T4

The Permian-Triassic tectonic evolution of Western Adria has been variously interpreted either as the first rifting phase that led to the opening of the Alpine Tethys or as the result of continental-scale strike-slip movements. Additionally, a common view on the age of inception of the rifting of the Alpine Tethys, its duration and its relationship with the antecedent Variscan tectonic phases, is still lacking. The European Western Southern Alps expose the basement and cover rocks of Western Adria and therefore represent a key area for understanding and testing the post-Variscan to pre-rifting evolution of this plate. We focus, in particular, on a relatively poorly deformed sector of Northern Italy (Varese Area), where the outcropping Permo-Carboniferous sequence and the overlying Triassic to Early Jurassic units allow to investigate the crosscut relationships between structures that were active during pre-rift and syn-rift tectonic phases. By means of a 3D geological model of the Varese Area, built on a brand-new geological map, firstly we restored Alpine tectonics and then performed a progressive geological restoration of faults, aided by new preliminary thermochronological data. We unveiled a polyphasic strike-slip Permian tectonic phase that switch to an unexpectedly early inception of the rifting.

How to cite: Scaramuzzo, E., Livio, F. A., and Fellin, M. G.: From Permian to rift-inception: new insight from the Western Southern Alps (Varese Area), 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-50, https://doi.org/10.5194/egusphere-alpshop2022-50, 2022.

The Canavese Zone (CZ) in the Western Alps represents the remnant of the distal passive margin of the Adria microplate, which was stretched and thinned up to mantle rocks exhumation during the Jurassic opening of the Alpine Tethys. Through detailed geological mapping, structural analysis, stratigraphic and petrographic observations, and documentation of relationships between tectonics and sedimentation, we redefine the multistage tectono-stratigraphic evolution of the CZ, which consists of a Variscan basement, post-Variscan magmatic bodies and a Late-Carboniferous to Cretaceous sedimentary succession (Festa et al., 2020, and references therein). The Variscan basement includes a Lower Unit, wherein micaschist and orthogneiss were metamorphosed under amphibolite-facies conditions and partly transformed into migmatitic gneiss during a post-Variscan high-temperature metamorphic event, and an Upper Unit, consisting of a metasedimentary succession metamorphosed under greenschist- to amphibolite-facies conditions during the Variscan orogeny. The two basement units were intruded by post-Variscan plutons and hypabyssal dykes of both mafic to acidic composition. The sedimentary succession, at its bottom, consists of continental fluvial deposits (Upper Carboniferous Basal Conglomerate Auct.) unconformably overlain by Permian volcanic and volcanoclastic rocks (Collio Formation), and it continues upward with Upper Permian to Lower Triassic conglomerates and sandstones (Verrucano Auct. and Servino Formation), which are followed by pre-rift Middle Triassic dolostone. The latter is overlain by Lower to Middle Jurassic syn-rift sediments (Muriaglio Formation) and by Middle Jurassic to Early Cretaceous post-rift sediments, consisting of Radiolarites, Maiolica micritic limestones and Palombini shale. We point out that (i) the whole CZ succession, since the Late Carboniferous, shows significant variations in both thickness and facies, documenting long-lived tectonic control on sedimentation, and (ii) Late Paleozoic – Triassic structural inheritances playing a significant role in the localization of both the Jurassic rifting of the Alpine Tethys and the subsequent convergent tectonics.

Festa, A., Balestro, G., Borghi, A., De Caroli, S. & Succo, A. (2020). The role of structural inheritance in continental break-up and exhumation of Alpine Tethyan mantle (Canavese Zone, Western Alps). Geosciences Frontiers, 11, 167–188.

How to cite: Balestro, G., Borghi, A., De Caroli, S., Barbero, E., and Festa, A.: Multistage tectono-stratigraphic evolution of the Canavese Zone (Western Alps), 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-14, https://doi.org/10.5194/egusphere-alpshop2022-14, 2022.

Component analyses of ancient Neo-Tethys mélanges along the Eastern Mediterranean mountain ranges allow both, a facies reconstruction of the Middle Triassic to Middle Jurassic outer passive margin of the Neo-Tethys and conclusions on the processes and timing of the Jurassic orogenesis. This Middle-Late Jurassic mountain building process in the Western Tethyan realm was triggered by west- to northwestward-directed ophiolite obduction onto the former passive continental margin (wider Adria) of the Neo-Tethys.

