Mantle convection, tectonics and the evolution of the Tethyan subduction zone.

International audienceMantle convection drives plate tectonics and the size, number and thermotectonic age of plates codetermines theconvection pattern. However, the degree of coupling of surface deformation and mantle flow is unclear. Most numericalmodels of lithospheric deformation are designed such that strain is a consequence of kinematic boundaryconditions, and rarely account for basal stresses due to mantle flow. On the other hand, convection models oftentreat the lithosphere as a single-layer stagnant lid with vertically undeformable surface. There is thus a gap betweenconvection models and lithospheric-scale geodynamic models. The transmission of stresses from the flowing mantleto the crust is a complex process. The presence of a ductile lower crust inhibits the upward transmission ofstresses but a highly extended crust in a hot environment such as a backarc domain, with no lithospheric mantleand a ductile lower crust in direct contact with asthenosphere, will be more prone to follow the mantle flow thana thick and stratified lithosphere. We review geological observations and present reconstructions of the Aegeanand Middle East and discuss the possible role played by basal drag in governing lithospheric deformation. InMediterranean backarc regions, lithosphere-mantle coupling is effective on geological time scale as shown by theconsistency of SKS fast orientations in the mantle with stretching directions in the crust. The long-term geologicalhistory of the Tethyan convergent zone suggests that asthenospheric flow has been an important player. Thecase of Himalaya and Tibet strongly supports a major contribution of a northward asthenospheric push, with nopersistent slab that could drive India after collision, large thrust planes being then decoupling zones between deepconvection and surface tectonics. The African plate repeatedly fragmented during its northward migration with theseparation of Apulia and Arabia. Indeed, extension has been active on the northern side of Africa from the Jurassicuntil the collision in the Oligocene, and even afterward when Arabia formed by opening of the Red Sea and theGulf of Aden. This also suggests a dominant role of an underlying flow at large scale, dragging and mechanicallyeroding plates and breaking them into fragments, then passively carried. Only during a short period of the LateCretaceous did the situation change drastically with the obduction event giving the large ophiolitic nappes observedfrom Oman to Turkey. This obduction event has never been really explained. It has been shown to be coeval withfaster plate velocities and more active formation of oceanic crust globally, which in turn suggests a link with deepmantle convection. We discuss this succession of events and propose to relate them with the basal drag induced byconvective mantle flow below the African continental lithosphere. We discuss the effects of convection on crustaldeformation at different scales from deep convection related to plumes and subduction zones to more local mantleflow due to slab retreat and tearing