Oceanic detachments are large-offset normal faults along the flanks of mid-ocean ridges. They represent a mode of accretion of the oceanic lithosphere that is fundamentally different from classical “magmatic” models, resulting in lithospheric composition and structure that are strikingly different from the classical model of a layered magmatic oceanic crust. Oceanic detachments, which exhume deep lithosphere, forming oceanic core complexes (OCCs), are scientifically interesting because they represent tectonic windows to deep-seated rocks and processes (mantle flow, melt generation and migration, strain localization, and crustal accretion) at mid-ocean ridges (Escartin & Canales, 2010).

In this ongoing study, we propose to compare two different ophiolitic series from the Tethys realm: The Chenaillet (Western Alps) and the Troodos (Cyprus) ophiolites. The two ophiolitic massifs are well known for the quality of their outcrop and the evidences of oceanic processes are still well preserved. This is particularly evident in their magmatic, tectonic and hydrothermal structures and textures. Both massifs have been little affected by events subsequent to their emplacement, excepted by the formation of some folds on a more or less large scale. In both cases, detachment-type intra-oceanic tectonics have been described in the literature (Troodos: Hurst et al. 1994; Chenaillet: Manatschal et al. 2011). Furthermore, both have been interpreted as oceanic core-complexes on the basis of isotopic (δD, δ¹⁸O) studies (Troodos: Nuriel et al. 2009; Chenaillet: Lafay et al. 2017).

This interpretation is identical for two ophiolitic massifs, despite significant differences in their crustal structure and geodynamic settings. The Troodos massif represents a complete supra-subduction ophiolitic series, characterised by the presence of well-developed layers of oceanic crust. In contrast, the Chenaillet massif, originating from the Piemont ocean, is predominantly composed of serpentinite and basalt, with widespread distribution of gabbros.

The objective of this study is to gain insight into the dynamics of these detachments by analysing new field data, petrological data and geochemical data. This will enable us to understand the relationship between the detachments and magmatic and hydrothermal processes. It can be demonstrated that, despite exhibiting certain similarities, these two detachments are in fact quite distinct and do not play the same role in the formation of the oceanic crust. The Chenaillet Massif provides an illustrative example of an OCC, whereas the detachments of the Troodos Massif post-date the formation of the oceanic crust and are linked to ridge jumps. 

How to cite: Bousquet, R.: Oceanic detachments in Tethys realm: core complexe or not?, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-78, https://doi.org/10.5194/egusphere-alpshop2024-78, 2024.

The Ligurian-Piedmont Ocean (LPO) is inferred as a relatively narrow oceanic basin in palaeogeographic restorations, but the actual amount of oceanic lithosphere generated and the timing of magmatic accretion are still debated. Magmatic ages obtained from LPO intrusive rocks predominantly range between 165 and 160 Ma, supporting the interpretation of the LPO as a magma-poor ocean. However, this relatively short timespan of magmatic accretion may also suggest that the orogen sampled older sectors of the oceanic lithosphere, while younger (and more oceanward?) sectors could have been deeply subducted without returning.

We therefore focus on studying a poorly-known stack of oceanic lithosphere (i.e., the Susa and Lanzo Valley Ophiolites; SLVO), which is exposed in the inner-central sector of the Western Alps and tectonically juxtaposed with the Gran Paradiso and Dora-Maira massifs. The SLVO were metamorphosed under eclogite-facies peak conditions and consist of large volumes of serpentinite hosting up to kilometer-sized metagabbro bodies, with Fe-Ti-rich differentiated masses and rare metaplagiogranite dykes. The metaophiolite sequence also includes widespread metabasaltic rocks and a metasedimentary cover consisting of minor quartzite and marble levels overlain by calcschist.

