Delamination in collisional orogens: Thermomechanical modeling

Abstract

Modes of mantle delamination in collision zones are discussed in the light of 2D numerical modeling. Freely evolving thermomechanical models take into account phase changes in the mantle and melting in the crust. Distinct modes are identified, in which orogens undergo mantle delamination concurrently or after collision. Delamination propagates along the Moho of the subducted plate together with the retreating trench, provided that slab pull is sufficient and that the meta‐stability of the crust‐lithospheric mantle interface is overcome. Topography is an instantaneous response to delamination and migrates with the focused and localized separation between crust and lithospheric mantle (delamination front). Early exhumation of high‐pressure rocks is followed by exhumation of high‐temperature, partially molten rocks. Convective stabilization of delamination outlasts slab break‐offs and impedes renewed build‐up of mechanically strong lithospheric mantle by static cooling. Parameter studies investigate the sensitivity of delamination to the age of the subducted oceanic plate, the collision rate, the mantle rheology, and explore the effects of melting and sedimentation. Mantle flow patterns that form around delamination and subduction are sensitive to and often limited by the semi‐permeable 670 km spinel‐perovskite transition, demonstrating that phase‐changes are essential factors. Delamination may account for the formation of complex boundary zones between continents, such as between Africa and Europe in the Mediterranean, where transient compression and extension, the lack of mantle lithosphere, and the exhumation of high‐grade metamorphic core complexes are features also obtained in our modeling results.