The Simplon normal fault in the Western Alps caused tens of kilometers of orogen-parallel extension during convergence of the European and Adriatic plates, but the slip rate of the fault and the time when normal faulting ended are still debated. Here, we constrain the slip history of the Simplon fault with low-T thermochronology and thermo-kinematic modeling (Wolff et al. 2024). Closely spaced samples from an elevation profile in the center of the fault yield zircon (U-Th)/He ages (ZHe) that are nearly invariant over an altitude of 1.4 km and cluster around ~6 Ma. In contrast, apatite (U-Th)/He ages (AHe) increase with altitude from 3.4±0.3 to 4.6±0.7 Ma, while the AFT ages range from 4.4±0.7 to 5.8±1.5 Ma. In addition, recently published ⁴⁰Ar/³⁹Ar ages constrain that our samples moved through the brittle-ductile transition (i.e., ~300°C) at 8–10 Ma. Our thermo-kinematic inverse modeling shows that these age data can be explained by a single phase of normal faulting, which lasted from 19.8±1.8 to 5.3±0.3 Ma and caused 45±10 km of extension. The slip rate of the 30°-dipping model fault is 3.5±0.3 km/Myr and equivalent to an exhumation rate of ~1.8 km/Myr. Our modeling reveals that the altitude-dependent difference between ZHe and AHe ages reflects the thermal relaxation after faulting stopped at ~5.3 Ma. Since then, exhumation by erosion continued at a rate of ~0.5 km/Myr. Remarkably, the end of slip on the Simplon fault coincides with the cessation of reverse faulting at 6±2 Ma in the external crystalline massifs of the Alps (Aar, Mont Blanc, Aiguilles Rouges) and with a decrease in strain rate by one order of magnitude at 5-4 Ma in the Swiss molasse basin and the Jura mountains. This temporal coincidence suggests that normal faulting in the internal part of the Alps ceased when plate convergence waned and the under-thrusting of European continental lithosphere beneath the Adriatic plate came to an end.

References

Wolff, R., Hölzer, K., Hetzel, R., Dunkl, I., Anczkiewicz, A.A., 2024. Late-orogenic extension ceases with waning plate convergence: The case of the Simplon normal fault (Swiss Alps). Journal of Structural Geology 179, 105049. doi:10.1016/j.jsg.2024.105049.

How to cite: Wolff, R., Hölzer, K., Hetzel, R., Dunkl, I., and Anczkiewicz, A.: Late-orogenic extension ceases with waning plate convergence:The case of the Simplon normal fault (Swiss Alps), 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-75, https://doi.org/10.5194/egusphere-alpshop2024-75, 2024.

A striking difference along the Alpine Orogen is the style of collisional tectonics during the Oligo-Miocene, with the onset of escape tectonics in the Eastern Alps. The indentation of the Adriatic Plate into the Eastern Alpine Orogen resulted in the formation of conjugate dextral and sinistral strike-slip faults in the vicinity of the Tauern Window. Moreover, major changes occurred in the foreland of the Eastern and Southern Alps in the Early Miocene, with the cessation of the northern Alpine front propagation and the onset of thrusting along the Southern Alpine Front. We present new results from structural, stratigraphic and subsidence analyses of the eastern North Alpine Foreland Basin (NAFB) to study the relationship between these Alpine tectonic events and basin dynamics.

Our results show a first phase of onset of foreland sedimentation in the eastern NAFB between c. 33-28 Ma, followed by a strong tectonic-driven subsidence between c. 28-25 Ma ending by a phase of erosion and the formation of a basin-wide Northern Slope Unconformity (NSU). During this time period, the rift-related Mesozoic normal faults of the European platform were reactivated and are capped by the NSU. We interpret this phase as an increase in the flexure of the subducting European Plate under the growing Alpine Orogen. Between 25-19 Ma, the eastern NAFB remained in a deep-marine, underfilled state with a gently increase in subsidence. A major shift took place around 19-17 Ma with strong tectonic-driven uplift, ranging from 200 m (absolute minimum) to 1200 m depending on uncertainties on paleo-water depths, and rapid sedimentary infill of the basin. We discuss the possible causes for this major tectono-sedimentary shift in the eastern NAFB in relation to contemporaneous changes in collisional tectonics within the Eastern and Southern Alps, and with a potential Early Miocene slab break-off event beneath the Eastern Alps.

