T9

The Adriatic Indenter is subdivided into a western and an eastern domain termed Canavese-Insubric Indenter and Dolomites Indenter, respectively, and offset for ~75 km from late Oligocene onwards by the NNE‐SSW‐trending sinistral-transpressive Giudicarie fault system (GFS). The N(NW)-directed movement of the Dolomites Indenter (DI) modifies the early Cenozoic nappe structure of the Alpine orogen as the accommodated shortening changes substantially, depending on the oblique shape of the indenter and its counter-clockwise rotation. The Austroalpine basement units northwest of the GFS experienced open folding of the Cretaceous nappe stack and preserved Cretaceous metamorphic ages. In contrast, the previously deep-seated Neoalpine metamorphic Subpenninic and Penninic units of the Tauern Window in front of the DI’s tip are exhumed and the Austroalpine units adjacent to the DI are brought into a subvertical or even overturned position.

The combination of several thermochronological methods and structural field work allows for constraining time on this tectonic evolution: The Austroalpine units directly adjacent to the DI belong to the uppermost nappe system of the Eoalpine orogeny (Drauzug-Gurktal Nappe System) and experienced an anchizonal to lowermost greenschist-facies metamorphic overprint during the Alpine orogeny resulting in an only partial reset of Variscan Rb/Sr Biotite ages (Pomella et al., 2022). Fission track data from the western Tauern Window and the Austroalpine units adjacent to the north-western corner of the DI, indicate cooling below 180-200°C (Zircon Fission track data) in the Early Miocene and below the 100-120°C (Apatite Fission track data) in the Late Miocene (Klotz et al., 2019). (U‐Th)/He on Apatite data, derived from a horizontal section of the Brenner Base Tunnel and reaching from the DI into the Austroalpine nappe stack, indicate continuous differential uplifting of the northern block along the, in this area, approximately E‑W striking Periadriatic fault system until the Pliocene (Klotz et al., 2019).

Earthquake focal solutions and satellite-based geodetic studies show, that indentation is ongoing today. The significant present-day seismotectonic activity concentrates in the Friuli area in the southeast, whereas there is currently no significant seismicity along the western and northern boundaries of the DI or in the northerly adjacent Austroalpine basement and the Tauern Window. Increased seismic activity can only be detected north of the Tauern Window, along, and north of the Inn Valley (Reiter et al., 2018). Based on field evidence and the thermochronological record, the recent seismic distribution indicates an important change in style and localisation of deformation compared to what is documented from the past.

Klotz, T., Pomella, H., Reiser, M., Fügenschuh, B., Zattin, M., 2019. Differential uplift on the boundary between the Eastern and the Southern European Alps: Thermochronologic constraints from the Brenner Base Tunnel. Terra Nova 31, 281-294.

Pomella, H., Costantini, D., Aichholzer, P., Reiser, M., Schuster, R., Tropper, P., 2022. Petrological and geochronological investigations on the individual nappes of the Meran-Mauls nappe stack (Austroalpine unit/South Tyrol, Italy). Austrian Journal of Earth Sciences 115, 15-40.

Reiter, F., Freudenthaler, C., Hausmann, H., Ortner, H., Lenhardt, W., Brandner, R., 2018. Active seismotectonic deformation in front of the Dolomites indenter, Eastern Alps. Tectonics 37, 4625-4654.

How to cite: Pomella, H., Klotz, T., Sieberer, A.-K., Reiser, M., Tropper, P., and Schuster, R.: The thermotectonic evolution in front of the Dolomites Indenter, 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-39, https://doi.org/10.5194/egusphere-alpshop2022-39, 2022.

The Badstub Formation is part of the Carboniferous of Nötsch sedimentary sequence, of the eastern Austroalpine domain. This formation outcrops in Carinthia (Austria), a few kilometres north of the Periadriatic line (Gailtal line), where a sequence of various conglomerates and breccias with interbedded sandstones, siltstones, and fossiliferous carbonatic schists is exposed. These rocks preserve pristine sedimentary features and even an outstanding fossil record, but multi-scale structural analysis revealed a tectonitic foliation localized in fine-grained rocks, different sets of mineralized faults and veins, and corona textures. Vein fillings and coronas are characterized by equilibrium mineral assemblages that include prehnite, pumpellyite, chlorite, phengite, winchite, and riebeckite. Chlorite-thermometry and thermodynamic modelling on mineralized veins and coronas revealed PT conditions of 260-310 °C and 0.25-0.50 GPa and testify that the Badstub Formation recorded a metamorphic imprint characterized by a low temperature/depth ratio (≈15 °C km^-1). The comparison between a 2D thermo-mechanical numerical model and the metamorphic conditions inferred with thermodynamic models suggest that the Badstub Formation underwent a thermal state consistent with that of the Alpine subduction. These results provide the first quantitative pressure constraints on Alpine subduction metamorphism on the Austroalpine Carboniferous covers nearby the Periadriatic line. Thus, within the Upper Austroalpine nappe system, pre-Alpine rocks were involved into the Alpine subduction at different structural levels and under metamorphic conditions, which therefore span from eclogitic to prehnite-pumpellyite facies.

How to cite: Zanoni, D., Filippi, M., Roda, M., Regorda, A., and Spalla, M. I.: Tectonic and metamorphic record in the Badstub Formation, Carboniferous of Nötsch, eastern Austroalpine, 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-30, https://doi.org/10.5194/egusphere-alpshop2022-30, 2022.

