Regional seismicity and on-land deformation in the Ryukyu arc: Implications for the kinematics of opening of the Okinawa Trough

The stress field evolution and the kinematics of opening of the Okinawa Trough are investigated on the basis of earthquake focal mechanisms and structural data in the Ryukyu are and the Okinawa Trough. Focal mechanisms show that the crust underlying the are and the trough undergoes extension along two suborthogonal directions: a regional are-perpendicular extension and a local al c-parallel extension. Both extensions are concurrent and related to the same regional stress field characterized by permutating horizontal sigma (2) and sigma (3) axes. Earthquake slip vectors reveal a southward motion of the Ryukyu are with respect to the south China block. The current pole of opening of the Okinawa Trough is located around 16 degreesN and 50 degreesE. Fracture analysis in Okinawa island allows identification of three episodes of extension: a late Miocene N40 degreesW to N20 degreesE extension (episode I), a late Pliocene to early Pleistocene N20 degreesE extension (episode II), and a latest Pleistocene to present-day N20 degreesW extension (episode III). Episodes II and III are characterized by permutations between the two horizontal sigma (2) and sigma (3) axes. By synthesizing regional deformation data and by comparing the geometry of the deformation with analogue models of oblique rifting we reconstruct the kinematics of opening of the Okinawa Trough since the late Miocene. The direction of divergence of the Ryukyu are has rotated clockwise from -N150 degreesE in the late Miocene to nearly N-S today.