Eo-Oligocene normal faulting in the Alps: Orogen-perpendicular subduction channel reactivation or orogen-parallel collapse following slab breakoff?

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Slab dynamics at mantle depth can force orogens into extension resulting in partial destruction of the mountain belt. In the Central Alps a set of pre-Miocene extensional structures are attributed to slab breakoff from the European plate at ~35 Ma. Structural correlations and cross-cutting relationships suggest this phase ended prior ~31 Ma. In contrast, a recently created detailed thermochronologic dataset suggests that extension continued until 29–18 Ma. Similar to the controversy about timing, the nature of extension is debated. The model based on structural observations suggests orogen-parallel orogenic collapse while the one mostly based on thermochronologic data postulates orogen-normal extension that reactivated the former plate interface/subduction channel, i.e. the Austroalpine-Pennine boundary.The herein proposed project will investigate extensional structures in the Central Alps by filling in gaps in the structural observations and directly dating extension. The proposed project combines - for the first time - geochronologic methods that experienced major improvements in recent years: U-Pb calcite geochronology, 40Ar/39Ar mica in situ analysis and 40Ar/39Ar dating of fluid inclusions in quartz mineralizations will be used to determine the timing of crustal deformation. The results will provide a detailed understanding of the geometry and longevity of extension. The proposed project will test the two described models. Furthermore, detailed knowledge of the chronology will allow contextualizing the extension in the Central Alps with the similar timed and well dated exhumation of high-pressure rocks, changes in igneous activity and far field deformation in the collision zone at the time (e.g. possible foreland fold-and-thrust belt propagation during extension). Integrating these processes affecting different levels of the crust in an absolute chronology will provide time resolved insights into the orogenic effects of slab breakoff.