The internal structure of the Dora-Maira Massif is of key importance for understanding exhumation mechanisms of continental-derived HP-UHP rocks in the Western Alps. Numerous petrological-geochemical studies have been done on the world-famous UHP Brossasco-Isasca Unit in the southern Dora-Maira Massif, and recent syntheses have provided an updated view of its metamorphic (Groppo et al. 2019) and structural (Michard et al., 2022) history.

By contrast, the northern Dora-Maira Massif has been much less explored. We are presently undertaking a multidisciplinary project aimed at better constraining its geometry and history. We studied in detail an area comprised between the Germanasca and the Chisone rivers. The first results are as follows:

• The nappe stack comprises, from bottom to top, the Pinerolo (Carboniferous metasediments intruded by dioritic and granitic plutons), Chasteiran (UHP), Muret (polycyclic unit, made of a Variscan basement overprinted during Alpine HP metmorphism) and Serre (Permian rhyolitic to granitic rocks, and the associated epiclastic rocks; slices of Mesozoic cover) Units.
• The pre-Alpine history of the Muret Unit (6-7 kbar, 650 °C) dated at 324 Ma (U-Pb LA-ICP-MS on monazite inclusions in garnet) is well preserved in undeformed volumes (Nosenzo et al., 2022).
• A new, colder (garnet + Fe-rich chloritoid +coesite), UHP unit (the Chasteiran Unit) has been discovered (Manzotti et al., 2022 and this meeting). This Unit, located in the immediate hangingwall of the Pinerolo Unit, occupies the same structural position than the UHP Brossasco-Isasca Unit, but records temperatures 200°C lower.

Geological mapping, structural data, and petrological investigations provide new constraints on the geometry and kinematics of this part of the Dora-Maira Massif. The main foliation D₁ developed at different peak PT conditions in the different units. During their stacking, a new foliation D₂ developed associated with kilometer-scale, E-W trending folds. Final doming of the Dora-Maira nappe stack (D₃) is associated to the westward displacement of the Adria mantle indentor. Detailed geological maps and cross-sections will be provided for illustrating the main steps of the history.

Groppo, C., Ferrando, S., Gilio, M., Botta, S., Nosenzo, F., Balestro, G., Festa, A., & Rolfo, F. (2019). What’s in the sandwich? New P–T constraints for the (U)HP nappe stack of southern Dora-Maira Massif (Western Alps). European Journal of Mineralogy, 31, 665–683.

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): a review and new data on the tectonometamorphic evolution of the Briançonnais distal margin. Swiss Journal of Geosciences, 115, in press.

Manzotti, P., Schiavi, F., Nosenzo, F., Pitra, P., Ballèvre, M. (2022). A journey towards the forbidden zone: a new, cold, UHP unit in the Dora-Maira Massif (Western Alps). Contributions to Mineralogy and Petrology, in press.

Nosenzo, F., Manzotti, P., Poujol, M., Ballèvre, M., & Langlade, J. (2022). A window into an older orogenic cycle: P-T conditions and timing of the pre-Alpine history of the Dora-Maira Massif (Western Alps). Journal of Metamorphic Geology, 40, 789-821.

How to cite: Nosenzo, F., Ballèvre, M., and Manzotti, P.: Nappe stacking and syn-nappe folding in the northern Dora-Maira Massif (Western Alps), 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-8, https://doi.org/10.5194/egusphere-alpshop2022-8, 2022.

The Adula and Cima Lunga units show the best preserved record of the deformation and metamorphic history of the Central Alps. Alpine studies lasting more than a hundred years documented a complex tectono-metamorphic evolution, including the presence of relicts of ultra-high pressure and high temperature metamorphism. Throughout this long history of researches, a few key questions stand out, still challenging the geological community. Major questions regard how to reconcile the structural pattern with the metamorphic path, as well as the timing relationships.

The occurrence of ultrahigh-pressure and/or high-temperature rocks embedded within significantly lower grade metamorphic rocks rises a major challenge for developing a consistent geodynamic model for exhumation of such deep-seated rocks. Subduction zones are, in fact, efficient player driving material from the surface down into the Earth's mantle. However, the mechanisms to exhume part of this material (and particularly the denser oceanic rocks) back to the shallow crust are still highly debated. Scientists generally invoke either mechanical decoupling within a tectonic mélange or variable metamorphic re-equilibration during the retrograde path.  These interpretations are based on the common assumption that the mineral assemblages form under lithostatic pressure and near-equilibrium regional geothermal gradients. Hence, the resulting metamorphic histories based on the estimation of the pressure and temperature conditions represent the major tool for tectonic reconstruction as proxies of the burial and exhumation history of the rocks during subduction-exhumation phases.

Alternative explanations highlight the role of deformation in promoting the coexistence of multiple local equilibria, which cease to correlate with lithostatic conditions and thus burial depths. In this view, the non-hydrostatic stress and the local temperature deviations are accounted as important components potentially modifying the metamorphic system.

In this contribution, we show new structural, petrological and thermochronometric data from the Adula and Cima Lunga units. The wide dataset comprises new field mapping covering the entire nappes extension (several hundred square kilometres) and structural-petrochronological analyses at the meso- to micro-scale. Our results show the highly variable pressure-temperature-time-deformation paths experienced by the compositionally heterogeneous rocks of the Cima Lunga and Adula nappes. We present evidence of contrasting metamorphic records among the rocks of these nappes, providing arguments to discuss pros and cons of the tectonic models proposed to explain these contrasting metamorphic records.

How to cite: Maino, M., Schenker, F., Casini, L., Corvò, S., Perozzo, M., Langone, A., and Seno, S.: Challenges in the interpretation of the structural and metamorphic record in the Adula and Cima Lunga units (Central Alps), 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-9, https://doi.org/10.5194/egusphere-alpshop2022-9, 2022.