Reinterpretation of the lithospheric structure beneath the Hidaka collision zone, Hokkaido, Japan 2. - Results for the northern collision zone from Hokkaido Transect 1998-2000 -

The Hidaka region in the central part of Hokkaido Island, Japan, is known as an arc-arc collision zone. Since the middle Miocene, the Kuril Arc (southern part of eastern Hokkaido) has been collided against the Northeast Japan Arc (western Hokkaido) from the east. This collision is a controlling factor for the formation of the Hidaka Mountains, the westward obduction of the crustal rocks of the Kuril Arc (the Hidaka Metamorphic Belt) and the development of the foreland fold-and-thrust belt. A series of seismic reflection/refraction surveys from 1994 to 2000 revealed the collision and deformation processes occurring in this region (e.g. Arita et al., 1998; Tsumura et al., 1999; Ito et al., 2002). Although these data sets were acquired more than 10 years ago, their high qualities provide promising potentiality for getting clearer structural image and new geological finding with the use of more advanced and sophisticated processing and interpretation techniques including CRS/MDRS method for seismic reflection data and refraction tomography both for reflection and refraction/wide-angle reflection data. This paper focus on the reanalysis for the data sets from the Hokkaido Transect Project from 1998 to 2000 in the northern part of the collision zone. The seismic expedition of this project was composed of a 227-km seismic refraction/wide-angle reflection profile running the middle part of Hokkaido with EW direction and three seismic reflection lines on the wide-angle profile, extending from the hinterland to the foreland crossing the Hidaka Mountains. The CRS/MDRS processing for the reflection data provided clearer images of the westward obduction of the upper half of the crust (including upper part of the lower crust) of the Kuril Arc and the deformation of shallow structural packages within the fold-and-thrust belt. The obduction starts at a depth of 25 km, probably corresponding to the brittle/ductile transition zone in the Kuril arc. Below the obducted part of the crust, there exists a 10-km thick reflective zone, expressing the deformation associated with the collision process. The most important finding in the present processing is a series of reflection events imaged at a 30-45 km depth below the obducted crust. These events, showing gradual increase in eastward dip to the east, are probably representing the lower crust and Moho within the Northeast Japan Arc descending down to the east under the collision zone. Refraction tomography was undertaken to map the deformed structure around the shallower part of collision zone. The obtained image is well consistent with the previous result (Iwasaki et al. 2004), showing a thick (4-5 km) undulated sediments in the hinterland, the outcrop of crystalline crust beneath the Hidaka Metamorphic Belt with higher Vp and Vp/Vs, probably expressing the obduction of the middle/lower crustal materials, and an enormously thick (>8-10 km) sedimentary package beneath the foreland. Weak later phases appearing at rather distant offsets (> 80-100 km) in the wide-angle data, which were not explained in the former study, are refractions/reflections from the descending lower crust and Moho of the Northeast Japan Arc as indicated by the CRS/MDRS imaging.