Two pairs of Fe-Ti metagabbro and metaplagiogranite s.l. sampled close to the Avigliana (lower Susa Valley) and Mondrone (middle Ala Valley) localities have been selected for zircon U-Pb dating. In each sample, the dated zircons yield magmatic ages falling within the uppermost Jurassic Period (~150 Ma). The common age, along with similar major and trace element compositions, suggests a cogenetic origin within differentiation trends for the two pairs of metagabbro-metaplagiogranite (De Togni et al., 2024). Consequently, the SLVO were sampled from a sector of the LPO characterized by magmatic activity at ~150 Ma, significantly younger than most of previously reported ages for the LPO magmatism. We argue that the SLVO represent the youngest oceanic lithosphere accreted in the Western Alps and they may provide new constraints on the structural architecture of the LPO.

De Togni, M., Balestro, G., Rubatto, D., Castelli, D., Gattiglio, M., & Festa, A. (2024). Late Jurassic magmatism in the Ligurian-Piedmont Ocean constrained by zircon ages of mafic and felsic meta-intrusives. Terra Nova, 00, 1–11. doi.org/10.1111/ter.12723

How to cite: De Togni, M., Balestro, G., Rubatto, D., Castelli, D., Gattiglio, M., and Festa, A.: New geochronological data from mafic and felsic meta-intrusives of the Western Alpine Ophiolites: the missing magmatism of the Ligurian-Piedmont Ocean?, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-44, https://doi.org/10.5194/egusphere-alpshop2024-44, 2024.

It is now well established that the tectonic structure and thermal properties inherited from the orogens and rifting play an important role in the subsequent collision. This study focuses on the thermal inheritance of crystalline massifs from the SW Alps (Pelvoux and Maures-Tanneron) and their geodynamical implications during the Mesozoic continental rifting. Thermochronometers, including U-Pb/Apatite, Zircon fission tracks (ZFT) and Apatites fission tracks (AFT), (U-Th)/He on zircon and apatite (Zhe, AHe), their QTQt modelling and Rb/Sr dating on phengite in one shear zone, show successive tectonic events. The ZFT in the Pelvoux indicates a complex thermal history with central ages ranging from 158 to 45 Ma, thereby revealing significant resetting and cooling in the Jurassic and Eocene periods. The thermal modelling of a separate block of the massif highlights a thermal history emphasized by three distinct periods of: (1) Jurassic-lower Cretaceous heating associated with the Alpine Tethys and Valaisan opening, (2) pre-Alpine upper Cretaceous to Priabonian cooling linked to tectonic inversion of the European margin, which agrees with onset of Pyreneo-Provençal phase of shortening during the upper Cretaceous as revealed by a Rb/Sr age of 79.7 ± 3.7 Ma in an E-W (top-to-the-South) shear zone, (3) Miocene Alpine cooling/exhumation event. In contrast, in the Maures-Tanneron Massif multiple thermal events are highlighted by thermochronology, including (1) a cooling phase at approximately 200 Ma associated with CAMP volcanism preserved in the Tanneron massif, which is followed by (2) a Mesozoic (120 Ma) cooling event, after which the massif remained close to the surface until a final Eocene cooling phase. These results provide insights on how the architecture of rifted domains of the European margin in the wide plate boundary between Adria, Iberia and Europe controlled exhumation between the Alps and the Pyrenees-Provence orogenic systems.

How to cite: Boschetti, L., Mouthereau, F., Schwartz, S., Rolland, Y., Milesi, G., Munch, P., Bernet, M., and Balvay, M.: Mesozoic tectonic events in the southern European basement revealed by thermochronology - insight for margin paleogeography (Pelvoux and Maures-Tanneron massif), 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-16, https://doi.org/10.5194/egusphere-alpshop2024-16, 2024.

Paleogeographic reconstructions of the Alps for the Jurassic suggest an important role of transform faults during rifting and drifting of the Piemont-Liguria and Valais oceans. A common characteristic of such reconstructions is a roughly S- to SW-trending southern segment of the ocean where rifting and drifting was orthogonal to its margins, and an E- to ESE-trending eastern part controlled by transform faults. In the N or NE, Adria (or Apulia) would be delimited by a transform margin against the ocean. According to published sequential reconstructions, these transform faults were active from the Jurassic to the early Late Cretaceous (Cenomanian). Here we investigate the impact of such transform faults on the tectonics of the Eastern Alps.

