Abstract
This study presents three multi-channel deep seismic reflection profiles located in the south Ryukyu margin between 122°30′E and 123°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 features which correspond to different stages of the oblique ridge subduction are observed. East of 123°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 along 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 of the Nanao and East Nanao forearc basins. Results from this study suggest that the oblique subduction of the Gagua Ridge has not only affected accretionary wedge structures but also the arc basement of the south Ryukyu margin.