Evolving temperature field in a fossil subduction channel during the transition from subduction to collision (Tauern Window, Eastern Alps)

Abstract

We investigate the evolution of the three‐dimensional thermal structure of a palaeo‐subduction channel exposed in the Penninic units of the central Tauern Window (Eastern Alps). Structural and petrological observations reveal a sheath fold with an amplitude of some 20 km that formed under high‐P conditions (~2 GPa). The fold is a composite structure that isoclinally folded the thrust of an ophiolitic nappe derived from Alpine Tethys Ocean onto a unit of the distal European continental margin, also affected by the high‐P conditions. This structural assemblage is preserved between two younger domes at either end of the Tauern Window. The domes deform isograds of the T‐dominated Barrovian metamorphism that itself overprints the high‐P metamorphism partly preserved in the sheath fold. Using Raman spectroscopy on carbonaceous material (RSCM), we are able to distinguish peak‐temperature domains related to the original subduction metamorphism from domains associated with the later temperature‐dominated (Barrovian) metamorphism. The distribution of RSCM temperatures in the Barrovian domain indicates a lateral and vertical decrease of peak temperature with increasing distance from the centres of the thermal domes. This represents a downward increase of palaeo‐temperature, in line with previous studies. However, we observe the opposite palaeo‐temperature trend in the lower limb of the sheath fold, namely an upward increase. We interpret this inverted palaeo‐temperature domain as the relic of a subduction‐related temperature field. Towards the central part of the sheath fold's upper limb, RSCM temperatures increase to a maximum of ~520°C. Further upsection in the hangingwall of the sheath fold, palaeo‐peak temperatures decrease to where they are indistinguishable from the peak temperatures of the overprinting Barrovian metamorphism. Peak‐temperature contours of the subduction‐related metamorphism are oriented roughly parallel to the folded nappe contacts and lithological layering. The contours close towards the northern, western and eastern parts of the fold, resulting in an eye‐shaped, concentric pattern in cross‐section. The temperature contour geometry therefore mimics the fold geometry itself, indicating that these contours were also folded in a sheath‐like manner. We propose that this sheath‐like pattern is the result of a two‐stage process that reflects a change of the mode of nappe formation in the subduction zone from thrusting to fold nappe formation. First, thrusting of a hot oceanic nappe onto a colder continental nappe created an inverted peak‐thermal gradient. Second, sheath folding of this composite nappe structure together with the previously established peak‐temperature pattern during exhumation. This pattern was preserved because temperatures decreased during retrograde exhumation metamorphism and remained less than the subduction‐related peak temperatures during the later Barrovian overprint. The fold ascended with diapir‐like kinematics in the subduction channel.

The Haidbach deposit in the Central Tauern Window, Eastern Alps, Austria: a metamorphosed orthomagmatic Ni-Cu-Co-PGE mineralization in the Polymetallic Ore District Venediger Nappe System – Hollersbach Complex

“…Such temperatures are also in agreement with the presence of pargasitic amphibole relics in mineralized hornblendite (Fig. 3,4a,4b), and thus clearly exceed temperature conditions reached during the Neoalpine metamorphic event (<450 °C; Schuster et al, 2004;Gross et al, 2020). Evidence from textures and mineral compositions thus indicates local preservation of pre-Alpine conditions at Haidbach.…”

Detrital apatite geochemistry and thermochronology from the Oligocene/Miocene Alpine foreland record the early exhumation of the Tauern Window

“…After the end of oceanic subduction in the Eastern Alps, the accretion of European crust at the base of the Adriatic plate culminated in subsequent Barrovian metamorphism in the thickened crust (Schmid et al., 2013). Peak metamorphic temperatures were variable in space and reached up to 600°C in the centres of the two Tauern subdomes (Figure 1) (Bousquet et al., 2012; Groß et al., 2021). Estimates for the timing of peak metamorphism range from 40–30 Ma (Christensen et al., 1994; Favaro et al., 2015; Glodny et al., 2005; Grundmann & Morteani, 1985; Ratschbacher et al., 2004; Schneider et al., 2015; Selverstone, 1988; Zimmermann et al., 1994) to ca.…”

2-D kinematic restoration of the western Tauern Window, Eastern Alps using thermochronological and P-T constraints

“…These nappes include the "Wolfendorn nappe" (Frisch, 1974), "Kaserer nappe" (Veselá et al, 2022) and "Flatschspitz nappe" (Brandner et al, 2008a;Töchterle, 2011). In the last decades, the "Modereck nappe system" (Schmid et al, 2013) was ascribed to these hanging-wall nappes as well, in particular in the central TW (Groß et al, 2020(Groß et al, , 2021(Groß et al, , 2022, as the structurally uppermost nappe is mainly composed of (Permo-)Triassic units (e.g., Rockenschaub et al, 2003;Schmid et al 2013 and references therein). However, in the course of the Brenner Base Tunnel construction, no evidence was found for a tectonic contact with the overlaying Glockner nappe system in the western TW (Brandner et al, 2008a(Brandner et al, , 2008bTöchterle et al, 2007;Töchterle, 2011;Töchterle et al, 2011).…”