Abstract
The Gagua Ridge, carried by the Philippine Sea Plate, is subducting obliquely beneath the southernmost RyukyuMargin. Bathymetric swath-mapping, performed during the ACT survey (Active Collision in Taiwan), indicatesthat, due to the high obliquity of plate convergence, slip partitioning occurs within the Ryukyu accretionary wedge.A transcurrent fault, trending N95◦ E, is observed at the rear of the accretionary wedge. Evidence of right lateralmotion along this shear zone, called the Yaeyama Fault, suggests that it accommodates part of the lateral componentof the oblique convergence. The subduction of the ridge disturbs this tectonic setting and significantly deforms theRyukyu Margin. The ridge strongly indents the front of the accretionary wedge and uplifts part of the forearc basin.In the frontal part of the margin, directly in the axis of the ridge, localized transpressive and transtensional structurescan be observed superimposed on the uplifted accretionary complex. As shown by sandbox experiments, theseN330◦ E to N30◦ E trending fractures result from the increasing compressional stress induced by the subduction ofthe ridge. Analog experiments have also shown that the reentrant associated with oblique ridge subduction exhibitsa specific shape that can be correlated with the relative plate motion azimuth.These data, together with the study of the margin deformation, the uplift of the forearc basin and geodetic data,show that the subduction of the Gagua Ridge beneath the accretionary wedge occurs along an azimuth which isabout 20◦ less oblique than the convergence between the PSP and the Ryukyu Arc. Taking into account the openingof the Okinawa backarc basin and partitioning at the rear of the accretionary wedge, convergence between the ridgeand the overriding accretionary wedge appears to be close to N345◦ E and thus, occurs at a rate close to 9 cm yr−1.As a result, we estimate that a motion of 3.7 cm yr−1 ± 0.7 cm should be absorbed along the transcurrent fault.Based on these assumptions, the plate tectonic reconstruction reveals that the subducted segment of the GaguaRidge, associated with the observable margin deformations, could have started subducting less than 1 m.y. ago