described in a material model. The material model describes the deformation behaviour of the material in dependence of the acting stress. Depending on the material model, the rheology is defined by material parameters. As we chose a linear elastic model, the ratio between stress and strain is linear (Figure 1). The material parameters for linear elastic models are expressed by the Young`s modulus (E-module) and the Poisson ratio. In geologic literature, the materials are often generalised as competent and less competent or incompetent rocks (e.g., Ramsay and Huber, 1987). In this study the competence is expressed by a higher E-module (stiffer) for competent rocks and a lower E-module (less stiff) for incompetent rocks.

Figure 1: Stress-strain diagram for the linear elastic material model.

87 All descriptions concerning the model were made in accordance with Priv.-Doz. Dr. tech. Barbara Schneider-Muntau from the Unit of Geotechnical and Tunnel Engineering, all descriptions concerning rocks are based on geological nomenclature.

3.1 Initial situation The prediction of structures in fold-and-thrust belts into the subsurface was driven by the exploration industry because fold-and-thrust belts are one of the principal sources of hydrocarbons, and increasingly also of hydrothermal energy (Goffey et al., 2010; Lacombe et al., 2007; Nemcok et al., 2005). The description of the internal geometry of fold-and-thrust belts often bases on geometric models, such as the fault-bend fold model (Suppe, 1983), or the fault- propagation fold model (Suppe and Medwedeff, 1990), where the geometry of the hanging wall is dependent on the geometry of the underlying fault (fault-related folding). It is, however, also known for a long time, that the dimensions of folds or lengths of horses in a duplex are dependent on the thickness of the competent units in the allochthon (Boyer, 1992; Currie et al., 1962; Trümpy et al., 1969), rendering the abovementioned prerequisites at least incomplete. If the geometry of the allochthon is dependent on the thickness of the competent layer, rheology of the deforming material must be taken into consideration. Therefore, also mechanical models need to be considered when describing fold-and-thrust belts. Buckling is a process, that affects layers when contracted along their lengths (Butler et al., 2019). Buckle folds depend on the rheology of the layers involved, and are often described as detachment or fault-propagation folds, which is not entirely accurate (see Butler et al. 2019 for discussion). In this study, we exclusively used the term buckle folds because we want to emphasise on the rheology dependence of the folds. The buckle fold model (Biot, 1961; Ramberg, 1960) is a mechanical model that explains fold shapes based on the rheology of the material involved. It was first described for single layer folding of a competent layer in a less competent matrix (Biot, 1961; Ramberg, 1960) but also for multilayer folding (Ramsay and Huber, 1987). The use of a buckle fold model in thrust belts has some advantages, as this model can explain the existence of folds with round hinges lacking kinks, as observed in many thrust belts, for example in the Helvetic nappes of Switzerland (Casey and Dietrich, 1997; Rowan and Kligfield, 1992; Sommaruga, 1999), and also in the study area. The development of a buckle fold train during progressive shortening is different from other types of folds, as sketched in Figure 2. Shortening from one side will buckle a competent layer above a décollement (Figure 2a). The buckle folds will have decreasing amplitude with increasing distance from the moving end, as part of the shortening will be consumed by folding.

88 This has been shown in analogue experiments (e.g., Price, 1975) and field examples (e.g., Espina et al., 1996). Deformation can progress until a limited interlimb angle then the folds lock (Figure 2b; Ramsay, 1974). Continued deformation may result in break-thrust faults (Woodward, 1997) in the internal part of the fold train (Figure 2c). Depending on the amplitude of the folds in the foreland of the newly developing break-thrust (Fischer et al., 1992; Woodward, 1997), the latter may truncate the folds in the foreland (Figure 2d).

(a) Buckle folding with amplitudes decreasing toward external (c) Nucleation of a new thrust across the locked folds folds

décollement break thrust (b) Locking of internal folds that have reached a minimum (d) Propagation of the thrust through existing and probably still opening angle growing folds break thrust

Figure 2: Schematic model of the development of a buckle fold train during progressive shortening from the left, pin line at the right end of model.

3.2 Aims of the study Structures in the Karwendel, Mieming and Wetterstein Mountains are the focus of this study. The main reason for this is that they offer exposed structures over kilometres, and the thrust boundaries are accessible. The timing of thrust movements is well known as thrusting and sedimentation took place at the same time and the syntectonic deposits monitor the ongoing thrust movements (Eisbacher and Brandner, 1995, 1996; Eisbacher et al., 1990; Gaupp, 1982, 1983; Ortner, 2003, 2016; Ortner and Gaupp, 2007). Thrusting in the Northern Calcareous Alps took place from the late Early Cretaceous onward as a consequence of closure of two oceans and subsequent collision (Schmid et al., 2004). In the Karwendel mountains the Karwendel thrust which brings Permian to Triassic sediments on top of Cretaceous sediments runs over kilometres bedding parallel to the hanging wall (Karwendel thrust sheet) and the footwall (Tannheim thrust sheet). The Mieming and Wetterstein Mountains show a different picture. Structures of the hanging wall (Karwendel thrust sheet) are truncated by the Karwendel thrust (see Figure 5). From south to north, the thrust cuts across a sedimentary succession of the hanging wall. Therefore, we assumed that the hanging wall was folded prior to stacking of thrust sheets, and the thrust must have propagated across the pre-existing folds (see also cross sections in Müller-Wolfskeil and Zacher, 1984; Ortner, 2003), as sketched in Figure 2. The observed structures in the Karwendel- and the Mieming and Wetterstein mountains are part of the same thrust sheet boundary (Kilian and Ortner, 2019; Study 2). The sedimentary

89 succession above and below the thrust is roughly the same. A very weak décollement (salt) may make the difference.

92 We see the two cross sections (Figure 4 and 5) as two possible end members of a deforming multilayer and formulate the following hypotheses for this study:

Hypothesis 1 For the Karwendel Mountains, we assume that transport happened without folding prior to thrust sheet transport where a salt bearing formation along the décollement is present. Buckle folds above the décollement develop after the emplacement of thrust sheets as structural data indicate (Kilian and Ortner, 2019).

Hypothesis 2 For the Mieming and Wetterstein Mountains, we suggest the absence of a salt bearing formation along the décollement. Rheology differences cause buckle folding, but in contrast to the Karwendel Mountains, already during thrust sheet transport. When the folds reach a maximum tightness, a thrust nucleates in the core of them. This thrust represents a new main thrust, which finally cuts through the pre-existing fold train (Figure 2a-d).

Based on these hypotheses we set up a numeric model to answer the following questions 1. What caused the development of large-scale buckle folds in the study area? 2. Are rheological differences along the décollement the reason for the different appearance of the thrust sheet boundary in the Karwendel- and in the Mieming and Wetterstein Mountains? Note that the hypotheses were developed before numeric modelling was performed. Hypothesis 2 had to be rejected. The new interpretation of the structures observed in the Mieming and Wetterstein Mountains is discussed in Study 2 and in the discussion (ch. 4).

3.3 Software and model set up The numeric simulations were realised with the finite element software Abaqus/CAE. Finite element methods (FE-Methods) are numeric solutions for problems with infinite degrees of freedom. By discretisation, the degrees of freedom become finite. A determined number of elements represents the complex model. A linear elastic material model was used for the following reasons. (1) The focus was on modelling folding and not on faulting. A material model that includes a failure criterion could be used for modelling faulting but stops the calculation before folding can develop. (2) Modelling concentrated on the Karwendel thrust sheet, which was eroded prior to transgression of the synorogenic Gosau Group of Late Cretaceous age (Krois and Stingl, 1994;

93 Ortner, 2003). We suggest a free surface at the time of the development of hanging wall folds and rocks deform elastically at shallow levels (Jeng and Huang, 2008). (3) The numeric model focused on the development of folds in dependence of rheological contrasts. Time was not considered, as is done in the case of viscous rheology.

Before we started modelling, we decided to model with three layers based on the sedimentary succession of the western Northern Calcareous Alps (Figure 6). Modelling with three layers is a strong simplification of the observed succession of rocks. The model focuses on groups of rocks, which are summarised in layers with specific material parameters. The thickest rock units in a group determine the material parameter of the whole layer. This means, e.g., that up to 3000 m thick limestone and dolomite successions of layer 2, determine the layer characteristic of layer 2 (Figure 6) even though shales and evaporites are intercalated, but are of minor thickness (about 400 m). These simplifications allowed us to concentrate on the research question and to test several settings with changing starting conditions. We tested e.g., the influence of element size (bigger elements give less accurate results, small elements increase calculation time), top load (a hypothetical overburden), material parameter (for linear elastic models, the Young´s Modulus and the Poisson ratio) and fixed nodes. We use elements with four nodes. Fixing nodes means that each element node can be fixed in x- and/or z-direction, and no displacement in x-and/or z-direction is possible (Figure 6). The final fixed nodes are close to an assumed behaviour of a thrust sheet. Along the base of the model, it is free to move in x-direction supposing that the upper thrust sheet glides along the décollement. From structural studies in the Karwendel Mountains we know that the Karwendel thrust is parallel to bedding and the fixed points along the bottom line therefore fit to field observations. The model is not allowed to move along the base in z-direction because then it would deform the underlying thrust sheet. The right end is fixed because from structural studies in the Karwendel we know that buckling of the hanging wall stops when the hanging wall and footwall are folded together (see cross sections in Study 3). Material parameter are determining for the development of buckle folds. We tested several variations of material parameter. Even if the model has just three layers there are many possible variations. We used published material parameter (Czech, 1990; von Soos and Engel, 2008). In pilot experiments it turned out that without a high competence contrast between layer 1 and layer 2 no buckling was possible. During the pilot tests, we further observed that the thickness of the layer with the highest E- module (layer 2) influenced the possibility to buckle. We simulated a possible erosion of layer

94 3 and 2 and the re-sedimentation of sediments on an eroded layer 2 (as known from the western NCA). The final model is based on a combination of literature values, experience from pilot- experiments and attempts to get as close as possible to natural conditions.

Figure 6: Final numeric three-layer model.

96 3.4 Study 3

Buckle folding in fold-and-thrust belts- a comparison between field observations and numeric experiments from the Northern Calcareous Alps

Sinah Kilian, Hugo Ortner, Barbara Schneider-Muntau In preparation for Submission

98 Buckle folding in fold-and-thrust belts - a comparison between field observations and numeric experiments from the Northern Calcareous Alps S. Kilian1, H. Ortner1, B. Schneider-Muntau2 1 Institute of Geology, University of Innsbruck, Austria 2 Unit of Geotechnical and Tunnel Engineering, University of Innsbruck, Austria Corresponding author: Sinah Kilian ([email protected])

Key words: Buckle folds, décollement folds, detachment folds, linear elastic, numeric modelling, Abaqus ______

Abstract We report the results of a numerical modelling study based on folds in the western Northern Calcareous Alps (NCA) fold-and-thrust belt. We study boundary conditions of folding based on the mechanical properties of the rocks involved. One key control on the model is stratigraphy that can be simplified to three layers: (1) an incompetent base along the décollement of the thrust sheet, and (2) an up to 3 km thick competent layer. The (3) incompetent top layer has no major control on folding. The incompetent base layer needs to be very weak to facilitate folding and is a salt-bearing evaporitic unit. Another key is erosion prior to folding. It needs to remove half of the competent layer to allow the creation of folds with limb lengths comparable to field observations. The results of the numeric model contribute to the understanding of the structural development in the western NCA. Folding within the upper thrust sheet was only possible above a salt-bearing décollement and after a decrease in lithostatic pressure related to Upper Cretaceous and/or Paleogene erosion. ______

1 Introduction Fold-and-thrust belts are one of the principal sources of hydrocarbons (Goffey et al., 2010; Lacombe et al., 2007; Nemcok et al., 2005), and increasingly, of hydrothermal energy. The understanding of the internal geometry is therefore a prerequisite to uncover hidden structures. The description of the internal geometry of fold-and-thrust belts often bases on geometric models, such as the fault-bend fold model (Suppe, 1983), or the fault-propagation fold model (Suppe and Medwedeff, 1990), where the geometry of the hanging wall is dependent on the geometry of the underlying fault (fault-related folding). However, foreland fold-and-thrust belts frequently show a basal evaporitic décollement horizon, e.g., Appalachian Plateau, Pyrenees, Carpathians, Zagros Mountains (Davis and Engelder, 1985). Fold geometries within in a salt- floored fold-and-thrust belts vary from the classical models (as proposed by e.g., Suppe 1983). The deformation style depends on the thickness and the mechanical properties of the involved layers (e.g., Davis and Engelder, 1985). A common attribute in salt-floored fold-and-thrust belts is the development of buckle folds above the décollement (e.g., Poblet et al., 1998). Buckle

99 folds develop due to competence contrast between the basal incompetent décollement (often salt) and the competent units above. The shape of buckle folds is entirely controlled by the physical and mechanical properties of the layers (Barani, 2012; Currie et al., 1962; Davis and Engelder, 1985).

The Northern Calcareous Alps (NCA) of the eastern European Alps are a thin-skinned fold- and-thrust belt. Numerous studies (e.g., Eisbacher and Brandner, 1996; Heißel, 1978; Kilian and Ortner, 2019; Linzer et al., 1995; Tollmann, 1976a) showed that the Permian salt-bearing succession (Haselgebirge) is the basal décollement for the upper thrust sheet (Karwendel thrust sheet) of the western NCA. Also other thrust sheets of the NCA were separated from their former basement along this décollement (Linzer et al., 1995; Eisbacher and Brandner, 1996; Granado et al., 2018), and the role of salt tectonic processes in the eastern NCA was highlighted recently (Granado et al., 2018). The focus of this study are the km-scale folds above one of the main thrusts (Karwendel thrust) in the Karwendel Mountains (Fig. 2). Our structural investigations of macroscale structures lead to the assumption that km-scale folds above the thrust plane are buckle folds, developed due to rheology contrast between the décollement and the sedimentary succession above.

