Post-collisional reorganisation of the Eastern Alps in 4D – Crust and mantle structure

In Neogene time, the Eastern Alps underwent a profound tectonic reorganisation. This featured northward indentation of the Alpine orogenic wedge by the Adriatic plate, eastward lateral extrusion between conjugate strike-slip faults, and a shift from thrust propagation on the European lower plate to the Adriatic upper plate. We investigate the triggers of this reorganisation with new sequentially restored orogen-scale cross-sections along the TRANSALP (12°E, western Tauern Window) and EASI (13.3°E, eastern Tauern Window) transects, plus an E-W orogen-parallel cross-section (46.5°E). We use a map-view reconstruction to restore the effects of out-of-section transport by lateral extrusion and compare our results with P-wave local earthquake (LET) and teleseismic tomographic models of the crust and upper mantle.

The geologic record reveals two phases of indentation: Phase 1 (c. 23-14 Ma): The Adriatic Plate was an undeformed indenter, with northward motion relative to Europe accommodated by sinistral motion along the Giudicarie Fault and shortening within the Eastern Alps orogenic wedge. Upright folding of nappes mostly derived from the downgoing European Plate, and lateral extrusion of the entire metamorphic edifice and North Calcareous Alps accommodated this N-S shortening. This shortening required ongoing subduction of European lithosphere, ruling out previous models involving north-dipping Adriatic subduction. A purported detachment below the Venediger Nappes may have served as the base of the laterally extruding wedge.

Phase 2 (c. 14 Ma-Present): The leading edge of the Adriatic indenter has been deforming since c. 14 Ma, forming the thick-skinned South Alps fold-thrust belt. The onset of S-directed thrusting is recorded by Langhian-Serravallian flysch in the footwall of the Valsugana thrust. The Adriatic lower crust was decoupled and transported northwards into the orogenic wedge, indenting and exhuming the deeply buried equivalents of the Venediger Nappes in the Tauern Window. A high-velocity (6.8 - 7.25 km/s) bulge in LET models of the TRANSALP section images this indenter, which comprises mostly Adriatic lower crust, but possibly also includes some accreted European lower crust.

In P-wave teleseismic tomography along the EASI section, the European slab appears to be detached at a locus marked by a Moho gap and a shallow discontinuity in the positive velocity anomaly beneath the orogenic wedge. In contrast, no such discontinuity occurs beneath the TRANSALP section, where S-dipping European lithospheric mantle still extends beneath and south of the orogenic wedge. If the southern end of this relict slab segment marks the locus of a detachment, we find that the current slab length is less than the amount of N-S shortening in the TRANSALP section since 23 Ma. To explain this mismatch, we propose that the most recent slab detachment in the Eastern Alps event occurred after 23 Ma, and likely after 14 Ma (Phase 2 indentation). Note that this does not preclude earlier detachment events, notably at 22-19 Ma when the eastern Molasse Basin rapidly filled and orogenic vergence shifted from north to south (see Handy et al., this session).