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Unroofing of the western Tauern window involved both low‐angle detachment faulting (Brenner Fault) and enhanced footwall erosion, contemporaneous with upright antiformal folding. This combination reflects orogen‐parallel (˜E–W) extension during continued ˜N–S Alpine convergence. New fission‐track ages establish the relative chronology of folding and faulting and demonstrate that displacement was not always accommodated on the same surface. During exhumation, some units migrated from the footwall to the hanging wall of the main detachment fault, due to the interplay between folding and faulting. The region can effectively be divided into 3 distinct domains. (1) The Penninic units of the western Tauern Window were always in the footwall to the fault, with maximum exhumation in the core of the dome, due to folding and erosion. (2) The Lower Austroalpine unit north of the Tauern Window was first part of the hanging wall to the Brenner Fault. At a later stage this unit was exhumed by a further 4–5 km as part of the footwall to a more discrete, through‐going fault (the Silltal Fault). (3) The Middle and Upper Austroalpine units west of the Tauern Window were always within the hanging wall. Exhumation of the footwall from an initial depth of ∼ 25 km led to a transition in mechanical behaviour. The curviplanar (folded) ductile shear zone marking the boundary to the Tauern window was eventually transected by a more planar discrete brittle fault (Silltal Fault, with unit 2 now in the footwall), along which the pre‐existing mylonites were passively exhumed to the surface.

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[1] Subducting oceanic plates carry a considerable amount of water from the surface down to mantle depths and contribute significantly to the global water cycle. A part of these volatiles stored in the slab is expelled at intermediate depths (70-300 km) where dehydration reactions occur. However, despite the fact that water considerably affects many physical properties of rocks, not much is known about the fluid flow path and the interaction with the rocks through which volatiles flow in the slab interior during its dehydration. We performed thermomechanical models (coupled with a petrological database and with incompressible aqueous fluid flow) of a dynamically subducting and dehydrating oceanic plate. Results show that, during slab dehydration, unbending stresses drive part of the released fluids into the cold core of the plate toward a level of strong tectonic under-pressure and neutral (slab-normal) pressure gradients. Fluids progressively accumulate and percolate updip along such a layer forming, together with the upper hydrated layer near the top of the slab, a Double Hydrated Zone (DHZ) where intermediate-depth seismicity could be triggered. The location and predicted mechanics of the DHZ would be consistent with seismological observations regarding Double Seismic Zones (DSZs) found in most subduction zones and suggests that hydrofracturing could be the trigger mechanism for observed intermediate-depth seismicity. In the light of our results, the lower plane of the DSZ is more likely to reflect a layer of upward percolating fluid than a level of mantle dehydration. In our models, a 20-30 km thick DSZ forms in relatively old oceanic plates without requiring an extremely deep slab hydration prior to subduction. The redistribution of fluids into the slab interior during slab unbending also has important implications for slab weakening and the deep water cycle. We estimate that, over the whole of Earth's history, a volume of water equivalent to around one to two oceans can be stored in nominally anhydrous minerals of the oceanic lithosphere and transported to the transition zone by this mechanism, suggesting that mantle regassing could have been efficient even without invoking the formation of high pressure hydrous minerals.

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