We investigate both the deep crustal structure of the Western and Central Alps orogenic wedge and the timing and amount of convergence accommodated since 32 Ma. The new structural interpretations are based on the most recent geophysical models (Vs and Vp tomography mainly) coupled to geological surface information. We show that first-order similarities in collision kinematics can be described from the Western to the Central Alps. After the subduction-collision transition (37-32 Ma), from around 32 Ma and until 22-20 Ma, the shortening consists of distributed deformation throughout the doubly verging orogenic wedge. From around 20 Ma until recent times, the orogen was controlled by localized west- or northwest-verging thrusts below the External Crystalline Massifs. This probably witnesses localization processes in the proximal European crust (i.e., below the Penninic Frontal Thrust) on a 10 Myr timescale. These structures (both distributed and localized ones) root in middle- to lower crustal low velocity (Vs) zones interpreted as a thick shear zone acting as a deep, crustal decollement. The low seismic velocity is most probably controlled by active fluid circulations, structural anisotropy, and/or metamorphic Alpine paragenesis (amphibolite facies). Thus, the 10 Myr timescale may correspond to characteristic time for the localization processes within the deep, ductile decollement.

Along-strike significant differences from Western to Central Alps can also be highlighted. Beyond collisional magmatism and amphibolite facies metamorphism only present in the Central Alps, kinematical differences can be quantified. In the Western Alps, after the first phase of collision, at around 20 Ma, the orogenic wedge consisted in a West-verging wedge while in the Central Alps, North- and South verging structures remained active. These differences imply significant contrasts in terms of convergence rates that can be quantified through balanced cross sections with realistic inherited Mesozoic structures. In Central Alps, convergence rates were about 1.2 +/- 0.2 cm/yr from 32 to 22 Ma and about 0.3 +/- 0.1 cm/yr from 22 to 0 Ma. This strongly suggests that before collision s.s., i.e. before 32 Ma, the convergence rate was higher than 1.2 cm/yr.

While similarities in terms of structural styles and kinematics in both parts of the orogen most likely reflect crustal rheology and localization processes, the differences allow discussing the influence of both the inherited Mesozoic structure and the kinematics of Adria after the subduction phase.

How to cite: Bellahsen, N., Rosenberg, C., Paul, A., Nouibat, A., Girault, J. B., Huet, B., Sonnet, M., Labrousse, L., Jolivet, L., Agard, P., Marquer, D., Bernet, M., and Pik, R.: Deep structure and collisional processes in the Western and Central European Alps, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-9, https://doi.org/10.5194/egusphere-alpshop2024-9, 2024.

Collisional dynamics, exhumation rates, and the large-scale geometry of orogenic belts are dependent on the relative strength contrast between colliding plates. In the Central Alps, the strong Adriatic lower crust indents into the thickened European upper crust, composed of stacked slices of weak, upper continental crust. The geological factors controlling this weak rheology and the timing of weakening are still debated.

To provide further constraints on what makes the European continental crust so weak, we have investigated the structural and tectonic evolution of the Rotondo granite through integrated field, microstructural, and in-situ (U-Pb on garnet, Rb-Sr on mica) petrochronological analyses. The Rotondo granite, an early Permian peraluminous granite (295 Ma, Rast et al., 2022), represents a strong inclusion in the polymetamorphic Gotthard nappe in the Swiss Central Alps. We have identified a sequence of four (D1-D4) main classes of deformation structures developed during the pre-Alpine, Alpine collisional, and exhumation history of the nappe.

D1 structures include brittle breccias, cataclasites and shear fractures, occurring pervasively throughout the pluton and pre-dating the Alpine peak metamorphic conditions. Garnet overgrew the brittle deformation fabric during Alpine peak metamorphic conditions at 580 ± 25 ºC and 0.9 ± 0.1 GPa at different times from 34 to 20 Ma (in-situ U-Pb dating on garnet). The following Alpine exhumation is recorded through the development of D2 reverse, ductile shear zones at 520 ± 40 ºC and 0.8 ± 0.1 GPa around 18-20 Ma (in-situ Rb-Sr on white mica). Exhumation perdured until 14 Ma, as inferred from in-situ Rb-Sr on synkinematic micas of D3 strike-slip brittle-ductile shear zones developed at 395 ± 25 ºC and 0.4 ± 0.1 GPa. The latest stages of upper crustal, brittle tectonics are shown by the development of D4 zeolite- and gouge-bearing fault zones at < 13 Ma (K-Ar illite dating).

