Alpine convergence record in the Carboniferous Badstub Formation, Upper Austroalpine basement nappes, Austria

ABSTRACT Within the Carboniferous of Nötsch, the Badstub Formation is a clastic sequence of the Upper Austroalpine basement nappes exposed few kilometres north of the Periadriatic Fault System (Carinthia, Austria). Although these rocks preserve pristine sedimentary features, multi-scale structural analysis disclosed a syn-metamorphic foliation in fine-grained rocks, sets of mineralized faults, veins, and corona textures. Vein fillings and coronas contain equilibrium mineral assemblages with prehnite, pumpellyite, chlorite, phengitic mica, winchite, and riebeckite. Thermodynamic modelling and geothermometry constrain metamorphic conditions at 260–310°C and 0.25–0.50 GPa that are consistent with a temperature/depth ratio of about 20°C km−1. In a 2D thermomechanical model this thermal state is reached either in the upper or lower continental plate during convergence. Thus, during the Alpine convergence, the Badstub rocks were buried from the shallow crust at depths between 13 and 18 km either by ablative oceanic subduction or by continental subduction of the passive margin, and eventually stacked into the orogenic wedge at the Adria margin. During the downward and upward paths, the Badstub rocks were translated as a coherent poorly strained block. This first quantitative constraint on metamorphism for these Carboniferous rocks is consistent with the Upper Austroalpine basement nappes being a tectonic system that recorded the Alpine convergence under eclogite to prehnite–pumpellyite facies conditions.


Introduction
The Austroalpine and Penninic domains of the Alps constitute the axial part of the Alpine chain that is confined by the Penninic front to the north and the Periadriatic Fault System to the south.During the Alpine convergence, rocks belonging to these domains recorded pervasive deformation and metamorphism at different structural levels and under various geothermal gradients (e.g.Frey et al. 1999;Schuster et al. 2004;Lardeaux 2014).However, in the Eastern Alps, the upper part of the Austroalpine nappe stack includes rock sequences that were only weakly to partially deformed and metamorphosed.Here, sedimentary rocks well preserve stratigraphic features.Among those rock units, the Carboniferous of Nötsch is part of the Palaeozoic sequences of the Drauzug-Gurktal Nappe System exposed between the Drau and Gail valleys (Figure 1a) and is even famous for the outstanding fossil records.As a result, many palaeontological and stratigraphic studies have been undertaken to shed light on the environmental significance of this formation in the frame of the Palaeozoic palaeogeography (Sieber 1972;Schraut 1999;Vachard et al. 2018).
Moreover, studies on the Carboniferous of Nötsch were focused on clasts for discussing their origin (Exner 1983;Krainer and Mogessie 1991).
The Carboniferous of Nötsch records low-grade metamorphism for which only temperature is constrained (Rantitsch 1995).This contribution includes new field, microstructural, and mineral chemical data from the Badstub Formation, a stratigraphic unit of the Carboniferous of Nötsch, to constrain the metamorphic temperature and pressure and the tectono-metamorphic record.Since the excellent rock exposure, we concentrate our analysis in the Jakomini quarry, which is in the Nötschbach valley, a few kilometres north of the Nötsch im Gailtal village in southern Carinthia, Austria (Figure 1b).Regardless the pristine sedimentary features clearly detectable at first glance on rock outcrops, the multi-scale structural analysis discloses tectonic and metamorphic overprints on sedimentary structures.We individuate microstructural domains with mineral assemblages suitable for quantifying metamorphic conditions by thermometrical estimates and thermodynamic modelling.The estimated metamorphic conditions are compared with the predictions of a 2D CONTACT Davide Zanoni davide.zanoni@unimi.itDipartimento di Scienze della Terra "A.Desio", Università degli Studi di Milano, via Mangiagalli, 34, Milano 20133, Italy Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2023.2206443 Figure 1.(a) Geological sketch map of the Eastern Alps that highlights the nappe systems within the Austroalpine domain (modified after Schmid et al. 2004); in the inset, the location of Eastern Alps (red rectangle) within the national borders of central southern Europe; SAM = Southern border of Alpine metamorphism (Hoinkes et al. 1999;Schuster et al. 2004); (b) Geological sketch map of the Carboniferous of Nötsch with regional fault systems, modified after Schönlaub (1985), Krainer and Mogessie (1991), and Vachard et al. (2018).Ages are from Piller (2014).
thermomechanical numerical model to test the most reliable geodynamic scenarios under which the detected tectono-metamorphic record of Badstub Formation developed.

Geological setting
The eastern Austroalpine domain is divided into the Lower and Upper Austroalpine basement nappes (Figure 1a) that both consist of basement and cover nappes generally interpreted as deriving from the outer and inner portions of the Adriatic passive margin, respectively (Schmid et al. 2004).The Upper Austroalpine basement nappes are divided into four stacked nappe systems of which the uppermost is the Drauzug-Gurktal Nappe System (Schmid et al. 2004).Most tectonic units of the Upper Austroalpine basement nappes record successive tectono-metamorphic imprints: a Variscan metamorphism characterized by a high-pressure peak; a Permian metamorphism characterized by low-pressure medium temperature conditions; a Cretaceous (Eo-Alpine) metamorphism characterized by high-pressure peak and P/T ratios and related to the Alpine subduction (Hoinkes et al. 1999;Neubauer et al. 1999Neubauer et al. , 1999;;Habler and Thöni 2001;Faryad and Hoinkes 2003;Kurz and Fritz 2003;Schuster and Stüwe 2008;Krenn et al. 2011;Roda et al. 2012;Rode et al. 2012;Sandmann et al. 2016;Hauke et al. 2019;Li et al. 2021).
The Upper Austroalpine basement nappes include Palaeozoic and Mesozoic rock assemblages.Within the Drauzug-Gurktal Nappe System, weakly metamorphosed Palaeozoic sequences outcrop in the Gurktal nappes, Carboniferous of Nötsch, Palaeozoic of Graz, Remschnigg and Sausal areas, and southern Burgenland (Piller 2014).The Carboniferous of Nötsch extends about 9 and 3 km in east-west and north-south direction, respectively, and is comprised between the Permian-Triassic carbonate and sandstone sequences of the Drau Range to the north, and the Gailtal crystalline basement to the south (Hauser et al. 1982;Schönlaub 1985;Krainer 1993).The Carboniferous of Nötsch outcrops adjacent to the Gailtal Line, which is the eastern segment of the Periadriatic Fault System, is separated from the crystalline basement by tectonic boundaries, and is capped by about 5 km-thick sequence of Permian to Mesozoic rocks at its eastern edge (Figure 1b) (Tollmann 1977).From bottom to top, the Carboniferous of Nötsch, consists of a continuous litostratigraphy (Supplemental Table S1): Erlachgraben Formation, Badstub Formation, and Nötsch Formation (Schönlaub 1985;Krainer 1992;Krainer and Vachard 2002;Vachard et al. 2018).The Carboniferous of Nötsch displays upper Visean and lower Kasimovian strata (Ebner et al. 2008;Piller 2014) and the Badstub Formation is Serpukhovian in age (Vachard et al. 2018).
The Carboniferous of Nötsch outcrops south of the SAM (southern border of the Alpine metamorphism, Figure 1a; Hoinkes et al. 1999;Schuster et al. 2004) line.North of the SAM line, the Upper Austroalpine basement nappes record Cretaceous metamorphism.The grade of the dominant metamorphic imprint increases from the Silvretta Seckau-Nappe System, which records anchizonal to epidote-amphibolite facies conditions, up to the centre of the Koralpe-Wölz Nappe System, which records greenschist to eclogite facies conditions.From the centre of the Koralpe-Wölz Nappe System, the grade of the dominant metamorphism decreases structurally upward and indeed the Ötztal-Bundschuh and Gurktal-Drauzug Nappe Systems record anchizonal to amphibolite facies conditions (Haüy 1822;Faryad and Hoinkes 2003;Schuster et al. 2004;Janák et al. 2004Janák et al. , 2015;;Froitzheim et al. 2008;Krenn et al. 2011;Schulz 2017;Hauke et al. 2019;Rantitsch et al. 2020;Li et al. 2021;Schorn et al. 2021;Schuster and Stüwe 2022).South of the SAM line, minerals such as prehnite and analcime are described in the Badstub Formation (Niedermayr et al. 2010) and very low-grade Alpine metamorphism in the Carboniferous of Nötsch is constrained based on illite crystallinity and vitrinite reflectance at 260°C and 6 km depth (Rantitsch 1995).In the Permian to Cretaceous sedimentary sequences of the western Drau Range, Alpine anchizonal metamorphism is constrained at temperatures lower than 270°C and pressures around 0.2 GPa based on illite crystallinity (Niedermayr et al. 1984), whereas Alpine greenschist facies metamorphism is inferred in basement rocks of Goldeck mountains (Deutsch 1988).In the regions surrounding the Carboniferous of Nötsch, diagenetic to very low-grade Alpine metamorphism is also documented in the Gurktal Nappe (Rantitsch and Russegger 2000;Rantitsch et al. 2020) and Carnic Alps just to the south of the Periadriatic Fault System (Läufer et al. 1997;Rantitsch 1997;Rantitsch et al. 2000).The low grade metamorphic peak temperatures are supposed to be Cretaceous in age and related to nappe staking during the Alpine convergence (Rantitsch et al. 2020), whereas they are constrained at the Oligocene in the Carnic Alps and related to the strike-slip deformation along the Periadriatic Fault System (Läufer et al. 1997).

