TS2.6 | Dynamics and evolution of the Alpine orogenic belt
Dynamics and evolution of the Alpine orogenic belt
Co-organized by GD9/GMPV11
Convener: Alexis Plunder | Co-conveners: M. R. Handy, Marco Giovanni Malusa', Ralf Schuster, Philippe Agard
Orals
| Wed, 17 Apr, 14:00–17:45 (CEST)
 
Room K1
Posters on site
| Attendance Thu, 18 Apr, 10:45–12:30 (CEST) | Display Thu, 18 Apr, 08:30–12:30
 
Hall X2
Posters virtual
| Attendance Thu, 18 Apr, 14:00–15:45 (CEST) | Display Thu, 18 Apr, 08:30–18:00
 
vHall X2
Orals |
Wed, 14:00
Thu, 10:45
Thu, 14:00
The Alps, a representative orogenic system, offers an exceptional natural laboratory to study the evolution of mountain-building processes from short- to long-term scales, including the evolution of a plate margin, from rifting to subduction, inheritance from previous orogenic cycles), ophiolite emplacement, collision and exhumation, upper-plate and foreland basin evolution.

Advances in a variety of geophysical 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 insight and observation on the record of subduction/collision, pre-Alpine orogenic stages; the influence of structural and palaeogeographic configuration; plate/mantle dynamics relationships; coupling between deep and surface processes.

Orals: Wed, 17 Apr | Room K1

Chairpersons: Alexis Plunder, Philippe Agard, M. R. Handy
14:00–14:05
Western Alps
14:05–14:15
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EGU24-4296
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On-site presentation
Gianreto Manatschal, Pauline Chenin, Gianluca Frasca, and Jean François Ghienne

The Western Alps, along the French-Italian border, are among the best investigated and imaged collisional belts worldwide. A major complexity of the Western Alps is their non-cylindricity and arcuate shape, as well as the occurrence of ultrahigh-pressure (UHP) metamorphic rocks. Our study shows that all these complexities are intimately linked to the interplay between the inherited rift architecture, the changing kinematics of convergence during the early stages of continental collision, and the complex 3D dynamics of the Alpine subduction system. Here we use a multi-disciplinary approach to investigate the evolution of the European/Briançonnais distal margin at the transition from subduction to early collision, which corresponds to the moment when rift inheritance and the paleogeographic configuration are the most important in controlling the orogenic structure and evolution.

In a first part, we reassess the architecture of the Western Alps based on a review of field and recent geophysical studies. This allows us to define the crustal architecture as well as the along and across strike position of the different Alpine units. The use of diagnostic petrologic, stratigraphic, and structural criteria allows us to identify the rift domains of the former European/Briançonnais margin, from which the different present-day orogenic units originated. This enables us to propose a first order, synthetic rifted margin template for the Western Alps. Of particular importance is the location of the necking zone, corresponding to the limit between the thick-crusted proximal and the thin-crusted distal margin. It also separates domains with different rheology and density/buoyancy/floatability, both of which control the subduction, exhumation and accretion behavior during subduction and early collision. We find that all units containing ultrahigh-pressure rocks derive only from the thin-crusted distal hyperextended domain.

In a second part, we revisit the paleogeography of the Alpine Tethys using a global kinematic restoration software (Gplates) and the new building block/rift domain concept that allows us to propose a tight fit restoration and evolution of the Atlantic Tethys junction during the Mesozoic.  In this restoration, the Briançonnais corresponds to a ribbon of slightly thinned continental crust that limits, along necking zones, two overstepping en-échelon rift basins, namely the Valais domain to the northwest and the Piemonte domain to the southeast. We affirm that this uneven-margin architecture can explain most of the Western Alps’ complexity. In our kinematic model, convergence between Adria and Europe was mainly accommodated by strike-slip movements until the late Eocene, which corresponds to the time of formation and exhumation of UHP metamorphic rocks. Early collision was diachronous along the margin and resulted first in the reactivation of the necking zone separating the Briançonnais and Prepiemonte domains. This fundamental structure, which we name the Prepiemonte Basal Thrust, floors the units preserving ultrahigh-pressure rocks. Once the distal margin was accreted, shortening mainly stepped inboard into the European necking domain, resulting in theformation of the Penninic Basal Thrust.

How to cite: Manatschal, G., Chenin, P., Frasca, G., and Ghienne, J. F.: Early Collisional Evolution of the Western Alps: how Important are Rift Inheritance and Paleogeography, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4296, https://doi.org/10.5194/egusphere-egu24-4296, 2024.

14:15–14:25
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EGU24-5930
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On-site presentation
Michel Ballèvre, Paola Manzotti, Francesco Nosenzo, Mikaela Krona, and Marc Poujol

High-pressure and ultra-high-pressure metamorphic terrains display an internal architecture consisting of a pile (or stack) of several coherent tectonic thrust sheets or units. Their identification is fundamental for understanding the scale and mechanisms active during subduction and exhumation of these crustal slices. This study investigates the geometry of the northern Dora-Maira Massif and the kinematics of the major tectonic boundaries, combining field and geochronological data. The tectonic stack of the northern Dora-Maira Massif comprises the following units. The lowermost unit (the Pinerolo Unit) is mainly characterized by Upper Carboniferous fluvio-lacustrine (meta-)sediments. The Pinerolo unit is overthrust by a pre-Carboniferous basement. The latter is subdivided in two tectonic units (the Chasteiran and Muret Units) with different Alpine metamorphism (ultra-high-pressure and high-pressure, respectively). The pre-Carboniferous basement of the Muret Unit is thicker than previously thought for two main reasons. Firstly, some paragneisses, traditionally assumed to be Carboniferous and/or Permian in age, display a dominant detrital zircon source at about 600 Ma. Secondly, three samples of the Granero Orthogneiss, previously assumed to be a Permian intrusive body, have provided zircon U-Pb ages of 447 ± 3 Ma, 456 ± 2 Ma and 440 ± 2 Ma, indicating a late Ordovician or early Silurian age for the protoliths. The uppermost unit (the Serre Unit) comprises porphyritic (meta-) volcanic and volcaniclastic rocks dated to the Permian (271 ± 2 Ma), on top of which remnants of the Mesozoic cover is preserved. Detailed mapping of an area about 140 km2 shows that (i) the ultra-high pressure Chasteiran Unit is localized at the boundary between the Pinerolo and Muret Units, (ii) the Granero Orthogneiss may be considered as the mylonitic sole of the Muret Unit, characterized by a top-to-W sense of shear, and (iii) the contact between the Muret and Serre Units displays ductile-to brittle structures (La Fracho Shear Zone), indicating a top-to-the-NW displacement of the hangingwall with respect to the footwall. A final episode of brittle faulting, cutting across the nappe stack (the Trossieri Fault), indicates an extensional stage in the core of the Alpine belt, as previously documented in more external zones. This work provides a necessary and robust basis for an accurate discussion of processes acting during continental subduction of the Dora-Maira Massif.

How to cite: Ballèvre, M., Manzotti, P., Nosenzo, F., Krona, M., and Poujol, M.: Tectonic architecture of the northern Dora-Maira Massif (Western Alps, Italy): field and geochronological data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5930, https://doi.org/10.5194/egusphere-egu24-5930, 2024.

14:25–14:35
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EGU24-8854
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ECS
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On-site presentation
Louise Boschetti, Frederic Mouthereau, Stephane Schwartz, Yann Rolland, Matthias Bernet, and Melanie Balvay

The Alpine orogenic belt in SE France is the result of the collision between the European, Adriatic and Iberian plates. The accreted Variscan continental crust, which now forms the external crystalline massifs (ECMs), recorded a complex Mesozoic thermal and tectonic evolution, that is not fully understood. In the Maures-Tanneron massif (MTM), the basement has undergone periods of subsidence and uplift, the latter indicated by stratigraphic gaps from the Albian and Upper Turonian to the Maastrichian. In the Ecrins-Pelvoux massif (EPM), differential subsidence is documented during Lower Jurassic by lateral variation from marine to continental environment, but most of the Cretaceous and Paleogene periods correspond to a stratigraphic hiatus that ends with the deposition of upper Eocene sediments. The link between these stratigraphic gaps and inheritance associated with the rifting, opening of the Alpine Tethys, and early convergence between Europe, Iberia and Adria is still not resolved. The goal of this study is to elucidate the thermal evolution of the European basement in SE France (EPM and MTM) during the Mesozoic using apatite and zircon fission track low-temperature thermochronology (AFT and ZFT). ZFT data from the southern EPM indicates a complex thermal history with central ages ranging from 158 to 45 Ma, thus revealing significant Jurassic to Eocene resetting and cooling. These ages are interpreted as resulting from several tectonic stages related to (1) Jurassic rifting (2) Mesozoic shortening and erosion and/or (3) incomplete Alpine reset during the main phase of underthrusting below the Penninic Frontal Thrust during the Oligocene. In contrast, the MTM shows several thermal events, comprising a major cooling stage at ca. 200 Ma coincident with the CAMP event preserved in the northern part of the massif. A final cooling event between 30 and 25 Ma, that is mostly represented to the South of the massif, is related to the opening of the Ligurian sea. Intermediate AFT ages between these two events are also identified, likely reflecting cooling events during the Mesozoic that can be resolved using thermal modelling. Finally, the long-term thermal evolution reported from SE France ECMs allows refining the geodynamics of this region from Pangea fragmentation to the onset of Alpine orogeny.

How to cite: Boschetti, L., Mouthereau, F., Schwartz, S., Rolland, Y., Bernet, M., and Balvay, M.: Mesozoic tectonic inheritance of the European crystalline basement (SE France) revealed by thermochronology, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8854, https://doi.org/10.5194/egusphere-egu24-8854, 2024.

14:35–14:45
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EGU24-10762
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ECS
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On-site presentation
Louison Mercier, Sébastien Migeon, Jean-Loup Rubino, Jenny Trevisan, Speranta-Maria Popescu, Mihaela Carmen Melinte Dobrinescu, Miroslav Bubík, Yamirka Rojas-Agramonte, Anna Hagen, and Romain Bousquet

Submarine canyons are commonly controlled by tectonic structures and, therefore, are key elements of the evolution of convergent margins such as the Southern French Alpine Foreland Basin. Here we use the outcrops of Grès d’Annot and Schistes à Blocs formations of the Sanguinière-Restefond and Trois Eveches sub-basins, to study the morphology of ancient canyons respectively in relation to extensive and compressive tectonics. The Grès d’Annot Upper Erosion Surface (GAUES) and faults have been mapped in the field and using airborne and drone pictures. Moreover, the deposition age of the Schistes à Blocs Formation has been constrained by the analysis of calcareous nannofossils and benthic foraminifera coming from 9 samples. We also compared ages of detrital zircons by U-Pb thermochronology from 4 samples. One of them was sampled within Annot Sandstones while the other come from the turbidites of the Schistes à Blocs Formation that seals the GAUES.

The Colombart Structure in the Sanguinière-Restefond area is composed of two normal faults with a N80°E orientation and a southern vergence, bordering a northward dipping rollover anticline. The Colombart Structure axially controls the 700 m deep La Bonette Canyon cutting through the underlying Annot Sandstones. The submarine canyon is made of a succession of sharp erosive features, such as erosive walls, ramps and terraces. The cross-section profile of the canyon exhibits a tectonic control at several scales: it is asymmetric as well as the thalweg is. Faults also commonly control smaller scale morphologies, but also the capture of tributaries at right angles with the canyon axis, which testifies for a rectangular drainage pattern. The preliminary study of the GAUES in the Trois Eveches Sub-basin also exhibits a strong relationship between tectonics and submarine erosion. The last shows a 300 m-high scarp frontally eroding a NW-SE oriented thrust which affects the underlying sandstones. Moreover, biostratigraphic dating of the Schistes à Blocs Formation indicates NP22-lower NP23 biozones, i.e. the Early Rupelian. Detrital zircons analysis by U-Pb method show that Annot Sandstones and Schistes à Blocs Formation have the same signal. Finally, within both sub-basins, the thin bedded turbidites of the Schistes à Blocs Formation exhibit paleocurrent directions which are almost opposed to those measured within the Annot Sandstones. Paleocurrents within the Trois Eveches Sub-basin also locally change depending on which thrusts is located below the Schistes à Blocs Formation.

Consequently, the GAUES mainly results from retrogressive erosion affecting partially lithified turbidites following two main triggering factors which are: i) the foreland deformation with a deformation direction that potentially locally changes, and ii) the 3rd order eustatic fall linked to the Oi1a δ18O event. The creation of submarine canyons affecting previously deposited turbidite lobes testifies of a strong paleogeographical modification of the foreland before the Autapie nappe emplacement. This change is also evidenced by the paleocurrent reorganization after the submarine erosion. Nevertheless, the complete understanding of the whole Early Rupelian source-to-sink system would need to enlarge the study of the Schistes à Blocs Formation to the whole foreland, including the use of other methods.

How to cite: Mercier, L., Migeon, S., Rubino, J.-L., Trevisan, J., Popescu, S.-M., Melinte Dobrinescu, M. C., Bubík, M., Rojas-Agramonte, Y., Hagen, A., and Bousquet, R.: Submarine canyons cutting through the Annot Sandstones, a key-element of the evolution of the Alpine Foreland Basin during the Rupelian, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10762, https://doi.org/10.5194/egusphere-egu24-10762, 2024.

