Trench‐parallel flow in the southern Ryukyu subduction system: Effects of progressive rifting of the overriding plate

Abstract

AbstractThe Okinawa trough in the Ryukyu subduction system is one of a few actively extending back arc basins within the continental lithosphere. Recent shear‐wave splitting measurements show variable fast directions along the trough suggesting a complex three‐dimensional mantle flow field. In this study we use numerical subduction models to demonstrate that the thickness variations of the continental lithosphere bounding the edge of a subduction zone can result in complicated mantle circulation and regional dynamics. The model results for the southern Ryukyu show a combination of two effects: Along the Okinawa trough, the increasing thickness of the overriding Eurasian plate toward Taiwan due to a gradually diminishing rifting induces pressure gradients that drive trench‐parallel flow to the edge in the shallow portion of the mantle wedge, and the thick Eurasian lithosphere to the west effectively blocks and diverts this along‐arc flow and limits the toroidal flow around the slab edge below it. The toroidal flow thus enters the mantle wedge at depths of more than about 100 km, opposite in direction with and largely below the along‐arc flow. These combined geometry effects of the Eurasian lithosphere create an intricate three‐dimensional flow structure at the southern edge of the Ryukyu subduction zone. Model predictions for lattice preferred orientations of olivine aggregates show rotation patterns that agree with the observed shear‐wave splitting patterns. This three‐dimensional scenario echoes the geochemical and seismological evidence worldwide that indicates complex, depth‐varying mantle circulations in subduction systems.