Organic metamorphism as a key for reconstructing tectonic processes: a case study from the Austroalpine unit (Eastern Alps)

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 reflectance measurements and Raman spectroscopy of carbonaceous materials (RSCM). Automated, interactive fitting 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 profile 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 confidence 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 profile 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 numerical model which emphasizes the significance of an increased thermal gradient in a continental margin towards the western Neotethyan ocean during Permo-Triassic lithospheric extension. It finally characterizes the Eoalpine metamorphic gradient during nappe stacking and a significant metamorphic jump related to exhumation and normal faulting.

Conodont thermometry by Raman spectroscopy on carbonaceous material: a case study from the Northern Calcareous Alps (Mürzalpen Nappe, Eastern Alps)

“…The study data compare conventional RSCM temperatures estimated on metapelites (Lünsdorf et al, 2017) to estimates from conodonts (McMillan and Golding, 2019). The RSCM estimates from the metapelites ranging between 290 and 310 °C are based on a lab-specific calibration, established by data from samples with well-constrained peak temperatures (Lünsdorf et al, 2017;Rantitsch et al, 2020). Therefore, they are used here as a reference and correlated to CAI values of 5.0-6.5.…”

Raman Spectroscopy of carbonaceous material thermometry in the Carnic Alps (Austria, Italy)

“…In comparison, the sediments of the Hochwipfel Nappe must have been obviously heated at a heat flow significantly exceeding the "normal" heat flow (63 mW/m 2 ) estimated for the Moravian-Silesian Belt (Jirman et al, 2018). This is supported by clay mineralogical data (Sassi et al, 1995;Rantitsch, 1997) and thermal modeling (Rantitsch, 1997) and was attributed to an increased post-Variscan heat-flow during post-Variscan times (Rantitsch, 1997;Rantitsch et al, 2020a).…”

“…Thus, despite a good data coverage, there is certain demand to improve the present view of metamorphism in the Carnic Alps by the use of Raman spectroscopy of carbonaceous material (RSCM), estimating metamorphic temperatures without reference to kinetic parameters and assumed boundary conditions, as needed e.g., to convert vitrinite reflectance data to burial temperatures. RSCM (reviewed by Henry et al, 2019) provides a well-established method to investigate a polymetamorphic unit to confidently estimate peak metamorphic temperatures (e.g., Rantitsch et al, 2020a;Rantitsch, 2023). The RSCM calibration used in this study (Lünsdorf et al, 2017) spans the temperature range that overprinted the pre-Variscan basement and post-Variscan cover units of the Carnic Alps.…”

Phases of Enhanced Exhumation During the Cretaceous and Cenozoic Orogenies in the Eastern European Alps: New Insights From Thermochronological Data and Thermokinematic Modeling

“…This Late Cretaceous phase of exhumation was presumably related to top‐to‐the‐ESE extension that occurred while WNW‐directed thrusting was still active at deeper crustal levels (Figure 8a) (Faupl & Wagreich, 2000; Kurz & Fritz, 2003; Mandl, 2000; Ratschbacher et al., 1989). In the Gurktal Alps, Late Cretaceous extension is documented by petrological and structural data, which show that Eo‐Alpine thrust faults were reactivated as normal faults (Figure 8a) (Koroknai et al., 1999; Rantitsch et al., 2020). Top‐to‐the‐ESE normal faulting may explain the difference in ZHe ages obtained from the central and northeastern Nock Mountains (Figure 4), because it placed the Drauzug‐Gurktal nappe in a hanging‐wall position relative to the Ötztal‐Bundschuh nappe in the footwall (Figures 3c and 8) (Koroknai et al., 1999; Rantitsch et al., 2020).…”

“…In the Gurktal Alps, Late Cretaceous extension is documented by petrological and structural data, which show that Eo‐Alpine thrust faults were reactivated as normal faults (Figure 8a) (Koroknai et al., 1999; Rantitsch et al., 2020). Top‐to‐the‐ESE normal faulting may explain the difference in ZHe ages obtained from the central and northeastern Nock Mountains (Figure 4), because it placed the Drauzug‐Gurktal nappe in a hanging‐wall position relative to the Ötztal‐Bundschuh nappe in the footwall (Figures 3c and 8) (Koroknai et al., 1999; Rantitsch et al., 2020). Late Cretaceous extension was, however, not limited to the Gurktal Alps but occurred orogen‐wide, as indicated by normal faulting in the entire Austroalpine nappe stack (e.g., Schuster et al., 2013), including Austroalpine units exposed in the Saualpe, Koralpe and other regions east of the Gurktal Alps (Krenn et al., 2008; Krohe, 1987; Kurz et al., 2002; Neubauer et al., 1995; Rantitsch et al., 2005; Schorn & Stüwe, 2016; Wiesinger et al., 2006), in the Kreuzeck block southeast of the Tauern window (Griesmeier et al., 2018; Hoke, 1990; Wölfler, Frisch, et al., 2015) and in the Ötztal Alps west of the Tauern Window (Froitzheim et al., 1997; Fügenschuh et al., 2000; Krenn et al., 2011; Ratschbacher et al., 1989).…”