This study aims to answer the following questions using a mechanical numeric model: (1) Is it mechanically possible to buckle the sedimentary succession of the western NCA during shortening? (2) Does erosion and later re-sedimentation influence the development of buckle folds? (3) Which contribution do the numeric results give to the understanding of the structural development in NCA?

1.1 Geologic setting The NCA belong to the Eastern Alps and the term itself is a regional term, not implicating a tectonic unit. In a plate tectonic context, the NCA belong to the Alcapian plate (Handy et al., 2010), meaning a part of Adria, which includes continental and oceanic parts of a microplate between the European, Iberian and the African plate. From the Cretaceous on to the Early Cenozoic Alcapia was separated from Adria. Alcapia derives from Alcapa (Alps-Carpathians- Pannonian basin) including the Austroalpine units of the Eastern and Western Capathians and remnants of the north-western end of the Meliata-Maliac Ocean (Schmid et al., 2004). The NCA represent the most external units of the Austroalpine (e.g., Schmid et al., 2004; Tollmann, 1976b), and constitute a thin-skinned fold-and-thrust belt. The sedimentary succession of the study area reaches from the Permian to the Cretaceous.

1.2 Stratigraphic development The Permian to Triassic sediments of the NCA were deposited on the passive continental margin of Pangea, towards the Meliata branch of the Neotethys ocean (e.g., Haas, 1995; Lein, 1987; Schmid et al., 2004; Stampfli et al., 1998). In the study area, the oldest sediments are

100 Permian sediments (Haselgebirge) represented by grey shales, cellular dolomites and reddish to greenish quartz-rich sandstones which can be found in surface outcrops. In the subsurface, large amounts of anhydrite and gypsum are known and salt has been mined (e.g., Schmidegg, 1951, Spötl, 1989). Lower Triassic quartz sandstones (Alpiner Buntsandstein) which are intercalated by marls and clays (Tollmann, 1976b) follow the Permian succession. The Lower Triassic sandstones are in contact to the Anisian limestone and evaporites (Reichenhall Fm.), represented by cellular dolomites, stromatolithic dolomites and limestones. The cellular dolomites are the residue of sulphate evaporites in the subsurface. Anisian to Ladinan limestones (or Alpine Muschelkalk Group; Bechstädt and Mostler, 1976) follow above the last cellular dolomites. These start with a succession of well-bedded micritic limestones that are often strongly bioturbated (Virgloria Fm.). Massive limestone represents an Anisian carbonate ramp (Steinalm Fm.; Nittel, 2006; Rüffer and Zamparelli, 1997). Break-up of this carbonate ramp initiates facies differentiation between basins filled by nodular cherty filament-bearing limestones intercalated with tuff layers (locally named Reifling Fm.; Nittel, 2006; Brühwiler et al., 2007), and the Ladinian carbonate platform (Wetterstein limestone). The Ladinan Wetterstein limestone is the first major carbonate platform in the NCA and reaches a thickness of up to 2200 m in the Karwendel Mountains (Kilian and Ortner, 2019). A succession of Carnian shales, limestone, dolomites and cellular dolomites (Northalpine Raibl Beds; Jerz, 1966) follows on top of the Wetterstein limestone. In the Norian another carbonate platform developed, the Norian Hauptdolomit, a well bedded, partly stromatolithic dolomite (Fruth and Scherreiks, 1982; Müller-Jungbluth, 1971). Toward the top, this Norian platform drowns, and an intercalation of dolomite and limestone which becomes more and more calcareous was deposited in a subtidal environment (Plattenkalk; Müller-Jungbluth, 1971). Ultimately, drowning led to the formation of basins filled with marly limestone (Kössen Fm.) interfingering with platform carbonates (Oberrhät limestone; Golebiowski, 1991) during the Rhaetian.

The Jurassic sedimentation is controlled by rifting and the opening of the Alpine Tethys (Handy et al., 2010; Schmid et al., 2004). Alcapia, including the future NCA, was separated from Adria (Handy et al., 2010). The new continental margins drowned, ending shallow marine deposition, and pelagic sediments settled throughout the Jurassic and Early Cretaceous. Rift-related normal faulting caused facies differentiation (e.g., Eberli, 1988; Jacobshagen, 1965; Nagel et al., 1976). Submarine highs are represented by condensed red nodular, micritic limestones (Adnet Fm.) and basins are filled with marly limestones reaching more than one kilometre of thickness (Allgäu Fm.; Jacobshagen, 1965). Synrift deposition ends at the turn to the Upper Jurassic when variegated cherts (Ruhpolding radiolarite) accumulated (Diersche, 1980). The Upper Jurassic to Lower Cretaceous sediments consist of dense pelagic, sometimes marly, well bedded, micritic limestones intercalated with thin marl layers (Oberalm Fm.; Tollmann, 1976a). The Lower Cretaceous sediments (Berriasian to Albian) consist of marls occasionally intercalated with sandstones (Schrambach Fm.; Nagel et al., 1976).

101

The Gosau Group represents synorogenic sediments of Late Cretaceous to Paleogene age, which are distributed over the whole width of the NCA (Faupl et al., 1987; Wagreich and Faupl, 1994). Their appearance ranges from clastic sediments (marls, sandstones and conglomerates), to shallow marine carbonates that were deposited on mixed carbonate-siliciclastic shelfs (e.g., Sanders and Höfling, 2000). The sedimentary succession described here is the base for the numeric model and is determining the material characteristics of the layers of the model.

Fig. 1: Stratigraphic succession of the NCA in the study area and its translation to mechanically homogeneous layers of the numeric model.

1.3 Deformation during Alpine orogeny Alpine orogeny is related to the closure of two oceans, the Neotethys and the Alpine Tethys (Handy et al., 2010): (1) Cretaceous (Eoalpine) orogeny occurred after obduction of Meliata ophiolites onto the southeastern Adriatic margin, with the Austroalpine, Adria-derived units being in lower plate position (Schmid et al., 2004; Stüwe and Schuster, 2010).

102 (2) Paleogene (Mesoalpine) orogeny was related to the extinction of the Alpine Tethys (Piemont-Liguria- and Valais ocean; Handy et al., 2010) in the Late Eocene, and this time the Austroalpine, including the NCA, was in upper plate position (Schmid et al., 2004; Stüwe and Schuster, 2010). Within the internal Austroalpine basement units, Eoalpine and Mesoalpine stacking is clearly separated by Upper Cretaceous extension (Froitzheim, 1994), but in the external thrust sheets (as the NCA are), Cretaceous to Cenozoic shortening is continuous, as documented by growth strata in different overlapping synorogenic successions (Ortner, 2001, 2003a and b; Ortner and Gaupp, 2007; Ortner et al. 2016). Nappe stacking in the western NCA started in the Hauterivian to Albian by imbrication of the Karwendel thrust sheet onto the Tannheim thrust sheet and propagated progressively into the more external parts of the NCA fold-and-thrust belt, incorporating the Cenoman-Randschuppe and the Penninic Arosa Zone in the Turonian to Coniacian, and the Rhenodanubian Flysch nappes in the Maastrichtian (Ortner, 2003a). The thrust sheets of the western NCA can be distinguished by their time of emplacement (Ortner, 2003a; Kilian and Ortner, 2019) using the youngest sediments below a thrust and transgression sediments on top of a thrust sheet. The tectonically deepest thrust sheet of the western NCA is the Tannheim thrust sheet, which is overlain by the Karwendel thrust sheet, separated by the Karwendel thrust. Eoalpine, mid-Cretaceous nappe stacking, transport and folding was NW- to NNW-directed, while subsequent Mesoalpine Late Cretaceous to Paleogene shortening was N- to NNE-directed (Eisbacher and Brandner, 1996; Ortner, 2003a). Neoalpine late Paleogene to Neogene shortening was NE-directed (Ortner, 2003b; Peresson and Decker, 1997). After Cretaceous thrust sheet emplacement, the thrusts of the NCA started to be folded as the active detachment shifted into the footwall. Locally, thrusts breaking across these folds evolved. Shortening can be as young as Miocene (Kilian and Ortner, 2019). As a consequence of nappe stacking and thickening of the orogenic wedge, regional erosion prior to deposition of synorogenic sediments affected the NCA (Branderfleck Fm. and Gosau Group; e.g., Gaupp, 1982; Ortner and Gaupp, 2007; Wagreich and Faupl, 1994). In the western NCA the Karwendel thrust sheet was affected by erosion down to the Ladinian platform carbonates prior to the transgression of Cretaceous (Gosau Group; Krois and Stingl, 1994; Ortner, 2003a) or Paleogene (Inneralpine Molasse; Schmidegg, 1951) synorogenic sediments. Therefore, shortening and folding took place after a decrease in overburden in the late Early Cretaceous. Late Cretaceous syntectonic sediments (Gosau Group) record folding into the Paleogene (Auer and Eisbacher, 2003; Ortner and Gaupp, 2007; Ortner, 2001, 2003a; Ortner, et al., 2016).

1.4 Local tectonic setting We studied the Karwendel thrust and the overlying Karwendel thrust sheet in the Karwendel Mountains (Fig. 2) of the western NCA. The Karwendel thrust was one of the first thrusts of the NCA recognized at the beginning of the 19th century (Ampferer, 1902). It emplaces Permian to Triassic rocks on top of Cretaceous rocks (Fig. 3). The thrust follows the Permian evaporites (remnants of salt bearing Haselgebirge), but is mostly found at the base of the Triassic

103 succession of the hanging wall (Reichenhall Fm.; Fig. 3). Only in the southernmost part of Karwendel Mountains larger volumes of the Permian succession (Haselgebirge) are preserved (Fig. 2 and 4).

104 In the Karwendel Mountains the Cretaceous nappe stacking was NW-directed. Mesoscale observations showed that shortening directions changed from NW (end-Albian) to N to NE in the Paleogene (Kilian and Ortner, 2019). The thrust is parallel to bedding in the immediate footwall and hanging wall, which is close to horizontal on hectometre scale (Fig. 3; Kilian and Ortner, 2019). However, the hanging wall succession above the thrust plane is folded on the kilometre-scale (Fig. 4). The W- to WNW-trending fold axes of these km-scale folds suggest a Paleogene age. Folds above the thrust plane are therefore younger than the Karwendel thrust, which is of Cretaceous age. Folds are mostly restricted to the hanging wall but also refold the Karwendel thrust locally (Fig. 4). This suggests that at least parts of the hanging wall were decoupled from the footwall during ongoing deformation after the emplacement of thrust sheets (Kilian and Ortner, 2019). The shape of the folds with round hinges and the accumulation of weaker material in the core of the folds (Fig. 4b) suggest that the folds are buckle folds caused by rheological differences between the detachment horizon (Permo-Triassic evaporites) and the competent sediment succession above (Anisian to Norian carbonates). Two cross sections (Fig. 4) summarize structural studies in the Karwendel Mountains.

Fig. 3: Key outcrop of the Karwendel thrust in the Karwendel mountains at Halftergraben (see Kilian and Ortner 2019 for a detailed description). Medium scale buckle folds in Triassic sediments (Reichenhall Fm.) above the Karwendel thrust developed due to the competence contrast between carbonate beds and cellular dolomites. The Karwendel thrust is parallel to bedding in the immediate footwall and hanging wall, which is close to horizontal on hectometre scale. The thrust cuts the sedimentary succession of the hanging wall (Reichenhall Fm) at a very low angle, truncating single beds (red arrow). For location see Fig. 4a.

105

Fig. 4: a) Cross section 1: In the south folds are restricted to the hanging wall, while in the north the hanging wall and the footwall are folded together. b) Cross section 2: Folds are restricted to the hanging wall. The weaker material (coloured in yellow) accumulates in the fold core. All folds show Paleogene (N-NNE) compression. fap = fold axial planes; fa = fold axes.

106 1.5 Buckle folding Buckle folds develop due to rheological differences in layered rock and scale from millimetres to kilometres (e.g., Abbassi and Mancktelow, 1992; Biot, 1961; Ramsay and Huber, 1987; Sherwin and Chapple, 1968). Buckle folds of isolated competent layers in a weaker matrix are quite common (e.g., Sherwin and Chapple, 1968; Hudleston and Holst, 1984). Large-scale buckle folds are usually referred to as detachment folds, which develop due to shortening and/or shearing of multilayers above a basal décollement (Butler et al. 2019; Rowan and Kligfield, 1992). The end-member of the buckle fold theory for multilayers with a low proportion of incompetent to competent layer thickness and a high viscosity contrast between competent and incompetent layers (e.g., Ramsay and Huber, 1987) is comparable to the geometry of fault-bend folding. In regional studies on the geometry and kinematics of fold-and-thrust belts, a continuous transition from thrusting with fault-bend folding to buckle folding on top of a décollement has been observed (e.g., Ortner et al., 2015; Pfiffner, 1993). There exists a wide range of numerical simulations on buckle folds (e.g., Frehner, 2011; Lan and Hudleston, 1991; Llorens et al., 2013; Schmalholz and Schmid, 2012). Most models describe the fold shape analytically in different settings (single layer, or multilayer) with different material rheologies (elastic, visco-elastic, viscous) (e.g., Biot, 1957; Biot, 1961; Ghassemi et al., 2010; Huang et al., 2010; Schmalholz and Schmid, 2012; Ramberg, 1963). Already Biot (1961) found that the wavelength (or arc length) of buckle folds depends on the competence contrast of the buckling layer (competent layer) and the surrounding matrix (incompetent layer). Generally, small-scale folds can ignore the effect of gravity whereas the development of large-scale folds is gravity dependent. In nature, both matrix and gravity resist folding but there is no simple analytical way to express that (Schmalholz et al., 2002).

