Variation in style of Adriatic lower crust indentation west and east of the Giudicarie Fault

Neogene tectonics of the Alps is marked by the indentation of the Adriatic Plate into the Alpine Orogen and onset of escape tectonics in the Eastern Alps. This resulted in the formation of a system of strike-slip faults, mainly the Periadriatic Fault (PF), separating the Eastern and Southern Alps, and the sinistral Giudicarie Fault (GF), which offsets the PF. The GF is kinematically related to Neogene shortening in the Southern Alps but questions remain on its geometry at depth, in particular its relation to the crust/mantle boundary (Moho).

In this study, we compare geological cross-sections and pre-existing geophysical datasets (controlled-source seismology, local earthquake tomography) with a new high-resolution 3-D local earthquake tomographic model from the AlpArray and SWATH-D experiment along two N-S profiles west and east of the GF, as well as a NW-SE oriented section across the GF. These sections reveal differences in the style of indentation tectonics, specifically in the behavior of the Adriatic lower crust, between the Central and Eastern Alps. West of the GF, the lower crust of the Adriatic plate detached from its mantle lithosphere and wedged within the Alpine orogenic crust, whereas to the east of the GF, the Adriatic lower crust forms a bulge just to the south of the PF. The Adriatic upper crust responded by shortening and formation of a fold-and-thrust belt, while the Europe-derived orogenic crust underwent upright, post-nappe folding and exhumation in the Tauern Window. We discuss the possible causes for such along-strike variations in terms of changes in crustal rheology and structural inheritance within the Adriatic Plate, contrasting metamorphic histories within the Alpine orogenic crust west and east of the GF, and potential Neogene slab break-off beneath the Eastern Alps.