How to cite: Le Breton, E., Bernhardt, A., Neumeister, R., Heismann, C., Borzi, A., Hülscher, J., Sanders, R., Grunert, P., and Handy, M.: Early Miocene tectono-sedimentary shift in the eastern North Alpine Foreland Basin and its relation to changes in tectonic style in the Eastern Alps, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-52, https://doi.org/10.5194/egusphere-alpshop2024-52, 2024.

A large body of geomorphological research has shown that topography records the history of both uplift and horizontal motions. Numerical landscape evolution models can be used to test the effects of various kinematic scenarios on landscape development. This enables the possibility of using landscape evolution models to solve an inverse problem and quantify tectonic motions based on observed features of a landscape. Various approaches of inverse landscape evolution modelling have been employed in recent years, with most previous work focusing on purely vertical motions, and/or the inversion of longitudinal stream channels in 1D rather than 2D landscapes. In this study, we introduce an approach using Ensemble Kalman Inversion – an efficient, ensemble-based data inversion method – to recover both vertical and horizontal kinematics from the present-day topography. Our approach is capable of handling large numbers of free parameters and quantifying uncertainty in the result. We use the average elevation and average normalised river steepness index (K_sn) calculated in a moving window along a profile across-strike of the orogen, to which we fit a large number of landscape evolution models. In this way, the models target first-order geomorphological features avoiding second-order variations characteristic in natural settings. Given the high data density and knowledge of the upper crustal structural evolution in the European Alps, we first demonstrate our method using a synthetic model set-up that emulates the structural geometry beneath the Tauern Window along the TRANSALP transect. We specifically include the possibility of long-wavelength surface uplift in our models, which may be derived from various mantle processes or isostatic responses. Our novel inversion approach can recover magnitudes and changes in surface uplift and horizontal advection rates in space and time. Not surprisingly, the ability of the method to determine past rates of deformation decreases the farther back in time they occur. However, initial results suggest that this effect is less pronounced for horizontal advection rates than for surface uplift rates, indicating potential limitations in modelling studies that do not consider horizontal advection. Our novel approach is now being applied to the topography along TRANSALP. We will use our inversion results to assess to what extent orogen-scale geomorphological features are able to record the tectonic and geodynamic evolution of Cenozoic mountain ranges.

How to cite: Oakley, D. and Eizenhöfer, P.: A Method for Recovering Fault Kinematics and Long-Wavelength Surface Uplift from the Inversion of Landscape Features and its Application in the Eastern European Alps, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-24, https://doi.org/10.5194/egusphere-alpshop2024-24, 2024.

The Carpathians are a typical Mediterranean-type orogen that resulted from slab roll-back, nappe accretion, collision and slab detachment. We will use basin-scale sedimentological and provenance analysis to reconstruct the Miocene to recent evolution of the Carpathian Foreland Basin. Paleontological observations will highlight the basin’s biogeographic relations with the surrounding regions, demonstrating the creation and destruction of topographic barriers with the Pannonian Basin and the Black Sea. Results from inverse modelling of thermochronological data from the Carpathians with PECUBE will be used to understand the timing and volume of sediment supply from different parts of the mountain belt, which will then be compared to deposition over time in the foreland basin. Finally we will establish a link between the geodynamic events in the subduction system, particularly collision and slab-detachment, and the evolution of the foreland basin. Since there are many similarities between the Carpathians and other roll-back orogens, our results might be interesting for researchers studying other Mediterranean-type orogens.

How to cite: de Leeuw, A., Matoshko, A., Roger, M., Vincent, S., Morton, A., van der Beek, P., Husson, L., Mandic, O., Stoica, M., and Krijgsman, W.: Evolution of the Carpathian Foreland Basin and its link with collision and slab detachment, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-80, https://doi.org/10.5194/egusphere-alpshop2024-80, 2024.

Neogene to ongoing N(W)-directed continental indentation of the Adriatic microplate into Europe controls the evolution of the European eastern Southern Alps (ESA). Despite the Adriatic plate acting as a rigid indenter, it has undergone internal deformation, with predominantly Miocene shortening being accommodated within a WSW-ENE striking, S-vergent fold-and-thrust belt. This deformation overprints a compositionally heterogeneous upper crust affected by several magmatic and tectonic events. We present new (Apatite (U-Th)/He (AHe) and Fission Track (AFT) data along a N-S profile in the western ESA to better understand the thermotectonic evolution of this complex area.