Lithostratigraphy
We focused our field survey in the Jakomini quarry (Figure 2) where the best outcrop of Badstub Formation is exposed (see also Schönlaub in Piller 2014).The mined rocks are entirely part of the middle sequence of the Badstub Formation of the Carboniferous of Nötsch.They consist of breccia, conglomerate, sandstone, and minor siltstone and pelite and are heterogeneously deformed, but with well-preserved primary textures.Therefore, protholit names are used except for finely foliated carbonatic rocks referred as fossiliferous schist.Based on the litostratigraphy and meso-and micro-textural features (see also Vignaroli et al. 2013), four zones have been distinguished in the quarry (Figure 2), which from north to south we name: CSS (conglomerate, sandstone, and siltstone), DCB (dominant conglomerate and breccia), FS (fossiliferous schist), and CB (conglomerate and breccia).Although the boundaries between DCB and FS, and FS and CB are sharp, the limit between CSS and DCB is gradational.From CSS to CB, the stratigraphic top of the sedimentary sequence is reached.
CSS: This zone is exposed in the northernmost part of the quarry (Figure 2) and is characterized by reddish breccia, conglomerate, conglomeratic breccia, sandstone, and siltstone.Matrix-and locally clast-supported conglomerate and breccia dominate in the northern and southern part of this sector (Figure 3a), whereas dozen metre-thick siltstone and sandstone sequences dominate the central part.Millimetre-to decimetre-sized angular to sub-rounded clasts consist of metabasite (amphibolite), and minor marble and metaintrusives (metadiorite and metatonalite).Metabasic clasts are more abundant in the upper portion of CSS and in the central southern part, decimetre-thick conglomerate beds are rich of marble clasts (Figure 3b).Leucocratic metagranitoid and marble clasts are also locally abundant in the lower part.Two decimetre-thick beds of fossiliferous schist with brachiopods outcrop nearby the junction of 'etage' 805 and 820 (Figure 2).Close to the transition with sector DCB, in 'etage' 790, decimetresized lenses contain metabasic clasts cemented by calcite.
DCB: Here rocks have intermediate composition compared to CSS and CB, and they mostly consist of conglomerate and breccia with dominant mafic components and are either clastic or sandy matrix-supported.Locally rocks contain a red silty matrix like in CSS.Angular to sub-rounded clasts are mostly metabasic with minor centimetre-to decimetre-sized marble clasts (Figure 3c).The conglomerate forms metre-thick beds interbedded with decimetre-thick mafic sandstone and red siltstone beds (Figure 3d) and forms up to metrethick sequences.Locally the conglomerate with centimetre-sized metabasite clasts shows calcite cement distributed along centimetre-thick and 1-to 2-metre-long layers and lenses (Figure 3e).

FS:
The rocks ('Zwischenschiefer') consist of black fossiliferous schists with up to decimetre-sized carbonatic bioclasts, such as brachiopods, foraminifera, and crinoids.These rocks are finely laminated and form a 10-to 15-metre-thick sequence (Figure 3f).Brachiopods are articulated and mostly iso-oriented with the convex side towards the stratigraphic bottom.Often these rocks contain a pervasive tectonic cleavage.
CB: Rocks consist of mafic breccia and minor conglomerate that form metre-thick beds with fining upward sequences.The boundaries between these sequences are marked by centimetre-thick layer of siltstone and pelite (Figure 3g).The texture of conglomerate and breccia is mainly clast-supported, and mainly rounded clasts consist of metabasic, and rare marble and leucocratic metagranitoid rocks (Figure 3h).Clast dimension varies between few millimetres to few decimetres and larger clasts consist of marble, which are more abundant close to the boundary with FS.Locally rocks display carbonatic cement.In CB, the rocks show the highest clast concentration (up to 90%).A carbonatic siltstone with bioclasts outcrops in the very southernmost edge of the quarry at lower elevation and is rich in crinoids.

Mesostructures
The dominant structure in the quarry is the bedding, S0, (Figure 4a) that shows dip direction towards south or southwest (Figure 2, inset a).A cleavage (S1) is developed mostly in the black fossiliferous schist in FS and locally in the sandstone and siltstone in CSS (Figure 4b,  4).In the fossiliferous schist, S1 foliation is asymptotic to calcite veins suggesting a shearing accommodated along the vein walls (Figure 4d) that intersects S1.S1 is  a slaty cleavage in fossiliferous schist and a slightly refracted spaced foliation in sandstone and siltstone.S1 is steeper than the bedding and dips generally southwestward (Figure 2, inset a).Due to the refraction in finer-grained rocks locally S1 parallels S0 especially at strata boundaries.The faults show different orientation and kinematics, and up to metre-thick cataclastic bands (Figure 4a).Calcite fibres developed also on fault surfaces sub-parallel to the quarry front (Figure 4e) and fault surfaces mineralized with haematite fibres are detected in CSS and DCB.Millimetre-to centimetrethick veins are filled mainly by calcite but also by chlorite and epidote (Figure 4f).Calcite veins crosscut chlorite and epidote veins.
The main fault system is steeply dipping and strikes NNE-SSW, sub-parallel to the quarry front (Figures 4a and 2, inset b) and sub-parallel to the main regional strike-slip fault systems (Figure 1b).Kinematic indicators suggest a dextral or sinistral transtensive displacement (Figure 2, inset b).A conjugate fault system shows north-northwest and northeast striking surfaces with dip between 30° and 50° (Figure 2, inset b).Both fault surfaces accommodate a reverse and normal kinematic and developed decimetre-thick cataclastic bands.

