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

Benjamin Huet,Ralf Schuster,Marianne Sophie Hollinetz,Gerd Rantitsch,Christoph Iglseder,Manuel Werdenich

doi:10.1007/s00531-020-01897-7

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.

Highlights

The metamorphic structure of a nappe pile reflects the spatial and temporal evolution of tectonic processes that shaped its history
Both approaches work best if a single monocyclic tectono-metamorphic event is explored. Their combination is hampered by a methodological gap in the temperature range between 150 °C and 350 °C, bridged by Raman spectroscopy of carbonaceous material (RSCM) thermometry
If a Raman spectroscopy of carbonaceous materials (RSCM) thermometer is not accurately calibrated or if the effects of strain (Barzoi 2015) are underestimated, major problems arise. This was claimed by Ferreiro Mählmann and Le Bayon (2016) to explain the controversial metamorphic map patterns of the Swiss Glarus Alps, where vitrinite reflectance discontinuities are not mirrored by RSCM data

Summary

Introduction

The metamorphic structure of a nappe pile reflects the spatial and temporal evolution of tectonic processes that shaped its history. If a RSCM thermometer is not accurately calibrated (see Lünsdorf et al 2014) or if the effects of strain (Barzoi 2015) are underestimated, major problems arise This was claimed by Ferreiro Mählmann and Le Bayon (2016) to explain the controversial metamorphic map patterns of the Swiss Glarus Alps, where vitrinite reflectance discontinuities are not mirrored by RSCM data (see Ferreiro Mählmann et al 2012). To avoid major methodological problems of RSCM thermometry (Lünsdorf et al 2014), Lünsdorf et al (2017) proposed the use of the IFORS (“Interactive fitting of Raman spectra”) software (Lünsdorf and Lünsdorf 2016) as a new technique to analyze Raman spectra of CM In this contribution, we provide a case study, demonstrating the benefits of this approach in the analysis of a regional metamorphic temperature pattern, covering very low- to medium- grade metamorphic conditions never explored before by a single geothermometric method

Results

Discussion

Conclusion