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

我们试图用可控震源地震反射法(Hidaka Vibroseis 94)、重力测量和大地电磁观测，沿236号公路上一条长达20公里的测线，对东高碰撞带(Hidaka变质带：HMB)的地壳结构进行成像。结果表明：(1)东高主冲断带在地表呈陡峭倾角，在45ﾟ处向北北东倾斜，在东侧约7公里深度处倾角较缓，为一条板状断裂。这表明HMB切割了HMB的岩性边界和片理。(2)虽然在上部看不到将波罗什里蛇绿岩带与Idon‘nappu混杂岩带分开的东达卡西冲断(HWT)，但在HMT反射层下方1公里处发现了反射体。这两个反射体之间的部分被推测为波罗什里蛇绿岩。(3)碰撞带前面的东纳普混杂岩带的浅部有多个NNE向倾斜的反射，指示了褶皱逆冲构造。(4)仅在地震剖面的NNE向部分存在13公里深的近水平反射层。该反射体可能对应于HMB北部地震折射剖面得到的速度边界(5.9-6.6公里/S)。(5)包括剖面线在内的该地区的重力数据显示沿日达卡山脉有较大的布格尔正异常，表明HMB浅层存在高密度物质。(6)根据大地电磁资料，日达卡变质岩层的高电阻率(10,000欧姆-米)厚约8公里。在层下面有一个2公里厚的相对导电层(100欧姆-米)，它的下面是一个非常高的电阻率(150,000欧姆-米)层。

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.