14:45–14:55
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EGU24-10978
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ECS
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On-site presentation
Stefano Ghignone, Federica Boero, Marco Bruno, Mattia Gilio, Emanuele Scaramuzzo, and Alessia Borghini

The occurrence of coesite, a Ultrahigh Pressure (UHP) index mineral,  in tectono-metamorphic belts is of paramount importance to pinpoint the depths attained during subduction. Such minerals are generally found as inclusions within garnets and are often the sole remnants of an UHP mineralogy in largely re-equilibrated rocks. UHP tectonometamorphic units in subducted oceanic lithosphere are of particular interest because they are natural laboratories to study element-exchange and fluid rock interactions occurring in a subducting slab at depths > 80 km. In this context, the meta-ophiolites of the Western Alps are a perfect case study, as they offer a continuous outcrop along the entire belt. Here, we focus on the UHP meta-ophiolites of the Internal Piedmont Zone (IPZ) in the Western Alps, where coesite inclusions in garnet have recently been found (Ghignone et al., 2023 and Boero, 2023). These localities lay on the same structural position of the Lago di Cignana Unit, wherein coesite was discovered in the early 90’s (Reinecke, 1991). In addition, these three UHP localities share similar metamorphic peak conditions and their PT estimates lie on the same metamorphic gradient (roughly 6°C/Km).

A targeted sampling campaign along the entire Western Alpine meta-ophiolitic belt allowed to better understand the distribution of coesite-bearing rocks. Metasediments (Grt-quartzite, Grt-Cld micaschist, Grt-calcschist) are the best lithotypes that preserved coesite, but also some meta-mafic lithotypes (eclogite, Grt-metabasite) contain it. Usually, garnets within metasediments are strongly zoned, whereas in meta-mafic lithotypes they have a more constant composition. Coesite was identified via µ-Raman spectroscopy, showing the typical vibrational modes of the phase (521, 427, 271 and 180 cm-1), slightly shifted due to elastic residual strain. Coesite occur as pristine tiny crystals (<40 µm) entirely trapped in garnet, both isolated and clustered. Their shape varies from well-faceted to strongly anhedral with morphological evidence of resorption (i.e., lobed morphologies with rounded shapes and/or embayment). Bigger inclusions of quartz (>40 µm) present the typical features of re-equilibration after coesite (i.e., radial cracks, polycrystalline aggregates). 

Among the different UHP localities, the presence of coesite is limited to a specific garnet shell (e.g., core, mantle), identifying a specific moment of garnet growth in UHP metamorphic conditions. The other shells, contrarily, preserve inclusions of quartz. These differences allowed to reconstruct the prograde or retrograde evolution through a detailed inclusion study of their preserved elastic properties (i.e., elastic geobarometry).

Our results highlight that the entire IPZ eclogite-facies meta-ophiolites underwent UHP metamorphism in the coesite stability field. This suggests that a large volume of oceanic lithosphere was subducted at ca. 100 km depth and then returned to the surface. This is an important constrain to create  reliable tectonic models of  subduction and exhumation of the oceanic lithosphere in collisional subduction/accretionary systems.

 

Boero, F., 2023. Master Thesis, University of Turin. 115 pp.

Ghignone, S., Scaramuzzo, E., Bruno, M., Livio, F., 2023. Am Mineral, 108(7), 1368-1375.

Reinecke, T., 1991. Eur J Mineral, 3, 7-17.

How to cite: Ghignone, S., Boero, F., Bruno, M., Gilio, M., Scaramuzzo, E., and Borghini, A.: Coesite in Alpine meta-ophiolites: hidden but widespread, and tectonically relevant , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10978, https://doi.org/10.5194/egusphere-egu24-10978, 2024.

14:55–15:05
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EGU24-16390
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ECS
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On-site presentation
Emanuele Scaramuzzo, Franz Livio, Alessia Borghini, Mattia Gilio, Michele Locatelli, Federica Boero, and Stefano Ghignone

Ultra-high pressure (UHP) units sample the deepest portion of a subduction zone that returned to the surface, escaping their fate of disappearing deep into the mantle. Several mechanisms have been proposed for the exhumation of UHP units in collisional orogens but the topic remains still controversial and poorly understood.

The models invoked for the exhumation of UHP units generally require a positive buoyancy as trigger of exhumation. However, Ghignone et al. (2023) reported for the first time the occurrence of a slice tens of kilometres in length of oceanic slab, i.e., the Lago Superiore Unit (LSU), that reached UHP depth. This latter represents a portion of the former Alpine Tethys oceanic lithosphere now accreted within the Western Alpine collisional system (Ghignone et al., 2023).

In this contribution we present new insights on the subduction-accretionary processes preserved in the UHP Lago Superiore Unit. Our study is based on i) a new structural map of the LSU considering new data, ii) structural and kinematic field data, and iii) new prograde and retrograde P-T estimations calculated combining quartz-in-garnet elastic thermobarometry with Zr-in rutile and Ti-in-quartz thermometry.

Our new integrated kinematic and thermobarometric model suggests that the primary process driving the exhumation of the UHP Lago Superiore Unit was the progressive extraction of a composite metamorphic wedge. Final extension as revealed by thermobarometric constrain, allowed the exhumation of the Lago Suepriroe Unit at shallow crustal levels.

Ghignone, S., Scaramuzzo, E., Bruno, M., Livio, F., 2023. Am Mineral, 108(7), 1368-1375.

How to cite: Scaramuzzo, E., Livio, F., Borghini, A., Gilio, M., Locatelli, M., Boero, F., and Ghignone, S.: Subduction and exhumation of an Ultra-High Pressure oceanic slab in the Western Alps, new insights from the Lago Superiore Unit, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16390, https://doi.org/10.5194/egusphere-egu24-16390, 2024.

15:05–15:15
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EGU24-17724
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ECS
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On-site presentation
Lorenzo Gemignani, Julian Hülscher, Michele Zucali, Edward R. Sobel, Klaudia Kuiper, and Irene Albino

The Cenozoic uplift evolution of the Western Alps has been examined from various perspectives. Several studies have suggested that a Late Miocene-Pliocene European slab break-off, coupled with increased erosion due to enhanced glaciation, serves as a driving factor controlling the Western Alps topography. Alternatively, strain partitioning resulting from Adriatic indentation and Oligocene clockwise rotation leads to contrasting kinematic regimes, segmenting the Western Alps into blocks with differential exhumation. Here, we analyze the evolution of the Dent Blanche Tectonic System (DBTS), an Austroalpine nappe in the Western Alps surrounded by oceanic units from the former Liguro-Piemontese ocean.

We apply Low-T thermochronology (apatite and zircon (U-Th)/He) and high resolution mica 40Ar/39Ar dating from the DBTS. ZHe sample ages from the DBTS are ~30 Ma, with an extremely low eU sample from the lower elevation of the Valpelline Valley as young as ~7 Ma. AHe samples are younger, ranging from ~20 Ma to ~3 Ma. Reliable mica Ar ages range from the Paleocene to Oligocene. Most of the samples' age distributions have low radiogenic Ar yields (low 40Ar*), and part of the analyzed muscovite shows low K/Ca ratios, likely indicating chloritization.

Inverse modelling of the cooling ages from selected samples from the core of the DBTS (Arolla Units) shows that the exhumation rate of the DBTS is one-fold lower than the exhumation rates derived in the units north of the nappe. These rates are comparable with slower exhumation rates south of the nappe.

We propose that the DBTS system underwent its highest exhumation rates in the Oligocene to Late Miocene, predating the proposed Pliocene slab break-off as well as Pliocene increased glaciation. The identification of Pliocene-Pleistocene ages from one sample is interpreted to reflect glacial erosion localized in the Valpelline Valley; this is aligned with similar increased denudation rates since Pliocene observed in other Western Alps regions. However, this single cooling age does not provide conclusive evidence that glaciation drove the DBTS’s exhumation from ~3 Ma ago.

How to cite: Gemignani, L., Hülscher, J., Zucali, M., Sobel, E. R., Kuiper, K., and Albino, I.: Late Cenozoic evolution of the Dent Blanche Tectonic Nappe in the Western Alps imaged by low-t thermochronology., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17724, https://doi.org/10.5194/egusphere-egu24-17724, 2024.

15:15–15:25
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EGU24-3428
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On-site presentation
Fritz Schlunegger and Edi Kissling

The change from a deep-marine, underfilled Flysch to a terrestrial and/or shallow-marine, overfilled Molasse stage of basin evolution is probably one of the major steps in the evolution of a foreland basin. Chrono-stratigraphic and sedimentologic data from the north Alpine foreland basin (NAFB), situated on the northern margin of the Alps, document that such a shift occurred at c. 30 Ma in the western (Swiss and German) part of the basin and c. 10 My later in the eastern (Austrian) segment. We relate these basin-parallel differences in the basin’s evolution to an orogen-parallel variation in subduction tectonics, that itself appears to be conditioned by the segmentation of the European plate during the Mesozoic phase of spreading preceding the build-up of the Alps (Schlunegger and Kissling, 2022). During the Mesozoic, the transition from the continental European plate to its extended margin farther South was most likely offset by a left-lateral fault in the vicinity of Munich, separating the future depositional realms of the NAFB into a western and an eastern segment. As a consequence, during the construction of the Alps from 35 Ma onward, continent-continent collision occurred earlier in the Western Alps (c. 32-30 Ma) than in the Eastern Alps (c. 20 Ma). This collision resulted in the delamination of the subducted European oceanic lithosphere from its continental counterpart beneath the Western Alps. As a consequence, the European continental plate beneath the Western Alps experienced a rebound, thereby causing the build-up of the Alpine topography and the increase in sediment supply to the foreland basin. This is recorded in the Western NAFB by a shift from Flysch- to Molasse-type of sedimentation at 30 Ma. Farther to the East, however, the subducted oceanic lithosphere slab of the European plate was still attached to the European continental plate, with the consequence that Flysch-type of sedimentation still prevailed in the Austrian part of the basin. The situation of sedimentation in an underfilled basin persisted until c. 20 Ma when the Austrian (eastern) part of the NAFB changed from a Flysch- to a Molasse-type of basin evolution. This is the main reason why we propose that continent-continent collision most likely occurred 10 My later in the Eastern Alps than in the Western Alps.

Schlunegger, F., Kissling, E. (2022). Slab load controls beneath the Alps on the source-to-sink sedimentary pathways in the Molasse Basin. Geosciences, 12, 226.

How to cite: Schlunegger, F. and Kissling, E.: Sedimentary records imply that continent-continent collision occurred later in the Eastern than in the Western Alps., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3428, https://doi.org/10.5194/egusphere-egu24-3428, 2024.

15:25–15:35
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EGU24-18901
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ECS
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On-site presentation
Armelle Ballian, Maud M. J. Meijers, Isabelle Cojan, Damien Huygue, Katharina Methner, Daniel Boateng, Sebastian G. Mutz, Walter Kurz, Emilija Krsnik, Horst Zwingmann, Yann Rolland, Todd Ehlers, Jens Fiebig, and Andreas Mulch

The European Alps, one of the most studied mountain ranges worldwide, are hypothesized to have experienced diachronous surface uplift resulting from slab-breakoff (Schlunegger and Kissling, 2018; Handy et al., 2015). However their surface elevation history is yet not well constrained (Campani et al., 2012; Krsnik et al., 2021; Botsyun et al., 2020). Quantifying surface elevation of an orogen through geological time is essential for our understanding of the geodynamic drivers, as well as the paleoenvironmental impacts of surface uplift.

Here, we present Early to Middle Miocene stable isotope-based paleoelevation reconstructions of the Western, Central, and Eastern Alps. Stable isotope paleoaltimetry (the 𝛿-𝛿 approach) is based on the systematic decrease of oxygen (𝛿18O) and hydrogen (𝛿D) isotopic composition of precipitation with increasing elevation and strongly benefits from contrasting high and low elevation records of past rainfall.

Accordingly, contrasting temperature-corrected near sea level pedogenic carbonate 𝛿18O values with time-equivalent 𝛿D values of K-Ar dated clay minerals from fault gouges allows for the calculation of the differential elevation between a foreland basin and an orogen’s interior through time. Recent paleoaltimetry research with focus on the Middle Miocene Central Alps indicates elevations exceeding 4 km (Krsnik et al., 2021).

With a spatiotemporally enhanced coverage of the European Alps, we present estimates of paleoelevation covering the time interval between ca. 23 and 12 Ma. In addition, paleoclimate simulations for a number of topographic scenarios allow for the isolation of contribution of local elevation complex climate change, and regional topographic configuration signals (Boateng et al., 2023).

Our quantitative stable isotope paleoaltimetry estimates indicate peak elevations of >4km in the Central Alps already during the earliest Miocene (ca. 23 Ma). 𝛿D values from fault gouge-derived illites are up to 25 ‰ higher in the Eastern Alps than in the Central Alps for the time interval between 21-16 Ma and suggest that the Eastern Alps were significantly lower during that time interval. Our results from the Mont Blanc massif are in line with isotopic measurements from fluid inclusions in quartz veins, which highlight the Mont Blanc massif in the Western Alps, did not exceed an average elevation of ca. 1 km until the end of the Miocene (Melis, 2023). Collectively, these results confirm a scenario of west-to-east surface uplift as suggested on the basis of slab-breakoff and tearing.

