Lateral structural variation across a collision zone in central Hokkaido, Japan, as revealed by wide-angle seismic experiments

Abstract

The crustal evolution in the central part of Hokkaido Island, Japan, has been dominated by a series of accretion and collision processes from the late Jurassic to the present. This region is divided into three main tectonic belts (the Sorachi-Yezo, Hidaka and Tokoro belts from west to east) formed in different Cretaceous subduction systems. Since the Miocene, the Sorachi-Yezo Belt and the western part of the Hidaka Belt (the Hidaka Metamorphic Belt, HMB) have suffered severe deformation during two tectonic events: the dextral oblique collision between the North American (Okhotsk) and Eurasian plates and the arc–arc collision of the Kuril forearc sliver with the western part of Hokkaido (the NE Japan Arc). A thrust belt along the western margin of the Sorachi-Yezo Belt is considered to represent the Miocene to Pliocene eastward subduction of the NE Japan Arc due to the compressional stress regime. This subduction jumped to the eastern margin of the Japan Sea in Pleistocene time. The reanalysis of wide-angle seismic data collected in central Hokkaido has provided a new image of crustal-scale structural variation and deformation in the tectonic environments mentioned above. The western part of the Sorachi-Yezo Belt is characterized by a 2–4.5 km thick forearc sedimentary layer with a velocity of 3.0–3.9 km s−1, beneath which a 3.9–5.8 km s−1 body forms an eastward-dipping boundary to the upper unit. This boundary may be the eastward extension of the thrust belt, representing the Miocene to Pliocene plate boundary. A 5.3–5.7 km s−1 body almost crops out beneath the eastern part of the Sorachi-Yezo Belt where ophiolite sequences and high-P metamorphic rocks are exposed. Our rather low velocity of 5.3–5.7 km s−1 may suggest that the original oceanic crustal rocks or the metamorphics were fragmented and merged with low-velocity accretionary prism rocks. Plausible tectonic events responsible for this process are fast uplift of a large amount of underplated materials in the Cretaceous subduction system of this region and/or severe deformation by collision tectonics since the Miocene. The HMB is characterized by high-T metamorphic rocks almost equivalent to those of continental or arc crust. The seismic velocity in the upper 5–7 km of crust in this part increases westwards from 5.6 to 5.85 km s−1, showing a good correlation with the change in metamorphic grade of the exposed rocks. The velocity structure in the eastern part of the Hidaka Belt is composed of 15–20 km thick upper crust overlying 6.6–6.7 km s−1 lower crust. The crystalline part of the upper crust is divided into a 6–7 km thick 5.6–6.0 km s−1 layer and an 8.5–11 km thick 6.2–6.35 km s−1 layer. In the depth range 25–45 km, there exist crustal laminations, probably forming a ductile and sheared zone. Good agreement of this velocity structure with physical properties of the metamorphics exposed in the HMB indicates that a 25–26 km thick brittle part of the Hidaka crust was obducted towards the west to form the HMB under the collision framework in this region. Major layer boundaries east of the HMB show a gentle eastward dip, which might have acted as a decollement at the time of the obduction. The Tokoro Belt, which is composed of accreted Jurassic seamounts, is marked by a high upper-crustal velocity (≥6.1 km s−1) in our model. This velocity represents relatively mafic rock compositions of the seamounts and/or oceanic crust trapped in this belt.