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

Authors

  • Marc Fournier,

  • Olivier Fabbri,

  • Jacques Angelier,

  • Jean-Paul Cadet


Abstract

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 arc and the Okinawa Trough. Focal mechanisms show that the crust underlying the arc and the trough undergoes extension along two suborthogonal directions: a regional arc-perpendicular extension and a local arc-parallel extension. Both extensions are concurrent and related to the same regional stress field characterized by permutating horizontal σ2 and σ3 axes. Earthquake slip vectors reveal a southward motion of the Ryukyu arc with respect to the south China block. The current pole of opening of the Okinawa Trough is located around 16°N and 50°E. Fracture analysis in Okinawa island allows identification of three episodes of extension: a late Miocene N40°W to N20°E extension (episode I), a late Pliocene to early Pleistocene N20°E extension (episode II), and a latest Pleistocene to present-day N20°W extension (episode III). Episodes II and III are characterized by permutations between the two horizontal σ2 and σ3 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 arc has rotated clockwise from ∼N150°E in the late Miocene to nearly N-S today.

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