Time-temperature path modelling confirms the Valsugana phase as the most significant period of tectonic exhumation within the western ESA. AFT data in the research area tend to cluster within consistent distinguishable tectonic blocks, however, they are quite scattered especially in the central part of the ESA, warranting further investigation. The geodynamic history of the ESA is characterised by multiple heat pulses, which must be considered when interpreting the AFT data especially as the temperature of this pulses likely did not significantly exceed the partial annealing zone of AFT. Potential heating events are the (1) Permian magmatism, (2) Ladinian magmatism, (3) Jurassic crustal extension, (4) sedimentary superimposition (maximum thickness in Cretaceous), Middle Eocene to Lower Oligocene (5) Periadriatic intrusions and (6) Veneto Volcanic Province magmatic event.

Modelled cooling paths indicate that nearly all samples experienced heating just above the AFT partial annealing zone during the Middle Triassic, preceding the Jurassic extension and Cretaceous maximum burial indicated by the stratigraphic record. Detailed analysis of AFT data reveals that only a small, single-digit percentage of analysed grains give a single grain age older than Middle Triassic. Accounting for the σ1 error, all single grain ages can be interpreted as post-Middle Triassic. This suggests that the Ladinian magmatic event caused a geothermal anomaly affecting the entire research area, not just the well-known magmatic centres (e.g. Predazzo area). During and after the subsequent relaxation of the geothermal gradient the aforementioned events (3) – (6) overprinted the geothermal field locally. Based on the modelled cooling paths, it can be assumed that most of the samples remained in the temperature range of the AFT partial annealing zone or at slightly cooler temperatures during this period. Finally, during Miocene, the entire area was affected by fast tectonic exhumation on thrusts related to the Valsugana phase. This sequence of regional and local, magmatic and tectonic events results in very complex cooling histories that can vary significantly even for closely situated samples, explaining the scattered AFT ages.

How to cite: Pomella, H., Klotz, T., Sieberer, A.-K., and Dunkl, I.: The complex thermotectonic history of the eastern Southern Alps, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-59, https://doi.org/10.5194/egusphere-alpshop2024-59, 2024.

The general geological structure of the eastern Southern Alps and the transition to the Dinarides is generally well understood. The main structural boundaries run in a west-east direction and coincide with the margins of the main Mesozoic paleogeographic units. The eastern Southern Alps are divided into two large-scale composite thrust units. The lower ones are the Tolmin nappes, which consist of deep-marine Slovenian Basin successions. They are further subdivided into the lowermost Podmelec, the middle Rud and the uppermost Kobla Nappe. Above these are the Julian nappes composed of the Julian Carbonate Platform successions. They are traditionally divided into the Krn Nappe and the Slatna thrust sheet. However, detailed stratigraphic investigations within the Bohinj range of the Julian Alps in the NW Slovenia, provide an alternative solution. In the Kobla Nappe, significant lateral variations can be observed throughout the basinal succession, especially within the best-studied Rhaetian Slatnik Formation. In the west, this formation shows a distal development dominated by hemipelagic limestone. Towards the east, the resedimented limestones become progressively abundant. Near the Soriška planina ski resort, they already dominate the sequence and the formation is characteristic of the lower slope sedimentary environment. According to the existing geological maps, further east the successions of the Slovenian Basin of the Kobla Nappe suddenly disappear and the area is dominated by Norian-Rhaetian platform carbonates, which often contain marginal reef limestones. This entire area (Jelovica Plateau) was traditionally considered part of the Krn Nappe. The described geological situation was the reason for a detailed geological mapping of the Soriška planina ski resort area. Preliminary results indicate that the Kobla Nappe does not wedge to the east. Instead, the succession of the Slovenian Basin (including the Slatnik Formation) passes laterally into the succession of the Julian Carbonate Platform within the same overthrust unit, namely within the Kobla Nappe. This is also supported by a thin, newly mapped Mačji potok thrust sheet composed of Jurassic basinal rocks that lies beneath the basinal successions in the west, but can be traced all the way to the platform limestones to the east. In such a structural reinterpretation, large part of the southeastern Julian Alps, previously considered part of the Krn Nappe, actually belongs to the Kobla Nappe. Therefore, the Krn Nappe is located exclusively in the western and central Julian Alps. The contact between the two nappes is clear in the west, where the nappes consist of successions belonging to a different paleogeographic unit. In the east, the structure was probably overlooked because it runs between the platform limestones. We emphasise that the structure is further complicated by post-thrusting stike-slip displacements. The proposed reinterpretation also opens new perspectives for some other regional problems, such as the occurrence of isolated Oligocene deposits in the central Julian Alps and the emplacement of the Bled Basin (paleogeographically distal Adria margin) in the northwestern Julian Alps.