2 Numerical modelling

2.1 Model set up: from field to model The numeric model is based on the sedimentary record of the hanging wall and the material characteristics of the layers. Generalizing the sedimentary succession, we distinguish three rheologically different layers. Describing the rheology of the layers, we differentiate between incompetent and competent rock (Fig. 1). 2.1.1 Layer 1 The décollement of the Karwendel thrust sheet are Permian to Triassic sediments (Haselgebirge, Alpiner Buntsandstein, Reichenhall Fm) and therefore the original thickness is not preserved. In the Halltal area (indicated by the salt mine in Fig. 2) the strongly deformed Permian succession is exposed at the Karwendel thrust. Cross sections of the Halltal area show a present-day thicknesses of at least 200 to 700 metres (Schmidegg, 1951), but the original stratigraphic thickness is difficult to assess due to tectonic deformation (Leitner and Spötl, 2017; Tollmann, 1976b, Schauberger, 1986). Salt itself flows over geologic time scales

107 (Jackson and Hudec, 2017), causing thickness and distribution variations of salt bodies and within the overlying Triassic sediments (e.g., Granado et al., 2018). In the southern part of the Karwendel Mountains, remnants of the Lower Triassic rocks (Alpiner Buntsandstein) are found at the décollement. In the most parts of the western NCA, however, Triassic cellular dolomites and dolomites of the Reichenhall Fm. (Miller, 1965; Nagel et al., 1976) are the basal unit on top of the décollement (see Fig. 3). In the study area the Permo- Triassic succession is truncated at the base by the Karwendel thrust (marked with a red arrow in Fig. 3). The remaining thickness on top of the thrust is 300 m. However, it may have been significantly thicker prior to thrust transport. We ultimately decided to use 600 m of thickness for the incompetent layer 1 as the real thickness is unknown. In pilot models we tested the influence of boundary effects and 600 m of thickness sufficed to minimise boundary effects from the bottom line (Fig. 5). 2.1.2 Layer 2 The Anisian to Ladinian limestone, the Ladinian carbonate platform, the Carnian shales and the Norian dolomites, follow above the Permo-Triassic succession. The Anisian to Ladinian limestone (Alpine Muschelkalk Group) is up to 500 m thick (Frisch, 1975; Miller, 1965). The thickness of the Ladinian carbonate platform (Wetterstein limestone) is variable, as the platform interfingers with basinal sediments in variable directions (e.g., Sarnthein, 1967). Up to 2200 m are present in the study area (Kilian and Ortner, 2019). Sarnthein (1965) measured a thickness of up to 1730 m for the Ladinian carbonate platform in the Karwendel Mountains north of Innsbruck. Based on 3D-modelling, Ortner (2015) deduced a thickness of 1400 m (measured) for the Ladinian carbonate platform in the Zugspitze massif of the Wetterstein Mountains, however the contact to the Carnian succession at the top is missing. An average thickness of 1500 m represents the Ladinian carbonate platform in the model. The Carnian succession, with approximately 400 m, appears between the Ladinian carbonate platform and the Norian dolomite. We omitted the Carnian rocks as an incompetent layer in the model because we assumed that the thick competent units (limestone and dolomite) determine the rheology. For the Norian dolomite (Hauptdolomit) Miller (1963a) described 1000 m of thickness in the Mieming Mountain chain. Donofrio et al. (2003) observed 2700 m in the Karwendel Mountains close to Seefeld (Tirol). Behrmann and Tanner (2006) specified 700 to 2500 m. The thickness of the Norian dolomite varies in the NCA, for the model we used a thickness of 1000 m defined by Miller (1963a) for the Mieming Mountains because in the Karwendel the upper part of the Norian dolomite mostly eroded and the real thickness is speculative. In total the competent layer 2 has a thickness of 3000 m. 2.1.3 Layer 3 The third layer divides in layer 3a and b. However, erosion exhumed the Ladinian platform carbonates to the surface during nappe stacking. Therefore, we tested two scenarios: (1) Layer 3a represents the Upper Triassic to Lower Cretaceous succession, which is preserved in the northern part of cross section 1 (Karwendel syncline of Fig. 4a). 108 (2) Layer 3b represents the Upper Cretaceous Gosau deposits on the top of the eroded thrust sheet after thrust sheet emplacement (see chapter 1.3). In scenario (1) the sedimentary succession of the upper thrust sheet continues with shallow marine to pelagic sediments of the Upper Triassic to the Cretaceous (Kössen Fm, Oberrhät limestone, Kendlbach Fm., Ruhpolding radiolarite, Oberalm Fm. and Schrambach Fm; Fig. 1). Layer 3a assumes continuous sedimentation throughout the Late Triassic into the Early Cretaceous. Layer 3a is incompetent, as the only competent unit, the Oberrhät limestone is thin, and it is absent in much of the area (Nagel et al., 1976; Palotai et al., 2017). We used a thickness of 800 m for layer 3a based on Palotai et al. (2017) and Nagel et al. (1976). In scenario (2) layer 3b represents the synorogenic Gosau Group, which was deposited unconformably on top of the NCA orogenic wedge after initial nappe stacking and related major erosion (e.g., Wagreich and Faupl, 1994; Ortner, 2003a). Erosion removed layer 3a, and parts of layer 2. Material characteristics of layer 3b are comparable to Layer 3a. The thickness is highly variable, and dependence on faulting (Wagreich and Decker, 2001) or folding has locally been demonstrated (Ortner, 2001; Ortner, et al., 2016). In spite of the present-day patchy distribution (Fig. 2), the Gosau Group is regarded to represent erosional remnants of an originally continuous sedimentary cover (Wagreich and Faupl, 1994). Even though no synorogenic Gosau sediments were observed in the immediate study area, we modelled them because in the Karwendel Mountains neptunian dykes of potential Late Cretaceous age were observed (Fig. 2; Krois and Stingl, 1994). Alternatively, these dykes are of Paleogene age (Schmidegg, 1951). For the model, we used different thicknesses for layer 3b from 200 m to 1800 m.

2.2 Model realisation The numeric simulations were realised with the finite element software Abaqus. Abaqus is a program designed for problems of solid mechanics and dynamics using a continuum approach. It allows interactive designing of models, running the models and analysing the results. Abaqus works dimensionless meaning the user has to define a coherent unit system, which is important when defining material parameters. Finite element methods (FE-Methods) are numeric solutions for problems with infinite degrees of freedom. By discretisation, the degrees of freedom become finite. A determined number of elements represents the complex model. Abaqus has been used previously successfully in studies on rock deformation (e.g., Eckert et al., 2014; Guest et al., 2007; Huang et al., 2010; Jeng et al., 2002; Liu et al., 2016). The numeric model was developed in cooperation with the Unit of Geotechnical and Tunnel Engineering from the University of Innsbruck. The wording of the geotechnical engineers and geologists partly differs. Some of the used terms are described here. The rheology of rocks is described in a material model. The material model describes the deformation behaviour of the material in dependence of the acting stresses. The rheology is thereby defined by material parameters. The material parameters for linear elastic models are expressed by the Young`s modulus (E-module) and the Poisson ratio. In geologic literature, the materials are often generalised as competent and less competent or incompetent rocks (e.g., Ramsay and Huber,

109 1987). In the numeric model the competence is expressed by a higher E-module (stiffer) for competent rocks and a lower E-module (less stiff) for incompetent rocks. The linear elastic material model was used for the following reasons. (1) The focus was on modelling folding (deformation) and not on faulting (failure). A material model that includes a failure criterion could be used for modelling faulting but stops the calculation before folding can develop. (2) Structural data showed that the large km-scale folds of the Karwendel Mountains are of Paleogene age and formed during Mesoalpine shortening. They were not formed during Lower Cretaceous, Eoalpine stacking of nappes. As a consequence of erosion following nappe stacking, we suggest a free surface at the time of the development of folds. At shallow levels, rocks deform elastically (Jeng and Huang, 2008). (3) The numeric model focused on the development of folds in dependence of rheological contrasts. Time was not taken into account, as is done in the case of viscous rheology. (4) As outlined above the studied folds are buckle folds, contemporaneous flexural slip is possible but neglected in the model. The model is a 2D plane strain model. Plane strain assumes no change in geometry with depth. This holds for problems where the variation with depth is not meaningful. Simplicity and a quick change of possible settings stood in the foreground. We did not predefine perturbations in the competent layer to produce folding, as many other studies do (e.g., Jeng et al., 2002; Schmalholz and Schmid, 2012).

2.3 Material parameters The model consists of three layer whereas each layer has different material characteristics but is in itself homogeneous. The Young’s modulus and the Poisson ratio determine the rock parameters in elastic material models. The Young’s modulus (E-module) is given in kilo Newton per square meter (kN/m²), the Poisson ratio is dimensionless. Rock parameters derive from geotechnical tests of homogenous rock samples and are always just an extract of natural properties of rocks. In pilot models, we tested the influence of the defined layer parameters (Young’s modulus and Poisson ratio for linear elastic models) on the possibility of folding. According to these pilot tests, competence contrast and the material parameters of the basal layer 1 control the ability of folding. Material parameters were chosen according to pilot models and published data (e.g., Czech and Huber, 1990; von Soos and Engel, 2008; see Table 1). The base layer 1 represents the Permo-Triassic evaporites (Haselgebirge, Alpiner Buntsandstein, Reichenhall Fm.). Published material parameters for the Young`s modulus declare values ranging from 4.500.000 to 28.600.000 kN/m² for the Permian evaporites (Haselgebirge; Czech and Huber, 1990). Li (2013) modelled the E-module for salt in triaxial numeric experiments and calculated a value of 10 GPa (1.000.000 kN/m², at temperatures of about 50°C). Typical depths for a basal detachment are around 2-6 km and a geothermal gradient of 20-35°C/km (Davis and Engelder, 1985). Modelled data (Li, 2013) would reflect a detachment in a depth of 2,5 km.

110 Our chosen value is 2.000.000 kN/m² because we do not know how much salt was in the system and we assume a mixture of salt and marls is the detachment horizon (see chapter 1.2). This value turned out to work best in the pilot models. If the E-Module is too high, the competence contrast between layer 1 and 2 is too low for buckling. If the E-Module is too low, only the base layer deforms until the large gradient in stresses between the layers stop calculation before layer 2 is folded. Values from literature Values used for the model

Rock E-module Poisson ratio Layer Layer E-module Poisso (kN/m²) (von Soos and thickness (kN/m²) n ratio (Czech, 1990) Engel, 2008) (m) Lower upper clay shales: Layer 3 800 10 000 000 0.25 Cretaceous 31 000 000 0.3 (TOP) rocks (Schrambach Fm.) Norian dolomite 37 000 000 - dense limestone: Layer 2 3000 80 000 000 0.2 (Hauptdolomit) 51 100 000 0.2 (MIDDLE)

Permian 4 500 000 - Layer 1 600 2 000 000 0.25 evaporites 28 600 000 (BOTTOM) (Haselgebirge)

Table 1: Selected values from literature review and values used for the simulation. The used values are composed values from published data and pilot-tests.

2.4 Discretisation Discretisation is the process that divides the whole model in subdomains, in this case in elements. All elements are plane strain elements with four nodes. Plane strain elements simulate strain in x- and z-direction, therefore only vertical and horizontal strain can be applied. Elements connect through nodes to build a mesh (Fig. 5Fig. ). The size of each element is 200 x 200 m. In pilot models, the influence of element size was tested: bigger elements reduced calculation time but also the accuracy of the results, small elements increased the calculation time but also the resolution of deformation. The used size is a compromise between calculation time and resolution.

2.5 Boundary conditions The boundary conditions define the freedom to move for the elements in the model. All elements can move in x- and z-direction, except fixed nodes of the elements. As the elements are four node-elements, any node can be fixed in x- and/or z-direction. The decision about the fixed nodes bases on an assumed behaviour of a thrust sheet. Along the bottom line, the model is fixed in vertical (z) direction but free to move in horizontal (x) direction. There is no friction across the bottom line, supposing that the thrust sheet glides along the décollement horizon. The surface is free, as folding happens after the emplacement of the hanging wall thrust sheet. The right side of the model is fixed in x- and z-direction, because structural studies in the

111 Karwendel Mountains showed that buckling of the hanging wall stops when the hanging wall and footwall are folded together (Kilian and Ortner, 2019). The model is thrusted in x-direction from the left side.

Fig. 5: Undeformed three layer model. The element size is 200 x 200 m, all elements together build the mesh. All length units in the model are given in meters. Orange triangles show the boundary conditions, the tip of the triangle indicates the fixed direction (see magnified detail to the right). The bottom line is fixed in z- direction, the right side is fixed in x-and z- direction.

