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[SCG50-03] A Framework to Understand Deformation in Japanese Islands: Inter-arc Deformation and Intra-arc Deformation
Keywords:inter-arc deformation, intra-arc deformation, GNSS, strain rate field, Japanese islands
Deformation of Japanese islands is quite complicated. Japanese islands are composed of five island arcs: Kuril, Northeast Japan, West Japan, Izu, and Ryukyu. In this paper, we propose to understand deformation of Japanese islands by decomposing it into inter-arc deformation and intra-arc deformation. Of course, we notice that many researchers have already used similar ideas consciously or unconsciously, but we propose to use this idea as a framework to consider Japanese island deformation with a help of a cutting edge result of GNSS data analysis (Okazaki, Fukahata & Nishimura, 2021, JpGU).
Before considering inter- and intra-arc deformation, we need to consider the relation between island arc deformation and subducting oceanic slabs. Subduction of oceanic slabs cause large crustal displacements, but most of them are cyclic. However, subducting oceanic slabs are often the causes of inter- and intra-arc deformation.
Kuril and Northeast Japan arcs are colliding in the EW direction in Hokkaido, which results in Hidaka Mountains. The cause of the collision is the southwestward motion of Kuril arc as a forearc sliver due to oblique subduction of the Pacific plate, but large active faults are not known to bound the forearc sliver. Probably because of that, large strain rates are observed in the forearc of eastern Hokkaido (Okazaki et al., 2021).
Northeast and West Japan arcs are contracting in the EW direction, due to the northwestward motion of the triple junction off Boso (Takahashi, 2006). The boundary of them are usually taken at the Itoigawa-Shizuoka tectonic line, but the result of GNSS data analysis (Okazaki et al., 2021) suggests a wider deformation zone.
Izu arc is colliding with Northeast and West Japan arcs. On the other hand, Ryukyu arc is splitting from West Japan arc in the NS direction in central and southern Kyusyu. According to recent GNSS data, the splitting is more active in southern Kyusyu than in central Kyushu, where Beppu-Shimabara graben passes through (Okazaki et al. 2021).
Representative intra-arc deformations are dextral movement of Median tectonic line, EW extension of Izu arc, and back-arc spreading of Ryukyu arc, which are caused by subduction of oceanic plates. We observe considerable contraction along the Ou-backbone range, while the forearc region from Ibaraki to Nara has relatively small deformation (Okazaki et al., 2021). Some volcanoes also show significant deformation locally (Okazaki et al., 2021).
Before considering inter- and intra-arc deformation, we need to consider the relation between island arc deformation and subducting oceanic slabs. Subduction of oceanic slabs cause large crustal displacements, but most of them are cyclic. However, subducting oceanic slabs are often the causes of inter- and intra-arc deformation.
Kuril and Northeast Japan arcs are colliding in the EW direction in Hokkaido, which results in Hidaka Mountains. The cause of the collision is the southwestward motion of Kuril arc as a forearc sliver due to oblique subduction of the Pacific plate, but large active faults are not known to bound the forearc sliver. Probably because of that, large strain rates are observed in the forearc of eastern Hokkaido (Okazaki et al., 2021).
Northeast and West Japan arcs are contracting in the EW direction, due to the northwestward motion of the triple junction off Boso (Takahashi, 2006). The boundary of them are usually taken at the Itoigawa-Shizuoka tectonic line, but the result of GNSS data analysis (Okazaki et al., 2021) suggests a wider deformation zone.
Izu arc is colliding with Northeast and West Japan arcs. On the other hand, Ryukyu arc is splitting from West Japan arc in the NS direction in central and southern Kyusyu. According to recent GNSS data, the splitting is more active in southern Kyusyu than in central Kyushu, where Beppu-Shimabara graben passes through (Okazaki et al. 2021).
Representative intra-arc deformations are dextral movement of Median tectonic line, EW extension of Izu arc, and back-arc spreading of Ryukyu arc, which are caused by subduction of oceanic plates. We observe considerable contraction along the Ou-backbone range, while the forearc region from Ibaraki to Nara has relatively small deformation (Okazaki et al., 2021). Some volcanoes also show significant deformation locally (Okazaki et al., 2021).