THE DISTRIBUTION OF FOREARC MAGMATISM ALONG SOUTHWEST JAPAN AND ITS CONSTRAINTS FOR THE RECONSTRUCTION OF THE PHILIPPINE SEA PLATE IN MIOCENE

During the Miocene, Southwest Japan rotated clockwise and moved away from the Asian continent. Currently, the part of the Philippine Sea Plate that expanded between 27 to 15 Ma, known as the Shikoku Basin, is subducting beneath Southwest Japan. Opinions differ regarding the configuration of the plate that faced Southwest Japan immediately after its clockwise rotation. Around the same time as this rotation, diverse igneous activities occurred in regions close to the trench, including the Setouchi volcanic rocks which contain high-Mg andesite derived from the arc mantle. These rocks are characterized by high Mg-number and elevated levels of Cr and Ni, and while also being rich in components derived from sediments. It is believed that the subduction of the newly formed, thermally anomalous Shikoku Basin facilitated the melting of slab surface sediments, leading to the generation of high-Mg andesite magma. Thus, by examining the distribution of Setouchi volcanic rocks along the island arc, it is possible to constrain the extent of the area that faced the Shikoku Basin immediately after the rotation of Southwest Japan. Through dating of middle Miocene igneous rocks in Southwest Japan, we have reevaluated the distribution of Setouchi volcanic rocks, confirming that they extend from the Kii Peninsula to the western part of Kyushu Island. While opinions vary on the timing of movements of TTT-type triple junction along Southwest Japan related to the clockwise rotation of the Philippine Sea Plate since Oligocene time, the distribution of the Setouchi volcanic rocks indicates that by about 15 Ma, it must have been positioned at least to the east of the Kii Peninsula.