Relating collisional kinematics to exhumation processes in the Eastern Alps

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

Synchronous Periadriatic magmatism in the Western and Central Alps in the absence of slab breakoff

“…In the Western Alps, Alpine subduction was active until the late Eocene, as attested by (U)HP rocks that reached the eclogitic peak at ~35 Ma (Rubatto & Hermann, ) and were rapidly exhumed during upper‐plate divergent motion by 32 Ma, corresponding to the age of the stratigraphic cover of the Voltri massif (Liao et al., ; Malusà, Faccenna, Garzanti, & Polino, ; Quaranta, Piazza, & Vannucci, ). In the Eastern Alps, Alpine subduction was active until the early Oligocene, as attested by eclogites of the Tauern Window that reached their pressure peak at ~31 Ma (Glodny, Ring, Kühn, Gleissner, & Franz, ) and then experienced crustal shortening (Rosenberg et al., ).…”

“…Rosenberg et al. (), based on the teleseismic tomography model by Mitterbauer et al. (), interpret the deep velocity anomaly beneath the Eastern Alps as stemming from an oceanic and detached slab.…”

Slab Break-offs in the Alpine Subduction Zone

“…(5) Shortening constraints: the total convergence since 25 Ma is estimated in the order of 190 km in the eastern Alps by assuming a 20 • counter-clockwise rigid-body rotation of Adria (Ustaszewski et al, 2008). However, collisional shortening estimates provide minimum estimates of 75 km in the European plate (Rosenberg et al, 2017) and 50 km in the Adriatic plate (Schönborn, 1999;Nussbaum, 2000).…”

Slab Break-offs in the Alpine Subduction Zone

“…(5) Shortening constraints: the total convergence since 25 Ma is estimated in the order of 190 km in the eastern Alps by assuming a 20 • counter-clockwise rigid-body rotation of Adria (Ustaszewski et al, 2008). However, collisional shortening estimates provide minimum estimates of 75 km in the European plate (Rosenberg et al, 2017) and 50 km in the Adriatic plate 25 (Schönborn, 1999;Nussbaum, 2000).…”