Geo-electromagnetic deep structure of old plate boundary-the Hidaka and Kamuikotan belts-

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Broad-band magnetotelluric investigations were made along 3 profiles across the Hidaka and Kamuikotan metamorphic belts, old plate boundary, and the following results were obtained.I. Southern part of the Hidaka-Kamuikotan belts (Hidaka profile) I-1. Shallow resistivity structure is well collerated with the surface geology: the conductive Sorachi-Yezo formations ((] SY.simeq.[) 10 ohm m) and the resistive Hidaka metamorphic belt ((] SY.simeq.[) 1000 ohm m). I-2. Old oceanic plate (trapped oceanic plate in the Jurassic) was detected as a resistive basements of the Sorachi-Yezo formations. I-3. The sediment to the west of the Kamuikotan belt is significantly thicker ((] SY.simeq.[)10 km) than that of the Shibetsu profile ((] SY.simeq.[) 3 km). This difference implies that the deep foredeep is created in the southern part of the Hidaka-Kamuikotan belts by the collision of Hokkaido and the Kurile arc since the Miocene time. I-4. There is eastward dipping conductor and overriding wedge-shape resister beneath the Hidaka mountains. The dipping conductor is consistent with the seismic low velocity and low Q anomaly.II. Northern part of the Hidaka-Kamuikotan belts (Shibetsu profile) II-1. Resistivity structure at shallow depth is generally common with that of Hidaka profile. II-2. Beneath this profile, westward dipping conductive layer was observed, suggesting the existence of the oceanic sedimentary lyer on the Kula plate.III. Eastwardly extended part of the Shibetsu profile (Rubeshibe profile) Although the resistivity structure is complex even at deeper crust of this region, the structure is tend to dip eastwardly. Therefore, the Cretaceous eastward subduction of the Kula plate against the Okhotsk block can be imaged.