Three-dimensional dynamic models of subducting plate-overriding plate-upper mantle interaction

We present fully dynamic generic three-dimensional laboratory models of progressive subduction with an overriding plate and a weak subduction zone interface. Overriding plate thickness (T_OP) is varied systematically (in the range 0-2.5 cm scaling to 0-125 km) to investigate its effect on subduction kinematics and overriding plate deformation. The general pattern of subduction is the same for all models with slab draping on the 670 kmdiscontinuity, comparable slab dip angles, trench retreat, trenchward subducting platemotion, and a concave trench curvature. The narrow slab models only show overriding plate extension. Subduction partitioning (ν_SP⊥ / (ν_SP⊥ + ν_T⊥)) increases with increasing T_OP, where trenchward subducting plate motion (ν_SP⊥) increases at the expense of trench retreat (ν_T⊥). This results from an increase in trench suction force with increasing T_OP, which retards trench retreat. An increase in T_OP also corresponds to a decrease in overriding plate extension and curvature because a thicker overriding plate provides more resistance to deform. Overriding plate extension is maximum at a scaled distance of ∼200-400 km from the trench, not at the trench, suggesting that basal shear tractions resulting frommantle flow below the overriding plate primarily drive extension rather than deviatoric tensional normal stresses at the subduction zone interface. The force that drives overriding plate extension is 5%-11% of the slab negative buoyancy force. The models show a positive correlation between ν_T⊥ and overriding plate extension rate, in agreement with observations. The results suggest that slab rollback and associated toroidal mantle flow drive overriding plate extension and backarc basin formation.