MELANGES ON GEOLOGICAL MAPS AT A SCALE OF 1:50,000

The author has conducted the detailed geological mapping on the mélanges in the Jurassic accretionary complexes since 1977. The geological maps of his recent works show the detailed structures of the mélanges based on the understanding of melange formation processes in the accretionary complexes.

The Jurassic accretionary complexes of Japan were composed mainly of Carboniferous to Triassic basalt and limestone derived from ancient seamounts, Carboniferous to Jurassic pelagic chert, P-T boundary claystone, Triassic to Jurassic hemipelagic siliceous shale, and Jurassic to early Cretaceous sandstone and shale of trench turbidite. These components were originally derived from the “Ocean Plate Stratigraphy” which were formed on the past ocean plate moving from ocean ridge to trench, and accreted to the continental margin.

The “Ocean Plate Stratigraphy” was deformed, disrupted and mixed during the accretionary processes to form melange structures. The disruption was caused mainly near the hemiplagic siliceous shale and P-T boundary claystone where decollement was developed during the accretionary processes. Lower part of chert, limestone and basalt were fragmentated and mixed near the P-T boundary claystone, while upper part of chert and trench turbidite were disrupted near hemipelagic siliceous shale. The middle part of chert formed large slabs in mélanges as remnants of disruption and fragmentation.

The structures of mélanges are not completely chaotic. The original structures of “Ocean Plate Stratigraphy” were preserved in the mélanges in some degree. The stratigraphic tops of the large slabs mostly show the same direction in the mélanges. Some of large tectonic slabs in the mélanges kept the original order of “Ocean Plate Stratigraphy” even though they were deformed in various degrees. It suggested that “Ocean Plate Stratigraphy” was tectonically stacked during the offscraping process, and was gradually disrupted along the decollement and out-of-sequence thrusts during the accretionary processes to form the mélange structures.

These structures of the melanges mentioned above were represented in the geological map at a scale of 1:50,000 recently published by Geological Survey of Japan. These geological maps clearly show the melange formation processes during ocean plate subduction along Mesozoic Asian continental margin.