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The Dolomites Indenter represents the eastern front segment of the Neogene to ongoing N(W)-directed continental indentation of Adria into Europe. Concomitant shortening is accommodated within a south-vergent thrust belt, situated between the Periadriatic Fault system and the South-Alpine front. A combination of low temperature thermochronological analyses, focussed mapping and analogue modelling is used for unravelling the Neoalpine history of the crustal and lithospheric scale tectonic processes during indentation.In our analogue models, extensional platform-basin geometries, formed at passive continental margins, are subject to subsequent shortening and orogenesis. Parallel to oblique (10 to 20 degrees with respect to the basin axes) contraction has been applied leading to the inversion of the pre-orogenic basins. The experiments show that the simple presence of an inherited platform-basin configuration controls the overall style of compressional deformation, no matter of including frictional or viscous basal décollements, of varying the rheology of the basin fill, or of changing platform-basin thickness ratios. Orientations of thrust faults change laterally across inherited platform-basin transitions throughout all experiments. New fault slip data and shortening directions from fold axes along the western segment of the Belluno thrust of the Valsugana fault system support variations of thrust fault orientation and a lateral change in shortening direction (from SSW to SSE along strike) along one single fault. Based on our modelling results, we infer that this variability of shortening directions depends on inherited structures and do not necessarily reflect different deformation phases.Our low-temperature thermochronological dataset (zircon and apatite (U-Th)/He, and apatite fission track analyses) focuses on the Dolomites Indenter and spans from the Periadriatic Fault System (Pustertal-Gailtal fault) in the north to the footwall of the Bassano thrust in the south. The results argue against an only in-sequence fault activity within the dominantly WSW – ENE striking thrust belt but indicate a more complex fault system development, including backstepping fault activity. This is locally supported by field observations of fault cross-cutting relationships (e.g., at the Moschesin fault near Agordo). Remarkable that west of the Transalp Corridor Mesozoic apatite fission track ages are preserved within fault delimited areas while further to east all apatite fission track data show Cenozoic ages indicating younger exhumation in the eastern part of the study area.

We have investigated the deformation history of the Keszthely Hills (Transdanubian Range, W Hungary), which belongs to the uppermost slice of the Austroalpine nappe system. This Upper Triassic to Upper Miocene sedimentary rock sequence documented the deformation of the upper crust during repeated rifting and inversion events. We investigated the structural pattern and stress field evolution of this multistage deformation history by structural data collection and evaluation from surface outcrops. Regarding the Mesozoic deformations, we present additional arguments for pre-orogenic (Triassic and Jurassic) extension (D1 and D2 phases), which is mainly characterized by NE–SW extensional structures, such as syn-sedimentary faults, slump-folds, and pre-tilt conjugate normal fault pairs. NW–SE-striking map-scale normal faults were also connected to these phases. The inversion of these pre-orogenic structures took place during the middle part of the Cretaceous; however, minor contractional deformation possibly reoccurred until the Early Miocene (D3 to D5 phases). The related meso- and map-scale structures are gentle to open folds, thrusts and strike-slip faults. We measured various orientations, which were classified into three stress states or fields on the basis of structural criteria, such as tilt-test, and/or superimposed striae on the same fault planes. For this multi-directional shortening we presented three different scenarios. Our preferred suggestion would be the oblique inversion of pre-orogenic faults, which highly influenced the orientation of compressional structures, and resulted in an inhomogeneous stress field with local stress states in the vicinity of inherited older structures. The measured post-orogenic extensional structures are related to a new extensional event, the opening of the Pannonian Basin during the Miocene. We classified these structures into the following groups: immediate pre-rift phase with NE–SW extension (D6), syn-rift phase with E–W extension (D7a) and N–S transpression (D7b), and post-rift phase with NNW–SSE extension (D8).

We reinvestigated parts of the northern Austroalpine margin and provided structural and kinematic field data in order to interpret the kinematic relationship between the Cenoman-Randschuppe (CRS) marginal slice, Falkensteinzug (FSZ), Tannheim- and Karwendel thrust sheets occurring in a narrow strip at the northern front of the northwestern Northern Calcareous Alps (NCA). As a consequence, we propose a revised model for the tectonic evolution of the northern Austroalpine margin. As thrusting propagates from SSE to NNW (Cretaceous orogeny), the Karwendel thrust sheet (including its frontal part, the FSZ) was emplaced onto the Tannheim thrust sheet in the Albian, deduced from (i) upper-footwall deposits, the youngest sediments below the Karwendel thrust (Tannheim- and Losenstein Fms.), and (ii) thrust-sheet-top deposits unconformably overlying the deeply eroded northern Karwendel thrust sheet (Branderfleck Fm.). The future CRS marginal slice was, at that time, part of the foreland of this Early Cretaceous Alpine orogenic wedge. Pervasive overprint by sinistral shear within the CRS marginal slice and northern Tannheim thrust sheet suggests sinistral W-E striking transform faults cutting across this foreland, decoupling CRS marginal slice and FSZ from the main body of the NCA and enabling an independent evolution of the CRS marginal slice from the Early Cretaceous onwards. Subsequent Late Cretaceous and younger shortening leads to successive incorporation of Arosa zone, Rhenodanubian Flysch (RDF) and Helvetic units into the Alpine nappe stack; the Tannheim thrust representing the basal thrust of the NCA. Growth strata within thrust-sheet-top deposits (Branderfleck-Fm.) give evidence for refolding of thrust sheet boundaries. In a typical thin-skinned fold-and-thrust belt, deformation should cease towards the thrust front, whereas within the NCA it increases. An Austroalpine thrust front controlled by E-trending transform faults could cause an increase in deformation towards the most external NCA and explain the absence of the Arosa zone between Allgäu and Vienna. Such faults would most probably also cut out Lower Austroalpine units. Therefore, RDF and CRS marginal slice are juxtaposed; the latter found in the tectonic position of the Arosa zone. The presence of transform faults underlines the strong imprint of the opening of the North Atlantic Ocean on the depositional setting and tectonic evolution of the NCA.