2.6 Initial stress distribution Before the model is shorted in x-direction, it is important to define the initial stress distribution to reach an equilibrium at the beginning. The stress is divided into a vertical (z) and a horizontal (x) component, which together cause an initial stress field before the model is shortened. The initial stress distribution depends on the height of the model and the material parameters of the layers. The model starts with an initial stress state free of deformation. Calculation of the vertical stress at rest The vertical stress is equal to geostatic stress or lithostatic pressure and defined by the model height (z) and the specific weight of the rock volume (훾). Specific weight: 훾 = 휌 ∙ 푔 푘푔 Density: 휌 = 2850 ∙ 푚3 푚 Gravity: 푔 = 10 ∙ 푠2 푘푔 푚 푘푔∙ 푚 훾 = 2850 ∙ ∙ 10 = 28500 ∙ = 28500 N/m³ 푚3 푠2 푚3∙푠2 훾 = 28,5 kN/m3 Vertical stress at the Top:

There is no vertical stress at the top because the surface is free (σtop = 0). The geostatic stress at the bottom is calculated exemplarily with 4000 m height:

σbottom = model height (z) ∙– 훾 (The algebraic sign is minus because the force acts in the negative z-direction ↓)

σbottom = 4000 m ∙ – 28,5 kN/m³ = –114 000 kN/m²

Calculation of the horizontal stress at rest The horizontal stress is dependent on the vertical stress and the coefficient of the earth pressure at rest (K0). The defined horizontal stress reflects the stress of the material caused by lithostatic pressure.

112 Horizontal stress: σh = K0 ∙ σz

σz = σtop + σbottom

σz top = 0

σz bottom = z ∙ 훾

Coefficient of earth pressure at rest (K0): 휗 K0 = (1−휗) The Poisson ratio 휗 derives from the geotechnical tests and is for the incompetent material 0.3, for competent material 0.2 (von Soos and Engel, 2008). We calculated the horizontal stress with a mean value of 휗 = 0.25. Poisson ratio: 휗 = 0.25

K0 = 0.33 4000 The horizontal stress is calculated exemplarily at the mean value of the model height ( ): 2 σh = K0 ∙ σz 4000 σh = 0.33 ∙ ( ∙ 28,5) = 18 810 kN/m² 2

2.7 Thrust amount The model was pushed in horizontal direction from the left side. The minimum thrust distance in the Karwendel Mountains is about 38 km (Kilian and Ortner, 2019), excluding folding and late out-of-sequence thrusting. Because folding postdated the emplacement of thrust sheets, and the focus of modelling was folding and not thrusting, we shortened the model without a specific limit. Due to the strong internal deformation of elements and high deviatoric stress differences between the elements, a maximum shortening of approx. 7 km was reached in the models (Fig. 6 and 7).

2.8 Erosion and sedimentation As described above, erosion and later sedimentation (Gosau or Inneralpine Molasse sediments) was contemporaneous with folding. We considered erosion and sedimentation in the numeric model by different model heights and changing overburden. The model series started with the maximum thickness of all three layers. Successive erosion was modelled by removing element rows from layer 3a and layer 2 until the latter was almost half as thick (1600 m) as compared to its initial thickness (3000 m). Removing one row of elements led to 200 m of erosion. The re-sedimentation starts on top of the eroded competent unit. New element rows (layer 3b) were added in each calculation on top of the eroded units. Given by the element size it starts with 200 m and reaches up to 1800 m syntectonic sediments.

3 Results Modelling erosion and sedimentation showed that fold geometries change between the single models geometries (Fig. 6 and 7). Figure 6 documents the influence of erosion on the development of folds. With increasing erosion, the number of folds increases while the limb lengths of the folds decrease. Two anticlines develop without erosion, or eroding only 0.2 km

113 (Fig. 6a and b, respectively). Between 0.4 km and 1.2 km of erosion (Fig. 6c to i) three anticlines develop. The model of Fig 6i (1.6 km of erosion) is an exception and most probably due to numerical issues, as it has the least shortening and only two anticlines are visible in this fold train. In the models of Fig. 6j to Fig. 6l four anticlines are developed. Thus, qualitatively the number of anticlines, which is connected to the limb length of folds, increases with the decreasing thickness of the competent layer. The model series depicted in Fig. 7a to i tests the influence of a growing overburden on folding. The thickness of layers 1 and 2 in the last erosion model (Fig. 6l) were used in the re- sedimentation models, and an increasing thickness of layer 3b between 0.2 km and 1.8 km. From the start, the model series of Fig. 7a to i has a very regular fold pattern with three or four anticlines.

114 Fig. 6: Results of erosion modelling. The material model is linear elasticity. Coloration represents the relative deviatoric stress distribution. Erosion removes progressively more element rows starting at the top in (a). Material parameters: Layer 1 (bottom): E-Module 2 000 000 kN/m2, Poisson ratio 0.25; Layer 2 (middle): E-Module 80 000 000 kN/m2, Poisson ratio 0.2; Layer 3 (top): E-Module 10 000 000 kN/m2; Poisson ratio 0.25. The grey background indicates models with irregular fold geometries (for explanation see chapter 3.3.1).

Fig. 7: Results from modelling folding with changing overburden. The material model is linear elasticity. Coloration represents the relative deviatoric stress distribution. The intermediate competent unit and the basal incompetent unit have constant thickness in the models, while the top incompetent unit ("Gosau overburden") increases in thickness. Material parameters: Layer 1 (bottom): E-Module 2 000 000 kN/m2, Poisson ratio 0.25; Layer 2 (middle): E-Module 80 000 000 kN/m2, Poisson ratio 0.2; Layer 3: E-Module 10 000 000kN/m2; Poisson ratio 0.25. The grey background indicates models with irregular fold geometries (for explanation see chapter 3.3.1)

3.1 Analysing folds – numeric results and field example A simple way to describe buckle folds is the determination of limb lengths (e.g., Biot, 1961; Ramberg, 1960; Ramsay and Huber, 1987). Buckle folds develop an initial arc length at the beginning of buckling which remains constant during ongoing shortening. The limb length of

115 a fold represents half of the arc length. During the model runs, we observed that the number of folds is stable, and the limb lengths in the models depend on the number of folds.

Fig. 8: Measured line, arc length and limb length of the folds. For the determination of the fold limb lengths a path in the middle of the competent layer was measured. From this path the limb lengths were calculated with the theorem of Pythagoras. All limb lengths along the measured path were determined and a mean value was calculated. The exported paths showed that in some models geometric irregularities develop (“irregular fold geometries” of Figs. 6 and 7, and, Fig. 9b; longer and shorter limbs occur). These irregularities start from the very beginning of the calculation. We suggest that these irregularities are an effect of the ratio between the sediment thickness, the length of the model and the initial stress state. It could also be an effect of two wavelengths, which develop at the same time and interfere. By now, we are not sure how to control this effect. We still calculated a mean value from the limb lengths, having in mind that the results might be distorted.

Fig. 9: Determination of limb lengths from numeric results. The limb length is calculated with the theorem of Pythagoras, assuming symmetric folds with vertical axial planes. This is not fully true in the case of irregular fold geometry. Limb length mean values were calculated for all models, even though in the case of irregular fold geometries longer and shorter fold limbs are present. (a) was generated from Fig. 7h, (b) from Fig. 6j. 3.1.1 Calculated limb length from numeric results The calculated limb lengths from the erosion model (Fig. 6) are summarised in Fig. 10. Layer 1 is of constant thickness whereas layer 2 and 3 are stepwise eroded. The limb length varies around the value of 4 to 5 km. Only the abovementioned model does not follow the trend, the

116 calculated limb length is 6.01 km (Fig. 6i). In contrast to the other models only two anticlines developed in the model of Fig. 6i. When the thickness of the model is about 2.6 km (Layer 2 eroded down to 2 km m instead of 3 km at the beginning) the limb length gets shorter and varies around 3 to 4 km. The standard deviations are larger when the fold geometry is irregular because then shorter and longer limbs occur.

Limb length Limblength (km) 2

1 Erosion of layer 3 Erosion of the competent layer 2

0 4,6 4,4 4,2 4 3,8 3,6 3,4 3,2 3 2,8 2,6 2,4 2,2 2 Thickness of the model (km)

Fig. 10: Calculated limb lengths from the numeric model of erosion and their standard deviation. Irregular fold geometries are coloured in grey. The results of limb lengths calculations for the models of re-sedimentation are shown in Fig. 11. The competent layer 2 is of constant thickness while the incompetent layer 3b becomes thicker. The limb length hardly varies, only the models with irregular fold geometries show limb lengths around 4 km, all others are around 3.7 km.

6,0

5,0

4,0

3,0

2,0 Limb length Limblength (km)

1,0

0,0 2,2 2,4 2,6 2,8 3 3,2 3,4 3,6 3,8 4 4,2 Thickness of the model (km)

Fig. 11: Calculated limb lengths from the numeric model of re-sedimentation and their standard deviation. Irregular fold geometries are coloured in grey. 117 3.1.2 Calculated limb lengths from field data To compare the modelled results with field examples limb lengths from the cross sections of the Karwendel Mountains (Fig. 4 and 12) were measured. As a reference we used the thickness of the competent layer measured from cross sections (Anisian to Ladinian limestone and the Ladinian platform) to compare with a modelled limb lengths having the same thickness of the competent layer 2. Because no synorogenic sediments occur in the study area, only results from the erosion model were used for comparison. Numeric results from the model vary between 3 and 5 km. The value for the abovementioned model Fig. 6i with 2.2 km thickness of layer 2 was not plotted because it deviates too much from the trend of the other results. The limb lengths measured in the cross section range between 2.4 and 6 km.

Field measured data 3

Numeric results Limb length Limblength (km) 2

0 1,6 1,8 2 2,2 2,4 2,6 2,8 Thickness of the competent layer (km)

Fig. 12: Modelled limb lengths versus field measured limb lengths.

4 Discussion The numeric model aimed to test if buckle folding is a possible deformation process for the Karwendel thrust sheet in the western NCA. The numeric model showed that buckle folding is only possible if there is a very weak layer at the base (décollement) and the rheology contrast between the competent layer and the incompetent base layer is high. Therefore, we conclude that the km-scale folds of the Karwendel thrust are buckle folds developed above the very weak décollement horizon of the salt bearing Permian to Triassic evaporites (Haselgebirge and Reichenhall Fm.). Further, we aimed to test the influence of erosion and sedimentation on the development of buckle folds. The numeric model showed that the erosion of the competent layer has a strong influence on the developing limb lengths. On the contrary, sedimentation of a less competent layer on an eroded surface had minor effects on the developing limb lengths.

118 This is in line with analytical models (e.g., Biot, 1961; Ramberg, 1960; Ramsay and Huber, 1987; Schmalholz et al., 2002) that showed that limb lengths of buckle folds are only dependent on the thickness of the competent layer. The comparison with limb lengths measured in cross sections showed that the numeric results fit in a range of 1 km deviation. The results showed further that a certain amount of erosion is needed to produce limb lengths, which are comparable to field measured data. Kilian and Ortner (2019) concluded that the Karwendel thrust sheet arrived of todays Karwendel mountains on top of a salt pillow, that allowed transport without folding. The redistribution of the salt along the décollement allowed folding on top of the décollement to start. Structural data already suggested that folding significantly postdated initial stacking of thrust sheets in the late Early Cretaceous. The numeric results allow a further interpretation. Folding is only possible after a decrease in lithostatic pressure related to Upper Cretaceous and/or Paleogene erosion. As shortening continued after initial emplacement of thrust sheets folds probably initiated in the late Upper Cretaceous to Paleogene (as fold axes indicate, see cross sections of Fig. 4). The effects of erosion and sedimentation in fold- and -thrust belts detached above a weak basal layer were also discussed in other numeric experiments (e.g., Simpson, 2006; Collignon et al. 2014, Yamato et al., 2011). For example, Collignon et al. (2014) showed that surface processes do have a minor effect on fold wavelengths (arc lengths) but enhance the amplification of anticlines by loading synclines with syntectonic deposits. However, surface processes modify the initial topography prior to deformation, which is determining the developing fold geometries. Post-Paleogene erosion might have favoured the growth of already existing anticlines in the Karwendel mountains. However, no new folds formed significantly later than Paleogene, based on fold axis orientations of km-scale folds. Newly formed Neogene folds should have NW- trending axes, and such folds were only observed on the small scale close to out-of-sequence thrusts (Kilian and Ortner, 2019). The results of the numeric model gave a contribution to the development of structures in the hanging wall of the Karwendel thrust. We observed two kinds of folds in the Karwendel Mountains: (1) Folds above the thrust plane and (2) folds folding hanging wall and footwall together. All folds developed during Paleogene N- to NNE-directed transport (see Fig. 4). The numeric model dealt only with the folds in the hanging wall, which developed as buckle folds above the very weak décollement horizon of the salt bearing Permian to Triassic evaporites (Haselgebirge and Reichenhall Fm.). However, folds folding hanging wall and footwall are not related to the weak décollement at the base of the Karwendel thrust sheets, but to a structurally deeper one within or at the base of the Tannheim thrust sheet. In Fig. 13 we sketched a possible development of field observed structures (cross section 1 of Fig. 4) based on the structural investigations of Kilian and Ortner (2019) and the numeric results presented here.

