The southwestern edge of the Ryukyu subduction zone: A high Q mantle wedge

The lateral edge of a subduction zone is usually depicted as an opening to the asthenosphere where invigorated dynamics and amplified magmatism take place. In this study we present evidence from seismic data suggesting the presence of a cold and dynamically sluggish edge environment at the southwest end of the Ryukyu subduction system. We measured attenuation, or 1/Q, for P waves from subduction zone events at ∼100 km depths received by OBSs in the Okinawa trough and land stations in NE Taiwan. In the Okinawa trough 100–200 km from the edge, Q values are lower than 100. In the vicinity of the edge, Q values increase from 100 to over 1000 towards Taiwan. To reconcile arguments from geophysical and geochemical observations, we propose that the mantle wedge near the edge has high Q values due to low temperatures and probably low water content. These may result from coupling of the slab laterally with the thick Eurasian lithosphere, which inhibits back-arc rifting, retards subduction, and reduces the water supply to the mantle wedge. The SW Ryukyu subduction system represents a subduction-zone edge type distinct from more commonly documented free or warm edges.