Polyphase deformation of continental crust is analysed through physical analogue models for settings where platform-basin geometries at passive continental margins are subject to subsequent shortening and orogenesis. In a first stage, segmentation of the brittle and brittle-ductile models into basins and platforms is achieved by extension. Basins are partly filled with brittle material to allow for a strength differences between basin and platform realms, simulating relatively weaker, incompetent deposits of grabens surrounded by competent pre-rift basement or carbonate platform rock, respectively. In a second stage of deformation, contraction parallel to oblique (10 to 20 degrees) with respect to the basin axes has been applied leading to the inversion of earlier formed basins. The experiments show that the simple presence of an inherited platform-basin configuration controls the overall style of compressional deformation, no matter of including frictional or viscous basal décollements, of varying the rheology of the basin fill, or of changing platform-basin thickness ratios. Orientations of thrust faults change laterally across inherited platform-basin transitions throughout all experiments; higher obliquity of basin inversion leading to stronger curvature of thrusts with respect to the pre-existing rift axes. Variations in the strike of thrust fronts are accompanied by changes of the shortening direction along one single fault and time step. Furthermore, our models support localisation of deformation in areas of lateral strength contrasts, as platform-basin transitions represent. Reactivation of normal faults occurs in oblique basin inversion settings only, favourably at platform-basin transitions where the normal faults face the shortening direction. The amount and style of fault reactivation depend on the material used. Both parallel and oblique inversion experiments can be applied to polyphase deformed continental crust, as, e.g., the Dolomites Indenter of the eastern Southern Alps. Our models involving two phases of deformation, suggest that the whole tectonic evolution of the Dolomites Indenter is controlled by inherited features. Fault slip data and shortening directions from fold axes from our field case study along the western segment of the Belluno thrust of the Valsugana fault system support variations of thrust fault orientation and a lateral change in shortening direction (from SSW to SSE along strike) along one single fault. Based on our modelling results, we infer that this variability of shortening directions depends on inherited structures and do not necessarily reflect different deformation phases.

We investigate the tectonic evolution of the Wetterstein and Mieming mountains in the western Northern Calcareous Alps (NCA) of the European Eastern Alps. In-sequence NW-directed stacking of thrust sheets in this thin-skinned foreland thrust belt lasted from the Hauterivian to the Cenomanian. In the more internal NCA major E-striking intracontinental transform faults dissected the thrust belt at the Albian–Cenomanian boundary that facilitated ascent of mantle melts feeding basanitic dykes and sills. Afterwards, the NCA basement was subducted, and the NCA were transported piggy-back across the tectonically deeper Penninic units. This process was accompanied by renewed Late Cretaceous NW-directed thrusting, and folding of thrusts. During Paleogene collision, N(NE)-directed out-of-sequence thrusts developed that offset the in-sequence thrust. We use this latter observation to revise the existing tectonic subdivision of the western NCA, in which these out-of-sequence thrusts had been used to delimit nappes, locally with young-on-old contacts at the base. We define new units that represent thrust sheets having exclusively old-on-young contacts at their base. Two large thrust sheets build the western NCA: (1) the tectonically deeper Tannheim thrust sheet and (2) the tectonically higher Karwendel thrust sheet. West of the Wetterstein and Mieming mountains, the Imst part of the Karwendel thrust sheet is stacked by an out-of-sequence thrust onto the main body of the Karwendel thrust sheet, which is, in its southeastern part, in lateral contact with the latter across a tear fault.

In the evolution of the Alps, the Adriatic plate is traditionally considered as rigid indenter and research on collision and extrusion tectonics mainly focused on the areas north of it. However, the structure of the northernmost part of the Adriatic microplate in the eastern Southern Alps of Italy and Slovenia, referred to as Dolomites Indenter (DI), demonstrates significant internal deformation of a continental indenter that contains the structural memory of Jurassic extension leading to the formation of the Alpine Tethys. Here we argue that these pre-existing NNE-SSW trending normal faults are of paramount importance for understanding and explaining Paleogene to Neogene crustal deformation of the DI. In particular, we demonstrate through physical analogue modelling that lateral changes of thrust fault orientations are controlled by the inherited fault bound basin and platform configuration (e.g., in the Cadore area, where the Trento platform merges into the Belluno basin). In our brittle and brittle-ductile analogue experiments, shortening is orthogonal or oblique to platform and basin configuration, which is represented by either (i) pre-scribed strength contrasts between platforms/basins or (ii) graben structures modelled by an initial extensional phase. This approach allows us to test various deformational wavelengths as well as timing and localisation of uplift of the DI’s upper to middle crust. Modelling results indicate that the localisation of deformation is controlled by lateral strength contrasts, as transitions from platforms to basins represent. Analyses of surface displacement vectors show that these areas are associated with changes in shortening directions, resulting in, curved faults. All models emphasise that the overall style of deformation is less dependent on the material of the basal décollement, but is ruled by the inherited platform and basin configuration, independent of orthogonal or oblique inversion. To compare analogue modelling results with deformation in the DI, structural fieldwork accompanied by thermochronological sampling was carried out. Examined cross-cutting criteria covering the entire DI comprise evidence for four distinguishable deformation phases during Paleogene (Dinaric) shortening and subsequent Neogene (Alpine) continental indentation: Top SW, Top (S)SE, Top S and Top E(SE). However, shortening directions along several of the studied faults, e.g. the overall SSE-vergent Belluno thrust (Valsugana fault system), change locally from top SSW to top SSE along strike. Based on our modelling results, we infer that the variability of shortening directions along these thrust faults may depend on inherited structures and do not necessarily reflect different deformation phases. As such the number of deformation phases in the Southern Alps may have been overestimated so far.

