Thermo-mechanical modeling of subduction of continental lithosphere

We consider two-dimensional thermo-chemical mantle convection models to investigate the deformation of the continental lithosphere that follows the oceanic lithosphere into the subduction zone. The models account for the compositional buoyancy forces by considering lithospheric plates with distinct crustal layers. Continental convergence results in crust–mantle detachment in the subducting plate and crustal thickening in both subducting and overriding plates. The depth of detachment ranges from 90 to 200 km, depending on the strength of the lithosphere and the density of the continental crust. As the crust thickens, the convergence velocity in the collision zone decreases and the locus of subduction gradually shifts toward the interior of the subducting plate. In models with greater viscosity, the subducting mantle lithosphere maintains its integrity and does not break up. In models with weaker rheology, the positive buoyancy of the thickened crust can overcome the strength of the subducting lithosphere, and causes the oceanic slab to break off and sink into the mantle. The breakoff occurs over a time interval of 10–20 million years, which is roughly the time needed for the crust to reach its maximum thickness. Crustal detachment takes place over a wide range of lithospheric strength, suggesting that crustal buoyancy has an important role in the dynamics of continental subduction.