How to cite: Ballian, A., Meijers, M. M. J., Cojan, I., Huygue, D., Methner, K., Boateng, D., Mutz, S. G., Kurz, W., Krsnik, E., Zwingmann, H., Rolland, Y., Ehlers, T., Fiebig, J., and Mulch, A.: Stable isotope paleoaltimetry reveals Early to Middle Miocene along-strike elevation differences of the European Alps, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18901, https://doi.org/10.5194/egusphere-egu24-18901, 2024.

15:35–15:45
Coffee break
Chairpersons: Ralf Schuster, Marco Giovanni Malusa'
16:15–16:25
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EGU24-9804
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ECS
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On-site presentation
Benedikt Braszus, Andreas Rietbrock, Christian Haberland, and Trond Ryberg

We present a new 3D P & S-wave model based on Local Earthquake Tomography (LET) of the European Alpine mountain chain using data from a total of more than 1100 broadband stations of the AlpArray Seismic Network and additional permanent and temporary stations. We use "SeisBench - A toolbox for machine learning in seismology" to assess the performance of the most commonly used AI pickers and find PhaseNet to be the most suitable. Our final data set comprises 2374 events of Ml >= 1.5 yielding 89,000 Pg-, 64,000 Pn-, 41,000 Sg- & 23,000 Sn-phases. Initially, we include observations from <130km epicentral distance to simultaneously relocate the quakes and invert for upper crustal velocity structure using the SIMUL2017 inversion algorithm. Subsequently, we add the remaining travel times to invert for velocities in the entire crust and upper mantle while fixing the hypocentres from the initial inversion run. 
First order features of our final vp model such as sediment basins and the Alpine orogenic root are in good agreement with previous tomographies and Moho studies of the area. In the Western Alps the well studied Ivrea Geophysical Body (IGB) is imaged as a high velocity anomaly where mantle velocities are present at depths of 15-20km. West of the IGB we find lower crustal velocities reaching depths of ~50km. Both observations are coinciding with the previously imaged Moho jump between deep European and shallow Adriatic Moho.
Similarly, we image the orogenic root in the Northern Apennines as an area of low vp with increased vp/vs-ratio. Beneath the Eastern Po plain we find mantle velocities at shallower depths than published Moho values would suggest. 

How to cite: Braszus, B., Rietbrock, A., Haberland, C., and Ryberg, T.: AI based 3D P- & S-wave velocity model for the Alpine mountain chain from Local Earthquake Tomography, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9804, https://doi.org/10.5194/egusphere-egu24-9804, 2024.

16:25–16:35
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EGU24-10513
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On-site presentation
Leni Scheck-Wenderoth, Ajay Kumar, Mauro Cacace, Judith Bott, Hajo Götze, and Boris Kaus

To address the question of how the present-day architecture of the lithosphere and the heterogenous density configuration of the uppermost mantle influence deformation in the Alpine orogenic system we use data-derived 3D configurations as input to dynamic simulations. This includes on the one hand the consideration of a detailed crustal model of the Alpine region and its forelands that resolves first-order contrasts in the thermophysical properties of the crust consistent with available geoscientific observables (active and passive seismic, gravity, geological, geothermal). In addition, we tested an ensemble of configurations of upper mantle thermophysical properties derived from published seismic tomography models. Using a Gibbs-free energy minimization algorithm (https://zenodo.org/records/6538257) we convert the results of regional shear-wave seismic tomography models to temperature models and define the base of the lithosphere and the geometry of slabs in the asthenosphere with a threshold temperature of 1300°C. As a first step we model topography and deformation velocities as resulting from buoyancy-forces driven by a quasi-instantaneous flow resulting from the first-order rheological structure of the lithosphere-asthenosphere system using the open source geodynamic code LaMEM (https://github.com/UniMainzGeo/LaMEM). The simulation results indicate that a slab detached beneath the Alps, but attached beneath the Northern Apennines captures first-order patterns in topography, vertical surface velocities, and mantle flow. The presence of an attached slab beneath the northern Apennines also explains the observed sub-crustal seismicity in contrast to the seismicity in the Alps restricted to the upper-crustal domain.

How to cite: Scheck-Wenderoth, L., Kumar, A., Cacace, M., Bott, J., Götze, H., and Kaus, B.: Where data-based structure meets process simulation - how heterogeneities relate to lithosphere deformation in the Alps, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10513, https://doi.org/10.5194/egusphere-egu24-10513, 2024.

Eastern and Southern Alps
16:35–16:45
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EGU24-15665
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ECS
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On-site presentation
María del Puy Papí Isaba, Elisa Buforn, Maurizio Mattesini, Gesa Petersen, Simone Cesca, Helmut Hausmann, and Wolfgang Lenhardt

Three peculiar seismic sequences occurred between March and May 2021 near Breitenau and Gloggnitz, about 50 km from Vienna, Austria. The seismic sequences’ mainshock epicentres are less than 15 km apart. In March 2023, seismic activity resumed in the Gloggnitz area and continued to be relatively high in comparison with the average background seismicity of the region.

The first of these sequences started on March 30th, 2021, with the occurrence of an ML4.6 and h = 9 km earthquake close to Breitenau. A period of increased seismic activity lasted ~2 weeks, before decreasing to the background seismicity rate by mid-April. On April 19th, 2021, an earthquake with similar magnitude and depth (ML4.4 and 9 km) occurred only 1 km northeast of the previous ML4.6. The following seismic sequence lasted until the end of May 2021. The third seismic sequence started on April 20th, 2021, ~15 km SW of the Breiteau sequences, with a shallow (h = 5 km) ML3.5 earthquake followed by the mainshock (ML3.8 and h = 5 km) on April 23rd, 2021 (ML3.8 and h = 5 km). Seismicity decayed to background rates by early May. On March 30th, 2023, the seismic activity resumed in the Gloggnitz area with a mainshock (ML4.2 and h = 10 km). Its epicentre was located between the 2021 Gloggnitz foreshock (ML3.5) and the mainshock (ML3.8). Compared to the 2021 Gloggnitz sequence, there was no significant surge in seismic activity following the ML4.2 event, and the seismicity levels remained moderately high, compared to the typical seismic activity observed in the year 2021, until the beginning of October.

In this study, we relocated all events using a non-linear location method and used a probabilistic full waveform inversion tool to derive full moment tensor solutions for the largest earthquakes of the sequences (ML4.6, ML4.4, ML4.2 and ML3.8). These neighbouring sequences, which cluster spatially along a narrow seismicity band, but show different focal mechanisms and different temporal evolutions, shed light on the segmentation of local seismogenic processes and complex fault system along the seismogenic lineament.

How to cite: Papí Isaba, M. P., Buforn, E., Mattesini, M., Petersen, G., Cesca, S., Hausmann, H., and Lenhardt, W.: The 2021 and 2023 Vienna Basin seismic sequences: Insights from earthquake relocation and moment tensor inversion, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15665, https://doi.org/10.5194/egusphere-egu24-15665, 2024.

16:45–16:55
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EGU24-21023
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On-site presentation
Peter McPhee and Mark Handy

In Neogene time, the Eastern Alps underwent a profound tectonic reorganisation. This featured northward indentation of the Alpine orogenic wedge by the Adriatic plate, eastward lateral extrusion between conjugate strike-slip faults, and a shift from thrust propagation on the European lower plate to the Adriatic upper plate. We investigate the triggers of this reorganisation with new sequentially restored orogen-scale cross-sections along the TRANSALP (12°E, western Tauern Window) and EASI (13.3°E, eastern Tauern Window) transects, plus an E-W orogen-parallel cross-section (46.5°E). We use a map-view reconstruction to restore the effects of out-of-section transport by lateral extrusion and compare our results with P-wave local earthquake (LET) and teleseismic tomographic models of the crust and upper mantle.

The geologic record reveals two phases of indentation: Phase 1 (c. 23-14 Ma): The Adriatic Plate was an undeformed indenter, with northward motion relative to Europe accommodated by sinistral motion along the Giudicarie Fault and shortening within the Eastern Alps orogenic wedge. Upright folding of nappes mostly derived from the downgoing European Plate, and lateral extrusion of the entire metamorphic edifice and North Calcareous Alps accommodated this N-S shortening. This shortening required ongoing subduction of European lithosphere, ruling out previous models involving north-dipping Adriatic subduction. A purported detachment below the Venediger Nappes may have served as the base of the laterally extruding wedge.

Phase 2 (c. 14 Ma-Present): The leading edge of the Adriatic indenter has been deforming since c. 14 Ma, forming the thick-skinned South Alps fold-thrust belt. The onset of S-directed thrusting is recorded by Langhian-Serravallian flysch in the footwall of the Valsugana thrust. The Adriatic lower crust was decoupled and transported northwards into the orogenic wedge, indenting and exhuming the deeply buried equivalents of the Venediger Nappes in the Tauern Window. A high-velocity (6.8 - 7.25 km/s) bulge in LET models of the TRANSALP section images this indenter, which comprises mostly Adriatic lower crust, but possibly also includes some accreted European lower crust.

In P-wave teleseismic tomography along the EASI section, the European slab appears to be detached at a locus marked by a Moho gap and a shallow discontinuity in the positive velocity anomaly beneath the orogenic wedge. In contrast, no such discontinuity occurs beneath the TRANSALP section, where S-dipping European lithospheric mantle still extends beneath and south of the orogenic wedge. If the southern end of this relict slab segment marks the locus of a detachment, we find that the current slab length is less than the amount of N-S shortening in the TRANSALP section since 23 Ma. To explain this mismatch, we propose that the most recent slab detachment in the Eastern Alps event occurred after 23 Ma, and likely after 14 Ma (Phase 2 indentation). Note that this does not preclude earlier detachment events, notably at 22-19 Ma when the eastern Molasse Basin rapidly filled and orogenic vergence shifted from north to south (see Handy et al., this session).

How to cite: McPhee, P. and Handy, M.: Post-collisional reorganisation of the Eastern Alps in 4D – Crust and mantle structure, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21023, https://doi.org/10.5194/egusphere-egu24-21023, 2024.

16:55–17:05
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EGU24-8184
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ECS
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On-site presentation
Anna-Katharina Sieberer, Ernst Willingshofer, Thomas Klotz, Hugo Ortner, and Hannah Pomella

Neogene to ongoing N(W)-directed continental indentation of the Adriatic microplate into Europe controls the evolution of the European eastern Southern Alps (ESA). The Adriatic microplate, traditionally considered as a rigid indenter, demonstrates significant internal deformation, with mostly Miocene shortening being accommodated within a WSW-ENE striking, S-vergent fold-and-thrust belt. The latter overprints a compositionally heterogeneous upper crust linked to Permian intrusives and extrusives and a pre-existing platform-basin geometry related to Jurassic extension.

We present new, multi-scale physical analogue experiments, to address the effect of lateral crustal heterogeneities on strain localization and deformation geometries of the ESA, which is key for establishing causal relations between crustal and lithospheric deformation and surface uplift patterns associated with Miocene basin inversion.

Brittle crustal-scale analogue experiments with inversion of pre-scribed platform-basin geometries, indicate that variations in thickness, shape, and basement structure have impact on timing and uplift of the ESA’s upper crust. Our modelling results demonstrate that experiments with a stronger upper crustal domain (representing Permian volcanic rock on Jurassic platforms) show a smaller number of thrust sheets, being in line with thrust sheet geometries across the natural example of the ESA, and continuous uplift patterns. The latter is supported by continuous exhumation within the last 15 Ma documented by low-temperature thermochronology data between Mauls and Bassano east of the Giudicarie belt (see contribution of Klotz et al., this session). The topographic evolution of the experiments is sensitive to a variation in crustal composition; additional, e.g., basement structures (modelled using a fixed and rigid basal plate whose boundaries represent Permian faults) result in limited uplift of northern model parts, which is consistent with documented little vertical movement of the western ESA north of the Valsugana fault system between Jurassic and Neogene times.

On the scale of the lithosphere, new analogue experiments with pre-scribed platform and basin geometries in the upper crust show similar lateral variations in thrust fault orientation across transfer zones as crustal-scale experiments (Sieberer et al., 2023). Variations in lithospheric strength lead to increasing wavelengths between thrust sheets in models with stronger rheologies, pre-existing heterogeneities in the upper crust to strain localisation at boundaries of strong domains. Additionally, lateral variability of ductile lower crustal thickness predicts stronger uplift in areas of thicker lower crust. A similar relationship has been documented for the northwestern ESA, where Miocene thickening of the lower crust is expected to correlate with higher uplift in the Tauern window (Jozi Najafabadi et al., 2022).

Jozi Najafabadi, A., Haberland, C., Le Breton, E., Handy, M. R., Verwater, V. F., Heit, B., and Weber, M.: Constraints on Crustal Structure in the Vicinity of the Adriatic Indenter (European Alps) From Vp and Vp/Vs Local Earthquake Tomography, Journal of Geophysical Research: Solid Earth, 127, 10.1029/2021jb023160, 2022.