This tectonic evolution is common to many other crystalline massifs of the External domains of the European Alps, and allow us to propose some large-scale implications on the rheological behavior of the continental (upper) crust during Alpine collision. The 34-20 Ma range of ages obtained from in-situ U-Pb dating on garnet suggests that the peak metamorphic conditions in the area likely persisted for more than 10 Myrs. After the peak, exhumation occurred at relatively fast rates (~3 mm/yr), and the internal deformation of the nappe was accommodated by weak ductile shear zones, localized on pre-existent (inherited) structural features. Indeed, meso- and microstructural considerations suggest that these shear zones were capable of sustaining differential stresses not larger than 10 MPa during collision and exhumation. The strength of the undeformed granite, limited by tensional veining, has been estimated to not exceed 60 MPa. This also demonstrates that the main weakening event of the crust occurred at retrograde conditions, during exhumation, after residing for a prolonged period of time at peak metamorphic conditions.

References:
Rast et al. (2022).  Swiss Journal of Geosciences, 115(1), 8. https://doi.org/10.1186/s00015-022-00409-w

How to cite: Ceccato, A., Behr, W. M., Zappone, A. S., Tavazzani, L., and Giuliani, A.: The structural evolution of the Rotondo granite (Gotthard nappe, Central Alps): constraints on the strength and timing of weakening of the European upper crust during Alpine collision, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-15, https://doi.org/10.5194/egusphere-alpshop2024-15, 2024.

The external parts of mountain belts, including their foreland basins, classically present a fold-and-thrust belt often detached on shallow decollement levels. These areas exhibit complex geometries with significant non-cylindrical components, necessitating a 3D approach to accurately determine the timing and style of deformation in the external zones.

The southwestern Alpine orogenic front is mainly characterized by the Digne Nappe, which thrusts over the deformed Mesozoic units. These Mesozoic units are unconformably overlain by the Cenozoic molasse deposits of the Valensole foreland basin, which are also deformed.

Despite the well-constrained sedimentary series of Barles and many of its structures, no study has yet fully explained the complex 3D geometries and processes that led to their formation. This region serves as an exceptional 3D example of a folded foreland, capturing much of the syn- and post-collisional history of the Alpine orogeny. The structural style, timing, and presence of salt structures remain challenging to specify, largely due to the non-cylindrical geometries that complicate simple 2D reconstruction. The Velodrome fold, formed by the initial deposits of the Valensole foreland basin, exemplifies a non-cylindrical structure whose understanding is still incomplete, leading to debates and various interpretations, including growth fold, post-sedimentary fold, and salt mini-basin.

To provide an accurate depiction and interpretation of the 3D geometries of the structures and to better characterize the style and timing of deformation in the Digne region, a combined approach of detailed structural field study and 3D geometric modeling using GeoModeller ©BRGM was undertaken. The 3D modeling was conducted at two scales: (i) regional, encompassing the Digne Nappe, the Robine unit, the Barles half-window, and the Valensole Basin, and (ii) more local, focusing on the Velodrome syncline. For the latter, GeoModeller was utilized to test hypotheses proposed in the literature. This approach enabled the reproduction of field-observed geometries in 3D, offering an interpretation of all formations consistent with surface observations. As a result, the contributions of regional tectonics and salt tectonics were assessed, and the timing of deformation was refined.

Elements of the geometry and timing of deformation in this frontal part of the Alps have been clarified. We show that south of the Barles half-window, the deformation of the Velodrome is early syn-depositional, starting earlier in the south of the basin (23 Ma) than in the north (18 Ma), requiring both regional tectonic control and halokinetic processes to account for the closure of the folded structures. The northern part of this half-window shows more cylindrical structures, but some faults appear localized and correlated with thickness variations of the Tithonian unit, indicating a role of inheritance in the localization of deformation. Finally, this study also demonstrated the power of GeoModeller as a 3D tool that is both predictive and useful for testing geological hypotheses in areas as complex as folded forelands.

How to cite: Faure, A., Loget, N., Jolivet, L., Bellahsen, N., Célini, N., Allanic, C., Gumiaux, C., and Callot, J.-P.: Western alpine orogenic front geometry : new insight from the Digne nappe area by a 3D geometrical modelling approach, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-5, https://doi.org/10.5194/egusphere-alpshop2024-5, 2024.

