International Journal of Earth Sciences (2020) 109:2235–2253 https://doi.org/10.1007/s00531-020-01897-7 ORIGINAL PAPER Organic metamorphism as a key for reconstructing tectonic processes: a case study from the Austroalpine unit (Eastern Alps) Gerd Rantitsch1 · Christoph Iglseder2 · Ralf Schuster2 · Marianne Sophie Hollinetz3 · Benjamin Huet2 · Manuel Werdenich3 Received: 9 March 2020 / Accepted: 12 June 2020 / Published online: 3 July 2020 © The Author(s) 2020 Abstract At the northwestern margin of the Gurktal Alps (Eastern Alps), Eoalpine (Cretaceous) thrusting of carbonaceous material (CM) bearing metasediments formed a very low- to low-grade metamorphic nappe stack above higher-grade metamorphic basement nappes. Sedimentary burial as well as progressive metamorphism transformed the enclosed CM to anthracite, metaanthracite and semigraphite. In a kinematically well-constrained section at the northwestern frontal margin of the nappe stack, this transformation has been investigated by vitrinite refectance measurements and Raman spectroscopy of carbo- naceous materials (RSCM). Automated, interactive ftting of Raman spectra estimates the metamorphic peak temperatures in a complete section through the upper part of the Upper Austroalpine unit. A RSCM trend indicates a temperature profle of ca. 250–600 °C. The top part of the gradient is reconstructed by one-dimensional thermal modeling. The certainty of ca. ± 25 °C at a confdence level of 0.9 resembles the data variability within a sample location. Due to the large calibration range, the method is able to reconstruct a thermal crustal profle in space and time. The study highlights the versatility of RSCM, which characterizes almost 250 Ma of a complex and polyphase tectonic history. RSCM data characterize the Variscan metamorphic grade in nappes now imbricated in the Eoalpine nappe stack. They additionally constrain a numeri- cal model which emphasizes the signifcance of an increased thermal gradient in a continental margin towards the western Neotethyan ocean during Permo-Triassic lithospheric extension. It fnally characterizes the Eoalpine metamorphic gradient during nappe stacking and a signifcant metamorphic jump related to exhumation and normal faulting. Keywords Eastern Alps · Upper Austroalpine Unit · Gurktal Alps · Organic metamorphism · Raman spectroscopy · Geothermometry Introduction are applied on metamorphic rocks formed at deeper crustal levels (e.g. Lanari and Duesterhoeft 2019, and references The metamorphic structure of a nappe pile refects the spa- therein), while diverse low-temperature thermometers (clay tial and temporal evolution of tectonic processes that shaped mineralogy, organic petrography and geochemistry) and its history. To investigate it, petrological methods applying thermochronometers are used to investigate the near-surface equilibrium thermodynamics on coexisting mineral phases parts of a crustal profle (e.g. Dunkl et al. 2011; Ferreiro * Gerd Rantitsch Manuel Werdenich [email protected] [email protected] Christoph Iglseder 1 Montanuniversität Leoben, Lehrstuhl für Geologie und [email protected] Lagerstättenlehre, Peter-Tunner Straße 5, 8700 Leoben, Ralf Schuster Austria [email protected] 2 Geologische Bundesanstalt Wien, Neulinggasse 38, Marianne Sophie Hollinetz 1030 Wien, Austria [email protected] 3 Department für Geodynamik und Sedimentologie, Benjamin Huet Universität Wien, Althanstraße 14, 1090 Wien, Austria [email protected] Vol.:(0123456789)1 3 2236 International Journal of Earth Sciences (2020) 109:2235–2253 Mählmann et al. 2012). Both approaches work best if a technique to analyze Raman spectra of CM. In this contri- single monocyclic tectono-metamorphic event is explored. bution, we provide a case study, demonstrating the benefts Their combination is however hampered by a methodologi- of this approach in the analysis of a regional metamorphic cal gap in the temperature range between 150 °C and 350 °C, temperature pattern, covering very low- to medium- grade bridged by Raman spectroscopy of carbonaceous material metamorphic conditions never explored before by a single (RSCM) thermometry. geothermometric method. Vitrinite reflectance data are a result of temperature For this purpose, the Upper Austroalpine nappe stack through time and cannot directly translated into a specifc in the northwestern part of the Gurktal Alps was selected temperature. In contrast, RSCM thermometry (Beyssac et al. (Fig. 1). This nappe stack consists of rocks showing a very 2002b) estimates low grade metamorphic peak temperatures. wide range of metamorphic conditions, from diagenesis to This method is now routinely used for characterizing the amphibolite-facies (Hoinkes et al. 1999; Oberhänsli et al. thermal