The Slab Puzzle of the Alpine-Mediterranean Region: Insights from a new, High-Resolution, Shear-Wave Velocity Model of the Upper Mantle

Mediterranean tectonics since the Lower Cretaceous has been characterized by a multi- phase subduction and collision history with temporally and spatially-variable, small-scale plate configurations. A new shear-wave velocity model of the Mediterranean upper mantle (MeRE2020) down to 300 km depth, constrained by ~200,000 broadband (8 - 350 s), inter-station, Rayleigh-wave, phase-velocities, illuminates the complex structure and the highly fragmented nature of the subducting slabs. The location of slab segments are consistent with and validated by the intermediate-depth seismicity, where it is present. We distinguish attached slabs reaching down to the model bottom from shallow slabs that terminate at shallower depths and from detached ones that are not connected to the lithosphere in the foreland. No evidence is found for a slab segment to the east of Cyprus within the upper 200 km of the mantle. We discuss evidence for continuous NE-dipping subduction beneath Antalya and NW-dipping subduction in the SE-Aegean. An attached slab is imaged beneath the Hellenides reaching down to at least 300 km depth whereas beneath the Dinarides a short slab is found down to ~150 km depth. A south-dipping slab is imaged in the central Alps. In the eastern Alps, the presence of Adriatic subduction towards the north and Eurasian subduction towards the south is proposed. In the western Alps, a shallow east-dipping Eurasian slab segment is in close proximity to an almost vertically dipping attached slab segment beneath the northern Apennines. A slab gap is found in the central Apennines. The NW-dipping Calabrian Slab is attached beneath Calabria and Sicily but detached beneath southern Apennines. Beneath Algeria, the Kabylides Slab appears to be attached to the North African lithosphere, while its detached part seems to extend towards southern Sardinia. At least down to about 250 km depth, the Kabylides Slab is clearly separated from the Calabrian Slab to the northeast and from the Gibraltar-Betic Slab to the west, the latter comprises two segments: a shallow Gibraltar and a detached Betic one. The new high-resolution tomography demonstrates the intricate relationships between slab fragmentation and the evolution of the relatively small and highly curved subduction zones and collisional orogens characteristic of the Mediterranean realm.