Orogenic lithosphere and slabs in the greater Alpine area – interpretations based on teleseismic P-wave tomography
Authors: Handy Mark R. , Schmid Stefan M. , Paffrath Marcel , Friederich Wolfgang ,   the AlpArray Working Group
Published: November 2021

Abstract. Based on recent results of AlpArray, we propose a new model of Alpine
collision that involves subduction and detachment of thick (∼ 180 km) European lithosphere. 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
lithosphere at an unprecedented resolution. The images call into question
the conventional notion that downward-moving lithosphere and slabs comprise only
seismically fast lithosphere. We propose that the European lithosphere is
heterogeneous, locally containing layered positive and negative Vp
anomalies of up to 5 %–6 %. We attribute this layered heterogeneity to
seismic anisotropy and/or compositional differences inherited from the
Variscan and pre-Variscan orogenic cycles rather than to thermal anomalies.
The lithosphere–asthenosphere boundary (LAB) of the European Plate therefore
lies below the conventionally defined seismological LAB. In contrast, the
lithosphere of the Adriatic Plate is thinner and has a lower boundary
approximately at the base of strong positive Vp anomalies at 100–120 km. Horizontal and vertical tomographic slices reveal that beneath the central
and western Alps, the European slab dips steeply to the south and southeast and is only
locally still attached to the Alpine lithosphere. However, in the eastern Alps and Carpathians, this slab is completely detached from the orogenic
crust and dips steeply to the north to northeast. 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 as representing hot upwelling asthenosphere that heated the
overlying crust, allowing it to accommodate Neogene orogen-parallel lateral
extrusion and thinning 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 down-dip length matches estimated Tertiary shortening in the
eastern Alps accommodated by originally south-dipping subduction of European
lithosphere. 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 highly attenuated, largely
detached sheets of continental margin and Alpine Tethyan oceanic lithosphere
that locally reach down to a slab graveyard in the mantle transition zone
(MTZ).