[S-GL24] Geologic structure and tectonic history of East Asia and Japanese Islands

[SGL24-01] Mode of migration of Southwest Japan during the Japan Sea opening: paleomagnetic constraints from the Tanabe Group, Kii Peninsula

*Hiroyuki Hoshi¹ (1.Aichi University of Education)

Keywords:Kii Peninsula, paleomagnetism, Southwest Japan, Tanabe Group, tectonics

Southwest Japan was split and then translated from the eastern margin of the Asian continent associated with the late Paleogene to early Neogene opening of the Japan Sea. However, two end-member hypotheses exist for the migration mode: (1) Southwest Japan migrated southward from the continental margin with clockwise rotation around a pivot near its western end; and (2) it was drifted southward without near-pivot rotation. Early Neogene and older paleomagnetic data from the Inner Zone of Southwest Japan suggest clockwise rotation relative to the Asian continent and thus have been treated as evidence of the clockwise rotation; however, the paleomagnetic rotation in the Inner Zone is interpreted to have been caused by intra-arc block rotation in the drift hypothesis. To determine which of the two views is correct, a practical approach is to obtain paleomagnetic data from the Outer Zone because regional geologic structures suggest little or no block rotation in the Outer Zone. Here I present paleomagnetic results from Miocene sediments of the Tanabe Group on the Outer Zone in the Kii Peninsula. This fiscal year, I have obtained data from a new geological section and merged them with previous data from other sections. In total, seven site-mean directions of characteristic remanent magnetization were determined for mudstones and fine-grained tuffs of the Asso Formation of the group. They showed a positive tilting test, demonstrating magnetization acquisition before tilting. Importantly, tilt-corrected site-mean directions are almost parallel to the directions of the same age reported from the Inner Zone, indicating no relative rotation between the Inner and Outer zones. Comparing the tilt-corrected directions with an early Miocene reference direction suggests clockwise rotation relative to the Asian continent in the study area. Therefore, our results are compatible with the hypothesis of the clockwise rotation of Southwest Japan and cast serious doubt on the drift hypothesis.