Kibi Plateau: A stable-coherent tectonic unit in the active Japanese Islands

Takafumi Sonehara,Kazumasa Aoki,Koshi Yagi,Yo-Ichiro Otofuji,Tetsumaru Itaya,Hiroyuki Takeshita,Shogo Aoki

doi:10.1038/s41598-020-60448-x

Takafumi Sonehara, Kazumasa Aoki + Show 5 more

Open Access

https://doi.org/10.1038/s41598-020-60448-x

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Abstract

The Kibi Plateau in the active Japanese Islands consists of mainly Permian to Cretaceous rocks that have been deeply weathered into a red soil, comprising a peneplain with U-shaped valley. Systematic geological analyses of the Eocene fluvial deposits revealed the paleo-rivers that existed in the eastern Asian continent and streamed out to the paleo-Pacific Ocean. Each paleo-river is traced in a flow line shape without any significant vertical and horizontal displacement. The Eocene shallow marine sediments in a possible coastal region have no relevant inclination. These geological data strongly suggest that the Kibi Plateau has been a stable-coherent tectonic unit since the Eocene through the opening of the Japan Sea and the associated quick rotation of SW Japan in the Middle Miocene. The Kibi Plateau region with a thick crust over 30 km existed as a stable eastern segment of the Asian continent in the Eocene. The Kibi Plateau tectonic unit drifted to the south without any destruction due to the peripheral successive tectonic events such as the Philippine Sea plate subduction and the reactivation of Median Tectonic Line. No subduction related arc volcanism since the Eocene has also influenced to preserve the stable tectonic unit.

Highlights

IntroductionThe Japanese Islands and surrounding areas consist of four plates (i.e., the Pacific, Eurasian, Philippine Sea and North American plates) with two triple junctions and have experienced active subductions
The Japanese Islands and surrounding areas consist of four plates with two triple junctions and have experienced active subductions
The high-V and high-Q anomalies in the Kibi Plateau area are visible in the upper crust down to an approximate 20 km depth

Summary

Introduction

The Japanese Islands and surrounding areas consist of four plates (i.e., the Pacific, Eurasian, Philippine Sea and North American plates) with two triple junctions and have experienced active subductions. A large and fresh granodiorite gravel (Fig. 4d–f) was collected to carry out K-Ar analyses, giving the ages (88.3 ± 2.0 Ma and 86.5 ± 1.9 Ma) of hornblende and plagioclase, respectively (Table 2) This suggests that the felsic to mafic plutonics in the conglomerates originated from the large-scale igneous activity region, thereby forming a caldera and batholith in the Cretaceous[11,15,16,17]. A sample of 11 grains gives concordant ages, with a weighted mean of 32.61 ± 0.49 Ma (Fig. 6c), thereby confirming the Eocene fluvial deposits This Eocene tuff originated from the large scale felsic igneous activity region, forming the Paleogene cauldrons in SW Japan[11,19,20]

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