Oblique subduction of the Gagua Ridge beneath the Ryukyu accretionary wedge system: Insights from marine observations and sandbox experiments

The Gagua Ridge, carried by the Philippine Sea Plate, is subducting oblique ly beneath the southernmost Ryukyu Margin. Bathymetric swath-mapping, perfo rmed during the ACT survey (Active Collision in Taiwan), indicates that, du e to the high obliquity of plate convergence, slip partitioning occurs with in the Ryukyu accretionary wedge. A transcurrent fault, trending N95 degree s E, is observed at the rear of the accretionary wedge. Evidence of right l ateral motion along this shear zone, called the Yaeyama Fault, suggests tha t it accommodates part of the lateral component of the oblique convergence. The subduction of the ridge disturbs this tectonic setting and significant ly deforms the Ryukyu Margin. The ridge strongly indents the front of the a ccretionary wedge and uplifts part of the forearc basin. In the frontal par t of the margin, directly in the axis of the ridge, localized transpressive and transtensional structures can be observed superimposed on the uplifted accretionary complex. As shown by sandbox experiments, these N330 degrees E to N30 degrees E trending fractures result from the increasing compressio nal stress induced by the subduction of the ridge. Analog experiments have also shown that the reentrant associated with oblique ridge subduction exhi bits a specific shape that can be correlated with the relative plate motion azimuth. These data, together with the study of the margin deformation, the uplift o f the forearc basin and geodetic data, show that the subduction of the Gagu a Ridge beneath the accretionary wedge occurs along an azimuth which is abo ut 20 degrees less oblique than the convergence between the PSP and the Ryu kyu Arc. Taking into account the opening of the Okinawa backarc basin and p artitioning at the rear of the accretionary wedge, convergence between the ridge and the overriding accretionary wedge appears to be close to N345 deg rees E and thus, occurs at a rate close to 9 cm yr(-1). As a result, we est imate that a motion of 3.7 cm yr(-1) +/- 0.7 cm should be absorbed along th e transcurrent fault. Based on these assumptions, the plate tectonic recons truction reveals that the subducted segment of the Gagua Ridge, associated with the observable margin deformations, could have started subducting less than 1 m.y. ago.