The Northern Calcareous Alps (NCA) are the Cretaceous external thin-skinned foreland fold-and-thrust belt of the Cretaceous Alpine orogenic wedge. The NCA were detached from their basement at an salt-bearing evaporitic décollement. Thrust sheets in typical foreland settings form salients if salt-floored (e.g., Subalpine Chains of southern France, Jura fold-and-thrust belt, South-Central Pyrenees).

In contrast, the northern margin of the externmost NCA is straight. Folds tend to be tight and symmetric, and local thrusts verge both to the N and the S and are upward concave. Such a structural style has been observed in wrench zones, suggesting that the northern margin of the NCA is a wrench fault. Kinematics of this wrench zone is sinistral. This wrench zone duplicates the hanging wall cutoff of one of the NCA thrust sheets. Therefore, the wrench zone postdates initial nappe stacking in the NCA. Based on overthrust sediments, local stacking has an Early Albian age. The age of the wrench zone can be dated by Albian basanitic dykes intruded into wrench faults within the NCA which are parallel to the NCA northern margin.

At several places within the NCA, sinistral shearing across roughly E-W trending faults has been observed. We list three faults here, but there are probably more: The Stanzertal fault delimits the Austroalpine basement (Silvretta nappe) against the southern margin of the NCA. Quartz fibres on outcrop-scale faults probably formed during peak metamorphic conditions in the Lower Cretaceous. The sinistral Puitental fault S of Zugspitze is intruded by Albian basanitic dykes (as the Stanzertal fault). The Trattberg fault S of Salzburg is a transpressive sinistral fault, associated with Late Jurassic folding and transpressive thrusting, and Upper Jurassic growth strata are observed in the synclines. During the early Cretaceous, kinematics changed to sinistral transtension.

In summary, there is evidence for sinistral shearing across E-W faults at the northern and southern boundaries of the NCA, and within. These faults have a Late Jurassic to Early Cretaceous age, and were active intermittent with Alpine shortening. Locally, they channelized basaltic melts from the subcontinental mantle to the surface, as in present-day transform zones (e.g., Dead Sea transform, Atlas system). Such faults pre-determined the shape of NCA. According to previous studies also Cretaceous subduction initiated along such a transform fault.

How to cite: Ortner, H. and Sieberer, A.-K.: Jurassic-Cretaceous transform faults control the present-day shape of the Eastern Alps, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-50, https://doi.org/10.5194/egusphere-alpshop2024-50, 2024.