Sieberer, A.-K., Willingshofer, E., Klotz, T., Ortner, H., and Pomella, H.: Inversion of extensional basins parallel and oblique to their boundaries: inferences from analogue models and field observations from the Dolomites Indenter, European eastern Southern Alps, Solid Earth, 14, 647-681, 10.5194/se-14-647-2023, 2023.

How to cite: Sieberer, A.-K., Willingshofer, E., Klotz, T., Ortner, H., and Pomella, H.: Control of inherited structures on deformation and uplift in the European eastern Southern Alps: a multi-scale analogue modelling study, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8184, https://doi.org/10.5194/egusphere-egu24-8184, 2024.

17:05–17:15
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EGU24-16377
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ECS
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On-site presentation
Thomas Klotz, Anna-Katharina Sieberer, Hugo Ortner, István Dunkl, and Hannah Pomella

The NW to N directed indentation of the Adriatic microplate into the European lithospheric domain, initiated in the upper Eocene following the closure of the Piemont-Liguria and Valais oceanic basins, constitutes a key feature of the Neoalpine orogenesis. The separation of the eastern Southern Alps (Dolomites Indenter) along the Giudicarie fault system from the late Oligocene (Middle Miocene at the latest) on and its increased northward push contributes significantly to major tectonic processes in the Eastern Alps north of the Dolomites Indenter: updoming, piggy-back top-N thrusting, and eastward lateral escape of the Tauern Window.

The interior of the Dolomites Indenter undergoes deformation as well, as documented, e.g., by the prominent, dominantly SSE-vergent fold and thrust belt of the Dolomites, as well as the top-WSW directed thrusts of the Dinaric chain and associated flysch sedimentation. New and compiled Apatite (U-Th)/He (AHe) and Fission Track (AFT) data allow the tracing of the exhumation history.

AFT data from the western Dolomites Indenter tend to cluster within consistent Dinaric and Neoalpine distinguishable tectonic blocks. However, the data are quite scattered. AHe data primarily indicate exhumation during the post-15 Ma Valsugana phase, showing a tendency of getting younger towards the east. A subordinate number of AHe datapoints document Eocene to Oligocene cooling as well.

Regional age-elevation profiles of consistent fault-delimited blocks exhibit (i) moderate cooling during the Mesoalpine Penninic subduction, (ii) fast Dinaric exhumation (in the Plose area), and (iii) fast Valsugana phase exhumation starting at approximately 15 Ma; Notably, this exhumation pulse starts earlier (Chattian/Aquitanian) in the northernmost tectonic block at the Indenter tip.

Time-temperature path modelling confirms the Valsugana phase as the most significant period of tectonic exhumation within the western Dolomites Indenter. According to the modeling, prior to this phase, a significant number of samples remained within the AFT annealing zone for an extended period of time, at least from Ladinian times onwards. This is due to a wide dispersion of single grain ages and suggests, the data does not necessarily represent a tectonic pulse. Moreover, many samples from sedimentary rocks of the Permian and Lower Triassic periods show a complete reset of the AFT system during the Middle Triassic, well before the maximum burial indicated by the stratigraphic record. This high-temperature anomaly could be attributed to the extensive Ladinian volcanism in the study area.

Based on the new thermochronological data, it can be inferred that the Middle Miocene Valsugana phase is the most significant exhumation phase in the Dolomites Indenter. Additionally, this phase begins earlier in the north than in the south. It is essential to consider the complex thermal history of the Dolomites Indenter and the possible long residence time of samples within the partial annealing zone prior to the Neoalpine exhumation when interpreting new data.

How to cite: Klotz, T., Sieberer, A.-K., Ortner, H., Dunkl, I., and Pomella, H.: The Alpine cooling history of the western Dolomites Indenter, European Southern Alps, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16377, https://doi.org/10.5194/egusphere-egu24-16377, 2024.

17:15–17:25
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EGU24-9709
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ECS
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On-site presentation
Lucija Golub, Stéphane Rondenay, and Josip Stipčević

Ever since the first regional teleseismic tomography images of the central Mediterranean region, one aspect that has stood out in nearly every model is the missing deep slab under the north and central Dinarides. In contrast, concurrent investigations of crustal formation have pointed to a deep crustal root under the whole of the Dinarides, supporting the hypothesis of a laterally continuous slab. In the last decade, several attempts have been made to untangle this conundrum but without much success. Nevertheless, these efforts have yielded some notable new findings, such as possible lithospheric delamination under the central Dinarides. This study aims to utilize all the available seismological results in combination with several new analyses to shed light on the upper mantle structure beneath the central Dinarides. We conducted the SKS shear-wave splitting analysis using 21 stations from the Croatian national seismic network and 7 stations from the AlpArray network. We considered events that occurred between 2010 and 2022 with magnitudes greater than MW = 6.0 and epicentral distances ranging between 85° and 120°. In parallel, a teleseismic Generalized Radon Transform (GRT) migration was conducted along a set of 2D profiles to provide structural insights into the subduction zone within the study area. Data from the Croatian national seismic network, the CRONOS temporary network, and the AdriaArray Temporary Network were used for the migration. For this approach, we considered events that occurred after January 2020 within the epicentral distance range of 30° - 100° and magnitudes greater than MW = 5.5. In addition to these two new analyses, we used other seismological results from previous investigations (including S-receiver functions and ambient noise tomography) to fill in the gaps in our investigation of the lithospheric structure under central Dinarides. Preliminary results exhibit distinctive patterns: the orientation of SKS fast axes, indicative of mantle flow, in the north and central External Dinarides aligns perpendicular to the mountain chain’s strike. However, this orientation abruptly transitions to a NW-SE direction further from the coast and continues in the northern part of Croatia. Results from converted/scattered-waves and ambient noise, for their part, point to a thickened crust under the central and southern External Dinarides, with a high-velocity anomaly reaching at least 100 km depth but a relatively thin lithosphere. Taken together these results suggest that the slab blocks the mantle flow up to depths of 100 – 150 km.

How to cite: Golub, L., Rondenay, S., and Stipčević, J.: Dinarides slab gap - fact or fiction, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9709, https://doi.org/10.5194/egusphere-egu24-9709, 2024.

17:25–17:35
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EGU24-3390
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ECS
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On-site presentation
Ludwik de Doliwa Zieliński, Jakub Bazarnik, Ellen Kooijman, Karolina Kośmińska, Tomáš Potočný, Stanisław Mazur, and Jarosław Majka

The collision of Europe (Laurusia) and Alcapa (part of Adria) lead to the formation and later erosion of high-pressure rocks in the Carpathian arc. Since metamorphic rutile requires relatively high pressure to crystallize, its formation during orogeny is indicative for a subduction setting. To better understand the closure of the Alpine Tethys Ocean in the Western Carpathians, U-Pb geochronology was applied to detrital rutile from medium grained sandstones of the Magura and Silesian Nappes. Twelve samples were collected along a transect through the Magura Nappe and three samples from the Silesian Nappe were added as a reference. Approximately 200 rutile grains were separated from each sandstone and around half of them were selected for further analysis. The dated rutile shows significant differences in age, as well as in appearance (shape, inclusions, zoning etc.) suggesting derivation from various sources.

The most prominent age peaks represent the Variscan (c. 400-280 Ma) and Alpine (c. 160-90 Ma) tectonic events, which are well-represented in all but the oldest dated sample. It is noteworthy that four distinct Alpine maxima were detected in the rutile dataset. The two most prominent peaks of 137-126 Ma and 115-105 Ma are found in the majority of the samples. In two sandstone samples, deposited in the Eocene – Oligocene and the Late Cretaceous – Paleocene, the youngest peak of 94-90 Ma appears. Another peak of 193-184 Ma is also present in these two samples, as well as in another sandstone deposited between the Paleocene and Eocene. In addition, most of the dated sandstones show some Proterozoic ages (approx. 1770 Ma, 1200 Ma, 680 Ma and 600 Ma).

Tentatively, we propose that recognizable events include the Jurassic subduction of the Meliata Ocean (~180-155 Ma), and the Cretaceous thrust stacking and exhumation of the Veporic and Gemeric domains (140-90 Ma). The abundance of Alpine rutile in all but the oldest dated sandstone suggests no physical barrier for supply of detrital material derived from the southern and central Alcapa (part of Adria) to a sedimentary basin developed north of the alleged Oravic (Czorsztyn) continental sliver within the Alpine Tethys Ocean. The lack of young Alpine ages in the oldest sandstone could be a result of either a natural boundary between the basin and the orogen or a lack of rutile-bearing rocks at the surface at that time.

In a broader sense, we propose that synorogenic deposits of the Outer Western Carpathians contain detritus from the formerly subducted, exhumed and imbricated oceanic and continental crustal domains at the southern margin of the ALCAPA microcontinent.

This research is funded by the National Science Centre, Poland, project no. 2021/43/B/ST10/02312.

How to cite: de Doliwa Zieliński, L., Bazarnik, J., Kooijman, E., Kośmińska, K., Potočný, T., Mazur, S., and Majka, J.: Detrital rutile U-Pb geochronology as a tracer of convergence in the External Western Carpathians, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3390, https://doi.org/10.5194/egusphere-egu24-3390, 2024.

17:35–17:45

Posters on site: Thu, 18 Apr, 10:45–12:30 | Hall X2

Display time: Thu, 18 Apr 08:30–Thu, 18 Apr 12:30
Chairpersons: Alexis Plunder, Marco Giovanni Malusa', Philippe Agard
X2.71
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EGU24-1504
Marián Putiš, Ondrej Nemec, Samir Ustalić, Jiří Sláma, Dražen Balen, Elvir Babajić, Ján Soták, and Peter Ružička

The Ozren and Borja-Mahnjača ophiolite complexes in Bosnia and Herzegovina are part of the Dinaridic Triassic-Jurassic ophiolite belt (Putiš et al., 2022; Minerals). Triassic oceanic crust was dated at 242±1 Ma from a relic zircon population in a plagiogranitic layer of partially melted eclogitic sole by LA-ICP-MS U-Pb method, while the main zircon population of 176±1 Ma dates the crystallization of this layer from a metamorphic-anatectic melt. The host sole (Cpx-Grt-Rt) eclogite yielded metamorphic, most likely exhumation zircon age of 168±5 Ma, while rutile gave an age of 165±3 Ma. Jurassic lower oceanic crust was dated from an isotropic gabbro (178±1 Ma, zircon) and plagiogranite (177±1 Ma, zircon). The mantle spinel lherzolites, harzburgites, and dunites are crosscut by Cpx-Pl and Amp-Pl gabbroic, gabbro-pegmatitic, leuco-gabbroic (174±1 Ma, zircon), and doleritic (174±5 Ma, apatite) dykes, all suggesting an advanced evolutional stage and a shallower level of ophiolites due to extension and the deeper mantle melting. The upper oceanic crust pillow basalts are alternating with Bajocian to Callovian radiolarites (~171-162 Ma; Ustalić, Soták et al., 2023; Newsletter of the Slovak Geological Society). The dated N-MORB type sole eclogites-amphibolites indicate the intra-oceanic subduction of the Triassic gabbroic oceanic crust to about 55-60 km that was estimated from Perple_X modelling of 1.9-2.1 GPa and 780°C. Partial melting of subducted slab and a mantle wedge initiated the formation of Jurassic supra-subduction ophiolitic complex detected at ~178-162 Ma. Inferred slab roll-back enhanced the sole extension exhumation between ~170-160 Ma that was coeval with the formation of the upper oceanic crust basalt-radiolarite section. The mineral chemistry-based discrimination diagrams of ultramafic rocks constrain an evolutional trend from MORB to supra-subduction types of ophiolites. An increased depletion of ultramafic rocks is indicated by an increase of Cr# in spinel from ~30 to 60, exceptionally to 75, suggesting transitional abyssal to supra-subduction peridotites and dunites. Relatively thin, often hydrated (Amp-rich) gabbro-dolerite layer of this ophiolite complex may have formed in a fore-arc/back-arc slow-spreading ridge. Ophiolitic breccia, with fragments of the Jurassic oceanic crust and rare Triassic radiolarites, indicates the closure of the Jurassic Neotethys from approximately 160 Ma.

Funding from The Slovak research and development agency projects (APVV-19-0065, APVV-20-0079, APVV-22-0092), VEGA agency (1/0028/24, 2/0012/24), and the RVO67985831 program is acknowledged.

How to cite: Putiš, M., Nemec, O., Ustalić, S., Sláma, J., Balen, D., Babajić, E., Soták, J., and Ružička, P.: Triassic-Jurassic ophiolites of Dinaridic Ozren and Borja-Mahnjača massifs in Bosnia and Herzegovina: Mineralogy, geochronology, and P-T estimates from subducted sole, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1504, https://doi.org/10.5194/egusphere-egu24-1504, 2024.

X2.72
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EGU24-1523
Liliana Minelli, Gaia Siravo, Fabio Speranza, Chiara Caricchi, Eugenio Fazio, Silvia Pondrelli, and Michele Zucali

The continental lower crust remains today the less known layer of the external Earth, and only relatively recently became the focus of researches addressing its structure, composition and magnetic characteristics as deduced from seismological, geophysical, geological, geochemical and petrological data. Particularly, very intense magnetic anomalies measured over cratons imply that strong magnetic source exists at lower crustal depths beneath the continents, but its nature has remained elusive so far.

