Advances in a variety of geophysical, geochronological, geochemical and geological fields provide a rich and growing set of constraints on the crust-lithosphere and mantle structure, tectonics and geodynamics of the entire mountain belt.
We invite contributions from different and multi-disciplinary perspectives ranging from the Earth’s surface to the mantle, and based on geology (tectonics, petrology, stratigraphy, geo- and thermochronology, geochemistry, paleomagnetism and geomorphology), geophysics (seismotectonics, seismic tomography and anisotropy) and geodesy and modelling (numerical and analogue). The aim is for contributions to provide new insights and observations on the record of subduction/exhumation/collision; pre-Alpine orogenic stages; the influence of structural and palaeogeographic configuration; plate/mantle dynamics relationships; coupling between deep and surface processes.
It is commonly accepted that the Late Jurassic marks the onset of convergent tectonics in the ALCAPA (Alps-Carpathians-Pannonia) domain. However, the lack of generalized metamorphism, and the absence of structures and features that can be ascribed to this event make it challenging to understand its relevance and extent. Two areas have been historically documented in the ALCAPA where Late Jurassic tectonic features can be recognized: in the Inner Western Carpathians (Meliata and Borka localities) and in the central Eastern Alps (Lower Juvavic tectonic units). The interpretation of the structure and geodynamic significance of both of these areas has been strongly conditioned by assumptions on the paleogeographic position of the units involved. In these two areas, the Juvavic and Silica tectonic units (successions of Permo-Mesozoic strata derived from the Triassic passive margin of the ALCAPA, in the Eastern Alps and Western Carpathians respectively) have been traditionally interpreted to represent the most distal units of the Triassic passive margin of the ALCAPA. This in turn implies that these units are interpreted to be tectonically far travelled and emplaced in a complex succession of in- and out-of-sequence thrusts.
In this contribution we propose a revision to the conventional interpretation of the Juvavic units, based on the structural re-interpretation of key localities. We focus on the central Eastern Alps, where we describe the geometry, timing and interplay of different structures related to the earliest phase of contractional deformation. We further integrate the modern understanding of salt tectonics and carbonate sedimentology in this area to show that the pre-contractional paleogeographic arrangement of the Juvavic was likely more complex than previously assumed. In particular, we argue that pelagic Triassic facies have been misinterpreted as evidence for the distality of the Juvavic and Silica units, and that they deposited not only in distal passive margin settings but also in proximal settings of limited crustal thinning. This is consistent with the fact that structures previously assumed to be north-directed thrusts are in fact south-directed back-thrusts, and challenges the conventional interpretation of the Juvavic units. The revised structural interpretation in the central Eastern Alps indicates that Late Jurassic contraction was part of a regionally-coherent system of deformation (over 100s of kilometers).
Furthermore, Late Jurassic deformation is observed to form a temporal continuum with Early Cretaceous, that was eventually sealed by the Gosau Gp sediments. We therefore propose that the Late Jurassic to Early Cretaceous contractional deformation that we document in the Eastern Alps represents the best documented record of the onset of Alpine orogenesis.
How to cite: Fernandez, O., Ortner, H., Sanders, D., and Grasemann, B.: Re-defining early Alpine orogenesis in the ALCAPA domain (Late Jurassic to Early Cretaceous), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21240, https://doi.org/10.5194/egusphere-egu25-21240, 2025.
The Tauern Windows in the Eastern Alps is one of the most spectacular tectonic window, which formed after opening and closure of the Alpine Tethys between Europe and the Adriatic micro-continent. The herein described paleogeographical model is based on a new lithostratigraphic unit, the Wörth Formation, which formed as a local Jurassic black shale deepwater trough below the CCD on a strongly attenuated crustal basement. It developed as an oblique depression between the European continent and the Permian-Jurassic metasediments of the Seidlwinkl Nappe, which became an isolated element during opening of the Alpine Tethys. The Wörth Formation trough terminated to the NW within the European continent but maintained an open connection to the main Alpine Tethys towards SE. Different clastic sediments were derived from both sides of the trough: detrital mica-rich sandstones intruded by gabbroic laccoliths (167 Ma), olistoliths and re-sedimentation of Keuper beds, yet no indication of Triassic carbonates were derived from the northern side. In contrast, the deposits on the southern side are characterized by carbonate-bearing quartz-schists, breccias of Triassic carbonates, arkoses and tectonic slivers from the basement (Modereck crystalline).
Radiolarites and “Aptychen” limestones are useful marker lithologies for better lithostratigraphic interpretations. Until now Aptychen limestones got little attention, but could be recognized in all environment and most tectonic units from the Matrei Zone to the Klammkalk Zone. Rare locations have been detected, where at the base of the Glockner nappe a primary sedimentary succession of siliciclastic Jurassic into the typical marly “Kalkglimmerschiefer” lithology has been preserved. The Sandstone-Breccia unit is now understood as a continuously pro-grading accretionary wedge, containing considerable portion of clastics, derived from the southern border of Alpine Tethys and emplaced during post-Albian times on the northern parts of the Wörth Formation. White mica Ar ages, clustering regionally at 30 my and only little younger restricted apatite FT ages (see Spalding et al. Poster Session GD9.1) indicate early cooling due to a detachment process (see Brunner et al. Poster Session GD9.1) at the frontal part of the accretionary wedge.
The complex paleogeography has also important consequences for the tectonic evolution history: It caused the contrasting structural architecture of the Glockner nappe W and E of the Rauris valley and the restriction of the HP-rocks (lawsonite pseudomorphs, eclogites) to the western side of Glockner nappe. The basement units of the eastern Tauern window should not interpreted as the direct continuation of the western basement. Earlier interpretations for different rifting ages in this part of Alpine Tethys, Jurassic in the S and Cretaceous in the N, lost their validity.
How to cite: Frank, W., Grasemann, B., Meisel, T., Spalding, J., Schneider, D., Huet, B., Iglseder, C., and Gallhofer, D.: Palaeogeography of the Eastern Tauern Window (Eastern Alps, Austria), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15744, https://doi.org/10.5194/egusphere-egu25-15744, 2025.
