The Southern Chichibu terrane fringes the southern margin of the Jurassic-Early Cretaceous accretionary complex of southwest Japan. Two subterranes, the Togano subterrane in the north and Sambosan subterrane to the south, are recognized by examination of the common features of the terrane. They are juxtaposed throughout the terrane from western Kyushu to the Kanto Mountains a distance of over 1000 km. The geology of the Sakawa area, central Shikoku, is described in detail to show an example of the terrane. The geology around Tsukumi in eastern Kyushu and Okutama in the Kanto Mountains is introduced to emphasize the common features of this terrane.
The Togano subterrane is composed of predominantly Triassic-Jurassic chert-clastic sequences associated with subordinate Jurassic olistostromal sequences and Upper Jurassic-Lower Cretaceous limestone-bearing clastic sequences. The chert-clastic sequences show an upward-coarsening sequence which reflects landward drift of the sea floor from a pelagic realm towards a trench. The olistostromal sequence contains abundant upper Paleozoic blocks. This subterrane is characterized by a south-vergent imbricate structure and south-younging polarity of the chert-clastic sequences.
The Sambosan subterrane consists of mainly Upper Jurassic-Lower Cretaceous olistostromal sequences associated with Upper Jurassic-Lower Cretaceous limestone-bearing clastic sequences. The olistostromal sequence includes abundant Triassic-Jurassic chert blocks and Triassic limestone blocks accompanied by greenstones. The limestone-bearing clastic sequences of both subterranes are regarded as shelf or inner-trench slope sediments deposited on the chert-clastic and olistostromal sequences.
Two major stages of tectonic development are discriminated. The Togano subterrane was formed by successive offscrape-accretion mainly during the Middle to Late Jurassic. The Sambosan subterrane was constructed by collision-accretion of seamounts mainly during the Late Jurassic to Early Cretaceous. The difference in the tectonic processes is considered to relate to topography of subducting oceanic plate; an oceanic plate with an abyssal plain for the Togano subterrane and an oceanic plate with seamounts for the Sambosan subterrane.