Crustal shortening of Southwest Japan in the Late Miocene

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Abstract
Tectonic deformation of an island arc is interpreted on the basis of geophysical data. Extensive reflection seismic, gravity, geomagnetic data around the back‐arc region of Southwest Japan delineate east‐west to northeast‐southwest folding, and imply conspicuous compression on the southern margin of the Sea of Japan. Because geological data of exploration boreholes indicate that the coinpressive regime was dominant in the late Miocene, the tectonic event seems to be linked with coeval resumption of subduction of the Philippine Sea Plate. Strong coupling of the young buoyant oceanic plate brought about north‐south shortening of the overriding continental lithosphere, and left wrench deformation at the southwestern corner of the Sea of Japan. Amount of shortening for the back‐arc shelf and mountainous ranges of Southwest Japan is estimated to be ca 10 km, adopting a uniform ratio of shortening (0.944) since the Miocene determined on the shelf from depth‐converted seismic profiles. Along the western side of a bend of boundary between the Eurasian Plate and Philippine Sea Plate, the middle Miocene and younger sediments upon the back‐arc shelf are much less deformed than the northern equivalents, and the fore‐arc Miocene strata are deformed by left wrenching, facts which are indicative of northerly initial convergence of the Philippine Sea Plate at the end of Miocene and crustal decoupling on the west of Kyushu Island.