Synorogenic sediments have often been used to constrain nappe movements and geodynamic processes. This contribution presents a case study from the Alps, in which synorogenic deposition (Lech-, Rossfeld-, Losenstein-, and Branderfleck Fms., Gosau Group) is affected by processes on different scales, that have led to a bewildering multitude of interpretations. We add another one.Presently, the Northern Calcareous Alps (NCA) are a thin-skinned fold-and-thrust belt in the external part of the Austroalpine unit, which represents the upper plate during Cenozoic Alpine orogeny. However, orogeny started in the late Early Cretaceous, when large parts of the Austroalpine and the entire NCA were in a lower plate position. This major geodynamic change also controlled deposition of synorogenic sediments.Prior to Cretaceous onset of subduction, the NCA were still in sedimentary contact with the underlying lithosphere. Most paleogeographic reconstructions show the NW edge of the Adriatic microplate at the transition from a passive margin in the SW to a transform-dominated margin in the N, as a consequence of Jurassic-Cretaceous opening of the Alpine Tethys. These transform faults apparently offset oceanic units dextrally to the east, however, they have sinistral kinematics, as a result of the northward propagating opening of the Atlantic Ocean.At the turn from the Early to the Late Cretaceous, the NCA of the external Austroalpine had already been affected by major nappe movements in the foreland of an intracontinental subduction that had initiated along sinistral, roughly E-striking intracontinental transform faults within the Adriatic microplate (Stüwe and Schuster, 2010). Thrusting had propagated across the Adriatic plate to its northern transform boundary (Ortner and Kilian, 2021 in press).As a consequence, oceanic crust in the N neighboured continental crust S of a transform zone. When shortening resumed in the early Late Cretaceous, continental lithosphere was subducted and replaced by oceanic lithosphere. Thus, the foreland thrust belt became an accretionary wedge. Its surface subsided to bathyal depth, as the surface of oceanic crust is isostatically in a depth of about 4.5 km below sea level, and the surface of continental crust is typically near sea level (e.g., Kearey et al., 2009).Synorogenic sediments were deposited throughout shortening. They were affected by (i) ongoing contraction associated with tear faulting on the local scale, (ii) thickening of the orogenic wedge by emplacement of thrust sheets on the regional scale, and (iii) subsidence of the thin-skinned wedge controlled by replacement of continental by oceanic lithosphere. Such a multi-scale explanation may solve the long-disputed question of the tectonic setting of the Cretaceous synorogenic sediments of the NCA.ReferencesKearey, P., Klepeis, K.A., Vine, F.J., 2009. Global tectonics (3rd ed.). Wiley-Blackwell, Oxford.Ortner, H., Kilian, S., 2021 in press. Thrust tectonics in the Wetterstein and Mieming mountains, and a new tectonic subdivision of the Northern Calcareous Alps of western Austria and southern Germany. Int. J. Earth. Sci. https://doi.org/10.1007/s00531-021-02128-3Stüwe, K., Schuster, R., 2010. Initiation of subduction in the Alps: Continent or ocean? Geology, 38, 175-178. https://doi.org/10.1130/G30528.

The Dolomites Indenter represents the front of the Neogene to ongoing N(W)-directed continental indentation of Adria into Europe. Deformation of the Dolomites Indenter is well studied along its rim, documented by important fault zones such as the Periadriatic fault system, the Giudicarie belt, and the Valsugana and Montello fault systems. With this study, we aim to investigate the internal deformation of the Dolomites Indenter, which has been much less studied so far but is important for understanding crustal-scale processes during the Alpine orogeny. Our approach to unravel the indenters exhumation and deformation history comprises (i) the compilation and acquisition of detailed structural and sedimentological field data within the Dolomites Indenter, (ii) a collection of a new and comprehensive low-temperature thermochronological dataset (this contribution), and (iii) crustal- to lithospheric-scale physical analogue modelling experiments (see contribution of Sieberer et al. in session TS7.2 – Internal deformation of the Dolomites Indenter, eastern Southern Alps: Orthogonal to oblique basin inversion investigated in crustal scale analogue models). New field data comprise evidence for four distinguishable shortening directions. Examined intersection criteria along N-S cross sections covering the indenters extend from Periadriatic to Bassano fault system support a succession of Top SW, Top (S)SE, Top S and Top E(SE) movement. However, preexisting geometry strongly seems to affect the regional expression of respective compression phases and along strike variation of lineation trends can be observed within coherent fault systems. The limited amount of existing thermochronological data already indicates the presence of relative vertical displacements within the Dolomites Indenter after the onset of indentation, including mostly Miocene apatite fission track (AFT) cooling ages along the Periadriatic and the Valsugana fault and several age clusters of Triassic to Jurassic AFT data. In order to obtain a detailed picture of the indenters thermotectonic evolution, an extensive set of samples has been collected along three roughly N-S striking corridors between Bolzano in the west and Tolmezzo in the east. In this contribution we present the new apatite (U-Th)/He and fission track data along the westernmost corridor (Mauls - Brixen - Valsugana - Schio). The results of field work, comprehensive modelling of time temperature paths, and physical analogue modelling substantially contribute to the understanding of internal deformation and thus enable conclusions to be drawn about the processes at lithospheric scale.

Salt welds are frequent features in basins with halokinesis. They are profusely observed on reflection seismic and are common features on maps of salt-rich geological provinces (even though they may not always have been identified as such). Despite this abundance, detailed studies of salt weld outcrops are remarkably scarce, in part due to outcrop conditions being hampered by vegetation and weathering. This results in a significant paucity in the description of the structure of salt welds at the meter- to centimeter-scale.In this contribution we present our observations on three non-primary salt welds from the Northern Calcareous Alps (NCA) in the Eastern Alps, an area that evolved from a Permian-Jurassic passive margin setting to the Cretaceous-Recent Alpine orogenesis. The NCA are dominated by Triassic to Jurassic shallow to deep water carbonates, underlain by an Upper Permian evaporitic unit (mainly salt, anhydrite and shales). The evaporite unit is preserved at present mostly in broad, poorly outcropping salt bodies (salt walls?) located between blocks of Triassic platforms, and in diapirs and allochthonous bodies that are mined or quarried for halite and anhydrite. These bodies have been affected by Alpine orogenesis to different degrees, but in general present a strong contractional and/or strike-slip overprint.In this presentation we discuss welds that are associated to two diapirs and potentially to a strongly overprinted salt wall. Outcrop conditions for the welds are outstanding, with two of these welds being observed in the galleries of two salt mines. All welds occur in Middle to Upper Triassic platform carbonates and contain no major traces of evaporites but do contain either highly sheared syn-evaporite shales or fragments of Lower to Middle Triassic post-salt sediments. Hand samples and outcrop observations have been used to describe the millimiter- to hectometer-scale structure of the welds and provide a unique insight into the detailed architecture of salt welds. Furthermore, it reveals a different tectonic evolution for each weld, with different relative contributions of halokinesis and faulting during the passive margin stage, and of re-activation during Alpine orogenesis.

