PALEOMAGNETISM AND FISSION-TRACK GEOCHRONOLOGY ON THE GOTO AND TSUSHIMA ISLANDS IN THE TSUSHIMA STRAIT AREA - IMPLICATIONS FOR THE OPENING MODE OF THE JAPAN SEA

Paleomagnetism of Neogene rocks in the Goto Islands and fission-track (FT) geochronology of Miocene igneous rocks in the Goto and Tsushima Islands were investigated in order to reveal Miocene tectonics in the Tsushima Strait area. Untilted directions of primary magnetic components from early to middle Miocene sedimentary rocks in the Goto Islands dominantly showed counter-clockwise (CCW) deflections to the expected direction of the geocentric axial dipole field, while paleomagnetic directions from Miocene igneous rocks and Quarternary basalts were concordant with the expected direction. Zircon FT ages determined on sixteen Miocene igneous rocks show good agreement at about 15 Ma. These results, in conjunction with previously-reported paleomagnetic data in the Tsushima Islands, suggest that the Goto Islands were rotated at the early to middle Miocene before about 15 Ma and that the CCW of the Tsushima islands occurred after about 15 Ma. The CCW rotations of these islands imply that the Tsushima Strait area did not belong to the Southwest Japan block in terms of clock-wise (CW) rotation at about 15 Ma, constraining the western margin of the CW-rotated block. The FT ages confine the time of the compressive deformation of pre-middle Miocene sediments in the area to be at the early to middle Miocene before about 15 Ma. The CCW rotations of these islands probably took place in repsonse to the movement of the fault system in the area associated with the compressive regime, especially a sinistral motion of the Tsushima-Goto tectonic line. The early to middle Miocene compressive deformation in the area occurred almost coeval with the opening of the Japan Sea. The compressive tectonic regime implies the convergence of the western margin of the Southwest Japan block to the Korean Peninsula during the CW rotation of the block at about 15 Ma, which gives a new constraint on the position of the rotation pivot of the block.