A new subduction initiation mechanism induced by lateral subduction propagation

• Experiments reveal a new subduction initiation mechanism via lateral propagation. • Strong plate coupling drives subduction initiation in adjacent oceanic plate. • Weak to moderate coupling develops transform plate boundary or tearing fault. • Findings elucidate the geodynamic evolution of plate-boundary subduction zones. Lateral subduction propagation is common in natural subduction zones, but its impact on side plates remains enigmatic. This study utilises scaled analogue models to investigate the behaviour of an ocean-ocean plate boundary subduction system, focusing on the effects of asymmetric lateral subduction propagation along a weak zone. Subduction experiments were conducted under varying degrees of interplate mechanical coupling (low, intermediate, and high). Our modelling results indicate that under high interplate coupling, the lateral propagation of the subducting slab can cause bending or dragging of part of the side plate into the mantle, initiating local subduction and leading to self-sustained subduction within the side oceanic plate. Conversely, under low interplate coupling, the side plate material is unlikely to be dragged into the mantle and a transform plate boundary fault activates to accommodate the motions between the two plates and simultaneously functions as a tear fault. The intermediate coupling scenario exhibits intermediary behaviour, with minor material being dragged into the mantle, leading to insufficient negative buoyancy to trigger subduction initiation within the side plate. Despite the simplifications, our modelling results provide new insights into the formation mechanism of highly curved plate-boundary-related subduction zones, such as the Banda subduction/collision zone, Lesser Antilles-Puerto Rico subduction zone and Scotia subduction zone. In particular, our models provide an explanation as to how the Puerto Rico subduction segment might have formed on the North American side of the transform plate boundary with the South American plate, as a consequence of downward drag forces exerted by the slab subducting along the Lesser Antilles subduction segment located south of the transform plate boundary.