Ophiolite obduction onto the former passive continental margin started in the Bajocian and trench-like deep-water basins formed in sequence within the northwest-/westward propagating nappe fronts in the footwall of the obducting ophiolites, i.e. in lower plate position. Deposition in these basins was characterized by coarsening-upward cycles, i.e. forming sedimentary mélanges as synorogenic sediments, in cases tectonically overprinted. In the Middle Jurassic, the oceanic realm and the most distal parts of the former passive margin were incorporated into the nappe stacking. Bajocian-Callovian ophiolitic and Meliata mélanges were formed as most oceanward preserved relics of trench-like basins in front of the propagating ophiolitic nappe stack, often with incorporated components from the continental slope (Meliata facies zone). In the course of ongoing ophiolite obduction, thrusting progressed to the outer shelf region (Hallstatt Limestone facies zone). In Bathonian/Callovian to Early Oxfordian times the Hallstatt nappes with the Hallstatt mélanges were established, expressed by the formation of the up to 900 m thick basin fills comprising its material mainly from the outer shelf region. In Callovian to Middle Oxfordian times the nappe stack reached the former carbonate platform influenced outer shelf region and the reef rim. Newly formed basins received material from this shelf region, occasionally mixed with material from the approaching ophiolite nappes. Ongoing shortening led to the formation of the proximal Hallstatt nappes with concomitant mobilisation of Hallstatt Mélanges. Persistent tectonic convergence caused the partial detachment and northwest- to west-directed transport of the older basin groups and nappes originally formed in a more oceanward position onto the foreland.

Comparison of mélanges identical in age and component spectrum in different mountain belts (Eastern Alps/Western Carpathians/Dinarides/Albanides/Pelso) figured out one Neo-Tethys Ocean in the Western Tethyan realm, instead of multi-ocean and multi-continent scenarios. The evolution of several independent Triassic-Jurassic oceans is unlikely considering the fact that re-sedimentation into newly formed trench-like basins in front of a west- to northwestward propagating nappe stack including ophiolite obduction is nearly contemporaneous along the Neotethyan Belt. The Middle to Late Jurassic basin evolutions with their sedimentary cycles and component spectra are comparable everywhere.

How to cite: Gawlick, H.-J.: Middle-Late Jurassic ophiolite obduction and formation of sedimentary mélanges in the Western Tethys Realm, 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-1, https://doi.org/10.5194/egusphere-alpshop2022-1, 2022.

The Pannonian Basin is a continental extensional basin system with various depocentres within the Alpine–Carpathian–Dinaridic orogenic belt. Along the western basin margin, exhumation along the Rechnitz, Pohorje, Kozjak, and Baján detachments resulted in the cooling of variable units of the Alpine nappe stack. This process is constrained by thermochronological data between ~25–23 to ~15 Ma (Fodor et al., 2021). Rapid subsidence in supradetachment sub-basins indicates the onset of sedimentation in the late Early Miocene from ~19 or 17.2 Ma. In addition to extensional structures, strike-slip faults mostly accommodated differential extension; branches of the Mid-Hungarian Shear Zone (MHZ) could also play the role of transfer faults.

During this period, the hanging wall margin of the detachment system, i.e., the pre-Miocene rocks of the Transdanubian Range (TR) experienced surface exposure, karstification, and terrestrial sedimentation. After ~14.5 Ma faulting, subsidence, and basin formation shifted north-eastward and reached the TR where fault-controlled basin subsidence lasted until ~8 Ma.

3D thermo-mechanical forward models analyze this depocenter migration and predict the subsidence and heat flow evolution that fits observational data. These models consider fast lithospheric thinning, mantle melting, lower crustal viscous flow, and upper crustal brittle deformation. Models suggest ~150–200 km of shift in depocenters during ~12 Myr.

Simultaneously with depocenter migration, the southern part of the former rift system, near or within the MHZ, underwent ~N–S shortening; the early syn-rift basin fill was folded and their boundary faults were inverted. Deformation was dated to ~15–14 Ma („middle” Badenian) and continued locally to ~9.7 Ma while north of the MHZ the TR was still affected by modest extensional faulting. The particularity of this shortening is that it happened during the post-rift thermal cooling stage. The low-rate contraction and related uplift rarely exceeded this regional thermal subsidence.

MOL Ltd. largely supported the research. The research is supported by the scientific grant NKFI OTKA 134873 and the Slovenian Research Agency (No. P1-0195).

Fodor, L., Balázs, A., Csillag, G., Dunkl, I., Héja, G., Jelen, B., Kelemen, P., Kövér, Sz., Németh, A., Nyíri, D., Selmeczi, I., Trajanova, M., Vrabec, M., Vrabec, M. (2021): Crustal exhumation and depocenter migration from the Alpine orogenic margin towards the Pannonian extensional back-arc basin controlled by inheritence. Global and Planetary Change 201, 103475. 31p. https://doi.org/10.1016/j.gloplacha.2021.103475

How to cite: Fodor, L., Balázs, A., Csillag, G., Dunkl, I., Héja, G., Kelemen, P., Kövér, S., Németh, A., Nyerges, A., Nyíri, D., Oravecz, É., Selmeczi, I., Soós, B., Tőkés, L., Vrabec, M., and Zadravecz, C.: Migration of basin formation and contrasting deformation style in the south-western Pannonian Basin (central Europe), 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-61, https://doi.org/10.5194/egusphere-alpshop2022-61, 2022.