The supercontinent Gondwana, assembled during late Neoproterozoic, was delineated by peripheral accretionary orogens and associated magmatic belts. The timing and geodynamic setting of the peri-Gondwana magmatism remain debated, especially in case of the Avalonian–Cadomian belt that straddled Gondwana´s northern margin. A great deal of this magmatism can be attributed to the Cadomian orogeny, however, the magmatic activity continued into the Ordovician. It has been well established that the Ordovician magmatism was widespread as abundant volcano-plutonic complexes are now found in almost all Cadomian terranes in the Variscan and Alpine orogens. Two different models have been proposed to explain the Ordovician magmatism in Western and Central Europe. One model, mainly based on observations in the Alps, invokes subduction and slab-rollback underneath north Gondwana (the Cenerian orogeny). The other model, based on work in the Bohemian Massif, assumes a hyperextended passive margin and rifting generated by a mantle plume or far-field slab pull. Whereas a large body of data exists for the latter, the Ordovician magmatism in the Alps remains worth of further investigation. In this study, we present new U­–Pb zircon ages from the Ötztal nappe, composed of a metasedimentary complex which encloses bodies of metagranitoid rocks. The U­–Pb detrital zircon age spectra obtained from paragneisses suggest deposition during the latest Ediacaran to Cambrian and were sourced from basement characterized by minor Archean and Paleoproterozoic, more abundant Tonian–Stenian, and dominant Ediacaran (Cadomian) ages, presumably the Arabian–Nubian shield. The metagranitoids, ranging in composition from tonalite through granodiorite to granite, were previously categorized into 5 groups and their intrusion ages (U–Pb on zircon) could now be constrained as follows: Group 1 (M-, I-, and A-type granitoids associated with metabasites) at ca. 500 Ma, Group 2 (Winnebach S-type granodiorite/tonalite) at ca. 640 Ma, Group 3 (Sulztal S-type granite suite) at ca. 470 Ma, Group 4 (Alpeiner I-type granitoids) at ca. 470 Ma, and Group 5 (Bassler S-type granite suite) at ca. 470 Ma. In addition, a mafic eclogite yielded zircon ages at ca. 480–450 Ma and a Variscan overprint at ca. 340 Ma. Although the timing of Ordovician magmatism is apparently similar in the Bohemian Massif and the Alps, we highlight the possibility of different geodynamic causes: in particular, we envisage a curved geometry of the north Gondwana margin, where rifting in its western segment (Bohemian Massif) was broadly coeval with subduction along its eastern segment which included also the present-day Ötztal nappe.

How to cite: Žák, J., Svojtka, M., Sláma, J., Zurbriggen, R., Schindlmayr, A., Vacek, F., and Finger, F.: New geochronologic constraints on the timing and geodynamic setting of Ordovician plutonism in the Ötztal nappe of the Eastern Alps, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-13, https://doi.org/10.5194/egusphere-alpshop2024-13, 2024.

Multiple rifting phases can strongly influence the structural architecture and stratigraphic evolution of a developing passive margin. Some stratigraphic intervals can be characterised by distinct changes in thickness, lithology and facies controlled by synsedimentary faults. These features profoundly modify and alter the classic “layer-cake” model. The Central Southern Alps, and the Lecco area in particular, are a first-class example of interaction between inherited and contractional structures (Gaetani & Jadoul, 1987). A Ladinian rifting phase caused the coexistence of both deep- and shallow-water successions of Middle Triassic age, as well as considerable changes in their thickness across the study area. During the Early Jurassic, another rifting phase caused the drowning of the Late Triassic to Hettangian carbonate platforms, leading to the formation of intra-basin structural highs and lows as well as extreme lateral thickness variations within the syn-rifting succession. The most striking evidence of the role of inherited structures during Alpine contraction are N-S trending transverse zones parallel to the main orogenic transport direction (Schönborn, 1992). During orogenic build-up, rift-related faults were passively transported along thrusts, preserving part of the post-rifting, pre-orogenic framework within the same tectonic unit or were reactivated as large displacement transfer faults separating tectonic sectors with different shortening. These transverse zones and the inherited, pre-orogenic structural architecture strongly influenced thrust development: lateral ramps, oblique thrusting, younger-on-older-relationships and lateral transfer of displacement occur throughout the entire study area. Geological mapping and structural analysis have been conducted to reconstruct kinematics and geometries of fault zones. Several geological cross-sections have been realized to constrain the fold-and-thrust belt geometry and reconstruct the structural evolution of Central Southern Alps. The complex pre- and syn-orogenic tectonic history of fault activity, particularly of the main thrusts and transverse zones, has been constrained from in-situ U-Pb dating of syn-tectonic carbonates. Inorganic thermal indicators were used to constrain the eroded overburden and the exhumation depth of the faulted succession. Another goal of our work is to reveal how fluid circulation may change from the high-angle dipping, inherited and misoriented transverse zones to the low-angle thrust faults, from the internal to the frontal sectors of the belt. C-O stable isotopes and clumped isotopes analyses on syn-tectonic carbonates collected along thrusts and transverse zones have been performed to assess fluid-host rock chemical and thermal (dis)equilibrium. We compare compressional mineralizations with those exposed in transverse zones where fluids might circulate in an open to semi-open system, with the ingress of cold (meteoric) and/or hot (deep) fluids.

