Lithospheric structure, buoyancy and coupling across the southernmost Ryukyu subduction zone: an example of decreasing plate coupling

The Okinawa Trough is a backarc basin located behind the Ryukyu are-trench system. The southernmost part of the Okinawa Trough (SPOT) displays differe nt tectonic features from the rest of the Okinawa Trough. The SPOT area inc ludes abundant seamounts with active hydrothermal venting and high heat-flo w values. To understand better the rifting and magmatism context of the SPO T area, we examine the lithospheric structure, buoyancy and coupling across the southernmost Ryukyu subduction zone. The results show that beneath the SPOT area the continental crust and mantle lithosphere thickness of simila r to 25-30 and 120 km, respectively, are thick with little continental thin ning. The analysis of mantle lithosphere buoyancy across the southernmost R yukyu subduction zone shows strong plate coupling between the overriding an d subducting plates. However, the two plates are actuary decoupled as indic ated by present-day interface earthquakes. This situation indicates that th e southernmost Ryukyu subduction zone displays a transitory case of a chang ing plate coupling, from a strong to a weak plate coupling. Such a coupling /decoupling transition is probably associated with the collision of the Luz on are with the Asian continental margin. Additionally, the curve of the ma ntle lithosphere buoyancy across the southernmost Ryukyu subduction zone in dicates that the submarine magmatism in the SPOT area is located within the volcanic are area? suggesting early are magmatism in the SPOT area. (C) 20 01 Elsevier Science B.V. All rights reserved.