One of the approaches to obtain valuable information on the continental lower crust is studying tectonically uplifted crustal cross-sections. The likely more complete continental lower crustal section exposed on Earth is the Ivrea-Verbano (IV) zone (NW Italy), considered as a petro-geophysical reference of the continental lithosphere. The IV exposes lower crust rocks of Adria (hence of African affinity) uplifted and tilted due to the Mesozoic and subsequent Alpine tectonics. Moving NW-ward along the section, originally deeper lower crust rocks are exposed, lying adjacent to the Insubric line marking the Alpine tectonic boundary. Three main lower crust types exist in the IV zone and their best exposures are along the Val d’Ossola, Val Strona and Val Sesia. Val d’Ossola and Val Strona outcrops show continental lithologies (mafic and felsic protoliths with few marbles) in both amphibolite and granulite metamorphic facies. The Val Sesia section hosts gabbros and diorites originated from a giant input of basaltic magmas underplated at crust-mantle interface in Permian times. Moving towards the Insubric line (lower part of the lower crustal section) few subordinate slices of peridotites are exposed (Megolo, Balmuccia and Finero, this latter in the northeastern most part of the IV zone). For instance, at Balmuccia (Val Sesia), a mantle slice of peridotites is tectonically embedded within the gabbros. Here seismic and gravimetric data suggest that paleo-Moho is very shallow.

We sampled the IV rocks along three sections exposed in the Val d’Ossola, Val Strona and Val Sesia at 34 paleomagnetic sites (eight oriented samples at each site) and 7 non-oriented sites (from two to eight hand-samples) for a total number of 306 samples and measured: 1) the magnetic susceptibility (k), 2) the direction and intensity of the natural remnant magnetization (NRM), 3) hysteresis loop parameters, and 4) density. These results will represent the input data for a forward magnetic model of the IV zone at a crustal scale, to be considered as an analogue for others lower continental crust settings.

These results were gathered in the frame of the Pianeta Dinamico "UNLOCK" INGV project, which aims at improving the knowledge on the structure, composition, magnetic properties and fluid content of the continental lower crust towards the mantle transition, by integrating new seismic, magnetic, mineralogical, petro-structural and geochemical data with unprecedented resolution from two worldwide known sampling localities, the Ivrea-Verbano and Serre (Calabria) lower crust sections.

How to cite: Minelli, L., Siravo, G., Speranza, F., Caricchi, C., Fazio, E., Pondrelli, S., and Zucali, M.: Magnetic characterization of the Ivrea-Verbano zone (NW Italy): A key to understand the magnetism and structure of the continental lower crust, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1523, https://doi.org/10.5194/egusphere-egu24-1523, 2024.

X2.73
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EGU24-4070
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ECS
Daniel Barrera, Giovanni Toscani, Chiara Amadori, Roberto Fantoni, and Andrea Di Giulio

The Po Plain in Northern Italy constitutes an elongated alluvial valley characterized by an intricate geological evolution, extending from the late Paleozoic era to recent times. Notably densely populated, this region accommodates approximately one-third of the Italian population and hosts critical industrial facilities, coupled with a substantial history of oil and gas exploration and production. Given these factors, the creation of a high-resolution subsurface geomodel is imperative for various applications in this region.

Tectonically, the Po Plain is located in the Adria Microplate and is bounded by two opposite verging orogens sharing the same foreland: the Northern Apennines (NA) to the south, and the Southern Alps (SA) and the Western Alps to the north and the west. The SA are a south-verging fold-and-thrust belt, while the NA are a north-northeast-verging fold-and-thrust belt; both belts have their outer thrust front buried beneath the Neogene-Quaternary sediments of the Po Plain. The front of the Northern Apennines is structured into three different arcs with increasing amounts of shortening, from northwest to southeast: the Monferrato Arc, the Emilia Arc, and the Ferrara Arc. Along the Emilia Arc, the juxtaposition of the buried Southern Alps and the buried Northern Apennines is notably close, allowing for a more detailed analysis of their frontal convergence (a few kilometers). Moreover, the influence on the thrust(s) geometry from the inherited and inverted structural highs from the passive Mesozoic platform can be observed more clearly. This combination of factors, among others, makes the central area of the Po-Plain one of the most prolific for oil and gas production, hosting several productive fields.
Despite the long story of hydrocarbon exploration and production, a large-scale comprehensive 3D model using seismic lines and well information has not yet been published, apart from a couple of very good seismic sections, that have been studied and analyzed multiple times. In particular, the Plio-Pleistocene architecture of the basin has been only partially described. In this study, we have used an extensive database provided by ENI Spa to create a high-definition static model and several balanced cross-sections to understand better the distribution of the deformation along the Emilia arc and to comprehend how the complex relationship between NA, SA, and the inherited structural highs have driven the actual architecture of the central Po-Plain subsurface. This new highly detailed 3D geomodel provides the necessary base to implement renewable energy developments (geothermic) in one of the most populated areas in Italy.

How to cite: Barrera, D., Toscani, G., Amadori, C., Fantoni, R., and Di Giulio, A.: High-resolution 3D geomodel of the central Po-Plain, Northern Italy., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4070, https://doi.org/10.5194/egusphere-egu24-4070, 2024.

X2.74
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EGU24-5735
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ECS
Yuantong Mao, Xiaobing Xu, Xiaotian Tang, Liang Zhao, Lei Yang, Stefano Solarino, Anne Paul, Silvia Pondrelli, Coralie Aubert, Simone Salimbeni, Elena Eva, and Stephane Guillot

The Western Alps are a crucial region for studying subduction-collision processes. The deep structure beneath the orogenic belts has been a topic of ongoing debate and has undergone continuous refined investigations. In this study, we utilized the most extensive dataset available, covering the period from 2012 to 2020, with 1093 stations. This dataset comprises 659 permanent stations, 110 CIFALPS and CIFALPS-2 temporary stations, along with 324 AlpArray temporary stations.

We employed the finite-frequency method to conduct inversion of the regional deep velocity structure. Meticulous waveform analyses were performed across various frequency bands for both P and S waves (P: 0.1-0.5Hz, 0.5-2Hz; S: 0.05-0.1Hz, 0.1-0.5Hz). Additionally, for regions with insufficient ray coverage, we utilized the LSBP_Alpscrust1.0 model [Lu et al., 2020], derived from ambient noise tomography, to correct crustal velocities.

We have presented for the first time the deep velocity results of S-waves, demonstrating a good consistency with the P-wave velocity structure. Additionally, we re-selected the dataset pairs for the inversion of Vp/Vs images. Our findings provide further insight into the underground structure beneath the Western Alps, uncovering the presence of a continuous subducted slab. Furthermore, in the southern part of the Western Alps, there is a potential indication of high Vp/Vs ratios within the depth range of 100-150 km.

How to cite: Mao, Y., Xu, X., Tang, X., Zhao, L., Yang, L., Solarino, S., Paul, A., Pondrelli, S., Aubert, C., Salimbeni, S., Eva, E., and Guillot, S.: Deep Structure of the Western Alps Derived from New Data — P and S wave velocity images from Finite Frequency Tomography, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5735, https://doi.org/10.5194/egusphere-egu24-5735, 2024.

X2.75
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EGU24-16584
Rens Hofman, Gesa Petersen, Jörn Kummerow, and Simone Cesca and the The AlpArray Swath-D Working Group

The installation of the temporary, large-N Swath-D seismic network in the years 2017-2019 (Heit et al., 2021) provided the basis for the recent compilation of a high-resolution, consistently processed seismicity catalogue for the eastern and southern Alps (Hofman et al., 2023). The catalogue contains more than 6,000 earthquakes with magnitudes down to −1.7 ML.

 

In the present study, we analyse in more detail several of the newly detected microseismic clusters in the study area, which includes the most active parts of the Alps as well as particularly quiet regions with very little previously reported seismicity. We combine inter-event waveform similarity clustering, catalogue statistics and rupture mechanisms to characterise the clustered seismicity swarms and mainshock-aftershock sequences. We apply a relative location technique based on differential Ts-Tp arrival times to better resolve the seismogenenic structures. For subgroups of microseismic events with magnitudes Mw 1.2-3.0, we obtain moment tensor solutions using the flexible probabilistic inversion framework Grond, which allows to combine different fitting targets and frequency bands, while providing meaningful estimates of uncertainties (Heimann et al., 2018, Petersen et al., 2021). This adds to resolve subtle, but systematic variations of the inner-cluster seismicity.

Thanks to the outstanding network density, we can report a variability of seismic sequences and microseismic event mechanisms across the study area and interpret them with in terms of long-term tectonic and intermediate triggering processes.

How to cite: Hofman, R., Petersen, G., Kummerow, J., and Cesca, S. and the The AlpArray Swath-D Working Group: Seismicity clusters in the Eastern Alps: New insights from the large-N Swath-D seismic network, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16584, https://doi.org/10.5194/egusphere-egu24-16584, 2024.

X2.76
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EGU24-21804
Riccardo Monti, Andrea Bistacchi, and Stefano Zanchetta

3D models are fundamental tools for studying the evolution of complex geological structures, such as shear zones in the metamorphic core of the Alps.
The study’s case is the area of the San Bernardino Pass (Swiss), focusing on the study of the structural and metamorphic evolution of HP units outcropping here.
The area of the San Bernardino Pass is part of the Penninic domain, an Alpine domain consisting of continental and oceanic crust derived from the distal margin of Europe, subducted during the Alpine orogeny.
In this area, the Adula nappe is in contact with the overlying Tambò nappe (part of the eastern flank of the Lepontine Dome) along a wide shear zone of several hundreds of metres.
This work is focused on the 3D modelling of the shear zone and the superposed fold system developed within the Adula nappe, in the hanging wall of the shear zone
Starting from original field data and available geological maps, structures were approximated and drawn using the open-source software QGIS to create a simplified geological-structural map.
These data are fundamental constraints for drawing geological sections using the open-source software PZero (https://github.com/andrea-bistacchi/PZero).
After careful reconstruction of serial geological cross-sections in PZero, advanced interpolation techniques such as implicit methods can be applied to develop accurate geological models.

PZero is an open-source software currently in development, dedicated to 3D geological modeling, featuring a user-friendly interface designed for structural geologists.

How to cite: Monti, R., Bistacchi, A., and Zanchetta, S.: 3D Geomodelling of Alpine structures: the Misox Shear Zone (Swiss), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21804, https://doi.org/10.5194/egusphere-egu24-21804, 2024.

X2.77
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EGU24-20494
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ECS
Simone Lombardi, Lorenzo Stori, Laura Federico, Laura Crispini, Seno Silvio, and Maino Matteo

The evolution of the European margin before and during the collisional phase of the alpine orogenesis is still a debated topic. The Western Ligurian Alps are a complex key area that can help to better understand this tectonic evolution. Here the contact between two different domains crops out: Briançonnais domain (European passive margin) and the Piedmont-Ligurian sedimentary covers (oceanic domain). These last units are characterized by several deformation stages, presented by many thrust sheets and non-cylindrical folds that make difficult to understand their relationships and their three-dimensional setting. Moreover, they are characterized by low-grade metamorphism that often masks their sedimentary structures and features, resulting in a challenging reconstruction of the pre-orogenic stratigraphic and structural setting. Previous works have hypothesized that these turbidite systems have been deposited in an abyssal plain resulted from the Piedmont-Ligurian Oceanic rifting and spreading. They are characterized by a lower part of basal complexes with thin bedded and very-thin bedded turbidites and often containing olistostromes. These basal complexes are overlain by sand- or carbonate-rich turbidite systems (Decarlis et al. 2014; Lanteaume et al.1990) that are interpreted as trench environment deposits (Di Giulio, 1992; Mueller et al. 2017). During the progressive advance of the accretionary wedge towards the European foreland, these units have migrated and stacked in reverse order, with the oldest one in the topmost part. The aim of the study is to review and integrate the previous works with new data following a multidisciplinary approach with a particular focus on the basal complexes of the flysch units. The CARG project is focused on the detailed fieldwork mapping that is already in progress with the aim of realizing the geological map of Ormea Sheet 244. During this activity, samples are collected for laboratory analysis. Specifically, petrographic characterization of samples collected in the basal complexes is currently carried out to better understand the source area of the sediments. Geochemical analyses are also in progress on basalt clasts found in the chaotic bodies. Another aim is to investigate the metamorphic grade by analysing fluid inclusions and vitrinite reflectance. Geochronological analysis will be performed with U/Pb analytical techniques on zircons to compare the results with surrounding crystalline basements to put an additional time constrain to the poor biostratigraphic data.

REFERENCES

Decarlis A., Maino M., Dallagiovanna G., Lualdi A., Masini E., Toscani G., Seno S., 2014. Salt tectonics in the SW Alps (Italy-France): from rifting to the inversion of the European continental margin in a context of oblique convergence. «Tectonophysics» 636, 293-314

Di Giulio A., 1992. The evolution of the Western Ligurian Flysch Units and the role of mud diapirism in ancient accretionary prisms (Maritime Alps, Northwestern Italy) «International Journal of Earth Sciences (Geologische Rundschau)» 81, 655-668

Lanteaume M., Radulescu N., Gavos., Feraud J., 1990. «Notice explicative, Carte Géol. De France (1/50000), feuille Viève-Tende» 948, Orleans, BRGM. 139 pp.

