Effects of Along-Trench Asymmetric Subduction Initiation on Plate Rotation and Trench Migration: A Laboratory Modeling Perspective

The impact of along-trench asymmetric subduction on plate kinematic evolution (e.g., plate rotation and trench migration) remains enigmatic. In this study, analog experiments were performed to investigate the effects of symmetric and asymmetric subduction initiation on slab kinematics and trench migration. In cases when subduction was started with a cylindric slab perturbation, the plate showed little rotation during the entire subduction process, resulting in a trench shape that was symmetric with respect to the center-line of the plate. However, if subduction started with a non-cylindrical slab perturbation, the trench shape changed substantially. During the free sinking stage, the more deeply subducted part of the slab had a higher trench-normal retreat velocity (V_T⊥) and subduction velocity (V_S⊥) than the shallow part, which induced trench and plate rotations in the same direction. This along-trench gradient in V_T⊥ increased until the deeper portion of the slab tip first touched the bottom, after which a marked decrease in V_S⊥ occurred at this location; the other side of the slab had not yet reached the bottom, so experienced no recorded reduction of subduction velocity at this time. This along-strike diachronous arrival of the slab tip could induce a marked along-strike reversal in magnitude of the subduction velocity and a rotation torque centered on the point of first contact between slab and 660-km discontinuity. This could lead to instability and rotation of the subducting slab, potentially causing a reversal in the direction of trench rotation direction, but rarely in the direction of plate rotation. Our modeling results may provide useful understanding for the processes driving the rotations of the trench and plate in natural subduction zones.