Microstructures
Microstructural analysis focused on deformation fabrics and metamorphic textures that overprinted the primary sedimentary features (sample locations are in Figure 2).Due to the heterogeneous deformation, metamorphic minerals constitute structures such as tectonitic foliations, mineral coronas, and veins.
In the studied sequence, the clasts mostly consist of foliated metabasite with amphibole, epidote, quartz, plagioclase, and minor titanite, chlorite, K-feldspar, and opaque minerals.Minor clasts consist of marble (calcite, minor chlorite, apatite, titanite, plagioclase), leucocratic metagranitoid (quartz, minor K-feldspar, and plagioclase and occasionally garnet), and epidosite (abundant epidote and minor titanite).Quartzite clasts are rare.The clast shape varies from rounded to angular.The texture of breccia and conglomerate is clast supported especially in CB and occasionally in DCB, whereas it is matrix supported in CSS.Matrix varies from sandy to silty and consists of the mineral phases forming the clasts.

CSS:
In conglomerate, the sedimentary matrix is locally foliated, and the grain size is even minor than 10 μm with albite, chlorite, and abundant interstitial haematite.Epidote crystallized in the fine-grained matrix and in micro-shear zones.Locally haematite-bearing stylolites define S1 foliation and mark clast boundaries that are parallel to S1; here chlorite, albite, and epidote grains show shape preferred orientation parallel to the stylolitic cleavage.Veins intersect clasts, matrix, and stylolite (Figure 5a) and are filled by calcite.Calcite forms inequigranular aggregates, and in places twinned crystals with SPO at high angle with the veins.In addition to calcite, locally veins contain albite and minor K-feldspar and haematite.Calcite veins, with a micro-crystalline leucocratic film developed along vein walls, intersect veins filled by coarse-grained calcite and fine-grained epidote.
DCB: Here locally leucocratic meta-granitoid and metabasite clasts show interlobate contacts that indicate dissolution (Figure 5b).In places, the conglomerate has calcite cement.Zoned calcite crystals and coarsegrained euhedral inequigranular polygonal aggregates in cement indicate recrystallization (Figure 5c).Calcite also shows thick straight twinning.Haematite and calcite are interstitial between detrital grains and locally coarsegrained euhedral quartz crystals are associated with interstitial calcite.
In corona textures, prismatic pumpellyite is radial with respect to the clast surfaces (Figure 5d,e) and generally associated with Fe-epidote.Here prehnite forms scattered crystals with irregular grain boundaries that are included in coarse-grained euhedral calcite (Figure 5e).Blue amphibole rims the detrital amphibole and is in contact with calcite (Figure 5f).
Veins filled by calcite, quartz, and minor epidote intersect the clast-supported conglomerate and are intersected by thicker calcite veins that contain coarsegrained crystals and minor quartz surrounded by finegrained calcite, which also includes host rock fragments.Straight twinning lamellae characterize calcite in this last group of veins.Cataclastic bands that intersect these veins are characterized by fragments as small as 10 μm and in turn sealed by millimetre-thick calcite veins.
Bedding in haematitic sandstone and siltstone is marked by different grain size of detrital amphibole, opaque minerals, plagioclase, epidote, quartz, and titanite.Calcite forms both detrital and interstitial grains.Locally also chlorite is interstitial to detrital grains.Chlorite and quartz-bearing shear zones locally offset the bedding (Figure 6a), which is also intersected by veins and cataclastic bands.In the veins, chlorite is randomly oriented but, if associated with quartz, forms inequigranular aggregates with crystals at high angle to the vein walls.These veins also intersect S1 stylolitic cleavage.
In mafic siltstone, calcite and quartz-bearing veins are cut by calcite veins.Veins are filled by coarse-grained polygonal mineral aggregates with calcite crystals FS: Fossiliferous schists are rich in carbonatic bioclasts consisting of centimetre-to decimetre-sized articulated brachiopods (Gigantoproductini), foraminifera, crinoids, and rare bryozoa with calcite shell.S1 foliation is at high angle to gently bent brachiopod shell, which are locally boudinaged with boudin necks filled by calcite and minor quartz, and at low angle with respect to S1. Geometric relationships between boudin necks and S1 suggest that boudinage predates foliation development.S1 is a wriggly to stylolitic foliation marked by opaque minerals (Figure 6b) and SPO of micrometre-sized calcite, chlorite, plagioclase, and quartz grains.Framboidal pyrite fills also veins with quartz and calcite and occurs in the rock matrix (Figure 6c).Bioclasts are elongated and dissolved along S1 (Figure 6b) or along mutual margins (Figure 6d) and shows shape preferred orientation parallel to S1. Around rare detrital plagioclase grains, albitebearing strain shadows developed (Figure 6c).Calcite replacing bioclast shells shows straight and thick twinning.Veins sub-orthogonal to S1 are interpreted as collectors of dissolved material.
CB: Breccias contain rare sandy matrix with local carbonatic cement, possibly derived from marble clast dissolution.Locally chlorite is interstitial to detrital matrix grains and, with K-feldspar, also in cataclastic bands.Locally clasts are compenetrated with lobated margins, suggesting dissolution; rare and discontinuous films nucleated at the clast margins, intersect the newly formed interstitial minerals, and are defined by SPO of chlorite and minor K-feldspar and albite.
Veins are filled by coarse-grained calcite with straight and thick twinning.Cataclastic bands are intersected by veins filled by chlorite grains at high angle with respect to the vein walls and by K-feldspar (Figure 6e).At the triple junctions of clasts calcite, chlorite, and locally radial prehnite (Figure 6f) occur.In places pumpellyite rims detrital amphibole (Figure 6f).
Mafic sandstone consists of laterally discontinuous bedding with dominant angular detrital amphibole grains and rare cm-sized marble clasts.Some samples contain up to 1.5 mm-sized clasts of metabasite and rare quartzite.Mafic siltstone contains detrital grains mostly of quartz, epidote, amphibole, and minor plagioclase.The bedding is marked by variation of detrital grain size with opaque minerals concentrated in finer-grained beds, which contain a sedimentary lamination.Locally they are folded and intersected by the dominant S1 fabric (Figure 7a).Two pervasive vein systems intersect S0 at low angle.The older system is filled by quartz and white mica (Figure 7b) and is intersected by calcite-filled veins.These are rimmed by Kfeldspar that also fills successive thinner vein sets.Between feldspar and calcite fillings, albite grains are randomly distributed as in the matrix.Besides these three vein systems, 30-μm-thick veins are filled with pumpellyite, phengitic mica, chlorite, and albite (Figure 7c).Locally, pumpellyite is interstitial between detrital grains.
In summary, metamorphic minerals overprinting Badstub rocks are as follows: pumpellyite, prehnite, chlorite, albite, K-feldspar, calcite, white mica, and blue amphibole.These minerals overgrew monomineralic and polymineralic clasts, fill successive sets of veins, and mark S1 foliation.Moreover, the carbonatic cement shows textures indicating re-crystallization and calcite twinning of type II (Burkhard 1993).These observations disclose the metamorphism recorded by the Badstub Formation rocks.(Hey 1954) with formula calculated on 14 oxygen atoms, according to Bourdelle et al. (2013); (c) Composition of epidote and prehnite with Fe 3+ estimated on charge balance and formula based on 12.5 oxygen atoms and 14 cations, respectively; (d) Composition of pumpellyite with formula based on 16 cations according to Cortesogno et al. (1984); (e) Composition of feldspar with formula based on 8 oxygen atoms; (f) Composition of white mica with formula based on 11 oxygen atoms.