The southern and eastern Alps are a fascinating target region for a seismological study because they include the deformation front of Adria-Europe convergence with historically significant events (e.g., M 6.0 Friuli 1976) as well as areas where seismicity seems more or less absent despite geologically mapped large fault systems and past deformation fronts. The large-N installations of the Swath-D (2017-2019) and AlpArray (2016-20219) seismic networks provide unmatched opportunities to study the microseismicity in the Eastern Alps in unprecedented detail. For the first time in the study area, the homogeneous station spacing allows a consistent analysis of seismicity across the entire area. These detailed seismological analyses provide the opportunity to characterize deformation in the upper 15 km of the crust.
We show how a combined workflow, including clustering, relocations, and MT inversions, sheds light on the seismicity and the ongoing active deformation. We observe strong zonations of seismic activity rates, sequence characteristics, and rupture mechanisms, coinciding with dominant tectonic deformation styles and subsurface properties such as Qp attenuation. We identify and characterize multiple likely unknown fault systems that experience local stresses deviating from the regionally dominant Adria-Europe convergence. Our findings agree well with the occurrence of large historical earthquakes while simultaneously shedding light on much smaller seismogenic features.
How to cite: Petersen, G., Hofman, L., Kummerow, J., and Cesca, S.: Microseismic activity in the Eastern Alps: Sequences, mechanisms, and active faults, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3294, https://doi.org/10.5194/egusphere-egu25-3294, 2025.
The geological map sheet Schlanders (Project CARG F012) offers the chance to carefully investigate the metamorphic evolution of the Austroalpine units in the Vinschgau and their tectonic contacts and to implement them into a tectonic model based on new petrological, geochronological and structural data. The Austroalpine nappe stack in the investigated area, located in the Vinschgau area (South Tyrol), comprises from bottom to top the Campo-, Texel-, Ötztal- and Matsch Units. The Matsch unit in the northern flank of the Vinschgau valley shows a clear polymetamorphic history (Variscan, Permian, Eoalpine) which can be well reconstructed with metapelites using the spatial distribution of alumosilicates (kyanite, andalusite, sillimanite), the chloritoid-isograd and the observation of chemical zoning patterns in garnets, which, depending on the geographical position and the geological setting, exhibit single-phase, two-phase or even three-phase compositions. The Ötztal and Texel Units (without the Lodner Unit) also show a polymetamorphic history (Variscan, Eoalpine) but without the Permian overprint. In contrast to the Ötztal Unit, the Texel Unit contains rare Eoalpine eclogites (e.g. Ulvas, Saltaus). Geothermobarometry from all three units yielded a strong increase in Eoalpine P-T conditions from ca. 450°C and 0.6 GPa in the west (Matsch valley) to 650°C and 1-1.2 GPa in the east (Naturns).
The study of amphibole composition is central to the understanding of metamorphic processes of metabasic rocks, especially when analyzing pressure and temperature conditions. This study analyzes the chemical composition of amphiboles along a W-E traverse along the Vinschgau Valley (South Tyrol). The composition of amphiboles changes from actinolite to hornblende along the prograde E-W-trending metamorphic gradient, and shows increasing chemical substitutions such as the edenite-, glaucophane- and tschermak vectors. This is also accompanied by an increase in Ti content (0.004 to 0.36 wt.% TiO2) in the amphiboles, as well as the XAn in the coexisting plagioclase from 0.1 to 0.2. Temperatures based on the Ti-in-hornblende- and the amphibole-plagioclase geothermometers yielded a T increase from 490°C to 600°C.
Tourmaline from Permian pegmatites in the Matsch unit show chemical evidence for the Eoalpine metamorphic overprint in the rim zoning along fractures and growth zones in tourmaline associated with muscovite (also showing Eoalpine growth rims), K-feldspar growing along veins, An-bearing plagioclase, quartz, and a second generation of garnet. The Permian tourmaline cores can be classified as schorl according to the [Y]-position and have the same composition in the entire area. The Eoalpine rims show compositionally a transition from schorl to dravite and show increasing contents of Ca[X] from 0.06 to 0.2, Mg/Fe[Y] from 0.02 to 2, and a significant decrease in Al[Y] from 0.4 to 0.1 from W to E. This confirms the from NW to SE increasing Eoalpine P-T conditions as reconstructed based on analysis of metapelitic rocks mentioned above.
The data show that lithologies such as amphibolites and pegmatites also show great potential to contribute significantly to our knowledge of prograde metamorphic evolution.
How to cite: Tropper, P., Erckert, A., Rudigier, C., Pomella, H., Morelli, C., and Mair, V.: Don’t always use metapelites: what do amphibolites and pegmatites reveal about the prograde Eoalpine metamorphic evolution of the Austroalpine nappe stack in the Vinschgau valley (S-Tyrol, Italy)?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9343, https://doi.org/10.5194/egusphere-egu25-9343, 2025.
Along-strike variations in deformation and structural build-up within fold-and-thrust belts are often controlled by pre-orogenic inheritance (e.g. Krabbendam & Leslie, 2010). This is the case of the south-verging Central Southern Alps in the Lecco area, where the E-W elongated belt is segmented along its strike by N-S oriented transverse zones, formed by the reactivation of early Mesozoic rift-related normal faults (Schönborn, 1992). These normal faults displaced the pre-rift sedimentary succession and controlled the facies distribution and thickness variation of syn- and post-rift Mesozoic carbonates. This led to the lateral juxtaposition of rocks with different rheological properties, which prompted the compartmentalization of the thrust system and the complex along-strike repartition of shortening across variable numbers of thrusts.
In this study, we reconstruct the early Mesozoic rift-related structures of the Lecco area and analyse their influence on the Alpine thrust system. Mesostructural analysis, geological cross-sections, burial history provided by the analysis of inorganic paleothermal indicators from clay-rich layers, and U-Pb dating of syn-tectonic carbonates have been integrated to investigate the role of inherited pre-orogenic structures within the Alpine orogenic context.
Three major tectonic phases were identified in the Early Mesozoic rifting processes by meso-structural analysis and U-Pb dating of syn-tectonic carbonates. N-S and E-W striking normal faults started to develop during the Ladinian marking the transition from isolated carbonate platform units to the basin successions. A second extensional pulse in Norian led to the formation of euxinic intra-platform basins within the massive Dolomia Principale carbonate platform, bounded by N-S and E-W striking normal faults. Finally, during the Early Jurassic, E-W and major N-S striking faults developed coevally with crustal thinning and the drowning of the carbonate platforms, leading to a generalized basinal sedimentation.
