[S22-1] Lithosphere structure and dynamics

[S22-1-03] Arc-arc collision structure in the southernmost part of the Kuril trench region -Results from integrated reanalyse

^○Takaya Iwasaki¹, Noriko Tsumura², Tanio Ito³, Hiroshi Sato¹, Eiji Kurashimo¹, Naoshi Hirata¹, Kazunori Arita⁴, Katsumi Noda⁵, Akira Fujiwara⁵, Susumu Abe⁶, Shinsuke Kikuchi⁶, Kazuko Suzuki⁷ (1.ERI, the University of Tokyo, Tokyo, Japan, 2.Graduate School of Science, Chiba University, Chiba, Japan, 3.Heisei Teikyo University, Tokyo, Japan, 4.The Hokkaido University Museum, Hokkaido University, Sapporo, Japan, 5.JGI, Inc., Tokyo, Japan, 6.JAPEX, Tokyo, Japan, 7.Schlumberger Ltd., Tokyo, Japan)

The oblique subduction of the Kuril plate is generating a unique tectonic environment in the southernmost part of the Kuril trench area. The Kuril forearc started to collide against NE Japan Arc from the east at the time of middle Miocene to form the Hidaka collision zone (HCZ) in the central part of Hokkaido, Japan. Our integrated reinterpretation for several seismic reflection/refraction experiments revealed detailed and new structural features within the HCZ.
In the southern part of the HCZ, the crustal delamination associated with the collision was clearly imaged by applying CRS/MDRS method to the seismic reflection data (Tsumura et al., 2014). Namely, the upper 22-23 km crust of the Kuril arc is obducted along the Hidaka Main Thrust (HMT), while the lower part of the crust is descending down to reach the subducted Pacific plate.
In the northern part of the HCZ, such delamination structure as obtained in the southern HCZ is not clearly seen. The most important finding in this region is a clear image of the NE Japan arc crust descending eastward to a depth of about 40 km under the hinterland side. Our refraction/ wide-angle reflection analysis revealed the strong dipping reflectors with a velocity contrast of 0.5-1 km/s at depths of 10-35 km just west of the HMT. The obduction of the upper Kuril crust starts at a deeper crustal level of at least 27-30 km and more easterly (~20 km) of the HMT as compared with the case in the southern HCZ. If the metamorphic rocks outcropped east of the HMT are the same crustal materials shallower than 22-23 km depth as in the case of the southern HCZ, the deeper crustal portion originally situated at 23-27~30 km depth must exist in the western side of the present HMT. The very strong and deep reflectors found west of the HMT might result from the mixture of these middle/lower crustal (high velocity) materials of the Kuril arc and upper crustal (low velocity) materials of the NE Japan arc.