No abstract

International audienceBased on a review of the surface and deep structure of the Eastern Alps, we link the timing and the inferred displacement fields to exhumation of upper and lower crustal units of the orogenic nappe stack during collision. The discussion focuses mainly on the Tauern Window and its country rocks, the only area of the Eastern Alps where the orogenic wedge, from its uppermost Austroalpine nappes down to its deepest European basement nappes is continuously exposed. We summarize and discuss the long-standing controversy on the mechanisms of exhumation of this nappe stack on the base of a synthesis of structural and geochronological data, and restorations of collisional displacements, both in cross-sections and map views. We conclude that the large amounts of exhumation assessed for the western Eastern Alps resulted from large amounts of thickening and erosion, not observed in the eastern part of the Eastern Alps. Extensional faults, laterally bounding the area of major thickening and exhumation are inferred to nucleate in order to accommodate displacement around the indenter corner in the west, and in order to reduce a large gradient of crustal thickness and surface elevation in the East.Restorations to the pre-indentation stage, document an amount of northward increasing orogen-parallel extension, varying 45 km to 85 km, corresponding to 15% of extension, that is partly accommodated along normal faults. N-S shortening between the Northern Calcareous Alps and the Dolomites Indenter attained 75 km in the west and decreased to 30 km in the east. 55 km out of these 75 were accommodated in the area of the Tauern Window. Our kinematic model shows that lateral extrusion accommodated along conjugate strike-slip faults requires large amounts of north-south shortening in the western part of the Eastern Alps. Such shortening is consistent with the reconstructed upright folding and erosion of the Tauern Window, thus explaining the largest amount of its exhumation. In contrast, the eastern termination of the Eastern Alps represents an area where collisional convergence was barely accommodated by crustal thickening. This transition from a highly shortened, thickened and exhumed wedge in the west, mainly affected by orogen-perpendicular displacements, to a barely shortened and exhumed wedge in the east, mainly characterised by orogen-parallel displacements, spatially coincides with a change in the deep structure of the European slab. Indeed, the inferred continental, European Slab, imaged in the west disappears into a low velocity anomaly, where no slab is detected in the east. An inherited step in the geometry in map view of the European passive margin, causing its crust to enter the subduction zone earlier than the area east of the Tauern Window, may explain the rapid decrease of shortening, of thickening, the different syn-collisional P-T gradients, and the disappearance of the continental slab east of the Katschberg Fault

Orogen‐parallel extension and orogen‐perpendicular shortening accommodated by folding acted at the same time to exhume the Tauern Window. In order to investigate the relative contribution of upright folding and erosion and of extensional denudation for exhumation, we provide compilations in map view of previous and new zircon and apatite fission track ages. These age maps show that isoage contour lines are subparallel to the axial planes of large‐scale, upright folds. On age versus distance diagrams, along a profile perpendicular to the dome axis, all thermochronometers show bell‐shaped curves with younger ages in the hinge area of the dome and age differences between different chronometers decreasing from the limbs to the hinge area. All these observations suggest that folding synchronous with erosion was largely responsible for exhumation of the Tauern Window. The younger ages and the higher fold amplitudes of the western subdome compared to the eastern one are corroborated by the results of inversion of cooling ages that show higher exhumation rates in the west. These reflect one and the same shortening and folding event that affected the entire Tauern Dome synchronously, but at higher rates than that in the western subdome. Only during Pliocene time were exhumation rates slightly higher along the normal faults bordering the window; hence, extensional unroofing may have dominated exhumation in the Pliocene. The northward displacement of the Dolomites Indenter was associated to a clockwise rotation, which caused increased amounts of shortening westward, hence higher uplift and exhumation rates in the western subdome.