[S-SS08] Crustal Structure

[SSS08-P02] Estimation of Resistivity Structure around the Horoman Peridotite Complex in the Hidaka Metamorphic Belt, Hokkaido

*Tomoka Ejiri¹, Hiroshi Ichihara¹, Kazuki Matsuyama ¹, Sota NAKAHAMA¹, Shinichiro Horikawa¹ (1.Nagoya University )

Keywords:Horoman Peridotite, Hidaka Collision Zone, Hidaka Main Thrust

The Hidaka Collision Zone was formed by the collision of the Kurile Arc and the Northeast Japan Arc in the Hidaka area of Hokkaido. At the southern end of the Hidaka Collision Zone, the Horoman Peridotite primarily consisting of mantle material such as peridotite, is exposed. The Horoman Peridotite is a fresh peridotite that has not been altered, and therefore it preserves important information about the upper mantle, making it geological significance. According to geological and petrological studies, the metamorphic rock stratigraphy of the Hidaka Metamorphic Belt indicates that the Kuril Arc was upturned by the collision. To clarify the actual underground structure, geophysical surveys have been conducted. Ito (2000) and Iwasaki et al. (2019) detected seismic reflectors in the lower crust based on seismic reflection surveys using controlled sources. They proposed the delamination model, in which the lower crust of the Kurile forearc is delaminated into upper and lower parts due to the collision of the two arcs; the upper part of the Kurile forearc thrust onto Northeastern Japan Arc, and the lower part descends. Kita et al. (2012) modeled seismic velocity structures based on seismic tomography using natural earthquakes and estimated low-velocity and high-velocity zones that were interpreted as crustal material and upper mantle material, respectively. They proposed a different model from the delamination model which the entire crust and uppermost mantle of the Kurile Arc thrust onto the Northeastern Japan Arc. However, neither model reveals the detailed structure around the Horoman Peridotite Complex. In the Magnetotelluric (MT) survey by Ichihara et al. (2016), a sheet-like high resistivity zone was identified in the shallow subsurface beneath the Horoman Peridotite outcrop area. Although this high resistivity area might reflect the Horoman Peridotite, Ichihara et al. (2016) did not discuss it due to absence of an MT station directly on the Horoman Peridotite, which led to insufficient accuracy of resistivity structure.
Therefore, in this study, we conducted MT surveys and deployed nine new observation points directly on the Horoman Peridotite. In the data analyses, we selected a time range with less noise from the time series of observational data and calculated the MT response. The MT response was estimated using the BIRRP code (Chave and Thomson, 2004), which transformed the MT data from time domain to frequency domain and calculated the apparent resistivity and the phase from the impedance at each frequency. As a result, the apparent resistivities are high at high frequencies and shift to low with decreasing frequency. Since apparent resistivity at lower frequency reflect the deeper structures, this indicates a tendency for high resistivity in the shallow subsurface and low resistivity in the deeper areas.