Mineral chemistry and metamorphism
We focus on the compositional variations of metamorphic minerals that overprint the Badstub Formation sequence (Figure 8, Supp.Table S2, and Supp.Table S3).Mineral chemical analyses have been performed on selected microstructural sites with a JEOL JXA-8200 microprobe, operating at Dipartimento di Scienze della Terra 'A.Desio' of Milano University, equipped with WDS and EDS systems; operating conditions were 5 nA and 15 kV and working distance was 11 mm.The pressure and temperature (PT) conditions recorded by metasedimentary rocks of the Badstub Formation were estimated by chlorite thermometry (Bourdelle et al. 2013) and thermodynamic modelling based on mineral assemblages.The compositions of selected equilibrium micro-domains were quantitatively estimated by modal analysis under the electron microscope (BSE images) and converted to mass.Phase diagrams were calculated using the Gibbs free energy minimization modelling software Theriak-Domino (de Capitani and Petrakakis 2010) and the internally consistent thermodynamic dataset 'ds6.2'(Holland and Powell 2011).The following a-x models were used: amphibole (Green et al. 2016), epidote (Holland and Powell 1998), feldspar (Holland and Powell 2003), chlorite, garnet, prehnite, pumpellyite, and white mica (White et al. 2014).
Chlorite: In the clasts, it consists mostly of pycnochlorite and minor ripidolite and brunsgivite (Figure 8b).Metamorphic chlorite shows generally higher Si content than detrital chlorite and in CB the composition is diabanitic.Only in FS, Si content in metamorphic chlorite is lower than in the detrital one.Chlorite aligned in microshear zones shows higher Si values than in the metamorphic matrix (Supp.Table S2).
Epidote: Al is higher in detrital than in metamorphic epidote (Figure 8c).In DCB, metamorphic epidote is associated with pumpellyite and shows a wider compositional range than epidote in CSS (Supp.Table S2).
Pumpellyite: In micro-veins of CB pumpellyite associated with phengitic mica, chlorite, and albite shows lower X Mg than crystals in the microcrystalline matrix (Supp.Table S2).Pumpellyite overgrowing detrital amphibole, both in DCB and CB, shows intermediate X Mg values (Figure 8d; Supp.Table S2).
Prehnite: In CB, prehnite with the highest and lowest Fe 3+ values (Figure 8c, Supp.Table S2) form lamellas in the same interspace.Irregular shaped crystals enclosed in calcite indicate a crystallization from CO 2 -poor fluids, predating calcite (see Wheeler et al. 2001).
Feldspar: Detrital plagioclase has a composition variable from albite to oligoclase.Metamorphic grains on the contrary consists of albite.K-feldspar in veins and in the metamorphic matrix shows a homogeneous composition (Figure 8e).
White mica: In CB, white mica in veins (if coexisting with pumpellyite and chlorite up to 3.43 apfu of Si) shows higher phengitic content with respect to detrital and metamorphic matrix white mica in FS (Figure 8f, Supp.Table S2).

Thermodynamic modelling
Two types of equilibrium assemblages were considered for phase diagrams in distinct micro-domains: spaces between clasts in meta-conglomerate (Figure 5e) and veins in meta-siltstone (Figure 7d).The interstices between metabasite clasts in meta-conglomerate (sample MA20) are filled by equilibrium assemblage of prehnite, pumpellyite, epidote, and quartz (cfr.Figure 5e).This micro-domain was modelled in the CFMASHO system (Figure 9a).The equilibrium assemblage Prh + Pmp + Ep + Qz + H 2 O (mineral abbreviations in Figures 5-7 caption) is stable between 260 and 310°C, and 0.25 and 0.50 GPa.The stability field is confined by heulandite-in reaction at low temperature, chlorite-in at low pressure, lawsonite-in and garnet-in at high pressure and temperature, respectively.
The pumpellyite-chlorite-white mica-albite-quartz assemblage in veins in the meta-siltstone (sample MA28; Figure 7d) was modelled in the KNCFMASHO system (Figure 9b).The equilibrium assemblage Pmp + Chl + Wm + Ab + Qz + H 2 O is stable for temperatures and pressures comprised between 200 and 380°C, and 0.1 and 0.7 GPa.The stability field is constrained by heulandite-in at low temperature, prehnite-in at low pressure, amphibole-in at high pressure, and garnet-in at high temperature.
The widespread twinning type in calcite grains in veins or cement (type II = 150-300°C according to Burkhard 1993) is coherent with the temperatures estimated in both modelled micro-domains.Moreover, in prehnite the molar fraction of Fe 3+ /(Fe 3+ + Al VI ) is up to 0.2 and consistent with crystallization temperature of about 250°C (see Wheeler et al. 2001).
According to amphibole phase relations (Banno et al. 1984), ferro-ferri-winchite with minor riebeckite and actinolite rimming detrital amphibole associated with albite, chlorite, haematite, and quartz in rock matrix suggests that the rocks of the Badstub Formation were metamorphosed at temperature lower than 400°C and pressure at around 0.4-0.6GPa.These conditions are consistent with the results of thermodynamic modelling (Figures 9 and  10a).Convergence of PT estimates suggests that synmetamorphic deformation in the metasedimentary rocks of the Badstub Formation occurred at 260-310°C and 0.25-0.50GPa (Figure 10a).

Thermal state of the metamorphic overprint in the Badstub Formation
Estimated metamorphic temperatures and pressures for the Badstub Formation are assessed at 260-310°C and 0.25-0.50GPa.These estimates are based on the metamorphic equilibrium mineral assemblages that developed in coronas and veins.Equilibrium microdomains including (1) intraclast spaces filled by quartz, pumpellyite, prehnite, and epidote in meta-conglomerate and (2) veins filled by chlorite, pumpellyite, phengitic mica, quartz, and albite in meta-siltstone have been considered for thermodynamic modelling.
In order to shrink such a large stability field and constrain the most reliable PT ratio, we consider phase compositions.Thermometry based on metamorphic chlorite from matrix, coronas, S1 foliation, and veins (10 samples, 37 analyses) results in 290 ± 50°C, except for chlorite localized in micro-shear zones that shows lower temperature around 210 ± 70°C (Supp.Table S2).In addition, in the pumpellyite-chlorite-white mica-albitequartz veins, the chlorite composition is consistent with temperature of 280 ± 20°C.In the same veins, the Si content in white mica allows tracking the 3.43 apfu Siisopleth in the pseudosection (Figure 9b).The intersection between those constrains based on phase compositions occurs at 290°C and 0.4 GPa and overlaps the two PT fields obtained with thermodynamic modelling (Figure 9, 10a).Therefore, these are considered the most probable conditions under which the Badstub Formation experienced metamorphism.
Although the full range of pressure would imply a depth between 9 and 18 km, considering a density of 2.8 g cm −3 and the pressure of 0.4 GPa, a burial of 14 km would result.From the depth of 14 km and the temperature of 290°C a thermal state of about 20°C km −1 results.This is the most likely thermal state under which the metamorphic mineral assemblages that overprinted the Badstub Formation developed.
Thus, in this work, we provide the first quantitative PT estimates of the metamorphic overprint in the Carboniferous of Nötsch.The temperatures we estimated are slightly higher than those previously estimated for the Carboniferous of Nötsch (Rantitsch 1995), whereas the depth of about 14 km is markedly higher than a few kilometres of subsidence that were indirectly derived from thermal estimates combined with a supposed thermal gradient of 40°C km −1 (Rantitsch 1995).Our independent quantitative PT estimates opens to new interpretations and scenarios of the tectono-metamorphic evolution within the Drauzug-Gurktal Nappe System.

