[SGL33-P09] North-South extension and compression juring the Japan Sea opening in the Amakusa, western Kyushu, Japan
Keywords:stress, Japan Sea opening, dike, Kyushu, Okinawa Trough
We observed altered andesite intrusions, i.e., 50 dikes and 18 sills. Their ages are loosely constrained to be 14–20 Ma by the radiometric ages of intrusive bodies in Amakusa (Nagao et al., 1992; Hamasaki, 1996) The method of Yamaji and Sato (2011), which fits a mixed Bingham distribution to the orientations of planar intrusions, was applied to the 68 data. As a result, we obtained normal-faulting and reverse-faulting stress regimes, indicating N-S extensiona and N-S compression. The latter is oblique to the fold axes, so that the previous researchers’ hypothesis about the folding accompanied by the Japan Sea opening is improbable. A few NNE-trending rhyolitic dikes in the island led Yamamoto (1991) to suggest horizontal maximam stress in the trend. The rhyolitic dikes may have been accompanied by the first phase rifting in the Okinawa Trough to the southwest of the island.