During S-verging thrusting and folding, the E-W striking faults were either passively translated and rotated or partially positively inverted. Some N-S striking inherited faults were also passively translated, while others underwent strike-slip reactivation; the latter are particularly evident within the N-S striking transverse zones, which exhibit complex tectonic settings with superimposed structures originated throughout different tectonic phases.
U-Pb dating of syn-tectonic carbonates from S-verging thrusts returns us Lower and Upper Cretaceous ages for the more internal structures, and Oligocene to Upper Miocene ages for the external thrusts and related folds. The latters suggest the reactivation of the Cretaceous orogenic structures. U-Pb dating of syn-tectonic carbonates along N-S striking transverse zones, instead, span unevenly from the Early Cretaceous to the Late Miocene, suggesting that these structures acted as long-lasting structural elements that remained active throughout all the stages of the orogenic build-up.
Krabbendam, M., & Leslie, A. G. (2010). Lateral variations and linkages in thrust geometry: the Traligill Transverse Zone, Assynt Culmination, Moine Thrust Belt, NW Scotland. Geological Society, London, Special Publications, 335 (1), 335–357
Schönborn, G. (1992). Alpine tectonics and kinematic models of the central southern alps. Memorie Di Scienze Geologiche, 44, 229–393
How to cite: Fiorini, A., Luca, A., Tavani, S., Rocca, M., Zanchetta, S., Zanchi, A., Kylander-Clark, A., and Carminati, E.: Pre- and syn-orogenic tectonic evolution of the transverse zones dissecting the Central Southern Alps (Lombardy, Italy), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3502, https://doi.org/10.5194/egusphere-egu25-3502, 2025.
The Alps formed as a consequence of the collision between Europe and the Adria-Africa plate starting from the middle-late Eocene. Despite most of metamorphism, deformation and nappe-stacking were localized in the N-vergent part of the Alps (i.e. N of the Periadriatic Fault), significant crustal shortening affected also the S-vergent retrobelt, with the development of a fold-and-thrust belt that extends from the Canavese zone in the W to the Dolomites to the E.
Late Cretaceous high-pressure metamorphism in the Africa-derived Austroalpine units and fault activity along major tectonic structures in the Southalpine domain (i.e. the Orobic Thrust), already posed a question on the occurrence of pre-collisional deformation and metamorphism in the upper plate of the alpine Thetys subduction.
New U-Pb dating of calcite tectonites, obtained on growth fibers, calc-mylonites and shear veins along major thrusts of the central Southern Alps, mainly result in Late Cretaceous to Paleocene ages, pointing out that N-S to NW-SE directed compression already affected the Southalpine domain at those times. Younger ages resulted from the Paleogene units which are involved in the exposed frontal part of the belt, mostly buried under the recent infilling of the Po Plain forming the Milan Belt. The resulting ages do not follow an in-sequence pattern, but instead reveal that several structures, from the inner to the external part of the belt, were episodically formed and re-activated in the Late Cretaceous - early Eocene time interval. All together, U-Pb ages confirm that S(SE)-directed thrusting and folding affected the central Southern Alps since the Late Cretaceous, well before the onset of the Alpine collision.
How to cite: Zanchetta, S., Rocca, M., Montemagni, C., Aldega, L., Kylander-Clark, A., Fiorini, A., Carminati, E., and Zanchi, A.: Pre-collisional Late Cretaceous-Paleocene development of the Alps retrobelt in the hangingwall of the Alpine Tethys subduction: U-Pb carbonate dating of major tectonic structures in the Southern Alps (N Italy), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5658, https://doi.org/10.5194/egusphere-egu25-5658, 2025.
Most paleotectonic reconstructions assume the indentation of Adria subsequent to the Periadriatic magmatism, after 32-26 Ma. Some consider an even younger (post 14-10 Ma) retrobelt of the Alps. These reconstructions contrast with evidence of a late Cretaceous to Eocene retro-belt in the western Southern Alps, intruded by the Adamello pluton and associated magmatic bodies. Recent work suggest this retro-belt continued eastwards into a relief extending from the Texelgroup towards the Transdanubian Range, allowing detritus to feed the retroforeland basin. In the eastern Southern Alps, remnants of this basin occur in the northernmost sectors, and recent work documented the Late Cretaceous northward flexuring of the Adria foreland.
Collectively, these observables confirm the occurrence of a Late Cretaceous retrobelt, subsequently cut in the Oligocene by the Periadriatic Line: the western part of the retro-belt remained in the Southern Alps, whereas, to the east, the Cretaceous double vergent belt was left north of the Periadriatic Line, only leaving the tip of the retro-foreland basin in the Southern Alps. This Eastern Alps Cretaceous belt is well recognized, following the so-called eclogite belt.
The Cretaceous retro-belt was sinistrally reworking the Jurassic Giudicarie fault system, finally defining it as first-order transverse range pre-existing the Periadriatic Line. This latter reworked the indented Adria plate in the west, where the crustal doubling prevented any possible deeper source for the Periadriatic magmatism. The lower plate break-off, therefore, seems a very unsuitable hypothesis.
How to cite: Picotti, V.: The Cretaceous retro-belt of the Alps and the early indentation of Adria, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16716, https://doi.org/10.5194/egusphere-egu25-16716, 2025.
Constraining quiescence intervals in tectonically active regions remains challenging, particularly in tectonic reconstructions, as these periods are often overprinted by extensional and/or compressional processes that remobilize geological materials, rendering access and dating of these intervals difficult.
Recent studies in tectonically stable regions on laterites and bauxites formed through weathering under tropical climates have demonstrated the efficacy of (U-Th)/He geochronology on Fe-oxyhydroxides (hematite and goethite) in constraining tectonic quiescence periods.
In the Mediterranean region, numerous bauxites have been preserved due to their remobilization into karst systems, allowing for their burial and protection during subsequent tectonic processes. This preservation offers a unique opportunity to better understand the geodynamics of the region. This study focuses on French bauxites from Bédarieux, Les Baux-de-Provence, and Brignoles, which constitute the Durancian Isthmus—a supposed Cretaceous paleosurface bordered by large inherited Variscan structures (Cévennes, Nîmes, and Durance faults)—whose geodynamic implications are still poorly understood.
The only available temporal constraints on the formation of this dismantled weathering profile rely on the sedimentary context of the karsts where they are trapped, with the most reliable timeframes established between the Hauterivian and Turonian. This transitional period is still poorly understood in the region, as various tectonic processes are at play, including Pyrenean rifting and its inversion, Alpine Tethys, and Massif Central exhumation.
