| Tue, 13 Sep, 11:20–12:40|Montanistika Building

Orals: Tue, 13 Sep | Montanistika Building

Chairperson: Mark Handy
Eline Le Breton, Mark R. Handy, Peter McPhee, Azam Jozi-Najafabadi, and Christian Haberland

Neogene tectonics of the Alps is marked by the indentation of the Adriatic Plate into the Alpine Orogen and onset of escape tectonics in the Eastern Alps. This resulted in the formation of a system of strike-slip faults, mainly the Periadriatic Fault (PF), separating the Eastern and Southern Alps, and the sinistral Giudicarie Fault (GF), which offsets the PF. The GF is kinematically related to Neogene shortening in the Southern Alps but questions remain on its geometry at depth, in particular its relation to the crust/mantle boundary (Moho).

In this study, we compare geological cross-sections and pre-existing geophysical datasets (controlled-source seismology, local earthquake tomography) with a new high-resolution 3-D local earthquake tomographic model from the AlpArray and SWATH-D experiment along two N-S profiles west and east of the GF, as well as a NW-SE oriented section across the GF. These sections reveal differences in the style of indentation tectonics, specifically in the behavior of the Adriatic lower crust, between the Central and Eastern Alps. West of the GF, the lower crust of the Adriatic plate detached from its mantle lithosphere and wedged within the Alpine orogenic crust, whereas to the east of the GF, the Adriatic lower crust forms a bulge just to the south of the PF. The Adriatic upper crust responded by shortening and formation of a fold-and-thrust belt, while the Europe-derived orogenic crust underwent upright, post-nappe folding and exhumation in the Tauern Window. We discuss the possible causes for such along-strike variations in terms of changes in crustal rheology and structural inheritance within the Adriatic Plate, contrasting metamorphic histories within the Alpine orogenic crust west and east of the GF, and potential Neogene slab break-off beneath the Eastern Alps.

How to cite: Le Breton, E., Handy, M. R., McPhee, P., Jozi-Najafabadi, A., and Haberland, C.: Variation in style of Adriatic lower crust indentation west and east of the Giudicarie Fault, 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-25, https://doi.org/10.5194/egusphere-alpshop2022-25, 2022.

Istvan Gyorfi, Laura Petrescu, and Felix Borlenu

Since the 1980s’ the geodynamic evolution of the Alpine-Carpathian-Pannonian (ACP) region has been clearly dominated by two models: McKenzie (1978) and Royden (1984). The model of McKenzie was the first numerical model to explain the continental extension in terms of lithospheric stretching and following thermal subsidence. The model has envisaged that these two processes are recorded by the intervening sedimentary processes: the initial syn-rift phase characterized by extensional growth sequences and the subsequent thermal phase best described in terms of tectonic quiescence with no, or little deformation of the sedimentary cover. Its first application to the North-Sea has brought serious breakthrough in the understanding of its geodynamic evolution, and became a strong predictive tool for the oil and gas exploration community. Further on, the model has been tested on the Pannonian Basin by Sclater et al (1980). The results were ambiguous, and Bally and Snelson (1980) have highlighted that the syn-rift phase is not responding properly to the model. In spite of these early concerns, the McKenzie model has been widely accepted for the coming decades. Evidences from reflection seismic data coupled with well data, however were to confirm that the style and timing of extensional deformation is indeed out of the reach of model predictions. Shortly afterwards, Royden has proposed that the extension of the Pannonian Basin System area would be coupled with the compressional tectonics of the Carpathians. Royden et al.  has proposed that the motor behind the two concurrent processes would be the subduction roll-back which they thought to be represented by the Vrancea Seismic Zone (VSZ). This model was simple and elegant, to that extent that has been unanimously adopted by the whole geoscientific community without reserves for the coming four decades. While it is clear, that the VSZ is a well-documented geodynamic entity, it is problematic to pursue how far can be applied to the whole Intra-Carpathian Region (ICR). There is a growing evidence coming from a variety of regions from the ICR, such as the Transylvanian Basin, Apuseni Mountains, East-Carpathians and ultimately from the Pannonian Basin suggesting that the subduction roll-back model cannot be retained anymore as the sole and only viable solution to explain the Miocene-Pannonian geodynamics of the ACP region. Moreover, possible alternative interpretation(s) of the VSZ is calling for a full revision of the mechanisms of basin and orogenic evolution.

