An attempt to reconstruct oceanic plate configuration offshore Japan in the late Mesozoic based on tectonically accreted rocks

Tectonically accreted oceanic rocks and ophiolites are fragmented remnants of oceanic crust once existed offshore continental margins. Although subducted slabs are now lying in untouchably deep interior of the Earth, the accreted fragments on the surface provide information on their age, geochemistry, topography, and interaction with other plates before subduction. We test geological approaches to plate reconstruction from accreted oceanic rocks in Japan. Late Mesozoic accretionary complexes (AC) together with cover sediments in Japan can be divided roughly into three terranes: Inner and Outer zones and Eastern Hokkaido. Inner Zone is characterized by wide occurrences of Jurassic to Early Cretaceous AC, and they together with cover sediments show close relation to Sino-Korea and Primorie (Fareast Russia). The Outer Zone also contain AC of similar ages, however, cover sediments yield tropical fauna and flora, inconsistent with present-day latitude. Triassic Megalodontoid limestones, characteristic of Tethyan realm, exclusively occur in the Outer Zone as accreted seamount fragments. Central Hokkaido in Northern Japan also belongs to the Outer Zone, however, Jurassic ophiolites with arc-derived detritus and partly with boninites are characteristic. Eastern Hokkaido is a part of Kurile arc-trench system on the southern margin of the Okhotsk Sea, distinct from the other terranes formed along the Eurasian margin. Duplicated structure of Inner and Outer zones and differences in paleobiogeography between them have been explained by northward drift of the Outer Zone relative to the Inner Zone. Following this scenario, central Hokkaido can be reconstructed to lie about the junction between the Outer Zone in the south and Inner Zone in the north. Supra-subduction zone nature of the central Hokkaido ophiolites, suggestive of intra-oceanic subduction between two oceanic plates, implies that oceanic plates responsible to accretionary growth of Inner and Outer zones were distinct. We assume that a subducted Tethyan plate provided Megalodontoid limestone blocks to the Outer Zone in the south, and a plate in the Panthalassa (presumably Izanagi) was subducted to grow the Inner Zone in the north. According to this hypothesis, the Hokkaido ophiolites could be formed in the junction area between Tethys and Panthalassa.