We combined petrological investigations on nine different Fe-Al-bauxitic duricrust samples, allowing for the determination of different hematite and goethite generations, prior to conducting (U-Th)/He dating on the identified sub-generations to quantify bauxite formation and evolution. The ages obtained for hematite and goethite pisolites span from the Cretaceous to the Oligocene, encompassing all generations, enabling the placement of Cretaceous bauxites within their Mediterranean geodynamic context—from their initial formation via basement alteration, to their reworking within karsts, sedimentary burial, and subsequent exhumation.
How to cite: Boschetti, L., Schwartz, S., Gautheron, C., Mouthereau, F., Rolland, Y., and Balvay, M.: Geodynamic of French bauxite through (U-Th)/He thermochronology on Fe-oxyhydroxides, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5941, https://doi.org/10.5194/egusphere-egu25-5941, 2025.
Evaporite distribution and mobility is a key parameter in the structuration of salt bearing sedimentary basins where these layers can generate halokinetic deformations. Recent revisions of compressive basin models, including those in the external Alps, highlight the significant role of salt tectonics. However, identifying pre-compression halokinetic deformations is often challenging due to erosion or misinterpretation. The “Baronnies provençales”, located in the Vocontian basin in the external western Alps (France), display a unique structure characterized by large E-W oriented synclines oblique to the NNW-SSE trend of the subalpine Alps, bordered by very tight E-W oriented anticlines, whether faulted or not, the origin of which remains debated. Several outcropping diapirs involving Triassic evaporites have been identified in this area, indicating halokinetic activity, either recent or ancient. This study aims to characterize the structural style of the region and the Mesozoic halokinetic structures in order to assess their impact on subsequent deformations.
Field observations, bedding measurements, and cross-sections illustrate that the sedimentary series in the synclines became abruptly steeper near the anticline axes, often adopting overturned dips, sometimes forming megaflap-type geometries. Several angular unconformities have been identified within the Early Cretaceous sequence, notably between the Barremian-Aptian and the Hauterivian, as well as between the Coniacian and the Turonian along strike of the present day anticline axes. N-S oriented slumps within the cretaceous succession highlight the presence of paleo-morphologies parallel to the modern anticline structures that were thus developing during the Early Cretaceous. E-W sediment gravity-flow (e.g. marly calcareous slumps, sandy- and calci- turbidites and debris flow) following submarine palaeocanyons more numerous than the N-S ones and located at the core of the synclines, reflect the regional paleoslope of the region. The structural analysis of the region shows that major thrust faults are located along and with the same orientation as the E-W oriented tight anticlines, thus positioned between the multi-kilometer wide and flat synclines. These thrust faults exhibit lateral variations in their vergence. Microtectonic analysis indicate normal faulting predating the formation of E-W folds, associated with a WNW-ESE extension likely linked to the reactivation of the major inherited NE-SW faults within the South French basin. A ~N-S compression, probably corresponding to the Pyrenean-Provençal phase, as well as a ~NE-SW to ENE-WSW compression, likely associated with the Alpine phase, have also been identified. These results highlight that the ‘Baronnies provençales’ area has thus recorded the main regional deformation phases of the Meso-Cenozoic, but has also been affected by renewed halokinetic activity during the Early Cretaceous, which was responsible for layer tilting forming the ‘megaflap’ – type structure and the formation of topographic anticline-like ridges that induced episodes of lateral sedimentary reworking. The Cretaceous salt-tectonics thus played a key role in shaping the structural style of the region and also probably in the thrust vergence. The paleo-diapirs were sutured while accommodating the shortening of later compressive phases.
Key words: Halokinesis, Structural inheritance, Tectonics, Vocontian basin.
How to cite: Ludovino Aranda, V., Homberg, C., Huyghe, D., Callot, J.-P., Rabaute, A., and Lasseur, E.: Mesozoic and Cenozoic tectono-halokinetic evolution in the Baronnies Provençales (Alps, France), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11458, https://doi.org/10.5194/egusphere-egu25-11458, 2025.
Understanding the evolution of convergent plate boundaries and the mechanisms of strain accommodation through time and space is made possible by studying exhumed subduction complexes within orogenic belts. This study uses the internal zones of the Western Alps, one of the largest and best-preserved fossil subduction complexes in the world, to track the transition from subduction to collision. We herein combine in-situ Ar-Ar and Rb-Sr data on white micas with pressure-temperature estimates derived from pseudosection modeling and Raman thermometry on carbonaceous material, along eleven transects crossing the mountain belt.
Results (i) confirm the preservation of similar peak pressure-temperature conditions on both sides of the Briançonnais/Liguro-Piemont contact (as proposed by Mendes et al., 2023), (ii) indicate that the Briançonnais cover units reached their metamorphic peak around 50 ± 5 Ma and likely correspond to the former cover of the Dora-Maira massif, (iii) document the progressive slicing of large basement units at the end of the subduction process and the evolution of deformation (from localized at interface-scale to distributed at crustal-scale), and (iv) allow refining the initial structure of the continental margin and its role during convergence.
This study also highlights the merits and limitations of Ar-Ar and Rb-Sr radiochronological systems, and in particular the complexity of the record associated with multiple metamorphic recrystallizations. Although the variable and in places marginal extent of excess argon complicates the interpretation of Ar-Ar ages, this study shows that the Ar-Ar system is likely more robust than the Rb-Sr system for tracking recrystallization history. The latter system appears sensitive to late re-equilibration episodes, potentially linked to fluid circulation.
Mendes, K., Agard, P., Plunder, A., Herviou, C., 2023. Lithospheric-scale dynamics during continental subduction: Evidence from a frozen-in plate interface. Geology 51, 1153–1157. https://doi.org/10.1130/G51480.1
How to cite: Mendes, K., Agard, P., Plunder, A., Bonnet, G., Herviou, C., and Gyomlai, T.: From oceanic to continental subduction and collision in the Western Alps: P-T-time evolution of the Briançonnais/Liguro-Piemont plate contact, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20088, https://doi.org/10.5194/egusphere-egu25-20088, 2025.