Gaetani, M., and F. Jadoul. "Controllo ancestrale sui principali lineamenti strutturali delle Prealpi lombarde centrali." Rendiconti della Società geologica italiana 10 (1987): 21-24.

Schönborn, G. (1992b). Alpine tectonics and kinematic models of the central Southern Alps. Memorie di Scienze Geologiche, 44, 229–393.

How to cite: Fiorini, A., Tavani, S., Aldega, L., Bernasconi, S. M., Dallai, L., Kylander-Clark, A., Rocca, M., Zanchetta, S., Zanchi, A., and Carminati, E.: Control of pre-orogenic inherited faults on Alpine thrusting and fluid circulation: an example from transverse zones of the Central Southern Alps (Lombardy, Italy), 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-12, https://doi.org/10.5194/egusphere-alpshop2024-12, 2024.

During the Cenozoic evolution of the Alps, the Adriatic plate is traditionally considered as a rigid indenter. However, in the eastern Southern Alps (ESA) of Italy and Slovenia, significant internal deformation is observed within the northernmost part of the Adriatic plate. Predominantly Miocene shortening is accommodated within a WSW-ENE striking, SSE-vergent fold-and-thrust belt, which overprints a pre-existing platform-basin geometry formed during Jurassic extension. Jurassic basins show a remarkable bend in eastern Italy and western Slovenia (i.e., Carnic and Julian Alps, respectively), where the N-S striking Belluno basin transitions into the E-W striking Slovenian basin north of the Friuli platform. The influence of this inherited basin geometry on Miocene shortening kinematics and geometries remains a topic of ongoing debate and is the focus of this study.

In this contribution we present a new series of 12 crustal-scale analogue models designed to investigate how inherited lateral crustal heterogeneities and basin geometries affect internal deformation within the ESA. The brittle and brittle-ductile analogue experiments for inversion parallel to the axes of pre-defined basins (areas of lower mechanical strength compared to accompanied platforms) particularly focus on the northeastern basin connected orthogonally to the eastern basin. Key parameters studied include the width of the northeastern basin (5 cm vs. 10 cm). For the case of oblique inversion experiments, contraction angles of 20° and 110° to the strike of the main basins and the northeastern basin, respectively, were applied. This approach allows us to test the influence of inherited basin widths and geometries on the style and timing of deformation within the evolving fold-and-thrust belt.

Our preliminary experimental results indicate that narrow northeastern basins primarily undergo inversion and are transported piggyback, leading to the formation of numerous faults in eastern model areas. The eastern platform south of the northeastern basin tends to be incorporated into the thrust belt. Increasing the width of the northeastern basin results in the eastern platform acting more as a barrier, thereby restricting the resulting fold-and-thrust belt to a smaller N-S extent. The latter is especially pronounced in oblique inversion experiments with large northeastern basins.

To compare analogue modelling results with deformation in the ESA, structural fieldwork was conducted along major fault systems within the eastern part of the ESA, specifically east of Lozzo di Cadore. The Dof-Auda-, Pinedo-Uccea- and Barcis-Staro Selo faults are overall SSE-vergent but show variations in strike direction across platform-basin boundaries. Comparative analysis of map-view observations from the ESA and oblique analogue experiments, particularly those including large northeastern basins, emphasises the significant influence of the eastern (i.e., Friuli) platform on the deformation style of the fold-and-thrust belt.

How to cite: Sieberer, A.-K., Willingshofer, E., Klotz, T., Ortner, H., and Pomella, H.: Relation between inherited basin size and fold-and-thrust belt deformation style in crustal-scale analogue models: implications for the evolution of the European eastern Southern Alps, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-27, https://doi.org/10.5194/egusphere-alpshop2024-27, 2024.