Selection of the tectonic scenarios for geodynamic modelling
The thermal state assessed for the metamorphism of the Badstub Formation points to the maximum term of the range 6-20°C km −1 derived from the pressure and temperature estimates of the Eo-Alpine metamorphic peak in the Upper Austroalpine basement nappes (Faryad and Hoinkes 2003;Schuster et al. 2004;Gaidies et al. 2008;Janák et al. 2015;Schulz 2017;Li et al. 2021).A similar thermal state of ca.18°C km −1 derives from constraints on the Late Cretaceous metamorphism in the Graz Palaeozoic of the Drauzug-Gurktal Nappe System, which point to higher temperature and pressure conditions (Krenn et al. 2008;Schantl et al. 2015) than those estimated in the Badstub Formation.Conversely, the thermal state estimated for the Badstub Formation is colder than 25-50°C km −1 derived in the Upper Austroalpine basement nappes for the Permian-Triassic metamorphism (Figure 10b) recorded during the lithospheric thinning (Stöckhert 1987;Thöni and Miller 2000;Habler and Thöni 2001;Schuster et al. 2001Schuster et al. , 2022;;Gaidies et al. 2006;Tenczer et al. 2006;Spalla and Marotta 2007;Spalla et al. 2014;Schulz 2017;Roda et al. 2019).As the thermal gradient is usually increasing close to the surface, rather than the Permian-Triassic extension, the thermal state we estimated for the metamorphism recorded in the Badstub Formation better agrees with lithosphere convergence and thickening (e. g.Cloos 1993;Ernst and Liou 2008;Regorda et al. 2020).Indeed, the metamorphic conditions experienced by the  Banno et al. (1984).The results converge to T = 260-310°C and P = 0.25-0.50GPa.(b) Metamorphic conditions determined for the Badstub Formation (cyan rectangle) compared with the petrogenetic PT grid (modified after Ernst 1976) with black-thick lines representing geothermal gradients traditionally associated to arc regions (1), plate interior (2), warm subduction zones (3), and cold subduction zones (4), according to Cloos (1993) and red-thick lines representing stable (V 0 ) and relaxed (V∞) geotherms, according to England and Thompson (1984).Labels of metamorphic facies: Z: zeolite; PP: prehnite-pumpellyite; BS: blueschist; GS: greenschist; EA: epidote amphibolite; A: amphibolite; E: eclogite; G: granulite.The pink area represents the PT field of Permian-Triassic metamorphic imprints recorded in the continental crustal units of the Alps (Roda et al. 2019 and references therein).
Badstub Formation are comprised between the warm subduction and the plate interior geotherms (Figure 10b) and definitely unrelated to the spreading ridge or volcanic arc geotherm, as expected for a continental rifting scenario.Therefore, we consider the Alpine convergence as responsible for the metamorphic overprint on the Badstub Formation.This interpretation is also consistent with the age of Carboniferous of Nötsch that excludes a Variscan and late Carboniferous age (Schulz 2017;Neubauer et al. 2022) for the metamorphism recorded in the Badstub Formation.
Palaeogeographical reconstructions of pre-Alpine times show that the Carboniferous of Nötsch formed at the western margin of the Palaeotethys embayment on the palaeo-Adria plate (Vachard et al. 2018;Neubauer et al. 2022).In Permian and Triassic times, the subduction and retreat of the Palaeotethys, led to the back-arc opening of Meliata-Hallstatt ocean, a part of the western Tethyan realm (Thöni and Jagoutz 1993;Schmid et al. 2004Schmid et al. , 2008;;Chang et al. 2020Chang et al. , 2022;;Belak et al. 2022;Neubauer et al. 2022).In the Eastern Alps, Middle Triassic radiolarite strata in the Northern Calcareous Alps were envisaged as the continuation of the Meliata-Hallstatt ophiolite from the Western Carpathians (Mandl and Ondrejičová 1991).Despite remnants of the Meliata-Hallstatt ocean are clear in the Western Carpathians, in many reconstructions this ocean is considered also for the Eastern Alps, although here ophiolitic remnants are scarce (Channell and Kozur 1997) and a clear suture zone has not been identified (Froitzheim et al. 2008).The subsequent subduction of the Meliata-Hallstatt ocean started at the end the Triassic, according to the former reconstruction (Kozur 1991).More recent works in the Eastern Alps and Western Carpathians asses the onset of this subduction in the Early Jurassic and the end in the Late Jurassic (Froitzheim et al. 1996;Faryad and Henjes-Kunst 1997;Missoni and Gawlick 2011).In the Internal Dinarides the subduction of the Meliata-Hallstatt ocean happened between the Middle and Late Jurassic (Slovenec and Šegvić 2019;Belak et al. 2022).At the time of Meliata-Hallstatt subduction, the Penninic ocean (i.e.Alpine Tethys) was opening (Mevel et al. 1978;Baumgartner et al. 1995;Stampfli et al. 1998;Bill et al. 2001;Schaltegger et al. 2002;Ratschbacher et al. 2004;Schmid et al. 2008;Handy et al. 2010;Li et al. 2013;Tribuzio et al. 2016;Rebay et al. 2018;Roda et al. 2019;Gleißner et al. 2021;Nicollet et al. 2022).Afterwards in the Eastern Alps, the subduction of the Penninic ocean below the Austroalpine Nappes was ongoing in Coniacian times (Stern and Wagreich 2013) and possibly started at ca. 110 Ma in the Albian times (Wagreich 2001) with the record of eclogitic metamorphism in the Tauern Window between 90 and 38 Ma (Ratschbacher et al. 2004;Kurz et al. 2008;Schmid et al. 2013).
In the evolutionary context depicted above, two main scenarios can be considered for the development of the metamorphism in the Badstub Formation and in general for the Alpine tectono-metamorphic evolution of the Upper Austroalpine basement nappes of the Eastern Alps.The first scenario accounts for the continental subduction either starting as an intracontinental subduction (Janák et al. 2004(Janák et al. , 2015;;Stüwe and Schuster 2010;Vrabec et al. 2012;Miladinova et al. 2021) or a continental collision of the passive margin once the oceanic lithosphere was consumed (Thöni and Jagoutz 1993;Chang et al. 2020Chang et al. , 2022)).The second scenario accounts for the ablative subduction of the Austroalpine upper plate that would have been operated by subducting oceanic lithosphere (Roda et al. 2012).