The Katschberg normal fault, which bounds the Tauern Window to the east, played a crucial role during Miocene lateral extrusion in the Eastern European Alps (Genser & Neubauer 1989; Scharf et al. 2013). We present new cooling ages from low-temperature thermochronology as well as thermo-kinematic models, which constrain the exhumation history of the Penninic units in the footwall of the Katschberg fault and its fault-slip history (Wolff et al. 2024). Zircon and apatite fission track and apatite (U-Th)/He ages from footwall units range from 16.0±1.9 Ma to 12.8±1.4 Ma, 10.4±1.8 Ma to 7.9±1.3 Ma, and 8.2±0.8 Ma to 3.9±0.4 Ma, respectively. Thermo-kinematic modeling indicates that the Katschberg normal fault was active with a total rate of 3.5±0.3 km/Myr from 21.1±1.8 Ma to 12.2±1.3 Ma and accommodated 27±6 km of crustal extension. After the end of normal faulting, exhumation continued with a rate of 0.21±0.06 km/Myr until 2.0±0.5 Ma and then accelerated to a rate of 0.84±0.08 km/Myr. A comparison with the Brenner low-angle normal fault at the western margin of the Tauern Window reveals that the amount of Miocene extension is higher in the west than in the east. This is consistent with an eastward decrease of N-S shortening in front of the Adriatic Indenter.
References
Genser, J., Neubauer, F. (1989) Mitt. Österr. Geol. Ges. 81, 233–243.
Scharf, A., Handy, M.R., Favaro, S., et al. (2013) Int. J. Earth Sci. 102, 1627–1654.
Wolff, R., Wölfler, A., Hampel, A., Dunkl, I. (2024) Tectonophysics 890, 230514.
How to cite: Wolff, R., Wölfler, A., Hampel, A., and Dunkl, I.: The slip history of the Katschberg normal fault (Eastern Tauern Window) from thermo-kinematic modeling and implications for the evolution of the Eastern European Alps, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1616, https://doi.org/10.5194/egusphere-egu25-1616, 2025.
Over the past few decades, the number of high-quality seismic stations monitoring the Euro-Mediterranean region has increased significantly, leading to a corresponding improvement in structural constraints. Hear, we present a new high-resolution Pn-wave anisotropic tomography model of the uppermost mantle beneath the Alps and surrounding areas, derived from the inversion of a large dataset of high-quality Pn arrival times, which were picked utilizing the PickNet deep learning method. Our model reveals strong lateral heterogeneities in both isotropic velocity and azimuthal anisotropy. Distinct high Pn velocities are observed under the Adriatic Sea, Mediterranean Sea, and Pannonian Basin, while prominent low Pn velocity anomalies are revealed beneath the orogenic belts, including the Alps, Apennines, and Dinarides. Generally, regions characterized by stable structures and low lithospheric temperatures exhibit high Pn velocities, whereas low Pn velocities indicate the upwelling of hot materials associated with plate subduction and continental collision processes. Pn anisotropic fast directions show consistent orientations subparallel to major orogenic structures, such as the Apennines, Calabrian Arc and Alps. Our newly obtained images of the uppermost mantle velocity and anisotropy structure provide further information and insights into continental collision processes and associated dynamic mechanisms.
How to cite: Du, M., Zhao, L., Tao, K., and Yang, L.: Pn anisotropic tomography of the Euro-Mediterranean region: new insight into subduction and mantle dynamics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6163, https://doi.org/10.5194/egusphere-egu25-6163, 2025.
Recent receiver function results from a passive seismic experiment have provided new insights into the geodynamic evolution of the Western Carpathians, the eastern extension of the Alps, formed in part by the closure of the Alpine Tethys. The Pieniny Klippen Belt (PKB) represents this closure at the surface, characterised by a narrow, elongated geometry dividing the external fold-and-thrust belt of the Outer Western Carpathians and the Central Western Carpathians. Unlike typical sutures, the PKB lacks ophiolites or high-pressure metamorphic rocks, instead it consists of resistant limestone blocks within a matrix of non-resistant flysch deposits, forming a distinctive “block-in-matrix” structure. This configuration has traditionally been attributed to the hypothesized Czorsztyn ridge, an island-like feature within the Alpine Tethys, where limestone deposition has been thought to occur. The ridge is supposed to correspond to the Briançonnais unit in the Alps, though evidence for its existence remains tenuous.
The current passive seismic experiment seeks to validate or refute the Czorsztyn ridge hypothesis. In May 2023, 18 broadband seismic stations were deployed along a north-south trending profile, under the umbrella of the Adria Array, complemented by 9 other permanent and temporary stations. This 27-station dense network enabled the extraction of receiver functions and the creation of Common Conversion Point (CCP) stack images to resolve the sub-surface geometry of the region.
Preliminary findings challenge the Czorsztyn ridge model. No distinct continental crustal body – interpretable as the Czorsztyn ridge basement and separate from the northern European platform or ALPCAPA – is evident beneath the PKB. Instead, subsurface structures appear complex, showing similarity to those in the Vienna Basin, located between the Eastern Alps and the Western Carpathians. A blind detachment fault occurs in the deep basement of the Outer Western Carpathians and connects southward with mid-crustal detachments in the Central Western Carpathians. Furthermore, a 40 km wide gap in Moho signature of the receiver functions beneath the PKB may reflect the position of the suture at a lower crustal level. Additionally, the Steimberg Fault in the Vienna basin likely correlates with the PKB, as both exhibit a displacement with partly strike-slip kinematics. Continued data collection and analysis will refine these interpretations and advance the understanding of the tectonic evolution of Western Carpathians.
How to cite: Soni, T., Schiffer, C., and Mazur, S.: Understanding the closure of Alpine Tethys in the Western Carpathians using Receiver Functions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6592, https://doi.org/10.5194/egusphere-egu25-6592, 2025.
For decades, Mesozoic tectono-halokinetic structures have been increasingly recognized in the peripheral French alpine basins. However, reconstructing the full history and mechanisms of halokinesis during the Mesozoic in these regions remains challenging due to the overprinting effects of Cenozoic compressive tectonics, which have erased much of the evidences of earlier deformations.This severely limits our understanding of the interplay between diapirism, tectonics, and sedimentary processes in sedimentary basins, and the role of pre-compressional inheritances in shaping the internal deformation of orogenic wedges. In the Baronnies (southern subalpine Alps), several Triasic diapirs that were reactivated during Cenozoic compression are exposed, raising questions about the earlier halokinetic activity in the area.
