STRUCTURAL INSIGHT INTO THE SOUTH RYUKYU MARGIN - EFFECTS OF THE SUBDUCTING GAGUA RIDGE

This study presents three multi-channel deep seismic reflection profil es located in the south Ryukyu margin between 122 degrees 30'E and 123 degrees 30'E, where a N-S-trending oceanic ridge the Gagua Ridge, is entering the subduction zone, for the purpose of examining the effects of ridge subduction on structures of the forearc region. Structural f eatures which correspond to different stages of the oblique ridge subd uction are observed. East of 123 degrees E, a short-lived sequence of indentation, tunneling, then resumption of frontal accretion occurred in the accretionary wedge (the Yaeyama Ridge) as the subducted portion of the Gagua Ridge swept the overriding Ryukyu margin from below alon g the northwesterly convergent direction. Under the forearc basin, the subducted portion of the Gagua Ridge is uplifting the arc basement to form the Nanao Basement Rise which separates the sedimentary strata o f the Nanao and East Nanao forearc basins. Results from this study sug gest that the oblique subduction of the Gagua Ridge has not only affec ted accretionary wedge structures but also the arc basement of the sou th Ryukyu margin. (C) 1998 Elsevier Science B.V. All rights reserved.