Set-up of the thermomechanical numerical model
Based on the discussions reported in paragraph 7.1, plate convergence (i.e.Alpine convergence) is inferred as the most likely tectonic setting responsible for the metamorphic thermal state recorded in the Badstub Formation.Thus, we perform a thermomechanical model for a generic ocean-continent subduction followed by continental collision, which allows comparing estimated metamorphic conditions with the thermal state resulting from either ablative oceanic subduction or continental subduction.The model combines linear viscous rheology for the sub-lithospheric mantle with linear viscoplastic rheology for the whole lithosphere (Marotta et al. 2020;Regorda et al. 2021).The physics is described by the equations of continuity of conservation of momentum and of conservation of energy, which includes the extended Boussinesq approximation to account for compressibility (e.g.Christensen and Yuen 1985;Gerya 2010;Ismail-Zadeh and Tackley 2010).Numerical integration has been performed by means of the 2D finite elements code SubMar (Marotta et al. 2006), in a rectangular domain 1400 km wide and 700 km deep (Figure 11).The Earth's surface has been treated by means of 10 km-thick sticky-air.The characteristic compositions of lower and upper continental crust, oceanic crust, and lithospheric mantle of the upper plate have been differentiated by means of markers distributed regularly with a density of 1 marker per 0.25 km 2 .The rheological weakening of the mantle wedge has been simulated by assuming a constant viscosity 10 19 Pa•s for the serpentinized mantle (Honda and Saito 2003;Arcay et al. 2005;Gerya and Stöckhert 2006;Roda et al. 2010Roda et al. , 2011)).The stability field of serpentine (Schmidt and Poli 1998) has been delimited inside the hydrated mantle wedge (see details in Regorda et al. 2017Regorda et al. , 2020;;Marotta et al. 2020).Rock properties and rheological parameters are as in Regorda et al. (2021) and here synthetized in Supp.Table S4.
The velocity boundary conditions correspond to no-slip conditions along the upper and the lower boundaries of the 2D domain and free-slip conditions along the right boundary.In addition, a velocity of 3 cm yr −1 has been prescribed at the bottom of the oceanic crust as already proposed as best scenario for the Alpine convergence (Roda et al. 2010 and references therein), and along a 45° dipping plane that extends from the trench to a depth of 100 km, to facilitate the trigger of the subduction.
The thermal boundary conditions correspond to fixed temperatures at the top (27°C) and at the bottom (1327° C) of the model and zero thermal flux at the vertical sidewalls.The initial thermal structure is characterized by a simple conductive thermal configuration throughout the lithosphere, with temperatures that increase linearly from the surface to the base at a depth of 80 km, and a constant temperature of 1327°C below the lithosphere.The initial geometry of the model is shown in Figure 11.The continental and oceanic crust is 30 and 10 km thick, respectively.In this way, we simulated 62 Myr of oceanic subduction followed by 60 Myr of continental subduction.This timing may apply for either the duration of the Meliata-Hallstatt or Penninic ocean subductions (see paragraph 7.1).However, this modelling does not involve the intra-oceanic subduction that some works suggest for the closure of Meliata-Hallstatt ocean below the Vardar ocean, which eventually would had been obducted onto the continental Adria margin (Schmid et al. 2004(Schmid et al. , 2008;;Handy et al. 2010;Slovenec and Šegvić 2019).

Comparison of model results
During the oceanic subduction, ablation of tectonic slices from the upper plate occurs with a consequent syn-subduction exhumation of subducted continental slices within the orogenic wedge (Figure 12a-c).The exhumation is driven by the contrast of low viscosity of sediments and hydration and serpentinization of the suprasubductive mantle wedge against the high viscosity of continental and oceanic slices.The ablation produces the bending of the upper plate with a consequent tectonic extrusion of the lithospheric mantle, which is not followed by a thermal rising in the lithosphere (as shown by the The characteristic compositions of lower and upper continental crust, oceanic crust, and lithospheric mantle of the upper plate have been differentiated by means of markers distributed regularly with a density of 1 marker per 0.25 km 2 .The velocity boundary conditions correspond to no-slip conditions along the upper and the lower boundaries of the 2D domain and free-slip conditions along the right boundary.In addition, a velocity of 3 cm yr −1 has been prescribed at the bottom of the oceanic crust and along a 45° dipping plane that extends from the trench to a depth of 100 km, to facilitate the trigger the of the subduction.The thermal boundary conditions correspond to fixed temperatures at the top (27°C = 300 K) and at the bottom (1327°C = 1600 K) of the model and zero thermal flux at the vertical sidewalls.The initial thermal structure is characterized by a simple conductive thermal configuration throughout the lithosphere, with temperatures that increase linearly from the surface to its base located at a depth of 80 km, and a constant temperature of 1327°C below the lithosphere.
configuration of the isotherms in Figure 12).The thermal gradient affecting the subduction zone decreases from initial to mature stages, because of the cooling induced by the burial of oceanic slab.During the collision, the continental crust of the lower plate is subducted to depths (Figure 12d-f); afterwards the convergence ends, and the gravitational re-equilibration begins.Therefore, the thermal gradient in the slab increases because of the radiogenic heating induced by the subducted continental crust.On the contrary, the thermal gradient in the orogenic wedge slightly decreases because of the reduction of the suprasubductive mantle flow.
The PT conditions, based on metamorphic mineral assemblages developed in the analysed Badstub Formation rocks, can be potentially achieved in the upper plate of the proposed model during different stages of the oceanic subduction starting from 15 Myr, as well as in the lower plate during the continental subduction after the ocean closure.According to the model, during the oceanic subduction the region Isotherms 260 and 310°C are reported as white dashed lines.The orogenic wedge is enclosed in the shaded area.The PT conditions inferred for the metamorphism of Badstub Formation (T = 260-310°C and P = 0.25-0.5 GPa) in the continental crust are indicated by cyan area.In the upper plate these conditions are confined within the orogenic wedge where tectonic burial occurs, whereas in the lower plate these conditions are constrained by 5 km of sedimentary sequences above the Badstub Formation indicated with the black dashed line.Model results indicate that the metamorphic conditions recorded by the Badstub Formation are potentially achieved in the upper plate during oceanic subduction or in the lower plate during continental subduction at a depth of 13-18 km and 16-18 km, respectively.Colour map after Crameri (2018).
where the PT conditions of metamorphism are achieved is within the orogenic wedge (cyan area in the shaded region of Figure 12a-c), where the continental crust, originally at 5 km depth, was buried at a depth between 13 and 18 km (Figure 12b; maximum depth range that satisfies the PT estimates) and then exhumed.In this way, the numerical simulation lets to shrink the depth range constrained in the paragraph 6.3, being anyway consistent with the value of 14 km.The region in which the PT conditions recorded in Badstub Formation are satisfied (Figure 12a-c) shrinks with time because (i) the orogenic wedge becomes colder and (ii) the mantle above the subduction channel rises (i.e.tectonic extrusion of the lithospheric mantle).After the onset of the continental collision and the subsequent continental subduction, the metamorphic conditions of the Badstub Formation are no longer matched (Figure 12d, f) except for a narrow zone in the lower continental plate about 20 Myr after the onset of continental collision (Figure 12e), where metamorphic conditions are matched at a depth between 16 and 18 km.

Discussion
In this section, we discuss the possible scenarios based on the tectonic, geodynamic, and palaeogeographical interpretations available from the literature.Without any absolute time constraint on the metamorphism overprinting the Badstub Formation, we can base our tectonic and geodynamic interpretations on the fitting between the PT conditions estimated with thermodynamic modelling and reproduced by the thermomechanical modelling.Since from the discussions in the previous chapters the plate convergence involving oceanic subduction is the reliable scenario for explaining the metamorphism in the Badstub Formation, in the following we discuss scenarios that involve both the Meliata-Hallstatt and Penninic oceans, which are considered in the literature for the evolution of the Eastern Alps.We compare the relative timing of PT condition fitting in the thermomechanical model with the timing constrains in the literature for the evolution of both oceans and concerned continental margins.