The Dolomites Indenter (DI) represents the front of the Neogene to ongoing N(W)-directed continental indentation of Adria into Europe. In this contribution, we focus on the internal deformation of the DI and its eastern continuation towards the Dinarides. Using a series of crustal scale analogue models, we investigate the effect of Jurassic E-W extension on the NW-SE directed shortening of the DI during Alpine orogeny.The brittle and brittle-ductile analogue experiments can be grouped in two sub-series. In sub-series A, the platform-basin topography has been created by pre-scribing an initial strength contrast between platforms and basins followed by one stage of indentation. In sub-series B, graben structures were developed through an initial extensional phase, subsequently followed by compression. The evolving grabens were syn-kinematically filled up to different thicknesses depending on the material used; either with quartz sand up to a platform/basin thickness ratio of 0,75 or with feldspar sand or glass beads up to the initial, non-stretched crustal thickness. The brittle upper crust of platforms was simulated with quartz sand, the brittle to ductile middle crust by either glass beads or by a mixture of polydimethylsiloxane (PDMS) silicon putty and quartz sand. In both sub-series, variations in the orientation of rheological boundaries with respect to the convergence direction have been modelled. This (oblique) basin inversion allows us to test various deformational wavelengths as well as timing and localisation of uplift of the DI’s upper to middle crust.Modelling results of (oblique) basin inversion confirm the localisation of deformation in areas of lateral strength contrasts (Brun and Nalpas, 1996), as transitions from platforms to basins represent. Spacing of in-sequence thrusts is larger on platforms and smaller in basins, visible in both, models of sub-series A (inversion of strength difference only) and B (inversion of strength difference and actual normal faults). The vergence of in-sequence structures varies from mostly foreland directed using glass beads as basal detachment, to pop-up structures using putty, to a combination of both using quartz sand only. Based on our modelling observations, we propose that the overall style of deformation is less dependent on the material of the basal décollement, but is ruled by the inherited platform/basin configuration.To compare analogue modelling results with deformation in the DI, structural fieldwork accompanied by thermochronological sampling was carried out (for details on structural and thermochronological data see the contribution of Klotz et al. in session GD8.4). According to field observations, the shortening direction along several of the studied faults, e.g. the overall SSE-vergent Belluno thrust (Valsugana fault system, external eastern Southern Alps, Italy), changes locally from top SSW to top SSE along strike. We infer that the variability of shortening directions along these thrust faults depends on inherited geometries and is not the result of different deformation phases. One possible conclusion from this observation is that the number of deformation phases in the Southern Alps may have been overestimated so far.References:Brun, J.-P., Nalpas, T. (1996). Graben inversion in nature and experiments. Tectonics. v. 15, no. 3, p. 677-687.

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The Dolomites Indenter (DI) represents the front of the Neogene to ongoing N(W)-directed continental indentation of Adria into Europe. Deformation of the DI is well studied along its rim, documented by important fault zones as, e.g., the Periadriatic fault system (PFS), the Giudicarie belt, and the Valsugana and Montello fault systems. With this study, we aim to investigate the internal deformation of the DI and its eastern continuation towards the Dinarides including the interference of Dinaric SW-directed and Alpine SE-directed folds and thrusts. What also remains unsolved at present is the relationship between deep-seated mantle dynamics and their control on the geometry and internal deformation of the DI. Our approach to unravel this tectonic history is a combination of (i) compilation and acquisition of detailed structural field data within the DI, (ii) collection of a new and comprehensive low-temperature thermochronological dataset covering the entire DI, and (iii) crustal- to lithospheric scale physical analogue experiments.The existing but limited thermochronological dataset already indicates the presence of relative vertical motions within the DI after the onset of indentation, including mostly Miocene Apatite fission track (AFT) ages along the PFS and the Valsugana fault and two age clusters of Triassic to Jurassic AFT data. One cluster represents the Monti Lessini east of Riva del Garda, the second is located SE of Bozen, in the footwall of the Truden line. Are these Mesozoic AFT age clusters resulting from tectonic vertical movements and/or are they linked to inhomogeneities within the DI, like the Mesozoic platform-basin geometries or the Permian Athesian Volcanic Complex? Ongoing thermochronological investigations aim to clarify these issues.By using crustal-scale (as a first step) physical analogue models, we aim to study (i) the impact of Jurassic E-W extension and (ii) the effect of crustal strengthening on the NW-SE directed deformation of the DI since Neogene times. Jurassic NNE-SSW trending normal faults led to a platform-basin-topography resulting, from west to east, in the Lombardian basin, Trento platform, Belluno basin, and Friuli platform (Winterer & Bosellini, 1981) but were inverted during Alpine orogeny. Moreover, the Trento platform approximately coincides with the extent of the up to ~2 km thick (Avanzini et al., 2013) Permian Athesian Volcanic Group. We simulate rigid Permian magmatic rocks, which could have led to a critical strengthening of the crust, in our analogue experiments by incorporating an additional strong domain to the lower upper crust. This, together with studying the influence of structural inheritance on the geometry and kinematics of Dinaric and Alpine deformation in the Southern Alps, allows us to model various deformational styles and -wavelengths of the DI during Neogene indentation.This study will contribute substantially to the understanding of internal deformation and thus enable conclusions to be drawn about the processes at lithospheric scale also addressed by AlpArray.References:Avanzini, M. et al. (2013): Note illustrative della carta geologica d'Italia, foglio 026 Appiano. Roma, Servizio Geologico d'Italia, 324 pp.Winterer, E. L., & Bosellini, A. (1981): Subsidence and Sedimentation on Jurassic Passive Continental Margin, Southern Alps, Italy. AAPG Bulletin, 65(3), 394-421.

We studied the Karwendel mountains of the Northern Calcareous Alps fold-and-thrust belt, that formed during the Upper Cretaceous. The study area exposes one of the principal thrusts that emplaces Triassic sediments on the top of Cretaceous rocks. Based on a detailed structural analysis (Kilian and Ortner, 2019) it has been demonstrated that the folds above the thrust are buckle folds.With the finite element modelling, we aimed to create folds with wavelengths comparable to those observed in the field. The model set up is a simple layer cake model based on the stratigraphy of the western Northern Calcareous Alps. We used ABAQUS a finite element software to create the model. The material model is linear elasticity. During modelling, we tested different material characteristics and layer thickness. In all model runs a very weak décollement (possibly salt) is necessary to produce folds. We further tested the influence of erosion and re-sedimentation on the development of structures. We concluded that the growth of folds having wavelengths comparable to the field examples depends on the thickness of the competent layer, whereas the thickness of the incompetent layer has negligible influence.We suggest that fold development in this part of the Northern Calcareous Alps is dependent on the interplay between the growth of evaporite-cored anticlines and surface erosion.ReferenceKilian, S., Ortner, H.: 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, Vienna, 2019, V.112/1, p.62-83, DOI: 10.17738/ajes.2019.0005.