119

Fig. 13: Sketch of a possible kinematic development of the observed structures. a) Stacking of thrust sheets in the late Lower Cretaceous along the detachment horizon (Permian to Triassic evaporites). b) Salt migration or a primary varying distribution of salt could cause the different behaviour during ongoing shortening. While in the south the folds are restricted to the hanging wall, the northern part shows a mechanical coupling of hanging wall and footwall. Based on fold axis orientation, we know that the mechanical coupling of the two thrust sheets was not before Paleogene. c) Out-of-sequence thrusts develop due to ongoing shortening and salt expulsion out of the thrust zone. For the study area, Kilian and Ortner (2019) showed that the out-of-sequence thrusts are of late Oligocene to Miocene age. As sketched in Fig. 13b folds deforming hanging wall and footwall together developed contemporaneous to the buckle folds above the Karwendel thrust. The coupling of hanging wall and footwall requires either a primary different distribution of the salt-bearing formations or salt evacuation out of the thrust zone. Both possibilities are feasible because from the Permian succession (Haselgebirge) it is known that it had a limited lateral extent (e.g., Leitner and Spötl, 2017; Spötl, 1989) and salt is mobile and migrates laterally or upward (Jackson and Hudec, 2017). Both scenarios lead to mechanical coupling of the hanging wall and the footwall. As soon as the thrust sheets are coupled they behave as a unit, a deeper décollement (base of the Tannheim thrust sheet) or an intern décollement (Carnian shales) in the lower thrust sheet might initiate folding. Due to ongoing shortening out-of-sequence thrusts developed (Fig. 13c) in the Oligocene to Miocene age in the Karwendel Mountains (Kilian and Ortner, 2019) and in the Mieming and Wetterstein Mountains (Ortner and Kilian, 2020, submitted). Out-of-sequence thrust strike trough hanging wall and footwall. Salt expulsion out of the Karwendel thrust might have strengthened the thrust itself, and forced the décollement into the footwall. In contrast to previous models (Mancktelow, 1999; Jeng et al., 2002; Huang et al., 2010; Llorens et al., 2013), we abstained from predefining errors in the model to produce folds. Homogeneous layers, as used in the model, do not reflect a natural rock state. Scattering rock parameters within the layer would be a possibility to come closer to a natural state of rocks. In such a case, each layer has a defined range of values, and then each element of the layer gets a random value out of the defined range.

120 Working with a multi-layer approach could also bring new insights into the deformation behaviour and would be closer to the “real” stratigraphic succession in the study area. Other numeric models, e.g., from the Zagros Mountains (Yamato et al., 2011), showed that multiple weak layers intensify the fold growth in comparison to thrusting.

5 Conclusion Although a numeric model is a strong simplification of reality, the model gave a new perspective on interpreting structures in the Karwendel Mountains. Km-scale folds in the hanging wall of the Karwendel thrust can be interpreted as buckle folds, developed after the emplacement of the Karwendel thrust sheet and after partial erosion of the latter. Our modelling demonstrated some key requirements of buckle folding: (1) The existence of a very incompetent décollement, which lies in the salt-bearing evaporitic Haselgebirge- Reichenhall horizon at the base of the Karwendel thrust sheet, and (2) major erosion prior to folding, as the full, up to 5 km thick sedimentary succession cannot be folded with limb lengths comparable to those observed in the field. Only the reduction of the competent carbonate platforms to roughly half of their original thickness facilitates folding with limb lengths around 3 to 4 km as observed in cross sections. This study also emphasises the need for geometric-rheological models for the construction of cross sections in salt-detached fold-and-thrust belts. Material characteristics strongly influence the geometry of structures, still many cross sections are entirely drawn by using geometric models.

6 Acknowledgments The Tiroler Wissenschaftsfonds and the Doctoral program of the University of Innsbruck supported this research.

7 References Abbassi, M. R., and Mancktelow, N. S., (1992): Single layer buckle folding in non-linear materials—I. Experimental study of fold development from an isolated initial perturbation. Journal of Structural Geology, v. 14, no. 1, p. 85-104. Ampferer, O. (1902): Bericht über die Neuaufnahme des Karwendelgebirges. Verhandlungen der Geologischen Bundesanstalt, p. 274-276. Ampferer, O. (1912): Gedanken über die Tektonik des Wettersteingebirges.- Verhandlungen der k.k. Geologischen Reichsanstalt, p. 197-212. Ampferer, O., and Hammer, W. (1911): Geologischer Querschnitt durch die Alpen vom Allgäu zum Gardasee. Jb. Geol. Reichsanst., v. 61, p. 531 - 710. Auer, M.; Eisbacher, G. H. (2003): Deep structure and kinematics of the Northern Calcareous Alps (TRANSALP profile).- International Journal of Earth Sciences, v.92, p. 92: 210-227. Barani, O. (2012): The Effect of Lower Detachment Zone on Buckle Folds Geometry. Bechstädt, T. & Mostler, H. (1976): Riff-Becken-Entwicklung in der Mitteltrias der westlichen nördlichen Kalkalpen.- Zeitschr. Dt. Geol. Ges., 127. 271-289. Behrmann, J. H., and Tanner, D. C., (2006): Structural synthesis of the Northern Calcareous Alps, TRANSALP segment: Tectonophysics, v. 414, no. 1-4, p. 225-240. Biot, M. A. (1957): Folding instability of a layered viscoelastic medium under compression. Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences, v. 242, no. 1231, p. 444-454.

121 Biot, M. A. (1961): Theory of folding of stratified viscoelastic media and its implications in tectonics and orogenesis. Geological Society of America Bulletin, v. 72, no. 11, p. 1595-1620. Butler, R. B., Clare E.; Cooper, Mark A.; Watkins, Hannah, 2019, Fold-thrust structures - where have all the buckles gone?: Bond, C.E. & Lebit, H.D. (eds) 2020. Folding and Fracturing of Rocks: 50 Years of Research since the Seminal Text Book of J.G. Ramsey. Geological Society, London, Special Publications, 487, 21-44. Brühwiler, T., Hochuli, P. A., Mundil, R., Schatz, W., Brack, P. (2007): Bio-and chronostratigraphy of the Middle Triassic Reifling Formation of the westernmost Northern Calcareous Alps. Swiss Journal of Geosciences, v.100, no.3., p.443-455. Collignon, M., Kaus, B., May, D., and Fernandez, N., (2014): Influences of surface processes on fold growth during 3‐D detachment folding: Geochemistry, Geophysics, Geosystems, v. 15, no. 8, p. 3281-3303. Currie, J., Patnode, H., and Trump, R. (1962): Development of folds in sedimentary strata: Geological Society of America Bulletin, v. 73, no. 6, p. 655-673. Czech, J. H., Helmut (1990): Gesteinskennwerte aus Laborversuchen. Felsbau, v. 8, no. 3. Davis, D. M., and Engelder, T. (1985): The role of salt in fold-and-thrust belts: Tectonophysics, v. 119, no. 1-4, p. 67-88. Diersche, V., 1980: Die Radiolarite des Oberjura im Mittelabschnitt der Nördlichen Kalkalpen. Geotektonische Forschungen, 58: 217 S. Donofrio, D. A., Brandner, R., and Poleschinski, W. (2003): Conodonten der Seefeld-Formation: Ein Beitrag zur Bio-und Lithostratigraphie der Hauptdolomit-Plattform (Obertrias, Westliche Nördliche Kalkalpen, Tirol). Geologisch-Paläontologische Mitteilungen Innsbruck, v. 26, p. 91- 107. Eberli, G.P. (1988): The evolution of the southern continental margin of the Jurassic Tethys ocean as recorded in the Allgäu Formation of the Austroalpine Nappes of Graubünden (Switzerland). Eclogae Geologicae Helvetiae, 81: 175 - 214. Eckert, A., Connolly, P., and Liu, X. (2014): Large‐scale mechanical buckle fold development and the initiation of tensile fractures. Geochemistry, Geophysics, Geosystems, v. 15, no. 11, p. 4570- 4587. Eisbacher, G. H., and Brandner, R. (1996): Superposed fold thrust structures and high angle faults, northwestern Calcareous Alps, Austria. Eclogae Geologicae Helvetiae, v. 89, p. 553 - 571. Faupl, P., Pober, E., and Wagreich, M. (1987): Facies development of the Gosau Group of the eastern parts of the Northern Calcareous Alps during the Cretaceous and Paleogene. Geodynamics of the Eastern Alps, p. 142-155. Frehner, M. (2011): The neutral lines in buckle folds. Journal of Structural Geology, v. 33, no. 10, p. 1501-1508. Frisch, J. (1975): Sedimentologische, lithofazielle und paläogeographische Untersuchungen in den Reichenhaller Schichten und im Alpinen Muschelkalk der Nördlichen Kalkalpen zwischen Lech und Isar: Jahrbuch der Geologischen Bundesanstalt, v. 118, p. 75-117. Froitzheim, N. (1994): Repeated change from crustal shortening to orogenparallel extension in the Austroalpine units of Graubunden. Eclogae Geol. Helv., v. 87, p. 559-612. Fruth, I., and Scherreiks, R. (1982): Hauptdolomit (Norian)—stratigraphy, paleogeography and diagenesis. Sedimentary Geology, v. 32, no. 3, p. 195-231. Gaupp, R. (1982): Sedimentationsgeschichte und Paläotektonik der kalkalpinen Mittelkreide (Allgäu, Tirol, Vorarlberg); Zitteliana, V.8, p.33-72. Ghassemi, M. R., Schmalholz, S. M., and Ghassemi, A. R. (2010): Kinematics of constant arc length folding for different fold shapes. Journal of structural geology, v. 32, no. 6, p. 755-765. Goffey, G., Craig, J., Needham, T., and Scott, R. E., 2010, Hydrocarbons in contractional belts, Special Publications, No. 348, Geological Society, London. Golebiowski, R. (1991): Becken und Riffe der alpinen Obertrias - Lithostratigraphie und Biofazies der Kössener Formation. In: Nagel D, Rabeder G (eds) Exkursionen im Jungpaläozoikum und Mesozoikum Österreichs. Österreichische Paläontologische Gesellschaft, Wien, 79-119. Granado, P., Roca, E., Strauss, P., Pelz, K., and Muñoz, J. A. (2018): Structural styles in fold-and-thrust belts involving early salt structures: The Northern Calcareous Alps (Austria). Geology, v. 47, no. 1, p. 51-54.

122 Guest, B., Guest, A., and Axen, G. (2007): Late Tertiary tectonic evolution of northern Iran: A case for simple crustal folding. Global and Planetary Change, v. 58, no. 1-4, p. 435-453. Haas, J., Kovács, S., Krystyn, L., Lein, R. (1995): Significance of Triassic facies zones in terrane reconstructions in the Alpine-North Pannonian domain. Tectonophysics, v. 242, p. 19-40. Handy, M. R., Schmid, S. M., Bousquet, R., Kissling, E., and Bernoulli, D. (2010) Reconciling plate- tectonic reconstructions of Alpine Tethys with the geological–geophysical record of spreading and subduction in the Alps. Earth-Science Reviews, v. 102, no. 3-4, p. 121-158. Heißel, G. (1978): Karwendel - geologischer Bau und Versuch einer tektonischen Rückformung. Geologisch-Paläontologische Mitteilungen Innsbruck, v. 8, p. 227 - 288. Huang, K.-P., Chang, K.-J., Wang, T.-T., and Jeng, F.-S. (2010): Buckling folds of a single layer embedded in matrix–Folding behavior revealed by numerical analysis. Journal of Structural Geology, v. 32, no. 7, p. 960-974. Hudleston, P. J., and Holst, T. (1984): Strain analysis and fold shape in a limestone layer and implications for layer rheology. Tectonophysics, v. 106, no. 3-4, p. 321-347. Jackson, M. P., and Hudec, M. R. (2017): Salt tectonics: Principles and practice, Cambridge University Press. Jacobshagen, V. (1965): Die Allgäuschichten (Jura-Fleckenmergel) zwischen Wettersteingebirge und Rhein. Jahrbuch der Geologischen Bundesanstalt, v. 108, p. 1-114. Jeng, F., and Huang, K. (2008): Buckling folds of a single layer embedded in matrix–Theoretical solutions and characteristics. Journal of Structural Geology, v. 30, no. 5, p. 633-648. Jeng, F., Lin, M., Lai, Y., and Teng, M. (2002): Influence of strain rate on buckle folding of an elasto– viscous single layer. Journal of Structural Geology, v. 24, no. 3, p. 501-516. Jerz, H. (1966): Untersuchungen über Stoffbestand, Bildungsbedingungen und Paläogeographie der Raibler Schichten zwischen Lech und Inn (Nördl. Kalkalpen). Geologica Bavarica, v. 56, p. 3- 100. Kilian, S., and Ortner, H. (2019): Structural evidence of in-sequence and out-of-sequence thrusting in the Karwendel mountains and the tectonic subdivision of the western Northern Calcareous Alps. Australian Journal of Earth Sciences, v. 112, p. 62-83. Krois, R., and Stingl, V. (1994): Kretazische „Augensteine": Notiz zu einem fraglichen Gosauvorkommen im Karwendel (Tirol, Österreich).-J b. Geol. Bundesanst, v. 137, p. 289-293. Lacombe, O., Lavé, J., Roure, F. M., and Verges, J. (2007): Thrust belts and foreland basins: From fold kinematics to hydrocarbon systems, Springer Science & Business Media. Lan, L., and Hudleston, P. J. (1991): Finite-element models of buckle folds in non-linear materials. Tectonophysics, v. 199, no. 1, p. 1-12. Lein, R. (1987): Evolution of the Northern Calcareous Alps during Triassic times. In: Flügel, H. W. & Faupl, P., Geodynamics of the Eastern Alps, Deuticke, Wien., p. 85-102. Leitner, C., and Spötl, C. (2017): The eastern Alps: Multistage development of extremely deformed evaporites, Permo-Triassic Salt Provinces of Europe, North Africa and the Atlantic Margins, Elsevier, p. 467-482. Li, S. (2013): Numerical studies of the deformation of salt bodies with embedded carbonate stringers. PHD Theseis, RWTH Aachen University. Linzer, H.-G., Ratschbacher, L., and Frisch, W. (1995): Transpressional collision structures in the upper crust: the fold thrust belt of the Northern Calcareous Alps. Tectonophysics, v. 242, p. 41 - 61. Liu, X., Eckert, A., and Connolly, P. (2016): Stress evolution during 3D single-layer visco-elastic buckle folding: Implications for the initiation of fractures. Tectonophysics, v. 679, p. 140-155. Llorens, M.-G., Bons, P. D., Griera, A., Gomez-Rivas, E., and Evans, L. A. (2013): Single layer folding in simple shear. Journal of Structural Geology, v. 50, p. 209-220. Mancktelow, NS (1999): Finite-element modelling of single-layer folding in elasto-viscous materials: the effect of initial perturbation geometry. Journal of structural Geology, v.21, no.2, p. 161-177. Miller, H. (1963a): Der Bau des westlichen Wettersteingebirges. Zeitschrift der Deutschen Geologischen Gesellschaft, v. 113, p. 409-425. Miller, H. (1963b): Die tektonischen Beziehungen zwischen Wetterstein- und Mieminger Gebirge. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen, v. 118, p. 291-320. Miller, H. (1965): Die Mitteltrias der Mieminger Berge mit Vergleichen zum westlichen Wettersteingebirge: Verhandlungen der Geologischen Bundesanstalt, v. 1965, p. 187-212.

