A new model of slab detachment in the Alps and its geodynamic consequences

Wholesale slab breakoff or detachment in the Alps has been invoked to explain Periadriatic
calc-alkaline magmatism (43-29 Ma), rapid exhumation of HP metamorphics, as well as
clastic infill of proximal parts of the Alpine Molasse basin (31-28 Ma). However, the 14 My
timespan of these events exceeds the duration of slab detachment estimated from
thermomechanical modelling (2-8 My) and from depocenter migration (~5 My) along
equivalent lengths of the Carpathians and Apennines. Moreover, wholesale slab
detachment does not explain major E-W differences in Alpine orogenic structure, basin
evolution, and kinematics of indentation in the Alps.
Recent V p tomography from AlpArray suggests that the slab segment beneath the
Central Alps comprises European lithosphere and remains attached down to the MTZ. The
~600km length of this segment suggests that it never ruptured and is still connected to
subducted lithosphere of Alpine Tethys. In contrast, the Alpine slab is detached beneath the
Eastern Alps and Pannonian Basin. The minimum time since detachment is bracketed at 25-
10 Ma based on a comparison of vertical detachment distance with global slab sink rates.
We propose a new model of slab detachment in the Alps that began with slab
steepening when the Adria-Europe convergence rate after collision at ~35 ma decreased to
<1 cm/yr. Periadriatic magmatism is no longer attributed to slab detachment and
asthenospheric upwelling, but to fluxing of the cold mantle wedge by fluids derived from
the devolatilizing Alpine slab (Müntener et al. 2021; doi: 10.2138/gselements.17.1.35). Slab
steepening and delamination were more pronounced in the Eastern Alps, possibly due to
the greater negative buoyancy of the slab in the absence of Brianconnais continental
lithosphere, which was never present in the eastern part of Alpine Tethys. Slab pull thus
drove subsidence and continued marine sedimentation in the E. Molasse basin from 29-19
Ma, while the western part of the basin filled with terrigeneous sediments already at 31-28
Ma.
Slab detachment was restricted to the part of the Alps east of the Giudicarie Fault in
Miocene time. Detachment coincided with a switch in the advancing orogenic front, from
the northern front in the Eastern Alps to the southern front in the eastern Southern Alps.
This also coincided with rapid exhumation in the Tauern Window and lateral eastward
escape of the orogenic crust toward the Pannonian Basin. Rapid W-to-E filling of the Eastern
Molasse basin between 19-16 Ma is interpreted to reflect eastward propagation of the slab
tear and the onset of rollback subduction in the Carpathians.
E-W differences in Alpine structure are thus attributed to the contrasting response of
the Alpine orogenic wedge to slab steepening, delamination and detachment. Whereas
steepening and delamination in the west in late Oligocene time induced horizontal
shortening and increased taper of the orogenic wedge with rapid exhumation and
denudation focused in the retro-wedge, Miocene detachment in the east triggered a
dramatic switch in the pro- and retro-wedges, such that rapid exhumation and denudation
was ultimately focused in the axis of the orogenic wedge.