Mappable features of mélanges derived from Ocean Plate Stratigraphy in the Jurassic accretionary complexes of Mino and Chichibu terranes in Southwest Japan

摘要

The mélanges of the Jurassic accretionary complexes of Japan have very complicated structures which were formed by the progressive disruption and mixing of Ocean Plate Stratigraphy (OPS) during various stages of accretionary processes along the ancient arc-trench system. The OPS is composed of Permian basalt-limestone-chert, Permian-Triassic boundary claystone, Early Triassic-Early Jurassic chert, Early Jurassic-earliest Cretaceous siliceous shale and Jurassic to early Cretaceous turbidite, in ascending order.The OPS was detached from the oceanic plate along a decollement, which developed within the P-T boundary claystone during the offscraping and underplating processes. Disruption and mixing first occurred along the decollement, then along successively developed out-of-sequence thrusts. Neighboring rocks along the faults are mixed to become various sized clasts and matrices of mélanges. Mappable slabs in the mélanges were survivors from tectonic disruption of the accreted OPS. On the surface of accretionary wedge, a part of the OPS was collapse by submarine sliding and mud diaprisim to form chaotic sediments on the top of the OPS. Thus, the full extent of accretionary complex was analyzed by age-stage development of mélange formation. In the mélanges, some of the large tectonic slabs in the mélanges kept the original order of the OPS even though they were deformed in various degrees. This suggests that the OPS was tectonically stacked during the offscraping process, and was gradually disrupted along the decollement and out-of-sequence thrusts during the accretionary processes that form the mélange structures. The mélanges structures mentioned above are shown in the geological maps at the scale of 1:50,000 recently published by the Geological Survey of Japan. These geological maps clearly show the mélange formation processes during ocean plate subduction along the Mesozoic Asian continental margin.