S. Dominguez et al., Oblique subduction of the Gagua Ridge beneath the Ryukyu accretionary wedge system: Insights from marine observations and sandbox experiments, MAR GEOPHYS, 20(5), 1998, pp. 383-402
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.