Geologic structure of Cretaceous accretionary complexes in the frontal Hidaka collision zone, Hokkaido, Japan

Abstract

Latest Jurassic to Cretaceous forearc and accretionary complexes are the major constituents of the frontal fold-and-thrust belt in the Hidaka collision zone, which formed between the Northeast Japan and the Kurile arcs. Their complicated geologic structure is here modeled through reconstruction of the original（pre-collision）tectonostrati- graphic relations between many geologic units. Constituent lithologic assemblages, fossil and radiometric ages, and geochemical discrimina- tion of metabasites suggest that the latest Jurassic - Cretaceous sys- tems originally had regional, near-horizontal pile-nappe structure. It comprises the Lower Sorachi ophiolite nappe with the Nitarachi-Yezo forearc basin cover sequence, the Naizawa-Iwashimizu composite nappe（Early Cretaceous accretionary complex） , the Horobetsugawa- Pankehoronai nappe（Late Cretaceous accretionary complex） , and the basal Poroshiri Ophiolite, structurally downward in ascending order. The Kurile Arc collision rearranged the pile-nappe structure mainly by folding, partly with strike-slip duplexes, rather than imbricate thrusting. Asymmetric unit distribution across the two major antiforms suggests the existence of the corresponding ramps of the main detach- ment thrust beneath them. Approximately 10 km high ramp may have lifted and tilted up the almost entire section of the immature forearc crust consisting of the nappe units around the Idonnappu Zone. The Hidaka Main Thrust, the major collision boundary, probably branched off this big ramp to the surface. The other 4-5 km high ramp may have formed an antiform consisting of the Naizawa-Iwashimizu composite nappe and sedimentary covers around the Kamuikotan Zone.