Long-term study of fault system activity is crucial for understanding the dynamics of orogeny structuring and the formation of peripheral basins, the impact of tectonic inheritance, seismic hazard assessment, and the estimating the coupling of deformation and erosion. At the junction of several orogenic domains, the foreland basin of the Western Alps exhibits a complex structural pattern inherited from the superposition of tectonic events since the late Paleozoic. Despite this knowledge, the absolute age of fault formation and reactivation remains poorly understood, primarily due to the difficulty of dating uranium-poor minerals typically found in sedimentary environments. This study proposes an integrated approach of structural analysis of deformations in the field combined to the recently developed U-Pb in-situ dating method on syn-tectonic calcite to fill this gap. By focusing on the subalpine massifs (from the Vaucluse massif to the Bornes massif), this work aims to constrain the dynamics of the Alpine foreland structuring over a wide temporal and spatial scale. Additionally, this study area presents diverse geodynamic characteristics, making it an ideal site to test the applicability of recent U-Pb in-situ dating methods.

How to cite: Bienveignant, D., Schwartz, S., Rolland, Y., Bernet, M., Léger, J., Vezinet, A., Bertauts, M., Boullerne, C., and Dumont, T.: Revealing the temporal dynamics of fault reactivation in the W-Alpine foreland using in-situ U−Pb dating on calcite, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-18, https://doi.org/10.5194/egusphere-alpshop2024-18, 2024.

The central Southern Alps (cSA) form a complex S-verging polyphase fold-and-thrust belt formed in response to the Alpine convergence to the S of the Periadriatic Fault System. Despite the onset of the final continent-continent collision in the Alps is constrained in the Late Eocene, evidence of Late Cretaceous deformation occur in the northern part of the belt, along the Orobic Thrust stacking the Variscan basement onto the Permo-Triassic cover. Here, pseudotachylytes associated to faulting close to the brittle-ductile transition display radiometric ages that trace back to 80 Ma.

No radiometric ages on structures in the central and southern part of the belt are until now available, with only indirect constrains (andesitic dikes and stocks cross-cutting tectonic structures) providing a pre-Late Eocene age of deformation related to the Alpine crustal shortening.

We present here new U-Pb radiometric ages of calcite tectonites located along the main structures of the central and southern sectors of the cSA that consist here of a thick pile of thrust sheets deforming the Lower to Middle Triassic carbonate successions. Our new U-Pb calcite ages obtained on growth fibers along fault planes, veins and calc-mylonites sampled along some of the most important regional thrust planes mainly result in Late Cretaceous ages, suggesting that N-S to NW-SE directed compression already affected the central part of the cSA at those times. Similar ages also occur within the southern portion of the belt, where the Norian “Dolomia Principale” thrust sheets, override the Rhaetian Riva di Solto Shale immediately to the north of the frontal portion of the belt. Younger ages resulted from the Paleogene units which are involved in the exposed frontal part of the belt, which is mostly buried under the recent infilling of the Po Plain forming the Milan Belt.

These data confirm that S- to SE-directed thrusting and folding affected the central Southern Alps since the Late Cretaceous, well before the onset of the Alpine collision.

How to cite: Zanchetta, S., Rocca, M., Montemagni, C., Fiorini, A., Carminati, E., Aldega, L., Kylander-Clark, A., and Zanchi, A.: Late Cretaceous S-verging thrusting in the central Southern Alps (N Italy) proved by U-Pb syn-tectonic calcite geochronology, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-56, https://doi.org/10.5194/egusphere-alpshop2024-56, 2024.

Internal Ligurian Units exposed in the Northern Apennines are regarded as fragments of the Ligure Piemontese oceanic lithosphere, which was interposed between the Europe and Adria Plates in the Middle to Upper Jurassic age. Starting from the Late Cretaceous, the convergence between the two plates led to the progressive closure of the Ligure-Piemontese Ocean and subsequently to the Europe margin continental subduction, until the collision in the Oligocene. The succession of the Internal Ligurian Units consists of an ophiolitic basement topped by pelagic deposits characterized by cherts, limestones, and shales (i.e., Chert, Calpionella Limestone, and Palombini Shale Fms.), and a thick turbidite sequence (Val Lavagna Shale Group and Gottero Sandstone Fm.) capped by chaotic deposits (Bocco Shale). Since the Palombini Shale Fm. occur in many of the Internal Ligurian Units, they are sampled to perform thermobaric estimates. Although the deformation history of the Internal Ligurian Units has been largely documented and regarded as reflecting their involvement in the alpine subduction zone, thermobaric estimates are poorly constrained, and the only available data come from semi-quantitative methods. Therefore, we use a multiequilibrium thermobarometry approach on 8 samples of metapelites. X-ray quantitative compositional maps were used for a detailed investigation of the sin-kinematic chlorite-white mica mineral chemistry to accurately apply classic low-grade metamorphic thermometers and barometers. The temperature values estimated with this approach are coherent both with the Raman results and the semi-quantitative white mica crystallinity index available in literature which, together with the estimated pressure strongly suggest a lower blueschist facies metamorphic conditions. Results yielded interesting and surprising pressure and temperature estimates associated with the metamorphic peak conditions of the Internal Ligurian Units and reflected their subduction signature. The calculated geothermic gradient is coherent with those proposed by previous authors for other oceanic units exposed in the Alps-Apennine orogenic system.

