European tectosphere and slabs beneath the greater Alpine area – Interpretation of mantle structure in the Alps-Apennines-Pannonian region from teleseismic V p studies

Abstract. Based on recent results of AlpArray, we propose a new model of Alpine collision that involves subduction and detachment of thick (180–200 km) European tectosphere. Our approach combines teleseismic P-wave tomography and existing Local Earthquake Tomography (LET) allowing us to image the Alpine slabs and their connections with the overlying orogenic crust at an unprecedented resolution. The images call into question the conventional notion that slabs comprise only seismically fast lithosphere and suggest that the mantle of the downgoing European Plate is heterogeneous, containing both positive and negative Vp anomalies of up to 5–6%. We interpret these as compositional rather than thermal anomalies, inherited from the Variscan and pre-Variscan orogenic cycles. They make up a kinematic entity referred to as tectosphere, which presently dips beneath the Alpine orogenic front. In contrast to the European Plate, the tectosphere of the Adriatic Plate is thinner (100–120 km) and has a lower boundary approximately at the interface between positive and negative Vp anomalies. Horizontal and vertical tomographic slices reveal that beneath the Central and Western Alps, the downgoing European tectospheric slab dips steeply to the S and SE and is only locally still attached to the Alpine crust. However, in the Eastern Alps and Carpathians, the European slab is completely detached from the orogenic crust and dips steeply to the N-NE. This along-strike change in attachment coincides with an abrupt decrease in Moho depth below the Tauern Window, the Moho being underlain by a pronounced negative Vp anomaly that reaches eastward into the Pannonian Basin area. This negative Vp anomaly is interpreted to represent hot upwelling asthenosphere that was instrumental in accommodating Neogene orogen-parallel lateral extrusion of the ALCAPA tectonic unit (upper plate crustal edifice of Alps and Carpathians) to the east. A European origin of the northward-dipping, detached slab segment beneath the Eastern Alps is likely since its imaged down-dip length (300–500 km) matches estimated Tertiary shortening in the Eastern Alps accommodated by south-dipping subduction of European tectosphere. A slab anomaly beneath the Dinarides is of Adriatic origin and dips to the northeast. There is no evidence that this slab dips beneath the Alps. The slab anomaly beneath the northern Apennines, also of Adriatic origin, hangs subvertically and is detached from the Apenninic orogenic crust and foreland. Except for its northernmost segment where it locally overlies the southern end of the European slab of the Alps, this slab is clearly separated from the latter by a broad zone of low Vp velocities located south of the Alpine slab beneath the Po Basin. Considered as a whole, the slabs of the Alpine chain are interpreted as attenuated, largely detached sheets of continental margin and Alpine Tethyan lithosphere that locally reach down to a slab graveyard in the Mantle Transition Zone (MTZ).