Scenarios involving continental collision
A possible scenario may imply that the whole Austroalpine domain was part of the northern margin of Meliata-Hallstatt ocean (Thöni and Jagoutz 1993;Chang et al. 2020).In this view, the convergence would have started in the latest Triassic (Kozur 1991) or Early Jurassic (Missoni and Gawlick 2011) with the ocean plate subducting beneath the Adria plate (e.g.Chang et al. 2020) for a time interval of 60-40 Myr.After the complete consumption of the Meliata-Hallstatt lithosphere in the Late Jurassic (Kozur 1991;Faryad and Henjes-Kunst 1997;Schmid et al. 2004;Froitzheim et al. 2008) the Austroalpine passive margin collided with the Adria margin (Neubauer et al. 1999).In this view, the Carboniferous of Nötsch could have stood on a thinned Austroalpine passive margin before entering in the footwall of the collisional margin (i.e.continental subduction).In the numerical model, this scenario is highlighted by the fitting during continental collision (Figure 12e).Following other reconstructions, the Meliata-Hallstatt ocean formed within the present-day Upper Austroalpine basement nappes so that the Drauzug-Gurktal Nappe System was placed at its southern margin (Kurz and Fritz 2003;Schmid et al. 2004;Neubauer et al. 2022).In this case, the Badstub Formation was placed in the upper plate during the continental collision.However, the numerical model does not reproduce this scenario (Figure 12) because PT conditions estimated for the Badstub Formation are not achieved in the upper plate (Figure 12d-f).Whatever the case, the metamorphic overprint recorded in the Badstub Formation should be related to the continental collision post-dating the closure of Meliata-Hallstatt ocean that took place in the Late Jurassic.This timing is consistent with the subduction timing of continental units such as the Sieggraben unit where the eclogitic peak was recorded between 113 and 86 Ma (Chang et al. 2022) and the Schöckel Nappe of the Graz Palaeozoic where the epidote-amphibolite facies peak is at 119-112 Ma (Schantl et al. 2015).On the other hand, since the closure of Meliata-Hallstatt ocean is much older than the cluster of ages between 95 and 90 Ma retrieved for the Alpine highpressure and ultrahigh-pressure metamorphic peak recorded in the Upper Austroalpine basement nappes (Thöni and Jagoutz 1992;Thöni et al. 2008;Sandmann et al. 2016;Schulz 2017;Miladinova et al. 2021;Li et al. 2021), some papers explain the high-pressure age cluster at 95-90 Ma as due to an intracontinental subduction.Such subduction would had been induced by the reactivation of the Permian rift that is testified by the Saualpe and Koralpe metagabbro bodies and it is supposed to have developed west of Meliata-Hallstatt ocean (Janák et al. 2004(Janák et al. , 2015;;Stüwe and Schuster 2010;Miladinova et al. 2021).The mechanical trigger of this intracontinental subduction would had been a gravitational instability due to a thermal thickening of the lithospheric mantle during the cooling of the rifted Adria lithosphere (Stüwe and Schuster 2010).Although the mechanism is given, a thoughtful analysis of the thermal implication of the intracontinental subduction is not available from the literature.Indeed, the thermal state estimated for the Eo-Alpine event in the Upper Austroalpine basement nappes is also consistent with that achieved during an active oceanic subduction (Roda et al. 2012;Penniston-Dorland et al. 2015;Regorda et al. 2020).

Scenarios involving ablative oceanic subduction
The geodynamic evolution of convergent systems that include ablative oceanic subduction has been thoroughly analysed and described by many authors (e.g.Gerya and Stöckhert 2006;Meda et al. 2010;Roda et al. 2012;Regorda et al. 2017;Stern and Gerya 2018).The possibility for the subducting oceanic lithosphere of ablating slices from continental crust was postulated about 30 years ago for the Western Alps (Polino et al. 1990).Afterwards, this hypothesis was supported by a thoroughly thermomechanical analysis of metamorphic conditions all over the Austroalpine domain (Stöckhert and Gerya 2005;Meda et al. 2010;Roda et al. 2012).In the numerical model here presented this scenario is highlighted by the fitting with the PT estimates starting 15 Myr after the subduction onset (Figure 12b,c).In order to explain the metamorphism recorded by the Badstub Formation, the oceanic ablation of the continental crust may apply to either Meliata-Hallstatt or Penninic oceans, because the duration of their subductions (ca.60-40 Myr) are comprised within the subduction time simulated in the thermomechanical model (62 Myr).In the first case, the most suitable palaeogeographical scenario considers the Drauzug-Gurktal Nappe System at the southern Meliata-Hallstatt ocean margin (Kurz and Fritz 2003;Schmid et al. 2004Schmid et al. , 2008;;Neubauer et al. 2022).In this case, the Badstub Formation was part of the upper plate during 60 to 40 Myr subduction and would have been dragged at depth by the Meliata-Hallstatt oceanic slab.The ablation induced by Meliata-Hallstatt oceanic slab can be consistent with radiometrical and petrological data indicating that continental material was subducted before the end of the subduction of Meliata-Hallstatt ocean in the Western Carpathians (Faryad and Henjes-Kunst 1997).Potentially the ablation may be operated also by the subduction of the Penninic ocean (Tauern Window) beneath the Austroalpine margin whose initiation may be assessed at ca. 110 Ma (Wagreich 2001).This age fits well with the ablation by the subducting Penninic slab that could have dragged the part of the Upper Austroalpine basement nappes, which record the eclogitic pressure peak at 95-90 Ma (see previous paragraph), whereas the collision of the European passive margin ended at ca. 33 Ma (Nagel et al. 2013).The duration of simulated subduction in the thermomechanical model is 62 Myr and thus includes the time interval necessary for the fitting of the PT estimates and to complete the subduction of both oceans, according to the literature cited in the previous paragraphs.

The shallow tectono-metamorphic evolution of the Badstub Formation
Either continental collision or oceanic subduction is considered for interpreting the tectonic significance of the metamorphism constrained in this work, the Badstub Formation was involved in the evolution of the orogenic wedge staked at the margin of Adria plate, but at significant shallower levels compared to the deeper tectonic systems of the Upper Austroalpine basement nappes.Within the Palaeozoic sequences of Drauzug-Gurktal Nappe System, the Badstub Formation recorded metamorphic peak pressures lower than those recorded in the Graz Palaeozoic (Schantl et al. 2015) and peak temperatures comparable with those recorded under subgreeschist facies in the Gurktal Range (Rantitsch et al. 2020).Since during Permian-Mesozoic times about 5 km of sedimentary sequences formed above the Badstub Formation (Tollmann 1977;Rantitsch 1995), the cumulative depth of 13-18 km (Figure 12b) includes a tectonic burial of about 8-13 km.Despite during the Alpine convergence these rocks experienced a tectonic burial of 8-13 km, the Badstub Formation rocks should have behaved as a solidly poorly strained block.This is demonstrated by the heterogeneous development of the S1 foliation only in the fossiliferous schist and locally in sandstone and siltstone.As a result, very pristine sedimentary features are preserved and even an outstanding palaeontological record survived.In addition, in the studied section the normal stratigraphic polarity is preserved and strata dip southward of about 45° (Figure 2, inset a).Thus, the most important component of the finite deformation of the Badstub Formation rocks was a translation and tilting, whereas accumulation of plastic strain was scarce.This observation agrees with the scarce possibility of mylonite development in a lowgrade metamorphic environment, as predicted by numerical modelling, which explores the relationship between metamorphic environments and strain gradients in subduction systems (Regorda et al. 2021).

Conclusions
The Badstub Formation includes meta-conglomerate and -breccia and minor -sandstone, -siltstone, and fossiliferous carbonatic schists.Although it is exposed in the axial part of the Alpine belt north of the Periadriatic Fault System, it preserves pristine sedimentary features, among which an outstanding Carboniferous fossil record.Even if sedimentary features are prominent and easily detectable at first glance, the multiscale structural analysis revealed an S1 foliation, which is pervasive in the fossiliferous carbonatic schists, veins, and different sets of fault systems, among which some are mineralized.Moreover, microscale analysis reveals very low-grade metamorphic assemblages in micro-veins, intraclasts spaces, coronas, and marking S1.Mineral assemblages are characterized by pumpellyite, prehnite, chlorite, phengitic mica, riebeckite, and winchite that are compatible with thermodynamic modelling revealing that the Badstub Formation re-equilibrated at 260-310°C and 0.25-0.50GPa.These results provide the first quantitative independent pressure constraints for the inferred temperature in the Carboniferous covers the Upper Austroalpine basement nappes south of the SAM line.In a 2D numerical thermomechanical modelling these temperature and pressure conditions demonstrate a cumulative burial (depositional and tectonic) of 13-18 km and are consistent with the development of the metamorphism recorded in the Badstub Formation during the Alpine convergence.According to the numerical model results, this metamorphism could have been recorded either in the upper or lower continental plate, either during oceanic subduction or continental collision.For the oceanic subduction, we advocate the tectonic ablation of the upper plate by the subducting oceanic slab, either operated by the Meliata-Hallstatt or Penninic ocean.The continental collision could have been developed by continental subduction of the passive Austroalpine margin beneath Adria as a consequence of the complete closure of the Meliata-Hallstatt ocean.Whichever geodynamic interpretation, during the downward and upward tectonic paths, the Badstub sequence recorded scarce plastic strain and was mostly translated and tilted as a coherent block that was eventually stacked into the Alpine orogenic wedge accreted at the Adria margin.Finally, these metamorphic conditions are consistent with the Upper Austroalpine basement nappes recording the Alpine convergence under conditions that span from eclogitic to prehnite-pumpellyite facies.