structure of orogens and sedimentary basins (e.g. 2004) formed during the Eoalpine (Cretaceous) collision Angiboust et al. 2009; Souche et al. 2012; Scharf et al. 2013; within the Adriatic microcontinent (Stüwe and Schuster Fauconnier et al. 2014; Vacherat et al. 2014). However, if a 2010). It records a long history with pre-, syn- and post- RSCM thermometer is not accurately calibrated (see Lüns- Eoalpine features (Neubauer 1987; Hoinkes et al. 1999; dorf et al. 2014) or if the efects of strain (Barzoi 2015) Koroknai et al. 1999; Schuster and Frank 1999; Thöni 1999; are underestimated, major problems arise. This was claimed Huet 2015). Any attempt to understand the Eoalpine tecton- by Ferreiro Mählmann and Le Bayon (2016) to explain the ics requires knowledge of the metamorphic grade and the controversial metamorphic map patterns of the Swiss Gla- timing of metamorphism in the individual tectonic units. rus Alps, where vitrinite refectance discontinuities are not This is however complicated by the polycyclic history. mirrored by RSCM data (see also Ferreiro Mählmann et al. The application of classical petrological geothermomet- 2012). To avoid major methodological problems of RSCM ric methods provided major constraints for reconstructing thermometry (Lünsdorf et al. 2014), Lünsdorf et al. (2017) the upper greenschist- to ultrahigh-pressure eclogite-facies proposed the use of the IFORS (“Interactive ftting of Raman metamorphic zonation in the metamorphic core zone of spectra”) software (Lünsdorf and Lünsdorf 2016) as a new an orogen, here represented by the Eoalpine high-pressure Fig. 1 Simplifed geological map of the study area modifed from Iglseder (2019). The nappe system nomenclature follows Schmid et al. (2004) and Froitzheim et al. (2008). The political borders of Austria are given as a geographic reference (Coordinate system: UTM 33 N, EPSG 32633) 1 3 International Journal of Earth Sciences (2020) 109:2235–2253 2237 belt (Janák et al. 2015), which derived from the tectonic Koroknai et al. 1999; Huet 2015). During this extensional lower plate and traces the Cretaceous suture (Schmid et al. event, Gosau Basins (middle Late Cretaceous to Eocene) 2004). Major units of the tectonically overlying upper-plate formed as collapse basins synchronously with the exhuma- segment, widely exposed in the study area, are however tion of formerly deeply buried basement rocks (Neubauer composed of very low-grade to lower greenschist-facies et al. 1995; Fügenschuh et al. 2000; Rantitsch et al. 2005; metamorphosed Paleozoic sediments (Hoinkes et al. 1999; Krenn et al. 2008). Oberhänsli et al. 2004). Due to the methodological gap, their The investigated individual nappes are composed of dif- peak temperatures have not been accurately constrained ferent types of basement and characterized by diferent strati- until now. The investigated section covers the nappe stack graphic ranges of their post-Variscan cover (Fig. 1). Major above the eclogite bearing units (Schmid et al. 2004). As parts of the basement record a polyphase history including carbonaceous material (CM) is present in all implicated Ordovician, Carboniferous (Variscan), Permo-Triassic and/ structural units, organic metamorphism is investigated in or Cretaceous (Eoalpine) tectono-metamorphic imprints an initially ca. 35 km thick (corresponding to peak pressure (Neubauer 1987; Hoinkes et al. 1999; Koroknai et al. 1999; of 10 kbar, Koroknai et al. 1999; Schuster and Frank 1999) Schuster and Frank 1999; Thöni 1999; Huet 2015). For this profle, bridging the methodological gap between petrologi- reason constraining the Eoalpine metamorphic conditions is cal geothermometers, applied in the deeper crust, and kinetic sometimes complicated. The post-Variscan cover deposited models of organic maturation, applied near to the surface. in three sedimentary cycles. The oldest cycle is represented The study data thus provide new evidence to understand the by Pennsylvanian (Stangnock Formation) to Cisuralian complex tectonic history of the Upper Austroalpine nappe (Werchzirm Formation) molasse type sediments deposited stack. within an intramontane basin (Krainer 1993). The next one RSCM data are used to cover the full temperature range comprises upper Permian to Lower Triassic siliciclastic of the rocks, verifed by phase equilibrium modeling, and transgressional series (e.g. Alpine Verrucano, Lantschfeld vitrinite refectance data constrain the Permo-Mesozoic ther- quartzite), overlain by Anisian to Jurassic successions mal evolution of the nappe stack. The study data also evalu- dominated by carbonaceous platform sediments (Pistotnik ate the limits of organic maturity data (vitrinite refectance) 1973/1974). During the Eoalpine event, sediments