A new model of slab detachment in the Alps and its geodynamic consequences
Authors: Handy Mark R. 
Published: March 2022

&lt;p&gt;Wholesale slab breakoff or detachment in the Alps has been invoked to explain Periadriatic&lt;br&gt;calc-alkaline magmatism (43-29 Ma), rapid exhumation of HP metamorphics, as well as&lt;br&gt;clastic infill of proximal parts of the Alpine Molasse basin (31-28 Ma). However, the 14 My&lt;br&gt;timespan of these events exceeds the duration of slab detachment estimated from&lt;br&gt;thermomechanical modelling (2-8 My) and from depocenter migration (~5 My) along&lt;br&gt;equivalent lengths of the Carpathians and Apennines. Moreover, wholesale slab&lt;br&gt;detachment does not explain major E-W differences in Alpine orogenic structure, basin&lt;br&gt;evolution, and kinematics of indentation in the Alps.&lt;br&gt;Recent V p tomography from AlpArray suggests that the slab segment beneath the&lt;br&gt;Central Alps comprises European lithosphere and remains attached down to the MTZ. The&lt;br&gt;~600km length of this segment suggests that it never ruptured and is still connected to&lt;br&gt;subducted lithosphere of Alpine Tethys. In contrast, the Alpine slab is detached beneath the&lt;br&gt;Eastern Alps and Pannonian Basin. The minimum time since detachment is bracketed at 25-&lt;br&gt;10 Ma based on a comparison of vertical detachment distance with global slab sink rates.&lt;br&gt;We propose a new model of slab detachment in the Alps that began with slab&lt;br&gt;steepening when the Adria-Europe convergence rate after collision at ~35 ma decreased to&lt;br&gt;&lt;1 cm/yr. Periadriatic magmatism is no longer attributed to slab detachment and&lt;br&gt;asthenospheric upwelling, but to fluxing of the cold mantle wedge by fluids derived from&lt;br&gt;the devolatilizing Alpine slab (M&amp;#252;ntener et al. 2021; doi: 10.2138/gselements.17.1.35). Slab&lt;br&gt;steepening and delamination were more pronounced in the Eastern Alps, possibly due to&lt;br&gt;the greater negative buoyancy of the slab in the absence of Brianconnais continental&lt;br&gt;lithosphere, which was never present in the eastern part of Alpine Tethys. Slab pull thus&lt;br&gt;drove subsidence and continued marine sedimentation in the E. Molasse basin from 29-19&lt;br&gt;Ma, while the western part of the basin filled with terrigeneous sediments already at 31-28&lt;br&gt;Ma.&lt;br&gt;Slab detachment was restricted to the part of the Alps east of the Giudicarie Fault in&lt;br&gt;Miocene time. Detachment coincided with a switch in the advancing orogenic front, from&lt;br&gt;the northern front in the Eastern Alps to the southern front in the eastern Southern Alps.&lt;br&gt;This also coincided with rapid exhumation in the Tauern Window and lateral eastward&lt;br&gt;escape of the orogenic crust toward the Pannonian Basin. Rapid W-to-E filling of the Eastern&lt;br&gt;Molasse basin between 19-16 Ma is interpreted to reflect eastward propagation of the slab&lt;br&gt;tear and the onset of rollback subduction in the Carpathians.&lt;br&gt;E-W differences in Alpine structure are thus attributed to the contrasting response of&lt;br&gt;the Alpine orogenic wedge to slab steepening, delamination and detachment. Whereas&lt;br&gt;steepening and delamination in the west in late Oligocene time induced horizontal&lt;br&gt;shortening and increased taper of the orogenic wedge with rapid exhumation and&lt;br&gt;denudation focused in the retro-wedge, Miocene detachment in the east triggered a&lt;br&gt;dramatic switch in the pro- and retro-wedges, such that rapid exhumation and denudation&lt;br&gt;was ultimately focused in the axis of the orogenic wedge.&lt;/p&gt;