For long, the continental lithosphere considered less dense than the mantle, was not supposed to be able to subduct. Nevertheless, continental subduction has been proposed for decades as a key process occurring during the long-lasting collision between India and Asia since ~50 Ma, allowing the subduction of the continental lower crust attached to the lithospheric mantle while the upper crust thickens and forms the Tibetan plateau (e.g. Mattauer, 1986; Tapponnier et al., 2001). In this talk, I will present the available data and models of this key process for the India/Asia collision, which requires to go beyond the paradigm of slab pull as a unique driver of plate tectonics, then I will compare to the Alps.

First, the deep intracontinental seismicity of the Pamir and Hindu Kush at the western extremity of the collision system reveals two subduction zones of opposite vergence. Global P-waves tomography shows the maximum depth extent of the two distinct slabs and the maximum depth of seismicity has been modeled, compatible with continental lithosphere subduction (Negredo et al., 2007). Further regional seismic studies reveal the continental nature of the slab beneath Pamir (Schneider et al., 2013) and the underplating of the Indian lithosphere below Pamir (Mechie et al., 2012). Beneath the Himalaya, the Indian lower crust, attached to its lithospheric mantle, is bent and is underplated below southern Tibet (Nabelek et al., 2008), with eclogitization of the Indian lower crust during the bending, which density could be close to mantle density (Hetenyi et al., 2007). The Asian lithosphere in central Tibet is inferred to subduct southward down to 300 km, with no related seismicity (Kind et al., 2002; Replumaz et al., 2013). During the early Tibetan collision stage, a first episode of subduction of the Asian lithosphere likely occurred, recorded by Cenozoic volcanics (Roger et al., 2000).

However, a dynamic explanation of continental subduction is still lacking. A low-density contrast between the continental lithosphere and the mantle, as inferred during the Indian plate bending, facilitates the subduction of the continental lithosphere attached to a dense oceanic slab (Capitanio et al., 2010). At the mantle scale, analogue models show that the continental subduction could occur in a context of convergence, due to far field forces instead of subduction related forces (Replumaz et al., 2016; Pitard et al., 2018), which could be due to the long lasting oceanic subduction on both sides of the Indian continent (Bose et al., 2022).

Bose et al., 2023, doi:10.1016/j.tecto.2023.229727

Capitanio et al., 2010, doi:10.1038/NGEO725

Hetenyi et al., 2007, doi:10.1016/j.epst.2007.09.036

Kind et al., 2002, doi:10.1126/science.1078115

Kufner et al., 2016, doi:10.1016/j.epsl.2015.11.046

Mattauer, 1986, BSGF, 2

Mechie et al., 2012. doi:10.1111/j.1365-246X.2011.05278.x

Nábĕlek et al., 2009, doi:10.1126/science.1167719

Negredo et al., 2007, doi:10.1016/j.epsl.2007.04.043

Pitard et al., 2018, doi:10.1016/j.epsl.2018.08.050

Replumaz et al., 2013, doi:10.1016/j.gr.2012.07.019

Replumaz et al., 2016, doi:10.1130 /G38276 .1

Roger et al., TerraNova, 12

Schneider et al., 2013, doi:10.1016/j.epsl.2013.05.015

Tapponnier et al., 2001, Science, 294

How to cite: Replumaz, A.: From orogenic range to orogenic plateau, what evolution along the Tethys subduction zone from the Alps to Tibet?, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-90, https://doi.org/10.5194/egusphere-alpshop2024-90, 2024.

The Western Alps exhibit a non-cylindrical lithospheric structure that cannot be understood using 2-D sections. To improve our understanding of the dynamics of the W-Alps and their peripheral basins, we have synthesised the latest geological and geophysical knowledge into a lithospheric-scale 3-D numerical geomodel. This work has been conducted by a team of geologists and geophysicists in the framework of the French Geological Reference Platform (RGF, http://rgf.brgm.fr), worksite “Alps and peripheral basins” (RGF-Abp). Our 3-D crustal-scale geological model covers the area [41.5°N - 48°N; 4°E - 10°E], from the Jura and Subalpine chains to the Ligurian-Provence basin and Corsica, and from the South-East basin of France to the western Po basin. We have modelled the crust-mantle boundary and the 2 boundaries of the highly metamorphosed subduction complex of the internal Alps, i.e. the Penninic front and the Insubric line.

