Peter McPhee , Mark R. Handy

The Eastern Alps were affected by a profound post-collisional tectonic reorganisation in Neogene time, featuring indentation by the Adriatic upper plate, rapid uplift and filling of the eastern Molasse Basin, rapid exhumation and eastward orogen-parallel transport of Tertiary metamorphic units in the orogenic core, and a shift from northward thrust propagation in the European plate to southward propagation in the Adriatic plate. We test the idea that these events were triggered by slab detachment by reconstructing the indentation process. This involved sequentially restoring N-S and E-W cross-sections of the orogenic wedge and correcting for out-of-section orogen-parallel transport with a map-view reconstruction. We propose two phases of indentation: Initially (23-14 Ma), the whole Adriatic crust acted as an indenter. Its northward motion was accommodated by upright folding and orogen-parallel extensional exhumation in the Tauern Window. This phase was followed (14 Ma-Present) by continued orogen-parallel transport of the orogenic edifice into the Pannonian Basin and deformation of the leading edge of the Adriatic indenter, forming the Southern Alps fold-thrust belt. The lower crust of the Southern Alps indented the base of the Venediger Nappes in the Tauern Window, forming a high-velocity (6.8-7.25 km/s) ridge at 30-45 km depth in map view. By correlating the post-23 Ma orogenic evolution with presently imaged European slab segments in P-wave teleseismic tomography, we discern two possible Neogene slab detachment events: One at 23-19 Ma triggering tectonic reorganisation of the Eastern Alps and its foreland basin, and potentially a second event after 14 Ma.