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

利用地震反射法（Hidaka Vibroseis 94）、重力测量和大地电磁观测等方法，在236号公路上沿20公里长的测量线对Hidaka碰撞带（Hidaka变质带：HMB）的地壳结构进行了成像。研究结果表明：(1)地表陡倾的Hidaka主逆冲（HMT）在HMB下方45‰处向NNE方向倾斜，在HMB东部下方约7 km处倾斜较为平缓。HMT是一个列表故障。(2)虽然在上部没有看到分隔波罗西里蛇绿岩带和伊顿那普混杂岩带的Hidaka西逆冲（HWT），但在HMT反射面下方1km处发现了反射。这两个反射体之间的部分被推断为波罗什里蛇绿岩。(3)碰撞带前方的don'nappu混杂带浅部存在大量北北东倾反射，为褶皱冲断构造。(4)仅在地震剖面的NNE部分在13 km深度处存在亚水平反射体。该反射面可能对应于HMB北部地震折射剖面得到的速度边界（5.9-6.6 km/s）。(5)包括剖面线在内的区域重力资料显示沿Hidaka山脉有较大的正异常，表明HMB浅部存在高密度物质。(6)根据大地电磁资料，Hidaka变质岩的高电阻率（10,000 ω -m）层厚度约为8km。在层的下面有一个2公里厚的相对导电层（100欧姆），下面是一个非常高的电阻率层（15万欧姆）。

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.