Tectonic subdivisions of larger geologic units reflect the geologic knowledge at the time of creation. In many thrust belts the original subdivisions had been created during the first comprehensive mapping campaigns at the end of the 19th to early 20th century and reflect the geologic knowledge at that time. Even if many thrusts were identified correctly, no formal framework existed to give guidelines of how to distinguish tectonic units. Nevertheless, these subdivisions are still in use.We analyze the thrust sheets of the Northern Calcareous Alps of western Austria and southern Germany and test the implicit assumptions underlying most tectonic subdivisions against field observations:Assumption 1: Thrust transport is large and thrusts do not end laterally. However, several major thrusts do loose stratgraphic offset and end laterally.Assumption 2: Allochthons are surrounded by thrusts on all sides. Unfortunately, any fault has been used to delimit allochthons.Assumption 3: Thrusting should bring old on young rocks. In some cases, allochthons have been delimited by out-of-sequence thrusts, that stack young on old rocks. In other cases, the allochthon is a mountain-size glide block that was buried by younger sediments, and the trace of the thrust is an unconformity in the field.As a consequence we propose a revised tectonic subdivision of the western part of the NCA, that avoids some of the problems discussed here, and is entirely based on the emplacement of old-on-young rocks across thrusts.

This study describes the inversion of a rift-related graben shoulder during emplacement and transport of a major thrust sheet in the external part of the western Northern Calcareous Alps (NCA). Structural fieldwork was carried out along the Lechtal thrust, separating the tectonically deeper Allgau thrust sheet from the Lechtal thrust sheet. An irregularly shaped Early Jurassic normal fault and an adjacent basin are present in the immediate footwall of the Lechtal thrust. The Early Jurassic age of the basin formation is indirectly established by thickness differences in the syntectonic deposits. Scaly fabric and small-scale drag folds document top S to top SW kinematics at the normal fault. During Alpine orogeny the Lechtal thrust is forced to recess around the graben shoulder in the footwall. A transpressive, dextral tear-fault develops in the hanging wall to compensate for lateral differences of shortening. The normal fault is too steep for inversion, and a shortcut thrust across the half-graben shoulder developed at a shallower angle. It transports the detached half-graben shoulder into the neighbouring basin. Albian to Paleogene transport of the Lechtal thrust sheet caused deformation below the Lechtal thrust, creating a shear zone with isoclinal folds, break thrusts, boudinaged beds and development of S–C fabric. Kinematic analyses of S–C fabrics and small-scale fold axes along the Lechtal thrust show consistent top NW to top N shortening directions in accordance with Cretaceous to Paleogene thrust directions in the NCA.

In studies of neotectonism, alluvial-fan and talus deposits commonly are used as deformation markers, but rarely are studied themselves and in relation to adjacent faults. Herein we report on the facies, diagenesis and deformation of a talus succession and fault cataclasites in the central Apennines of Italy. The study site is located near the western end of the Assergi normal fault zone that accommodates >2000 m of vertical throw, but was dormant since a longer interval of time. The preserved talus succession is confined to the fault hangingwall. Deposition and deformation of the talus overlapped with the terminal phase of fault activity. The talus accumulated mainly from grain flows, cohesive debris flows and ephemeral fluid flows; it comprises (i) two superposed units of scree breccias, partly cemented before deformation, and intercalated with (ii) an interval of unlithified scree and soils. The exposed succession accumulated between ≥33–30 cal ka BP to less than ~22 cal ka BP. Talus breccias record complex diagenetic successions including eluviation/dissolution of primary matrix, growth of interstitial cements interrupted by episodes of dissolution and/or fracturation, and late-stage dissolution porosity development. Downthrow by faulting produced two types of folds, (i) a syncline-anticline pair with inclined fold axes roughly normal to the fault plane, and (ii) a recumbent fold with an axis subparallel to depositional strike of the scree slope; the complicated stratigraphic architecture of the deformed succession is truncated along and hidden beneath the topsoil of the present, uniform slope surface. Further deformation structures include conjugate fractures locally coated by speleothem flowstones, deformation bands, sediment fabrics characterized by planar clast contacts, and lithoclasts crushed in situ while embedded in the sediment. Cementation overlapped with and post-dated in-situ crushing of clasts. The core of the normal fault is an ultracataclasite that (i) shows different degrees of diagenetic recrystallization into micro- to pseudospar, and (ii) that is riddled with solution pores fringed or filled with successive ‘generations’ of calcite cement. These cements are locally sharply capped along discrete levels, and overlain by ultracataclasite with floating chunks of calcite cement crystals ripped off during increments of faulting. Stable isotopes of oxygen and carbon indicate that the ultracataclasites lithified under influence of meteoric waters, but at variable degrees of rock buffering; the calcite cements within the talus breccias and the flowstones along fractures, in turn, precipitated from meteoric-derived waters with low to negligible rock buffering. The structural juxtaposition of ultracataclasites (probably formed in 1–2 km depth) with talus breccias indicates that the preserved fault/talus ensemble records only the terminal phase of total faulting. The interstitial cements in talus, the flowstones on fracture walls, and the cements in ultracataclasites should be datable with the 234U/230Th errorchron method. This promises to be a new approach to derive age constraints on talus cementation and fracturation, and on palaeoactivity and final dormance of normal faults.

Slump structures in Upper Jurassic pelagic limestones of the Northern Calcareous Alps were studied using methods of ductile structural geology. The early deformation observed cannot be explained by conventional models for slumping, commonly describing slump complexes with an extending upper and contracting lower part. Instead, the structures suggest distributed bedding-parallel surficial stretching, and folding localized by roll-over normal faulting. The initially isoclinal and recumbent folds are pervasively overprinted by stretching, causing reorientation of fold axes into the downslope direction. Transport of folds separates antiformal and synformal hinges, and the resulting isolated hinges resemble intrafolial folds in metamorphic terranes. Only folds formed late in the process of lithification allow insight in the early stages of fold evolution. We suggest that this type of early deformation represents sediment creep that may be characteristic for slump structures in pelagic carbonates.

