Global back-arc extension due to trench-parallel mid-ocean ridge subduction

Subduction of young seafloors straddling mid-ocean ridges (MOR) is an inevitable consequence of plate tectonics. Surprisingly, this process correlates globally with prolonged back-arc extension when the MOR is largely trench-parallel. We investigate the underlying mechanism by analyzing the East China Sea Basin (ECSB) whose Cenozoic tectonic history consists of three syn-rift stages with the rift center progressively migrating oceanward. Global geodynamic models satisfying the past subduction history and present-day mantle structures successfully reproduce the lithospheric stress states of the evolving ECSB. We show that segmented removal of the Mesozoic Izanagi slab due to subduction of the young seafloor initiated Paleocene rifting within the western ECSB. Detachment of the former slab facilitated a strong landward mantle wind driven by the large pressure gradient across the slab. The resulting mantle traction pushed the thickened upper plate landward while entraining the young seafloors behind to slowly subduct, a process causing long-lasting Eocene extension of the central ECSB. The waning mantle wind after 30 Ma reduced basal traction and upper plate extension. A final phase of ECSB extension since the late Miocene formed the Okinawa Trough, when the subducting plate became old enough to trigger slab retreat. A similar dynamic scenario is also predicted in other circum-Pacific margins. We conclude that this enduring back-arc extension during MOR subduction represents an important mechanism for continental evolution during the closure of major ocean basins.