How to cite: Rožič, B., Žvab Rožič, P., Slapnik, L., and Gale, L.: Detailed stratigraphic studies encourage geostructural reinterpretation of the eastern Southern Alps , 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-11, https://doi.org/10.5194/egusphere-alpshop2024-11, 2024.

The transition from the Dinarides to the S-Alps and the position of the S-Alpine Trust-front east of Ljubljana is debated. Previous research agrees that the contact runs in the central Slovenian Sava Folds, they follow the analogy of western Slovenia, where the S-Alpine Thrust front (SATF) has been determined as the base of the Tolmin Thrust sheet (Placer 2008, Schmid et al. 2020). This tectonic unit incorporates the deep-water Middle Triassic to Cretaceous sedimentary succession of the Slovenian Basin (SB) which has been thrust over the Dinaric Platform. The presence of SB sediments east from Ljubljana, in the Sava Folds was suggested before (Buser 1989), but the presence of Jurassic deep-water sediments have only been proved recently (Rožič et al. 2022, Scherman et al. 2023). However, the area lying north of the discontinuous occurrence of the SB rocks was postulated to be of SB origin (Buser 2010, Placer 2008).
Detailed stratigraphic and structural observations confirmed the following succession: Ladinian siliciclastic sediments with volcanites (Pseudozilian Formation) are followed by Ladinian to Carnian Platform limestone (Schlern Formation). With a large gap Late Jurassic to Early Cretaceous pelagic limestone layers follow, containing resedimented limestone beds (Biancone Formation sl.). Covered by Early Cretaceous marlstone, with occasional calcarenitic interlayers (Lower Flyschoid Formation). The succession ends with pelagic limestones (Volče Limestone Formation).
The succession resembles the External Dinaric succession considered by Placer (2008) as the “Transitional zone” between the External and Internal Dinarides originally lying east from the Dinaric Carbonate Platform. With this discovery there is evidence for units of Dinaric origin north of the SATF, along the northern margin of the Sava Folds region, near the Sava Fault.
Tectonically this newly identified unit of External Dinaric origin is thrust over the SB succession in a south-vergent direction, which occurred likely during the Early Miocene, prior to the folding of the Sava Folds in a N-S contractional phase. The emplacement of the SB over the Dinaric units of the Sava folds is the oldest of the three events. Indirect evidence suggest that this thrusting was Palaeocene-Eocene, in SW direction (pre Oligocene post “mid” Cretaceous). The pre-Oligocene formations are folded. Over the erosional contact following Oligo-Miocene formations are folded in a following phase.
It is evident that the structural and the older palaeogeographical-stratigraphical boundaries have to deviate from each other east of Ljubljana. This also suggests that the SATF should be defined on structural basis (Schmid et al. 2020) and could represent a Miocene south-vergent thrust front. Emplacement of the deep-water Mesozoic basin over the platform seems to be a different deformation east of Ljubljana.

The research was supported by the OTKA (134873), Hantken and The Papp Simon Foundations.

Buser, S. 1989: Memorie della Società Geologica Italiana, 40:313–320.
Buser, S. 2010: Geological map of Slovenia 1:250.000.
Placer, L. 2008: Principles of the tectonic subdivision of Slovenia. Geologija, 51/2:205–217.
Rožič et al.2022: Geologija, 65/2:177–216.
Scherman et al. 2023 Geologija, 66/2:205–228.
Schmid et al. 2020: Gondwana Research 78:308–374.

How to cite: Scherman, B., Rožič, B., Görög, Á., Kövér, S., and Fodor, L.: Dinaric thrust sheet over the Slovenian Basin. Where is the contact of Southern-Alps and Dinarides? Structural and stratigraphical constraints., 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-20, https://doi.org/10.5194/egusphere-alpshop2024-20, 2024.