The thrust sheets of the Northern Calcareous Alps were emplaced during Late Cretaceous thrustdominated transpression expressed by thrust sheets segmented by closely spaced tear faults. Thrust sheet-top sediments were deposited during thrusting and associated fold growth and were controlled by active folding and tearing. We observe two types of angular unconformities: (1) Angular unconformities above folds between tear faults conform with the model of progressive unconformities. Across these unconformities dip decreases upsection. (2) Here, we define progressive unconformities that are related to tear faults and are controlled by both folding and tearing. Across these unconformities both strike and dip change. In growth strata overlying folds dissected by high-angle faults, such unconformities are expected to be common. We used analogue modelling to define the geometry of the tear faults and related unconformities. Within the syn-tectonic sediments, a steep, upward flattening thrust within a broader, roughly tulip-shaped drag zone develops. The thrust roots in the tear fault in pre-tectonic deposits and is curved upward toward the downthrown block. Vertical offset on the thrust is related to differential vertical uplift caused by, for example, growth of folds with different wavelength and amplitude on either side of the tear fault. Formation of progressive unconformities is governed by the relationship between the rates of deposition and vertical growth of a structure. Fault-related progressive unconformities are additionally controlled by the growth of the vertical step across the tear fault. When the rates of vertical growth of two neighbouring folds separated by a tear fault are similar, the rate of growth across the tear fault is small; if the first differ, the latter is high. Episodic tear fault activity may create several angular unconformities attached to a tear fault or allow the generation of angular unconformities near tear faults in sedimentary systems that have a rate of deposition too high to generate classical progressive unconformities between the tear faults.

In this paper we review the structure of the most external thrust belt of the Alps between the Rhein valley and Salzburg based on a new tectonic map and the (re)interpretation of seismic sections. Specifically we address the correlation between deformation in the Subalpine Molasse and the Alpine thrust belt in general and focus on the control of sedimentary facies on the structural style. A dramatic change in architecture from a ramp-flat structure to buckle folding is related to a change from coarse-grained fans to fine-grained deposits within the Subalpine Molasse. Additionally the interaction of escape tectonics with postcollisional shortening controls the decrease of late Early Miocene and younger shortening within the Subalpine Molasse from 50 km near the Rhine valley to almost zero near Salzburg. Transfer of shortening into the hinterland, which is the zone of lateral escape, ended foreland propagation of the Alpine thrusts and initiated a general break-back sequence of thrusting. Throughout this time the thrusts remained active. In such a scenario, tectonic units on top of the Subalpine Molasse are expected to undergo clockwise rotation around vertical axes. As thrusting in the Subalpine Molasse is closely related to contemporaneous transport and shortening within the tectonically higher Helvetic thrust sheets, amounts of Miocene differential shortening and related clockwise vertical axis rotation are minimum amounts. True clockwise vertical axis rotation is probably larger than the 12° deduced from the Subalpine Molasse thrust belt.

Based upon tectonic as well as facies arguments, two different Helvetic nappes can be distinguished in western Austria (Vorarlberg) and in southwestern Germany (Upper Allgau): the Hohenems nappe and the overlying Vorarlberg Santis nappe. Both encompass Middle Jurassic to Eocene strata deposited on the internal to external shelf of the southward deepening European margin of the Eurasian plate. Synsedimentary normal faults caused changes in thickness and facies of the various strata, which play a crucial role in deformation behavior. Arcuate fold axes in map view and an almost 3 km thick sequence of stacked Middle Jurassic shales and sandstones drilled below a Jurassic anticlinorium in the southern part of the Bregenzerwald are thought to be indicative of an inverted Jurassic basin. Inversion occurred during the Cenozoic Alpine nappe formation along synsedimentary normal faults, reactivated as ramps and tear faults. A lateral ramp, segmented by tear faults, running along the Iller Valley, and a supposed lateral ramp in the subsurface of the Rhine Valley mark the extension of the inverted former basin. Fault deformation style changes across the Rhine Valley. East of it, i.e. in Vorarlberg, the Vorarlberg Santis nappe comprises a coherent succession of Jurassic and Cretaceous strata detached along Middle Jurassic sediments. In the west, on the other hand, Cretaceous strata of the Swiss Santis nappe were largely detached from their Jurassic substrate (Gonzen-Walenstadt imbricates) along the Santis thrust. This allows to correlate the Helvetic nappe stack of eastern Switzerland, comprising the Swiss Santis nappe (together with the Gonzen-Walenstadt imbricates) and the underlying Murtschen nappe, with the Vorarlberg Santis nappe and the Hohenems nappe of Austria.

Based on an N–S-oriented crustal-scale cross section running east of the Rhine Valley in Vorarlberg, western Austria, we address the Alpine nappe stack and discuss the boundary between Central and Eastern Alps. For our cross section, we used surface geology, drillings and reinterpreted seismic lines, together with published sections. The general architecture of the examined area can be described as a typical foreland fold-and-thrust belt, comprising the tectonic units of the Subalpine Molasse, (Ultra-)Helvetic, Penninic and Austroalpine nappes. These units overthrusted the autochthonous Molasse along the south-dipping listric Alpine basal thrust. The European Basement, together with its autochthonous cover, dips gently towards the south and is dissected by normal faults and trough structures. The seismic data clearly show an offset not only of the top of the European Basement, but also of the Mesozoic cover and the Lower Marine Molasse. This indicates an activity of the structures as normal faults after the sedimentation of the Lower Marine Molasse. The Subalpine Molasse is multiply stacked, forming a triangle zone at the boundary with the foreland Molasse. The shortening within the Subalpine Molasse amounts to approximately 45 km (~67 %), as deduced from our cross section with the Lower Marine Molasse as a reference. The hinterland-dipping duplex structure of the Helvetic nappes is deduced from surface and borehole data. There are at least two Helvetic nappes needed to fill the available space between the Molasse below and the Northpenninic above. This is in line with the westerly located NRP20-East transect (Schmid et al., Tectonics 15(5):1047–1048, 1996; Schmid et al., The TRANSMED Atlas: the Mediterranean Region from Crust to Mantle, 2004), where the two Helvetic nappes are separated by the Santis thrust. Yet in contrast to the Helvetic nappes in the NRP20-East transect, both of our Helvetic nappes comprise Cretaceous and Jurassic strata. This change is explained by an eastward down-stepping of the Santis thrust along a pre-existing, approximately N–S striking lateral ramp bounding an inverted Jurassic graben structure below the Rhine Valley. This causes the Santis thrust to detach the base Cretaceous west of the Rhine Valley and the base Jurassic units east of it. This graben-controlled change in detachment level leads to the formation of quite different nappe stacks on either side of the Rhine Valley and a “fault-controlled” appearance of the boundary between the Central and Eastern Alps.