Mueller P., Patacci M., Di Giulio A., 2017. A Hybrid event beds in the proximal to distal extensive lobe domain of the coarse-grained and sand-rich Bordighera turbidite system (NW Italy). «Marine and Petroleum Geology» 86, 908-931

How to cite: Lombardi, S., Stori, L., Federico, L., Crispini, L., Silvio, S., and Matteo, M.: Pre to syn orogenic evolution of the Piedmont-Ligurian oceanic covers: clues on the Flysch units of the Western Ligurian Alps (CARG project – Ormea sheet 244). , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20494, https://doi.org/10.5194/egusphere-egu24-20494, 2024.

X2.78
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EGU24-5390
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ECS
Kevin Karner-Ruehl, Walter Kurz, Hauzenberger Christoph A., Harald Fritz, Gallhofer Daniela, and Etienne Skrzypek

Pre-Alpine basement units, derived from the northeastern Gondwana margin, are incorporated within the Eastern Alps and were overprinted during Alpine nappe stacking. However, some of these units were only slightly affected by Alpine metamorphism and could therefore provide significant information about the pre-Alpine history of these basement units. The Kaintaleck Metamorphic Complex as part of the Eastern Greywacke Zone and the Silvretta-Seckau Nappe System experienced greenschist facies metamorphic conditions during Eo-Alpine times and are nowadays affiliated to the Upper Austroalpine Subunit.

The Kaintaleck Metamorphic Complex comprises a mafic suite of amphibolite, garnet-amphibolite, greenschist and serpentinite, and a felsic suite, mainly composed of gneiss and mica-schist, some of them garnet-bearing. Geochemical results of metabasites indicate a tholeiitic basalt source with MORB affinity. U-Pb zircon dating of a garnet-bearing amphibolite yields an Early Devonian age of 414 ± 5.6 Ma, interpreted as age of protolith formation. Chemical U-Th-Pb dating of monazites from the felsic suite revealed Late Devonian to Early Carboniferous ages of 362 ± 6 Ma, 358 ± 15 Ma, 351 ± 4 Ma and 349 ± 3 Ma, reflecting peak metamorphic conditions during Variscan orogeny. A two-stage metamorphic history of a HT/LP and a subsequent LT/HP metamorphic event, indicated by Zr-in-rutile thermometry and thermodynamic modeling, relates the Kaintaleck Metamorphic Complex to the opening and closure of the short-lived Balkan-Carpathian Ocean and implies a correlation to other ophiolitic relicts of Devonian age, exposed in the North-Gemeric Klatov and Rakovec Complexes in the Western Carpathians. The Seckau Complex, a part of the Silvretta-Seckau Nappe System is characterized by various metagranitoids, which have been extensively analyzed in recent studies. Based on these studies, the metagranitoids of the Seckau Nappe are subdivided into the Late Cambrian to Early Ordovican Hochreichart Plutonic Suite and the Late Devonian to Early Carboniferous Hintertal Plutonic Suite. The host rock for these large intrusions is the so-called Glaneck Metamorphic Suite, which is mainly composed of fine-grained paragneiss and mica-schist, some of them garnet-bearing. U-Pb zircon ages of the paragneisses indicate a detrital origin and ages of the cores cluster in the Neoarchean, Paleoproterozoic and Ediacaran, between 2.7 Ga and 559 Ma. A migmatized paragneiss yields an age of 505 Ma, which indicates, that migmatization was probably triggered by the intrusion of the Hochreichart Plutonic Suite. The timing of pre-Alpine metamorphism can therefore be constrained to have happened between 559 Ma and 505 Ma. Some samples do show a distinct two-phase garnet growth, suggesting an additional metamorphic event possibly during Variscan times. The Schladming Crystalline Complex, also part of the Silvretta-Seckau Nappe System, again comprises paragneisses, that were intruded by various metagranitoids. In contrast to the Seckau Complex, these metagranitoids do not only show Cambrian and Late Devonian to Early Carboniferous ages, but also Permian ages.

In order to complement the knowledge of the pre-Alpine metamorphic history of the Eastern Alps, new geochronological, geochemical and geothermobarometric data from various metapelitic and metabasic rocks within the Silvretta-Seckau Nappe system are being examined to reconstruct the tectonic development of these units in pre-Alpine times.

How to cite: Karner-Ruehl, K., Kurz, W., Christoph A., H., Fritz, H., Daniela, G., and Skrzypek, E.: Pre-Alpine Metamorphism in Alpine low-grade metamorphic units in the Eastern Alps, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5390, https://doi.org/10.5194/egusphere-egu24-5390, 2024.

X2.79
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EGU24-7625
Simone Masoch, Michele Fondriest, Nereo Preto, Francesca Prando, and Giulio Di Toro

Alpine Corsica is an accretionary wedge formed during the Alpine orogenesis and exhumed through Oligocene-Miocene lithospheric extension controlled by the eastward migration of Apenninic subduction. Here we integrate field geological surveys with microstructural and carbonate stable isotope (δ18O–δ13C) analyses of fault zone rocks to constrain the evolution of the W-dipping extensional Patrimonio Fault System (PFS). The PFS consists of multiple gouge-bearing fault core strands and splay faults in the footwall damage zone, and exhumed the Schistes Lustrés (e.g., impure quartzites, marbles, calcschists, serpentines) and slices of Hercynian granitoids in the footwall block, accommodating ~6 km of cumulative displacement.

We describe a deformation sequence during exhumation consisting of D1 mylonitic shearing, D2 seismogenic faulting and D3 shallow veining events. D1 mylonitic shearing produced a decameter mylonitic zone forming the roots of PFS, coeval with localized brittle-ductile shear zones and quartz ± chlorite vein arrays observed in the footwall metamorphic units. Ductile shearing was accommodated by low-temperature quartz and calcite crystal-plasticity, and pressure-solution mechanisms at greenschist conditions (i.e., 300-400 °C). D2 seismogenic faulting either overprinted or cut the D1 structures. Ancient seismic faulting is attested by occurrence of (i) altered pseudotachylytes and (ii) cockade-bearing fault-veins injecting into the host-rocks and mutually overprinting dolomite-rich veinlet mesh and mirror-like slip surfaces observed in the footwall splay faults. Seismic faulting is also accommodated by dolomite-quartz(-chalcedony) crack-seal veins, which have isotopic compositions similar to those of the carbonate-rich units of the Schistes Lustrés. These structural and geochemical observations indicate that ancient seismicity was cyclically modulated by overpressured fluids which isotopic composition was buffered by the host-rocks. The later D3 shallow (≤ 1 km depth) veining event consists of calcite-bearing veins and concretions filling open fractures, which have distinct isotopic compositions compared to the Schistes Lustrés units, suggesting percolation of meteoric fluids at depths. Based on these observations, we speculate that the D2 faults may represent a fossil analogue of the extensional faults active in the Apennines where seismicity is driven by CO2-rich deep-sourced fluids.

How to cite: Masoch, S., Fondriest, M., Preto, N., Prando, F., and Di Toro, G.: Deformation processes and origin of fluids during Oligocene-Miocene post-orogenic extension in Alpine Corsica, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7625, https://doi.org/10.5194/egusphere-egu24-7625, 2024.

X2.80
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EGU24-9322
Giridas Maiti, Alexander Koptev, Paul Baville, Taras Gerya, Silvia Crosetto, and Nevena Andrić-Tomašević

It is assumed that slab tearing (or the lateral propagation of slab break-offs) in collisional belts controls the progressive along-strike uplift of mountains and the development of adjacent basins. However, differential continental collision due to obliquity or other irregularities of the original passive margin can introduce additional complications and influence the progressive topographic growth. Here, we use a 3D thermo-mechanical numerical modelling approach to distinguish the topographic response to slab break-off propagation from the surface uplift caused by along-strike differential collision. To this end, we examine the effects of several key factors, including (1) the obliquity of the passive margin, (2) the age of the oceanic slab, (3) the rate of convergence between colliding plates, and (4) the presence of a microcontinental block between passive and active margins. In all experiments, slab break-off initiates earlier than continental collision due to the transition from oceanic to continental subduction beneath the fore- and back-arc domain formed during the previous retreat of the subduction zone. The topographic uplift associated with slab tearing is more pronounced and spreads laterally much faster than in the subsequent collision phase. The parametric analysis shows that the lateral migration of the continental collision is controlled by the convergence rate, while the horizontal velocity of slab tearing depends mainly on the obliquity angle and slab age. The presence of additional structural complexity - a microcontinental block that has detached from the passive margin - leads to a transition from horizontal to vertical slab tearing and to more intense syn-collisional mountain growth.

How to cite: Maiti, G., Koptev, A., Baville, P., Gerya, T., Crosetto, S., and Andrić-Tomašević, N.: 3D thermo-mechanical modelling of oblique continental collision: relative role of slab tearing in along-strike topography evolution, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9322, https://doi.org/10.5194/egusphere-egu24-9322, 2024.

X2.81
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EGU24-8582
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ECS
Tanishka Soni, Christian Schiffer, and Stanisław Mazur

The Pieniny Klippen Belt (PKB) in the Western Carpathian branch of the Alpine-Carpathian-Dinaridic orogenic system is considered to be the surficial representation of the Alpine-Tethys suture. It is a few kilometres wide and about 600 km long unit between the Outer Western Carpathians and Central Western Carpathians and does not show typical characteristics of a suture (Plašienka et al., 1997; Schmid et al., 2008). In fact, the structural relationship between the PKB and surrounding units is ambiguous. The PKB is a sub-vertical unit with mainly shallow marine limestone and flysch deposits in a conspicuous “block-in-matrix” structure (Plašienka et al., 1997). This structure has been explained mainly by two theories: sedimentary structures formed by gravity sliding; and post-sedimentation tectonic shearing due to strike-slip movement affecting the heterolithic composition of the unit (Plašienka et al., 2012; Golonka et al., 2015). The presence of “exotic” sediments in the PKB and the southernmost units of the OWC along with their shallow marine deposition environment led to the theory proposing the presence of a continental sliver called the Czorsztyn Ridge in the Alpine Tethys, dividing it into two oceanic/marine basins: the Magura Ocean to the north and the Vahic Ocean to the south (Plašienka, 2018).

A passive seismic experiment was designed and installed to provide insight into the deep lithospheric structure across the PKB, testing the presence of a tectonic suture along with relaminated remnants of the Czorsztyn Ridge, and potential remnants of subducted or underthrusted lithosphere. Eighteen broadband stations have been deployed in a ~N-S transect under the umbrella of the AdriaArray initiative, cutting across the PKB and the Neotethian Meliata suture to the south. The data obtained during up to three years will complement 10 other permanent and temporary broadband stations, forming an approximate 250 km long profile and will be primarily used to perform receiver function analysis and to build structural and velocity models of the lithosphere (i.e., Schiffer, 2014; Schiffer et al., 2023) beneath the Western Carpathians.

Gravity and magnetic data will be used to construct a 3-D model of the subsurface complementing the seismic experiment. Preliminary assessment of the data has shown that the PKB is represented by an anomaly reaching at least until the 15 kms depth and, therefore, is a deep-seated feature. It leads to a tentative conclusion that the PKB’s “block-in-matrix” structure is rather of tectonic origin. The qualitative analysis of potential field data reveals the presence of three major elements in the deep basement of the northern Carpathians corresponding to the ALCAPA, European Platform, and a previously undefined wedge-shaped block under the Eastern Carpathians. The PKB follows the boundary between the ALCAPA and the remaining two domains.

How to cite: Soni, T., Schiffer, C., and Mazur, S.: Unraveling the collisional history of the Western Carpathians through deep geophysical sounding, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8582, https://doi.org/10.5194/egusphere-egu24-8582, 2024.

X2.82
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EGU24-9371
Julia Rudmann, David Colin Tanner, Michael Stipp, Hannah Pomella, and Christian Brandes

The Tauern Window (TW) in the European Alps is one of the largest tectonic windows in the world. Its formation started in the Cretaceous with subduction of the Penninic realm beneath the northern margin of Adria leading to the collision between Europe (Subpenninic) and the Adria margin (Austroalpine). The resulting Penninic and Subpenninic nappe stack was exhumed by ca. 20 km by the approach of the Dolomites Indenter (Eastern Southern Alps) in the Miocene. This last deformation stage resulted in synkinematic N-S shortening of the western TW (ca. 70 km), W-E extension and lateral extrusion towards the east. However, how the Subpenninic core (Venediger Duplex; VD) and the Penninic and Austroalpine nappes (PN and AN, respectively) in the hanging-wall were tectonically stacked, upright folded and emplaced is poorly understood. This study investigates the deformation accommodated by each major tectonic basement unit of the western TW, and contributes to a better understanding of orogenic processes in general.