123 Müller-Jungbluth, W.-U. (1971): Sedimentologische Untersuchungen des Hauptdolomits der östlichen Lechtaler Alpen, Tirol, Geologisches Institut der Universität. Nagel, K. H., Schütz, K. I., Schütz, S., Wilmers, W., and Zeil, W. (1976): Die geodynamische Entwicklung der Thiersee- und Karwendelmulde (Nördliche Kalkalpen): Geologische Rundschau, v. 65, p. 536-557. Nemcok, M., Schamel, S., and Gayer, R. (2005): Thrust belts - Structural archtitecture, thermal regimes, and petroleum systems, Cambridge, Cambridge University Press 541 p. Nittel, P. (2006): Beitrage zur Stratigraphie und Mikropaläontologie der Mitteltrias der Innsbrucker Nordkette (Nordliche Kalkalpen, Austria): Geo.Alp, v. 3, p. 93-146. Ortner, H. (2001): Growing folds and sedimentation of the Gosau Group, Muttekopf, Northern Calcareous Alps, Austria: Int. J. Earth Sci. (Geol. Rundsch.), v. 90, p. 727-739. Ortner, H. (2003a): Cretaceous thrusting in the western part of the Northern Calcareous Alps (Austria)– evidences from synorogenic sedimentation and structural data. Mitteilungen der Österreichischen Geologischen Gesellschaft, v. 94, p. 63-77. Ortner, H. (2003b): Local and far field stress – analysis of brittle deformation in the western part of the Northern Calcareous Alps, Austria. Geologisch-Paläontologische Mitteilungen Innsbruck, 26: 109-131. Ortner, H., Aichholzer, S., Zerlauth, M., Pilser, R., and Fügenschuh, B. (2015): Geometry, amount, and sequence of thrusting in the Subalpine Molasse of western Austria and southern Germany, European Alps. Tectonics, v. 34, no. 1, p. 1-30. Ortner, H., and Gaupp, R. (2007): Synorogenic sediments of the western Northern Calcareous Alps. Geo. Alp, v. 4, p. 133-148. Ortner, H., and Kilian, S. (2020): A new tectonic subdivision in the Northern Calcareous Alps of western Austria and southern Germany resolves a 100 year old controversy. International Journal of Earth Sciences. Submitted 2020. Ortner, H., Kositz, A., Willingshofer, E., Sokoutis, D. (2016): Geometry of growth strata in a transpressive fold belt in field and analogue model: Gosau Group at Muttekopf, Northern Calcareous Alps, Austria. Basin Research, 28(6): 731–751. Palotai, M., Pálfy, J., and Sasvári, Á. (2017): Structural complexity at and around the Triassic–Jurassic GSSP at Kuhjoch, Northern Calcareous Alps, Austria. International Journal of Earth Sciences, v. 106, no. 7, p. 2475-2487. Peresson, H., and Decker, K. (1997): The Tertiary dynamics of the northern Eastern Alps (Austria): changing palaeostresses in a collisional plate boundary. Tectonophysics, v. 272, p. 125 - 157. Pfiffner, O. A. (1993): The structure of the Helvetic nappes and its relation to the mechanical stratigraphy. Journal of structural Geology, v. 15, no. 3-5, p. 511-521. Poblet, J., Muñoz, J. A., Travé, A., and Serra-Kiel, J. (1998): Quantifying the kinematics of detachment folds using three-dimensional geometry: Application to the Mediano anticline (Pyrenees, Spain): Geological Society of America Bulletin, v. 110, no. 1, p. 111-125. Ramberg, H. (1960): Relationships between length of arc and thickness of ptygmatically folded veins. American Journal of Science, v. 258, no. 1, p. 36-46. Ramberg, H. (1963): Fluid dynamics of viscous buckling applicable to folding of layered rocks. AAPG Bulletin, v. 47, no. 3, p. 484-505. Ramsay, J. G., and Huber, M. I. (1987): The techniques of modern structural geology, v.2: Folds and fractures. Academic press. Rowan, M. G., and Kligfield, R. (1992): Kinematics of large-scale asymmetric buckle folds in overthrust shear: an example from the Helvetic nappes: In: McClay, K. R., Thrust Tectonics, 165-173, Chapman & Hall, London., p. 165-173. Rüffer, T., and Zamparelli, V. (1997): Facies and biota of Anisian to Carnian carbonate platforms in the Northern Calcareous Alps (Tyrol and Bavaria). Facies, v. 37, no. 1, p. 115-136. Sanders, D., Höfling, R. (2000): Carbonate depostion in mixed siliciclastic-carbonate environments on top of an orogenic wedge (Late Cretaceous, Northern Calcareous Alps, Austria). Sedimentary Geology, 137: 127-146. Sarnthein, M. (1965): Sedimentologische Profilreihen aus den mitteltriadischen Karbonatgesteinen der Kalkalpen nördlich und südlich von Innsbruck: Verhandlungen der Geologischen Bundesanstalt, v. 1965, p. 119-162.

124 Sarnthein, M. (1967): Versuch einer Rekonstruktion der mitteltriadischen Paläogeographie um Innsbruck. Österreich: Geologische Rundschau, v. 56, no. 1, p. 116-127. Simpson, G. (2006): Inﬂuence of erosion and deposition on deformation in fold belts: Tectonics, Climate, and Landscape Evolution, v. 398, p. 267. Suppe, J. (1983): Geometry and kinematics of fault-bend folding: American Journal of science, v. 283, no. 7, p. 684-721. Suppe, J., and Medwedeff, D. A. (1990): Geometry and kinematics of fault-propagation folding: Eclogae Geologicae Helvetiae, v. 83, no. 3, p. 409-454. Schauberger, O. (1986): Bau und Bildung der Salzlagerstätten des ostalpinen Salinars. Archiv für Lagerstättenforschung der Geologischen Bundesanstalt, v. 7, p. 217-254. Schmalholz, S., Podladchikov, Y., and Burg, J. P. (2002): Control of folding by gravity and matrix thickness: Implications for large‐scale folding. Journal of Geophysical Research: Solid Earth, v. 107, no. B1, p. ETG 1-1-ETG 1-16. Schmalholz, S. M., and Schmid, D. W. (2012): Folding in power-law viscous multi-layers. Philosophical Transactions of the Royal Society A, Mathematical, Physical and Engineering Sciences, v. 370, no. 1965, p. 1798-1826. Schmid, S. M., Fügenschuh, B., Kissling, E., and Schuster, R. (2004): Tectonic map and overall architecture of the Alpine orogen. Eclogae Geologicae Helvetiae, v. 97, p. 93-117. Schmidegg, O. (1951): Die Stellung der Haller Salzlagerstätte im Bau des Karwendelgebirges. Jahrb Geol Bundesanst, v. 94, p. 159-207. Sherwin, J.-A., and Chapple, W. M. (1968): Wavelengths of single-layer folds; a comparison between theory and observation. American Journal of Science, v. 266, no. 3, p. 167-179. Spötl, C. (1989): Die Salzlagerstätte von Hall in Tirol: ein Überblick über den Stand der geologischen Erforschung des 700jährigen Bergbaubetriebes. Tiroler Landesmuseum Ferdinandeum. Stampfli, G., Mosar, J., Marquer, D., Marchant, R., Baudin, T., and Borel, G. (1998): Subduction and obduction processes in the Swiss Alps. Tectonophysics, v. 296, no. 1-2, p. 159-204. Stüwe, K., and Schuster, R. (2010): Initiation of subduction in the Alps: Continent or ocean?. Geology, v. 38, no. 2, p. 175-178. Tollmann, A. (1976a): Der Bau der Nördlichen Kalkalpen: Monographie der Nördlichen Kalkalpen, Teil III, 449 p., Deuticke, Wien. Tollmann, A. (1976b): Analyse des klassischen nordalpinen Mesozoikums: Monographie der Nördlichen Kalkalpen, Teil II, 580 p., Deuticke, Wien. von Soos, P., and Engel, J. (2008): Eigenschaften von Boden und Fels- ihre Ermittlung im Labor: In: Grundbau Taschenbuch Teil 1: Geotechnische Grundlagen, 7. Auflage; Karl Josef Witt (Hrsg.). Wagreich, M., and Faupl, P. (1994): Palaeogeography and geodynamic evolution of the Gosau Group of the northern Calcareous Alps (Late Cretaceous, eastern Alps, Austria). Palaeogeography Palaeoclimatology Palaeoecology, v. 110, no. 3, p. 235-274. Wagreich, M., and Decker, K. (2001): Sedimentary tectonics and subsidence modelling of the type Upper Cretaceous Gosau basin (Northern Calcareous Alps, Austria): Int. J. Earth Sci. (Geol. Rundsch.), v. 90, p. 714-726. Yamato, P., Kaus, B. J., Mouthereau, F., and Castelltort, S. (2011): Dynamic constraints on the crustal- scale rheology of the Zagros fold belt, Iran. Geology, v. 39, no. 9, p. 815-818.

125

126 3.5 Analysing folds Modelling folds raised the question about the geometric description of the developing buckle folds. In study 3 we compared the folds with field data. Here, we tried to compare the modelled folds also with analytical solutions knowing that these solutions do not exactly fit to the model (as explained below). To determine the wavelength (or arc length) of buckle folds a wide range of analytical models exist (e.g., Biot, 1964; Jeng et al., 2002; Jeng and Huang, 2008). Most models describe buckle folds of single layers (e.g., Biot, 1961; Ramberg, 1960; Jeng and Huang, 2008) or multi-layer buckling in a uniform matrix (Schmalholz and Schmid, 2012; Ramsey, 1987). Because we modelled a linear elastic material model, the Ramberg (1960) or the Ramsey (1987) formulae, which both describe the initial wavelength of single layer folds at the moment of buckling, could be used to predict the wavelength of the modelled folds. However, the analytical solutions were developed for small scale models, where the competent layer is embedded in an incompetent matrix, and only two rheologies occur. The performed numeric model is a 3-layer model (with 3 different rheologies) and as it is large scale, the effect of gravity must be considered but is ignored in both formulae.

In Ramberg´s (1960) calculations the initial wavelength (wi) is equal to the arc length (wa) of a buckle fold. The arc length remains constant during progressive shortening. Ramsey and Huber (1987) worked on the base of this formula and described the development of folds in more detail. They defined that wi and wa are only equivalent when the competence contrast (for linear elastic models the E-module) between the competent layer and the 퐸−푀표푑푢푙푒 표푓 푡ℎ푒 푏푢푐푘푙𝑖푛푔 푙푎푦푒푟 incompetent matrix is high ( >50). In our calculation, the ratio 퐸−푀표푑푢푙푒 표푓 푡ℎ푒 푚푎푡푟𝑖푥 80 000 000 푘푁/푚2 between the competent layer 2 and the incompetent layer 1 is 40 ( = 2 000 000 푘푁/푚2 40) because the low competence contrast is defined as <10, we suggest that our calculation is comparable to a high competence contrast.

Figure 7: Buckled competent layer in an incompetent matrix. Wi is the initial wavelength developed by layer parallel shortening, wa is the arc length measured through the middle layer (modified from Ramsey 1987). At the very beginning of buckling wi is almost equal to wa. In a later stage the wavelength wi is decreasing due to fold amplification but the arc length (wa) and consequently also the limb length (wa/2) is constant.

It is assumed that buckled folds maintain the initial arc length during ongoing shortening and only the amplitude increases (Price, 1990). That means that the initial arc length is constant during ongoing shortening which in turn is the prerequisite for the comparison between analytical results and measured modelled arc lengths.