This study analyses and discusses well preserved examples of Late Jurassic structures in the Northern Calcareous Alps, located at the Loferer Alm, about 35 km southwest of Salzburg. A detailed sedimentary and structural study of the area was carried out for a better understanding of the local Late Jurassic evolution. The Grubhorndl and Schwarzenbergklamm breccias are chaotic, coarse-grained and locally sourced breccias with mountain-sized and hotel-sized clasts, respectively. Both breccias belong to one single body of breccias, the Grubhorndl breccia representing its more proximal and the Schwarzenbergklamm breccia its more distal part, respectively. Breccia deposition occurred during the time of deposition of the Ruhpolding Radiolarite since the Schwarzenbergklamm breccia is underlain and overlain by these radiolarites. Formation of the breccias was related to a major, presumably north-south trending normal fault scarp. It was accompanied and post-dated by west-directed gravitational sliding of the Upper Triassic limestone (“Oberrhatkalk”), which was extended by about 6% on top of a glide plane in underlying marls. The breccia and slide-related structures are sealed and blanketed by Upper Jurassic and Lower Cretaceous sediments. The normal fault scarp, along which the breccia formed, was probably part of a pull-apart basin associated with strike slip movements. On a regional scale, however, we consider this Late Jurassic strike-slip activity in the western part of the Northern Calcareous Alps to be synchronous with gravitational emplacement of “exotic” slides and breccias (Hallstatt melange), triggered by Late Jurassic shortening in the eastern part of the Northern Calcareous Alps. Hence, two competing processes affected one and the same continental margin.

Styles of soft-sediment deformation in a syngrowth coarse-grained sediment gravity flow system are described from outcrops of the Upper Gosau Subgroup of the Northern Calcareous Alps. The deepwater sediments were deposited along the flanks and top of growing folds. Beside load and fluid escape structures, which are commonly found in sediment gravity flow systems, soft-sediment deformation is mainly related to fluidization of coarse-grained beds and downslope gliding of meter-thick sediment packages. Common structures affecting single or several beds are: hydroplastic folding of fine-grained beds in a coarse-grained fluidized matrix, symmetric or asymmetric mullions at the base of coarse-grained beds, and folding of heterolithic units into coarse-grained beds. Folds have varying geometries and styles, and fold axes are scattered. Soft-sediment folds also affect thick sediment packages. These folds are uniformly verging asymmetric metric to dekametric folds, formed at different degrees of lithification. Rheology of sandstones in such folds ranges from ductile to semi-brittle and brittle. Fold axes tightly cluster about one direction. The first group of deformational structures is attributed to slump-related deformation, whereas the second group represents folds related to tectonic deformation. A dislocation model and a shear zone model are used to interpret orientation of folds related to slump deformation. This considers processes taking place on the scale of the whole slump sheet and within the shear zone at the base of gliding units. Both models predict rotation of fold axes toward the transport direction of the slump. Field data support that fold axes scatter either about the downslope transport direction or about the strike of the slope.

We present new paleomagnetic data from the Northern Calcareous Alps and the Central Alps of Austria. All new data are overprint magnetizations and can be subdivided into two groups: In rocks older than earliest Rupelian, two remagnetizations reflecting both clockwise and counter-clockwise rotation were detected. In rocks of late Rupelian and younger ages, only a counter-clockwise rotated remagnetization was found. Our results together with results from previous paleomagnetic studies from the Eastern and Southern Alps suggest two main phases of vertical axis rotation. The first, clockwise rotation affecting the Northern Calcareous Alps was active between earliest to Late Rupelian. We propose a model where the Northern Calcareous Alps are segmented into individual blocks. Within a dextral shear corridor these blocks rotated clockwise due to the counter-clockwise rotation of the Southern Alps and Central Alps. The second, counter-clockwise rotation occurred in the Late Oligocene to Middle Miocene, affecting Eastern and Southern Alps. In this stage of orogeny, the internal massifs of the Western Alps were already accreted to the upper plate and therefore included in counter-clockwise rotation. This rotation is contemporaneous with counter-clockwise rotation in the Apennines and opening of the Balearic basin, and a genetic relationship is suggested. A second step of counter-clockwise rotation, reconstructed from published data, is observed in the sedimentary basins at the southeastern margin of the Eastern Alps, where counter-clockwise rotated Miocene and Pliocene sedimentary rocks are present. This rotation is seen in connection to a young counter-clockwise rotation of the Adriatic plate.

A new interpretation of the Inntal–Tauern sector of the TRANSALP seismic section is presented. One of the most prominent contrasts in reflectivity in the TRANSALP seismic section is the contact between the Bajuvaric unit in the footwall and the overlying Tirolic unit and its basement across a moderately south-dipping interface. We trace this contact from the surface at the southern margin of the Inn valley to a depth of 5 km. There, the contact is deformed or cut by the Tauern Window northern margin. We define the contact between Bajuvaric and Tirolic units as Brixlegg thrust, which is older than Miocene Tauern window exhumation and has a Paleogene age. The sub-Tauern ramp connects with the Inntal fault system at the surface and roots below the Tauern window. Oblique thrust movements across this fault system in the Miocene caused exhumation of the hanging wall, where the fault has a ramp geometry, which is in the area of the TRANSALP cross section and west of it. East of the TRANSALP cross section, the fault system merges with Alpine basal thrust, which is a flat. No Miocene exhumation occurred above the flat.

