[S-GL18] Geologic structure and tectonic history of East Asia and Japanese Islands

[SGL18-05] Paleomagnetic pole from the Early Cretaceous Sasayama Group and apparent polar wander path for southwest Japan

*Koji Uno¹, Hayao Morinaga² (1.School of Human Science and Environment, University of Hyogo, 2.Professor Emeritus, University of Hyogo)

Keywords:Paleomagnetism, Sasayama Group, Southwest Japan, Cretaceous

Paleomagnetic data are useful for investigating the geodynamic activity of geological domains at various scales. In the study of large geological domains such as cratonic blocks or island arcs, paleomagnetic poles are indispensable to provide tectonic scenarios for the regions. Paleomagnetic poles for the inner arc of the southwestern Japanese islands (referred to as southwest Japan) have been proposed for the Early Cretaceous onward to determine the tectonic movement in this region. However, over the past 30 years, the tectonic scenario for the Cretaceous in southwest Japan continues to be rewritten because of insufficient data accumulated from this region during the Cretaceous. The purpose of this study is to obtain more reliable 110 Ma paleomagnetic data to establish a precise paleomagnetic pole position for southwest Japan. Red beds from the Early Cretaceous (~110 Ma) Sasayama Group were collected at 23 sites from the Sasayama basin and five sites from the Kamitaki-Shimotaki area, where the dinosaur fossil Tambatitanis amicitiae has been discovered. A high-temperature magnetization component with unblocking temperatures of up to 690 °C is isolated from all the sites. The site-mean directions of the component fail the fold test, and incremental unfolding for the tilt yields an optimum mean direction. Previously reported site-mean directions of the Sasayama basin from six sites are incorporated with those of this study to provide a grand-mean direction for the Sasayama Group. Furthermore, the dataset of the grand-mean direction is combined with a contemporaneous dataset consisting of 11 sites from the Inakura Formation in Okayama Prefecture to yield a representative 110 Ma paleomagnetic pole for southwest Japan. We compared this representative pole for southwest Japan with the 110 Ma pole for East Asia and the Korean Peninsula. The pole for southwest Japan shows significant discrepancy with respect to both the East Asia pole and the Korean Peninsula pole. Comparing the southwest Japan pole with the East Asia pole, we obtain 63.9º ± 5.7º rotation for the reference point of southwest Japan at 34°N, 129°E. Similarly, the southwest Japan pole shows, with respect to the Korean Peninsula pole, 40.1º ± 5.0º rotation for the reference point. These observed rotations are considered to have occurred after the Cretaceous, and southwest Japan was in tectonically quiescent settings during 110-70 Ma in terms of rotation.