The Shimanto Belt of Japan: Cretaceous-lower Miocene active-margin sedimentation

Abstract

Summary South-east of the two paired metamorphic belts of Kyushu, Shikoku and Honshu, and separated from them by the Chichibu and Sambosan belts and the Butsuzo Tectonic Line, lies a belt of mildly metamorphosed sedimentary rocks called the Shimanto Supergroup. Their area, extending from the Nansei Islands through Kyushu, Shikoku, Kii Peninsula, Akaishi and Kanto Mountains to the Boso Peninsula, is comparable in size with the combined Franciscan Formation and the Great Valley sequence in California. A Cretaceous lower group is flanked on the Pacific side by an upper group of Palaeogene to early Miocene age. The Shimanto sediments, mostly sandstones and mudstones, were deformed by cyclic subduction into open and isoclinal folds which close south-eastwards, and thrust slices which in places incorporate basaltic pillow lavas and radiolarian cherts. Sandstones from Kyushu and Shikoku show marked changes of composition with stratigraphic position and the feldspar content can be used to distinguish Cretaceous from Tertiary sandstones. Data on sandstone petrography and palaeocurrents indicate that sediments of the Shimanto Supergroup were probably derived from the NW. Precambrian gneisses and older Mesozoic granitic rocks of the Korean Peninsula may have contributed sediments in Early Cretaceous times, when the Shimanto Terrane was located to the east of southern Korea. The unroofing of granites in the Inner Zone of SW Japan may also have contributed to the Cretaceous and later sedimentation. Quartz-rich, well-sorted sands in younger Shimanto sediments may have been recycled from older Shimanto formations. The thick Shimanto sedimentary sequences were laid down in a variety of environments within a forearc basin on accretionary complex, trench-slope break, trench inner slope and trench settings; first in the Cretaceous and again, in a more southerly position, in the Palaeogene and early Miocene. In the shallower waters on the inner (arc) side of the Palaeogene forearc basin, coarsening- and thickening-upward deltaic sequences were deposited. In deeper water further offshore, submarine channel and fan complexes, base-of-slope slump deposits and red shales with cherts accumulated. Mélanges with basalts and slump olistostromes occupied the trench-slope break, while inner trench wall perched basins on accretionary basement were filled with coarsening-upward flysch, slump-olistostrome facies and mélanges with metabasalts: similar lithologies probably occupied a trench fill. Sedimentation and deformation were controlled by intermittent cyclic subduction of the Kula Plate towards the north. Phases of tension and down-faulting, forming long, narrow intra-arc basins, alternated with those of compression. Comparable sedimentary environments are found today off SE Japan on the submarine terraces, trench-slope break, inner slope (including perched basins) and fill of the Nankai Trough.