We kinematically restore the cross-section of [1] along the Brenner Base Tunnel (W of the TRANSALP seismic profile) using the software MOVEtm (Petroleum Experts), focusing firstly on the VD. We choose area balancing as minimum criteria, because we do not know how much material was transported out of the plane of cross-section by extension and lateral extrusion. We integrate zircon fission-track data (ZFT) as a temporal constraint and test different geothermal gradients. Petrological data are used to define the maximum depth the VD reached at the time of indentation and as marker for the transition from brittle to viscous conditions of the felsic rocks of the VD (lowest temperature for folding). Finally, we reconstruct the hanging-wall nappes above the restored VD, thereby precisely constraining the position of the AN at that time. The surface samples taken from the AN must have reached thermal conditions between the annealing zones of apatite fission-tracks and ZFT (115°C and 180°C, respectively) as only the former system was reset in the Miocene.

We first displace the entire VD down along the Sub-Tauern Ramp below the 300°C isotherm (brittle to viscous transition of felsic rocks). For this, the geothermal gradient of 50°C/km fits well to the petrological data. ZFT ages reveal upright folding of the VD terminated at ca. 17 +/- 2 Ma. Subsequent unfolding of the gneiss cores, while conserving surface area, reveals the model to be extended ca. 70 km to the south (i.e. thus equaling indenter shortening), which means that no material left the plane of cross-section by W-E extension or lateral extrusion. However, the situation for the hanging-wall nappes is different: The total thickness of the northern limbs of the AN and the PN together is twice as much after restoration compared to today. We postulate that the extension on the Brenner Normal Fault mainly caused this tectonic thinning, which is approximately 10 km.   

References

[1] Reiter, F., Freudenthaler, C., Hausmann, H., Ortner, H., Lenhardt, W., & Brandner, R. (2018). Tectonics, 37(12), 4625-4654.

How to cite: Rudmann, J., Tanner, D. C., Stipp, M., Pomella, H., and Brandes, C.: Kinematic restoration of the western Tauern Window, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9371, https://doi.org/10.5194/egusphere-egu24-9371, 2024.

X2.83
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EGU24-12811
Tobias Diehl, Julia Heilig, Carlo Cauzzi, Nicolas Deichmann, John Clinton, Sandro Truttmann, Marco Herwegh, and Stefan Wiemer

The base data for any seismotectonic study consist of accurate and precise hypocenter information, consistent magnitude estimates, and focal mechanisms derived either from the analysis of first-motion (FM) polarities or moment-tensor (MT) inversions. In this study, we present a new baseline seismotectonic earthquake catalog of Switzerland and surrounding regions (SECOS24), which covers the Central Alps (CA) region between 45.4°N/5.6°E and 48.4°N/11.1°E. The SECOS24 catalog includes instrumental seismicity routinely detected and located by the Swiss Seismological Service (SED) between 1975 and 2024 (about 49 years). For the digital era of the SED bulletin (phase picks and seismograms available in digital form) starting in 1984, hypocenters were consistently relocated in absolute terms using a recent Pg and Sg 3-D velocity model. Starting from these improved hypocenters, double-difference relative relocations were performed at different scales (single clusters as well as at regional scales), combining differential times from manual picks and waveform cross correlations. Based on available solutions and resulting location quality, a preferred solution was selected for each hypocenter of the SECOS24 catalog, in order to provide the maximum possible hypocenter accuracy and precision for each event. The SECOS24 catalog contains about 36,000 earthquakes with magnitudes ranging between ML -0.7 to 5.3. In addition to ML, the catalog reports complementary magnitudes for a subset of events. For 71 events, an MW magnitude was derived from a revised MT inversion for events starting in 1999. For events since 2009, a spectral MW was calculated if possible. This magnitude compilation allows for the assessment and improvement of existing ML-MW scaling relations. Finally, we linked each hypocenter with the revised MT catalog as well as solutions of an augmented FM catalog, which contains 492 high-quality, manually reviewed mechanisms based on P-wave first-motion polarities.

The SECOS24 catalog is used for down-stream seismotectonic analysis of the CA region. In this presentation, we show updated maps of seismicity and moment release in the CA and their foreland. In addition, we provide updated maps of deformation regimes and stress orientations derived from the analysis and inversion of the FM data. Besides previously known features, the SECOS24 catalog reveals several new features in the CA and their foreland like newly imaged seismogenic fault zones, lateral changes in the deformation regime along the Alpine Front of the CA, and ongoing shortening at shallow crustal levels in the Jura fold-and-thrust belt. In addition, the updated stress inversion provides more stable results and, in several places, higher spatial resolution in comparison to previous studies. The SECOS24 catalog therefore contributes to an improved understanding of present-day tectonic processes in the CA region and is crucial input for next-generation seismic hazard models of the region.

How to cite: Diehl, T., Heilig, J., Cauzzi, C., Deichmann, N., Clinton, J., Truttmann, S., Herwegh, M., and Wiemer, S.: SECOS24: New insights into seismicity, deformation and crustal stresses in the Central Alps Region from a baseline seismotectonic earthquake catalog, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12811, https://doi.org/10.5194/egusphere-egu24-12811, 2024.

X2.84
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EGU24-16705
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ECS
Judith Confal, Paola Baccheschi, and Silvia Pondrelli

Complex tectonics and strong heterogeneity due to thickened crust, subducting lithosphere, and the movement of the surrounding asthenosphere can be well described by seismic anisotropy, a good indicator for active and past tectonic events. Most of methodologies adopted so far to reconstruct anisotropy have a poor depth resolution. To overcome this problem we are using splitting intensity, which is related to the energy on the transverse component of the waveform and is linearly related to the mediums elastic perturbations through 3D finite-frequency sensitivity kernels. Here, we have paid special attention to three regions: the Western Alpine orogen; the Upper Rhine Graben and the still active oceanic subduction in Southern Tyrrhenian region. We used 822 stations in the Central Mediterranean to compute 12480 splitting intensity measurements, afterwards they were inverted for depth dependent anisotropy. The 3D anisotropy models show a complex pattern in the shallower parts (60-100 km depth), becoming more aligned parallel to the slabs in the deeper parts (100-350 km depth) and influenced only by strong mantle flows. In the Upper Rhine Graben we are finally able to appoint an anisotropy pattern of NNW-SSE oriented fast polarisation directions, which are parallel to faults in the graben structure, to the lithosphere and a lower layer with orientations pointing NE-SW, to asthenospheric flow. While between 100 and 250 km depth the strength of anisotropy is very small. In the Western Alps we see complex shallow anisotropy pattern and possible mantle flow around the Alpine slab. Beneath the southern Tyrrhenian subduction system looking at the anisotropy tomography images we are able to identify circular mantle flow directions around the edge of the slab (beneath the Sicily Channel) and possible break-offs in the continuity of the slab.

How to cite: Confal, J., Baccheschi, P., and Pondrelli, S.: Changes in anisotropy with depth revealed by splitting intensity tomography beneath the Alps and surrounding regions, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16705, https://doi.org/10.5194/egusphere-egu24-16705, 2024.

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EGU24-11066
Klaus Bauer, Benjamin Schwarz, Rahmantara Trichandi, Britta Wawerzinek, Peter McPhee, and Mark R. Handy

The TRANSALP project carried out around the Millenium provided unique geophysical sections across the orogenic structure in the Eastern Alps. Active and passive seismic experiments were conducted along a 300 km long profile between Munich and Venice. From North to South, the transect covered parts of the eastern Molasse Basin, the Northern Calcareous Alps and European Alpine crust, the Peninnic units of the Tauern Window, the Periadriatic Fault System (PFS), the Dolomite Mountains and Adriatic crustal indenter, and the foreland basin of the Venetian-Friulian plain. The comprehensive data sets were used to derive seismic velocity models, structural images from processing of seismic reflection data and Receiver Function analysis, azimuthal anisotopy from shear wave splitting, and to provide constraints for density modelling with gravity data.

More recently, new geophysical, mostly seismological experiments were conducted in the Central and Eastern Alps within the framework of the priority programme "Mountain Building Processes in Four Dimensions" (4D-MB) as part of the AlpArray mission. The general scope of this programme is to image the structure of the Alps from their surface down to lithospheric depth. A multi- and interdisciplinary approach is used to improve understanding of linked processes between surface and mantle beneath mountain belts, where integration of geophysical and geological observations with modeling enable to look backward and forward in time during these processes.

In the Eastern Alps, the pre-existing geophysical transects along TRANSALP (around 12°E) and EASI (around 13.3°E) are often used as reference sections to compare and discuss new 3D and 4D models along these 2D high resolution profiles. However, there is still controversy on the interpretation of these previous cross-sections. Of particular interest are crustal structures which can be used to test the hypothesized change of subduction polarity from S-directed subduction along TRANSALP towards N-directed subduction along the EASI profile, more eastward. Hence, in our sub-project we reprocess the pre-existing seismic reflection data along TRANSALP with promising, more recently developed methods that were not applied to this data set so far. The first approach is based on the extraction and usage of diffractions for the seismic imaging of the subsurface. Controlled numerical simulations explain the workflow and demonstrate the performance of the method. Application to the northernmost part of the TRANSALP seismic line reveals a number of sub-vertical structures which match with the location of known faults and fracture systems both in the Molasse and the Northern Calcareous Alps. The second approach is based on coherency analysis of pre-stack data. For the subsequent depth migration we test a wide range of existing velocity models, both from previous work and new results from the 4D-MB project. Most prominent sub-vertical structures are imaged in the central part of the Tauern Window and around the PFS. Ongoing tests with different velocity models are used to derive robust images of these key structures in the central part of the TRANSALP profile. The results are reconciled with surface geology and other geophysical studies, and will ultimately provide additional constraints for 3D and 4D geological modeling.

How to cite: Bauer, K., Schwarz, B., Trichandi, R., Wawerzinek, B., McPhee, P., and Handy, M. R.: New insights into seismic structures around the Tauern Window and the Periadriatic Fault System from reprocessing of TRANSALP seismic reflection data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11066, https://doi.org/10.5194/egusphere-egu24-11066, 2024.

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EGU24-16122
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ECS
Johannes Rembe, Edward Sobel, Susanne Schneider, and Axel Gerdes

The Permo-Triassic shallow marine to continental deposits of the Alpine Verrucano and the Lantschfeld quartzite discordantly overly the Variscan basement in the Eastern Alps. The late Permian Alpine Verrucano is characterized by fine- to coarse-clastic (meta-)sediments with local calcareous and/or conglomeratic layers. The well sorted and more mature, early Triassic Lantschfeld quartzite is carbonate free, shows pale green to white coloring and rare conglomeratic layers. Both provide an important detrital record of post-Variscan landscape evolution. Investigations on non metamorphic Permo-Triassic units of the Northern Calcareous Alps (Haas et al., 2020) provided zircons connected to processes of the Pan-African, Cadomian and Variscan Orogenies. However, they show large disparities between different nappes. This underlines the varied character of the Variscan basement units, and a better understanding may provide interesting hints for the assignment of tectonic slivers to certain nappe complexes.

In this contribution we present detrital age spectra from the metamorphic Lantschfeld quartzite of the Lower Austroalpine Radstadt Nappe and the Upper Austroalpine Silvretta-Seckau Nappe System. By combing detrital zircon U-Pb dating with detrital rutile U-Pb and rutile geochemistry data, we can better trace the metamorphic history of the Variscan basement units contributing to the early Triassic basin fill.

Haas I, Eichinger S, Haller D, Fritz H, Nievoll J, Mandl M, Hippler D and Hauzenberger C 2020 Gondwana Research 77 204–22

How to cite: Rembe, J., Sobel, E., Schneider, S., and Gerdes, A.: Provenance analysis of the Permo-Triassic Lantschfeld quartzite in the Austroalpine of the Radstädter Tauern, European Eastern Alps, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16122, https://doi.org/10.5194/egusphere-egu24-16122, 2024.

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EGU24-16199
Simone Salimbeni, Judith Confal, Silvia Pondrelli, and György Hetényi and the DIVENet Team

As part of the ICDP-DIVE project (www.dive2ivrea.org), the temporary seismic network DIVEnet has been installed across the northeastern part of the Ivrea Verbano zone (IVZ). The DIVE project aims to find answers to fundamental questions about the lower continental crust and its transition to the mantle with two scientific boreholes and a combination of geochemical, geological and geophysical analyses. Since due to Alpine collision lower crustal rocks are at the surface, and the Ivrea geophysical body is at a very shallow depth (locally ~1±1 km b.s.l.), the site is unique and offers an excellent frame for new discoveries. The DIVE project includes a first drillhole DT-1B which has been completed in Ornavasso, and a second, currently ongoing DT-1A in Megolo. To monitor natural seismicity as well as drilling-induced noise and possible signals in the area, we have deployed DIVEnet in Autumn 2021, a temporary seismic network consisting of 13 seismometers. In September 2023, a broadband borehole instruments has been lowered in the first, completed borehole and is now recording at 250 m depth. This long-term monitoring produced a catalog of local seismicity that shows that the main seismic activity is located around the well-known principal tectonic lines of the region, i.e. the Insubric Line, which, geologically speaking, are considered as inactive. Seismic monitoring techniques have been redefined to improve the detection ability, which has become possible thanks to tested and continuously improved quality checks. Additionally we use the data for various geophysical analyses. Together with other permanent and temporary seismic stations in the region, receiver function analysis and its back-azimuthal harmonics are being calculated to get a better image of the IVZ by checking the presence of anisotropy in this anomalous body and its surrounding lithosphere.

