Balancing a cross-section through the western Tauern Window using non-plane strain

The Tauern Window in the European Alps has a high tectonic complexity. It is a key area to understand a number of important orogenic processes, including nappe stacking, exhumation, indentation as well as escape tectonics. The polyphase Alpine deformation history of the Tauern Window began with subduction and accretion of the Penninic realm beneath the northern margin of Adria (Austroalpine) in the Cretaceous. Ongoing convergence led to collision between Europe (Subpenninic) and the Adria margin and to the formation of the Penninic and Subpenninic nappe stack in the southward dipping orogenic wedge from Eocene to early Oligocene. The W-E trending Periadriatic Fault System (PFS) located within the Adriatic units south of the Tauern Window was active as dextral strike-slip fault at this time, as indicated by the deformation of the Eocene and Oligocene Periadriatic intrusions [1]. Indentation of the Dolomites Indenter (Eastern Southalpine) bent the primarily PFS and finally caused this fault system to be sinistrally offset by the NNE-SSW striking Giudicarie fault system in the Miocene. This last deformation stage (D5 after [2]) caused strong N-S shortening (~65 km) of the western Tauern Window in front of the Dolomites Indenter, accompanied by lateral extrusion towards the east of at least ~100 km involving major strike-slip faults (e.g., Inntal Fault, PFS, SEMP). W-E extension further led to the formation of the Katschberg and Brenner Normal Fault (on the eastern and western borders of the Tauern Window, respectively). The latter, perhaps in combination with slab break-off and mantle upwelling, led to rapid exhumation of the Tauern Window.

Balancing a cross-section is an excellent tool to analyze the kinematic evolution of mountain belts. Therefore, we collected a structural dataset along a N-S trending cross-section through the western Tauern Window based on the Brenner Base Tunnel profile [3]. Before balancing, however, basic assumptions have to be typically considered: (1) Whether plane-strain deformation is applicable, which means that no material should move lateral into or out of the cross-section plane (2) Conservation of the area (or volume), and (3) line-length should be preserved. Hence, such a balancing is not simply possible in the western Tauern Window because of the last deformation stage (D5 after [2]), when contemporaneous N-S shortening, W-E extension, and vertical uplift led to penetrative deformation and non-plane strain conditions, respectively. We focus on the restoration of the last deformation stage; first with plane-strain and second with non-plane, oblate strain. The results reveal the effect of the W-E extension on the nappe geometry in the footwall of the Brenner Normal Fault – a topic that is controversially discussed. This is the basis for further backward restoration that needs to incorporate all the tectonic movements out of the cross-section plane, and will be carried out as balancing in 3-D at a later stage of the project.

 

 

References

[1] Pomella, H. et al. (2011). International Journal of Earth Sciences, 100(8), 1827-1850.

[2] Schmid, S. M. et al. (2013). Swiss Journal of Geosciences, 106(1), 1-32.

[3] Brandner, R. et al. (2008). Geo Alp, 5, 165-174.