Evolution of subduction zone curvature and its dependence on the trench velocity and the slab to upper mantle viscosity ratio

Three-dimensional laboratory models of upper mantle subduction are presented investigating the effect of the trench velocity (v_t) and the slab to upper mantle viscosity ratio (_SP/_UM) on trench curvature and slab curvature. One set of experiments varies _SP/ _UM from 66 to 1375. Another set of experiments modifies v_t through applying different velocities at the trailing plate. The results show that the radius of trench curvature (R_TC) progressively decreases with continuous trench retreat due to quasi-toroidal mantle return flow from the subslab region around the lateral slab edges and toward the mantle wedge region, but starts to increase when trench retreat changes to trench advance due to quasi-toroidal return flow in the opposite direction. Furthermore, R _TC increases with increasing _SP/_UM (i.e., progressively stronger slabs are progressively less curved) following a cubic root function. The force required to curve the slab and trench varies with progressive subduction, but is only 0.1-2.5% of the total negative buoyancy force of the slab, while the viscous dissipation rate due to progressive slab curvature is only 0.01-0.60% of the potential energy release rate. Comparison of trench curvature in the models (scaled slab width w = 750 km) with that of the Scotia subduction zone (w 800 km) indicates that for the Scotia subduction zone the effective _SP/_UM is of the order 1-2 × 10 ². Finally, the elastic sphere indentation model for arc curvature is revisited, demonstrating that four main predictions are not met by observations, implying that the model should be rejected.