In orogenic domains, salt-tectonics is generally inferred from geometric evidences, which are not always well preserved. To address this limit, we developed a geochemical and regional approach, applied to the Mesozoic deposits in the Baronnies. Specifically, we used the strontium (Sr) content of pelagic carbonates deposited in the Vocontian basin (today incorporated in the Alpine prism) as a tracer of potential salinity anomalies associated to submarine diapirism. Rocks samples were collected from Oxfordian to Turonian sedimentary sequences in the deep environments of the Vocontian basin and along its northern (Vercors and Chartreuse), southern (Ventoux) and western (Ardèche-Languedoc) shallow margins. Sr content was measured using X-ray fluorescence (XRF) in the field and then lab-based XRF on both bulk samples and their carbonate fractions. Selected samples were also measured using ICP-OES spectrometry. Sr values were compared to the mean contemporaneous oceanic values of the reference curve established by Renard (1975) to identify possible anomalies.
The Sr content exhibits spatial and temporal variability, with both normal and abnormal values relative to the reference curve. Normal values characterise the Late Jurassic and basal Cretaceous periods. In contrast, Valanginian to Aptian values are significantly higher than the reference curve. The largest anomalies are observed in the deep Vocontian basin and suggest local contamination of the sediments by saline material flows. Comparison of the geochemical signal, sedimentary remobilization events (slumps, calciturbidites,…) and the structural and paleo-stresses frameworks point to a renewed halokinesis activity after the Liasic rifting, with diapirs piercing or not the seafloor. In the Baronnies, this Mesozoic activity has significantly deformed the contemporaneous sedimentary sequences, with local overtuned dips and megaflap-type geometries associated with angular unconformities and pitching of the sequences close to the paleodiapir bodies. At the basin scale, wide synclines were flanked by EW submarine ridges which, together with the inherited NE-SW faults divided the Vocontian basin. We corelate these structures with the Early Cretaceous tectono-halokinesis activity in the South East French Basin, with the Vocontian rift forming a major structure between the Valaisan Ocean and the Altlantic rift. This study supports the existence of sutured diapirs in the meridional subalpine Alps with an enhanced tectono-halokinetic activity during Early Cretaceous. The last one created regional weak salt inheritances in the pre-compression Mesozoic sedimentary pile, preconditioning it for deformation during the later compressional phases.
How to cite: Homberg, C., Huyghe, D., Ludovino Aranda, V., Le Callonnec, L., Rabaute, A., Lefebvre, G., and Alix, O.: Strontium (Sr) signal in the Mesozoic Southeastern French Basin (Alps) and its relation with pre-compression tectono-halokinetic activity , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10551, https://doi.org/10.5194/egusphere-egu25-10551, 2025.
In the NE Tauern Window of the Eastern Alps, new mapping in Nordrahmen Zone and Glockner nappes reveals significant deformation associated with W-E extension and a component of N-S shortening during the transition from high-pressure metamorphism and nappe stacking to extensional deformation. Kinematic indicators, including winged inclusions, tiling, and climbing pinch-and-swell veins, reveal a clear top-to-E shear sense, and deformation is further highlighted by progressively deformed quartz-calcite-dolomite veins, whose rotation was used to quantify flow parameters. The deformed veins used to quantify the flow parameters related to ductile deformation reveal that pure shear and simple shear contributed relatively equally. Moreover, the sub-horizontal axial planes of DIII fold structures is indicative of vertical flattening, which is signifies vertical shortening during ductile deformation. The ductile deformation is overprinted by E-dipping shear bands and faults, which transition into brittle-ductile faults compatible with incremental strain axes also indicating vertical shortening during top-to-E extension. Raman spectroscopy data show a temperature gradient with higher structural levels exhibiting paleotemperatures <450°C, increasing to >500°C at deeper levels. White mica Ar-Ar analyses in both shear veins and recrystallized fabrics yield Oligocene deformation dates (25-34 Ma). Distributed ductile thinning is a characteristic feature in the footwall of detachment systems, and prompted further investigation up section. At higher crustal levels at the upper limit of the Nordrahmen Zone, the deformation gradient progressively increases towards the newly discovered top-to-E Schuhflicker Detachment, defined by a knife-sharp fault surface of ultramylonites and cataclasites. The hanging wall is defined by slightly deformed quartzites and dolomites of the Lower Austroalpine Unit. The Schuhflicker Detachment developed at mid-crustal levels during the Oligocene, and during progressive exhumation, W-E extension was transferred to the structurally higher Katschberg Fault and Katschberg Shear Zone System during the Miocene. Collectively, these structures form the East Tauern Detachment System, which accommodated tens of kilometers of exhumation of the Tauern Window, facilitating the incipient stages of its exhumation during the Oligocene and subsequent erosion-dominated unroofing in the Miocene.
How to cite: Schneider, D., Spalding, J., Huet, B., Grasemann, B., and Rantitsch, G.: Opening the window slightly earlier: Oligocene east-directed extension along the East Tauern Detachment System, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12147, https://doi.org/10.5194/egusphere-egu25-12147, 2025.
One one the most prominent examples of shallow continental Moho is related to the Ivrea Body, in the Western Alps, with its formation and deformation still debated. Several recent temporary seismic deployments as well as the permanent station networks of Switzerland and Italy provided this study with sufficient teleseismic P-to-S converted waveforms to perform Receiver function analysis and retrieve anisotropic parameters for the Ivrea Body.
In continuity with the work done by Salimbeni et al. (2021), where the anisotropic properties of the southern part of the Ivrea Geophysical Body were determined, here we present the results of the same analysis applied to the stations over the entire Ivrea body itself, from the south toward its northern margin.
In this study, therefore, we present the result of this new Receiver Function analysis applied to 63 new broadband seismic stations deployed across the region. Our preliminary results show that, for the 35 stations located directly above the high gravity anomaly of the area, generally referred as a signature of the Ivrea Geophysical Body, the anisotropic properties of the shallow crustal materials have all similar affinities, with high degree of anisotropy and coherent angular pattern which displays a change in direction from South to North.
How to cite: Confal, J. M., Pondrelli, S., Salimbeni, S., and Agostinetti, N. P.: Anisotropy from Receiver Function Analysis shed light into the Deformation Style of the Ivrea Body, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11524, https://doi.org/10.5194/egusphere-egu25-11524, 2025.