127 We calculated arc lengths using the Ramberg (1960) and the Ramsey and Huber (1987) formulae and determined the limb lengths (wi/2). We compared the analytical results with limb lengths from numeric results (Figure 8 and 9). The Ramberg formula (1960) Calculation of the initial wavelength:

3 2 휇1 푤i= 2휋ℎ √ ∙ 3 휇2

푤𝑖 = is equal to the arc length of a fold, 휇1=competent (80.000.000 kN/m²), 휇2= incompetent (2.000.000 kN/m²), h= half the thickness of the competent layer (m)

The Ramsey formula (1987) Calculation of the initial wavelength after Ramsey and Huber (1987) who determined the following formula from studies on buckled layers:

3 휇1 푤i= 2휋퐷 √ 6휇2

푤i = is equal to the arc length of a folds, 휇1=competent (80.000.000 kN/m²), 휇2= incompetent (2.000.000 kN7m²), D= thickness of the competent layer (m)

19 18 17 16 15 14 13 12 11 10 Ramsey 9 8 7 6 Ramberg 5 limb length limblength (km) 4 3 2 1 0 3 2,8 2,6 2,4 2,2 2 1,8 1,6 Thickness of the competent layer (km)

Figure 8: Comparison of limb lengths calculated with the Ramberg (yellow) and the Ramsey (orange) formulae.

128 7 6 6,02

5 4,91 4,98 4,43 4 3,33 3,80 3 3,28

2 Limb length Limblength (km) 1 0 3 2,8 2,6 2,4 2,2 2 1,8 1,6 Thickness of the competent layer 2, after the erosion of the incompetent layer 3.

Figure 9: Limb length from numeric results. For better visibility, only some results are plotted. All results can be found in Study 3.

In some numeric models geometric irregularities in the fold geometries developed (see Study 3). More precisely, in some models two or more limb lengths occurred, whereas partly one limb length is only half the length of another limb length. The used formulae describe only regular folds. Ramsay and Huber (1987) hypothesised that the development of irregular fold geometries in the same layer are overlapping areas of in-phase wavelengths. In two sectors along the buckling layer two sectors of in-phase folds could develop. These sectors could be out-of-phase to each other. In overlapping areas, a new shorter wavelength develops (Ramsey and Huber, 1987, p. 391). By now, we are not sure why in some models regular and in others irregular folds geometries develop. We suggest that these irregularities are an effect of the ratio between the layer thickness, the length of the model and the initial stress state. It could also be an effect of two wavelengths, which develop at the same time and interfere (according to Ramsay and Huber, 1987). As long as we are not sure about the source of the irregularities, the comparison between modelled folds and calculated folds is difficult, although solutions for irregular fold patterns exist (Jeng et al. 2002; Ramberg, 1970; Ramsay and Huber, 1987, p. 424ff.). Limb lengths calculated using the Ramberg (1961) or the Ramsey and Huber (1987) formula are systematically longer (approx. 18 – 7.5 km) than limb lengths in the numeric model (approx. 6 – 3.8 km). Field data from the Karwendel showed limb lengths between 6 and 2.4 km. In comparison to the analytical results, the numeric results are pretty closer to field data, although the numeric model is a simplified 2D version of a complex 3D structure. The numeric results showed, as analytical solutions, the dependence of the limb length on the thickness of the competent layer.

129 3.6 From folding to faulting At the beginning of the numeric modelling, we attempted to model the whole process from folding to faulting. Running the model with a failure criterion showed that calculation stopped before folding was reached. Therefore, we used a linear elastic model to produce folding. The results of modelling folding were taken as input geometry to model faults (Figure 10). Faults were modelled with an ideal plastic model with the failure criterion according to Mohr- Coulomb.

Figure 10: Input geometry for the modelling using the Mohr Coulomb material model. The model starts without the deviatoric stress distribution reached at the end of the linear elastic modelling. In the first step, gravity force was applied before thrusting started.

Figure 11: Numeric modelling of the folded 3-Layer model using the Mohr Coulomb material model. The input model was already shortened (17,4%) and therefore folded. The figure shows the plastic strain distribution after shortening in x-direction (transport was 2,1 m along the x-axis). Layer 2 with the highest plastic strain distribution fails. Layer 1: Thickness: 600m, E-Module: 2 000 000 kN/m²; Poisson ratio: 0,25, friction angle: 42°; Layer 2: Thickness: 1500m, E-Module: 80 000 000 kN/m², Poisson ratio: 0,2, friction angle: 41°; Layer 3: Thickness: 1600m, E-Module: 10 000 000kN/m², Poisson ratio: 0,25, friction angle: 42°.

Testing different settings using different material parameters and boundary conditions showed that faults developed in the layer with the highest E-module and in dependence of the angle of internal friction with respect to gravity. Neither the fold shape nor the orientation of the boundary between the incompetent to competent layer, which is even in continuation of the failed elements (Figure 11), seem to influence the failure behaviour. These findings lead to a new interpretation of structures observed in the Mieming and Wetterstein Mountains (see Study 2).

130 4 Discussion This thesis consists of two parts, a field geology part and a numeric modelling part. The results of the field investigations in the context of the nappe subdivision in the western NCA will be summarised here. Further, the results of the numeric model will be related to the formulated hypothesis and their implications on the structural development of the western NCA will be given. The thrust sheets of the western NCA were defined at the beginning of the 20th century by Ampferer and Hammer (1911) and Ampferer (1912). This tectonic subdivision is in use ever since. In the nineteen seventies Tollmann (1970; 1976b) reviewed the structure of the NCA and unified the classifications. Based on the work of Hahn (1912; 1913), he defined nappe systems to which every nappe could be assigned to. Following Ampferer, Tollmann (1970; 1976b) defined three thrust sheets in the western NCA, which are from base to top: the Allgäu-, the Lechtal- and the Inntal thrust sheet. Ever since this subdivision was proposed, it was controversially debated, especially the boundary of the Inntal- against the Lechtal thrust sheet in the western NCA (Ampferer, 1912; 1914; 1931; 1942; Heißel, 1958; Loesch, 1915; Mylius, 1914; Richter, 1929; Rüffer and Bechstädt, 1995; Schlagintweit, 1912a; b). Study 1 and 2 are both located at the border between the Inntal- and the Lechtal thrust sheets (Tollmann, 1976b). We analysed the boundary between the Inntal and the Lechtal thrust sheet using macroscopic structural record and demonstrated that out-of-sequence thrusts have been used to delimit the Inntal thrust sheet against the deeper Lechtal thrust sheet. These findings lead to a new tectonic subdivision in the western NCA, which is based on two principles: (1) Thrusts should bring old-on-young rocks. (2) The distribution of synorogenic sediments, i.e., thrust sheets top deposits and upper- footwall deposits should be used to define thrust sheets. Applying this concept systematically in western NCA showed that only two thrust sheets (instead of three) are necessary (Ortner, 2016; Study 1 and 2). We renamed the thrust sheets to a lower Tannheim thrust sheet and an upper Karwendel thrust sheet, according to their largest surface exposure. The name Karwendel thrust sheet follows Ampferer (1902), who was the first to recognise and name the Karwendel thrust flooring the Karwendel thrust sheet, prior to his nappe definitions (see above). In this revised subdivision the Karwendel thrust sheet represents the former Inntal- and part of the Lechtal thrust sheet, the Tannheim thrust sheet represents the former Allgäu- and part of the Lechtal thrust sheet. The superordinate systems would be the

131 Bajuvaric nappe system for the Tannheim thrust sheet and the Tirolic-Noric nappe system for the Karwendel thrust sheet (after Mandl et al., 2017). This concept is largely in line with the new definition of thrust systems in the NCA presented by Mandl et al. (2017), who suggest a tectonic subdivision on the base of syn- and post tectonic sediments. But, in contrast to Mandl et al. (2017), we emphasise on the need for a new nomenclature, because defining new systems with old names, which were used without a clear definition of the terms and sometimes in a wrong context (e.g., for out-of-sequence thrust), produces confusion for all workers in the western NCA.

The numeric modelling aimed to model some key features of folding and thrusting in the western NCA (Karwendel- and Mieming and Wetterstein Mountains). According to our results we need to reject hypothesis 2.

Hypothesis 1: Folds above the Karwendel thrust in the Karwendel Mountains are buckle folds, which developed during or after thrust sheet transport. The salt-bearing décollement (Permo- Triassic evaporites) facilitated buckle folding above the thrust plane during shortening. The numeric model showed that the development of buckle folds is only possible when a salt bearing formation (or a very weak décollement) is present and is in accordance with hypothesis 1. The numeric model further showed that erosion and sedimentation influence the possibility of buckling and the limb length of developing folds. This confirms analytical solutions where limb lengths of buckle folds are dependent on the thickness of the buckling layer (Ramberg, 1960, Ramsay and Huber 1987). The numeric results contribute to understanding the structural development of the NCA. In the Karwendel Mountains folding of the hanging wall succession did not happen prior to thrust sheet transport. Folds above the thrust plane indicate N to NNE directed Cretaceous to Paleogene transport directions (Kilian and Ortner, 2019). Buckle folds in the Karwendel Mountains developed therefore after the initial stacking in the late Lower Cretaceous and after a decrease in lithostatic pressure due to Upper Cretaceous erosion. Numeric modelling showed that a very weak décollement is necessary to produce buckle folds. Buckle folding without a weak décollement is very unlikely. Therefore, the Permian evaporites (Haselgebirge) are indispensable for the development of buckle folds in the Karwendel thrust sheet. Today only remnants of the salt bearing Haselgebirge are preserved along the thrust plane, but based on the structural record we suggest that salt was present and salt tectonics was active at least into the Paleogene.

132 Hypothesis 2: The Karwendel thrust in the Mieming and Wetterstein Mountains formed out-of- sequence during ongoing shortening and cut buckle folds, which developed during the emplacement of thrust sheets (for explanation see Figure 2 and chapter 3.2). As already stated, numeric modelling showed that in absence of a very weak décollement horizon buckling is not possible. The sketched development of structures in the Mieming and Wetterstein Mountains (Figure 2b-d) is therefore not probable. Modelling the development of faults after folding showed that new faults develop in the competent units rather than following the anisotropy of the décollement horizon. These findings lead to a new (speculative) interpretation of the structures in the Mieming and Wetterstein Mountains (c.f., Study 2): The Mieming- and the Zugspitze thrust are not out-of- sequence thrusts but represent the main thrust, which is the Karwendel thrust (Figure 5). The bedding above the Karwendel thrust in the Mieming and Wetterstein Mountains probably onlapped a salt-anticline (e.g., Jackson and Hudec, 2017). Similar to the Karwendel Mountains no folding happened during the Cretaceous nappe stacking but thrusting was along the weak décollement horizon (Permian evaporites). Today the Permian evaporites are not preserved in the Mieming and Wetterstein Mountains. Large normal faults in the Anisian to Ladinian carbonates (e.g., Ortner, 2020) could have developed due to salt flow-out, but this idea was not further explored here. Salt flow-out of the thrust plane could also initiate the jump from the Eoalpine décollement (Karwendel thrust) into the Upper Triassic marls (Kössen Fm.) of the footwall (Tannheim thrust sheet), as observed in the Karwendel, Mieming and Wetterstein Mountains. The evacuation of salt strengthened the original thrust and forced it to the next décollement in its footwall. Younger thrusts crosscutting older thrusts are out-of-sequence by definition (e.g., Morley, 1988). This interpretation is in accordance with the results of the structural investigations and the new subdivision of the western NCA. Finally, it can be stated that the NCA are a fold-and-thrust belt, which is almost inactive today but an excellent area to study thrust geometries. The detailed structural investigations in combination with the numeric model helped to understand the development of geologic structures. The model itself was simple and a more precise model (e.g., more layers) could improve understanding the development of structures. One possibility to come closer to the natural composition of rocks would be to scatter the material parameters within the layer. In this case, each layer becomes a range of values assigned to and then each element of the layer becomes a random value out of the defined range. To understand the development from folding

133 to later out-of-sequence faulting the model should be able to change from a linear elastic to an ideal plastic behaviour during ongoing shortening. Simultaneously material parameter would have to change to simulate the salt flow-out of the thrust zone. By now, we did not test one of these ideas. The used model showed that material parameter strongly influence the development of folds and faults. This is important because many cross sections in fold-and-thrust belts are entirely drawn with geometric models. Classic models (e.g., ramp-flat model) need to be complemented by rheological models, to predict hidden structures in fold-and-thrust belts. By now, we are not able to derive general rules for the construction of cross sections in fold-and-thrust belts.

5 Conclusion This work intended to unravel some structural specialties in the western Northern Calcareous Alps. In the Karwendel, Mieming and Wetterstein Mountains we showed that in-sequence and out-of-sequence thrusts were both used to delimit thrust sheets even though these were active at different times. Based on the structural analysis we suggest a new tectonic subdivision with only two major thrust sheets. To avoid confusion with other nomenclatures, we named the tectonically deeper thrust sheet “Tannheim thrust sheet” and the tectonically higher “Karwendel thrust sheet”, based on their largest exposure. The Karwendel thrust sheet was possibly detached on salt. We suggest that before and after the emplacement of the Karwendel thrust sheet the salt layer influenced the structures in the western NCA. Paleogene folds in the Karwendel Mountains, which are younger than the Karwendel thrust and restricted to the hanging wall, are rheology dependent folds. Numeric results support the idea that these folds are buckle folds induced by the high competence contrast between the décollement horizon and the competent Anisian to Ladinian limestone above. In the Mieming and Wetterstein Mountains, salt might have influenced the structural style of the hanging wall in form of sedimentary onlap structures, which cause the impression of an anticline cut by the Karwendel thrust. Salt possible also influenced the development of Mesoalpine and Neoalpine out-of- sequence thrusts. From numeric models, we know that a weak décollement prevents the development of out-of-sequence thrusts. It therefore needs a salt flow-out of the thrust zone to enable mechanical coupling of hanging wall and footwall and the development of out-of- sequence thrusting. Structural studies suggest that salt tectonics in the Karwendel Mountains were at least active until the Paleogene.

