Keywords:Hidaka collision zone, serpentine, peridotite, magnetotelluric, kamui-kotan belt, 2018 eastern Iburi earthquake
Middle axis of Hokkaido Island is located on a tectonic boundaries between NE Japan arc and Kurile arc. In the south part of the boundary, collision due to migration of fore-arc aliver is known (Hidaka Collision zone). Huge serpentine rock bodies are exposed in the high P/T metamorphic belt (kamui-kotan belt) located on the NE Japan arc side near the boundary. In addition, a fresh peridotite rock body (Horoman periodotite) is also distributed in the south part around the boundary. However, tectonic process to bring the surface has not been understood although it is the key to understand the tectonics. In this study, we discuss the crustal structure beneath the arc-arc boundary especially for ultra-mafic rock area based on electrical resistivity distribution obtained in following three areas: (1) middle part of Hidaka collision zone where serpentine is exposed, (2) north part of Kamui-kotan belt where serpentine is exposed but the collision is not occurred, and (3) south part of Hidaka collision zone where the Horoman peridotite is distributed. In the area (1) and (2), very low resistivity (1-10 ohm-m) anomalies area found in the depth around 10-40 km beneath the surface serpentine body but they are not significant beneath out of the serpentine area. We interpreted them as high fraction of aqueous fluid area related to serpentine forming and bring them to the surface. In the area (3), we found thin sheet-like high resistivity body near the surface embedded to low resistivity body (Ichihara et al., G3). The surface high resistivity correspond to the peridotite because unaltered igneous rocks are basically resistive. The underlying conductive area possibly reflect cretaceous sedimentary rock based on integrated interpretation of geomagnetic anomaly, gravity anomaly and the resistivity distribution. If these interpretations in the area (3) are true, the Horoman peridotite covers over the sedimentary rock, implying the unknown tectonics processes occurred on the south part of Hidaka collision zone.