Résumé
Both newly acquired GPS and structural data in the southern Ryukyu subduction zone allowed us to point out some first-order geodynamic implications of the present-day kinematics. Off Hualien, the Hoping Basin lies on a wide zone undergoing north-south transform motion and associated extension between NE Taiwan and the Yonaguni Island. Indeed, this westernmost segment of the Ryukyu Arc, which includes the Yonaguni Island, is moving southward 1.4 cm/yr faster than NE Taiwan with respect to South China. The basin could thus have developed as a pull-apart basin controlled by en-echelon N-S-trending right-lateral strike-slip faults and associated N40 degrees E-trending normal faults during the last 2 m.y., based on present divergence rate. A N50 degrees W-striking valley cutting the southern Ryukyu Arc slope between Yonaguni and Hateruma Islands is explained as a graben caused by the still ongoing differential motion between Yonaguni and Hateruma Ryukyu Arc segments, during the last 400,000 years. If we extrapolate back the present-day kinematics, about half of the southernmost Okinawa Trough might have been formed during the last 2 m.y. The convergence rate between the southern Ryukyu Arc and the Philippine Sea Plate has been estimated to be about 10.5 cm/yr in a direction of N312 degrees east of 123 degrees 30'E, and N325 degrees west of 123 degrees 30'E. The convergence obliquity is thus estimated to be about 40 degrees and 53 degrees , with respect to the N275 degrees striking Ryukyu Trench, west and east of 123 degrees 30'E, respectively. The convergence obliquity increases to 60 degrees west of 122 degrees 40'E, because of the northward turn of the trench close to Taiwan. The Ryukyu accretionary wedge (Yaeyama Ridge) can undergo about 3 to 5 cm/yr of transcurrent motion toward Taiwan assuming a mean degree of partitioning of 0.5.