Thermo-mechanical modeling of subduction of continental lithosphere

In this thesis I use 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 of the crustal layers by considering lithospheric plates that consist of a low-density crustal layer and a high-density mantle part. They also allow the basalt-eclogite phase change in the subducted oceanic crust. Two sets of kinematic and dynamic plate models are studied. In the kinematic models, the flow is driven by an imposed surface velocity, and rigid lithospheric plates are generated by adopting temperature-dependent viscosity. In the dynamic models, the buoyancy forces due to internal density variations provide the driving force of convection, and temperature, depth, and stress-dependent viscosity is used to achieve uniform plate motion.; The results of the kinematic models show that the subduction of the continental crust is strongly controlled by the imposed surface velocity and the lithospheric viscosity. For most models with realistic rheologies, the continental lithosphere subducts to lower mantle depths as a coherent layer. Crustal detachment occurs only when crust-mantle coupling is weak. In such a case the mantle lithosphere continues its descent, while the crust detaches at depths <150 km and accumulates at the top.; In contrast, the dynamic models show that 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 100 to 170 km. As the thickness of the crust increases, the convergence velocity in the collision zone decreases and the location 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 subsequently a state of tensional stress develops in the upper part of the slab that ultimately causes the oceanic slab to break off and sink into the mantle. The breakoff occurs over a time interval of 10 to 20 m.y. (depending on plate velocity), which is roughly the time needed for the crust to reach its maximum thickness. The models indicate no rapid asthenospheric upflow and related heating of the base of the crust as a consequence of slab breakoff. Crustal detachment takes place over a wide range of lithospheric strength values, suggesting that in the dynamic models crustal buoyancy has an important role in the dynamics of continental subduction.