Japan Geoscience Union Meeting 2015

Presentation information


Symbol S (Solid Earth Sciences) » S-CG Complex & General

[S-CG57] Structure, evolution and dynamics of mobile belts

Wed. May 27, 2015 4:15 PM - 6:00 PM A06 (APA HOTEL&RESORT TOKYO BAY MAKUHARI)

Convener:*Toru Takeshita(Department of Natural History Sciences, Graduate School of Science, Hokkaido University), Hiroshi Sato(Earthquake Prediction Research Center, Earthquake Research Institute, The University of Tokyo), Koichiro Obana(Research and Development Center for Earthquake and Tsunami, Japan Agency for Marine-Earth Science and Technology), Takuya NISHIMURA(Disaster Prevention Research Institute, Kyoto University), Yukitoshi Fukahata(Disaster Prevention Research Institute, Kyoto University), Aitaro Kato(Graduate School of Environmental Studies, Nagoya University), Jun Muto(Department of Earth Sciences, Tohoku University), Katsushi Sato(Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University), Shuichi Kodaira(Institute for Research on Earth Evolution Japan Agency for Marine-Earth Science and Technology), Takeshi Sagiya(Disaster Mitigation Research Center, Nagoya University), Tatsuya Ishiyama(Earthquake Research Institute, University of Tokyo), Makoto MATSUBARA(National Research Institute for Earth Science and Disaster Prevention), Yasutaka Ikeda(Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo), Chair:Bunichiro Shibazaki(International Institute of Seismology and Earthquake Engineering, Building Research Institute)

4:15 PM - 4:30 PM

[SCG57-06] Lithospheric Structure of the Hidaka Collision Zone, Hokkaido, from Reanalysis of 1998-2000 Hokkaido Transect Data VI

*Takaya IWASAKI1, Noriko TSUMURA2, Tanio ITO3, Hiroshi SATO1, Eiji KURASHIMO1, Naoshi HIRATA1, Kazunori ARITA4, Katsuya NODA5, Akira FUJIWARA5, Susumu ABE5, Shinsuke KIKUCHI6, Kazuko SUZUKI7 (1.Earthquake Research Institute, the University of Tokyo, 2.Graduate School of Science, Chiba University, 3.Heisei Teikyo University, 4.The Hokkaido University Museum, Hokkaido University, 5.JGI, Inc., 6.JAPEX, 7.Schlumberger Ltd.)

Keywords:Hidaka Collision Zone, Kuril Arc, NE Japan Arc, Delamination

The Hokkaido Island, located in the southernmost part of the Kuril trench region, has been under a unique tectonic environment of arc-arc collision due to the oblique subduction of the Pacific (PAC) plate. At the time of middle Miocene, the Kuril forearc sliver started to collide against Northeast (NE) Japan arc to form the Hidaka collision zone (HCZ) with complex structural features including the westward obduction of the crustal rocks of the Kuril arc (the Hidaka metamorphic belt (HMB)) along the Hidaka main thrust (HMT) and a thick foreland fold-and-thrust belt.
Re-analyses for a series data sets of seismic reflection/refraction experiments in HCZ, which started in 2012, are aimed to construct a more detailed collision model through new processing and interpretation techniques. A multi-disciplinary project of the 1998-2000 Hokkaido Transect, crossing the northern part of the HCZ in EW direction, collected high-quality seismic data on a 227-km seismic refraction/wide-angle reflection profile and three seismic reflection lines. Our reprocessing/re-analyses revealed interesting collision structure ongoing in the northern part of the HCZ. The westward obduction of the Kuril arc crust was clearly imaged along the HMT. This obduction occurs at a depth of 27-30 km, much deeper than in the southern HCZ (23-25 km). The CRS/MDRS processing to the reflection data firstly succeeded in imaging clear reflection events associated with the lower crust/Moho within the NE Japan arc descending down to the east. Gently eastward dipping structures above these events (in a depth range of 5-10 km) are interpreted to be fragments of Cretaceous subduction/arc complexes or deformation interfaces branched from the HMT.
The refraction/wide-angle reflection analysis including amplitude calculations revealed a series of eastward dipping interfaces at depths of 15-30 km east of the HMT, some of which show a very large Vp contrast exceeding 0.5-1.0 km/s. The subducted NE Japan arc meets the Kuril arc 20-40 km east of the HMT at a depth of 20-30 km. The above mentioned high Vp contrasts may result from the mixture of the upper crustal (low Vp) materials of the NE Japan arc and lower crustal (high Vp) materials of the Kuril arc.
Seismic reflection image in the southern HCZ reprocessed by almost the same techniques confirms a clear crustal delamination, where the upper 23-km crust is thrust up along the HMT while the lower part of the crust descends down to the subducted PAC plate. At the moment, the results in the northern HCZ do not provide positive evidence on shallow crustal delamination as found in the case of the southern HCZ, probably presenting important information on "3D structure" of the HCZ.