How to cite: Sanità, E., Di Rosa, M., Meneghini, F., Michele, M., and Luca, P.: Subduction imprint in the Internal Ligurian Units (Northern Apennines, Italy): evidence from multi-equilibrium thermobarometry, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-32, https://doi.org/10.5194/egusphere-alpshop2024-32, 2024.

In the Southwestern Alps, tectonic units derived from the Briançonnais passive margin are exposed in two domains (Michard et al., 2022 and references therein): (i) the “Briançonnais Zone s.l.”, dominated by Mesozoic sequences either overlying Permo-Carboniferous deposits or Variscan basement, described by the early French authors (e.g., Termier, 1903), and (ii) the pre-Mesozoic basement-dominated “Internal Crystalline Massifs” (ICM). We present a new tectonic map covering a large portion of the diverse units of the “Briançonnais Zone s.l.” and immediately adjacent units. These comprise, from W to E: (1) Briançonnais, Subbriançonnais and Dauphiné units and far-travelled Ligurian Helminthoid units, structurally located in the footwall of the out-of sequence Penninic Frontal Thrust (PFT), (2) a stack of Briançonnais s.str. units in the immediate hangingwall of the PFT dominated by W-directed fore-thrusting (e.g., the classical “1.-4. écailles” of Termier, 1903), (3) a  complex intermediate zone characterized by large-scale backfolds leading to (4) a pile of backthrusted units, comprising units referred to as “Acceglio-type” and “Prepiemonte" units, and (5) units derived from the Piemont-Liguria Ocean. Units (4) and (5) tectonically overlie ICM (Michard et al., 2022 and references therein; Dana et al., 2023). We use existing geological maps and literature combined with own fieldwork data to construct a tectonic map, and associated geological cross-sections, of the above-mentioned units between Modane (north of Briançon) and the Argentera-Mercantour Massif. Our compilation aims at revealing the structural architecture of the “Briançonnais Zone s.l”, allowing for a classification of these units over longer distances along strike on the basis of structural and metamorphic criteria, avoiding as much as possible the traditional mixing of paleogeography and tectonics and preventing the use of a wealth of multiple names for one and the same unit. These units have been deformed by a polyphase structural evolution (from D1 up to D3) associated with Alpine metamorphic conditions ranging from greenschist to blueschist facies. Particularly, we highlight the frequently underestimated importance of backfolding and backthrusting (D3) and its effect on the structures associated with the previous deformation phases linked to subduction and fore-thrusting (D1-D2). A comprehensive review in the form of a map and profiles of the “Briançonnais Zone s.l.” tectonic units is essential for future and more detailed studies, on the one hand, and for better understanding of the highly non-cylindrical large-scale structure of the Western Alps arc and its transition into the E-W striking Central Alps.

Dana, D., Iaccarino, S., Schmid, S.M., Petroccia, A., & Michard, A. (2023). Structural and metamorphic evolution of a subducted passive margin: insights from the Briançonnais nappes of the Western Alps (Ubaye–Maira valleys, France–Italy). Swiss Journal of Geosciences, 116, 18.

Michard, A., Schmid, S.M., Lahfid, A., Ballèvre, M., Manzotti, P., Chopin, C., Iaccarino, S. & Dana, D. (2022). The Maira–Sampeyre and Val Grana Allochthons (south Western Alps): review and new data on the tectonometamorphic evolution of the Briançonnais distal margin. Swiss Journal of Geosciences 115, 19 https://doi.org/10.1186/s00015-022-00419-8

Termier, P. (1903). Les montagnes entre Briançon et Vallouise: (écailles briançonnaises, terrains cristallins de l’Eychauda, massif de Pierre-Eyrautz, etc.), 182 pp. https://hal-insu.archives-ouvertes.fr/insu-00848081

How to cite: Dana, D., Schmid, S. M., Iaccarino, S., and Michard, A.: Tectonic map and structural architecture of the "Briançonnais Zone" (Western Alps), 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-10, https://doi.org/10.5194/egusphere-alpshop2024-10, 2024.