Shear-Dominant Continental Rifting in Northern Ryukyu Revealed by Ambient Noise Tomography

Abstract

In this study, we reveal the deformational structure of the crust of the northern part of the Ryukyu Arc and Okinawa Trough using ambient noise tomography. Compared with southern Ryukyu, the northern segment exhibits a wide and shallow basin, a crust without localized thinning, slow extension rates, and highly arc-oblique, right-lateral retreat of the Ryukyu Arc. We present both isotropic and azimuthally anisotropic shear-wave velocity models using data recorded by an ocean-bottom seismometer array and nearby island stations. The isotropic model demonstrates a monotonic decrease in velocity from the backarc to the forearc, in accord with the accretionary-prism origin of the latter. The resolved azimuthal anisotropy exhibits arc-parallel and arc-perpendicular fast shear-wave polarization directions in the upper to mid-crust and the lower crust to uppermost mantle in much of the arc and backarc, respectively. We interpret the arc-parallel anisotropy as resulting from the anisotropic fabrics aligned by the vertical shearing imposed by the right-lateral motion of the Ryukyu Arc. The underlying arc-perpendicular anisotropy may be attributed to horizontal shearing driven by corner flow in the mantle wedge. We found arc-perpendicular anisotropy in the forearc upper crust, which may reflect crack alignment caused by the collision of the Amami Plateau. The oblique arc retreat and the resolved deformation fabrics in the arc and backarc together attest to the shear-dominant, transtensional nature of the northern Ryukyu continental rift system. Some of the features in northern Ryukyu may be better explained from the perspective of transtensional rifting.