 

 

How to cite: Salimbeni, S., Confal, J., Pondrelli, S., and Hetényi, G. and the DIVENet Team: Seismicity recorded by DIVEnet, a temporary network covering the northern Ivrea-Verbano Zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16199, https://doi.org/10.5194/egusphere-egu24-16199, 2024.

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EGU24-18831
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ECS
Alessia Tagliaferri, Enrico Pigazzi, Sabrina Morandi, Paola Tartarotti, and Filippo Luca Schenker

In the Central Alps, the Penninic domain is formed by Europe-derived crystalline basement units that experienced a complex geodynamic history. This geodynamic history spans from subduction-related HP-LT (high pressure-low temperature) at ca. 38 Ma up to Barrovian metamorphic conditions peaked at ca. 31 Ma, followed by a more or less pervasive greenschist facies re-equilibration more evident in the northern units. This history led to the piling of polymetamorphic crystalline basement nappes that nowadays are up arched forming the Lepontine dome.

The Lepontine dome is a structural and metamorphic dome characterized by a widespread Barrovian metamorphic imprint. The temperatures of the Barrovian metamorphism increment towards the south and have a concentric distribution, locally intersecting the nappe contacts.

Here, we present a critical review of petrological data from the literature within the Lepontine dome, coupled with new temperature data computed with Raman spectroscopy acquired on the E-NE margin (up to the Tambo nappe) of the Lepontine dome. This work aims to identify the finite shape of isotherms at the base and on the roof of the Adula HP nappe and to trace the peak temperature conditions according to their relation to the Adula nappe emplacement (pre-, syn- or post- deformation). Two endmembers are envisaged in the literature: (1) a history where the temperature peak is attained during deformation and, according to thermodynamic studies, evolves from a single prograde PT loop, and (2) a post-deformation thermal peak formed after the HP deformation. The spatial distribution of rocks recording these different thermo-mechanical histories and the geochronological ages of the peak thermal conditions will help to postulate coherent geodynamic scenarios.

Petrological data from the Lepontine crystalline basement nappes point to peak conditions developed during nappe emplacement. On the other hand, the metamorphism and deformation of the northern metasedimentary covers suggest that a second thermal imprint is responsible for the peak temperatures registered close to the Adula nappe. This might suggest that the heat surplus developed during deformation of the Adula nappe was diffused to the close units also after its emplacement.

How to cite: Tagliaferri, A., Pigazzi, E., Morandi, S., Tartarotti, P., and Schenker, F. L.: A critical review of petrological data in the Penninic domain of the Central Alps (Lepontine dome and its E-NE metasedimentary covers) , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18831, https://doi.org/10.5194/egusphere-egu24-18831, 2024.

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EGU24-20770
Ralf Schuster, Gerald Schuberth-Hlavač, Tanja Knoll, Heinrich Mali, and Annika Geringer

The investigated area of the Eastern Alps consists of Austroalpine basement units composed of various types of mica schist and paragneiss with intercalations of amphibolites or eclogites, marbles and quartzites. Furthermore, Permian magmatic rocks are represented by gabbros, pegmatites and the Wolfsberg orthogneiss. All rocks belong to the Koralpe nappe system and are characterized by main imprints during Permian HT/LP metamorphism and Cretaceous LT/HP overprint and nappe stacking in the course of the Eoalpine orogenic event. Geological maps show only a rudimentary tectonic subdivision, although there are indications for a complex internal structure. To study the internal structure a W-E orientated section across the Großer Speikkogel (2140 m) is most suitable. Along this section the E-dipping Cretaceous schistosity is gently folded by WNW-ESE trending axes with steep axial planes. From bottom to the top two amphibolite-facies, three eclogite-facies and again amphibolite-facies nappes can be identified. This indicates an inverted metamorphic field gradient in the footwall and an upright gradient in the hanging wall.

The tectonically deepest part crops out in the Wolfsberg Window, where amphibolite-facies rock units of the northerly-situated Gleinalpe Mountains reappear at the surface. They comprise the Vordergumitsch nappe, which is mainly built up by biotite-rich mica schist, paragneiss and amphibolite of the Klining Complex. Additionally it includes the Wolfsberg orthogneiss. Above the Pusterwald nappe is situated. It is built up by the Rappold (Preims) Complex, mainly composed of garnet-bearing mica schist, paragneiss and marble with additional amphibolite, quartzite and pegmatites. The latter show relatively low grades of fractionation.

The lowermost eclogite-bearing unit is the several hundred meters thick Brandhöhe nappe. It represents an upright section through the lower and middle part of the Permian crust. Its lower part mainly consists of paragneiss, which experienced high amphibolite-facies and anatexis during the Permian event. Usually it contains several millimetres large aggregates of fine-grained kyanite ("Disthenflasergneis"). In the upper part paragneiss with up to several decimetres long kyanite pseudomorphs appears, which developed from Permian greenschist-facies schists with chiastolitic andalusite ("Paramorphosenschiefer”). Within the metasedimentary matrix some huge eclogite bodies appear. While pegmatitic mobilisates and weakly fractionated simple pegmatites occur in the lower part of the succession, fractionated pegmatites and spodumene pegmatite dikes of the Weinebene locality are situated in the upper part. Maybe the spodumene pegmatites from Trahütten and Klementkogel are also at this level. The overlying Hoher Speikkogel nappe is characterised by migmatic paragneiss again. Its upper boundary is masked by the Plattengneis shear zone. Therein, thick evolved pegmatites occur near the base, whereas above only millimetre thick pegmatitic mobilisates appear. This transition marks the base of the overlying Deutschlandsberg nappe. In its less deformed upper part bodies of Permian gabbro-eclogite, the leucogranite of Trahütten and most probably the spodumene pegmatite of the Gupper quarry are situated.

Along the western foothills of the Koralpe Mountain ridge amphibolite-facies rock assemblages are dominated by garnet mica schist. It is yet not clear whether they represent one continuous nappe sheet or several nappe sheets.

How to cite: Schuster, R., Schuberth-Hlavač, G., Knoll, T., Mali, H., and Geringer, A.: Tectonostratigraphy of the Koralpe Mountain ridge (Eastern Alps), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20770, https://doi.org/10.5194/egusphere-egu24-20770, 2024.

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EGU24-4686
Mark R. Handy and the members of the 4D-MB and AlpArray Groups

            Teleseismic Vp tomography from AlpArray suggests that the slab segment beneath the Central Alps comprises European lithosphere, is attached to its orogenic lithosphere and extends down to ~250 km depth, in parts possibly even to the Mantle Transition Zone. This marks a first phase of partial slab detachment, probably in late Paleogene time based on comparing slab length with shortening in the Central Alps and of Adria-Europe convergence since 35 Ma. In contrast, the slab segment beneath the Eastern Alps is detached between 80-150 km depth. The age of this second phase of slab detachment is bracketed at 23-19 Ma by criteria below and by comparing vertical detachment distance with global slab sink rates.

We propose a new model of Alpine mountain-building that features the northward motion of subduction singularities above delaminating and detaching Alpine slab segments, respectively in the Central and Eastern Alps, to explain E-W differences in Oligo-Miocene structure, magmatism, and foreland sedimentation. Mountain-building began at ~35 Ma with a decrease in Adria-Europe convergence to <1cm/yr collision, causing the European slab to steepen and detach beneath both the Central and Eastern Alps. Periadriatic magmatism may have initiated prior to slab detachment due to fluxing of the cold mantle wedge by fluids from devolatilizing crust along the steepened Alpine slab. Thereafter, the Central and Eastern Alps evolved separately. Northward motion of the singularity during slab delamination in the Central Alps increased both horizontal shortening and the taper angle of the orogenic wedge, with rapid exhumation and denudation in the retro-wedge. Slab steepening and delamination are inferred to have been more pronounced in the Eastern Alps, possibly due to the greater negative buoyancy of the slab in the absence of Brianconnais continental lithosphere in the eastern part of Alpine Tethys. Slab delamination in the east drove subsidence and continued marine sedimentation in the Eastern Molasse basin from 29-19 Ma, while the western part of the basin in the Central Alps filled with terrigeneous sediments. Slab detachment beneath the Eastern Alps at ~20 Ma coincided broadly with several dramatic events in the interval 23-17 Ma: (1) a switch from advance of the northern thrust front to indentation of the E. Alps by the eastern Southern Alps along the Giudicarie Fault; (2) rapid exhumation of Penninic nappes in the core of the orogen (Tauern Window) and orogen-parallel escape of orogenic crust toward the Pannonian Basin; (3) rapid filling of the Eastern Molasse basin. These events are attributed to a northward and upward shift of the singularity to within the orogenic crust during Adriatic indentation. Eastward propagation of the uplifting depocenter in the Eastern Molasse basin is interpreted to reflect orogen-parallel slab tearing beneath the Eastern Alps. This tearing ultimately accompanied Miocene rollback subduction in the Carpathians, as inferred from the migrating depocenter around the orogenic foredeep. An possible later slab detachment event (< 20 Ma) is inferred for the Eastern Alps from 3D-tectonic balancing of the Eastern and Southern Alps (McPhee et al., this session).

How to cite: Handy, M. R. and the members of the 4D-MB and AlpArray Groups: A new 4D model of Alpine orogenesis based on AlpArray, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4686, https://doi.org/10.5194/egusphere-egu24-4686, 2024.

Posters virtual: Thu, 18 Apr, 14:00–15:45 | vHall X2

Display time: Thu, 18 Apr 08:30–Thu, 18 Apr 18:00
Chairpersons: Alexis Plunder, Ralf Schuster
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EGU24-15277
Zorica Petrinec, Lucian Mureta, and Dražen Balen

Mt. Moslavačka Gora (MG) is a small crystalline exposure in the western segment of the Sava suture zone (SSZ) that divides the Europe-derived Tisia and Dacia from the Adria-derived units. The MG differs from other crystalline exposures of the SSZ by the presence of Cretaceous LP/HT metamorphic rocks and Alpine S-type granitic pluton. Our study of geochemical variability, source characteristics and geodynamic setting is based on geochemical dataset for the two predominant Late Cretaceous granite types sampled throughout the northern and central part of the pluton: two-mica granites (TMG; Bt>>Ms) that comprise the main plutonic body and subordinate muscovite (± tourmaline) granites i.e. leucogranites s.s. (LG) that crosscut the pluton. Most of the samples are highly peraluminos granites (ASI 1.1-1.6) with high SiO2 content (70-77 wt %). They correspond to magnesian to ferroan alkali-calcic and calc-alkalic granites. Major element characteristics show decreasing TiO2 (0.42-0.03 wt %), MgO (1.09-0.04 wt %), FeOtot (2.47-0.38 wt %), Al2O3 (15.34-13.18 wt %) and CaO (1.45-0.19 wt %) with increasing SiO2, with lowest abundances in the LG type. The Zr, Th and La quantities decrease from TMG toward the LG samples, consistent with petrological observations and fractionation of accessory phases (zircon, monazite and apatite). REE patterns point to vapour-absent partial melting of metasedimentary source, presence of residual feldspar during partial melting and retention of monazite within residual biotite in the source, more pronounced in the case of LG. Our data suggests that LG samples are generated as minimum melts by reactions involving predominantly breakdown of muscovite. TMG samples show geochemical variability indicative of involvement of biotite in melting reactions. Rb, Ba and Sr content are consistent with the observed mineralogy and further corroborate low melt fraction vapour-absent or vapour-deficient melting conditions. Multiple diagrams (e.g. Al2O3/TiO2 vs. CaO/Na2O, A-B discrimination diagram) point to Pl-enriched source and higher melting temperatures for the TMG source whilst LG source corresponds to Pl-poor/clay-rich source and lower melting temperatures which is in good agreement with Zr saturation temperatures for both types (c. 730 °C for TMG and c. 650 °C for LG, respectively). Based on geochemical, mineralogical and field characteristics of Bt-dominated (TMG) and Ms (±Tur)-dominated (LG) granites, partial melting of different portions of crustal source composed of felsic igneous rock or immature metasediments under similar melting conditions seems like a plausible genetic model. Studied samples categorize predominantly as collision-related peraluminous granites. Previous research tentatively ascribed the origin of MG granitoids to partial melting induced by (localized) mafic magma underplating in a subduction/collisional setting of the SSZ. However, the presence of regionally metamorphosed metasedimentary rocks of amphibolite to granulite facies in the parts of the pluton supports the idea that localized strain heating has also contributed to the Late Cretaceous crustal melting and granite magmatism in the studied area or even had a dominant role. This is further corroborated by our geochemical data that point to derivation of TMG and LG from metasedimentary source similar to the exposed metamorphic rocks.

How to cite: Petrinec, Z., Mureta, L., and Balen, D.: Late Cretaceous peraluminous collisional granites from the Sava Suture Zone (Moslavačka Gora, Croatia): geochemical variability, source characteristics and geotectonic interpretation, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15277, https://doi.org/10.5194/egusphere-egu24-15277, 2024.