Evolution and deformation process of the Hidaka Collision Zone, Central Hokkaido, Japan.

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Our study in 1999 was concentrated in the data processing of seismic reflection data. Our data revealed a clear image of deformation style associated with the arc-arc collision' between the kuril Arc and Northeastern Japan Arc. The obtained reflection section is characterized by(1) strong westward dipping reflector in range of 3-4 sec in IWT under the eastern flank of the Hidaka Mountains,(2) eastward dipping reflector at 2 sec in TWT under the. western flank of the Hidaka Mountains.(3) westward dipping reflector at 4-6 sec in TWT under the middle part of the Hidaka Mountains,and(3) Deep events at 10-1 1 sec in TWT under the eastern edge of the profile.Data collected on off-line recorders were also completed for the refraction/wide-angle crustal study in the collision zone. A combined analysis of the seismic refection and refraction data indicates the following structural features in the surveyed area.(1) In the eastern part of the Hidaka Mountains, a material with a velocity of 5.8-6.0 km/s is almost outcr6pped.(2) The obtained velocity of 5.8-6.0 km/s is rather small as those of the middle crustal or lower crustal rocks which are expected to expose in the Hidaka Mountains. This suggests that the obducted crustal rocks are highly deformed and merged into the surrounding low-velocity upper crustal material.The seismic image obtained forms a part of the delamination-wedge structure associated with the arc-arc collision. The reflectors in the eastern part of the Hidaka Mountaines Is a upper part of the reflective lower crust of the Kuril Arc overriding westward. Our results provide important key for understanding collision process occurring in this region.