The high-quality 3-D S-wave velocity models computed from ambient-noise tomography using data of the AlpArray, Cifalps and Cifalps-2 temporary seismic networks were key elements in our geomodelling (Nouibat et al. 2022, 2023). We have also used the receiver-function profiles along the Cifalps and Cifalps-2 transects (Paul et al. 2022), and the recent active seismic reflection-refraction and wide-angle profiles SEFASILS (Dessa et al. 2020) and LOBSTER-P02 (Dannowski et al.,  2020) in the Ligurian-Provence basin. All previous models or data have been taken into account, including 3-D P-wave velocity models (Diehl et al. 2009; Solarino et al. 2018), the ECORS-CROP deep seismic reflection profile, Moho depth models from active and passive seismic imaging (e.g. Spada et al. 2013), a Moho depth model of the Ligurian basin from gravity inversion (Chamot-Rooke et al. 1999) and results of active seismic profiles in the Ligurian basin (e.g. Rollet et al. 2002). The digital terrain model, geological map, and geophysical models have been input in the GeoModeller software to share data in the same reference frame, and model the geological boundaries in 3-D (Calcagno et al. 2008). All available models have been carefully cross-checked along reference 2-D cross-sections to determine which Moho proxy should be picked in the 3-D Vs model. The Penninic Front and the Insubric line were picked by geologists based on their surface trace and extended to depth based on velocity heterogeneities. The geological boundaries, Moho and boundaries of the subduction complex, have then been interpolated from interpreted 2-D cross-sections to 3-D surfaces by GeoModeller.

Unlike previously published Moho models in the W-Alps, our 3-D geomodel highlights the downthrusting of the European Moho in the subduction of Europe beneath Adria, and it emphasizes its strong shape changes along the arc. The 3-D shape of the Adriatic Moho on top of the Ivrea geophysical body is also clearly highlighted. This first version of the crustal-scale geomodel of the W-Alps will be later augmented by the addition of earthquake hypocentres and focal mechanisms. It will also be used as initial model in the inversion of gravity data. Within the RGF-Abp programme, the geomodel will help to integrate results of local studies in a crustal reference frame.

How to cite: Paul, A., Bellahsen, N., Dessa, J.-X., Bader, A.-G., and Calcagno, P. and the RGF-Abp geomodel working group: A 3-D geological model of the Western Alps and Ligurian basin from joint interpretation of geological data and seismic imaging models, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-65, https://doi.org/10.5194/egusphere-alpshop2024-65, 2024.

The Swiss Geological Survey (SGS) is the competence centre for the investigation of the subsurface and georesources of the Swiss Confederation. It provides up-to-date, high-quality spatial reference data for the entire country in the form of nationwide geological 2D datasets and 3D geological models. Between 2024 and 2030, the SGS is funding the Swiss Alps 3D (SA3D) project, which consists of eight research projects involving multiple universities and aims to develop a consistent large-scale underground 3D geological model of the main contacts and structures of the Central Alps.

In this presentation we show the workflow that will be used to build the SA3D model and the project plan until 2030. The main challenge for 3D modelling in Alpine regions is the lack of subsurface data (seismic data, borehole data, etc.). However, the high relief, the sparse vegetation and the large number of scientific studies make these regions an excellent site for advanced surface-based 3D geological modelling. In addition, researchers from several universities in Switzerland and Europe, as well as the SGS, have a wide range of expertise in regional geology and 3D geological modelling. SA3D aims to bring all this know-how together in teams of people with diverse expertise. The result will be a large-scale 3D geological model validated by scientific arguments.

Based on the new Tectonic Map of Switzerland 1:500'000 (swisstopo, 2024), the target area is divided into eight 3D modelling projects according to their paleogeographic origin and structural evolution. The resulting models will be then compiled into a single large-scale 3D model. Within each project, the target structural and lithostratigraphic contacts are modelled at the equivalent scale of 1:25’000. A network of regularly spaced (1000 m) geological cross sections and scientific concepts, discussed and reviewed by the different modelling teams, are then developed to strengthen the modelling interpolation. The workflow developed for the SA3D project offers the chance to gain validation approaches for domains only weakly constrained by/ or with no subsurface data available, by generating a 3D model that integrates multiscale geological data unified by a common dataset provided by the Tectonic Map.

SA3D will generate key knowledge by establishing an experienced modelling community and 3D visualization of the main geological structures and lithostratigraphic boundaries of the Central European Alps. The development of such a project will provide a framework model of the area as a basis for higher resolution 3D geological models to be used for infrastructure planning, groundwater studies, natural hazard assessment, education and research purposes. In addition, the models will facilitate access to strategic subsurface knowledge, which is essential for the management and exploration of geo-resources and geo-energy.

How to cite: Musso Piantelli, F., Ursprung, A., Baland, P., and Baumberger, R.: Swiss Alps 3D: building a large-scale 3D underground model of the Central European Alps, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-49, https://doi.org/10.5194/egusphere-alpshop2024-49, 2024.

