Abstract
The deformational structures (outcrop-scale normal faults and extensional joints) observed in the upper Osaki Formation of the middle Miocene (16-10 Ma) Kukinaga Group exposed on Tanegashima Island, northern Ryukyu arc, result from the superposition of two stages of extension : (1) a NE-SW (N43°E on average) extension, well documented by fault-slip data from eight localities and by extensional joints or non-striated normal faults from eight localities ; (2) a NW-SE (N134°E on average) extension, inferred from fault-slip data at one locality and by joints or non-striated normal faults from five localities. These two orthogonal extensions probably correspond to a unique stress field with, in most localities, σV=σ1 and permutating σ2 and σ3 axes. Like elsewhere in the Ryukyu arc (Miyako Island region), the permutation between the intermediate principal axis and the least principal axis can be accounted for by arc-parallel stretching in response to the increasing curvature of the oceanward-migrating arc.The tensional stress field has been active since the time of deposition of the Osaki Formation (ca. 11-10 Ma). At present, it is expressed by NW-SE normal faulting (apparent N55°E extension) of Quaternary coastal terraces.