TectonicVB for Macintosh and TectonicsFP for Windows TM operating systems are two menu-driven computer programs which allow the shared use of data on these environments. The programs can produce stereographic plots of orientation data (great circles, poles, lineations). Frequently used statistical procedures like calculation of eigenvalues and eigenvectors, calculation of mean vector with concentration parameters and confidence cone can be easily performed. Fault data can be plotted in stereographic projection (Angelier and Hoeppener plots). Sorting of datasets into homogeneous subsets and rotation of tectonic data can be performed in interactive two-diagram windows. The paleostress tensor can be calculated from fault data sets using graphical (calculation of kinematic axes and right dihedra method) or mathematical methods (direct inversion or numerical dynamical analysis). The calculations can be checked in dimensionless Mohr diagrams and fluctuation histograms.

We present new apatite fission track and apatite (U–Th)/He data from the Alpine orogenic front, the Austrian Subalpine Molasse. We show that the cooling signal reported from the Swiss part of the basin since 10 Ma is also present farther east. Hence, it appears to be independent of the kinematic relation to thrusting in the external thrust belt present further west, the Jura Mountains. By reconstructing the Central Alpine pro-wedge geometry at 10 Ma, we show that the taper of the Central Alps has not changed significantly and presumably remained close to the critical state since then. From cooling offsets at faults, the present day shortening rate of the pro-wedge ranges between 1.0 and 2.0 mm/a and appears to have been constant since at least the Late Miocene. In conjunction with the observations of repeated out-of-sequence thrusting in the Subalpine Molasse, a stationary deformation front, constant shortening and erosion rates and the constant taper, we argue that the Central Alpine pro-wedge is at kinematic, as well as at mass flux steady-state since 10 Ma. Our results suggest that distinct climate-driven erosion events – in contrast to more continuous erosion – are not distinguishable in the deformation record of the last 10 Ma. Hence, kinematic and mass flux steady state in the Central Alpine pro-wedge along with stable localization of deformation restricted to the Subalpine Molasse may indicate a feedback between ongoing shortening and erosion at low rates during the Late Neogene to present.

Analysis of the three-dimensional geometry of Upper Cretaceous clastics in the Muttekopf area (Northern Calcareous Alps, Austria) indicate fold and fault structures active during deposition. Coniacian continental to neritic sedimentation (Lower Gosau Subgroup) was contemporaneous with displacements on NW-trending faults and minor folding along NE-trending axes. From the Santonian onwards (sedimentation of the deep-marine Upper Gosau Subgroup) the NW-trending faults were sealed and large folds with WSW-trending axes developed. The direction of contraction changed to N-S after the end of Gosau deposition in the Danian (Paleocene). Synorogenic sedimentation patterns indicate continuous contraction from the Coniacian to the Late Maastrichtian/?Danian. Therefore, large-scale extension as observed in the central part of the Eastern Alps cannot be documented in the western parts of the Northern Calcareous Alps. A combination of subduction tectonic erosion for the frontal parts and gravitational adjustment of an unstable orogen after nappe stacking for the internal parts possibly accounts for the different development of Gosau basins in the frontal and trailing regions of the Austroalpine wedge.

In the Eastern Alps compression during orogeny in the Upper Cretaceous caused crustal thickening, isostatic uplift and gravitational adjustment of the unstable orogenic wedge. This process triggered extensional basin formation on the back of the orogen (Gosau Basins). The basin fill of the Muttekopf Gosau Basin is arranged in megacycles, the first one comprising alluvial fan sediments and “Inoceramus marls” of the Lower Gosau Complex (Faupl et al. 1987) of Santonian age. Three other cycles follow (Upper Gosau Complex, Campanian to Maastrichtian), consisting of turbiditic fining upward sequences, that are indicative for extensional tectonics during basin formation, as subsidence events prevent formation of autocyclic coarsening upward sequences and therefore prograding of the turbidite system. Deposition of the 1st and 2nd Megacycle occured below the CCD (Carbonate Compensation Depth). The carbonate rich 3rd Megacycle was deposited probably below the CCD after a period of palaeogeographic reorganisation (uplift?) in the source area.

The investigation of geologically complex settings in Alpine or mountainous terrains is still dominated by traditional data collection and analytical techniques. The application of computer-aided geometric design and three-dimensional (3D) visualization and interpretation is rarely applied to such settings, despite its significant benefits. This contribution uses the Gosau Muttekopf Basin (Eastern Alps, Austria) to demonstrate that the application of 3D geospatial models can both provide new insights into our understanding of such settings and result in a more robust and reproducible synthesis of a complex region. The objective of studying the Muttekopf Basin is to investigate the 3D structural control on the deposition of the deepwater sedimentary basin fill. Data for the investigation only consist of that which would be collected in a traditional field study (e.g., structural mapping, stratigraphic logging, and data localities derived from hand-held GPS [global positioning system]). The 3D basin configuration is initially derived using traditional analysis techniques (e.g., cross-section construction, photo-panel mapping, block diagrams, etc.). Using these analysis techniques, significant thickness variations are observed the basin fill and are related to temporal and spatial variations in displacement of the controlling structure on the southern basin margin. However, there are significant limitations to this approach. In particular, because of the uncertainty in projection and spatial positioning, these techniques can only be used in an illustrative or qualitative fashion. To overcome these limitations, a 3D geospatial model is constructed from the same input data and illustrates that 3D geospatial modeling is a powerful technique for understanding complex geological settings. Integration of map data, stratigraphic section data, photographic images, structural data, and rock property data (gamma ray) into a single geospatial model maximizes the constraints of the limited data set. It also facilitates a deeper data analysis by significantly decreasing the time involved in generating multiple surfaces required for isopach generation. The use of the isopach maps in the Muttekopf Basin provides significant insights into the basin9s evolution. In the Schlenkerkar section, the isopach maps reveal: (1) there was very little sediment thickness variation across the basin during the early basin fill; (2) the intermediate episode was characterized by a very thick accumulation in the basin9s axis with significant thinning onto the southern uplifted margin; and (3) a northward migration of accumulation occurred during the late stage of the basin fill. Overall, the isopach maps suggest that the structure on the southern margin was the primary control on accommodation space creation and that it was most active during the intermediate basin-fill episode. Using similar observations from isopach maps for the entire basin reveals that the change in structural style of the southern margin from a fold- to a fault-dominated system plays a significant role both on internal deformation of the basin as well as the sedimentology of the syngrowth basin fill. Geospatial models, therefore, provide a more robust technique for analyzing and interpreting data within a 3D environment. In addition, they enable analysis that would be impossible with traditional techniques, such as probabilistic geocellular model construction and input models for 3D structural restorations.

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