<国立科博専報>北海道周辺海域の地球物理学的研究

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1. From seismic profiler records, three reflectors were read in sediments on the island shelf and slope. Layer A, above reflector A, is generally trapped on the indented topographies. Layer B, from reflector A to reflector B, is generally unconformable to Layer C which has itself many reflectors under reflector C. Reflectors B and C undulate fairly parallel. One of the reflectors within Layer C, is assumed to be a boundary between sediments and crystalline rocks (basement) from seismic refractoin data in the adjacent area. On the slope brake, Layer C outcrops and thickness of the sediments above crystalline basement decrease. In the basin area, there is no distinct reflectors in sediments above acoustic basement. As the surface of the basement is generally smooth, it is probably said that thin opaque sediments are covering the oceanic layer 2. On the Shatsky Rise, the boundary between transparent sedimentary layer and the underlying opaque one corresponds to the boundary between Paleogene and upper Cretaceous, chronologically, according to JOIDES. Therefore, it is assumed that age of oceanic crust south of Hokkaido is probably uppermost Cretaceous. 2. In the sea around Hokkaido, topographic, geologic and geomagnetic trends are traced as shown in Figs. 17 and 20. Extension of those trends are traced on land and assumed to be geological trends of pre-Miocene. From those data, geological patterns of Hokkaido are presumably extending to the SSE-direction and approach to the axis of trench. In the Okhotsk Sea, trend α is assumed to be a belt connecting Mesozoic unit of the Naka-Shiretoko Peninsula in Sakhalin and one of the Tokoro belt in Hokkaido. Trend δindicates a southwest boundary of the Okhotsk Basin. 3. From the magnetic and gravity anomaly patterns (Figs. 21 and 22,respectively) a geological boundary is presumed in the area south off Kushiro. Assuming that the boundary coincides with the Kushiro Canyon, the southernmost part of X-Y line in Fig. 22 was drawn. The authors take a hypothesis that the Japanese Islands split from the Asiatic Continent and also the Kurile arc did from the Okhotsk continental shelf. At the time of the splitting, a straight line X-Y in Fig. 2 might be a shear fault which was assumed to have bent as shown in Fig. 20 by the succeeding tectonics of the Kurile Arc. The central part of the curved line X-Y was drawn tentatively in oder to complete the line. The authors wish to call this line "Kushiro Canyon Tectonic Line". 4. As shown in Fig. 21,clear magntic anomaly patterns exist, which trends to run ENE-WSW and are detectable on both the sides of the trench wall. There is no distince differences in amplitude of the magnetic anomaly patterns on both the walls, but on the landside wall the amplitude decreases and becomes undetectable at the distance of 50-60km from the trench axis. If the decrease of amplitude is assumed to be brought by sinking of oceanic plate under the island crust, dip angle of the plate is calculated to be about 10 degrees within the range of 50-60km from the axis. It is important, geophysically, that the oceanic plate is not strongly disturbed near the axis. Along the Kurile Trench, magnetic patterns indicate short waves superposed on the original long waves. Those short waves may have some relations with faults which are frequently found parallel to the trench axis. 5. From patterns of geological trends and presumable tectonic line "Kushiro Canyon Tectonic Line", it may be assumed that a fairly broad area of island crust had existed outside the present trench axis and was brought under the island arc by the process of island arc tectonics. There is such a possibility that the Japanese Islands had a straight form and a geological belt corresponding to the Hidakas had existed on the outermost boundary of the Outer Belt of Japan before the Japanese Islands split from the Asiatic Continent. There is a small difference in geological trends between the ones belonging to NE Japan-SW Ho

<国立科博専報>北海道周辺海域の地球物理学的研究

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