Extension in the southern Ryukyu arc (Japan): Link with oblique subduction and back arc rifting

Compilation of earthquake focal mechanisms, morphological analysis, and examination of mesofractures help to clarify the Cenozoic and recent stress history in the southern Ryukyu are between the Ryukyu trench and the southern Okinawa Trough. (1) An older, pre-Miocene, intermediate-type stress field (vertical stress component sigma(v) = sigma(2)) is attested by conjugate strike-slip faults and shear joints in Eocene rocks of Ishigaki Island. The sigma(1) axis trends N90 degrees E to N110 degrees E in present-day coordinates. (2) Normal faults and extensional joints in Mio-Pliocene strata of Iriomote and Yonaguni Islands and fault scarps in Pleistocene strata at Yonaguni allow us to reconstruct a Miocene to Pleistocene stress field of tensional type (sigma(v) = sigma(1)). The sigma(3) axis trends about NW-SE. The effects of this stress field can be traced back in Eocene rocks of Ishigaki Island. (3) Evidence for the youngest, Holocene to present-day stress field comes from the analysis of the Harvard Moment Tensor Catalog (1976-1997) for shallow earthquakes, Quaternary fault scarps visible on the two islands of Miyako and Yonaguni, and extensional joints in Holocene rocks of Yonaguni. This stress field is tensional. Two directions of extension are inferred: a regional, arc-perpendicular (N-S) one and a local, are-parallel (N60 degrees E) one in the easternmost part of the area near Miyako. The record of Neogene to Quaternary tensional stress fields demonstrates that the extension in the Okinawa Trough can be traced until the are region Are-oblique to arc-perpendicular stretching can be directly correlated with oceanward rifting in the back are basin, while the are-parallel stretching reflects the increase of the are curvature. The predominance of extensional features, the absence of shortening or transcurrent deformation structures, and a weak interplate, coupling do not support the forearc sliver model for the present-day southern Ryukyu are.