134 6 References Ampferer, O., 1902, Bericht über die Neuaufnahme des Karwendelgebirges: Verhandlungen der Geologischen Bundesanstalt, p. 274-276. Ampferer, O., 1912, Gedanken über die Tektonik des Wettersteingebirges.- Verhandlungen der k.k. Geologischen Reichsanstalt, p. 197-212. Ampferer, O., 1914, Besprechung mit O. Schlagintweit, K. Ch. v. Loesch und H. Mylius über das Wettersteingebirge.: Verhandlungen der k.k. Geologischen Reichsanstalt, v. 1914, p. 338-352. Ampferer, O., 1931, Zur neuen Umgrenzung der Inntaldecke: Jb. Geol. Bundesanst, v. 81, p. 25-48. Ampfer, O., 1942, Geologische Formenwelt und Baugeschichte des östlichen Karwendelgebirges: Denkschriften d. Akad. d. Wiss:, v. 106 no. 1, p. 1-95. Ampferer, O., and Hammer, W., 1911, Geologischer Querschnitt durch die Alpen vom Allgäu zum Gardasee: Jb. Geol. Reichsanst., v. 61, p. 531 - 710. Biot, M. A., 1961, Theory of folding of stratified viscoelastic media and its implications in tectonics and orogenesis: Geological Society of America Bulletin, v. 72, no. 11, p. 1595-1620. Biot, M. A., 1964, Theory of internal buckling of a confined multilayered structure: Geological Society of America Bulletin, v. 75, no. 6, p. 563-568. Boyer, S. E., 1992, Geometric evidence for synchronous thrusting in the southern Alberta and northwest Montana thrust belts., in McClay, K. R., ed., Thrust Tectonics: London, Chapman & Hall, p. 377-390. Butler, R. B., Clare E.; Cooper, Mark A.; Watkins, Hannah, 2019, Fold-thrust structures - where have all the buckles gone?: Bond, C.E. & Lebit, H.D. (eds) 2020. Folding and Fracturing of Rocks: 50 Years of Research since the Seminal Text Book of J.G. Ramsey. Geological Society, London, Special Publications, 487, 21-44. Casey, M., and Dietrich, D., 1997, Overthrust shear in mountain building, Evolution of Geological Structures in Micro-to Macro-scales, Springer, p. 119-142. Currie, J., Patnode, H., and Trump, R., 1962, Development of folds in sedimentary strata: Geological Society of America Bulletin, v. 73, no. 6, p. 655-673. Czech, J. H., Helmut, 1990, Gesteinskennwerte aus Laborversuchen: Felsbau v. 8, no. Nr.3. Eisbacher, G. H., and Brandner, R., 1995, Role of high-angle faults during heteroaxial contraction, Inntal thrust sheet, Northern Calcareous Alps, western Austria: Geologisch-Paläontologische Mitteilungen Innsbruck, v. 20, p. 389 - 406. Eisbacher, G. H., and Brandner, R., 1996, Superposed fold thrust structures and high angle faults, northwestern Calcareous Alps, Austria: Eclogae Geologicae Helvetiae, v. 89, p. 553 - 571. Eisbacher, G. H., Linzer, G.-H., and Meier, L., 1990, A depth extrapolated structural transect across the Northern Calcareous Alps of Western Tirol: Eclogae Geologicae Helvetiae, v. 83, no. 3, p. 711 - 725. Espina, R. G., Alonso, J. L., and Pulgar, J. A., 1996, Growth and propagation of buckle folds determined from syntectonic sediments (the Ubierna Fold Belt, Cantabrian Mountains, N Spain): Journal of Structural Geology, v. 18, no. 4, p. 431-441. Fischer, M., Woodward, N., and Mitchell, M., 1992, The kinematics of break-thrust folds: Journal of Structural Geology, v. 14, no. 4, p. 451-460. Gaupp, R., 1982, Sedimentationsgeschichte der kalkalpinen Mittelkreide (Allgäu, Tirol, Vorarlberg): Zitteliana, v. 8, p. 33 - 72. Gaupp, R., 1983, Die paläogeographische Bedeutung der Losensteiner Schichten (Alb, Nördliche Kalkalpen): Zitteliana, v. 10, p. 155-171. Goffey, G., Craig, J., Needham, T., and Scott, R. E., 2010, Hydrocarbons in contractional belts, Special Publications, No. 348, Geological Society, London.

135 Hahn, F. F., 1912, Versuch einer Gliederung der austroalpinen Masse westlich der österreichischen Traun: Verhandlungen der Geologischen Reichsanstalt, v. 1912, p. 337-344. Hahn, F. F., 1913, Grundzüge des Baues der nördlichen Kalkalpen zwischen Inn und Enns, Geologische Gesellschaft. Heißel, W., 1958, Zur Tektonik der Nordtiroler Kalkalpen Mitt Österr Geol Ges 50 (1957), p. p. 95- 132. Jackson, M. P., and Hudec, M. R., 2017, Salt tectonics: Principles and practice, Cambridge University Press. Jeng, F., and Huang, K., 2008, Buckling folds of a single layer embedded in matrix–Theoretical solutions and characteristics: Journal of Structural Geology, v. 30, no. 5, p. 633-648. Jeng, F., Lin, M., Lai, Y., and Teng, M., 2002, Influence of strain rate on buckle folding of an elasto– viscous single layer: Journal of Structural Geology, v. 24, no. 3, p. 501-516. Kilian, S., and Ortner, H., 2019, Structural evidence of in-sequence and out-of-sequence thrusting in the Karwendel mountains and the tectonic subdivision of the western Northern Calcareous Alps: Australian Journal of Earth Sciences, v. 112, p. 62-83. Krois, R., and Stingl, V., 1994, Kretazische „Augensteine": Notiz zu einem fraglichen Gosauvorkommen im Karwendel (Tirol, Österreich).-J b. Geol. Bundesanst, v. 137, p. 289- 293. Lacombe, O., Lavé, J., Roure, F. M., and Verges, J., 2007, Thrust belts and foreland basins: From fold kinematics to hydrocarbon systems, Springer Science & Business Media. Loesch, K. C. v., 1915, Der Schollenbau im Wetterstein- und Mieminger- Gebirge.: Jahrbuch der k. k. Geologischen Reichsanstalt, 64, p. 1-98. Mandl, G., Brandner, R., and Gruber, F., 2017, Zur Abgrenzung und Definition der Kalkalpinen Deckensysteme: Arbeitstagung der Geologischen Bundesanstalt 2017, Bad Ischl, Hallstatt, Gmunden. Morley, C. K., 1988, Out-of-sequence thrusts: Tectonics, v. 7, p. 539-561. Müller-Wolfskeil, P., and Zacher, W., 1984, Neue Ergebnisse zur Tektonik der Allgäuer und Vilser Alpen: Geologische Rundschau, v. 73, p. 321-335. Mylius, H., 1914, Berge von scheinbar ortsfremder Herkunft in den bayrischen Alpen: Landeskundliche Forschungen, v. 22, p. 1-14. Nemcok, M., Schamel, S., and Gayer, R., 2005, Thrust belts - Structural archtitecture, thermal regimes, and petroleum systems, Cambridge, Cambridge University Press 541 p.: Ortner, H., 2003, Cretaceous thrusting in the western part of the Northern Calcareous Alps (Austria) - evidences from synorogenic sedimentation and structural data: Mitteilungen der Österreichischen Geologischen Gesellschaft, v. 94, p. 63-77. Ortner, H., 2016, Field Trip 4: Deep water sedimentation on top of a growing orogenic wedge - interaction of thrusting, erosion and deposition in the Cretaceous Northern Calcareous Alps: Geo.Alp, v. 13, p. 141-182. Ortner, H., 2020, Bericht 2018 über geologische Aufnahmen im Karwendelgebirge auf den Blättern UTM NL 32-03-17 Hinterriß und UTM NL 32-03-23 Innsbruck: Jb. Geol. Bundesanst., v. 132 (2019), p. 392-396. Ortner, H., and Bitterlich, L., 2016, The Zugsitze cross section and the structure of the Northern Calcareous Alps.: In: Ortner H. - Abstract Volume of GeoTirol 2016 - Annual Meeting of DGGV and PANGEO Austria, Institute of Geology, University of Innsbruck, Innsbruck, v. 248. Ortner, H., and Gaupp, R., 2007, Synorogenic sediments of the western Northern Calcareous Alps: Geo.Alp, v. 4, p. 133-148. Ortner, H., and Kilian, S., 2013, In-sequence and out-of-sequence thrusts: nappe structure of the western Northern Calcareous Alps revisited: Berichte der Geologischen Bundesanstalt, v. 99 no. 11th Alpine Workshop, Schladming, Abstract Volume, p. 72.

136 Price, N. J., 1975, Rates of deformation: Journal of the Geological Society, v. 131, no. 6, p. 553-575. Price, N. J. C., J.W., 1990, Analysis of geological Structures: Cambridge University Press. Ramberg, H., 1960, Relationships between length of arc and thickness of ptygmatically folded veins: American Journal of Science, v. 258, no. 1, p. 36-46. Ramberg, H., 1970, Folding of laterally compressed multilayers in the field of gravity, I: Physics of the Earth and Planetary Interiors, v. 2, no. 4, p. 203-232. Ramsay, J. G., 1974, Development of chevron folds: Geological Society of America Bulletin, v. 85, no. 11, p. 1741-1754. Ramsay, J. G., and Huber, M. I., 1987, The techniques of modern structural geology: Folds and fractures, Academic press. Richter, M., 1929, Die Struktur der nördlichen Kalkalpen zwischen Rhein und Inn: Neues Jahrbuch für Mineralogie, Geologie und Paläontologie, v. Beilagenband, 63, no. Abteilung B, p. 1-62. Rowan, M. G., and Kligfield, R., 1992, Kinematics of large-scale asymmetric buckle folds in overthrust shear: an example from the Helvetic nappes: In: McClay, K. R., Thrust Tectonics, 165-173, Chapman & Hall, London., p. 165-173. Rüffer, T., and Bechstädt, T., 1995, Interpretation des Deckenbaus in den westlichen nördlichen Kalkalpen: Widerspruch zwischen tektonischen und sedimentologischen Daten: Jahrbuch der Geologischen Bundesanstalt, v. 138, p. 701 - 713. Schlagintweit, O., 1912a, Die Mieminger-Wetterstein Überschiebung: Geolog. Runschau, v. 3, no. 2, p. 73-92. Schlagintweit, O., 1912b, Zum Problem des Wettersteingebirges: Verhandlungen der k. k. Geologischen Reichsanstalt, p. 313-327. Schmalholz, S. M., and Schmid, D. W., 2012, Folding in power-law viscous multi-layers: Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, v. 370, no. 1965, p. 1798-1826. Schmid, S. M., Fügenschuh, B., Kissling, E., and Schuster, R., 2004, Tectonic map and overall architecture of the Alpine orogen: Eclogae Geologicae Helvetiae, v. 97, p. 93-117. Sommaruga, A., 1999, Décollement tectonics in the Jura forelandfold-and-thrust belt: Marine and Petroleum Geology, v. 16, no. 2, p. 111-134. Suppe, J., 1983, Geometry and kinematics of fault-bend folding: American Journal of science, v. 283, no. 7, p. 684-721. Suppe, J., and Medwedeff, D. A., 1990, Geometry and kinematics of fault-propagation folding: Eclogae Geologicae Helvetiae, v. 83, no. 3, p. 409-454. Tollmann, A., 1970, Tektonische Karte der Nördlichen Kalkalpen: 3. Teil: Der Westabschnitt: Mitteilungen der Österreichischen Geologischen Gesellschaft, v. 62 (1969), p. 78-170. Tollmann, A., 1976b, Analyse des klassischen nordalpinen Mesozoikums, Wien, Deuticke, Monographie der Nördlichen Kalkalpen, Teil II, 580 p.: Trümpy, R., Geologist, S., Trümpy, R., and Géologue, S., 1969, Die helvetischen Decken der Ostschweiz: Versuch einer palinspastischen Korrelation und Ansätze zu einer kinematischen Analyse, Birkhäuser. von Soos, P., and Engel, J., 2008, Eigenschaften von Boden und Fels- ihre Ermittlung im Labor: In: Grundbau Taschenbuch Teil 1: Geotechnische Grundlagen, 7. Auflage; Karl Josef Witt (Hrsg.). Woodward, N. B., 1997, Low-amplitude evolution of break-thrust folding: Journal of Structural Geology, v. 19, no. 3-4, p. 293-301.

137

138 7 Appendix Authors' contributions for Study 1 Sinah Kilian wrote the manuscript. Figures were created by Sinah Kilan and Hugo Ortner. Hugo Ortner checked the manuscript.

Authors' contributions for Study 2 Hugo Ortner wrote the manuscript and created most of the figures. Sinah Kilian performed the numerical modelling presented in Figure 10, contributed to the generation of the new tectonic subdivsion and checked manuscript and figures.

Authors' contributions for Study 3 Sinah Kilian wrote the Manuskript. Sinah Kilian and Hugo Ortner created the figures. Hugo Ortner checked the manuscript. Barbara Schneider-Muntau checked the numeric modelling part.

139