A collision boundary imaged by magnetotellurics, Hidaka Mountains, central Hokkaido, Japan

Abstract

Magnetotelluric (MT) data were acquired across the Hidaka Mountains, which are thought to have formed by collisional orogenic processes. A regional electrical strike of N18°W, consistent with surficial geology, was derived from impedance tensor decomposition. Two‐dimensional resistivity models were constructed by forward model fitting with preference for reducing the misfit of transverse magnetic mode. The final modeling imaged a collision boundary between Cretaceous forearc basin rocks and continental crust. The major features of the model are (1) a 5‐km‐thick surficial conductive layer corresponding to Cretaceous forearc basin and the underlying resistive basement that shallows and is exposed to the east where Sorachi formations, characterized by oceanic crustal rocks, crop out. Therefore we interpret the underlying resistive layer as oceanic crust. (2) A thin (500 m thick) conductive (3 Ω m) anomaly corresponds to serpentinite exposure in the Kamuikotan metamorphic belt. (3) Exposed deep crustal rocks in the Hidaka belt showed high resistivity (1000–2000 Ω m) at the surface. The subsurface westward extension inferred from the MT data is supported by seismic refraction data. (4) In the Hidaka Mountains, a relatively conductive (500–1000 Ω m) layer implying accretionary prism sequences, was inferred at 2‐to 10‐km depth beneath a more resistive (1000–2000 Ω m) surface layer. (5) A wedge‐shaped resistivity anomaly was found at 5‐to 50‐km depth beneath the Hidaka belt. The resistivity of this wedge decreases eastward from 30,000 Ω m to 2000 Ω m, consistent with the surface decrease of metamorphic grade. The spatial distribution is also consistent with a seismic high‐velocity anomaly. (6) The western side of the resistive wedge is fringed by a dipping conductor. Correlations with a low‐velocity anomaly and high seismicity suggest that this conductor is an enhanced porosity region, and that it may be working as a detachment zone at the collision boundary. The high resistivity contrast between oceanic crust and high‐temperature metamorphosed continental crust permitted us to image the interfingered complex structure beneath the Hidaka Mountains from 300‐m to 50‐km depth.