How to cite: Gyorfi, I., Petrescu, L., and Borlenu, F.: Alpine-Carpathian-Pannonian Geodynamics: McKenzie and Royden, how far?, 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-26, https://doi.org/10.5194/egusphere-alpshop2022-26, 2022.

Anna-Katharina Sieberer, Ernst Willingshofer, Thomas Klotz, Hugo Ortner, and Hannah Pomella

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.

How to cite: Sieberer, A.-K., Willingshofer, E., Klotz, T., Ortner, H., and Pomella, H.: Internal deformation and tectonic evolution of the Dolomites Indenter, eastern Southern Alps: A combined field and analogue modelling study, 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-45, https://doi.org/10.5194/egusphere-alpshop2022-45, 2022.

Quentin Brunsmann, Claudio Rosenberg, Nicolas Bellahsen, and Giancarlo Molli

The arc of the western Alps forms the western termination of the Alpine Chain. The E-W striking Austro-Italian-German and Swiss Alps turn into a N-S direction along the western margin of the Po plain, finally rotating back to an E-W strike along the Italian-French Mediterranean coast. The origin of this enigmatic shape was originally attributed to a variscan inheritance (Argand, 1916), but the vast majority of the present-day literature suggests that it results from the indentation of Adria during collision, as a result of a significant W-directed component of convergence. We briefly review previous interpretations and suggest a new kinematic model based on retrodeformation of syn-collisional shortening, on paleomagnetic results, on structural analysis of maps on the arc-scale, and on field-based structural investigations.

Retrodeformation of syn-collisional shortening around the arc of the Western Alps points to the existence of an arc of significant amplitude before the onset of collision. Paleomagnetic results from the External Zone (Dauphinois) suggest that most rotations around vertical axes only affect the Mesozoic cover above the Triassic, hence they do not provide an information on the kinematic of the entire crust. In the area of the Argentera Massif, where paleomagnetic data were derived from Permian beds, hence allowing to interpret rotations of the entire crustal block, it is shown that no significant rotations around vertical axes affected the area during Alpine orogeny. Structural analyses of maps indicate that the transition between the N-S and E-W striking parts of the arc in the External Zone is abrupt, taking place along the Var Valley. No progressive rotations of structures are observed there, instead N-S striking folds and thrusts appear to be interrupted by the E-W striking ones which continue all along the southern coast of France until the Pyrenees. In several localities, stratigraphic and structural evidences show that these E-W structures were initiated before the onset of Alpine collision, and amplified during Alpine collision.

Our field-based structural data and compiled ones point to the occurrence of a large-scale widely distributed system of sinistral shear zones, striking ENE-WSW, which affect the area north of the Argentera Massif including part of the Internal Zone. Such structure was often assumed to be the prime site accommodating the west-directed indentation of Adria. In spite of its significant extent, its newly mapped location within the Arc precludes such such a 1st order kinematic role of this structure during collision.

To conclude, we suggest that the arc of the Internal Zone (Penninic Units) showing a progressive rotation of structures is not similarly observed in the External Zone, and we infer that this progressive, continuous curvature largely existed or formed during subduction. The arc of the western Alps as observed in the External Zone mainly reflects the existence of such a structure at the end of subduction and the transition between the Alps s.s. and the Pyrenean Chain, reactivated during Miocene time.

How to cite: Brunsmann, Q., Rosenberg, C., Bellahsen, N., and Molli, G.: The arc of the western Alps: a review and new kinematic model, 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-55, https://doi.org/10.5194/egusphere-alpshop2022-55, 2022.