Our research is part of the ongoing geological fieldwork aimed at creating the 1:50,000 scale "Ormea Sheet" (n. 244) within the framework of the CARG Project (Italian National Geological Cartography). The goal is to integrate existing data and observations to reconstruct the structure and evolution of the European (Alpine) margin. The study area is located in the Ligurian Alps, the southeastern end of the Western Alps. This region is crucial for understanding its geodynamic evolution, as the contact between lithological units from different domains is preserved (e.g., the Briançonnais domain representing the European passive margin and the Piedmont-Ligurian sedimentary covers corresponding to the oceanic domain). The structural framework is characterized by thrust sheets, superimposed non-cylindrical folds, and local deformations, which provide evidence of a complex polyphase tectonic evolution. These units underwent low-grade Alpine metamorphism, partially overprinting and reworking the original sedimentary structures and features.
Our work focuses on the Flysch Units outcropping within the Ormea Sheet, particularly the formations that constitute the Colla Domenica-Leverone unit, as referred to in the literature. Previous authors have hypothesized that these turbidite systems were deposited in an abyssal plain, resulting from the rifting and spreading of the Piedmont-Ligurian Ocean. These systems are characterized by basal complexes made up of thinly bedded turbidites, often containing olistostromes, followed by sand- or carbonate-rich turbidite systems (Decarlis et al., 2014), which are interpreted as trench environment deposits (Di Giulio, 1992).
During the advancement of the accretionary wedge towards the European foreland, these sedimentary units underwent a migration and stacking process, resulting in an inverted stratigraphy, with the oldest unit at the topmost part of the nappe pile. Our intense fieldwork revealed an erosional boundary between the Arnasco-Castelbianco and Borghetto units and the overlying Colla Domenica and Leverone formations, differently from previous assumptions of a tectonic surface, essentially revisiting the idea of classifying the Colla Domenica-Leverone as a tectonic unit.
Moreover, we are investigating the origin of the sediment supply through various analyses. Different basalt samples collected from the chaotic event in the Colla Domenica Shale, analyzed using ICP-MS and XRF instruments, show compositional similarities with the results proposed by Saccani et al. (2008) for the basalts from the Balagne region (Northern Corsica). This preliminary evidence suggests that these formations filled the closing oceanic basin (as proposed by the model of Pandolfi et al., 2016) through mass transport events originating from different areas, likely from the European margin and the front of the accretionary wedge.
Additionally, petrographic analyses of sandstone samples are in progress to identify the source areas, and biostratigraphic analyses have been performed to provide additional time constraints to complement the limited existing data. We are currently working on stratigraphic logs to describe in detail the facies and boundaries of these formations.
How to cite: Lombardi, S., Stori, L., Federico, L., Crispini, L., Silvio, S., and Matteo, M.: Pre to syn orogenic evolution of the European margin: clues from the Flysch units of the Ligurian Alps (CARG Project – Ormea sheet 244)., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18979, https://doi.org/10.5194/egusphere-egu25-18979, 2025.
Garnet has an extensive pressure-temperature (P–T) stability field for a wide variety of rock compositions, and its compositional changes reflect changes during its prograde P–T evolution. The beauty of garnet petrochronometry is thus the ability to extract P–T and temporal (t) information from a single rock-forming mineral with relatively well-known thermodynamic properties.
While still in its infancy, U–Pb dating of garnet by LA–ICPMS is an evolving petrochronological tool with a vast potential and a plethora of possible applications. To assess its reliability and potential systematic differences in comparison with the well-established Sm-Nd ID-TIMS dating technique, we applied garnet U-Pb dating by LA-ICPMS to garnet specimen that were previously dated by Sm-Nd ID-TIMS. The investigated samples include Paleozoic to Cenozoic garnet samples from diverse geotectonic settings and bulk rock compositions, including blueschists, eclogites, metapelites, and meta-rodingites.
Our results indicate that the two dating techniques mostly yield similar results, demonstrating the accuracy of the in situ U-Pb method. We further demonstrate that garnet U-Pb dating by LA-ICPMS can resolve dates from thin garnets rims (<300 µm), which are too narrow to be dated by ID-TIMS. In the case of the meta-rodingite sample, we found that garnet veinlets formed during two events, which were not clearly resolved by ID-TIMS dating. These spatial resolution advantages of the LA-ICPMS technique are contrasted, however, by generally less precise garnet dates compared to the ID-TIMS data. Furthermore, in situ U-Pb dating can be rendered unfeasible mainly by two factors: (1) the presence of and contamination by (inherited) U-rich inclusions (e.g. zircon and monazite); (2) garnet contains more Pb than U, thereby severely limiting the spread in 238U/206Pb which produces significant uncertainties and geologically meaningless dates.
How to cite: Millonig, L. J., Beranoaguirre, A., Albert, R., Marschall, H., Baxter, E., and Gerdes, A.: Accuracy of garnet U–Pb LA–ICPMS compared to Sm-Nd TIMS dating, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19672, https://doi.org/10.5194/egusphere-egu25-19672, 2025.
The area around the Semmering pass (Austria) is of particular importance for the geology of the Eastern Alps as it was here that the sequence from the crystalline rocks of the Central Eastern Alps to the Mesozoic sediments of the Northern Calcareous Alps was tectonically subdivided for the first time. This took place during the construction of the railway line in the 1880s, but the current work on the railway base tunnel has also brought new insights into the regional geology. This article deals with a variegated lithological association that was excavated in the course of the tunnel construction under the Hocheck Mountain, but which is also known from surface outcrops. According to the available geological maps it locally forms the base of the Stuhleck-Kirchberg Nappe, directly above the Wechsel Nappe. Both nappes are part of the Austroalpine Unit.
