Exhumation mechanisms of the eastern Tauern Window, Austria

Geologists have been exploring the Alps for more that 200 years. Our fundamental understanding of mountain building processes stem from these endeavours, however many concepts remain unclear.

Within major mountain chains, the highest peaks often comprise packages of rock that were formed deep below the surface, often >35 km in depth. A limited number of mechanisms are capable of bringing these rocks to the surface, a process referred as exhumation. These processes are difficult to quantify, however technological advances have facilitated innovative research that can elucidate the timescale on which these processes occur. In conjunction with field observations, it is possible to identify the dominant mechanisms contributing to exhumation.

This research serves as a case study to understand the mechanisms that have contributed to exhumation within the eastern Alps by:

1) Applying innovative thermochronological techniques to constrain the timing and rate of exhumation

2) Integrate structural data with thermochronology results to interpret and quantify the relative contribution of various exhumation mechanisms

In the eastern Alps, the most recent period of mountain building occurred over the last 70 million years. Prior to continental collision, during the Mesozoic, a small ocean separated the African and Eurasian tectonic plates ( Trümpy, 1960 ;  Tricart, 1984) . During the Cretaceous, the Adriatic plate broke away from Africa and migrated northward, eventually colliding with Eurasia in the Miocene (c. 22 Ma). The Adriatic plate acted as a rigid indenter and is responsible for large scale deformation of the eastern Alps, including the formation of the Tauern Window ( Rosenberg et al., 2004 ).

The Tauern Window is a tectonic window, meaning erosion has exposed structurally deep rock formations surrounded by shallower rocks separated by a major fault. For simplicity, the regional geology can be separated into three major domains based on their origin: Austroalpine units are derived from the Adriatic plate, Penninic units derived from oceanic domains, and Sub-Penninic units derived from Europe. During subduction, these major domains have been imbricated where Sub-Penninic units form the deepest structural level and were metamorphosed under ultra-high pressure (60-70 km) conditions. Structurally overlying are Penninic units, yet the highest peaks in Austria (>3500 m) exposed in the core of window are the high pressure Sub-Penninic units. A complex network of faults bounds the Tauern Window and its hanging wall consists of Austroalpine unit ( Schmid et al., 2004) .

Various mechanisms are known to have contributed to exhumation of the Tauern window

Erosion: Collison of the Adriatic plate with Eurasia resulted in broadly E-W trending high-amplitude folds. High topography facilitates erosion ( Bertrand et al., 2017 ).

Extrusion: The rock units that form the Tauern Window were imbricated within a subduction zone. The preservations of ultra-high pressure rocks (eclogites) indicates that extrusion contributed to the early stages of exhumation ( Ratschbacher et al., 2004 ).

Normal faulting: The eastern margin of the Tauern Window is bound by a high angle normal fault (the Katschberg fault) and is interpreted to have accommodated up to 25 km of displacement during the final phases of exhumation (since 5 Ma;  Scharf et al., 2013 ).

Ductile thinning: A vertical shortening mechanism during the early stages of exhumation. This mechanism is difficult to quantify and not entirely understood.

Identifying ductile thinning and quantifying its contribution to exhumation is the focus of this research.

Thermochronological methods resolve when the rock cooled through a specific temperature. As Earth has geothermal gradient, rocks naturally cool as they are brought to the surface and exposed. The apatite fission track system (AFT) is sensitive to temperatures of ~100°C, therefore AFT ages can be interpreted as the time when samples were exhumed to upper crustal conditions ( Donelick et al., 2005 ).

Throughout the 2020-2021 field season, thirteen samples were collected for AFT analysis along a N-S traverse that transects the Tauern Window. Preliminary results show predominantly young (Miocene) cooling ages. In the northern section of the field area, two samples preserve older (Oligocene) ages, however these two samples are separated by a younger a sample. A large statistical uncertainty is associated with sample BH/20/61 and is being re-analyzed.

Brittle-ductile structures are important as they record progressive deformation that occurred from deeper structural levels (flowing under ductile conditions) and are overprinted by deformation occurring at shallows levels (breaking under brittle conditions).

Detailed analysis of brittle-ductile structures can elucidate information on exhumation processes. Conjugate shear bands are the most prevalent brittle-ductile structure preserved in the study area, and if their spatial distribution and magnitude can be documented, ductile thinning can be invoked as part of the exhumation history.

At suitable outcrops, conjugate shear bands and the schistosity were measured. The data was projected on stereonets and rotated relative to the outcrops average schistosity to visualize the principle stress axis during their formation.

Fourteen outcrops were deemed suitable for structural analysis of conjugate shear bands. Notably, the locations of these outcrops follow an E-W striking greenschist unit, which may have accommodated ductile thinning.

• Interpret AFT data from preliminary samples
• Start numerical modeling of AFT data to resolve the thermal history and rate of exhumation within the Tauern Window
• Logistical planning for summer field work

• Map brittle-ductile structures between Fusch and Rauris valleys, proximal to greenschist unit (30 km ²  area; Fig. 10)
• Verify other lithologies on published geologic maps
• Collect new samples for additional analyses that will complement our dataset ( ⁴⁰ Ar/ ³⁹ Ar and Raman spectroscopy on carbonaceous material)

LocusGIS is used in the field to track sample and outcrop locations. In addition, published geologic maps serve as a base layer and act as a guide to the region’s complex geology. On a daily basis, information recorded in LocusGIS is imported to ArcMap.

ArcMap serves as an interactive platform that helps visualise a complex data set that is continuously evolving. This helps identify new trends and foster new ideas which is important, as developing new hypotheses in the field allows us to quickly reassess our field plan to target new research interests.

Deliverables to collaborators require data in ArcMap files. In addition, the maps produced will help create journal quality figures and will also be used for my PhD thesis.

This work is partially funded by NSERC and  iMAGE-CREATE .