Subduction Mode Selection During Slab and Mantle Transition Zone Interaction: Numerical Modeling

Global seismic tomography of the subduction zones shows that the subducting slabs could either stagnate around the 660-km discontinuity, or penetrate into the lower mantle. The stagnating slabs also have various morphologies, which are directly related to the interaction between the subducting slabs and the mantle transition zone (MTZ). Using a 2-D thermo-mechanical model, we systematically investigated the modes of subduction in the mantle transition zone and explored the key constraints of various subduction styles. Four basic subduction modes are obtained in the numerical experiments, including one with slab penetrating through the 660-km discontinuity and three other modes with slab stagnating in the MTZ (i.e. folding, lying and rolling-back). The numerical models indicate that the age of subducting oceanic plate, the thickness of overriding continental lithosphere and the convergence velocity play crucial roles in the dynamics of subducting slab and MTZ interaction. In general, the young subducting slab favors the penetration or folding mode, whereas the old subducting slab tends to result in lying or rolling-back mode, although other parameters can also affect. The selection of lying or rolling-back mode in the models is further strongly dependent on the thickness of the overriding continental lithosphere (OCL): slab lying in the models with thick OCL, while slab rolling-back with thin OCL. Our models also show a strong correlation between the subduction mode selection and the dip angle of the slab tip when first touching the 660-km phase boundary.