Effect of Plate Length on Subduction Kinematics and Slab Geometry: Insights From Buoyancy-Driven Analog Subduction Models

Subduction style is controlled by a variety of physical parameters. Here we investigate the effect of subducting plate length on subduction style using laboratory experiments of time-evolving buoyancy-driven subduction in 3-D space. The investigation includes two experimental sets, one with a lower (~740) and one with a higher (~1,680) subducting plate-to-mantle viscosity ratio (η_SP/η_M). Each set involves five models with a free-trailing-edge subducting plate and variable plate length (20–60 cm, scaling to 1,600–4,800 km), and one model with a fixed-trailing-edge subducting plate representing an infinitely long plate. Through determining the contact area between subducting plate and underlying mantle, plate length affects the resistance to trenchward motion of the subducting plate and thus controls the partitioning of the subduction velocity (v_S) into the subducting plate velocity (v_SP) and trench velocity (v_T). This subduction partitioning thereby determines the subduction style by controlling the dip angle of the slab tip once it first touches the 660 km discontinuity. The low η_SP/η_M models display two subduction styles. Short plates (≤40 cm) induce a higher subduction partitioning ratio (v_SP/v_S), promoting trench advance and slab rollover geometries, whereas longer plates (≥50 cm) lead to a lower v_SP/v_S, producing continuous trench retreat and backward slab draping geometries. In contrast, the high η_SP/η_M models exclusively show trench retreat with draping geometries, as the high η_SP/η_M enables less slab bending before its tip touches the 660 km discontinuity. Our study indicates that future modeling work should consider the effects of plate length on the style and evolution of subduction.