ORIGIN OF THE SOUTH KITAKAMI PALEOLAND AND SUCHLIKE ALLOCTHONOUS CONTINENTAL BLOCKS IN THE JAPANESE ISLANDS

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There were subduction-related granitic-gabbroic igneous activities in the South Kitakami and Hayachine Tectonic Belts of Northeast Japan, which are remnants of the South Kitakami Paleoland, during the latest Ordovician to earliest Silurian age. The South Kitakami Paleoland and Kurosegawa Landmass are closely related in their geotectonic history and paleobiogeography during the Paleozoic to Mesozoic, and are considered to be originated and developed in a closely located region with same tectonic settings. The Hida-Gaien Belt differs from the South Kitakami in lithofacies of the Middle-Upper Paleozoic and, particularly, in Mesozoic geotectonic history. Based on the coral fauna and ammonoid ones, the South Kitakami situated in a gateway between the Panthalassa and Tethys Oceans at early Carboniferous, and near South China during the Permian to Triassic, both at a low latitude.The major and trace elements analyses on the in situ basalts in the accretionary complex of the Shimanto Belt suggest that these green rocks were formed by igneous activity in a subduction zone at and close to the ancient trench-trench-ridge triple junction. Following the migration of the triple junction, which forms of a slab window and interrupts volcanic arc-type igneous activity, forearc rocks were uplifted to the surface, forming a high mountain range. The Oshima Orogeny in the Kitakami Massif, which took place at the early Cretaceous, is one of this type orogenies.