日高衝突帯前縁部における白亜紀付加体の地質構造(<特集>日高衝突帯研究の最近の進歩(1)-その深部過程と上昇過程)  [in Japanese] Geologic structure of Cretaceous accretionary complexes in the frontal Hidaka collision zone, Hokkaido, Japan(<SPECIAL ISSUE>Recent Advances in Research of the Hidaka Collision Zone, Hokkaido, Northern Japan: Deep-Seated Processes and Tectonic Uplift (Part 1))  [in Japanese]

東北日本弧と千島弧の間に形成された日高衝突帯の前縁部では,ジュラ紀末〜白亜紀の前弧海盆堆積物および付加体が広い面積を占める.これらは,初生的には広域的かつ低角なパイルナップ構造をとっていたと考えられる.ナップユニットは,一部に横ずれデュープレックスが形成されたほかは,主として褶曲による構造再配列を被っている.2つの主要な背斜構造のそれぞれで,軸を挟んで非対称な岩相分布を示すことから,これら背斜構造の地下に剥ぎ取り衝上断層のランプが推定される.イドンナップ帯周辺では未成熟な前弧地殻がめくれ上がっていると考えられ,落差10km近い大規模なランプによって持ち上げられたと推察される.日高主衝上断層は,このランプから派生したのであろう.一方神居古潭帯周辺では,恐らく落差4〜5kmのランプにより,低温高圧変成を受けた付加体と被覆層がアンチフォームを形成したと考えられる.

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) tectonostratigraphic relations between many geologic units. Constituent lithologic assemblages, fossil and radiometric ages, and geochemical discrimination of metabasites suggest that the latest Jurassic-Cretaceous systems 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 detachment thrust beneath them. Approximately 10km 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 4-5km high ramp may have formed an antiform consisting of the Naizawa-Iwashimizu composite nappe and sedimentary covers around the Kamuikotan Zone.