Vibroseis Seismic Reflection Profiling, Crustal Strructure and Tectonics of Hidaka Collion Zone, Hokkaido, Japan

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We tried to image the crustal structure of the Hidaka collision zone (Hidaka metamorphic belt : HMB) along a 20km-long measurement line on Route 236 across the zone by using Vibroseis seismic reflection method (Hidaka vibroseis 94), gravity survey and magnetotelluric observations. The following conclusions are rearched :(1) The Hidaka Main Thrust (HMT) which shows steep dip at the surface dips NNE at 45ﾟ beneath the HMB,and dips more gently at about 7-km depth beneath the eastern part of the HMB.The HMT is a listric fault. This suggests the HMB to cut the lithology boundary and schistosity of the HMB(2) Although the Hidaka Western Thrust (HWT) separating the Poroshiri ophiolite belt from the Idon'nappu melange belt is not seen at the upper part, a reflectior is detected 1km below the HMT reflector. The part between these two reflectors is inferred to be the Poroshiri ophiolite.(3) The shallow portion of the don'nappu melange belt at front of the collision zone has a number of NNE-dipping reflections, indicative of a fold-and-thrust structure.(4) There is a subhorizontal reflector at 13-km depth only in the NNE part of the seismic profile. This reflector may correspond to the velocity boundary (5.9-6.6 km/s) obtained from seismic refraction profiling in the northern HMB.(5) The gravity data in the area including profiling line show large positive Buoguer anomalies along the Hidaka Mountains, suggesting an existance of high-density material at shallow part of the HMB.(6) Based on magnetotelluric data a high resistivity (10,000 ohm-m) layr of the Hidaka metamorphic rocks reaches about 8km in thickness. Below the layr there is a 2-km thick relatively conductive (100 ohm-m) layr which is underlan by a very high resistivity (150,000 ohm-m) layr.