The variegated lithological association consists of micaschist and paragneiss with intercalations of granitic orthogneiss and frequently amphibolite. Of special interest are weakly deformed alkalifeldspar and quartz phyric dikes, which are Permian in age due to regional considerations. Further, small ore deposits containing pyrite, galena and chalcopyrite or quartz veins with haematite occur. Partly the micaschist is rich in muscovite and contains garnet up to 1.5 mm in diameter. Paragneiss often shows a layering due to a varying biotite content. Subordinate quartz and feldspar rich types with garnet and/or amphibole occur. Sometimes the latter are interlayered with amphibolite. The orthogneiss is mostly hololeucocratic with chloritisised biotite and alkalifeldspar porphyroclasts up to 3 cm in length. Based on the observed mineral assemblage upper greenschist to amphibolite facies conditions were reached at the metamorphic peak. A later greenschist facies overprint caused intense retrogression and phyllonitisation at a variable grade. Retrogression is indicated by chloritisation of biotite, garnet and amphibole in the paragneiss and sericitisation of feldspar. In the amphibolite actinolithe or chlorite formed. Phyllonitisation occurred along internal shear zones but especially along the basal nappe contact and towards the monotonous phyllonitic micaschist and orthogneiss overlying the variegated lithological association. The lithological layering as well as the main schistosity are mostly dipping towards south and a frequently observed stretching lineation is SW-NE orientated. An overprinting folding and crenulation shows E-W orientated axes.
With respect to the lithological association and the characteristics of the lithologies we argue that the investigated sequence is not part of the Stuhleck-Kirchberg Nappe, but a part of the Vorau Nappe (Silvretta-Seckau Nappe System, Austroalpine Unit), which appears in between the Wechsel and Stuhleck-Kirchberg Nappe further in the south. The Vorau Nappe is built up by the Waldbach Complex. The latter experienced an upper greenschist to amphibolite facies Variscan metamorphic imprint in the Late Devonian and Carboniferous. In Permian time, it was at the Earth’s surface and covered by Permian acidic volcanics and siliciclastic sediments. Subsequently Early Triassic sandstones (Semmering quartzite) and carbonates were deposited. During the Eoalpine event in the Cretaceous the whole sequence experienced greenschist facies conditions and deformation during nappe stacking and folding with E-W trending axes.
How to cite: Ranftl, E.-M., Schuster, R., and Vanek, R.: A new occurrence of the Vorau Nappe in the Semmering area (Eastern Alps), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20445, https://doi.org/10.5194/egusphere-egu25-20445, 2025.
The Tauern Window (TW) in the European Eastern Alps is one of Earth’s largest tectonic windows. It comprises nappes that were formed by the southward subduction of the European plate beneath the Adriatic plate. These nappes were stacked during the Late Eocene and, subsequently refolded during the Miocene due to the northward push of the eastern Southalpine Indenter. This process exhumed the western TW by up to 25 km, and coevally caused lateral escape and extensional tectonics. However, the Miocene deformation history of the western TW is still under ongoing debate. This study focuses on the Miocene deformation history of the western TW using 2-D, 3-D, and 4-D approaches.
We first restore a N-S oriented cross-section along the Brenner Base Tunnel using published zircon fission-track and P-T data. Restoration reveals two deformation phases: upright folding of the top of the nappe stack started to cease around 17 Ma, followed by thrusting of the entire nappe stack along the Sub-Tauern ramp. Contemporaneously, the hanging-wall nappes experienced 44–50% thinning due to W–E extension.
Our static 3-D reconstruction of the present-day structure of the western TW integrates published maps, cross-sections, and structural field data. The model discloses lateral structural changes, e.g., the transition of upright folds in the east into overturned folds in the west with varying plunge of the fold axes. We hypothesize that detachment of the lower crust of the eastern Southalpine Indenter caused different styles of deformation in front of it during indentation.
To prove our hypothesis, we restore the western TW in 4-D using the same method as for our 2-D reconstruction. We displace the nappe stack of the western TW downwards along the Sub-Tauern ramp (ca. 10 km over 15 Ma), followed by unfolding under high-temperature conditions, which allows viscous deformation. Finally, we will integrate strain information to restore the component of lateral escape.
How to cite: Tanner, D., Rudmann, J., Stipp, M., Pomella, H., Brandes, C., and Eizenhöfer, P.: 4-D kinematic restoration of the western Tauern Window (European Eastern Alps), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10544, https://doi.org/10.5194/egusphere-egu25-10544, 2025.
The Tauern Window in the Eastern Alps (Austria) is one of the most prominent tectonic windows, which exposes Subpenninic and Penninic nappes derived from the European margin and Alpine Tethys respectively below the Austroalpine Unit derived from the Adriatic continent. Along the northeastern margin of the window, in the so-called Nordrahmen Zone (NRZ), subvertical W-E striking marble mylonites, graphitic schists and phyllonites with a subhorizontal stretching lineation record intense ductile shear deformation. Previous studies suggested that these structures record the ductile history of a major sinistral strike-slip fault (i.e. the Salzach-Ennstal-Mariazell-Puchberg Fault System), which accommodated the Miocene lateral extrusion of the central parts of the Eastern Alps towards the Pannonian Basin.
In this work, we investigated a N-S section along the Grossarl valley, which demonstrates that the subvertical mylonitic rocks are deformed into upright folds with wavelengths and amplitudes on the order of several hundreds of meters and fold axes that are parallel to the mylonitic W-E trending stretching lineation. Reversal of the apparent strike-slip shear sense in the fold limbs suggests that the mylonites have been folded after shear deformation and that mylonites record top-E shearing when unfolded. Ductile subvertical flattening is recorded by a second fold generation with similar W-E trending fold axis but subhorizontal axial planes forming Type 3 refold structures. Ductile top-E shearing is documented by low-angle E-dipping ductile shear zones, shear bands, SC and SCC’ fabrics and brittle ductile conjugate N-S striking high-angle faults. Shear deformation intensifies towards higher structural levels localizing in ultramylonites and cellular dolomite cataclasites below almost undeformed klippen of quartzites and dolomites (Mt. Schuhflicker and Mt. Saukarkopf), which belong to the Lower Austroalpine Unit. Using Raman Spectroscopy of Carbonaceous Materials, we constrain the temperature of mylonitization between 350°C and 400°C. Comparison with published Ar/Ar ages from the Nordrahmen Zone suggests that mylonitization operated around 30 Ma.
We therefore suggest that the mylonites along the northeastern margin of the Tauern Window are not part of a strike-slip fault system. They actually belong to a major top-E detachment system, which records an early stage of the exhumation of the Tauern Window before deformation localized along the Miocene Katschberg Normal Fault at the eastern margin of the Tauern Window.
How to cite: Brunner, J., Grasemann, B., Huet, B., Schneider, D., Rantitsch, G., and Frank, W.: Detachment versus strike-slip ductile shearing in the Nordrahmen Zone of the Tauern Window, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9894, https://doi.org/10.5194/egusphere-egu25-9894, 2025.