In the Western Alps, rocks belonging to the fossil subduction zone are exceptionally well exposed, and structures related to the (U)HP exhumation stage are still preserved. Some recent studies have analyzed the along-strike variations in the deep tectonic structure of the Western Alps, but analysis was not extended southward to the Ligurian Alps, where geodynamic reconstructions have predicted the strongest upper-plate divergent motion that may have favored exhumation of (U)HP metamorphic rocks and associated mantle-wedge rocks. Here we analyze the deep tectonic structure of the Ligurian Alps as revealed by the first receiver-function profile and a new local earthquake tomography model based on data collected during the passive seismic experiments CIFALPS and CIFALPS2. We provide evidence for an exhumed mantle wedge and a former subduction channel preserved at shallow levels beneath the Ligurian Alps, above a shallow-dipping lower-plate Moho. We found that the lower boundary of the exhumed subduction channel is the most evident seismic-velocity interface beneath the Ligurian Alps, which may be easily misinterpreted as a Moho. Similar Moho-like interfaces are found beneath the exhumed (U)HP domes of eastern Papua New Guinea and the Dabie Shan, which suggests that the results of the CIFALPS experiments may be used as a reference case to improve the interpretation of the deep tectonic structure of other (U)HP terranes worldwide.

How to cite: Malusa', M. G., Solarino, S., Eva, E., Paul, A., Guillot, S., and Zhao, L.: Geological interpretation of the CIFALPS2 seismic tomography data in the Ligurian Alps, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-79, https://doi.org/10.5194/egusphere-alpshop2024-79, 2024.

Positive P-wave velocity V_p anomalies beneath the Alps resolved with teleseismic tomography (TEL, Paffrath et al. 2021) and local earthqake tomography (LET, Jozi Najafabadi et al. 2022) from AlpArray have geometries that are reminiscent of drips rather than slabs. These slab drips are still attached to the European orogenic lithosphere in the Central Alps, but appear to be partly detached in the Eastern Alps (Handy et al. 2021). The amount of Neogene crustal shortening during north-directed Adriatic indentation of the latter corresponds to the length of the slab drip within the limits of vertical resolution (20-50 km) down to a depth of 300 km (McPhee & Handy, in press). This allows us to establish the ages of the drip (14-21 Ma) and of a horizontal negative V_p-anomaly (≤ 14 Ma) separating this drip from its orogenic lithosphere.

Using these ages, we estimate the sink rate of the hanging part of the European slab drip to be 6-11 mm/yr. A higher sink rate (32 mm/yr) is obtained by using the clockwise migration of depocenter uplift and thrusting in the foreland basin around the Carpathian arc as proxies for the timing of slab detachment from the European orogenic lithosphere (Meulenkamp et al. 1996). Accordingly, the slab drip detached between 19 and 11.5 Ma and is currently entrained in the Mantle Transition Zone, MTZ (Wortel & Spakman 2000). Taken together, these relations indicate that the sink rate of slab drips increased with depth and time. However, we are unable to ascertain whether these drips reached a terminal sink velocity.

The lack of Neogene magmatism in the Alpine orogen suggests that detachment and sinking of the slab drips occurred in the absence of melting of the mantle. Taking the sink rates above at face value, we apply a modified form of the Stoke’s Law equation to obtain a dynamic viscosity of some 10⁸ MPa-s for asthenosphere undergoing solid-state flow around the slab dripping beneath the Eastern Alps. This is several orders of magnitude greater than the generally cited range of asthenospheric viscosities in subduction settings (10¹² – 10¹³ MPa-s) as well as viscosities obtained for polycrystalline dunite undergoing grainsize-insensitive creep (10¹¹-10¹³ MPa-s, grainsize = 1cm), but similar to viscosities for grainsize-sensitive creep (10⁹ MPa-s, grainsize = 50 microns, Handy et al. 1989) in the temperature range (1270-1345 °C) of the aforementioned negative V_p-anomaly (≤ 14 Ma) under the Eastern Alps. While we can only speculate on the temperatures and syntectonic grainsize in the asthenospheric mantle beneath the Alps, our findings indicate that suborogenic asthenophere is anomolously weak, even in the absence of melting, and facilitates the rapid detachment and sinking of slabs at the end of convergence and indentation.

How to cite: Handy, M. R.: Mantle rheology inferred from seismic tomography and foreland basin uplift around the Alps-Carpathian arc, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-69, https://doi.org/10.5194/egusphere-alpshop2024-69, 2024.