Constraints on backstop geometry of the southwest Ryukyu subduction based on reflection seismic data

Based on the analysis of 45 seismic reflection profiles, the top of the Ryu kyu Are basement is mapped in the forearc region east of Taiwan. The Ryukyu Are slope dips 10 degrees trenchward, while beneath the forearc basins, th e are basement shows a series of morphologic along-are 'highs-and-lows'. On e of the basement highs, the Nanao basement rise, has been described as an area uplifted by the subduction of the Gagua Ridge. The other one, the Hopi ng basement rise. is suggested in this study to be an uplifted feature caus ed by the subduction (or the underplating) of some local asperity. The natu re of this asperity could either be a detached block of the Luzon Are. an i solated seamount or some material removed from the frontal part (southernmo st extremity) of the Ryukyu margin. From seismic reflection profiles, we infer that the right-lateral transcurr ent fault system located at the rear of the accretionary wedge defines the surface projection of the southern termination of the Ryukyu Are basement. Two crustal sections across the Ryukyu subduction system demonstrate that t he backstop terminates in the form of a near-vertical 'wall' with its heigh t over 5 km. Such an unusual geometry could be produced by crustal truncati on at the toe of the backstop. (C) 2001 Elsevier Science B.V. All rights re served.