Figure 2 .
Figure 2. Geological map of the Jakomini quarry mapped in mid-December 2017 and N-S geological cross-section.Worsche Vermessung provided the base map.Pale colours indicate the inaccessible parts of the quarry during the fieldwork.In the inset equal area stereographic projection, lower hemisphere, with number of orientation data (N): (a) pole to plane of bedding (S0) and S1 foliation; (b) planes of fault surface (FP) and linear kinematic indicators (FL).

Figure 3 .
Figure 3. (a) Metabasic clasts embodied in the red matrix intersected by fracture systems in CSS.(b) Decimetre-thick conglomeratic breccia bed with angular marble clasts and minor metabasic clasts, in CSS.(c) Angular to subrounded metabasic and marble clasts in the mafic conglomerate of DCB.(d) Decimetre-thick red sandstone and siltstone beds within the mafic conglomerate of DCB.(e) Centimetre-thick layer of conglomerate in DCB that is cemented by calcite.(f) Boundary between brachiopod-rich shale strata of sector FS and the greenish sandstone of DCB.(g) Centimetre-thick mafic siltstone and pelite beds (white arrows) marking the boundary of conglomerate beds in CB.(h) Rounded metabasic and marble clasts in the conglomerate of CB.

Figure 4 .
Figure 4. (a) Bedded red conglomerate, sandstone, and siltstone of CSS that are intersected by cataclastic bands and fracture systems.(b) Decimetre-thick beds of siltstone and sandstone of CSS with S1 foliation recorded only in siltstone; (c) Brachiopod-rich shale that records the S1 foliation intersecting the bedding (S0) in FS; coin for scale.(d) Calcite veins in a centimetre-thick shear zone deflecting S1 foliation in fossiliferous carbonatic schist of FS; ink marker for scale.(e) Fault surface with calcite fibres in the schist of FS.(f) Centimetre-thick vein filled by epidote and chlorite in the conglomeratic breccia of CB.

Figure 5 .
Figure 5. (a) Sample MA17, CSS, meta-conglomerate with sub-rounded to angular clasts surrounded by haematite matrix and intersected by calcite veins; plane polarized light; (b) Sample MA12, DCB, meta-conglomerate with dominant rounded metabasic clasts in recrystallized calcite cement; plane polarized light; (c) Sample MA12, DCB, detail on twinning in polygonal calcite around a foliated metabasic clast; plane polarized light; (d) Sample MA20, DCB, interstitial coarse-grained quartz and calcite crystals with pumpellyite radial to the clast surface; plane polarized light; (e) Sample MA20, DCB, close up of previous photo with pumpellyite (Pmp) crystals radial to the clast surface and associated with epidote (Ep) in contact with euhedral coarse-grained quartz (Qz) crystal; prehnite (Prh) is enclosed in calcite (Cc); backscattered electron image; (f) Sample MA23, sector DCB, portion of brecciated amphibolite clast with amphibole (Amp) from the clast with a rim of coronitic blue amphibole (light grey) in contact with calcite (Cc); also chlorite (Chl) partly replace amphibole; backscattered electron image.Sample location is shown in Figure 2.

Figure 7 .
Figure 7. (a) Sample MA28 meta-siltstone of CB in which the original stratigraphy (S0) is poorly preserved and S1 is the dominant fabric; S1 is intersected by subsequent generation of veins; plane polarized light; (b) Sample MA28 meta-siltstone of CB; vein filled by white mica (Wm), quartz (Qz), and minor chlorite (Chl); close to the vein pumpellyite (Pmp) formed in the rock matrix; backscattered electron image; (c) Sample MA28 meta-siltstone of CB; microscopic vein filled by pumpellyite (Pmp), albite (Ab), white mica (Wm), and chlorite (Chl); backscattered electron image.Sample location is shown in Figure 2.

Figure 9 .
Figure 9. (a) CFMASHO phase diagram that models the stability field of mineralized interstices between metabasite clasts in MA20 metaconglomerate.The bulk composition determined on the modal amount of metamorphic minerals is reported at the top of the diagram.The Pre + Pmp + Ep + Qz + H 2 O stability field is coloured in green.Univariant lines are double thickness.(b) KNCFMASHO phase diagram that models the stability field of mineralized veins in MA28 meta-siltite.The bulk composition determined on the modal amount of metamorphic minerals is reported at the top of the diagram.The Pmp + Chl + Wm + Ab + Qz + H 2 O stability field is coloured in light blue.Si-in white mica isopleths (apfu) are tracked in dark blue.Small fields were not labelled.Univariant lines are double thickness.

Figure 11 .
Figure 11.Model set-up.The model domain is 1400 km wide and 700 km deep.The Earth's surface has been treated by means of a 10 km-thick sticky-air.The characteristic compositions of lower and upper continental crust, oceanic crust, and lithospheric mantle of the upper plate have been differentiated by means of markers distributed regularly with a density of 1 marker per 0.25 km 2 .The velocity boundary conditions correspond to no-slip conditions along the upper and the lower boundaries of the 2D domain and free-slip conditions along the right boundary.In addition, a velocity of 3 cm yr −1 has been prescribed at the bottom of the oceanic crust and along a 45° dipping plane that extends from the trench to a depth of 100 km, to facilitate the trigger the of the subduction.The thermal boundary conditions correspond to fixed temperatures at the top (27°C = 300 K) and at the bottom (1327°C = 1600 K) of the model and zero thermal flux at the vertical sidewalls.The initial thermal structure is characterized by a simple conductive thermal configuration throughout the lithosphere, with temperatures that increase linearly from the surface to its base located at a depth of 80 km, and a constant temperature of 1327°C below the lithosphere.

Figure 12 .
Figure 12.Thermal field of the model for 122 Myr of convergence, from subduction (a-c) to continental collision and gravitational requilibration (d-f).Black solid lines represent crustal boundaries (OC = oceanic crust; CC = continental crust; S = trench sediments).Isotherms 260 and 310°C are reported as white dashed lines.The orogenic wedge is enclosed in the shaded area.The PT conditions inferred for the metamorphism of Badstub Formation (T = 260-310°C and P = 0.25-0.5 GPa) in the continental crust are indicated by cyan area.In the upper plate these conditions are confined within the orogenic wedge where tectonic burial occurs, whereas in the lower plate these conditions are constrained by 5 km of sedimentary sequences above the Badstub Formation indicated with the black dashed line.Model results indicate that the metamorphic conditions recorded by the Badstub Formation are potentially achieved in the upper plate during oceanic subduction or in the lower plate during continental subduction at a depth of 13-18 km and 16-18 km, respectively.Colour map afterCrameri (2018).