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.

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.

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.

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.

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 M_L.

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.

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.

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.

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.

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.

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.

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.

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 MOVE^tm (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.

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 M_L -0.7 to 5.3. In addition to M_L, the catalog reports complementary magnitudes for a subset of events. For 71 events, an M_W magnitude was derived from a revised MT inversion for events starting in 1999. For events since 2009, a spectral M_W was calculated if possible. This magnitude compilation allows for the assessment and improvement of existing M_L-M_W 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.

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.

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.

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.

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.

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.

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.

            Teleseismic V_p 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.

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 %), FeO_tot (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.